ΠΑΝΕΠΙΣΤΗΜΙΟ ΚΡΗΤΗΣ
UNIVERSITY OF CRETE

G. Goslovich

D. Manske

Abstract Proceedings

ULTRAFAST DYNAMICS & METASTABILITYULTRAFASTBANDGAPPHOTONICS

2023

X Symposium
1Professor Michael K.Rafailov
University of Alberta

Professor Ilias E Perakis
The Foundation Research & Technology – HELLAS
University of Alabama at Birmingham

Abstract Proceedings comprising summaries of research papers prepared for two closely related Conferences – Ultrafast
Bandgap Photonics and Ultrafast Dynamics and Metastability of Transient States, forming the Symposium. The Symposiumbuilds a bridge between cutting edge modern physics and emerging applications which are desperately in need. Symposium isvertically integrated, covering the area of interaction of high intensity and relatively low energy pulses with condensed matter,from fundamental physics to practically applicable energy sources, devices and technologies.

This workshop is the 10th in a series of International symposia, conferences and workshops on Ultrafast Dynamics and closely
related and application technology driven Ultrafast Bandgap Photonics which we are running from 2016. University of Creteand Foundation for Research and Technology - FORTH are hosting the 10th Symposium on the island of Crete.

The Symposium is organized by sections that are focused either on research field or on phenomena. The division is pretty muchconditional, while providing direct access to general topics of interest for the research community and applications as well.
Phenomenology topics, like ultrafast dynamic in hterostructures and spin-and orbital ultrafast phenomena may overlap areas ofstudies like photoinduced high temperature superconductivity and ultrafast magnetism, creating multiple entries into theProceedings General reviews of Ultrafast Dynamics and Ultrafast Bandgap Photnics progresss are provided in 7 Plenary
papers, while the most interesting recent discoveries are presented here as the keynote papers. Each section begins with theplenary and keynote papers, where results and considerations of most common interests are presented, while the Astract
Proceedings is compiled based alphaetic list of the speakers in Program -as it was presented at the Symposium. Plus to Tableof Contents the Abstract Prosceeding has list of authors where the authors of submitted papers are listed with their papers
associated page numbers.

The Abstract Proceeding is actually a snapshot of most interesting and noticeable research results in Ultrafast Transient
Phenomena and Ultrafast Bandgap Photonics which one can get in 2023. a snapshot of up to date research results and progressin Ultrafast Dynamics and Metastability and the applications in Ultrafast Bandgap Photonics. It is an Encyclopedia of Ultrafast
Dynamics, Metastability and Ultrafast Bandgap Photonics that presents the status quo in the disciplines ranging from
Theoretical Physics to Ultrafast Laser and covering practically all phenomena of interests in interaction of ultrashort highintensity-low energy interaction with condensed matter .

In Loving Memory of Elena

Contents
Peter Abbamonte“Observation of Pines’ demon in Sr2RuO4 with momentum-resolved EELS"………………………………………………………………….8
Elsa Abreu"Conductivity dynamics in THz driven spin-ladders"………………………………………………………….……………………………….............9Martin Aeschlimann"Ultrafast dynamics of charge carriers in energy and momentum space"…………………………………………………………………...10
Andrey Akimov"Hybrid phonons in the structures with van der Waals nanolayers"……………………………………………………………………………12
Nadia Antoniadis"Observation of giant nonlinearities and photon bound-states using a single artificial atom"………………………………………14
Mete Atatüre"Spin-photon quantum interfaces and their applications"…………………………………………………………………………………………15
Ed Barnes"Entangled photon factories and multinuclear gates for quantum networks and computing"……………………………………...16
Marco Battiato"Emergence in out of equilibrium systems: theoretical treatment of complex out-of-equilibrium
dynamics and the ensuing phenomena"……………………………………………….………………………………………………………...…..…….17
Michael Bauer"Non-equilibrium carrier dynamics and carrier-phonon interaction in 2D quantum materials"………………………………….....19
Nadia Belabas"Quantumness in a tight space: Randomness, n-reconfigurability and multiparty entanglement on chip"……………….....….21
Jens Biegert"Energy conversion pathways and multi-body physics revealed by attosecond soft x-ray core-level
spectroscopy "……………………………………………………………………………………………………………………………………………..…...........22
Rolf Binder"The polaritonic Bardeen-Cooper-Schrieffer state"………………………………………………………………………………………..………….24
Fabio Boschini"Advances in time-resolved photoemission spectroscopy with high-intensity mid-IR
excitation at ALLS"………………………………………………………………………………………………………………………………………………...26
Sangam Chatterjee"Ultrafast dynamics and applications of type II semiconductor heterostructures"……………………………………………………….27
Iannis Chatzakis"Ultrafast hot-carrier cooling in quasi-free-standing bilayer graphene with hydrogen intercalated atoms"………….………28
Ebert Chia
"Controlling THz emission in topological materials"………………………………………………………………………………………………....29
Giacomo Coslovich“Light-enhanced charge density wave coherence in a high-temperature superconductor”………………………………..................30
Carl Davies"Ultrafast magnetization reversal driven by circularly-polarized optical phonons"………………………………………………….….31
Jeffrey Davis"Effects of Floquet Engineering on the Coherent Exciton Dynamics in Monolayer WS2". ………………………………………………32
Mark Dean"Ultrafast dynamics from the perspective of resonant inelastic x-ray scattering"………………………………………………………...34
Jure Demsar"Non-equilibrium superconductivity in THz driven two-band superconductor MgB2"………………………………………………….36
Sukhdeep Dhillon"Ultrafast properties of THz lasers in THz spintronics "..…………………………………………………………………………………………...37
Scott Diddams“Few-cycle pulses for optical frequency comb spectroscopy from the ultraviolet to mid-infrared”…………………….,,………....39
Hermann Durr
"Ultrafast spin dynamics in antiferromagnets"…………………………………………………………………………………………………………40
Hubert Ebert"Theoretical investigations on ultra-fast demagnetization using ab-initio and simulation methods"…………………….………41
Majid Ebrahim-Zadeh"Ultrafast optical parametric oscillators spanning the ultraviolet to mid-infrared"……………………………………………………43
Martin Eckstein“Theory of ultrafast disordering in photo-induced phase transitions”…………………………………………………….…………………...46

Stefan Eisebitt".Probing magnetization dynamicswith atomic selectivity in the fs- to pc range"…………………………………………………..……47
Sebastian Fava
" Search for the ultrafast Meissner effect in driven YBa2Cu3O6.48"………………………………………………………………………...........49
Jon Finley“Deterministic generation of quantum states of light using single and coupled QDs”………………………………………………..….50
Herbert Fotso"Tracking the thermalization of an interacting disordered system under an interaction quench"………………………………...51
Rafail Frantzeskakis"Time-crystalline behavior in central spin models with Heisenberg interactions"………………………………………………..………52
Jim Freericks"Theoretically describing pump-probe experiments in electron-phonon coupled systems out to ps time scales"……………..53
Josef Freudenstein"Attosecond chronoscopy of many-body correlations in quantum materials"…………………………………………………………….55
Nuh Gedik"Time, energy and momentum resolved probing of charge order dynamics in a Kagome metal"…………………………………..57
Matthias Geilhufe"Transient magnetism and entropy production"………………………………………………………………………………………………….……58
Romain Geneaux“Direct observation of few-femtosecond spin currents across metallic layers”………………………………………………..……………59
Giacomo Ghiringhelli“First results of high-resolution pp-RIXS on correlated materials from the European XFEL” .........................................................60
Isabella Gierz"Non-equilibrium carrier dynamics and band structures of 2D materials"………………………………………………………………….62
Alexander Gray
"Atomic-level design and ultrafast THz E-field control of the emergent ferromagnetism at oxide interfaces"……………...…63
Uwe Griebner"Generation of few-cycle pulses beyond 10 µm wavelength with 70 µJ energy at 1 kHz repetition rate"……………………..…64
Kenan Gundogdu,"Hidden order and high temperature superfluorescence in lead-halide perovskites"………………………………………..………….65
David Hagan“Multi-photon absorption in direct-gap and indirect-gap semiconductors"…………………………………………………………………66
Zahid Hasan“Ultrafast studies of novel topological quantum matter”…………………………………………………………………………………………...68
Niklas Hofmann"K-dependent band gap renormalization in a WS2-graphene heterostructure."…………………………………………………............69
Ulrich Höfer"Birth, rise and collapse of Floquet-Bloch bands on subcycle time scale"……………………………………………………………………..70
Philip Hofmann“Line shapes in ultrafast XPS"………………………………………………………………………………………………………………………………….72
Mark Hogg"Cavity-enhanced single-shot readout of a quantum dot spin state on nanosecond timescales"…………………………………….73
Michael Horn von Hoegen"Nonthermal melting of a charge density wave in atomic wires at the quantum limit"……………………………………..………….74
Wanzheng Hu"Transient gap generation in iron-based superconductors driven by coherent lattice vibrations"…………………………………76
Anton Husakov"Linking high-harmonic generation and strong-field ionization in bulk crystals"……………………………………………………..…77
Amiel Ishaaya" Highly efficientfFirst StokesgGeneration ingGas-filled hollow-core photonic bandgap fibers"…………………………………….79
Shinichiro Iwai
"Ultrafast coherent dynamics in strongly correlated conductors and superconductors "………………………………………………..81
Allan Johnson"All optical control of an insulator-to-metal phase transition by dynamic reduction in dimensionality"……………………..... 82
Steven Johnson,"Ultrafast dynamics and control of structure in charge-density-waves“……………………………………………………………………...83
Long Ju"Infrared spectrosocpy of 2D moire superlattice materials"…………………………………………………………………………….………...84
Dominik Juraschek"Magnetic resonances of chiral phonons"……..……………………………………………………………………………………………………..……85

Nick Karpowicz"Field-resolved metrology for attosecond-scale carrier dynamics”. ……………………………………………………………..……………..86
Guru Khalsa..
“New strategies for ultrafast structural control in the mid- and far-infrared".……………………………………………………..……...87
Tobias Kippenberg"Ultra low loss Silicon nitride integrated photonics:
from chipscale combs, traveling wave parametric and Erbium amplifiers to frequency agile low noise lasers”………………88
Sota Kitamura"Floquet topological superconductivity induced by chiral many-body interaction "…………………………………………….............89
Junichiro Kono
"Spectroscopy of vacuum-driven materials"……………………………………………………………………………………………….…………….90
Roopali Kukreja"Unraveling optically induced ultrafast distortions of magnetic textures using fs x-rays"…………………………………………….91
Pierrick Lample“Time resolved studies of ultrafast dynamics in wide band-gap materials”………………………………………………………………....92
Eric Landahl"Ultrafast dynamics in semiconductors across the Brillouin zone measured by time-resolved diffuse scattering"…………...93
Yannis Laplace"Tunable THz surface plasmon cavities in the deep plasmonic regime"………………………………………………………………...........95
Albert Liu"Distilling the nonlinearity of a cuprate superconductor using 2-D terahertz spectroscopy"…………………………………………96
Wolfgang Löffler"Synthesis of artificial states of light from true single photons". ………………………………………………………………………………97
Paul van Loosdrecht"Ultrafast dynamics in 2D TMDCs"…………………………………………………………………………………………………………………..………98
Dirk Manske"How to activate and detect the Higgs mode in superconductors"………………………………………………………………………...........99
Masakazu Matsubara"Spin-driven optical nonlinearities for detection and generation of spin currents"……………………………………….….................100
Johan Mentink"Challenging space-time-energy limits of magnetism by harnessing fluctuations"………………………………………………………..101
Matteo Michiardi"Optical manipulation of Rashba-split 2-dimensional electron gas"………………………………………………………………………....102
Dragan Mihailovic"Space-time texture of topologically trapped states in a doped Wigner crystal"………………………………………………………...103
Rostislav Mikhaylovskiy“Light-induced nonlinear spin dynamics in iron oxides”………………………………………………………………………………….............104
Sergey Mirov"Ultrafast lasers based on transition metal doped chalcogenides"……………………………………………………………………...........105
Mateo Mitrano"Ultrafast x-ray spectroscopy of light-driven quantum materials"……………………………………………………………………...........108
Claude Monney"Transient enhancement of the ferroelectricity in the Rashba semiconductor α-GeTe”……………………………………………….109
Marco Arrigoni“Status and perspective of commercial off-the shelf USP laser amplifiers and their component"………………………………....110
Joshua Mornhinweg,"Multi-mode-mixing deep-strong light-matter interaction"……………………………………………………………………………….........111
Andrey Moskalenko"Subcycle dynamics of quantum light"…………………………………………………………………………………………………………………...113
Yuta Murotani"Ultrafast spectroscopy of optical and electrical properties of Dirac semimetal driven by periodic light field"…………….115
Nathalie Nagl
"Albatross – a single-cycle infrared light source with unique waveform stability"………………………..……………………………116
Keith Nelson"New experimental methods for driving and monitoring quantum material transformations"………………………………......118
Gal Orenstein"Sub-diffusive topological defects of a charge density wave probed by an x-ray laser"…………………………………………….…121
Marcus Ossiander"Extreme ultraviolet metaoptics"…………………………………………………………………………………………………………………………..123

Lazaro Padilha"Using heterostructure engineering to tailor two-photon absorption in nanomaterials"……………………………………………124
Evangelos Pappaioannu"Spin and charge current dynamics in spintronic THz emitters"……………………………………………………………………...............126
Evangelos Papalazarou"Probing the pseudospin texture of Dirac states using circular dichroism in photoemission"……………………………………127
Vivek Pareek"Imaging the constituent electrons and holes of interlayer excitons in semiconductor heterostructure"…………………..128
Giacomo Passetti"The role of current fluctuations in the creation of Light-matter entanglement"…………………………………………….………...129
Ernest Pastor"Probing ultrafast disorder in photo-induced phase transitions"……………………………………………………………………………..130
Ilias Perakis"Imaging of light-controlled superconductivity using THz multi-dimensional coherent spectroscopy"……………………..…131
Luca Perfetti"Theoretical and experimental results in ultrafast dynamics and Ultrafast bandgap photonics"………………………………...133
Ilie-Elian Radu
”Terahertz lightwave-driven control of magnetism". ……………………………………………………………………………………………134
Theo Rasing,"All optical control of magnetism for energy efficient and brain inspired computing"……………………………………….............135
David Reis
"Towards imaging optically induced charge density with atomic resolution"…. ………………………………………………………..136
Dmitry Reznik,"Time-resolved Raman scattering as a window into ultrafast dynamics of quantum materials"………………………..............138
Michael Alexander Rübhausen"Optically induced avoided crossing in Graphene studied by MIR pump/Raman probe spectroscopy“………………..............140
Charlotte Sanders“Time-resolved photoelectron diffraction: mapping atomic motion in Bi2Se3 phonon oscillations"…………………………..142
Clara Saraceno"From high-power THz sources to fast nonlinear THz spectroscopy"………………………………………………………………………..143
Pavlos Savvidis“Polariton condensate lattices: A novel quantum
simulator platform”………………………………………………………..………..144
Alexey Scherbakov"Giant photoelasticity of polaritons for detecting coherent phonons with quantum sensitivity". ……………………………....145
Heinrich Schwoerer“Structural dynamics during singlet exciton fission in pentacene single crystals”…………………………………………….………...148
Ryo Shimano"On the origin of coherent c-axis charge carrier responses in photoexcited cuprate superconductors"………………………..149
Khalid Siddiqui"Ultrafast surface melting of electronic phase transition"………………………………………………………………………………………150
Andrej Singer"Photoinduced phase transformations in a nano-textured Mott insulator." ………………………………………………………………151
Irina Sorokina"En route to next generation high energy high repetition rate ultrafast laser sources in the mid-IR"………………………….152
Antoinette Taylor“Ultrafast dynamics in quantum materials”………………………………………………………………………………………………..……….155
Samuel Teitlebaum“Mesoscale structure in nonenquibrium charge density wave systems visualized with X-ray free electron lasers"………...157
Takami Tohayama"In-gap spectral weight of the optical conductivity induced by a strong subcycle pulse in
low-dimensional Mott insulators"……………………………………………………………………………………………………………..…………159
Rick Trebino"Measuring everything you’ve always wanted to know about a laser pulse"……………………………………………………………...160
George Tsibidis"Ionization dynamics and damage conditions in fused silica irradiated with Mid-Infrared femtosecond pulses"….............162
Mattia Udina“Light-induced collective- excitations: from phonons to electronic modes”……………………………………………….…………….…163
Konstantin Vodopyanov“High-precision high-resolution spectroscopy with frequency combs: from mid-infrared to terahertz"……….…………….…165
Alexander von Hoegen"Light induced metastable magnetization in a 2D antiferromagnet."……………………………………………………………………….167

Eryin Wang,"Probing photo-induced superconductivity on a chip". ………………………………………………………………………………….………..168
Jigang Wang
"Ultrafast bandgap control by geometric motive force at topological instability"…………………………………………………..….169
Nan-Lin Wang
"Nonlinear terahertz spectroscopy study on the interplay between
Superconductivity and pseudogap in cuprate superconductor"……………………………………………………………………………...170
Yao Wang"Controlling magnetic excitations and entanglement using ultrafast laser"…………………………………………………..............171
Martin Weinelt"Ultrafast spin excitations in 3d and 4f metals"…………………………………………………………………………………………..............…172
Wolf Widdra“Ultrafast electron and spin dynamics in NiO upon above bandgap excitation”…………………………………………………………173
Martin Wolf
" Dynamics of many-body exctiations from a k-space and real space perspective"……………………………………………………..174
Kazuhiro Yabana"Propagation of extreme pulsed light: First principles computational study"………………………………………………..…………175
Katsumasa Yoshioka"Ultrafast intrinsic optical-to-electrical conversion dynamics in graphene
investigated using on-chip THz spectroscopy"………………………………………………………………………………………………..............177

Observation of Pines’ demon in Sr2RuO4
A. A. Husain1, E. W. Huang2, M. Mitrano3, M. S. Rak1,S. I. Rubeck1, X. Guo1, H. Yang4, C. Sow5
Y. Maeno5, B. Uchoa6, T.C. Chiang1, P.E. Batson7, P. W. Phillips2, P. Abbamonte1
1
University of Illinois, Urbana, IL, 61801, USA
2
Institute for Condensed Matter Theory, Urbana, IL, 61801, USA
3
Harvard University, Cambridge, MA, 02138, USA
4
Rutgers University, Piscataway, New Jersey, 08854, USA
5
Kyoto University, 606-8502 Kyoto, Japan
6
University of Oklahoma, Norman, OK, 73019, USA
7
Rutgers University, Piscataway, NJ, 08854, USA
The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of
its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a“demon,” could exist in three-dimensional metals containing more than one species of charge carrier[1]. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic,
electrically neutral, and do not couple to light, so have never been detected in an equilibrium, threedimensional metal. Nevertheless, demons are believed to be critical for diverse phenomena including
phase transitions in mixed-valence semimetals [2], optical properties of metal nanoparticles[3] ,soundarons in Weyl semimetals[4], and high temperature superconductivity in, for example, metal
hydrides[3,5,6,7]. Here, we present evidence for a demon in Sr2RuO4 from momentum-resolved
electron energy-loss spectroscopy (M-EELS). Formed of electrons in the β and γ bands, the demon isgapless with critical momentum qc = 0.08 reciprocal lattice units and room temperature velocity v =(1.065 ± 0.12) × 105 m/s, which undergoes a 14% renormalization upon cooling to 20 K due tocoupling to the particle-hole continuum. The momentum dependence of the intensity of thedemon confirms its neutral character.

Fig. 1: Dispersion
curve of the demonin Sr2RuO4 observedwithomentumresolved EELS.

Our study confirms a 66-year old prediction and suggests that demons may be a pervasive feature ofmultiband metals[8].
References[1] D. Pines, Canadian Journal Physics 34, 1379 (1956).
[2] C. M. Varma, Review of Modern Physics 48, 219 (1976).
[3] R. Akashi, R. Arita, Journal of Physical Society of Japan 83, 061016 (2014).
[4] A. N. Afanasiev, A.A. Greshnov, D. Svintsov, Physical Review B 103, 205201 (2021).
[5] J. Ihm, M.L Cohen, S.F. Tuan, Physical Review B 23, 3258 (1981).
[6] J. Ruvalds, Advances in Physics 30, 677 (1981).
[7] E.A. Pashitskii, V.I. Pentegov, A.V. Semenov, Low Temperature Physics 48, 26 (2022).
[8] A. A. Husain, E. W. Huang, M. Mitrano, M. S. Rak, S. I. Rubeck, X. Guo, H. Yang, C. Sow, Y. Maeno, B. Uchoa, T. C. Chiang, P. E. Batson
P. W. Phillips, P. Abbamonte, arxiv:2007.06670.

Conductivity dynamics in THz driven spin-ladders

J. Dössegger1, T. Suter1, L. Lanini1, M. Glantschnig1, P. Puphal2, E. Pomjakushina3, S. L. Johnson1
E. Abreu1
1
ETH Zürich, 8093 Zürich, Switzerland
2
Max-Planck Institute for Solid State Research, 70569 Stuttgart, Germany
3
Paul Scherrer Institut, 5232 Villigen, Switzerland

The nature of the superconducting state in unconventional superconductors, in particular hightemperature superconducting cuprates, remains to be fully understood. The fundamental building
blocks in cuprates are two-dimensional CuO2 layers. One approach to simplifying the problem
consists in lowering the dimensionality of the system. Sr14-xCaxCu24O41 (SCCO) compounds have aquasi-one-dimensional structure characterized by alternating layers of Cu2O3 ladders and CuO2
chains. These spin-ladder systems exhibit a rich phase diagram, where conductivity, charge order andmagnetic order can be controlled by varying temperature, external pressure and the level of Sr
substitution by Ca, x. In particular, a superconducting phase arises for temperatures below 15 K, anapplied pressure of 3-10 GPa and x > 11.5. The conductivity in these inherently hole doped
materials, and in particular the superconducting phase, is believed to be controlled by the distributionof holes between the chains and the ladders. [1]Fig. 1. Temperature dependence of the THz transmission through an a-cut sample of Sr14Cu24O41 (x = 0), with
the THz electric field E aligned along (a) the b-axis and (b) the c-axis of the material.

We investigate the low frequency response of bulk single crystal SCCO samples with Ca contents x= 0, 7 and 12. I will first present our characterization of the temperature and doping dependent
anisotropic conductivity response, obtained using THz time domain spectroscopy in the 0.3-2.7 THz
range. [2] I will then discuss the ultrafast nonlinear conductivity transient that arises following
excitation by a strong THz field pulse, and how this behavior depends on the intensity andpolarization of the pump and on the temperature and doping level of the sample. [2]References[1] T. Vuletić, B. Korin-Hamzić, T. Ivek, S. Tomić, B. Gorshunov, M. Dressel, J. Akimitsu, Physics Reports 42, 169 (2006).
[2] J. Dössegger, T. Suter, L. Lanini, M. Glantschnig, P. Puphal, E. Pomjakushina, S. L. Johnson, E. Abreu, (to be submitted) (2023).
* Acknowledgement(s): JD and EA acknowledge support from the Swiss National Science Foundation through Ambizione Grant PZ00P2_179691.

Ultrafast dynamics of charge carriers in energy and momentum space.
Ultrafast control of spin-dependent energy level alignment ofmolecular/WSe2 heterostructures
M. Aeschlimann
University of Kaiserslautern-Landau, 67663 Kaiserslautern, Germany

One of the great challenges in information technology is the development of novel concepts tocontrol charge and spin carriers in active functional units at ever shorter lengths and fastertimescales. The easiest way to realize such nanoscale devices is to use intrinsic two-dimensional(2D) materials such as transition metal dichalcogenides (TMDCs) and to design their spin-dependent
band structure by chemical functionalization or optical engineering. For TMDCs, this is mainly
achieved by forming 2D heterostructures with either other TMDCs or 2D honeycomb materials.
While this approach has been successfully used to tune the interfacial properties of 2D
heterostructures, it still shows limitations in the tunability of the interfacial energy level alignment aswell as in the lateral dimensions of the heterostructures. In this contribution, we present analternative way to functionalize the properties of TMDCs by adsorption of molecular materials. As
an exemplary case, we focus on a heterostructure consisting of the TMDC bulk crystal WSe2 and theprototypical molecule C60. Both were chosen because of their interesting optical and spin-dependent
electronic properties. On the one hand, the valence band structure of WSe2 shows an overall
vanishing spin polarization in spite of the layer- and valley-dependent spin polarization of theindividual tri-layers (see Fig. 1a). On the other hand, the excited state dynamics of C60 and otherfullerenes is dominated by charge transfer excitons (as shown in the sketch in Fig. 1b), which cantransiently manipulate the energy level alignment of the surrounding material [1,2].

Fig.1 (a) Sketch of the layer- and valley-dependent local valence band structure of the first two WSe2 tri-layers.
Bands with opposite spin character are marked in red and blue. (b) Illustration of the investigated C60/WSe2
heterostructure and of the resonant optical exaction of the C60 layer. (c) Exemplary energy vs. momentum cutsof the tr-ARPES data set along the K-Σ- high symmetry direction. The photoemission yield of the excited states
is illustrated as difference map with intensity accumulation in red and depletion in blue. Energy level alignment,
optical excitation scheme and scattering processes of the ultrafast dynamics of the C60/WSe2 heterostructure.

The studied material system of a single C60 layer on a 2H-WSe2 bulk crystal is shown in Fig. 1b. For
a resonant optical excitation of the C60 layer with 3.1eV photons, we uncovered the existence of ahighly efficient interfacial charge transfer process that can transiently lift the spin degeneracy of thebulk WSe2 band structure near the interface. Our conclusions are based on spin-, time-, andmomentum-resolved fs-XUV photoemission spectroscopy experiments performed along the Σ-Khigh symmetry direction of the WSe2 crystal. This particular high symmetry direction reveals thecharacteristic hole-like valence bands (VB) and electron-like conduction bands (CB) of WSe2, aswell as the non-dispersive highest occupied molecular orbital (HOMO) and excitonic states of C60.
An exemplary time and momentum resolved photoemission (tr-ARPES) data set is shown in Fig. 1c,
the corresponding energy level alignment in Fig. 1d. We will show that optical excitation of the
C60/WSe2 heterostructure with 3.1eV photons leads to a resonant optical transition from the C60
HOMO into the manifold of excitonic C60 states and to the formation of charge transfer excitons(CT2). These CT2 excitons decay within 100fs leading to electron transfer into the K-valley of thefirst WSe2 tri-layer. This interlayer scattering is followed by interlayer scattering from the K to the Σvalley of the WSe2 CB. These scattering processes together with the extracted scattering times are
summarized in Fig. 1d. For example, we will provide evidence for a significant increase of theinterval scattering time between the K- and the Σ-valley upon adsorption of C60 [3] and demonstrate
a depopulation time of the Σ-valley of about 10ps.The most important observation, however, is theabsence of any interlayer hole transfer between the C60 layer and the WSe2 bulk crystal. This allow
us to confirm that only electrons are transferred across the interface. This electron transfer leads to atransient charging of the C60 (positive charges) and the first WSe2 layer (negative charges) as
illustrated in the sketch in Fig. 2a. This charge distribution results in an interfacial dipole at the C60
and WSe2 interface and causes the transient shifts with different signs of the C60 and WSe2 valence
states observed in our tr-ARPES experiments, see Fig. 2b.

Fig.2. (a) Sketch of the charge transfer process across the C60/WSe2 interface and the resulting charge
distributions for two characteristic time steps. (b) Transient energy shifts of the C60 (HOMO) and WSe2 valenceband structure after optical excitation with 3.1eV photons.

Finally, we will turn to our spin-, time-, and momentum-resolved photoemission spectra that wererecorded at the K-point of the WSe2 valence band structure for characteristic time delays after the
optical excitation. These spectra contain characteristic signatures of the spin polarization of the WSe2
valence bands that can be unambiguously attributed to the first and second WSe2 tri-layer. In thiscontribution, we discuss the spectral line shape of our spin-resolved tr-ARPES data in details anddemonstrate different energy shifts for the valence band of the first and second WSe2 tri-layer. The
layer dependent magnitude of these valence band shifts can be attributed to the strength of theinterfacial dipole field, which decreases towards the bulk of the WSe2 crystal. In conclusion, ourstudy of C60/WSe2 heterostructures has revealed an ultrafast charge separation at the C60/WSe2
interface. This results in an interfacial dipole responsible for a layer-dependent (Stark-like) shift ofthe spin-polarized WSe2 valence band structure. This layer-dependent shift transiently lifts the spindegeneracy of the WSe2 crystal valence band structure. Thus, our results provide a clear pathway tomanipulate the spin functionalities of a molecular/TMDC heterostructure by optical excitation on theultrafast timescale.
References[1] B. Stadtmüller, S. Emmerich, D. Jungkenn, N. Haag, M. Rollinger, S. Eich, M. Maniraj, M. Aeschlimann, M. Cinchetti, S. Mathias, Naure
Communications 10, 1470 (2019).
[2] S. Emmerich, S. Hedwig, M. Cinchetti, B. Stadtmüller, M. Aeschlimann, Journal of . Electron Spectroscopy and Related Phenomena 252, 147110(2021).
[3] R. Bertoni, C. W. Nicholson, L. Waldecker, H. Hübener, C. Monney, U. De Giovannini, M. Puppin, M. Hoesch, E. Springate, R. T. Chapman
C. Cacho, M. Wolf, A. Rubio, R. Ernstorfer, Phyics Reiew Letters 117, 277201 (2016).

Hybrid phonons
in the2 structures withvan der
Waals nanolayers
1
3
3
3
W. Yan , V. E. Gusev , D. D. Yaremkevich T. L. Linnik , S. M. Kukhtaruk
A. Patanè1, A. J. Kent1A. Nadzeyka4, M. Bayer5, A. V. Scherbakov5
A. V. Akimov1
1
University of Nottingham, Nottingham NG7 2RD, UK
2
Le Mans Université, 72085 Le Mans, France
3
V.E. Lashkaryov Institute of Semiconductor Physics, 02000 Kyiv, Ukraine.
4
Raith GmbH, 44263 Dortmund, Germany
5
Technische Universität Dortmund, 44227 Dortmund, Germany.

Two-dimensional van der Waals crystals (2D-vdW) consist of chemically bonded atomic layers held
together by weak vdW forces. They provide the perfect atomic-size “Lego-type” toy models for the
exploration of new classical and quantum phenomena in solid state physics. Their versatile homoand heterostructures created by stacking, twisting, stretching and bending of vdW layers provide
means of manipulating optical, electrical, magnetic, piezoelectric and spin properties. Coherent THz
and sub-THz phonons, which carry dynamical strain, could become an instrument to control classicaland quantum phenomena in the unexplored picosecond temporal and nanometer spatial
regimes.Coherent phonons in vdW nanolayers fabricated from different materials andheterostructures are intensively studied during the last decade (for a review see Ref.[1]). The
experiments use the picosecond ultrasonics pump-probe technique which allows measurement of thevibrations of nanolayers with sub-picosecond resolution. In the present talk we describe a series ofexperiments where different phonon modes are studied in various vdW nanolayers [2-4]. Special
attention is paid to the role of elastic interaction with the substrate and between the vdW layers ofdifferent materials. The frequencies of breathing phonon modes are strongly dependent on the
nanolayer/substrate coupling, which allows us to estimate the stiffness of the contact with thesubstrate [2]. Figure 1 demonstrates this method for obtaining the image of the elastic contact
between InSe layer and sapphire substrate [3].
abcFig.1. Images of elastic contact InSe nanolayer/ sapphire
substrate [3].
a. Temporal evolutions (insets) and their fast Fouriertransforms measured at three different positions on an
InSe/InSe homojunction on a sapphire substrate. Position
coordinates inside brackets are in microns. The lowest red
spectrum corresponds to a broken interface; the black
middle spectrum corresponds to a perfect interface; the top
blue spectrum corresponds to a position where both broken
and perfect interfaces coexist.
b-c. Spectral density images at frequencies of 32 GHz and
27 GHz corresponding to the phonon resonances for
broken (b) and perfect (c) interfaces. The areas marked bydashed squares and ovals indicate homogeneous regions ofbroken and perfect interfaces, respectively.

In another experiment, we study in-plane coherent phonon modes in MoS2 nanolayers transferred ona nanograting - Fig. 2 [4].
Fig. 2. Picosecond acoustics of van der
Waals layers on nanogratings [4]a, Schematic illustration of the pump-probe
setup and the MoS2 layer on the grating.
b, Fundamental antisymmetric (A0, top),
symmetric (S0, middle) and breathingmodes (bottom) of the free standing layer.
c, Scanning electron microscopy (SEM)
image of the FeGa grating with 150 nm
period (left), AFM image of a 200 nm
period nanograting with a 13 nm thick
MoS2 (middle). A zoomed image of thearea enclosed by the black square is shown
to the right, and the height profile of thelayer on the grating along a white line isshown below. d, The same as in c but for
the Si grating: SEM ( period 100 nm);
AFM (layer thickness 8.3 nm on gratingwith period 200 nm).

We can generate and detect coherent phonons propagating with the LA sound velocity withfrequency up to 40 GHz and hybrid flexural phonons with frequencies up to 10 GHz (Fig. 3 and 4).

Fig. 3. Phonon modes in hybrid nanostructures [4]. a. The measured frequencies of phonon modes as afunction of grating vector for various hybrid nanostructures (symbols). The correspondence of different symbols
to the frequency group (f1, f2 and f3) and grating material (FeGa, Si, and Cr) are shown in the inset table. The
solid lines are the calculated dispersion curves for S0 (black straight line) and A0 modes (green curved lines)
for the free standing MoS2 layers. The shaded area includes phonon modes calculated for A0 modes in thelayers with thethicknesses between 3 and 16 nm. The inset shows the temporal evolution of the measured ΔR(t)after high pass filtering which emphasizes the detection of high frequency S0 and breathing modes. b. The
dependence of frequencies f1 and f2 on the vdW layer thickness measured in FeGa grating with a period d = 200
nm (symbols) and the corresponding theoretical dependences H1 and H2 (lines) calculated by Comsol
Multiphysics software. The dotted lines are the calculated thickness dependences for A0 modes with qx = 𝐺 andqx = 2G in a free-standing layer.

The latter arise from the periodic modulation of the elastic coupling of the vdW layer at the grooves
and ridges of the nanograting. The simulation of hybrid phonons is demonstrated in Fig. 4. This
creates a new type of a tailorable 2D periodic phononic nanoobject, a flexural phononic crystal,
offering exciting prospects for the ultrafast manipulation of states in 2D materials.
c

a

H1

a (nm)H2d

Z

1.6 GHz

2.8 GHz

3.3 GHz

b

5

15

30

7.7 GHz

8.7 GHz30.8 GHz

S0

30.8 GHz

S0*

9.5 GHzH2

5.6 GHz

H2*

7.8 GHze

2.3 GHz

H1

Fig. 4. Theory of phonon mode hybridization [4] a. The dependence of normalized frequencies of theexperimentally relevant hybrid eigen modes on the normalized grating vector 𝐺̅ in the hybrid nanostructure.
Inset: the scheme of the spring model. b. Calculated dependence of the mode frequencies on the layer
avoided crossing of modes H2 and H3 is highlighted by an arrow. The dotted line corresponds
thickness. The
to the mass-on-spring
type oscillation. c. The layer motion for H1 and H2 hybrid modes displayed for threelayer thicknesses. d. The dispersion curves and layer motion for the phononic crystal showing even (S0, H1
and H2) and odd (S0* and H2*) phonon modes near the centre of the Brillouin zone. e, The dependences ofthe frequencies for H1 and H2 modes on
the spring stiffness. The arrow indicates the frequency of A0
resonance for free-standing MoS2 layer. a and b are theresults of analytical calculations for infinitively
rigid grating and q x a<<1; other panels are numerical calculations
performed by COMSOL Multiphysics
for FeGa grating with d=200 nm.

The experiments with coherent phonons in vdW nanolayers pave a way to control strain- induced
processes in 2D-vdW materials on a picosecond time scale. Based on static and low frequency straininduced effects, high frequency phonons could be used in a similar way as done in epitaxial
nanostructures to manipulate light emission[5], conductivity[6] magnetization[7] and plasmons [8].
References[1] F. Vialla, N. Del Fatti, Nanomaterials 10, 2543 (2020).
[2] J. Greener,A.Akimov,V.Gusev,Z.Kudrynskyi,P.Beton,Z.Kovalyuk,T.Tanigichi,K.Watanabe,A.Kent,A.Patane,Physical Review B98,075408 (2018)[3] J..G.Greener,E.De Lima Savi,A.Akimov,S.Raetz, S.Kudrynskyi, S.Kovalyuk, N.Chigarev, A.Kent, A.Patane,V.Gusev, ACS Nano 13,11530 (2019).
[4] W. Yan, A.Akimov, M.Barro-Burilla, M. Bayer, J. Bradford, V. Gusev, L. Hueso, A Kent, S. Kuktaruk, A.Nadzeika, Nano Letters 22, 6509 (2022).
[5] A. V. Akimov, A. V. Scherbakov, D. R. Yakovlev, M. Bayer, Ultrasonics 56, 122 (2015).
[6] A. V. Akimov, C. L. Poyser, A. J. Kent, Semiconductor Science and Technology 32, 1 (2017).
[7] J. Jager. A.Scherbakov, B. Glavin, A.Salasyuk, R.Campion, A.Rushforth, D.Yakovlev, A.Akimov, M. Bayer,Physical Review B 92, 020404 (2015).
[8] V. V. Temnov, Nature Photonics 6, 728 (2012).

Observation of giant nonlinearities and photon bound-states using asingleartificial
atom 1
1
1
2
1
3
N. O. Antoniadis , N. Tomm , S. Mahmoodian , M. Janovitch , M. Brunelli , R. Schott ,S. R. Valentin3, A. D. Wieck3, A. Ludwig3, P. P. Potts1, A. Javadi1, R. J. Warburton1
1
University of Basel, 4056 Basel, Switzerland
2
The University of Sydney, AU-2006 Camperdown, Australia
3
Ruhr-Universität Bochum, DE-44780 Bochum, Germany

A coherent and efficient light-matter interface at the level of single emitters and single photons is themain requirement for photonic quantum gates. The β-factor determines the efficiency of the lightmatter interface. Here, we use a semiconductor quantum dot in an open microcavity (Fig. 1a) torealise a one-dimensional atom [1]. We achieve an extinction of 99.2% in the transmission at low
input power. This demonstrates that the β-factor is high, in this case 92%. The transmission dip
depends strongly on the input laser power and in fact vanishes at “high” powers (Fig. 1b). Thisnonlinearity is provided by the atom and means that the light-matter interaction depends strongly on
the number of photons interacting with the two-level system within its emission lifetime. The tunable
nature of the microcavity allows the β-factor to be tuned, which enables control over the photon
statistics from strong bunching (g(2)(0)=587) to anti-bunching [2]. In the nonlinear regime, photon
bound states can arise, strongly correlated quasi-particles of the system. We probe the bound states ina direct way by using pulsed excitation (pulse width comparable to the radiative decay time). We
observe a photon-number-dependent time delay in the scattering from the quantum dot-cavity
system. By scattering a weak coherent pulse and measuring the time-dependent output power andcorrelation functions, we show that single photon, two- and three-photon components incur different
time delays of 144.02 ps, 66.45 ps and 45.51 ps respectively (Fig.1c) [3].

Fig. 1. (a) Schematic of the open microcavity. A quantum dot is embedded in the bottom mirror. We can either
measure the transmission (port 1 to port 2) or the back-reflection (port 1 to port 1). (b) Transmission throughthe system as a function of quantum dot detuning. We observe an extinction as high as 99.2 %. (c) Delay of
scattered single-, two- and three-photon components for back-reflected light.

The reduced time delay of the two-photon component with respect to the one-photon component is afingerprint of the celebrated example of stimulated emission, where the arrival of two photons within
the lifetime of an emitter causes one photon to stimulate the emission of the other. Furthermore, atthe optimal pulse width we show that the two-photon scattering results in the efficient creation of
two-photon bound states with a temporal wave function that matches theoretical predictions very
precisely.
References[1] N. Tomm, A. Javadi, N. O. Antoniadis, D. Najer, M. C. Löbl, A. R. Korsch, R. Schott, A. D. Wieck, A. Ludwig, R. J. Warburton, Nature
Nanotechnology 16, 399 (2021).
[2] N. O. Antoniadis, N. Tomm, T. Jakubczyk, R. Schott, A. D. Wieck, A. Ludwig,R. J. Warburton, A. Javadi, npj Quantum Information 8, 27 (2022).
[3] N. Tomm, S. Mahmoodian, N. O. Antoniadis, R. Schott, A. D. Wieck, A. Ludwig, A. Javadi, R. J. Warburton, Naure. Physics (in print) (2023).

Spin-Photon Quantum Interfaces and their Applications
M. Atatüre
University of Cambridge, Cambridge CB3 0HE, UK

Optically active spins in solids are often considered prime candidates for scalable and feasible
quantum-optical devices. They are formed by a confined spin in a solid-state matrix coupled to
optical transitions using higher-lying orbital states. These transitions typically have energies
significantly smaller than the bandgap energy, so the host material acts as a natural trap for holding aquantum object isolated. This sets a great advantage over the atomic counterparts of such lightmatter interfaces requiring complex trapping and isolation infrastructure. This advantage can thus beleveraged towards miniaturization, on-chip integration, and scalability using a plethora of material
host platforms. Choice of host material will also be influenced by the intended applications
straddling the broad areas from quantum networks and communication to the development of
quantum sensors. There are numerous forerunning material platforms including diamond,semiconductors, and atomically thin 2d materials today [Fig. 1]. Each promising material platform
brings its own advantages along with their challenges. Semiconductors are arguably the mostinvestigated candidate material and offer great in-depth knowledge on how to design, fabricate andscale quantum-enabled devices. Layered materials offer immense opportunities in integration andfeasible heterostructure device design, yet material quality has been an apparent limitation until
today. Diamond comes across as a material platform that can deliver acceptable performance both asa host for spin-photon quantum interfaces and a material suitable for heterogeneous on-chip
integration. The substantially weaker spin noise due to the only 1.1% nuclear-spin-active carbon
isotope is a great advantage for the confined spins in color centers, while the large bandgap energy
and the high stiffness of the material renders the optical properties equally attractive.

Fig. 1. Forerunning material systems that can host quantum devices linking single spins and single photons.

While none of the forerunning material platforms is a clear winner in this pursuit of scalable
quantum technologies [1], diamond makes the most appearance across different applications. It isthus worth investigating diamond colour centres in detail as a platform with a strong focus on thefamily of group-IV color centres for quantum networks and nitrogen-vacancy centers for sensing.
References[1] M. Atature, D. Englund, N. Vamivakas, S.-Y. Lee, J. Wrachtrup, Nature Reviews Materials 3, 38 (2018).

Entangled photon factories and multinuclear gatesfor quantum networks and computing
E. Barnes
Virginia Tech, Blacksburg, VA 24061, USA

Multi-photon entangled graph states are a fundamental resource in quantum communication
networks, distributed quantum computing, and sensing. These states can in principle be created
deterministically from quantum emitters such as optically active quantum dots or defects, atomic
systems, or superconducting qubits. However, finding efficient schemes to produce such states has
been a long-standing challenge. I will present an algorithm that, given a desired multi-photon graphstate, determines the minimum number of quantum emitters and precise operation sequences that can
produce it. The algorithm itself and the resulting operation sequence both scale polynomially in thesize of the photonic graph state, allowing one to obtain efficient schemes to generate graph states
containing hundreds or thousands of photons. Among the most mature and promising platforms for
multi-photon entangled graph state generation are nitrogen-vacancy (NV) centers in diamond andother color centers in solids. One of the challenges in using these systems for graph state generationand networking applications is to controllably manipulate entanglement between the electron and thenuclear spin register despite the always-on nature of the hyperfine interactions, which makes this an
inherently many-body quantum system. I will present a general formalism to quantify and control thegeneration of entanglement in an arbitrarily large nuclear spin register coupled to a color centerelectronic spin. I will describe a reliable measure of nuclear spin selectivity, by exactly incorporating
into our treatment the dynamics with unwanted nuclei. I will also show how to realize direct
multipartite gates through the use of dynamical decoupling sequences, drastically reducing the total
gate time compared to protocols based on sequential entanglement with individual nuclear spins. We
quantify the performance of such gate operations in the presence of unwanted residual entanglement
links, capturing the dynamics of the entire nuclear spin register.

Fig. 1. (a) Schematic of a defect spin (purple) hyperfine-coupled to a nuclear spin register (orange) and driven
by a dynamical decoupling sequence with 2N pulses and pulse spacing t.
(b) An example four-photon graph state. Each red dot corresponds to a photonic qubit, and lines represent CZ
gates;(c) Arranging the graph state in a 1D array and computing the entanglement entropy as a function of the
bipartition point x;(d) reveals that two quantum emitters (green squares) are needed to produce the four-photon state in (b);(e) Time-reversed sequence of operations that systematically absorb all photons into the emitters. Reversing thissequence gives a precise protocol for generating the graph state in (b) from the two emitters.

Finally, using experimental parameters of a well-characterized 27 nuclear spin register device, weshow how to prepare with high-fidelity entangled states for quantum error correction. While in thisanalysis we focus on a particular NV-diamond-based register, our framework is completely generaland applicable to other defects in diamond and in SiC.
References[1] B. Li, S. E. Economou, E. Barnes, npj Quantum Information 8, 11 (2022).
[2] E. Takou, E. Barnes, S. E. Economou, Physical Review X 13, 011004 (2023).
[3] E. Takou, E. Barnes, S. E. Economou, arXiv:2302.05580 (2023).
* Acknowledgments: This work was supported by the National Science Foundation (grant nos. 1741656 and 2137953).

Emergence in out of equilibrium systems:
Theoretical treatment of complex out-of-equilibrium dynamics andthe ensuing phenomena
M. Battiato
Nanyang Technological University, Singapore 639798, Singapore

Emergence is the phenomenon by which a system with many heterogeneous degrees of freedom
develops behaviours that are qualitatively different from its simpler parts. A famous example is life.
Describing and predicting emergence requires handling the system in its full complexity: if
simplified, or if degrees of freedom are removed, entire behaviours will simply not happen.
Femtosecond laser pulse-generated out-of-equilibrium states of materials are a very fertile ground for
emergence, as being away from the rather strict requirements of equilibrium or near equilibrium
conditions frees up a large number of degrees of freedom. Spectacular examples of emergence inthese cases are, among others, the super-diffusive spin transport, [1-4] spintronic THz emitters [4]and the giant spin injection in semiconductors [5, 6].Such effects are the results of the complex
interplay of the far-from-equilibrium state of the system, spin-, band- and momentum-dependent
thermalization, transport of excited quasiparticles and interaction with electromagnetic fields. To
address the cull complexity of the situation without losing the emergent behaviours, one has to movebeyond usual treatments. We have developed the, so far, only available numerical algorithm to solve
the full Boltzmann transport and scattering equation for realistic band structures, and, for the firsttime, no close to equilibrium approximation, as well as several essential computational properties(see left panel of Fig. 1) [7-9].
(a)

0.25
0.20
0.15
0.10
0.05
0
1.00

(b)

0.95
0.90
0.85
0.80
0.75

(c)

1.0

1.0

1.0

1.0

0.8

0.8

0.8

0.8

0.6

0.6

0.6

0.6

0.4

0.4

0.4

0.4

0.2

0.2

0.2

0.2

0.0

0.0
4 -4

0.0
4 -4

-4

-2

0

2

-2

0

2

-2

0

2

0.0
4 -4

-2

0

2

4

Fig. 1. Left: Snapshots of thermalizing population through electron-electron scatterings is semiconductor.
Top and middle left: momentum-resolved population in the conduction and valence band.
Bottom left: energy resolved population [7].
Right: Schematics of the theoretical treatment of laser excitation and thermalization under bias of CNTs [10].

Ab sor p t ion (a.u .)More recently we have shown how disorder in carbon nanotubes can dramatically impact thethermalization dynamics [11].
Interestingly the same numerical method can be employed to produce spectra and, in connection with
time-resolved Boltzmann simulations, time- resolved spectra (see left panel of Fig. 2) [12].
The developed approach allows us to tackle the thermalisation dynamics of different types ofquasiparticles: we managed to describe and identify the unconventional thermalisation pathways in
GeTe driven by spin-selective electron-electron and electron-phonon scatterings (see right panel ofFig. 2). [10]1.4 a)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4 b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4 d)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0 1.2

pht //↔ Eii

Total theory paral.
Direct E11 and E22
Phonon Stokes
Phonon Anti-Stokes
0.8 1.0 1.2 1.4 1.6 1.8

pht //+ 𝔸↔ Eii
pht //↔ 𝔸+ Eii

c)

10-1
10-2
E11 bandextremum 10-3
10-4
10-5

e)

pht //+ 𝔼 1 ↔ Eij
pht //↔ 𝔼 1 + Eij

10-1
10-2
10-3

E12 bandextremum

10-4
10-

5

10-6
1.4

1.6 1.8
Energy (eV)

2.0

2.2

2.4 1.4 1.6 1.8 2.0 2.2
Energy (eV)Fig.2. Left: Absorption spectrum for light parallel to the CNT's axes. Each panel shows a different processcontributing to the total spectrum [12]. Right: Change with temperature of the scattering rate (inverse lifetime)
due to electron-electron scattering in GeTe [10].

Finally, more recently, we have been able to address the failure of generating lasing with carbon
nanotubes. We showed that reaching the population inversion necessary for lasing to start at the main
E11 exciton transition is not achievable. We have however shown that at frequencies corresponding
to anti-Stokes phonon-assisted optical generation of excitons, lasing can be triggered way below the
population inversion threshold.
References[1] M. Battiato, K. Carva, P.M. Oppeneer, Physical Review Letters 105, 027203 (2010).
[2] D. Rudolf, C. La-O-Vorakiat, M. Battiato, R. Adam, J. M. Shaw, E. Turgut, P. Maldonado, S. Mathias, P. Grychtol, H. T. Nembach, T. J. Silva
M. Aeschlimann, H. C. Kapteyn, M. M. Murnane, C. M. Schneider, P. M. Oppeneer, Nature Communications 3, 1037 (2012).
[3] A. Eschenlohr, M. Battiato, P. Maldonado, N. Pontius, T. Kachel, K. Holldack, R. Mitzner, A. Fohlisch, P. M. Oppeneer, C. Stamm, Nature
Materials 12, 332 (2013).
[4] T. Kampfrath, M. Battiato, P. Maldonado, G. Eilers, J. Notzold, S. Mahrlein, V. Zbarsky, F. Freimuth, Y. Mokrousov, S. Blugel, M. Wolf, I. Radu,P. M. Oppeneer, M. Munzenberg, Nature Nanotechnology 8, 256 (2013).
[5] M. Battiato, K. Held, Physical Review Letters 116, 196601 (2016).
[6] L. Cheng, X. Wang, W. Yang, J. Chai, M. Yang, M. Chen, Y. Wu, X. Chen, D. Chi, K. E. J. Goh, J.-X. Zhu, H. Sun, S. Wang, J. C. W. SongM. Battiato, H. Yang, E. E. M. Chia, Nature Physics 15, 347(2019).
[7] M. Wais, K. Held, M. Battiato, Computer Physics Communications 264, 107877 (2021).
[8] I. Wadgaonkar, R. Jain, M. Battiato, Computer Physics Communications 263, 107863 (2021).
[9] I. Wadgaonkar, M. Wais, M. Battiato, Computer Physics Communications 271, 108207 (2022).
[10] F. R. Bagsican, M. Wais, N. Komatsu, W. Gao, L. Weber, K. Serita, H. Murakami, K. Held, F. A. Hegmann, M. Tonouchi, J. Kono, I. Kawayama,
M. Battiato, Nano Letters 20, 3098 (2020).
[11] M. Wais, F. R. Bagsican, N. Komatsu, W. Gao, K. Serita, H. Murakami, K. Held, I. Kawayama, J. Kono, M. Battiato,M. Tonouchi, under review[12] S. Dal Forno, N. Komatsu, M. Wais, A. Mojibpour, I. Wadgaonkar, S. Ghosh, Y. Yomogida, K. Yanagi, K. Held, J. Kono, M. Battiato
Carbon 186, 465 (2022).
[13] O. J Clark, I. Wadgaonkar, F. Freyse, G. Springholz, M. Battiato, J. Sánchez Barriga, Advanced Materials 34, (2022).
* Acknowledgements: M.B. acknowledges financial support from the Nanyang Technological University, for the grant NAP-SUG, and MOE
Singapore for the grants Tier 1 RG75/22, Tier 1 RT13/22, and Tier 2 T2EP50222-0047.

Non-equilibrium carrier dynamics and carrier-phonon interaction in
2D quantum materials
M. Bauer
Kiel University, 24098 Kiel, Germany

Electron-phonon interaction is one of the most fundamental quasiparticle interaction in solids
governing for instance transport and thermodynamic properties of materials, but being also
responsible for unconventional phenomena such as low-temperature superconductivity and theformation of charge-ordered phases. In this presentation I will discuss the results of two time- andangle resolved photoemission (trARPES) studies of our group that addressing electron-phonon
interactions and there effect on electron and spin degrees of freedom on ultrafast timescales in two
different types of 2D transition metal dichalcogenides (TMDC). In a singly oriented film of singlelayer WS2 deposited on a Au(111) surface we studied the spin- and valley-selective photoexcitationand decay of free carriers at the K and K’-points (Fig. 1) [1]. The results reveal that in the valenceband (VB) maximum an ultimate valley polarization of free holes of 84% can be achieved upon
excitation with circularly polarized light at room temperature. For the photoexcited free electrons in
the conduction band (CB) minimum, we observe a significantly smaller valley polarization. Clear
differences in the carrier dynamics between electrons and holes imply intervalley scattering
processes into dark states being responsible for the efficient depolarization of the excited electron
population. The observed characteristic timescale for this process match very well values predictedby theory for the formation of momentum forbidden intervalley dark excitons in tungsten-based
single-layer TMDCs due to electron-phonon interaction [2].

Fig. 1. Left: Illustration of the optical excitation at K and K’ in WS2 using circularly polarized pump pulses.
Right: Transient valley-selective VB and CB) populations created by band gap-resonant absorption of circularpolarized light as probed in a trARPES experiment. The figure displays the photoemission signal contrast
observed at the K and the K’ point upon excitation with left (σ-) and right (σ+) circularly polarized pump pulses,
respectively. The contrast inversion between K and K’ indicates the valley selectivity of the excitation processwith respect to the pump polarization.

In the second example I will discuss results on the electron-phonon interaction in the Weylsemimetal Td-WTe2 as probed in a trARPES experiment following the excitation of coherent
phonons using NIR pump pulses [3].

We can show that a phonon-frequency selective analysis of the experimental data provide highresolution information on strengths and types of couplings of the individual modes to the electronic
bands (Fig. 2). I particularly will focus on the observation of a transient modulation of a Dresselhaustype spin splitting of electronic bands driven by the selective coupling of an interlayer shear mode ofthe layered compound. The latter results reveal real-time insights into electron-phonon coupledprocesses that are of vital importance for a light-driven topological phase transition in Td-WTe2 [4].

Fig. 2. Left: trARPES snapshot of Td-WTe2 at a NIR-pump UV-probe temporal delay of 120 fs (left spectrum) incomparison to energy-momentum maps derived from a Fourier-analysis of a complete trARPES scan. Here, fdenotes the frequencies of coherent phonon peaks identified in the Fourier-spectrum of the pump-probe scans.
The Fourier amplitude is a measure for the coupling strength of the respective phonon mode to the electronic
bands of WTe2.

In summary, the examples illustrate the potential of trARPES in providing relevant information onelectron-phonon couplings in quantum materials. In particular, the concept of a frequency-domain
analysis of trARPES data, which are modulated due to the excitation of coherent phonons, may
provide in the future quantitative and electronic band- and phonon mode-resolved views onto
electron phonon interactions in general [5, 6].
References[1] H. Beyer, G. Rohde , A. Grubišić Čabo , A. Stange, T. Jacobsen, L. Bignardi , D. Lizzit , P. Lacovig, C. E. Sanders, S. Lizzit, K. Rossnagel, P.
P. Hofmann, M. Bauer, Phyical Review Letters 123, 236802, (2019).
[2] M. Selig, G. Berghäuser, A. Raja, P. Nagler,C.Schüller,T.F.Heinz,T.Korn,A.Chernikov,E.Malic,A. Knorr, Nature Communications 7, 13279 (2016).
[3] P. Hein, S. Jauernik, H. Erk, L. Yang, Y. Qi, Y. Sun, C. Felser, M. Bauer, Nature Communications 11, 2613, (2020).
[4] E.J. Sie, C.M. Nyby, C.D. Pemmaraju, S.J. Park, X. Shen, J. Yang, M.C. Hoffmann, B.K. Ofori-Okai, R. Li, A.H. Reid, S. Weathersby,
E. Mannebach, N. Finney, D. Rhodes, D. Chenet, A. Antony, L. Balicas, J. Hone, T.P. Devereaux, T.F. Heinz, X. Wang, A.M. Lindenberg,
Nature 565, 61 (2019).
[5] T. Suzuki, et al., Physical. Review B 103, L121105 (2021).
[6] U.D. Giovannini, H. Hübener, S.A. Sato, A. Rubio, Physical Review Letters 125, 136401 (2020).
* Acknowledgement(s) : M. Bauer acknowledges support DFG, (grants 239392151, 266380790, and 389191527).

Quantumness in a tight space:
Randomness, n-reconfigurability and multiparty entanglement on chip
N. Belabas
Université Paris-Saclay, 91120 Palaiseau France

Quantum-dot based semiconductor sources offer unprecedented brightness. Feeding synchronizedindistinguishable photons emitted by such an efficient single photo source (figure, left) into
integrated reconfigurable photonic chips (figure, middle column) enables the implementation ofmultiphoton protocols. These on-chip protocols harness quantumness with remarkable performancesand represent small-scale quantum computation in the "noisy intermediate scale" regime with linear
optical gates. We detail three instances and associated protocols:
1. N-indistinguishability is required for N-photon quantum information protocols and is non-trivial
to measure beyond N=2. We proposed a method to measure multi-photon N-indistinguishability on
chip and implemented a N=4 demonstration (top blue row of figure) [1].
2. We generated with high fidelity a 4-photon entangled state, |GHZ4>, a state of the class of thegraph states useful for quantum information. Owing to the high photon rate of our demonstration, we
could perform a 4-photon state full tomography on chip and achieved a record generation rate for
integrated generation of |GHZ4> (medium green row in figure) [2].
3. Harnessing quantumness on chip also comes at the cost of the sacrifice of space-like separationthat is typically relied upon for certification of randomness.
Fig.1. Left:
A semiconductor
quantum-dot-based quasi-deterministic
efficient source of photons. Blue top –4-photonindistinguishabilityasmeasured by the contrast ofinterference at the output of a 2-layerinterferometer as a function of asingle phase [1].
Green middle – Post-selected |GHZ4>
preparation and full characterization
with a reconfigurable glass circuit,here the real part of the density matrix
we achieve a Fidelity to|𝐺𝐻𝑍4 ⟩ 𝑜𝑓 ℱ𝑒𝑥𝑝 = 86.0±0.4%.
and a purity of the generated state
𝒫𝑒𝑥𝑝 = 76.3±0.6%. [2].
Red bottom – Alice and Bob play acontextual game (equivalent to CHSH
inequality testing) to guarantee
randomness. On-chip and thus without
space-like separation, the crosstalkσ between the devices enabling thechange of bases for both parties
prevent the application of thisframework. We introduce a new
metrics andprotocol to certify
randomness in this context that isclose to real-world applications [3].

By introducing a dedicated protocol which is based on witnessing contextual correlations, andmetrics that account for an amount of signaling in the absence of space-like separation, we could forthe first time certify randomness in a tight space (bottom orange row in Fig.1) [3].
References[1] M. Pont, G. Corrielli, A. Fyrillas, I. Agresti, G. Carvacho, N.Maring, P.-E. Emeriau, F. Ceccarelli, R.Albiero, P.H. D. Ferreira, N. Somaschi
J. Senellart, I. Sagnes, M.Morassi, A. Lemaitre, P. Senellart , F. Sciarrino, M. Liscidini, N. Belabas R.Osellame, arXiv quant-ph 2211.15626[2] M. Pont, R.Albiero, S.E. Thomas, N. Spagnolo, F.Ceccarelli, G. Corrielli, A.Brieussel, N.Somaschi, H. Huet, A.Harouri, A.Lemaître, I.Sagnes
N. Belabas, F. Sciarrino, R. Osellame, P.Senellart, A. Crespi, Physical Review X 12, 031033 (2022)[3] A. Fyrillas, B.Bourdoncle, A.Maïnos, P.-E. Emeriau, K.Start, N. Margaria, M.Morassi, A.Lemaître, I.Sagnes, P.Stepanov, T. H.Au, S. Boissier
N. Somaschi, N. Maring, N. Belabas, S. Mansfield, arXiv:2301.03536.
* Acknowledgements: This work is partly supported the European Union’s Horizon 2020 FET OPEN project PHOQUSING (Grant ID 899544),
the European Union’s Horizon 2020 Research and Innovation Programme QUDOT-TECH under the Marie Sklodowska-Curie Grant Agreement
No. 861097, the French RENATECH network, the Paris Ile-de-France Region in the framework of DIM SIRTEQ. Fabrication of one of thephotonic chips was partially performed at PoliFAB - www.polifab.polimi.it

Time-resolving multi-body dynamics in materials withattosecond
soft
x-rays11
1
1
1
1

T.P.H. Sidiropoulos , N. Di Palo , D.E. Rivas , S. Severino , M. Reduzzi , B. Nandy , B.
Bauerhenne2 S. Krylow2, T. Vasileiadis3, T. Danz4, P. Elliott5, S. Sharma5, K. Dewhurst6, C. Ropers4,
Y. Joly7, M. E. Garcia2, M. Wolf3, R. Ernstorfer3, J. Biegert1
1
Barcelona Institute of Science and Technology,08860 Castelldefels, Spain
2
Universität Kassel, 34132 Kassel, Germany
3
Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
4
University of Göttingen, 37073 Göttingen, Germany
5
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany
6
Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany
7
Université Grenoble Alpes, 38000 Grenoble, France

We show that core-level x-ray absorption near edge structure (XANES) spectroscopy withattosecond soft x-ray (SXR) pulses [1] can image the flow of energy inside a material in real time[2]. We photoexcite graphite with a 11 ± 1 fs pump pulse at 1850 nm, or with a 15 ± 1 fs pulse at 800
nm, for various pump fluences between 2.8 ± 0.2 mJ/cm2 and 81 ± 5 mJ/cm2. Figure 1(a) shows themeasured differential x-ray absorption ΔA(E) (pumped minus unpumped) from which striking
changes of up to 15% are immediately apparent. We identify these features as π bonding state and as
π* and σ* antibonding states. Attosecond-resolved measurement with a pump-probe delay step sizeof 0.6 fs show the buildup of coherent charge oscillations, i.e., polarization of the material. Theseoscillations occur at occupied states below and unoccupied states above the Fermi level
predominantly at the pump carrier frequency. We identify the incoherent background due to thedephasing of coherent charge oscillation. This background rises within a few oscillations of the lightfield, signifying the ultrafast transfer of energy from the light field into the electron and hole
excitation of the material.

Fig. 1 Time-dependent XANES measurement. (a) Shown is the differential absorption A(E) between the
XANES spectrum with and without pump pulse. The observed features are identified as arising from electrons at
the bottom of the CB ( ), holes at the top of the valence band ( ), and predominately from optical phonons( *); note that the material is n-doped to 650 meV. The pump pulse bandwidth and duration are indicated inpink. (b) Sum of the differential absorption ∑over the respective energy range for (red),(blue),and (yellow). The curves are fitted with a double exponential convolved with a Gaussian.

Further, we find that ultrafast dephasing of the coherent carrier dynamics is governed by impact excitation(IE) for electrons, while holes exhibit a switchover from impact excitation to Auger heating (AH) alreadyduring the 11-fs duration of the infrared light field.

We further analyze the coherent phonon signal (see Fig. 2) by analyzing the oscillatory pattern (Fig.
2b) exhibited by the σ* data with a short-time Fourier transform (STFT) analysis (Fig. 2c-e).

Fig. 2 Phonon dynamics retrieved from attosecond-XANES. (a) and (b) show results from the TTM-MD
simulation (solid lines) together with experimental data from Fig. 1. The evolution of the electronic temperature
is shown in (a) while (b) shows the SCOPs and lattice subsystems;(c) shows results from a Fourier analysis of the experimental
data (shown in (b)). A multi-peak fit reveals
the predominant modes; positions of theare shown in red and blues, respectively.
(d) Short-Time Fourier Transform (STFT) of thedata (b) over a time range of 250 fs, visualizing coherent
phonon contributions. A comparison with the calculated phonon dispersion (e), for different electronic
temperatures (see SI), allows to identify the phonon modes, their main branches (dashed lines), and theirdispersion (dashed arrows). Striking are the high frequency oscillations at twice the phonon frequencyof , the simultaneous and fast rise of the coherent Raman activemode, concomitantly with the nonRaman-activemode, and the rapid decay into coherent low-frequency phonons and their temporal dynamics.

The STFT analysis shows that already during and shortly after the laser excitation, coherent motion
emerges over a broad range of frequencies. A comparison with the phonon dispersion from twotemperature-model molecular dynamics (TTM MD) simulations [4] [Fig. 1b] identifies them as theRaman-active Γ − E2g and the non-Raman-active K – A’1 SCOPs at 46.4 ± 2.7 and 42.7 ± 1.1 THz,
respectively. The surprising early contribution from the (non-Raman-active) A’1 mode originate fromthe very strong electron-SCOPs coupling, thus acting almost impulsively. We demonstrate the ability
to track energy flow upon light absorption between electrons, holes, and phonons in real-time. We
applied the method to graphite and demonstrate its capability by disentangling the coherent andincoherent contribution to the multi-body dynamics. We expect that the general applicability of themethod will prove valuable to address questions such as, for instance, the energy dissipation in lightharvesting, organic electronic and energy storage systems, or to re-examine long-standing questions
in non-equilibrium multi-body physics such as phase-transitions and superconductivity.
References[1] S. M. Teichmann, F. Silva, S. L. Cousin, M. Hemmer, J. Biegert, Nature Communications 7, 11493 (2016).
[2] T.P.H. Sidiropoulos, N. Di Palo, D.E. Rivas, S. Severino, M. Reduzzi, B. Nandy, B. Bauerhenne, S. Krylow, T. Vasileiadis, T. Danz, P. Elliott
S. Sharma, K. Dewhurst, C. Ropers, Y. Joly, K. M. E. Garcia, M. Wolf, R. Ernstorfer, J. Biegert, Physical Review X. 11, 041060 (2021).
[3] B. Buades, D. Moonshiram, T. P. H. Sidiropoulos, I. Leon, P. Schmidt, I. Pi, N. Di Palo, S. L. Cousin, A. Picon, F. Koppens, J. Biegert
Optica 5, 502 (2018).
[4] S. Krylow, F. Valencia Hernandez, B. Bauerhenne, M. E. Garcia, Physical Review B 101, 205428 (2020).

The polaritonic Bardeen-Cooper-Schrieffer state
R. Binder, M. Spotnitz, N.H. Kwong
University of Arizona, Tucson, AZ 87721, USA
Exciton-polaritons in semiconductor microcavities are well known for their ability to undergo Bose-Einsteincondensation (BEC) as a consequence of the bosonic character of polaritons in the low-density regime. One of
the signatures for BEC is the emergence of coherent emission as a form of spontaneous symmetry breaking.
There are, however, other forms of condensation possible. One is the conventional semiconductor lasing,
sometimes called 'photon lasing', in which Coulomb interaction effects can be largely ignored, and the lasing
process is driven by fermionic gain. In-between the two is the regime of the Bardeen-Cooper-Schrieffer (BCS)
state, where the interaction is important, but the pairing does not lead to tightly bound bosonic quasi-particles.
In the BCS state the electron-hole pairs are not well separated, and understanding the spontaneous symmetry
breaking requires a theory that contains the fermionic degrees of freedom of the electrons and holes, since thepolariton BCS laser is a hybridized state between bosonic cavity photons and Cooper-pair fermions (electrons
and holes). It is the purpose of this talk to discuss the conceptual foundation of the polaritonic BCS state andto present a microscopic theory that helps understand the physical properties of the polaritonic BCS state. In
particular, we have developed a comprehensive theory of the fluctuation modes, from which we obtain
information of the frequencies of the linear excitations of that state. This knowledge, in turn, can help us
develop experimental proposals for spectroscopic approaches to the linear excitations of the polaritonic BCS
state and to predict measurable quantities, such as absorption spectra, that can probe those excitations.

Fig. 1 Schematic of the cavity indicating the cavity confinement of the laser radiation in the near-infrared,
and the absence of cavity effects in the THz regime.
Experimental demonstration of a polariton laser operating in the BCS regime has been achieved only recently[1] in the group of Hui Deng, Univ. Michigan. For the theoretical analysis of that experiment, we used a
microscopic many-particle theory to compute experimental observables such as emission frequency. The
observation of fermionic gain enabled us to rule out BEC, and the large detuning of the emitted light from thecavity resonance enabled us to rule out photonic lasing. We have since extended the theoretical analysis andstudied linear response and fluctuation modes of the polariton laser, i.e., a driven-dissipative quantum manyparticle system prepared in a spontaneous broken-symmetry steady state, using optical frequencies (interband
transitions) [2] and terahertz (THz) frequencies (intraband transitions) [3,4]. In this talk, we focus mainly on
the rich landscape of optical fluctuation modes, that exhibits discrete and continuum soft modes in a twoparameter (pump density and cavity decay rate) space. Since we obtain a complete set of eigenvalues andeigenvectors of the linear response including discrete states and spectral continua, our analysis gives
unprecedented insight into the microscopic physics of a polariton laser and its excitation spectrum. For
example, as the external parameters such as cavity decay rate and pump power are varied, we find thatcollective modes can 'collide' at exceptional points, and that there is a continuum of exceptional points in thetwo-dimensional plane of pump density and cavity decay rate.
Fig. 2 shows an example of the eigenvalues of the fluctuation modes when excited by THz radiation(a) and near-infrared radiation (b).
The modes shown in (b) contain the Goldstone mode 𝐺𝐺0 , as well as various collective (spectrally discrete)modes and spectral continua. A detailed analysis of the collective modes reveals that these collective modes
are not pure Higgs modes [4].

All modes in (b) involve only interband transitions where the photon angular momentum of ±1ℏ is transferredto the electrons that undergo transitions from the valence to the conduction band with angular momentum
difference of the lattice-periodic part of the Bloch wavefunctions of ±1ℏ. Hence, the orbital angularmomentum of the modes in (b) is the same as that of the Goldstone mode, namely zero (s-wave solutions).

Fig. 2 (a) & (b). The eigenvalues of the fluctuation matrix of a polariton laser in the BCS regime, showing
spectral continua and discrete modes (collective states). The modes can be triggered by THz radiation (a) or
near-infrared radiation (b). The fluctuation modes of the light field and the interband polarization oscillate at
frequencies close to the fundamental band gap of GaAs, i.e., the near infrared. The fluctuation modes of thecarrier distributions and carrier density oscillate at THz frequencies in both cases. From Ref. [4] .

In contrast, the modes in (a) have orbital angular momentum of ±1ℏ, since these are related to intrabandtransitions where the intraband photon angular momentum is transferred to the angular momentum of theelectrons and holes that is related to the envelope motion (the plane-wave part) of the Bloch states (the angularmomentum related to the lattice-periodic part of the Bloch states remains unchanged) and thus to the relative
motion of the electron-hole pairs. It is interesting to note that the modes in (a), triggered by THz probes,
contain collective modes (called 𝑇𝑇0 and 𝑇𝑇1 here), which is a pure Coulomb correlation effect. The gaps in thecontinuum energies relative to the energy of the Goldstone mode correspond to the gaps in the excitation
spectrum of the BCS state in superconductors. We note, however, that that the polaritonic system is an openpumped-dissipative system and therefore not equivalent to a superconductor in thermal equilibrium.
In addition to GaAs microcavities, we are also studying analogous effects in microcavities containing twodimensional materials such as MoSe2.
References[1] R. Binder, N.H. Kwong, Physical Review B 103, 085304 (2021).
[2] J. Hu, Zh. Wang, S. Kim, H. Deng, S. Brodbeck, Ch. Schneider, S. Hoefling, N.H. Kwong, R. Binder, Physical Review X 11
011018 (2021).
[3] M. Spotniz, N.H. Kwong, R. Binder, Physical Review. B 104, 115305 (2021).
[4] M. Spotniz, N.H. Kwong, R. Binder, Physical Review B (accepted for publication) (2023).
* We gratefully acknowledge financial support from NSF under grant number DMR 1839570, and CPU time at HPC

University of Arizona.

Advances in time-resolved photoemission spectroscopy
with high-intensity mid-IR excitation at ALLS
J.M. Parent, G. Jargot, B. Frimpong, A. Longa, F. Légaré, F. Boschini
Institut national de la recherche scientifique, Varennes, QC J3X 1P7, Canada

The Advanced Laser Light Source (ALLS) laboratory at the Institut national de la recherche
scientifique (INRS), Montreal (QC, Canada), is a national user facility offering a coherent rainbow
of ultrashort light pulses spanning seven orders of wavelength, from the THz to the hard X-rays,
coupled to several endstations. After a brief summary of ALLS’ experimental capabilities, I will
discuss the new time- and angle-resolved photoemission (TR-ARPES) endstation that will enablenovel investigations of light-induced electron dynamics in quantum materials [1-3]. At the moment,
the TR-ARPES endstation relies on a low photon energy UV probe (6 eV) with ~10 meV bandwidth,
but an HHG-based beamline, comprising a time-preserving monochromator operating in the 10-40
eV energy range, is under construction and will soon enable comprehensive studies of mid-IRinduced electron dynamics over the entire momentum space of quantum materials. Intense near- andmid-infrared optical pulses are generated
via a home-build three-stage opticalparametric amplifier followed by difference
frequency generation (see scheme in Fig.
1).
Here I will present some preliminary 6-eVprobe TR-ARPES data acquired at ALLS.
In particular, I will show how mid-IR light(in the 150-300 meV photon energy range)transiently modifies the dispersion of the
topological surface state of Bi2Te3,
prototypical 3D topological insulator.
Figure 2 displays iso-energy contour maps Fig. 1. Sketch of the optical modules for the generation of nearin the kx-ky momentum space at zero pump- and mid-infrared pump and 6-eV probe at the TR-ARPES
probe delays above the equilibrium Fermi endstation of ALLS.
level (E=0 eV).

Fig. 2. (a): ARPES map of Bi2Te3 at zero pump-probe delay upon 300-meV pump excitation. (b): Iso-energy
contour maps at 0, 0.2, and 0.4 eV above the Fermi level (as highlighted in a), at zero pump–probe delay.

I will also discuss the capabilities of our state-of-the-art hemispherical analyzer (ASTRAIOS 190,
SPECS) that enables the detection of photoelectrons within a 60-degree emission cone without theneed of mechanically move the sample.
References[1] D. Golež, S.K.Y. Dufresne, M.-J. Kim, F. Boschini, H. Chu, Y. Murakami, G. Levy, A.K. Mills, S. Zhdanovich, M. Isobe, H. Takagi, S. Kaiser
P. Werner, D.J. Jones, A. Georges, A. Damascelli, A.J. Millis, Physical Review B 106, L121106 (2022).
[2] M. Michiardi, F. Boschini, H.-H. Kung, M.X. Na, S.K.Y. Dufresne, A. Currie, G. Levy, S. Zhdanovich, A.K. Mills, D.J. Jones, J.L. Mi,
B.B. Iversen, Ph. Hofmann, A. Damascelli, Nature Communications 13, 3096 (2022)[3] M. Zonno, F. Boschini, A. Damascelli, Journal of Electron Spectroscopy and Related Phenomena 251, 147091 (2021)* Acknowledgements: F. Boschini acknowledges support from the Fonds de recherche du Québec – Nature et technologies (FRQNT) and the
Natural Sciences and Engineering Research Council of Canada (NSERC)

Ultrafast dynamics and applications of type II
Semiconductor
heterostructures
1
1
1
2
3

Markus Stein , Felix Schäfer , Daniel Anders , K. Volz , T. Meier , S. Chatterjee1
1
Justus Liebig University Giessen,35392 Giessen, Germany
2
Philipps University Marburg, 35043 Marburg, Germany
3
Paderborn University, 33098 Paderborn, Germany

Low-dimensional quantum structures of compound semiconductors such as type-I quantum wells are
the backbone of the established optoelectronic devices. Their fundamental properties have been
intensely studied for more than half a century [1]. The accompanying immense development inmaterials quality are indispensable for reliable long term device performance and true scalability,
presumably the two prime advantages of semiconductor technology. These developments have beenaccompanied by stringent development of microscopic theories such as the Semiconductor Bloch
Equations [2]. The interplay of experimental and theoretical research has successfully fostered design
and understanding of this class of quantum structures. Regardless, even more than two decades old
predictions remain discussed as they have not been demonstrated experimentally [3].
More recently, heterostructures featuring type-II band offsets offer additional degrees of freedom intailoring their optoelectronic response. From a fundamental stand point, these type-II heterostructuresoffer the potential to investigate charge-transfer-like excitations. Their nonlinear optical response isinvestigated experimentally using polarization resolved four wave mixing, optical-pump optical-probe,and optical-pump Terahertz-probe spectroscopy. The four-wave mixing data reveal clear signatures
which are consistent with of coherent biexcitons obeying the suitable polarization selection rules. TypeI quantum well reference samples show the well-known beating signatures in the transients as well asclear spectral signatures [4]. The latter are also present in type-II samples. However, the smaller excitonand biexciton binding energies infer longer beating times which are not observable due to fasterdephasing of type-II exciton coherences.
Fig. 1. Left: linear absorption spectrum(grey) and shaped pulses tuned resonantly
to the CT exciton resonance (red), as wellas the Ga(As,Sb) and the (Ga,In)As
quantum well exciton resonances, show ingreen and blue, respectively. Right:
degenerate transient four-wave mixing(FWM) data for excitation resonant to the
CT exciton.

For applications, such type-II active devices combine the advantages of a spectrally broad, temperaturestable efficient gain with the potential for electrical injection pumping. The intrinsic charge-carrier
relaxation dynamics limit the feasible repetition rates beyond constraints of cavity design and heatremoval. Here, we investigate the initial buildup of gain after optical excitation as well as its recovery
after a stimulated emission process in a (Ga,In)As/GaAs/Ga(As,Sb) heterostructure optimized for thenear infrared. This experimentally simulates the operation condition of a pulsed laser or semiconductor
optical amplifier which are ideally limited by the materials charge-carrier dynamics. The gain
bandwidth in these heterostructures exceeds 50 meV full-width-at-half-maximum and should support
very temperature stable operation as well as ultrafast pulsing. We use an optical pump - optical probe
setup where a first optical pulse injects hot charge carriers that eventually build up spectral gain in thesample. The energies are chosen such to mimic typical electrical injection surplus energies.
Subsequently, a second laser pulse tuned to the broad spectral region in which gain is observed is used
to stimulate emission and thus eliminate the gain. Analysis of the absorption spectra after stimulated
emission reveals gain recovery times in the vicinity of 5 ps [5], which defines a physical limit for thehighest laser repetition rate possible with this material system in the range of 100 GHz under a directelectrical injection scheme. The understanding of the ultrafast dynamics of such type-II heterostructures
is important for designing and optimizing next generation mid-infrared light sources. Tailoring the band
offsets will reduce the emission energies of active devices and is envisioned to reduce if not eliminate
any Auger losses. Furthermore, such structures area ideal model systems to study the structure anddynamics of excitations across internal interfaces.
References[1] J. Shah, Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures, Springer Series in Solid-State Sciences 115, (2013)[2] M. Lindberg, S. W. Koch, Physical Review B 38, 3342, (1988).
[3] C. Ciuti, C. Piermarocchi, V. Savona, P. E. Selbmann, P. Schwendimann, A. Quattropani. Physical Review Letters 84, 1752 (2000).
[4] M. Fey, M. Stein, C. Fuchs, W. Stolz, K. Volz, S. Chatterjee Physical Review B 106 165303, (2022)[5] F. Schäfer, M. Stein, J. Lorenz, F. Dobener, C. Ngo, J. T. Steiner, C. Fuchs, W. Stolz, K. Volz, T. Meier, J. Hader, J. V. Moloney, S. W. Koch
S. Chatterjee, Applied Physics Letters 122, 082104 (2023).
* Acknowledgment: Financial support from the Deutsche Forschungsgemeinschaft via the Collaborative Research Center No. 223848855-SFB 1083 andEuropean Regional Development Fund (ERDF) through FPG990 0005/2018 is gratefully acknowledged.

Observation of a longer decay time constant and deviation from theSupercollision model of the carriers’ cooling in bilayer graphene
A. I. Chatzakis
Texas Tech University, Lubbock, TX 79409, USA

Optical-pump THz-probe spectroscopy is employed to investigate the cooling dynamics of hot
carriers in quasi-free standing bilayer epitaxial graphene with hydrogen intercalation. The epitaxial
graphene layer on SiC is sitting on top of a graphene-like layer called the buffer layer (BL). About
30% of the carbon atoms in BL are covalently bound with the Si atoms on the SiC surface, which
prevents the typical formation of the linear dispersion of π bands in graphene. With the penetrationof the hydrogen atoms under the BL, the bonds between the C and Si break, and the Si dangling
bonds are saturated with hydrogen atoms. The BL turns into quasi-free-standing epitaxial grapheneand the monolayer graphene into decoupled bilayer [1] (Fig.1 left)

Time DelayΔτ
THz Probe
Graphene on
SiC

800nm Pump

Quasi-freestanding
graphene

Bufferlayer
Hydrogen
atoms
SiC

Fig. 1. Left Schematic of the measurement setup and bilayer graphene; Right: Differential transmission of the T
Hz waveform under various excitation fluences.

Upon optical excitation, electrons in graphene are in a nonthermal distribution that undergoesinternal thermalization through Coulomb interaction. This occurs on a time scale of tens offemtoseconds and leads to a broadened Fermi-Dirac distribution with an elevated electron
temperature, . Due to the small heat capacity of electrons, the electronic temperature can besignificantly high. Subsequently, the hot carriers relax towards equilibrium throughscattering mechanisms including defects-assisted acoustic phonons scattering (supercollisions) [2],heat diffusion, cooling via optical phonons, and interactions with the substrate phonons. We identify
the dominant cooling mechanism of hot carriers and demonstrate (i) that the predictions of thesupercollision cooling model significantly deviate from the experimental data and, (ii) that therelaxation of the hot carriers primarily occurs due to electron–optical phonon interactions.
Furthermore, the observed experimental dynamics exhibit the expected longer timescales, in therange of 2.6 to 6.4 ps, which increase nonlinearly with excitation intensity. The increased relaxation
times are due to the decoupling of the graphene layer from the SiC substrate after hydrogenintercalation which increases the distance between graphene and substrate [3]. In addition, we
observe an increase in pump-induced conductivity.
References[1] C. Riedl, C. Coletti, T. Iwasaki, A. A. Zakharov, U. Starke, Physical Review Letters 103, 246804 (2009).
[2] J. C. W. Song, M. Y. Reizer, L. S. Levitov, Physical Review Letters 109, 106602 (2012).
[3] M. T. Mihnev, F. Kadi, C. J. Divin, T. Winzer, S. Lee, C.-H. Liu, Z. Zhong, C. Berger, W. A. de Heer, E. Malic, A. Knorr, T. B. Norris
Nature Communications 6, 8105 (2015).

Controlling THz emission in topological materials
E. E. M. Chia
Nanyang Technological University, Singapore 637371, Singapore

In this talk I will show terahertz (THz) emission data on a few topological materials. In the
ferromagnet-semiconductor Co/MoS2 heterostructure, by making use of the strongly out-ofequilibrium character of the injected spins, we demonstrate a highly-efficient spin injection from aferromagnet into a semiconductor, thus overcoming the crippling problem of impedance mismatch.
Astonishingly, we measure a giant spin current that is orders of magnitude larger than typicalinjected spin current densities using currently available techniques [1]. In thin polycrystalline filmsof the centrosymmetric Dirac semimetal PtSe2, we observe a giant and highly tunable THz emissionthat is rapidly turned on at oblique incidence. Strikingly, we find the THz emission to be locked toboth the in-plane photon momentum and polarization state of the incident pump beam, where theTHz sign and amplitude are fully controlled by the incident pump polarization, helicity and photon
momentum. Moreover, the emitted THz efficiency is two orders of magnitude larger than that of thestandard THz-generating nonlinear crystal ZnTe, and approaches that of the record-setting
topological material TaAs.

Fig. 1 (a) (THz peak)/(absorbed fluence) of Co/MoS2 under different pump wavelengths. The red, white andpurple regions represent different spin injection processes illustrated in panels (b), (c) and (d), respectively [1].

Fig. 2 (a) Schematic of THz emission setup for Dirac semimetal PtSe2. (b) THz emission amplitude for different
incident pump angle [2].

Our work demonstrates how photon drag activates a rich and pronounced directional optical linearitythat are available even in centro-symmetric and polycrystalline Dirac materials [2].
References[1] L. Cheng, X. Wang, W. Yang, J. Chai, M. Yang, M. Chen, Y. Wu, X. Chen, D. Chi, K. E. J. Goh, J.-X. Zhu, H. Sun, S.Wang, J. C. W. SongM. Battiato, H. Yang, E. E. M. Chia, Nature Physics 15, 347 (2019).
[2] L. Cheng, Y. Xiong, L. Kang, Q. Chang, M. Chen, J. Qi, H. Yang, Z. Liu, J. C. W. Song, E. E. M. Chia, Science Advances 9, 7856 (2023).

Light-enhanced charge density wave coherence inHigh-temperature superconductor
G. Coslovich
SLAC National Accelerator Laboratory, Menlo Park, California 94720, USA

The use of ultrashort optical and X-ray pulses offers new opportunities to study fundamental
interactions in materials exhibiting unconventional quantum states, such as stripes, charge density
waves and high-temperature superconductivity. To understand the microscopic interdependence
between these order parameters, a probe capable of discerning their interaction on its natural lengthand time scales is necessary. In this talk, I will focus on high temperature superconductors and recent
ultrafast resonant soft x-ray scattering results tracking the transient evolution of charge density wave
correlations in YBa2Cu3O6+x [1]. In this study, ultrashort infrared pulses produce a non-thermal
quench of the superconducting state while X-ray pulses detect the reaction of charge density waves.
The response happens on a picosecond timescale and is characterized by a large enhancement of
spatial coherence of charge density waves, nearly doubling their correlation length, accompanied bya smaller increase of their amplitude (Fig.1). Such behavior can be reversed at higher fluences and at
higher temperatures (above TC, 65 K) where the photo-induced CDW melting process takes placeinstead.

Fig. 1. Left: X-ray scattering profile of the CDW along the H direction in reciprocal space at 65 K before(orange) and after (red) laser excitation. The data show the well-known melting dynamics of the CDW order.
Right: X-ray scattering profile at 12 K before (grey) and after (purple) laser excitation, highlighting the lightinduced enhancement of CDW correlations. See Ref. [1] for further details.

This ultrafast snapshot directly reveals thecoupling between superconductivity and charge
density waves on its natural timescale. It
demonstrates that their competition manifests
inhomogeneously at the nanoscale level, as
disruption of spatial coherence, indicating therole of superconductivity in stabilizing
topological defects within charge density
waves domains. The study highlights a path forenhancing spatial coherence (order) by using
Fig.2.Illustration of the light-induced enhanced
coherence in a CDW system.(Image credit: Greg
light pulses changing the balance between
Stewart, SLAC)
intertwined orders (Fig.2).
In conclusion, I will also discuss ongoing developments at the LCLS to further expand on theseresults and unlock novel capabilities to control and observe quantum materials with light pulses.
References[1] S. Wandel, F.Boschini, E. H. da Silva Neto, L. Shen, M. X. Na, S. Zohar, Y. Wang, S. B. Welch, M. H. Seaberg, J. D. Koralek, G. L. Dakovski
W. Hettel, M.-F. Lin, S. P. Moeller, W. F. Schlotter, A. H. Reid, M. P. Minitti, T.Boyle, F. He, R. Sutarto, R. Liang, D. Bonn,W. Hardy
R. A. Kaindl, D. G. Hawthorn, J.-S. Lee, A. F. Kemper, A. Damascelli, C. Giannetti, J.J. Turner, G. Coslovich, Science 376, 860 (2022).
* Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

Ultrafast magnetization reversal driven by circularly-polarized
Optical phonons
1
1
2
3
1
1
C.S. Davies , F.G.N. Fennema , A. Tsukamoto , I. Razdolski , A.V. Kimel , A. Kirilyuk
1
Radboud University, 6525 Nijmegen, the Netherlands
2
Nihon University, 101-8360 Tokyo, Japan
3
University of Bialystok, 15-328 Bialystok, Poland

All-optical switching (AOS) of magnetization – the process through which ultrashort optical pulses
reverse magnetic ordering without any bias magnetic fields – offers a promising and potentiallydisruptive paradigm for future data-storage technologies operating at ultrafast speeds with highefficiency. Multiple avenues for AOS have emerged in recent years [1] ranging from thermal singleshot exchange-driven switching in ferrimagnets [2] and helicity-dependent switching in ferromagnets[3] to non-thermal switching mechanisms driven by the resonant excitation of electronic [4] orphononic [5] subsystems. Here, we explore further how the resonant excitation of infrared-active
optical phonons can switch magnetization. Specifically, using narrow-band transform-limited
infrared optical pulses delivered by the free-electron laser facility FELIX, with frequency ranging
between 6 THz and 40 THz (wavelength 7.5-50 µm) [6], we drive transverse optical phonons atresonance in different systems. We have recently discovered that optical phonons in paramagnetic
substrates can, when driven at resonance by circularly-polarized infrared pulses, realize directional
AOS in an adjacent film [7]. In our experiments, we irradiate thin GdFeCo films mounted on fusedsilica, sapphire and silicon substrates with circularly-polarized optical pulses. The resulting helicitydependent magnetic switching scales in efficiency with the transverse optical phonon spectrum
characteristic not of the metal but rather the substrate (see Fig. 1).

Fig. 1 Spectral dependence of helicity-dependent switching of magnetization in a nanolayer of GdFeComounted on a fused-silica substrate (blue line, typical images shown in inset). Also overlaid is the spectrum of
longitudinal and transverse optical phonons characteristic of the fused-silica substrate (black and red lines
respectively).

Since this approach is based on the excitation of phonons in a dissimilar material, it could beuniversally applied to manipulate magnetic ordering in any conceivable system.
References[1] A. Kirilyuk, A.V. Kimel, T. Rasing, Review of Modern Physics 82, 2731 (2010).
[2] C. S. Davies, T. Janssen, J. H. Mentink, A. Tsukamoto, A. V. Kimel, A. F. G. van der Meer, A. Stupakiewicz, A. Kirilyuk, Physical Review
Applied 13, 024064 (2020).
[3] C.-H. Lambert, S. Mangin, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, M. Hehn, M. Cinchetti, G. Malinowski, K. Hono, Y. Fainman,
M. Aeschlimann, E. E. Fullerton, Science 345, 1337 (2014).
[4] A. Stupakiewicz, K. Szerenos, D. Afanasiev, A. Kirilyuk, A. V. Kimel, Nature 542, 71 (2017).
[5] A. Stupakiewicz, C. S. Davies, K. Szerenos, D. Afanasiev, K. S. Rabinovich, A. V. Boris, A. Caviglia, A. V. Kimel, A. Kirilyuk
Nature Physics 17, 489 (2021).
[6] G. M. H. Knippels X. Yan, A. M. MacLeod, W. A. Gillespie, M. Yasumoto, D. Oepts, A. F. G. van der Meer, Phyical Review Letters 83, 1578
(1999).
[7] C. S. Davies, F. G. N. Fennema, A. Tsukamoto, I. Razdolski, A. V. Kimel, A. Kirilyuk, submitted (2022).

Effects of Floquet engineering on the coherent exciton
Dynamics
in 2monolayer
WS2
1
1
2
2
1

M.A. Conway , S.K. Earl , T.-H.-Y. Vu , M.S. Fuhrer , M.T. Edmonds , J.A. Davis
1
Swinburne University of Technology, Hawthorn VIC3122, Australia
2
Monash University, Clayton VIC3800, Australia

Coherent light-matter interactions offer a versatile means to optically control nonlinear phenomena[1, 2]. In particular, Floquet-engineering can utilise the periodicity of a light-field to alter theproperties of quantum-matter. In 2D materials, interactions between the n-photon-dressed band
replicas, called Floquet-Bloch bands, can facilitate ultrafast manipulation of excitonic properties. A
simple example of this for a two-level system is the AC-Stark effect(Fig. 1(a)) [3, 4].
Semiconducting monolayer transition metal dichalcogenides (TMDCs) are ideal platforms for band
structure engineering as they possess an optically accessible bandgap with intrinsic, but opposite,
Berry phase at the K and K′-valleys. Combining the nontrivial Berry phase with appreciable spinorbit splitting of atomic orbitals, these time-reversal symmetric valleys couple to different opticalselection rules: K-valley excitons (XK) are dressed by left-circularly polarized light (σ−), while K′valley excitons (XK′) are dressed by right-circularly polarized light (σ+). When excited by linearlypolarized light, the superposition of σ+ and σ− polarized components that comprise the light-field
prepare a coherent superposition of excitons in the K- and K′-valley [5]. Optical manipulation of thisinter-valley exciton coherence, |XK⟩ ⟨XK′|, has been indicated in experiments driving a valley
selective AC-Stark shift to break the valley degeneracy. This causes the phase of the inter-valley
exciton coherence to rotate, resulting in a measurable rotation of polarization of the measured
photoluminescence[6]. However, there are ambiguities in these time-integrated photoluminescencemeasurements; whether or not the process proceeds adiabatically cannot be resolved; and subsequent
investigations of a Bloch-Siegert effect (Fig. 1(b)) suggest there is a counteracting shift induced in
the opposite valley which is not separable from the dominant AC-Stark effect (Fig. 1(a)) [7]. Here
we utilise multidimensional coherent spectroscopy (MDCS) [8] to directly measure the pump
induced changes to the amplitude and phase of a coherence in monolayer WS 2. To isolate theinfluence of the AC-Stark and Bloch-Siegert effects on the signal phase, φsig, we can measure apump induced phase rotation of the 1Q coherence during t1, wherein φsig ∝ −ω1t1: After the arrival ofthe σ+-polarized first pulse (labelled k1) a 1Q-coherence between the ground state, g, and the K’valley exciton state, XK’, will be created. This |g⟩⟨XK’| 1Q-coherence evolves with a frequency givenby the A-exciton energy in monolayer WS2 of 2.07 eV, over the t1 time period, until the second pulse
arrives. Placing a red-detuned optical pump during t1 that is co- or cross-circularly polarized relative
to k1, we can induce an additional phase rotation of the signal due to the energy shift induced by the
AC-Stark or Bloch-Siegert effect, respectively. The evolution of the measured phase difference as afunction of t1, obtained by taking a slice at the exciton energy, is shown in Fig. 1(c). For a σ+polarized pump during t1 we induce an appreciable change in the phase of the |g⟩⟨XK’| coherence up
to a π phase shift (red lines). With a σ− polarized pump a much smaller phase shift is observed (bluelines). In the case of the σ+ polarized pump the polarization is the same as the |g⟩⟨XK’| coherence
induced by k1. In this configuration, the AC-Stark effect will blue-shift the XK’ state [3, 4], whichcauses the phase to shift over the pulse duration, as measured. Conversely, with the σ− polarizedpump the Stark shift is occurring in the XK state, which should not affect the measured phase. The
Bloch-Siegert effect will, however, cause a blue-shift of the XK state [7], and is the source of thephase shift observed in this case. For 250 < t1 < 400 the measured phase shift increases as the k1
pulse is moved through the pump pulse. The rate of change of the phase is determined by the induced
energy shift, i.e. the coherence evolves as. The time dependent energy shift can thus bedetermined by differentiating the measured phase shift with respect to t1. This is shown in Fig. 1(d)for a σ+ polarized pump in t1. A Gaussian fit to the instantaneous Stark shift (red lines) closely
resembles the temporal profile of the measured cross-correlation between the pump and the MDCS
excitation beams (black line). This indicates that within the temporal resolution afforded by the 31 fs
k1 pulse the AC-Stark effect follows the instantaneous intensity of the pump. The peak Stark shiftmeasured is > 16meV for the highest pump fluence of 751.7 μJ/cm2.

Fig. 1. (a) and (b) schematically represent an energy diagram of the K’-valley as a function of time t, upon
arrival of the red-detuned σ+ and σ− polarised pump an AC-Stark or Bloch-Siegert blue-shift is induced,
respectively. (c) Phase dynamics at the exciton energy driven by a σ + and σ– circularly polarized pump arriving
during t1; (d) derivative of the phase shift φ for the σ+ pump case in (c) yields the instantaneous Stark shift, dφ/dt(e) signal amplitude integrated around the exciton energy as a function of t1 with a log y-axis. Over pumpduration the amplitude decreases due to power broadening to which we can fit a slope with a decay rate 1.7
times the unpumped decay; (f) signal amplitude in (e) of the pumped data divided by the unpumped reference
highlights the flat region beyond the pump duration. The lines follow the smoothed data points.

The smooth evolution of the phase through the pump-induced Stark shift suggests the process isadiabatic. On the other hand, however, Fig. 1(e) shows the normalized signal amplitude integrated
around the exciton energy, as a function of t1, in which a decrease in the signal amplitude can be seen
over the pump duration, suggesting the pump causes a loss of coherence. After the pump pulse, thedecay rate once again matches the unpumped data, as indicated by the slope of the data and fits in
Fig. 1(e). We rule out the possibility that the loss of coherence is due to the red-detuned pumpexciting carriers through two-photon absorption as the increased scattering and faster decoherence
would persist as long as the photoexcited carriers remain, which is expected to be well beyond themeasurement time. Thus, we conclude that the decrease of the macroscopic coherence is a field
driven effect persisting only over the finite pump duration.
Varying the intensity of the pump pulse shows that the magnitude of the added coherence loss islinearly proportional to the pump fluence and is quantitatively consistent with power broadening. In
the case of short pulses, where the pulse duration is less than the decoherence time, quantifying thepower broadening can be more challenging [10]. An experimental estimate of the average power
broadening throughout the pulse is obtained from the data in Fig. 1(e). A linear fit to the data (on thesemi-log axes) over the duration of the pulse yields a decay rate that is 1.7 times faster than theunpumped decay. This suggests an average power broadened linewidth of ≈5 meV.
By using multidimensional coherent spectroscopy (MDCS) to create excitonic coherences inmonolayer WS2 we have been able to measure the changes to the amplitude and phase induced
through a process of Floquet engineering. While the average phase evolves smoothly, and follows
what would be expected from Floquet theory, even though the pulses are short, there is a loss inmacroscopic coherence due to power broadening caused by the electric field of the pump pulse. This
places some limitations on the possible use of this approach for applications requiring adiabatic
control, but also points to means by which the effects can be minimized. The experiments describedhere will provide a valuable tool to quantify the effectiveness of such approaches, including
suggestions for inducing proposals to induce spectral narrowing with shaped pump pulses.
References[1] J. W. McIver, B. Schulte, F.-U. Stein, T. Matsuyama, G. Jotzu, G. Meier, A. Cavalleri, Nature Physics. 16, 38 (2020).
[2] Y. Kobayashi, C. Heide, A.C. Johnson, V. Tiwari, F. Liu, D.A. Reis, T.F. Heinz, S. Ghimire, Nature Physics 19, 171 (2023).
[3] J. Kim, X. Hong, C Jin, S-F Shi, C-Y.S. Chang, M-H Chiu, L-J Li, F. Wang, Science 346, 1205 (2014).
[4] E.J. Sie, J.W. McIver, Y-H. Lee, L. Fu, J. Kong, N. Gedik, Nature Materials 14, 290-294 (2015).
[5] A. M. Jones, H. Yu, N.J. Ghimire, S. Wu, G. Aivazian, J.S. Ross, B. Zhao, J. Yan, D.G. Mandrus, D. Xiao, W. Yao, X. Xu, Nature
Nanotechnology 8, 634 (2013).
[6] Z. Ye, D. Sun, T. F. Heinz, Nature Physics. 13, 36 (2017).
[7] E.J. Sie, C H Lui, Y-H Lee, L. Fu, J Kong, N. Gedik, Science 355, 1066 (2017).
[8] J. O. Tollerud, J. A. Davis, Progress in Quantum Electronics 55, 1 (2017)[9] M.A. Conway, S.K. Earl, J.B. Muir, T-H-Y Vu, J.O. Tollerud, K. Watanabe, T. Taniguchi, M.S. Fuhrer, M.T. Edmonds, J.A. Davis,
arXiv:10.48550/arXiv.2301.12599 (2023)[10] N.V. Vitanov, B.W. Shore, L. Yatsenko, K. Böhmer, T. Halfmann, T. Rickes, K. Bergmann, Optics Communications 199, 117-126 (2001).
[11] I.I. Boradjiev, N.V. Vitanov, Optics Communications 288, 91(2013).

Ultrafast dynamics from the perspective of resonant inelastic
X-ray scattering
M. P. M. Dean
Brookhaven National Laboratory, Upton, NY 11973, USA

Resonant inelastic x-ray scattering (RIXS) is a state-of-the-art spectroscopic technique with uniquecapabilities for probing a wide variety of electronic excitations in quantum materials [1]. The adventof x-ray free electron lasers has opened new opportunities to apply this technique to transient states,
as demonstrated by pioneering experiments that measured the short-range magnetic correlations inphoto-excited iridates [2, 3]. It has long been recognized that very strong mixing or “hybridization”
between Cu and O orbitals, coming from small charge-transfer energy Δ, is crucial for the physics ofcuprates. Controllably modifying Δ represents an attractive potential means to modify the propertiesof correlated materials. In this talk, I will address the prospects of using RIXS to probe the chargetransfer properties of quantum materials. Figure 1 illustrates the use of O K-edge RIXS to probe thecharge-transfer energy and hybridization in quantum materials and shows the different regimes of acharge-transfer insulator, a mixed charge-transfer/Mott-Hubbard insulator, and a Mott-Hubbard
insulator.

Fig. 1. Schematic of the O K edge RIXS process. (a)–(c)RIXS processes for different values of the Hubbard U
and charge-transfer energy Δ. For a charge-transfer insulator (Δ ≪ U), the doped holes are mostly in theoxygen 2p orbitals, while in a Mott-Hubbard insulator (Δ ≫ U), the doped holes mainly occupy the TM
3dstate. In the mixed charge-transfer/Mott-Hubbard regime (Δ ∼ U), the doped
holes are spread among boththe TM and oxygen sites. Arrows show examples of x-ray transition pathways, which, because of x-ray dipole
selection rules, can involve either O states or TM-O hybridized states, making this process ideal to distinguishsituations (a)–(c).

Thus far, we have managed to apply RIXS to study charge transfer properties in equilibrium and have used it
to solve several open problems in the physics of the recently discovered low valence superconductors
nickelates and how they relate to cuprates. Some important questions include: Do these materials haveappreciable oxygen charge-transfer character and superexchange akin to the cuprates or are they in a distinct
Mott-Hubbard regime where oxygen plays a minimal role and superexchange is negligible?

Figure 2 illustrates the use of O K-edge RIXS to quantify the role of oxygen in these materials in which wecompare the low valence nickelate La4Ni3O8 to prototypical cuprate La2−xSrxCuO4. As expected, the cupratelies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen (ZSA) scheme. The nickelate, however,is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer–MottHubbard character with the Coulomb repulsion U of similar size to the charge-transfer energy Δ.
Nevertheless, the transition-metal-oxygen hopping is larger in La4Ni3O8 than in La2−xSrxCuO4, leading to asignificant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La4Ni3O8
despite its larger Δ [4]. This explains our prior direct measurements of magnetic exchange in this material [5].

Fig. 2. Cluster exact diagonalization results. (a) Sketch of the clusters used in the case of the cuprates andnickelates. (b) Measured O K-edge RIXS spectra for La2-xSrxCuO4 and La4Ni3O4. (c) Calculated spectra forthese materials. The feature at 0.5 eV in the La2−xSrxCuO4 sample is known to come from a plasmon, so is notexpected to appear in our model calculations.

I will end the talk by discussing how to extend this approach to study charge density waves in these materials[6] and prospects to ultrafast pump-probe measurements of this type.
References[1] Y. Cao, D.G. Mazzone, D. Meyers, J.P. Hill, X. Liu, S. Wall, M.P.M. Dean, Philosophical Transactions of the Royal Society A 377, 2145 (2019).
[2] M. P. M. Dean, Yue Cao, X. Liu, S. Wall, D. Zhu, R. Mankowsky, V. Thampy, X. M. Chen, J. G. Vale, D. Casa, J. Kim, A. H. Said
P. Juhas, R. Alonso-Mori, J. M. Glownia, A. Robert, J. Robinson, M. Sikorski, S. Song, M. Kozina, H. Lemke, L. Patthey, S. Owada
T. Katayama, M. Yabashi, Y. Tanaka, T. Togashi, J. Liu, C. R. Serrao, B. J. Kim, L. Huber, C.-L. Chang, D. F. McMorrow, M. Först
J. P. Hill, Nature Materials 15, 601(2016).
[3] D. G. Mazzone, D. Meyers, Y. Cao, J. G. Vale, C. D. Dashwood, Y.Shi, A. J. A. James, N. J. Robinson, J. Lin, V. Thampy, Y. Tanaka
A. S. Johnson, H. Miao, R.Wang, T. A. Assefa, J. Kim, D. Casa, R. Mankowsky, D.Zhu, R. Alonso-Mori, S. Song, H. Yavas, T.Katayama,
M. Yabashi, Y. Kubota, S. Owada, J. Liu, J. Yang, R. M. Konik, I. K. Robinson, J. P. Hill, D. F. McMorrow, M. Först, S. Wall, X. Liu
M. P. M. Dean, Proceedings of the National Academy of Sciences 118, e2103696118 (2021).
[4] Y. Shen, J. Sears, G. Fabbris, J. Li, J. Pelliciari, I. Jarrige, Xi He, I. Bozovic, M. Mitrano, Junjie Zhang, J. F. Mitchell, A. S. Botana, V. Bisogni
M. R. Norman, S. Johnston, M. P. M. Dean, Physical Review X 12, 011055 (2022)[5] J. Q. Lin, P. Villar Arribi, G. Fabbris, A. S. Botana, D. Meyers, H. Miao, Y. Shen, D. G. Mazzone, J. Feng, S. G. Chiuzbaian, A. Nag
A. C. Walters, M. Garcia-Fernandez, Ke-Jin Zhou, J. Pelliciari, I. Jarrige, J. W. Freeland, Junjie Zhang, J. F. Mitchell, V. Bisogni, X. Liu
M. R. Norman, M. P. M. Dean, Physical Review Letters 126, 087001 (2021)[6] Y. Shen, J. Sears, G. Fabbris, J. Li, J. Pelliciari, M. Mitrano, W. He, Junjie Zhang, J. F. Mitchell, V. Bisogni, M. R. Norman, S. Johnston
M. P. M. Dean, Physical Review X 13, 011021 (2023).

Non-equilibrium superconductivity in a prototype two-band
Superconductor
MgB2 driven
by narrow-bandTHzpulses
1
1
1,2
1
3
3
3

S. Sobolev , A. Lanz , T. Dong , A. Pokharel , A. Pashkin , S. Winnerl , M. Helm , V.V. Kabanov2
T-Q. Xu4, Y. Wang4, Z. Z. Gan4, L.Y. Shi2, N-L. Wang2, J. Demsar1
1
Johannes Gutenberg-University Mainz, 51099 Mainz, Germany
2
Peking University, Beijing 100871, China
3
Jozef Stefan Institute, 1000 Ljubljana, Slovenia
4
Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany

Excitation of a superconductor (SC) with a low energy electromagnetic pulse may lead to a nonequilibrium state, which may profoundly differ from a quasi-thermal one, driven by opticalexcitation. Such a non-equilibrium may be especially pronounced in multi-band superconductors,
such as MgB2. Here, we report on studies of the dynamics of the SC order in ultra-clean thin films of
MgB2 [1], the prototype two-band SC with two distinct superconducting gaps opening in the twobands with weak interband coupling [2,3]. We performed systematic time-resolved studies of gap
dynamics following excitation with intense narrow-band THz pulses with photon energies tuned
between the two superconducting gaps. We demonstrate that the temperature and excitation density
dependent dynamics qualitatively follows the behavior predicted by the phenomenological RothwarfTaylor model for dynamics in a single gap BCS superconductor [4]. This implies strong coupling
between the two condensates on the ps timescale. Tracking the dependence of the amplitude of theTHz driven gap suppression, however, displays a pronounced minimum near ~ 0.6 T c (see Fig. 1).
This cannot be accounted by the phenomenological model.

Fig. 1. (a): The amplitude of photoinduced suppression of the effective gap (D) as a function of temperature andabsorbed energy density, A (mJ/cm3), when pumping with 2.7 THz narrow-band THz pulses. (b) Photoinduced
suppression amplitudes normalized to A presented in the contour plot. Star-like marks in the color plot
represent the data points. Areas that could not be accessed in the experiment are masked with grey.

Comparison of the results to those obtained by excitation with near-infrared pulses suggests that
excitation with narrowband THz pulses results in long-lived (100 ps timescale) non-thermal
quasiparticle distribution, which gives rise to Eliashberg-type enhancement of superconductivity[5,6], competing with pair-breaking.
References[1] S. Kovalev, T. Dong, L-Y. Shi, C. Reinhoffer, T-Q. Xu, H-Z. Wang, Y. Wang, Z-Z. Gan, S. Germanskiy, J-C. Deinert, I. Ilyakov
P.H.M. van Loosdrecht, D. Wu, N-L. Wang, J. Demsar, Z. Wang, Physical Review B 104, L140505 (2021).
[2] A.Y. Liu, I.I. Mazin, J. Kortus, Physical Review Letters 87, 087005 (2001).
[3] S. Souma, Y. Machida, T. Sato, T. Takahashi, H. Matsui, S.-C. Wang, H. Ding, A. Kaminski, J. C. Campuzano, S. Sasaki, K. Kadowaki,
Nature 423, 65-67 (2003).
[4] V.V. Kabanov, J Demsar, D Mihailovic, Physical Review Letters 95, 147002 (2005).
[5] G.M. Eliashberg, Journal of Experimental and Theoretical Physics-JETP Letters 11, 114 (1970).
[6] M. Beck, I. Rousseau, M. Klammer, P. Leiderer, M. Mittendorff, S. Winnerl, M. Helm, G. N. Gol’tsman, J. Demsar, Physical Review Letters.
110, 267003 (2013).
* Acknowledgement(s): This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Grant No. TRR
173-268565370 (project A05) and TRR 288−422213477 (project B08). T.D. acknowledges support by the Alexander Humboldt Foundation.

Atomic layer-controlled nonlinear THz valleytronics in
Semi-metal and semiconductor PtSe2
S. S. Dhillon
Paris-Sorbonne Université, 75005 Paris, France

Platinum diselenide (PtSe2) is a promising two-dimensional (2D) material for the terahertz (THz)range as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned
from a semiconductor in the near-infrared to a semimetal with the number of atomic layers. This
gives the material unique THz photonic properties that can be layer-engineered. Here, we
demonstrate that a controlled THz ultrafast nonlinearity - tuned from monolayer to bulk PtSe2 - canbe realised in PtSe2 through the generation of ultrafast photocurrents and the engineering of the
bandstructure valleys. Further, we show layer dependent circular dichroism, where the sign of theultrafast currents can be controlled through the excitation of different bandstructure valleys. As well
as showing that PtSe2 is a promising material for THz generation through layered opticalnonlinearities, this work opens up new class of circular dichroism materials, beyond the monolayer
limit that has been the case of traditional TMDs, and impacting a range of domains from THz
valleytronics, THz spintronics to harmonic generation. Over the past few years, a large number of 2D
transition metal dichalcogenides (TMDs) semiconductors have been studied, with uniquebandstructures and distinct properties from their bulk counterparts [1]. The terahertz (THz) spectral
region, however, remains relatively unexplored for novel TMD applications since most TMDs havetunable bandgap energies residing in the optical region. However, a newly discovered TMD material,
platinum diselenide (PtSe2), has sparked considerable interest. Unlike other TMDs, its bandgap canbe tuned from a semiconductor in the near-infrared to a semimetal depending on the number of
atomic layers, making it an attractive THz candidate [2]. Although THz emission time-domain
spectroscopy (TDS) has provided valuable insights into transport phenomena in 2D materials, thetransition between semiconductor and semimetal behavior and their effects on THz photocurrents
and nonlinearities has not been studied in a single material system. These phenomena can be
explored in PtSe2 owing to its layer-controlled bandgap, large electronic mobilities, and tunability
from a semiconductor to a Dirac semimetal [3]. This also opens perspectives for functionalizing THz
emission properties with number of Atomic Layers. Very recent studies have investigated THz
photocurrents in PtSe2 but only for a specific thickness, limiting the understanding of the effect of
the bandstructure. Further, these studies have assumed an entirely centrosymmetric nature andneglected the effect of the substrate [4]. The latter is primordial to consider in 2D materials, such thatthe origin of the generated photocurrents can be understood in the context of the layer-dependent
bandstructure. Figure 1a,b shows the principle of our approach, where we exploit the layer-dependent energy
gaps and the valley properties of PtSe2.

Fig. 1: Schematic of helicity-dependent generation of THz photocurrents in substrate-coupled PtSe2.
Simulated spin-resolved band structure (red for spin up and blue for spin down) of a) ML and b) multilayer
PtSe2 on a substrate under the circular right (RCP) and left (LCP) femtosecond excitation pulse at 1.5 eV. For
the ML, indirect interband excitations around Γ point (orange) are possible, whilst, for multilayer PtSe2, directinterband transitions in the vicinity of K(K’) under the circular left (right) polarized light can be excited.

It shows the simulated spin-resolved bandstructure for the two extremes of monolayer (ML) and multilayer
PtSe2 on a substrate. The PtSe2 is excited with an optical pulse at 1.5 eV, generating ultrafast THz
photocurrents that radiate as a short THz pulse. In the ML case, the states at the K points are not accessible forphoton energies of 1.5 eV, and the interband response is governed by (indirect) transitions around the Γpoint. Here the bands are not spin-polarized and thus the optical absorption is not selective to a
specific circular polarization (LCP or RCP) of the incident light. However, this situation changes
drastically with multilayer PtSe2 since the material slowly becomes semi-metallic with an increasing
number of layers, and the interband transitions at the K points become accessible for photon energies
of 1.5 eV, dominating the optical response. Through symmetry breaking owing to the substrate, K
and K′ points have a strong and opposite spin nature that results in a strong circular dichroism (CD)in PtSe2 i.e. interband transitions in the vicinity of the K(K′) points couple exclusively to LCP (RCP)
light. We show that this results in the generation of THz photocurrents and the resulting THz
emission with opposite signs when excited with LCP or RCP light [5]. To highlight the observed
trends for different PtSe2 thicknesses under circularly polarized light, the linear effects wereextracted as (E⟲ + E⟳)/2 and the circular effects as (E⟲ − E⟳)/2 from the experimental
measurements of the THz electric field (E) with LCP (⟲) and RCP (⟳) excitations. These are shownin Figure 2 for 3.85 nm and 38.5 nm PtSe2 for TE polarizations. As expected a circular effect is only
observed for the thickest films. This further highlights the strong effect of the PtSe2 bandstructure on
the THz properties.

Fig 2: Contributions of circular (E⟲ −E⟳)/2 and linear (E⟲ +E⟳)/2 effects to PtSe2 THz emission.
Circular and linear effects for THz TE emission for a) 3.85 nm PtSe2 and b) 38.5 nm PtSe2 shown by blue andred lines, respectively, are obtained from measurements in THz-TDS reflection geometry with LCP and RCP
excitations.

This work has investigated the first observation of layer-dependent, polarization and valley-selective
excitation of ultrafast photocurrents in ML to multilayer PtSe2. We show the amplitude and phaseresolved THz emission from this 2D material under optical femtosecond excitation, permitting
access to the processes of ultrafast photocurrent generation. We demonstrate that this is a result of asecond-order nonlinear response with the THz pulse generated from the ultrafast photocurrents
induced by the nonlinear conductivity of PtSe2. In particular, we show the critical role of theenvironment, with the substrate inducing a structural asymmetry in the inherent centrosymmetry of
PtSe2. This is strongly illustrated by THz photocurrents that show an opposite phase change with left
and right circular polarization (LCP and RCP) optical excitation for semi-metal multi-layer PtSe2 but
remain the same for the few layers semiconducting cases. These results are corroborated theoreticallythrough extensive DFT simulations of the layer-dependent bandstructure showing important circulardichroism (CD) for semimetal PtSe2 owing to the excitation of opposite valleys and the strong
interaction with the substrate. This work shows that CD is not limited to the ML limit as in other
TMDs with natural non-centrosymmetry, and can be finely controlled in the novel layer-dependent
bandstructure of PtSe2 [5].
References[1] S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V. Yazyev, A. Kis, Nature Reviews Materials 2, 17033(2017).
[2] Y. Wang, L. Li, W. Yao, S. Song, J. T. Sun, J. Pan, X. Ren, C. Li, E. Okunishi, Y. Wang, E. Wang, Y. Shao, Y. Y. Zhang, H. Yang, E. F. Schwier
H. Iwasawa, K. Shimada, M. Taniguchi, Z. Cheng, S. Zhou, S. Du, S. Pennycook, S. Pantelides, H. Gao, Nano Letters 15, 6 4013(2015).
[3] L nsari
onaghancEvoy C Coilea in C P Cullen Lin R iris
timpel-Lindner, K. F. Burke, G. Mirabelli, R. Duffy
E. Caruso, R. E. Nagle, G. S. Duesberg, P. K. Hurley, F. Gity, npj 2D Materials and Applications 3, 1 33(2019).
[4] L hang D hang
u
u
hao
un
u
Lu
ang
hao Advanced Optical Materials 10, 2201881(2022).
[5] M. Hemmat,(submitted).
* Acknowledgement: he authors acknowledge funding from European Union’s orizon 2020 research and innovation program under grant
agreement No 964735 (FET-OPEN EXTREME-IR).

Few-cycle pulses for optical frequency comb
Spectroscopy from the ultraviolet to mid-infrared
S. A. Diddams
University of Colorado, Boulder, CO 80309 USA

Optical frequency combs are a unique spectroscopic tool, providing an unparalleled combination of
frequency accuracy, high-resolution and broad spectral coverage. The field of frequency comb
spectroscopy has grown rapidly to encompass multiple spectroscopic scenarios, ranging from trace
gas detection for atmospheric sensing to fundamental spectroscopy in chemistry, physics andbiology. In this talk, I will highlight our recent development of frequency comb spectroscopy tools
and techniques that span from the ultraviolet (~200 nm) to the infrared (~25 microns).
Our typical frequency comb spectroscopic system start with a straightforward approach togenerate few-cycle optical pulses with robust 1550 nm Er:fiber laser frequency comb
technology [1,2]. With modest average power, we generate >0.5 MW pulses at 100 MHz rate. The
high peak power allows us to exploit the second-order nonlinearities in infrared-transparent,
nonlinear crystals (LiNbO3, GaP, GaSe, and CSP) to provide a robust source of phase-stable infrared
ultrashort pulses with simultaneous spectral brightness exceeding that of an infrared synchrotron.
Additionalcascaded second-order nonlinearities in LiNbO3 lead to comb generation with foura.
octaves of simultaneous coverage (0.350 to 5.6 μm). With a free-running comb-tooth linewidth of 10
kHz at 193 THz, we realize a notable spectral resolving power exceeding 10 10 across 0.86 PHz of
bandwidth. Recently, we have also been exploring non-perturbative harmonic generation in ZnO to
produce light at even shorted wavelengths—at and below 200 nm [3]. This is achieved with very
tight focusing of the few-cycle pulse in the ZnO to peak intensities exceeding 1 TW/cm2.
Spectroscopic scenarios we have been exploring with this unique frequency comb source include
trace gas and atmospheric sensing, spectroscopy of biologically relevant proteins and hyperspectral
microscopy [1,4,5]. Readout of spectral information imprinted on the infrared frequency comb isenabled by either dual-comb spectroscopy or dual-comb electro-optic sampling [4]. In this later case,a second comb at a wavelength of 1550 nm, and a slightly different repetition rate, stroboscopicallysamples and upconverts the mid-infrared comb to the near-infrared. This approach has the benefit ofshot-noise limited detection with robust 1550 nm detectors that have significantly greater efficiency,
and spectral and electrical bandwidth when compared to mid-infrared detectors.

Fig. 1: (Left) An erbium fiber mode-locked laser comb at 1550 nm seeds the generation of a few-cycle pulse
to drive frequency conversion in second-order χ(2) nonlinear crystals. The resulting composite spectrum,
generated in three nonlinear crystals, spans six octaves [2]. (Right) Example of using the mid-IR portion ofthis comb for high-resolution gas phase spectroscopy of NH3 with dual-comb electro-optic sampling [4].

Finally, a new direction involves the direct heterodyne of incoherent thermal light with the frequency
comb itself, thereby bringing the power of telecommunications photonics and the precision of
frequency comb metrology to laser heterodyne radiometry, with implications for solar andastronomical spectroscopy, remote sensing, and precise Doppler velocimetry [6].
References[1] H. Timmers, A. Kowligy, A. Lind, F. C. Cruz, N. Nader, M. Silfies, G. Ycas, T. K. Allison, P. G. Schunemann, S. B. Papp
S. A. Diddams, Optica 5, 727 (2018).
[2] D. M. B. Lesko, H. Timmers, S. Xing, A. Kowligy, A. J. Lind, S. A. Diddams, Nature Photonics 15, 281 (2021).
[3] D. M. B. Lesko, K. F. Chang, S. A. Diddams, Optica 9, 1156 (2022).
[4] A. S. Kowligy, H. Timmers, A. J. Lind, U. Elu, F. C. Cruz, P. G. Schunemann, J. Biegert, S. A. Diddams, Sci Adv 5, eaaw8794 (2019).
[5] H. Timmers, A. Kowligy, A. J. Lind, N. Nader, J. Shaw, D. Zalvidea, J. Biegert, S. A. Diddams, OSA Technical Digest , SF1E.4(2019) .
[6] E. Tsao, A. Lind, P. Chang, C. Fredrick, N. Hoghooghi, F. Quinlan, S. Diddams, Optica Publishing Group, JW3A.17(2022).

Ultrafast manipulation of the magnetic order inCorrelated insulators via sub-gap optical excitation
H.A. Dürr
Uppsala University, 751 20 Uppsala, Sweden.

Strongly correlated magnetic insulators provide a unique opportunity to investigate the interaction
between different degrees of freedom in a system possessing several exciting interaction
mechanisms, including spin-orbit coupling and coulomb correlations. This talk will provide an
overview how time-resolved optical and x-ray spectroscopy methods can contribute to ourunderstanding of several such materials systems.For wide-band-gap insulators such as NiO
theoretical predictions promise the coherent manipulation of electronic correlations with strong
optical fields [1]. Contrary to metals where rapid dephasing of optical exci- tations via electronic
processes occurs, the sub-gap excitation in charge-transfer insulators has been shown to couple tolow-energy bosonic excitations such as phonons and magnons [2]. We use the prototypical chargetransfer insulator NiO to demonstrate that sub-gap excitation does not lead to a renormalization ofelectronic correlations up to peak electrical field strengths of 0.3 V/Å (see Fig. 1). However, whenthe excitation becomes resonant with a phonon-assisted dd-excitation we find a renormalized NiO
band-gap in combination with a significant reduction of the antiferromagnetic order. We employ
element-specific x-ray absorption spectroscopy at the FLASH free electron laser [4] to demonstrate
the reduction of the upper band-edge at the O 1s-2p core-valence resonance whereas theantiferromagnetic order is probed via x-ray magnetic linear dichroism (XMLD) at the Ni 2p-3d
resonance. Comparing the transient XMLD spectral lineshape to ground-state measurements allows
us to extract a spin temperature rise of more than 60 K for time delays longer than 400 fs. At earlier
times a non-equilibrium state is formed characterized by O 2p mid-gap states. These results can beunderstood in terms of a transient Ni-O charge transfer during the optical driving field. CrI3 is a
prototype of a new class of 2D magnetic insulator materials displaying ferromagnetic order down to
the monolayer limit with a potential for device applications [5]. The optical and magneto-opticalproperties are proposed to be dominated by the presence of several excitonic dd-transitions
characterized by spin polarized electron and hole pairs [6] with evidence of strong coupling to
optical phonons [7].
Fig. 1: (a) Measured XAS spectra
for O 1s-2p transitions and (b) for
Ni
2p-3d
transitions.
[3]
Measurements were obtained at
the LCLS free electron laser using
2mm optical radiation as pump.
While the more itinerant O 2p
orbitals respond to laser excitation
as indicated by the change intransient absorption in the excitedstate in (a), Ni 3d levels remainunchanged indicating a negligible
change of electron correlations up
to the employed pump electricfield strengths of 0.3 V/Å (b).

We will show how time resolved spectroscopic techniques can be used to investigate the optical andmagneto-optical response of CrI3 to understand their origin. This could open up the exciting prospect
to extend sub-bandgap opto-magnetism down to the monolayer limit.
References[1] N. Tancogne-Dejean, M. A. Sentef, A. Rubio, Physical Review Letters 121, 097402 (2018).
[2] F. Novelli, G. De Filippis, V.Cataudella, M.Esposito, I.Vergar, F.Cilento, E. Sindici, A. Amaricci, C. Giannetti, D. Prabhakaran, SWall,
A. Perucchi S. Dal Conte, G.Cerullo, M.Capone, A. Mishchenko, M. Grüninger, N.Nagaosa, F. Parmigiani, D. Fausti, Nature Communications 5,5112 (2014).
[3] O. Grånäs, I. Vaskivskyi, X. Wang,, S. Ghimire, R. Knut,, L. Kjellsson, D. Turenne, R. Y. Engel, M. Beye, J. Lu
D. J. Higley, A. H. Reid, W. Schlotter, G. Coslovich, M. Hoffmann, G. Kolesov,-Langeheine, A. Styervoyedov, N. Tancogne-Dejean
M. A. Sentef, D. A.Reis, A. Rubio, S. S. P.Parkin, O Karis, J.-E.Rubensson, O.Eriksson, H. A.
, Physical Review Research 4, L032030 (2022).
[4] X. Wang, R. Y Engel, I. Vaskivskyi, D.Turenne, V.Shokeen, A.Yaroslavtsev, O. Grånäs, R.Knut, J. O. Schunck, S. Dziarzhytski, G. Brenner
R.-P. Wang, M.Kuhlmann, F.Kuschewski, W.Bronsch, C. Schüßler-Langeheine, A. Styervoyedov, S. SP Parkin, F. Parmigiani, O. Eriksson
M.Beye, H. A Dürr, Faraday Discussions 237, 300 (2022).
[5] B. Huang, G. Clark, E.Navarro-Moratalla, D.R. Klein, R. Cheng, K.L. Seyler, D. Zhong, E.Schmidgall, M. A. McGuire, D. H. Cobden, W.Yao
D. Xiao, P.Jarillo-Herrero, X. Xu, Nature 546, 270 (2017).
[6] M. Wu, Z. Li, T. Cao, S. G. Louie, Nature Communications 10, 1 (2019).
[7] P. Padmanabhan, F. L. Buessen, R. Tutchton, K. W. C. Kwock, S. Gilinsky, M. C. Lee, M. A. McGuire, S. R. Singamaneni, D. A. Yarotski
A. Paramekanti, J.-X. Zhu, R. . Prasankumar, Nature Communications 13, 4473 (2022).

Theoretical investigations on ultra-fast demagnetization using
Ab-initio
andsimulation
methods11
1
1
1
E. Hubert , J. Braun , S. Mankovsky, S. Polesya , H. Lange ,M. Weißenhofer2, U. Nowak2
1
Ludwig2 Maximilian University of Munich, 80539 Munich, Germany
University of Konstanz, 78464 Konstanz, Germany

The field of ultra-fast demagnetization started with the pioneering work of Beaurepaire et al. [1] who demonstrated the rapid decay of the magnetization of Ni after a short laser pule via the magneto-optical Kerr effect(MOKE). Similar experimental work was done later using for example the transverse MOKE [2], the X-ray
magnetic circular dichroism (XMCD) [3] or the spin and angle resolved photo emission spectroscopy (ARPES) [4]. Theoretical investigations on ultra-fast demagnetization were first performed on a model level followed by simplified quantum-mechanical studies. A great step forward could be made by using an implementation of a scheme based on time-dependent density functional theory (TD-DFT) that allowed to describe in an
ab-initio or parameter-free way the demagnetization after a strong laser pulse [5]. As electronic spectroscopies
sensitive to the magnetization play a crucial role to monitor ultra-fast demagnetization processes the scheme
has been extended in the meantime to calculate spectroscopic properties directly and to make this way directcontact with experiment. As an alternative to this all-in-one approach, we use the time and spin dependent
potential and occupation functions from TD-DFT calculations as an input to our own programs that work on a
fully relativistic level within the Green function formalism implemented by means of multiple scattering theory. This approach allows us to calculate spectroscopic and many other magnetic properties in a time-resolved
but quasi-static way. Concerning spectroscopy first applications have been made for time-resolved XMCD
calculations. As will be shown, the resulting sequence of spectra reflect in a one-to-one manner the decay ofthe magnetization with the connection between spectra and magnetization as suggested by the so-called
XMCD sum rules. Another application is the calculation of spin resolved ARPES spectra on the basis of the
one-step model of photo emission. Fig. 1 shows a corresponding sequence for ferromagnetic Fe that reflect inparticular the diminishing of the exchange splitting that was observed in experiment.

Fig. 1. Top: Magnetization m(t) versus time with five time markers. Bottom: photo emission spectra from fer
romagnetic Fe for normal emission along the path G-X calculated via the one-step model using the TD-DFT
based potential for the marked times. The dashed lines mark the exchange splitting at the G-point at t=0.

As will be shown, using the two-time Keldysh non-equilibrium Green function formalism [6] allows
to go beyond the quasi-static approach concerning a description of the XMCD but also ARPES. First
results of corresponding applications of this very powerful and flexible scheme will be presented for
two-photon photo emission experiments involving surface states. Because of the large computationalcosts and other restrictions of TD-DFT calculations they allow only to cover the time window of thedemagnetization and the first stage of the subsequent relaxation of the system. For that reason, simulations that are based on simplified parametrized models are and will always be indispensable tools
for studies that account at the same time for the electronic, magnetic and lattice degrees of freedom.
So far such studies used either empirical parameters or parameters derived from the electronic
ground state. Using TD-DFT based potentials and occupation functions, on the other hand, should
lead to more realistic parameters. Fig. 2 shows as an example the exchange coupling parameters Jij(middle row) of ferromagnetic Co as a function of the distance Rij calculated for a sequence of times
when a laser pulse is applied (top row together with the time dependent magnetic moment). These
parameters have been used for subsequent Monte Carlo simulations to get the effective time dependent critical temperature Tc that is shown in the bottom row of Fig. 2

Fig. 2. Exchange coupling parameters Jij of ferromagnetic Co as a function of the distance Rij of 2 moments onsite i and j (middle) and resulting effective critical temperature Tc (bottom) for a sequence of times.

The data show obviously a very pronounced diminishing of Tc due to the laser pulse as it is exploited in heat-assisted magnetic writing. Similar results have been obtained and will be shown for themagneto-crystalline energy.
An important aspect in this context and for the magnetization dynamics in general is the coupling of
the electronic system carrying the magnetization and the lattice. As will be shown, the corresponding
spin-lattice coupling parameters Jijk as well as higher order terms can be obtained in a similar manneras the conventional exchange parameter Jij by a generalization of the so-called Lichtenstein formula.
Using TD-DFT based potentials and occupation functions, again the time-dependence of the parameters Jijk as well as of any other parameters entering the model Hamiltonians or equations of motion
used within magnetization dynamics simulations can be accounted for. An interesting aspect of corresponding combined spin lattice dynamics simulations based on the parameters Jijk that will be addressed is the angular momentum transfer between the spin and lattice systems.
References[1] E. Beaurepaire, J.-C. Merle, A. Daunois, J.-Y. Bigot, Physical Review Letters 76 4250 (1996)[2] S. Mathias, C. La-O-Vorakiat, P. Grychtol, P. Granitzka, E. Turgut, J. M. Shaw, R. Adam, H. T. Nembach, M. E. Siemens, S. EichC. M.Schneider, T. J.Silva, M.Aeschlimann, M. M.Murnane, H. C.Kapteyn, Proceedings of National Academy of Sciences-PNAS109, 4792 (2012).
[3] I. Radu, C. Stamm, A. Eschenlohr, F. Radu, R. Abrudan, K. Vahaplar, T. Kachel, N. Pontius, R. Mitzner, K. Holldack, A. Föhlisch, T. A. Ostler
J.H. Mentink, R. F. L. Evans, R. W. Chantrell, A. Tsukamoto, A. Itoh, A. Kirilyuk, A. V. Kimel, Th. Rasing, SPIN 5, 1550004 (2015).
[4] S. Eich, M. Plötzing, M. Rollinger, S. Emmerich, R. Adam, C. Chen, H. C. Kapteyn, M. M. Murnane, L. Plucinski, B. Stadtmüller, M. Cinchetti
M. Aeschlimann, C. M. Schneider, S. Mathias, Science Advances 3, e1602094 (2017).
[5] K. Krieger, J. K. Dewhurst, P. Elliott, S. Sharma, E. K. U. Gross, Journal of Chemical Theory & Computations 11, 4870 (2015).
[6] J. Braun, H. Ebert, Physical Review B 98, 245142 (2018).
[7] S. Mankovsky, S. Polesya, H. Lange, M. Weißenhofer, U. Nowak, H. Ebert, Physical Review Letters 129, 067202 (2022).

Ultrafast optical parametric oscillators spanning the
Ultraviolet to mid-infrared
M. Ebrahim-Zadeh
ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels, Spain

The latest developments in ultrafast wavelength conversion sources based on optical parametricoscillators covering spectral regions from the ultraviolet to deep-infrared, and temporal domains
from the continuous-wave to few-cycle pulses are described. The advent of novel nonlinear opticalmaterials over the past decade combined with progress in solid-state and fiber laser technology has
had a tremendous impact on frequency conversion sources, enabling their advancement to new
spectral and temporal domains. The developments have led to the realization of a new generation ofadvanced solid-state sources of tunable coherent radiation providing unprecedented capabilities withregard to wavelength coverage, output power and efficiency, spatial, spectral and temporal quality,
and operating in all time scales from the continuous-wave (cw) to the ultrafast few-cycle pulses. In
particular, optical parametric oscillators (OPOs) are now firmly established as truly viable andpractical sources of broadly tunable coherent radiation capable of accessing extended spectral
regions from the ultraviolet (UV) to the deep-infrared (deep-IR), not available to conventional solidstate lasers. Harnessing the superior linear and nonlinear optical properties of new birefringent andquasi-phase-matched (QPM) crystals and advanced laser pump sources, together with the applicationof innovative design concepts, OPOs have transformed solid-state laser technology in new directions
and towards new frontiers. The exploitation of birefringent crystals such as BiB3O6 (BIBO) and
CdSiP2 (CSP), and QPM materials such as MgO-doped periodically-poled LiNbO3 (MgO:PPLN),
stoichiometric LiTaO3 (MgO:sPPLT), KTiOPO4 (PPKTP), orientation-patterned GaAs (OP-GaAs)
and GaP (OP-GaP), combined with ultrafast solid-state and fiber pump lasers, have enabled tunableultrashort pulse generation across vast spectral regions from ~250 nm in the UV up to ~12 µm in the
deep-IR, with pulses down to a few-cycle duration [1-6]. In the femtosecond time-scale, operation ofsynchronously-pumped OPOs into deep-IR at wavelengths as long as ~8 µm has been realized by
exploiting internal cascaded parametric generation based on MgO:PPLN and CSP using the KLM
Ti:sapphire laser as pump source [4] or by external cascaded pumping [5] using a commercial nearIR femtosecond OPOs as the intermediate stage, as shown in Fig. 1. At the same time, by employing
ultrashort pump pulses of ~20 fs duration, with careful control of dispersion, operation of
femtosecond OPOs has been extended to new temporal limits, providing broadband mid-IR pulses
with
as few as 3.7 optical cycles and bandwidths spaning over ~500 nm [6].

Fig. 1. (Left): Schematic of the cascaded CSP femtosecond OPO tunable in the deep-IR. (Middle):
Corresponding (a) signal, and (b) idler spectra across the tuning range. (Right): Representative idler spectrum
centered at 6440 nm [5].

Successful operation of femtosecond OPOs has been further extended to wavelengths beyond 8 µm
in the deep-IR by direct pumping with the KLM Ti:sapphire laser, and without the need forintermediate cascaded step [7,8]. The configuration of such an OPO is shown in Fig. 2. By exploiting
type-I (e→oo) critical phase-matching in combination with angle-tuning in CSP, the high-repetitionrate femtosecond OPO can provide continuous wavelength coverage across 7306–8329 nm (1201–
1369 cm-1) in the deep-IR. The oscillator delivers up to 18 mW of idler average power at 7306 nm
and >7 mW beyond 8 µm at 80.5 MHz repetition rate, with spectra exhibiting bandwidths of >150
nm across the tuning range. With an equivalent spectral brightness of ~5.6×1020 photons s-1 mm-2 sr-1
0.1% BW-1, this OPO represents a viable alternative to synchrotron and supercontinuum sources fordeep-infrared applications in spectroscopy, metrology and medical diagnostics. In anotherdevelopment, the advent of the new QPM nonlinear material, OP-GaP, has enabled femtosecond
pulse generation across 3.6–8 μm in the deep-IR from a single OPO system [9], as shown in Fig. 3.
Average output powers of 54 mW at ~3.8 μm in the mid-IR signal, with >10 mW across the entire
deep-IR idler tuning range, was obtained. The measured quantum conversion efficiency of 28.9%
from pump to idler at 3.8 μm is the highest recorded from any OPO based on OP-GaP or CSP.

Fig. 2. (Left): Schematic of the experimental setup for the Ti:sapphire-pumped CSP femtosecond OPO.
(Middle): Idler and signal tuning as a function of the internal phase-matching angle in the CSP crystal, for apump wavelength of 993 nm, superimposed on the parametric gain map. (Right): Deep-IR idler spectrum
centered on 7387 nm with a FWHM bandwidth of 152 nm. Insets: Corresponding signal spectrum centered at ~1150 nm with FWHM bandwidth of 13 nm, TEM00 beam profile at 7314 nm [7, 8].

In another development, the advent of the new QPM nonlinear material, OP-GaP, has enabled
femtosecond pulse generation across 3.6–8 μm in the deep-IR from a single OPO system [9], asshown in Fig. 3. Average output powers of 54 mW at ~3.8 μm in the mid-IR signal, with >10 mW
across the entire deep-IR idler tuning range, was obtained. The measured quantum conversion
efficiency of 28.9% from pump to idler at 3.8 μm is the highest recorded from any OPO based on
OP-GaP or CSP.

Fig. 3. (Left): Schematic of femtosecond OPO based on OP-GaP. (Middle): Signal and idler average power
and optical spectra across the tuning range. (Right): Signal spectra at a central wavelength of 1281.7 nm
measured with (filled area) and without (dashed line) prisms in the cavity. (b) Corresponding intensity
autocorrelations with (filled area) and without (red line) prisms [9].

In the picosecond time scale, has also become possible into the deep-IR spectral range by direct
pumping using Yb-fiber lasers at 1.064 µm. The major improvements in the optical quality of the
CSP crystal with low transmission loss over long interaction lengths have made it possible toovercome the low nonlinear gains under low pumping intesities, thus enabling the realization ofhigh-repetition-rate picosecond OPOs at wavelegnths out to 6.7 µm [10]. The configuration of suchan OPO is shown in Fig. 4.

Fig. 4. (Left): Schematic of the experimental setup for the Yb-fiber laser pumped high-average-power deep-IR
picosecond OPO. (Middle): Power across the idler tuning range of the picosecond CSP OPO. Inset: Signal spectra acrossthe temperature tuning range. (Right): Long-term power stability of the deep-IR idler from the picosecond CSP
OPO.
Inset: deep-IR idler beam profile at an operating wavelength of 6205 nm [10].

The OPO is based on a 16.3-mm-long CSP crystal cut at θ=90º (φ=45º) for type-I (e→oo) noncriticalphase-matching and synchronously pumped by ~20 ps pulses from a mode-locked Yb-fiber laser
79.5 MHz repetition rate. The OPO is tunable across 6205-6710 nm in the idler, providing as muchas 105 mW of average power at 6205 nm, with >55 mW over nearly the entire tuning range. The
deep-IR idler output exhibits a passive power stability better than 2.3% rms over 12 hours in highbeam quality. With the extended wavelength coverage, practical average powers, high beam quality,and good passive stability, this OPO represents an attractive source of high-repetition-ratepicosecond pulses in the deep-IR for many applications including spectroscopy and imaging. The
latest progress in picosecond OPOs include broadband phase-locked pulse generation[11], see Fig. 5.

Fig.5. (Left): Yb-fiber-pumped phase-locked degenerate picosecond OPO based on MgO:PPLN for broadband
generation in the near-infrared. (Middle): (a) Output power and stability in transition from unlocked to phaselocked regime; (b) spectral stability of ps-OPO output in transition from unlocked to phase-locked regime.
(Right): OPO output spectrum in the absence of phase-locking and in degenerate phase-locked state [11] .

This talk will provide an overview of the latest progress at the forefront of OPO technology, enabled
by new nonlinear materials, advanced solid-state and fiber laser pump sources, and novel desing
concepts. Some emerging applications of OPO sources in optical microscopy, imaging andspectroscopy will also be highlighted.
References[1] M. Ebrahim-Zadeh, S. C. Kumar, IEEE Journal of Selected Topics in. Quantum Electronics 20, 7600519 (2014).
[2] M. Ebrahim-Zadeh, S. C. Kumar, K. Devi, IEEE Journal of Selected Topics in Quantum Electronics 20, 0902823 (2014).
[3] S. C. Kumar, P. G. Schunemann, K. T. Zawilski,M. Ebrahim-Zadeh, Journal of Optical Society of America-JOSA B 33, D44 (2016).
[4] V. R. Badarla, S. C.Kumar, A. Esteban-Martin, K. Devi, K. T. Zawilski, P. G. Schunemann, M. Ebrahim-Zadeh, Optics Letters 41, 1708 (2016).
[5] S. C. Kumar, A. Esteban-Martin, A. Santana, K. T. Zawilski, P. G. Schunemann, M. Ebrahim-Zadeh, Optics Letters 41, 3355 (2016).
[6] S. C. Kumar, A. Esteban-Martin, T. Ideguchi, M. Yan, S. Holzner, T. W. Hänsch, N. Picqué, M. Ebrahim-Zadeh, Laser & Photonics Review 8
86 (2014).
[7] C. F. O’Donnell, S. C. Kumar, K. T. Zawilski, P. G. Schunemann, M. Ebrahim-Zadeh, Optics Letters 43, 1507 (2018).
[8] C. F. O'Donnell, S. C. Kumar, K. T. Zawilski, P. G. Schunemann, M. Ebrahim-Zadeh, IEEE Journal of Selected Topics in. Quantum
Electronics 24, 1 (2018).
[9] C. F. O'Donnell, S. C. Kumar, P. G. Schunemann, M. Ebrahim-Zadeh, Optics Letters 44, 4570 (2019).
[10] S. C. Kumar, K. T. Zawilski, P. G. Schunemann, M. Ebrahim-Zadeh, Optics Letters 42, 3606 (2017).
[11] B. Nandy, S. Chaitanya Kumar, M. Ebrahim-Zadeh, Optics Letters 45, 3981 (2020).
* Acknowledgments: Spanish Government through Project Nutech PID2020-112700RB-I00 (MCIN/AEI/10.13039/501100011033), and “Severo
Ochoa” Center of Excellence, CEX2019-000910-S) – Funded by the Spanish State Research Agency; Generalitat de Catalunya (CERCA);
Fundación Cellex; Fundació Mir-Puig.

Theory of ultrafast disordering in photo-induced phase transitions
M. Eckstein
University of Hamburg, 22607 Hamburg, Germany

Using ultrashort laser pulses, it has become possible to probe the dynamics of long-range order insolids on microscopic timescales. In the conventional description of symmetry-broken phases within
time-dependent Ginzburg-Landau theory, the order parameter evolves coherently along an average
trajectory. Recent experiments, however, indicate the profound effect of order parameter fluctuations
on the dynamics. An extreme scenario is ultrafast inhomogeneous disordering [1], where the averageorder parameter is no longer representative of the state on the atomic scale. While this has a profoundeffect on the dynamics, a theoretical approach which takes into account atomic scale
inhomogeneities of both the electronic structure and the order parameter is challenging. In my talk, I
will report on results for the Holstein model, which are based on a nonequilibrium generalization of
statistical dynamical mean-field theory, coupled to stochastic differential equations for the orderparameter [3]. The results show that ultrafast disordering can occur already in this minimal model for
the Peierls charge-density wave transition (Fig. 1) [2].

Fig. 1 (a): Dynamics of the order parameter XA-XB for the charge density wave transition in the Holstein model
after a sudden excitation of the electrons. The order parameter is the difference of an atomic displacement
between two sublattices A and B of a bipartite lattice (see inset). After the excitation, the CDW order isquenched, and recovers on a timescale of 100ps. The figure shows a strong excitation, in which the sign of theorder can be reversed. (b) to (e) Probability distribution of the local lattice displacement on the A sublattice atfour representative times (see vertical lines in (a): Already at an early time when the average order parameteris still close to zero, one finds a bimodal distribution (c): The order parameter is locally restored, while thesystem is still in a disordered state, which is the hallmark of ultrafast disordering.

Similar techniques may help in future to solve the coupled electron lattice dynamics for stronglyinteracting electrons.
References[1] S. Wall, S. Yang, L. Vidas, M. Chollet, J. Glownia, M. Kozina, T. Katayama, T. Henighan, M. Jiang, T. Miller, D. Reis, L. Boatner
O. Delaire, M. Trigo, Science 362, 572 (2018).
[2] A. Picano, F. Grandi, M. Eckstein, arXiv:2112.15323.
[3] A. Picano, F. Grandi, Ph. Werner, M. Eckstein, arXiv:2209.00428.
* Acknowledgement(s): We acknowledge financial support from the ERC starting grant No. 716648.

State of the art in soft x-ray lab sources:
Probing magnetization dynamics with atomic selectivity
in the femto- to picosecond range
S. Eisebitt
Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, 12489 Berlin, Germany

Soft x-rays are a unique tool to study magnetism due to presence of circular and linear x-ray
dichroism, allowing to quantitatively access the spin (S) and orbital (L) magnetic moments in anelement specific fashion via the exploitation of suitable resonant and polarization dependent
electronic transitions. Furthermore, the short wavelength as compared to visible radiation allows toaccess magnetic ordering on the nanometer scale spatial scale. Scattering and imaging approaches
are exploited to access order from the unit cell range to the mesoscale. With magnetization dynamics
on these spatial scales typically covering the few femtosecond to picosecond temporal scale, pulsed
sources in the soft x-ray spectral range are key to interrogate dynamical processes in pump-probe
experiments. So far, experiments of this kind have almost exclusively been performed at large scale,accelerator-based facilities. However, with many synchrotron radiation sources upgrading to higher
brightness with associated electron bunch length exceeding 100 ps, shorter pulses of soft x-rays areavailable only at free electron x-ray lasers (FELs) and a few synchrotron beamlines.
Laser-based secondary sources of soft x-rays have started to fill this gap. Sources based on highharmonic generation (HHG) provide < 50 fs pulses, allowing to access ultrafast magnetization
phenomena. Their dominant use in magnetism research has been in the photon energy range up to
about 100 eV (λ=12.4 nm), predominantly in spectroscopy (see e.g. [1, 2]) but also with a few proofof-principle scattering and diffractive imaging experiments reported. [3,4] While HHG to photon
energies in the water window and even above have been reported, applications in magnetism
research have been very sparse so far, due to the very strong decrease in photon flux for increasing
photon energies. In particular, the “workhorse photon energies” allowing to exploit the transitions
form the 2p3/2,1/2 levels of the 3d transition metals could not be exploited for the study of ultrafast
magnetization dynamics at laser based soft x-ray sources yet. With the photon flux demands
typically increasing from spectroscopy via resonant scattering to resonant imaging, the more photon
hungry methods are so far practically limited to photon energies below 100 eV, with the notable
exception of two recent resonant magnetic scattering experiments with HHG sources at around 150
eV photon energy, i.e. accessing 4d to 5f transitions in lanthanides. [5, 6]
We report on a development of a high-flux HHG source based on an OPCPA system providing a
2 µm HHG driver at 10 kHz repetition rate, extending up to 600 eV photon energy with sub-25 fs
pulses as shown in Fig. 1 [7]. First experiments on magnetization dynamics on this system are
discussed. Exploiting the magneto-optical Kerr effect with soft x-rays in angle resolved
measurements, we monitor (transient) spectral changes in the entire broad spectrum at an absorption
resonance. These changes encode depth information on the (transient) magnetization profile which
can be extracted in conjunction with scattering simulations [8]. In this fashion we are able to trace
the transient magnetization depth profile during ultrafast demagnetization after laser excitation,
demonstrated in a prototypical FeGd thin film system with seed and cap layers [6]. Developments toextend these capabilities to the photon energy range up to 800 eV using a 3 µm-driver system withsimilar architecture of the laser system are under way.
To reach the photon energies up to 900 eV with suitable photon flux in order access 3d elements attheir L-edges or even up to 1500 eV to reach the M-edges of lanthanides is of large importance in the
study of magnetic materials via x-ray dichroism in spectroscopy and scattering - but also a
formidable challenge, in particular via HHG. Such work has so far been the exclusive domain oflarge scale facilities. For studies where fs temporal resolution is not required, laser-driven plasma xray sources (PXS) provide an alternative. We report on the development of such a soft x-ray source
specifically for studies in magnetism with a pulse duration of slightly below 10 ps, together with soft
x-ray optics and endstations to measure x-ray absorption and resonant magnetic scattering [9]. The
driver for the PXS is an in-house-developed double-stage Yb:YAG-based thin-disk amplified laser,
producing pulses with a duration of 2 ps full width at half-maximum at a wavelength of 1030 nmwith a pulse energy of 150 mJ at 100 Hz repetition rate [10]. Soft x-rays are produced in the plasma
generated when the pulses hit a tungsten target. A flux curve of the source is included in Fig. 1. Notethat such a source is intrinsically unpolarized.

Fig. 1 Photon flux curves of the 2µm-driven OPCPA-based HHG source (yellow, red and green lines for the useof different gases in the HHG process) [7]. Photon flux curve of the laser-driven plasma x-ray source, operated
with a tungsten target [12]. In both cases, only the photon flux corresponding to the solid angle actuallyaccepted by the subsequent optical elements in the beamline is shown. All photon flux measurements werecarried out with a detector calibrated by the German national metrology institute Physikalische Technische
Bundesanstalt (PTB).

Nevertheless, we demonstrate for the first time the ability to measure (i) x-ray magnetic circulardichroism absorption spectra (Fe L3, 707 eV) [11], (ii) resonant magnetic scattering from
antiferromagnetic order in Fe/Cr superlattices (Fe L3, 707 eV) [12], (iii) small angle scattering fromferromagnetic domains in Fe/Gd multilayers (Fe L3, 707 eV and Gd M5, 1189 eV) [13] at such
elevated photon energies with a laboratory source.
Not only does such a source allow for the static characterization of magnetic samples during
competitive data acquisition times, but in all three modes we are able to acquire transients ofmagnetization dynamics after laser excitation in pump-probe-experiments with a temporal resolutionof slightly below 10 ps.
The source and instrument developments will be discussed together with the new capabilities toprobe magnetization dynamics with atomic selectivity and reciprocal space information in alaboratory setup, detailing and extrapolating from the first demonstration experiments.
References[1] C. La-O-Vorakiat, M. Siemens, M. M. Murnane, H. C. Kapteyn, S. Mathias, M. Aeschlimann, P. Grychtol, R. Adam, C. M. Schneider, J. M. Shaw
H. Nembach, T. J. Silva, Physical Review Letters 103, 257402 (2009).
[2] F. Willems, C. V. Schmising, C. Struber, D. Schick, D. W. Engel, J. K. Dewhurst, P. Elliott, S. Sharma, S. Eisebitt, Nature Communications 11
871 (2020).
[3] B. Vodungbo, J. Gautier, G. Lambert, A. B. Sardinha, M. Lozano, S. Sebban, M. Ducousso, W. Boutu, K. Li, B. Tudu, M. Tortarolo, R. Hawaldar
R. Delaunay, V. Lopez-Flores, J. Arabski, C. Boeglin, H. Merdji, P. Zeitoun, J. Luning, Nature Communications 3, 999 (2012).
[4] O. Kfir, S. Zayko, C. Nolte, M. Sivis, M. Moller, B. Hebler, S. S. P. K. Arekapudi, D. Steil, S. Schafer, M. Albrecht, O. Cohen, S. Mathias
C. Ropers, Science Advances 3, (2017)[5] G. Fan, K. Legare, V. Cardin, X. Xie, R. Safaei, E. Kaksis, G. Andriukaitis, A. Pugzlys, B. E. Schmidt, J. P. Wolf, M. Hehn, G. Malinowski,
B. Vodungbo, E. Jal, J. Luning, N. Jaouen, G. Giovannetti, F. Calegari, Z. Tao, A. Baltuska, F. Legare, T. Balciunas, Optica 9, 399 (2022).
[6] M. Hennecke, D. Schick, T. Sidiropoulos, F. Willems, A. Heilmann, M. van Moerbeck-Bock, L. Ehrentraut, D. Engel, P. Hessing, B. Pfau
M. Schmidbauer, A. Furchner, M. Schnuerer, C. von Korff Schmising, S. Eisebitt, Physical Review Research 4, L022062 (2022).
[7] M. van Mörbeck-Bock, T. Feng, A. Heilmann, L. Ehrentraut, H. Stiel, M. Hennecke, T. Sidiropoulos, C. von Korff Schmising, S. Eisebitt,
M. Schnürer, Proceedings SPIE 11777, 117770C (2021); doi: 10.1117/12.2591309.
[8] D. Schick, Computer Physics Communications 266, (2021).
[9] M. Borchert, J. Braenzel,1 R. Gnewkow, L. Lunin,1 T. Sidiropoulos, J.Tümmler, I. Will, T. Noll, O. Reichel, D. Rohloff, A. Erko, T. Krist
C. von Korff Schmising, B. Pfau, S. Eisebitt, H. Stiel, D. Schick, (submitted), (2023).
[10] R. Jung, J. Tümmler, I. Will, Optics Express 24, 883(2016).
[11] M. Borchert, D. Engel, C. von Korff Schmising, B. Pfau, S. Eisebitt, D. Schick, Optica (accepted), (2023). DOI: 10.1364/OPTICA.480221[12] D. Schick, M. Borchert, J. Braenzel, H. Stiel, J. Tummler, D. E. Burgler, A. Firsov, C. V. Schmising, B. Pfau, S. Eisebitt, Optica 8, 1237 (2021).
[13] L. Lunin, M. Borchert, N. Schneider, K. Korell, D. Sommer, S. Eisebitt, B. Pfau, D. Schick, (to be submitted), (2023)* Acknowledgement: I am indebted to all authors of Refs. 6,7,9,11,12,13 who have significantly contributed to the results discussed in thispresentation.

Search1for the1 ultrafast
Meissnereffect1 in driven
YBa
2Cu3O26.48
1
1
2
2
S. Fava , G. Jotzu , M. Buzzi , G. De Vecchi , T. Gebert , Y. Liu , S. Nakata , B. Keimer ,A. Cavalleri 3
1
Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany
2
Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany
3
University of Oxford, Oxford OX1 3PU, UK

Resonant excitation of certain phonon modes in high-Tc cuprates has been shown to induce transient
properties reminiscent of superconductivity at temperatures far above equilibrium Tc [1-3]. These
out-of-equilibrium superconducting-like states have been investigated in time-domain terahertz spectroscopy experiments, that revealed the dissipation-less nature of the electronic transport. A fundamental question remains open: do these non-equilibrium superconducting-like states also feature an
ultrafast Meissner effect and expel a statically applied magnetic field? To address this question, wedeveloped an experimental technique based on the Faraday effect in a magneto-optic crystal adjacentto the sample. Tracking the polarisation changes in an ultrashort near-infrared pulse reflected off thiscrystal, allows us to reconstruct the strength of the magnetic field surrounding the photo-excited region (Fig. 1). We make use of this technique to investigate the possible expulsion of a statically applied magnetic field in the photo-induced superconducting-like state of YBa2Cu3O6.48, a responsethat would amount to an ultrafast Meissner effect.

Fig. 1: Schematic representation of the experimental setup. The YBa2Cu3O6.48 sample (green puck at the center)is embedded in an external magnetic field (not shown) and the Cu-O apical phonon mode is driven resonantly
by a mid-infrared laser pulse (orange). The local magnetic field changes due to the superconducting
diamagnetic response are encoded in a red or blue shading for positive and negative values, respectively. Thesechanges are measured in close proximity of an edge of the sample in a GaP crystal (orange slab) by virtue ofthe Faraday effect using a near-infrared probe beam (blue pulse).

We discuss how the measured field expulsion depends on temperature and external magnetic field,
leading to a more complete understanding of this exotic state of matter.
References[1] W. Hu, S. Kaiser, D. Nicoletti, C.R.Hunt, I.Gierz, M.C.Hoffmann, M.Le Tacon, T. Loew, B. Keimer, A.Cavalleri, Nature Materials 13, 705(2014).
[2] B. Liu, M. Först, M. Fechner, D. Nicoletti, J. Porras, T. Loew, B. Keimer, A. Cavalleri, Physical Review X 10, 011053 (2020).
[3] A. von Hoegen, M. Fechner, M. Först, N. Taherian, E. Rowe, A. Ribak, J. Porras, B. Keimer, M. Michael, E. Demler, A. Cavalleri, PhysicalReview X 12, 031008 (2022).

Ultrafast electrical switching and charge state control of silicon
Vacancy centers in diamond
M. Rieger, V. Villafañe, L. Todenhagen, S. Appel, M. Brandt, K. Mueller
J. J. Finley
Technical University of Munich, 85748 Garching, Germany
Optically-active point defects in diamond feature atomic-sized two-level systems, constituting a promisingsolid-state platform for the development of novel quantum technologies. Example applications are thegeneration of non-classical light, such as single photons and photonic cluster states, and nodes in quantum
networks [1, 2]. One of the most intensively studied color centers in diamond is the nitrogen vacancy complex(NV), which exhibits a remarkable coherence time and a spin-selective dark shelving state that proves usefulfor non-resonant spin pumping and readout [3]. However, NVs lack efficient emission into the zero-phononline (ZPL), and are susceptible to external environmental noise due to their inherent symmetry, leading toinstability of the optical transition energy. Conversely, group IV vacancy complexes in diamond (G4V) (withsilicon, germanium, tin or lead) show excellent optical properties due to their crystallographic symmetry
which favors emission into ZPL [4]. In this group, the silicon vacancy (SiV−) has been so far the most studiedcomplex and T2 spin coherence times exceeding ~10ms have been demonstrated at low operation
temperatures (T~100mK) [5]. Even though the energy levels of the G4V, forming a double-lambda systemwith spin S=1/2, can be used to store and process quantum information, the lack of optical spin selection rules
makes their optical readout difficult and inefficient. In this contribution, we present our efforts towards
controlling and understanding the SiV− charge state mechanisms towards implementing spin-to-charge
conversion protocols. By means of voltage pulses applied from a pair of interdigital metal contacts on the
diamond surface we develop a method to dynamically manipulate the charge state of SiV− centers anddemonstrate that they can be switched reversibly between SiV0 and SiV− at MHz-rates (see Fig. 1).

Fig. 1. a) Optical micrograph of the diamond in planar-interdigital configuration. b) SiV- photoluminescenceintensity upon changes in the applied voltage between V=±200 V. c) Time-resolved photoluminescence during
an optical and electrical sequence depicting the change of the SiV- charge state. d) SiV- switching rate as afunction of the applied optical power.

We furthermore fully explore the charge cycle of the SiV− center spectroscopically and find that it is aided by
the presence of P1 and divacancy centers in the host lattice. Our results shed light on the charge cycle
mechanisms on G4V and elucidate the potential for realizing an opto-electronic readout of their spin state.
Finally, we will discuss current strategies to enhance the SiV− spin coherence times beyond theconventionally used ~mK temperatures. Specifically, 1D optomechanical structures containing single SiVcenters can be engineered to achieve optical cooling of the host diamond lattice and therefore extend the SiV−
spin coherence time. Such a hybrid physical platform for coherent cavity acousto-optics (AO) is based on
SiV− in diamond and confined GHz phonons. The strong AO coupling in these structures will be exploited toperform unconventional spin initialization techniques in SiV− centers while cooling down the host lattice andthus increase the spin coherence time at higher temperatures.
References[1] M. Ruf, N.H. Wan, H. Choi, D. Englund, R. Hanson, Journal of Applied Physics 130, 070901 (2021).
[2] S. Prawer, A.D. Greentree, Science 320, 1601(2008).
[3] N. Bar-Gill, L. M. Pham, A. Jarmola, D. Budker, R.L. Walsworth, Nature Communications 4, 1743 (2013).
[4] B. Pingault, D.D. Jarausch, C. Hepp, L. Klintberg, J.N. Becker, M. Markham, C. Becher, M. Atatüre, Nature Communications 8, 15579 (2017).
[5] D D. Sukachev, A. Sipahigil, C.T. Nguyen, M.K. Bhaskar, R.E. Evans, F. Jelezko, M.D. Lukin, Physical Review Letters 119, 223602 (2017).

The Effect of Disorder on the Thermalization of Correlated Quantum
Systems under an Interaction Quench
H. F. Fotso
University at Buffalo SUNY, Buffalo, NY 14260, USA

To enable the investigation of the nonequilibrium dynamics of interacting systems in the presence ofdisorder, we recently introduced the nonequilibrium DMFT+CPA method [1]. An embedding
scheme that combines the nonequilibrium extensions of both the dynamical mean field theory(DMFT) [2] and the coherent potential approximation (CPA) [3,4]. This framework enables
investigations of the important interplay of interaction and disorder on the nonequilibrium dynamics
of quantum systems. The approach was previously benchmarked on the equilibrium solution of theAnderson-Hubbard model revealing among other features the disorder-induced insulator to metallic
phase transition that occurs for strong interactions when the disorder strength is increased. Here, we
employ this framework to analyze the effect of disorder on the nonequilibrium dynamics of acorrelated system, described by the Anderson-Hubbard model, under an interaction quench. The
system, initially in equilibrium at a given temperature, Tinitial = 1/βinitial , has the interaction abruptly
switched from zero to a finite value at a given time. To investigate the role of disorder, we use oureffective medium approach to calculate, through the nonequilibrium Green’s functions, for different
values of the final interaction and for different disorder strengths, the distribution functions as thesystem evolves in time. This allows us to determine the effective temperature after the quench and toanalyze the effects of disorder on the thermalization for various interaction strengths.

Fig. 1: Inverse effective temperature as a function of the final interaction strength (U 2) for different disorderstrengths (W). The systems is initially at a temperature such that βinitial = 15. Inset: Effective inverse temperature
β as a function of the disorder strength for different interaction strengths. Increased disorder strength formoderate interaction strengths leads to a lower long-time temperature.

The analysis shows that, for moderate interaction strengths after the quench, disorder can tune thefinal temperature of the system across a broad range of values; With increased disorder strength
leading to lower effective temperature [5].
References[1] E. Dohner, H. Terletska, K.-M. Tam, J. Moreno, H. F. Fotso, Physical Review B 106, 195156 (2022).
[2] J. K. Freericks, Physical Review B 77, 075109 (2008).
[3] Y. Zhu, L. Liu, H. Guo, Physical Review B 88, 205415 (2013).
[4] A. V. Kalitsov, M. G. Chshiev, J. P. Velev, Physical Review B 85, 235111 (2012).
[5] E. Dohner, H. Terletska, H F Fotso, arXiv:2303.06495 (2023).
* Acknowledgement(s) : We acknowledge support from National Science Foundation under Grant No. PHY-2014023.

Time-crystalline behavior in central spin models with
Heisenberg
interactions
1
2
3
4
5

R. Frantzeskakis , J. Van Dyke , L. Zaporski , D. A. Gangloff , C. Le Gall , M. Atatüre3S. Economou 2, E. Barnes 2
1
University of Crete, Heraklion, 71003 Greece
2
Virginia Tech, Blacksburg, VA 24061 USA
3
University of Cambridge, Cambridge, CB3 0HE, United Kingdom
4
University of Oxford, Oxford, OX1 3PJ, United Kingdom
5
Microsoft Research, Cambridge, CB1 2FB, United Kingdom

Time crystals have been experimentally realized recently [1, 2]. Different physical non-equilibriumdriven systems can create a time crystal phase. Quantum dots have been proposed as one of the best
candidates to protect quantum information [3]. In addition, color centers are well known for theirenormous dephasing times and the stabilization of their quantum states [2,4]. We study how we cantake advantage of nuclear-electron spin/defect hyperfine interactions to create a time crystal phase ina central-spin model. This nuclear hyperfine interaction is unavoidable in spin systems [5-7]. We
focus on stabilizing multi-qubit computational basis pure quantum states in the presence ofHeisenberg hyperfine interactions.We propose applying a strong enough on-site magnetic field onelectron spin/defect (central spin) to create a time crystal phase in the central-spin model. An on-site
constant magnetic field can effectively decouple the Heisenberg interactions, achieving thesubharmonic response which can be seen through a phase diagram (Figure 1a). However, an on-sitemagnetic field may not be applicable in different spin systems, so we propose another way tostabilize multi-qubit states (Figure 1b). We focus on applying a sequence of π pulses in the electron
spin to observe subharmonic behavior. This sequence of H2I π pulses (Heisenberg-to-Isingpulses) can make a Heisenberg Hamiltonian behave like an Ising one [8]. Electrons/defects andnuclear spins can be stabilized by quantum control of the central spin (magnetic field or Heisenberg
to Ising pulses) and periodic driving of electrons and nuclei.

Fig. 1. Left (or a): Phase diagram with an on-site constant central spin magnetic field; Right (or b):
Phase diagram with application of H2I π pulses.

This may be beneficial for using physical central spin systems with hyperfine interactions like color
centers (defect coupled to nuclei), multi-dots systems opening the road of using quantum dots, andcolor centers in protecting quantum information. More details can be found in our recent arXiv
preprint [9].
References[1] J. Zhang, P.W. Hess, A. Kyprianidis, P. Becker, A. Lee, J.Smith, G. Pagano, I. D. Potirniche, A.C. Potter, A. Vishwanath, N.Y. Yao, Nature
543, 217 (2017).
[2] J. Randall, C.E. Bradley, F.V. van der Gronden, A. Galicia, M.H. Abobeih, M.Markham, D. J. Twitchen, F. Machado, N.Y. Yao, TH. Taminiau
Science 374, 1474 (2021).
[3] J.S. Van Dyke, Y.P. Kandel, H. Qiao, J.M. Nichol, S.E. Economou, E. Barnes, Physical Review B 103, 245303 (2021).
[4] E. Bauch, C.A. Hart, J.M. Schloss, M.J. Turner, J.F. Barry, P. Kehayias, S. Singh,R. L. Walsworth, Physical Review X 8, 031025 (2018).
[5] L.V. Assali,H.M. Petrilli, R.B. Capaz, B. Koiller, X. Hu, S.D. Sarma, Physical Review B 83, 165301 (2011).
[6] G. Gillard, I.M. Griffiths, G. Ragunathan, A. Ulhaq, C. McEwan, E. Clarke, E.A. Chekhovich, E.A., npj Quantum Information 7, 43 (2021).
[7] S. Pezzagna, J. Meijer, Applied Physics Reviews 8, 011308 (2021).
[8] E. Barnes, J.M. Nichol, S.E. Economou, Physical Review B 99, 035311 (2019).
[9] R. Frantzeskakis, J. Van Dyke, L. Zaporski, D.A. Gangloff, C.L. Gall, M. Atatüre, S.E. Economou, E. Barnes, arXiv:2303.00893.
* This work was supported by DARPA (grant no. D18AC00025), by NSF grant no. 1847078 and by EU Horizon 2020 programme (GA 862035
QLUSTER).

Theoretically describing pump-probe experiments inElectron-Phonon coupledsystems
out to ps
time scales
1
1
2

J. K. Freericks M. Petrovic ,Manuel Weber
Georgetown University, Washington, DC 20003, USA
2
Max Planck Institute for the Physics of Complex Systems, 01187 Dresden, Germany
1

In this talk, we describe coupled nonequilibrium electron-phonon systems semiclassically—using
Ehrenfest dynamics for the phonons and quantum mechanics for the electrons—via a classical Monte
Carlo approach that determines the nonequilibrium response to a large pump field. The semiclassicalapproach is quite accurate, because the phonons are excited to average energies much higher than thephonon frequency, eliminating the need for a quantum description of their behavior. The energy forthe electrons is still in a quantum-degenerate regime, so quantum mechanics is critical to describe
their behavior. The numerical efficiency of this method allows us to perform a self-consistent time
evolution out to very long times (tens of picoseconds) enabling us to model pump-probe experimentsof a charge density wave (CDW) material. Our system is a half-filled, one-dimensional (1D) Holsteinchain that exhibits CDW ordering due to a Peierls transition. The chain is subjected to a timedependent electromagnetic pump field that excites it out of equilibrium, and then a second probe
pulse is applied after a time delay. By evolving the system to long times, we capture the complete
process of lattice excitation and subsequent relaxation to a new equilibrium, due to an exchange ofenergy between the electrons and the lattice, leading to lattice relaxation at finite temperatures. We
employ an indirect (impulsive) driving mechanism of the lattice by the pump pulse due to the drivingof the electrons by the pump field. We identify two driving regimes, where the pump can either
cause small perturbations or completely invert the initial CDW order. Our work successfully
describes the ringing of the amplitude mode in CDW systems that has long been seen in experiment,
but never successfully explained by microscopic theory. We also describe the fluence-dependent
crossover that inverts the CDW order parameter and changes the phonon dynamics.
We work with a Holstein model, which involves a coupling of the local electronic charge density tothe phonon coordinate at each lattice site. We work with spinless electrons on a one-dimensionallattice with nearest neighbor hopping γ. The phonons are local Einstein modes with a frequency
given by Ω = 0.01 γ/ℏ. The system has a charge-density-wave ground state at T=0, but does not
display long-range order for any finite temperature. However, because we work in a finite system,
once the correlation length is larger than the system size the system appears ordered. We always
work with a proper superposition of the two translationally invariant charge-density-wave states, so
our system only displays broken symmetry when we look at correlation functions. Using this code,
we can compute Green’s functions and from that we can extract the photoemission spectroscopy(PES). As observed in experiments (see, e.g., Ref. 1), the lattice motion causes the gap energy tooscillate, which isreflected in the computed PES at low temperatures in Fig. 1. The equilibrium
spectrum before the pump isgapped (due to the Peierls distortion), which implies that only the lower
band is populated. After excitation, the system will stabilize to a new equilibrium with a reduced gap
energy. Increasing the temperature of the lattice has two major effects on the computed PES. The
first is the evident damping of the gap oscillations, so the system relaxes faster to a new equilibrium
PES for higher initial temperatures. The second effect is the "washing out" of the finer details in the
spectrum at higher temperatures, increasing at longer times. The period of initial PES oscillations in
Fig. 1(a) is much larger (around 1000 ℏ/γ) than what one would expect for Ω = 0.01 γ/ℏ which is aclear sign that electron-phonon interaction significantly modifies the intrinsic phonon frequency. The
advantage of our self-consistent MC approach is evident from the time scale of Fig. 1, where thetime resolution must be kept at 0.1 ℏ/γ to capture the electron dynamics, still the fast evolution
scheme allows us to average over 3000 MC configurations. Translating these units to the ones inexperiments, for γ = 1 eV, the time step is ∆t ≈ 0.07 fs, while the simulation time is around 7 ps.
Most other methods can only approach maximal times on the order of 10s to 100s of fs.
As one can see in Fig. 1, exciting the electron impulsively excites the amplitude mode phonon as
well, whose oscillation is damped as a function of time. The band edges of the PES oscillate with thefrequency of the amplitude mode (which is renormalized from its bare value)Fig. 1. Photoemission spectra for a chain of L = 30 sites at different temperatures. The electron-phonon
coupling is λ = 0.6 and the phonon frequency is Ω = 0.01 γ/ℏ. The pump parameters are E0 = 0.33, σp = 10ℏ/γ, and ωp = 0.1 γ/ℏ.

As the initial temperature is raised, the damping is more rapid. In the talk, I will discuss thisphenomenon and describe more pump/probe experiments on electron-phonon coupled systems.
References[1] F. Schmitt, P. S. Kirchmann, U. Bovensiepen, R. G. Moore, L. Rettig, M. Krenz, J.-H. Chu, N. Ru, L. Per- fetti, D. H. Lu, M. Wolf, I. R. Fisher,
Z.-X. Shen, Science 321, 1649 (2008).
* Acknowledgements: This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) underAward DE-FG02-08ER46542. J.K.F. was also supported by the McDevitt bequest at Georgetown University. This research used resources of the
National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated underContract no. DE-AC02-05CH11231.

Attosecond chronoscopy of correlations between
conducting electrons in quantum materials
J. Freudenstein1, M. Meierhofer1, M. Borsch2, D. Afanasiev1, M. Liebich1, M. Knorr1, M. Kira2, R. Huber1
1
University of Regensburg, 93040 Regensburg, Germany
2
University of Michigan, Ann Arbor, Michigan 48109, USA

Correlations between delocalized Bloch electrons are the driving force behind key properties ofsolids as well as intriguing phase transitions [1]; to directly follow how many-body interactions
affect intrinsic electron motion, sub-femtosecond temporal resolution is desirable [2]. Multi-terahertz(THz) photon energies are perfectly matched to resonantly probe millielectronvolt (meV)
correlation-induced excitations [3] and THz fields have been used to control charge carriers onsubcycle time scales [4]. Yet, resolving the influence of correlations on the attosecond dynamics ofdelocalized Bloch electrons has remained an open challenge.
Here we directly reveal the effect of many-body interactions on the motion of delocalized electrons,in the time domain, by combining attosecond resolution with meV energy selectivity, for the firsttime [5]. Our clocking concept (Fig. 1a) employs the force of an intense few-cycle THz pulse (darkgrey shaded area). At a tunable delay time, tex, a 9-fs near-infrared excitation pulse creates electron–
hole (e−h) pairs by resonant interband transitions. As the freshly generated e−h pairs are separated by
the THz field, their relative momentum, ℏ k, and their relative coordinate, x, increase. Upon reversalof the polarity of the THz field, both ℏ k and x decrease. For a favorable excitation time, the e−hpairs can recollide at x = 0 (blue trajectory), generating high-order sideband (HSB) radiation. For less
optimized tex, the e−h pairs will not recollide, yielding weak HSB emission.
Whereas quasi-free e−h pairs follow ballistic trajectories governed by the single-particle band
structure (Fig. 1a), strong many-body correlations (Fig. 1b), such as excitonic binding (purple fieldlines) and interactions with other carriers modify the trajectories and, thus the optimal excitation
time. Coulomb attraction, for instance, acts as a restoring force. In order to reach large relative
momenta and therefore enable highly energetic recollisions, an enhanced action of the THz field
during the separation phase of the e−h pair is needed. This can be achieved by optical excitation atearlier tex (Fig. 1b, blue trajectory). Overall, the different dynamics for quasi-free versus correlated
charge carriers should manifest in a measurable shift, ∆t, of the optimum injection time tex.
The vastly different strength of Coulomb interactions in bulk and monolayer WSe2 offer a perfectbenchmark to test this concept. By actively stabilizing our setup [6], even shifts as small as 300 as ofFig. 1. a, b, Attosecond correlation clocking. An e–h pair created at excitation time tex is accelerated by theforce of an intense THz waveform (dark-grey shaded area), changing e–h distance x and relative crystal
momentum ℏ k. Uncorrelated e–h pairs (a) generated shortly after a THz field crest ballistically recollide at adelay time Tcoll (blue trajectory), emitting HSB radiation. Many-body interactions (b), such as excitonic e–h paircorrelations (purple field lines) modify trajectories and reduce Tcoll. ∆t indicates the difference between
optimum tex with and without many-body interactions. c, Experimental THz driving field centered at 25 THz. d,Experimentally recorded HSB intensity, IHSB, from bulk (blue) and monolayer WSe2 (orange). e, Close-up of two
IHSB maxima compares experiment (spheres) with QDCE-computations (solid lines). The timing differences
extracted from measurements are labelled. f, Measured subcycle delay δsc in bulk (blue) and monolayer
𝐛𝐛𝐛𝐛𝐛𝐛𝐛𝐛(orange) WSe2. Shaded areas indicate peak-averaged values 〈δ𝐬𝐬𝐬𝐬〉 = 3.2 ± 0.2 fs and 〈δ𝐬𝐬𝐬𝐬𝐌𝐌𝐌𝐌 〉 = 2.1 ± 0.2 fs.

the optimal tex can be detected, corresponding to 0.7% of the driving field’s oscillation period. In afirst set of experiments, a peak THz field of Epeak = 4.9 MV cm-1 (Fig. 1c) was used. Both monolayer
and bulk show a strongly modulated high-order sideband emission as a function of tex. Close
inspection of the peaks of the HSB generation reveals a small temporal shift between the bulk andmonolayer case. For a quantitative analysis, the subcycle delay δsc, which is the timing difference
between the HSB emission peaks and the nearest THz field crest, is extracted. Figure 1d shows themeasured HSB emission (data points) for two distinct peaks. Full quantum dynamic cluster
expansion (QDCE) computations (solid lines) corroborate the temporal shape of the HSB emissionMLand link the significant reduction of 〈δbulksc 〉 = 3.2 ± 0.2 fs to 〈δsc 〉 = 2.1 ± 0.2 fs between bulk andmonolayer (Fig. 1e) to enhanced excitonic correlations in the monolayer case [5].
To explore how excitonic attraction competes with lightwave-driven charge separation, we measure
the average subcycle delay in bulk and monolayer WSe2, for various fields Epeak. (Fig. 2a).

Fig. 2. a, Measured (triangles and shaded areas) and QDCE-computed (solid lines) subcycle delay δsc and theirrespective standard error (areas) for bulk (blue) and monolayer WSe2 (orange) as a function of the peak THz
MLbulk
field strength Epeak. b, Averaged subcycle delay difference, ∆t = δsc− δsc
and their respective standard
error (areas), measured between bulk and monolayer for linearly (grey triangles) and right circularly polarized
excitation (purple triangles) as a function of the e–h pair density in the monolayer.

The monotonic increase of δsc confirmed by our QDCE computations (solid lines); strong fields
ionize the excitonic coherence faster, which reduces the probability of recollision in the subsequentTHz half-cycle. Thus maximum IHSB is reached for coherent excitons excited closer to a field zerocrossing (i.e. larger tex), explaining the observed δsc trend.
Additionally, even the valley pseudospin can be exploited to switch the strength of the Coulombinteraction Vk,k’ between electrons at wave vectors k and k’. Specifically, the presence of electrons at
k effectively reduces Vk,k’ via Pauli blocking. Such a reduction is expected to be particularly strong
in monolayer samples when circularly polarized light excites e−h pairs and blocks k states only in asingle valley. Indeed, we find that ∆t = δscML − δscbulk , which ranges from -150 as to -1.4 fs, issignificantly less negative (Fig. 2b), corresponding to a stronger reduction of Vk,k’ for circular thanfor linear polarization. For large excitation fluences, ∆t approaches zero – the quasi-free limit for thebulk. Our computations predict that δsc could even resolve correlation changes associated with theMott transition from excitons to an electron–hole plasma. In conclusion, attoclocking delocalized
Bloch electrons reveals many-body correlations in a completely new way – directly in the time
domain – as demonstrated by correlation tuning in transition metal dichalcogenide monolayers andbulk crystals. Deciphering tailored correlation dynamics in artificial van-der-Waals heterostructures
or naturally occurring correlated states in quantum materials could revolutionize our understandingof unexpected phase transitions and emergent quantum-dynamics for future electronic,
optoelectronic, and quantum information technologies.
References[1] I. A. Perakis, M. K. Rafailov, J. M. Auxier, Proceedings “VII Ultrafast Dynamics and Metastability & Ultrafast Bandgap Photonics” (2022).
[2] A. L. Cavalieri, N. Müller, T. Uphues, V. S. Yakolev, A. Baltuška, B. Horvath, B. Schmidt, L. Blümel, R. Holzwarth, S. Hendel, M. Drescher
U. Kleineberg, P. M. Echenique, R. Kienberger, F. Krausz, U. Heinzmann, Nature 449, 1029 (2007).
[3] T. Kampfrath, K. Tanaka, K. A. Nelson, Nature Photonics 7, 680 (2013).
[4] J. Reimann, S. Schlauderer, C. P. Schmid, F. Langer, S. Baierl, K. A. Kokh, O. E. Tereshchenko, A. Kimura, C. Lange, J. Güdde, U. Höfer
R. Huber, Nature 562, 396 (2018).
[5] J. Freudenstein, M. Borsch, M. Meierhofer, D. Afanasiev, C. P. Schmid, F.Sandner, M. Liebich, A. Girnghuber, M. Knorr, M. Kira, R. Huber
Nature 610, 290 (2022).
[6] M. Meierhofer, S. Maier, D. Afanasiev, J. Freudenstein, J. Riepl, J. Helml, C. P. Schmid, R. Huber, Optics Letters 48, 1112 (2023).
* The work in Regensburg has been supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through Project ID
422 314695032-SFB 1277 (Subproject A05) as well as Research Grant HU1598/8. Work in Ann Arbor has been supported by ARO throughAward W911NF1810299, W.M. Keck Foundation and College of Engineering Blue Sky Research Program.

Time, energy and momentum resolved probing of charge order
Dynamics
in a Kagome
metal 2
1
1
1
1
3
3
2

D.Azoury , A. von Hoegen , Y.Su , K.H. Oh , T. Holder , H. Tan , B. R. Ortiz , A. C. Salinas , S. D. Wilson2
B. Yan3, N. Gedik1
1
Massachusetts Institute of Technology, Cambridge, MA02139, USA
2
University of California, Santa Barbara, CA 93106, USA
3
Weizmann Institute of Science, Rehovot 7610001, Israel

The recently discovered group of superconducting kagome metals, AV3Sb5 (A = K, Rb, Cs) exhibit avariety of intertwined unconventional electronic phases, which emerge from a puzzling charge
density wave (CDW) phase1-3. Understanding of this parent charge order is crucial for deciphering
the entire phase diagram. However, the mechanism of the CDW is still controversial, and its primarysource of fluctuations – the collective modes – have not been experimentally observed. Here, I will
present recent results in which we used time- and angle- resolved photoemission spectroscopy toinvestigate the CDW phase of CsV3Sb5 (see Figure 1 top). After excitation with an ultrafast laser
pulse, we obtained momentum resolved tomographic images of the CDW gap which shows themelting of the charge order within 250 fs (see Figure 1 bottom). Furthermore, we detect coherent
oscillations at various high symmetry points across the electronic band-structure, and identify thesemodes as CDW induced phonons, in good agreement with recent Raman measurements4. In contrast,
measuring the in-gap intensity dynamics reveals two new modes that were not previously detected byany other spectroscopic method. Our experimental results distinguish these modes from the phononmodes, and using DFT calculations, we show that they correspond to the collective excitations of theorder parameter.

Fig. 1. Top: Schematic of the pump-probe high-harmonic angle-resolved photoemission experiment next to afalse-color plot of the Fermi surface of CsV3Sb5 in the charge density phase. Bottom: Time-resolved
intensity change at the Γ and Μ bands, the latter hosts the CDW, for two energies above (+130 meV) andbelow (-20 meV) the Fermi energy. The dynamics at the Μ band show a delay of 250 fs with respect to the
dynamics at the Γ band. The dynamics at the Μ band are superimposed with coherent oscillations withfrequencies of 1.6 THz and 2.4 THz. The faded line is the raw data, the black line is the data smoothed by athree-point average (provided as a guide to the eye) and the dashed line is a fit to the data.

These observations together with ab-initio calculations provide clear evidence for a structural rather
than electronic mechanism of the charge density wave. Our findings pave the way for better
understanding of the unconventional phases hosted on the kagome lattice.
References[1] B. R. Ortiz, L. C. Gomes, J. R. Morey, M. Winiarski, M. Bordelon, J. S. Mangum, I W. H. Oswald, J. A. Rodriguez-Rivera, J. R. Neilson
S. D. Wilson, E. Ertekin, T. M. McQueen, E. S. Toberer, Physical Review Materials 3, 094407 (2019)[2] H. Zhao, H. Li, B. R. Ortiz, S. M. L Teicher, T. Park, M. Ye, Z. Wang, L. Balents, S. D. Wilson, I. Zeljkovic, Nature 599, 216 (2021).
[3] H. Chen, H. Yang, B. Hu, Z. Zhao, J. Yuan, Y. Xing, G. Qian, Z. Huang, G. Li, Y. Ye, S.Ma, S. Ni, H. Zhang, Q. Yin, C. Gong, Z. Tu
H. Lei, H. Tan, S. Zhou, C. Shen, X. Dong, B. Yan, Z. Wang, H.-J. Gao, Nature 599, 222 (2021).
[4] S. Wu, B. R. Ortiz, H. Tan, S. D. Wilson, B. Yan, T. Birol, G. Blumberg, Physical Review B 105, 155106 (2022).
* Acknowledgement(s) : Alexander von Hoegen acknowledges support from the Alexander von Humboldt foundation. We acknowledge
support from the US Department of Energy and Gordon and Betty Moore Foundation’s EPiQS Initiative grant GBMF9459. B.Y.
acknowledges the financial support by the European Research Council (ERC Consolidator Grant “NonlinearTopo”, No. 815869) and the
ISF - Personal Research Grant (No. 2932/21). S.D.W., B.R.O., and A.C.S. gratefully acknowledge support via the UC Santa Barbara NSF
Quantum Foundry funded via the Q-AMASE-i program under award DMR-1906325.

Ultrafast time-reversal symmetry breaking:
Magnetism and entropy production
R.M. Geilhufe
Chalmers University of Technology, 41296 Gothenburg, SwedenHigh intensity THz lasers allow for the coherent excitation of individual phonon modes. The ultrafast
control of emergent properties by means of phonons opens up new tuning mechanisms for functional
materials. Such properties are often related to the targeted breaking of symmetries. Here, we focus
on the breaking on time-reversal symmetry, connected to two phenomena. First, the emergence ofmagnetism due to chiral phonons. Second, the process of entropy production for laser induced
phonons. Chiral phonons are lattice excitations with finite angular momentum. As individual ions ina crystal are charged, a circular ionic motion induces a magnetic field. However, the expected fieldstrength due to an ionic motion is tiny and of the order of the nuclear magneton [1,2]. Astonishingly,
recent experiments show the opposite effect [3-5]. Independently of the material class, a hugephonon magnetic moment has been observed, being in the order of the Bohr magneton, i.e., 4-5
orders of magnitude larger than predicted for an ionic effect. Hence, a coupling and angularmomentum transfer to electrons should be involved. The linear coupling of phonon angularmomentum and spin angular momentum is a consequence of inversion and time-reversal symmetry.
However, the exact coupling constant is unknown. A part of this coupling constant is related to thespin-orbit interaction of electrons combined with conventional electron-phonon coupling. Here, weshow that an additional effect emerges, based on the accelerating ionic motion. In fact, inertial
effects on electrons couple the ionic angular momentum to the electron spin, in a quantum version of
the classical Coriolis force. We derive this contribution and estimate its effect on the spinpolarization of electronic states in KTaO3 (see Fig 1 left).In the second part of the talk we pay
attention towards ultrafast thermodynamic processes connected to the laser excitation. Even though,it is known that an intense laser field induces heat in a sample, the microscopic mechanisms for heator entropy production connected to a laser excitation have only sparsely been discussed. In particular
the dynamics of heat and entropy production during the excitation process is unknown in most cases.

Fig. 1. Left: Transient magnetization in KTaO3 due to the coupling of laser induced chiral phonons with theelectron spin. The magnetization depends on the doping level (colored lines in units of fermi level energy
against conduction band edge) and the electronic temperature due to the laser excitation. Right: Entropy
production in SrTiO3 obtained using the formalism of Ref [8] and experimental data from Ref [9]. Comparisonof theoretical and experimental laser profile (a) and phonon amplitude (b). The computed entropy production isshown in (c), the displacement-displacement correlation in (d).

To make progress in this direction, I will introduce a model to calculate the entropy production in a
pump-probe experiment. While entropy has been known for almost 200 years, it cannot be measured
directly. Here, we show that tracking the ionic motion, e.g., in a time-resolved X-ray scatteringexperiment allows us to reproduce the process of entropy production in the presence of an appliedlaser field. This opens the prospect for an ultrafast thermodynamics approach to nonequilibrium
quantum materials.
References[1] D. M. Juraschek, N. A. Spaldin, Physical Review Materials 3, 064405 (2019).
[2] R. M. Geilhufe, V. Juričić, S. Bonetti, J.-X. Zhu, A. V. Balatsky, Physical Review Research 3, L022011 (2021).
[3] B. Cheng, T. Schumann, Y Wang, X. Zhang, D. Barbalas,S. Stemmer,and N. P. Armitage , Nano Letters 20, 5991 (2020).
[4] F. G. G. Hernandez, A. Baydin, S. Chaudhary, F. Tay, I. Katayama, J. Takeda, H. Nojiri, A. K. Okazaki, P. H. O. Rappl, E. Abramof
M. Rodriguez-Vega, G. A. Fiete, J. Kono, arXiv:2208.12235 (2022).
[5] M. Basini, M. Pancaldi, B. Wehinger, M. Udina, T. Tadano, M. C. Hoffmann, A. V. Balatsky, S. Bonetti, arXiv:2210.01690 (2022).
[6] R. M. Geilhufe, Physical Review Research 4, L012004 (2022).
[7] R. M. Geilhufe, W. Hergert. Physical Review B 107, L020406 (2023).
[8] L. Caprini, H. Löwen, R. M. Geilhufe, arXiv:2302.02716 (2023).
[9] M. Kozina, M Fechner, P. Marsik, T. van Driel, J. M. Glownia, C. Bernhard, M. Radovic, D. Zhu, S. Bonetti, U. Staub, M. C. Hoffmann, Nature
Physics 15, 387 (2019).
* Acknowledgement(s) : RMG acknowledges support from the Swedish Research Council (VR starting grant No. 2022-03350) and Chalmers
University of Technology. Computational resources were provided by the Swedish National Infrastructure for Computing (SNIC) via the National
Supercomputer Centre (NSC).

Direct observation of few-femtosecond
Spin 2currents acrossmetallic
layers2
3
3
4

R. Géneaux1, H.-T. Chang , A. Guggenmos , F. Légaré , K. Légaré , J. Lüning , T. Parpieev2
B. R. de Roulet2, M. W. Zürch2, S. Sharma6, M. Schultze7, S.R. Leone2
1
Université Paris-Saclay, 91191 Gif-sur-Yvette, France
2
University of California, Berkeley, CA 94720, USA
3
Institut National de la Recherche Scientifique, Varennes, QC J3X 1P7, Canada
4
Sorbonne Université, 75005 Paris, France5
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
6
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany
7
Graz University of Technology, 8010 Graz, Austria

Femtosecond laser pulses are able to act on the magnetic moments of solids, as amply demonstrated
by magneto-optical experiments [1, 2]. However, the vast majority of control schemes make use ofincoherent interactions (scattering, diffusion) between electrons, spins and phonons. On the contrary,
the observation of direct and coherent interactions between light and spins has been scarce, despite
first hints discovered more than ten years ago [3].Here we examine the ultrafast spin response of
Co/Pt multilayers when driven by 4 fs laser pulses. The material response is captured by magnetic
circular dichroism (MCD) using broadband attosecond pulses, an extension of the well-developed
attosecond transient absorption method [4]. This recently developed approach [5] is extended here bymeasuring MCD across both the Pt O2,3 and the Co M2,3 edges in a spectrally continuous way, as
shown in Figure 1. The method thus combines outstanding time resolution and element-specificity,
providing a layer-resolved probe of electronic and spin dynamics. Importantly, the experiment canunambiguously separate electronic and magnetic contributions to both the absorption and MCD of
the sample. We observe that the electronic excitation is simultaneous in Pt and Co layers (within an
uncertainty of 1.5 fs). However, the rise time of carrier signal differs between the two elements, withthe rise time in Pt being systematically 1.5 times faster than in Co. This rise time can be ascribed toelectron thermalization times, and thus this difference can be interpreted as different pre-thermal(sub-15 fs) electron processes in either metal. This is possibly linked to different screening
environment in each metals [6].The spin response of each layer uncovered by attosecond magnetic
circular dichroism is markedly different. The system shows a sub-5 fs spike of magnetization in theplatinum layer, simultaneously with a strong magnetization decrease in cobalt (see right of Fig. 1).

Fig. 1. Left: Broadband magnetic circular dichroism measured across both Pt and Co edges(indicated on top). Right: Measured dynamics of the Co and Pt magneto-optical index at very earlytime delays. The brown shaded area indicates the measured pump pulse intensity profile.

After that, both magnetic moments begin to decay with two distinct timescales, ascribed to coherent
and incoherent demagnetizing processes. Simulations in the framework of fully noncollinear timedependent density functional theory (TD-DFT) [7] reproduce this behavior. We interpret ourobservations as a coherent light-driven spin injection across the metallic layers of the junction,
making it one the shortest spin current ever observed. Our findings suggests that our ability ofshaping light fields in refined ways could be translated into the shaping of spin currents in materials.
References[1] E. Beaurepaire, J.-C. Merle, A. Daunois, J.-Y. Bigot, Physical Review Letters 76, 4250 (1996).
[2] C.D. Staunciu, Physical Review Letters 99, 047601 (2007).
[3] J.-Y. Bigot, M. Yomir, E. Beaurepaire, Nature Physics 5, 515 (2009).
[4] R. Geneaux, H.J. B. Marroux, A. Guggenmos, D. M. Neumark, S.R. Leone, Philosophical Transactions Royal Society A 377, 20170463 (2019).
[5] F. Siegrist, J. A. Gessner, M. Ossiander, C. Denker, Y.-P. Chang, M. C. Schroder, A. Guggenmos, Y. Cui, J. Walowski, U. Martens,
J. K. Dewhurst, U. Kleineberg, M. Munzenberg, S. Sharma, M. Schhultze, Nature 571, 240 (2019).
[6] B.Y. Mueller,B. Rethfeld, Phyical. Review B 87, 035139 (2013).
[7] J. K. Dewhurst, P. Elliott, S Shallcross, E. K. U. Gross, S. Sharma, Nano Letters 18, 1842 (2018).

First results of high-resolution pp-RIXS on
Correlatedmaterials
from the European
XFEL
2
3
13
2
1

G. Ghiringhelli , L. Adriano , A. Alic , D.R. Baykusheva , R. Carley , G.S. Chiuzbaian3
M.P.M. Dean4 ,O. Duros3, A. Foelisch5, M. Först6, B.K. Freelon7, N. Gerasimova2, X. Jiang8, D. Jost9
M. Kusch5, T. Laarman10, V. Lebedev1, W.S. Lee9, C.Y. Liu5, L. Martinelli1, L. Mercadier2, G. Merzoni1
M. Minola12, M. Mitrano13, S. Molodtsov2, C.S. Pathiraja7, S. Parchenko2, J.N. R. Pelige7, Y.Y. Peng8
Q. Qiu7, T. Schmitt14, J. Sears4, A. Scherz2, S. S. N. Lalithambika10, S. Techert10, M. Teichmann2
S.F.R. TenHuisen13, B. van Kuiken2, Z. Yin2, J. Schlappa2
1
Politecnico di Milano, 20133 Milano, Italy
2
European XFEL, 22869 Schenefeld,, Germany
3
Sorbonne University, 75252 Paris, France
4
Brookhaven National Laboratory, Upton, NY 11973, USA
5Helmholtz-Zentrum Berlin für Materialien und Energie, 12489 Berlin, Germany
6
Max-Planck-Institut für Struktur und Dynamik der Materie, 22761 Hamburg, Germany
7
University of Houston, Houston, TX 77021, USA
8
Peking University, Beijing 100871, China
9
Stanford University, Menlo Park, CA 94305, USA
10
Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany
11
University of Limerick, V94 T9PX, Ireland
12
Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany
13
Harvard University, Cambridge, MA 2138, USA
14Paul Scherrer Institut, 5232 Villigen, Switzerland

Pump-probe spectroscopy has a rich tradition in the optical regime but is quite a novelty in the xrays. X-ray free electron lasers (XFEL) provide brilliant sources of ultrashort x-ray pulses that offer
new opportunities in this direction. In the first generation of XFELs the low repetition rate has been
the main limiting factor when dealing with photon hungry techniques such as photoelectron
spectroscopy and inelastic scattering. The constraints are even more stringent when working withsolid samples that can easily be damaged by very energetic x-ray pulses, thus further reducing theuseable average flux. Moreover, some difficulties in scanning the x-ray photon energy and inchanging their polarization have slowed down even the simplest of all x-ray spectroscopies, i.e., xray absorption spectroscopy (XAS). Among the spectroscopic techniques available at synchrotron xray sources, resonant inelastic x-ray scattering (RIXS) spectra are particularly rich in information
about the electronic and magnetic properties of materials, because they host several excitations, oforbital, charge spin and vibrational character. In particular, the current state of the art RIXS
instruments at synchrotrons (20-40 meV total linewidth) allow resolving not only the crystal field
excitations, but also magnons and paramagnons, optical phonons, particle-hole pairs and charge
density fluctuations. Two decades of instrumentation development have therefore boosted RIXS to
be the most innovative x-ray spectroscopy for the study of quantum materials, in which different
degrees of freedom are all relevant and multiply entwined. Therefore, RIXS is probably the mostpromising spectroscopy for XFELs, if sufficient resolution can be achieved both in energy and intime. So far the pioneering work realized with low repetition rate x-ray FELs [1,2,3] suffered fromlow resolution and low statistical quality. In this context, the availability of the hRIXS spectrometerat the spectroscopy and coherent scattering (SCS) instrument of the European XFEL in Hamburg [4]is a major step forward. Thanks to the high repetition rate, the average flux on the sample iscomparable if not larger than the typical one at storage ring x-ray sources, even when the number ofphotons per pulse stays below the damaging threshold. The necessary flux reduction is obtained not
only with gas attenuators, but also by the action of the beam line monochromator that selects a very
narrow bandwidth in energy. In the first months of 2022, thanks to hRIXS we could measure, for the
first time, pump probe (pp) RIXS in the soft x-ray range, combining time resolution around 100 fs
and energy resolution around 100 meV at Cu and Ni L3 edges. This achievement was possible thanks
to the joint efforts of the SCS staff and of the hRIXS User Consortium who designed and realized thebeam line and the spectrometer in close collaboration. The commissioning runs were open to thebroad RIXS community, who could thus provide help while directly learning how to operate theinstrument. For this very first high-resolution pp-RIXS experiment, we selected two charge transfer
antiferromagnetic insulators based on divalent Ni and Cu, the cubic NiO and the layered perovskite
La2CuO4 (LCO). We used an optical pump able to promote electrons across the Mott gap, as longer
wavelength pulsed sources in the far IR and THz range, more suitable for the fine tuning of sometransport properties [5,6] were not available. The temporal resolution was about 100 fs and theenergy resolution 93 meV for Cu L3 (931 eV) and ≈ 80 meV for Ni L3 (853 eV) edges. We acquired
spectra changing the pump delay between -0.2 ps and 50 ps, and the laser fluence on the sample
between 1 mJ/cm2 up to 35 mJ/cm2.Although issued from a technical commissioning, where thepriority was on the instrumentation, the results are extremely exciting. First, the static RIXS spectrataken at EuXFEL are in all consistent with those measured with similar resolution at synchrotrons.
This means that the short pulses do not influence the RIXS process itself and the high power peakdoes not damage the sample. Second, a few ps dynamics is detected at the elastic, quasi-elastic(phonon envelope) and magnon peaks in both samples. Also, the dd excitations get modified by the
optical pump over a longer timescale of tens of ps, more related to the thermalization from theelectronic bath to the lattice. Moreover, in NiO a transient excitonic state is populated by the opticalpump, which is clearly detected in XAS and leads in RIXS to new spectral features, both in theenergy-loss and energy-gain sector of the spectrum. The data analysis is still ongoing, also with thetheoretical support by several groups. These results demonstrate the huge potential of high-resolution
pp-RIXS for the study of quantum matter: the superior specificity and sensitivity of RIXS withrespect to more established pp-techniques can be exploited in multiple ways. Moreover, when lower
photon-energy pump pulses will be available pp-RIXS will also be an exceptional tool for the
exploration of transient metastable states in complex materials. We note that the hRIXS spectrometercan be used also to study molecular systems thanks to an in vacuum liquid-jet system, therefore
providing access to the dynamics of photo-dissociation and of charge transfer mechanisms inmolecules.
References[1] M. P. M. Dean, Y. Cao, X. Liu, S. Wall, D. Zhu, R. Mankowsky, V. Thampy, X. M. Chen, J. G. Vale, D. Casa, J. Kim, A. H. Said, P. Juhas
R. Alonso-Mori, J. M. Glownia, A. Robert, J. Robinson, M. Sikorski, S. Song, M. Kozina, H. Lemke, L. Patthey, S. Owada, T. Katayama
M. Yabashi, Yoshikazu Tanaka, T. Togashi, J. Liu, C. Rayan Serrao, B. J. Kim, L. Huber, C.-L. Chang, D. F. McMorrow, M. Först J. P. Hill
Nature materials 15, 601 (2016).
[2] M. Mitrano, S. Lee, A. A. Husain, L.Delacretaz, M. Zhu, G. De La Peña Munoz, S. X.-L. Sun, Y. I. Joe, A. H. Reid, S.F. Wandel, G.Coslovich
W. Schlotter, T.Van Driel, J.Schneeloch, G. D. Gu, S. Hartnoll, N. Goldenfeld,P. Abbamonte, Science Advances 5, 3346 (2019).
[3] E. Paris, C. W. Nicholson, S. Johnston, Y. Tseng, M. Rumo, G. Coslovich, S. Zohar, M. F. Lin, V. N. Strocov, R. Saint-Martin, A. Revcolevschi
A. Kemper, W. Schlotter, G. L. Dakovski, C. Monney, and T. Schmitt, NPJ Quantum Materials 6, 51(2021).
[4] N. Gerasimova, D. La Civita, L. Samoylova, M. Vannoni, R. Villanueva, D. Hickin, R. Carley, R. Gort, B.E. Van Kuiken, P. Miedema,
L. Le Guyarder, L. Mercadier, G. Mercurio, J. Schlappa, M. Teichmann, A. Yaroslavtsev, H. Sinn, A. Scherz, Journal of Synchrotron Radiation.
29, 1299 (2022)[5] D. Fausti , R.I.Tobey, N.Dean, S. Kaiser, A. Dienst, M.C.Hoffmann, S. Pyon,T. Takayama, H. Takagi, A. Cavalleri, Science 331, 6014 (2011).
[6] R. Mankowsky, A.Subedi, M. Först, S.O Mariager, M.Chollet, H.T Lemke, J.S Robinson, J.M Glownia, M. P Minitti, A.Frano, M. Fechner
N.A Spaldin, T. Loew, B.Keimer, A.Georges, A.Cavalleri, Nature 516, 71 (2014).
* Acknowledgement(s) : the hRIXS project was funded by Helmoltz Association (Germany), European X-FEL, European Research Council,
Italian Ministry of Research (Italy), FIRI (Finland) and supported by University of Potzdam, HZB and DESY (Germany), Politecnico di Milano .

Influence of twist angle on ultrafast charge separation in WS2Graphene
heterostructures
1
1
2
2
2
3
4
N. Hofmann , J. Gradl , N. Mishra , G. Orlandini , S. Forti , C. Coletti , A. Kleiner ,S. Refaely-Abramson4, I. Gierz1
1
University of Regensburg, 93053 Regensburg, Germany
2
Istituto Italiano di Tecnologia, 56127 Pisa, Italy
3
Istituto Italiano di Tecnologia, 16163 Genova, Italy
4
Weizmann Institute of Science, Rehovot 7610001, Israel

Since the discovery of graphene in 2004 the family of 2D materials has been growing steadily suchthat, by now, a whole zoo of high-quality 2D materials with a broad range of electronic properties isreadily available. These 2D crystals can be stacked at will allowing for the fabrication of novel
artificial materials with tailored electronic properties. Crucially, due to interlayer interactions, newelectronic properties emerge that go beyond the sum of the electronic properties of the individual
layers. One commonly encountered example with great relevance for optoelectronic applications is
ultrafast charge separation following photoexcitation. The main driving force for charge separation isthe band alignment (see Fig. 1). Provided that the band alignment is favorable, charge transfer occurs
at those points in the Brillouin zone where the bands of the two participating layers hybridize and thewave function is delocalized over both layers. [1].

Fig. 1. Ultrafast charge transfer in vdW heterostructures with type I (a) and type II (b) band alignment.

Recently, is has been realized that the twist angle between the layers is a crucial parameter forcontrolling the band structure and therefor the electronic properties of various 2D van-der-Waals(vdW) stacks [2, 3]. The twist angle determines the orbital overlap between adjacent layers andthereby directly affects interlayer hybridization and – likely – non-equilibrium charge transfer.
We investigated the influence of the twist angle on ultrafast charge separation and recombination in
WS2-graphene vdW heterostructures using time- and angle-resolved photoemission spectroscopy(trARPES). We observe pronounced differences in the momentum- and energy-dependent nonequilibrium carrier and band structure dynamics for twist angles of 0° and 30°, that we interpret withthe help of density functional theory band structure calculations. Our findings suggest that adjusting
the twist angle in a vdW heterostructure can be used to tailor electron and hole transfer rates for
specific applications.
References[1] N. Hofmann, L. Weigl, J. Gradl, N. Mishra, G. Orlandini, S. Forti, C. Coletti, S. Latini, X. Lian, A. Rubio, D. Perez Paredes, R. Perea Causin,
S. Brem, E. Malic, I. Gierz, arXiv:2303.11856 (2023).
[2] Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrero, Nature 556, 43 (2018).
[3] D. M. Kennes, M. Claassen, L. Xian, A. Georges, A. J. Millis, J. Hone, C. R. Dean, D. N. Basov, A. N. Pasupathy, A. Rubio, Nature Physics 17,
155 (2021).

Atomic-level design and ultrafast THz E-field control of the emergent
Ferromagnetism at oxide interfaces
A. X. Gray
Temple University, Philadelphia, PA 19122, USA

0.5

0.5

0.0

0

0.0

-0.5

-0.5

ky (Å-1)

0.5

-0.5

-0.5 0 0.5
kx (Å-1)-0.5

Mn 3s splitting (eV)

b.

0.0

0.5

Ni 3d eg

-0.5 0 0.5
kx (Å-1)
-0.5

0.0

0.5

Mn 3s SW-XPS

4.8

c.

3

0.0
0.0

-0.5
-0.5
ILCP-IRCP
ILCP+IRCP
-1.0
-1.0

23

24

-1.5
X-rays -1.5
0.05 0.1
0.10
0.15 0.2
0.20
0.25 0.3
0.30
0.35 0.00

qz
ϴ

Bulk

25

Incidence angle(°)

26

THz

Experiment
ExperimentBest fitFitBest

qqz (1/Å)(A-1)

Mn3.86+

THz

0.5
0.5

Interface

4.6 Mn3.70+

d. s-pol
IR

1.0
1.0
Mn L3L XRR
Asymmetry
Mn
edge

CaMnO3
LaNiO3zϴKerr Rotation φK (mrad)

2 u.c. LaNiO3

8 u.c. LaNiO3

Asymmetry at 642.7 eVa.

XRR XMCD Asymmetry

Interfacial charge transfer in oxide heterostructures gives rise to a rich variety of emergent electronic
and magnetic phenomena [1]. As an example, epitaxial superlattices consisting of antiferromagnetic
CaMnO3 and paramagnetic LaNiO3 exhibit emergent ferromagnetism that can be tailored bysuppressing such charge transfer across the interface [2]. The ability to switch and tune thisphenomenon enables precise control of the ferromagnetic state and, thus, has far-reachingconsequences on the future strategies for the design of next-generation spintronic devices.
Here, we report a direct observation of the tunable character of interfacial charge transfer in
LaNiO3/CaMnO3 superlattices using a combination of the depth-resolved soft x-ray standing-wave
photoelectron spectroscopy [3], angle-resolved photoemission, and resonant magnetic x-ray
scattering. Our results establish a connection between the depletion of the Ni 3d eg states resulting in
the metal-insulator transition in LaNiO3, a charge-transfer-induced valence-state change of theinterfacial Mn cations, and the concomitant suppression of the interfacial ferromagnetic state in
CaMnO3.We then use a combination of time-resolved magneto-optic Kerr effect, optical reflectivity,
and transmissivity spectroscopies of variable-thickness LaNiO3/CaMnO3 superlattices to disentangle
multiple interrelated electronic and magnetic processes driven by ultrafast high-field THz electricfield pulses.

IR

-0.05

-0.10

-1

0

1

2

3

4

5

Time (ps)Fig. 1. (a): Angle-resolved photoemission measurements of the LaNiO3 layers in the LaNiO3/CaMnO3
superlattices grown in situ. Fermi-surface maps show depletion of the Ni 3d eg states resulting in the metalinsulating transition in ultrathin (2 u.c.) LaNiO 3. (b): Standing-wave x-ray photoelectron spectroscopy
measurements of the Mn 3s core-level multiplet splitting show charge-transfer-induced valence-state change (by0.16 e-) of the interfacial Mn cations in CaMnO3. (c): Specular angle-dependent soft x-ray reflectivity XMCD
and the best fit to the experimental data measured at T = 20 K at the resonant Mn L3 absorption thresholdenergy. The best fit model yields the magnetic interface thickness of 3.41 Å. (d): Schematics and the delay traceof the THz-pump IR MOKE probe response of the magnetic interface decomposed into several electronic andmagnetic dynamical components.

Our findings provide a new recipe for designing next-generation spintronic devices using chargetransfer and ultrafast external stimuli for efficient tuning and switching of low-dimensional
electronic and magnetic states at interfaces.
References[1] H. Chen, A. J. Millis, Journal of Physics: Condensed Matter 29, 243001 (2017).
[2] R. U. Chandrasena, C. L. Flint, W. Yang, A. Arab, S. Nemšák, M. Gehlmann, V. B. Özdöl, F. Bisti, K. D. Wijesekara, J. Meyer-Ilse, E. Gullikson
E. Arenholz, J. Ciston, C. M. Schneider, V. N. Strocov, Y. Suzuki, A. X. Gray, Physical Review B 98, 155103 (2018).
[3] C.-T. Kuo, G. Conti, J. E. Rault, C. M. Schneider, S. Nemšàk, A. X. Gray, Journal of Vacuum Science &. Technology A 40, 020801 (2022).
* Acknowledgements: authors acknowledge support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences,
Materials Sciences and Engineering Division, under Award No. DE-SC0019297.

High energy few-cycle pulses around 12 µm for nonlinear
Spectroscopy in the longwave-infrared
U. Griebner, P. Fuertjes, M. Bock, T. Elsaesser
Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, 12489 Berlin, Germany

Few-cycle pulses at wavelengths beyond 10 µm are important for fundamental studies of the nonequilibrium properties of condensed matter, i.e., solids and liquids, and exhibit a high application potential,
for example in optical materials processing. Our particular interest is on the study of molecular vibrations
and/or for performing time resolved two-dimensional infrared spectroscopy [1]. The comparably small
vibrational absorption cross sections require ultrashort pulses of sufficient energy in order to induce anonlinear vibrational response. A technological challenge, however, is the limited availability of nonlinear
crystals being transparent for the pump and the idler pulses and exhibiting high damage threshold. Thus,
there has been limited improvement of ultrashort pulse longwave-infrared (LWIR) sources over the last two
decades [2, 3]. Here we present a new light source that delivers femtosecond pulses beyond 10 µm
wavelength with record parameters. The extremely compact system is based on the concept of opticalparametric chirped pulse amplification (OPCPA). The LWIR source combines a novel type of front-end
with the technology of picosecond 2-µm Ho:YLF regenerative amplifiers. The achieved idler peak power of
0.35 GW at 11.4 µm wavelength was sufficient to demonstrate the potential of this source for ultrafast
spectroscopy beyond 10 µm in a nonlinear transmission experiment with liquid water.The OPCPA front-end
consists exclusively of a commercial Cr:ZnS oscillator (IPG) operating at a repetition rate of 79 MHz. The
oscillator provides pulses as short as 30 fs with a spectrum spanning from 1.9 µm up to 2.6 µm (30 dB-level)and 12.5 nJ energy. The emitted spectrum is sufficiently broad to seed the pump and the OPCPA at its signal
frequency in parallel. The spectral components below 2.1 µm are separated with a dichroic mirror and seed
the 2-µm pump, whereas the part above 2.1 µm is used directly as the signal input. The pump is a Ho:YLF
regenerative amplifier providing pulses of a 3 ps duration and 13 mJ energy at a 1 kHz repetition rate [4].
Prior to amplification, the signal pulses of 46 fs duration are stretched in sapphire and phase shaped by an
acousto-optic programmable dispersive filter (AOPDF). The first amplification stage is equipped with a 2
mm thick GaSe crystal, the second and the third with a 1 mm thick crystal. GaSe exhibits a high second
order nonlinearity and a comparably high damage threshold. In all stages, the type II parametric process isphase matched at an internal angle of 12.2° (external angle: 36°). In total, only 6 mJ of the available pump isused for the three OPA stages in GaSe. All stages are pumped with a fluence of about 50 GW/cm 2. In thefirst stage, the signal pulses are amplified to 14 µJ. The second collinear stage generates idler pulses of
15 µJ energy at 11.4 µm, subsequently seeding the third stage. In the last amplification stage, the noncollinearity allows for a clean separation of the pump and the idler pulses.
Fig. 1. (a) Characterization of the LWIR OPCPA pulse performance,long-term pulse stability measurement. Left inset: Far-field intensity
distribution. Right inset: Retrieved temporal pulse shape of the fewcycle pulse. (b, c) Nonlinear transmission of liquid water (12 µm thickfilm held between two transparent BaF2 windows) at the librational (L2)band (vibration indicated by the blue circular arrow). (b) L2
absorption of water (blue line) and incident (black line) andtransmitted (magenta line) spectra of the 11.4 µm pulses (energy:
25 µJ). (c) Transmission of the water sample as a function ofincident pulse energy, showing a nonlinear transmission increase.

The idler pulses are amplified to an energy exceeding 70 µJ.The spectrum has its center at 11.4 µm (877 cm) and a full width at half maximum (FWHM) of 74 cm-1 (Fig. 1b) supporting a pulse duration of 172 fs. In
principle, type II phase matching supports amplification beyond 12 µm. However, the spectral part above
13 µm is not accessible due to the group velocity mismatch in GaSe between the involved pulses. The idler
pulses are characterized by the second-harmonic generation frequency-resolved optical gating technique.
After compression in ZnSe and additional phase shaping with the AOPDF, we retrieve an idler pulse
duration of 185 fs which corresponds to less than five optical cycles of the light wave (Fig. 1a, right inset).
The compressed idler pulses have an energy of 65 µJ, corresponding to a peak power of 0.35 GW. In the
1 kHz train the pulses are highly stable with a rms-value of 1.9% and of excellent optical beam quality(Fig. 1a) [5]. The potential of this unique source for nonlinear spectroscopy was demonstrated. A
fundamental and so far unaddressed problem of liquid water research is studied, the nonlinear response oflibrational excitations. These hindered rotations of water molecules were excited to such an extent that their
optical absorption decreased significantly (Fig. 1b,c). The analysis of this absorption saturation suggests a
lifetime of the librational excitation of 20 to 30 fs [5].
1References[1] A. Ashihara, N. Huse, A. Espagne, E. T. J. Nibbering, T. Elsaesser, Journal of Physical Chemistry A 111, 743 (2007).
[2] M. Duda, L. von Grafenstein, M. Bock, D. Ueberschaer, P. Fuertjes, L. Roškot, M. Smrž, O. Novák, U. Griebner, Optics Letters 47, 2891 (2022).
[3] A. Dubietis, A. Matijošius, Opto-Electronic Advances 6, 2200460 (2023).
[4] P. Fuertjes, L. von Grafenstein, C. Mei, M. Bock, U. Griebner, T. Elsaesser, Optics Express 30, 5142 (2022).
[5] P. Fuertjes, M. Bock, L. von Grafenstein, D. Ueberschaer, U. Griebner, T. Elsaesser, Optica 9, 1303 (2022).

Quantum analog of vibration isolation, hidden order and high T
Superfluorescence in lead-halide perovskites
K. Gundogdu
North Carolina State University, Raleigh, NC 27695, USA

As the demand for quantum approaches in computing, communication, and cryptology continues tosoar, the quest for discovering new "quantum materials" has become an unparalleled necessity.
While we possess an understanding of the required quantum properties for most applications, theguidelines for designing these materials still remain unclear, and the existence of quantum materials
that function at room temperature remains scarce. A significant challenge we encounter is the limited
lifespan of quantum coherent states at temperatures that are practically relevant. The fragility of thequantum phase, attributed to thermal scattering events, presents a fundamental obstacle. This
prompts us to inquire: Do thermal processes inherently impede the design of quantum materials withextended coherence? Is there a means to safeguard quantum coherence despite the occurrence ofthermal scattering? Remarkably, we have recently made an astonishing observation of roomtemperature superfluorescence in lead-halide perovskites [1,2]. Superfluorescence (SF) arises fromthe macroscopic quantum state of optically excited dipoles, resulting in collective emission.
Previously, SF has solely been observed in materials with discrete electronic transitions, such as twolevel systems like quantum dots [3,4], defects in solids [5], or transitions between Landau levels
formed under high magnetic fields (>10T) [5]. In all these instances, SF is confined to lowtemperatures due to the rapid thermal dephasing of electronic excitations. Thus, the discovery ofroom temperature SF in hybrid perovskites with extended electronic states (bands) is highly
unexpected. This observation suggests the existence of a mechanism that protects the quantum phaseof a system against thermal disturbances in lead-halide perovskites. Here, I will introduce theQuantum Analog of Vibration Isolation (QAVI) model, which we have proposed as a mechanism toprotect the quantum phase of dipole oscillators from thermal dephasing. This model facilitates thephase transition of electronic dipoles into a superradiant state. In lead-halide perovskites, electronic
excitations give rise to polaronic lattice distortions. These distortions manifest as low-energy latticemodes with LO-phonon characteristics. In such a configuration, the oscillations of polarons due tothermal phonon interactions depend on the frequency of the polaron and the thermal phonon
involved. When a low-frequency thermal phonon scatters with the polaron, the polaron responds inphase. Conversely, when a high-frequency thermal phonon scatters with the polaron, the polaronresponds out of phase.

Fig. 1 (a) Energy levels of a particle in a box simulates the quantum oscillator. The black line is a superposition
state that oscillates within the box. The fluctuations in the box potential due to thermal phonons is illustrated aswaves scattering off the walls and lead to dephasing of the quantum oscillator. (b) Once the polaron forms,
electronic oscillator and the lattice distortion oscillator are strongly bound. This situation is illustrated as aspring that modulates the left wall in the box. Since incoming scattering phonons are molecular vibrations, they
will impact the lattice distortion mode. This situation is a quantum analogue of vibrational isolation.

This behavior effectively isolates the dipole from high-frequency thermal modes. In thispresentation, I will share our studies conducted on lead-halide perovskites with different halide
compositions.
References[1] M. Biliroglu, G. Findik, J. Mendes D. Seyitliyev, L. Lei,Q. Dong,Y. Mehta,V.V. Temnov,F. So, K. Gundogdu, Nature Photonics 16, 324 (2022).
[2] M. Findik, M. Biliroglu, D.Seyitliyev, J.Mendes, A. Barrette, H. Ardekani, L. Lei,Q. Dong, F. So, K.Gundogdu, Nature Photonics 15, 676 (2021).
[3] G. Rainò, M.A. Becker, M.I. Bodnarchuk, R.F. Mahrt, M.V. Kovalenko, T.Stöferle, Nature 563, 671 (2018).
[4] L. Phuong, K. Miyajima, K. Maeno, T. Itoh, M. Ashida, Journal of Luminescence 133, 77 (2013).
[5] G. T. Noe II, J.-H. Kim, J. Lee, Y. Wang, A.K. Wójcik, S.A. McGill, D.H. Reitze,A.A. Belyanin, J.Kono, Nature Physics 8, 219(2012).

Multi-photon absorption in direct-gap and indirect-gap
Semiconductors
N. Cox, S. Faryadras, E. W. Van Stryland, D. J. Hagan
University of Central Florida, Orlando FL 32816, USA

Semiconductors have long been known to exhibit large two photon absorption (2PA) coefficients. It
has also been shown that for extremely nondegenerate pairs of photons (energy ratios ofapproximately 10:1) the 2PA in direct gap semiconductors (e.g. GaAs, CdTe, ZnSe, ZnO, GaN) is
enhanced over the degenerate value by up to 3 orders of magnitude [1]. This large nondegenerate(ND) 2PA has been shown be desirable for mid infrared detection, providing sensitive Mid-IR
detection with uncooled, large bandgap semiconductors [2]. Large 2PA coefficients also correspondto large two-photon gain coefficients under population inversion, enabling the possibility of twophoton gain and lasing [3]. This nondegenerate enhancement has been shown to be even larger inquantum well systems, as demonstrated experimentally by Cox, et. al., [4]. In that paper, awaveguide geometry was used to allow propagation of TM waves, with electric field transverse to
the waveguide, this allowing for allowed inter-subband transitions. In figure 1, we show theory andexperimental results for a GaAs/AlGaAs quantum well structure using pump radiation at 1960 nm
and a probe tunable between 1176 and 1326 nm.

Fig. 1. TM-TM ND-2PA coefficients vs sum wavelength (nm). Black circles: measured coefficients withtotal pulse energy
Black triangles: measured coefficients with pulse energy
Solid line:
theory with 14-band basis. Dashed line: theory convolved with 4 nm Gaussian to approximate bandwidth
effects.

The ND2PA coefficients measured in Fig. 1 are not particularly large, but this particular experimentis not strongly nondegenerate. More nondegenerate experiments, with much longer pumpwavelengths, are underway, but these are rater challenging due to the need to guide two waves ofvery different wavelength. Experiments with These results simply provide experimental proof of thetheory. For much larger nondegeneracy, as the smaller photon energy approaches the inter-subband
resonance, we anticipate that the ND2PA coefficients will grow rapidly, as shown in Fig. 2.
15000

10000

5000

0
5.5

6

6.5

7

7.5

8

8.5

Fig. 2. Maximum calculated TM-TM ND-2PA coefficient vs pump wavelength (nm) for the same
quantum well structure used for Figure 1.

With quantum well structure, we anticipate an enhancement of the 2PA coefficient of approximately
a factor of 10 compared to bulk, and approximately a factor of 1000 over the degenerate case. While
direct gap semiconductors have many advantages in photonics, Silicon is of practical interest forphotonics applications due to its low cost and possibility for integration with electronics. However,its indirect bandgap provides a challenge in theoretical modeling since 2PA requires a phonon
scattering process. This additional process means that there are many more possible transition
schemes for 2PA in an indirect gap semiconductor, than for the direct-gap case.

Fig. 3. Diagrams for different transitions across the band structure of silicon, including, (a) allowedforbidden, (b) allowed-allowed, and (c) another allowed-allowed transition.

Despite this, the degenerate 2PA scaling in Silicon has been shown to be comparable to that of direct
gap semiconductors with similar band gap energy. [5] As with direct gap semiconductors, theseenhancements indicate the applicability of using nondegenerate 2PA as possible method for ultrafast,
gated mid-IR detection using off-the-shelf Silicon photodiodes. It has also been shown that Mid-IR
imaging can be achieved via ND2PA in Silicon cameras. [6] We present a model for nondegenerate
2PA (ND-2PA) in indirect semiconductors that is nearly identical to the direct gap theory with aphonon transition added as a perturbation step. We also experimentally investigate the enhancementof ND-2PA in Silicon for the extremely nondegenerate case using a femtosecond pump-probe
arrangement, with several different Mid-IR pump wavelengths. In these experiments, thetransmittance of a tunable near infrared probe pulse is monitored in the presence of a mid-IR pumppulse generated by difference frequency generation while varying the relative delay between pumpand probe pulses. Our experimental results reveal that the 2PA enhancements due to nondegeneracy
are similar to those seen in direct-gap semiconductors. However, the very low density of states near
the indirect band edge is likely to restrict the enhancement for highly nondegenerate cases.
Additionally, this nondegenerate 2PA data sheds light on which of the many transition schemes are
dominant in the 2PA process in Silicon, something which up until now has remained somewhat
unknown.
References[1] C. M. Cirloganu, L. A. Padilha, D. A. Fishman, S. Webster, D. J. Hagan, E. W. Van Stryland, Optics Express 19, 22951 (2011).
[2] D. A. Fishman, C. M. Cirloganu, S. Webster, L. A. Padilha, M. Monroe, D. J. Hagan, E. W. Van Stryland, Nature Photonics 5, 561 (2011).
[3] M. Reichert, A. L. Smirl, G. Salamo, D. J. Hagan, E. W. Van Stryland, Physical Review Letters 117, 073602 (2016).
[4] N. Cox, J. Wei, H. Pattanaik, T. Tabbakh, S.-P. Gorza, D. Hagan, E. W. Van Stryland, Physical Review Research 2, 013376 (2020).
[5] A. D. Bristow, N. Rotenberg, H. M. VanDriel, Applied Physics Letters 90, 191104 (2007).
[6] D. Knez, A. M. Hanninen, R. C. Prince, E. O. Potma, D. A. Fishman, Light: Science and Applications 9, 125 (2020).

Ultrafast spectroscopic studies of topological quantum matter
M. Z. Hasan, I. Belopolski, S.-Y. Xu, M. Neupane, T. A. Cochrane, B. Kim
Princeton University, Princeton, NJ 08544, USA

Over the last decade, ultrafast techniques have been crucial to gain fundamental insights into the
topological properties of quantum materials. We employed ultrafast time-resolved angle-resolved
photoemission spectroscopy to probe and elucidate surface electronic structure and electron
dynamics of Z2 topological insulators, Dirac-Weyl semimetals, knotted quantum matter and exotic
spin-orbit superconductors [1, 2, 3].
In this talk, I plan to present a few recent examples where ultrafast techniques have been crucial to
discover the topological behavior: (1) It was long been speculated that MoxW1−xTe2 materials may
realize Lorentz violating topological Weyl fermions. In order to explore its topology, we first showed
theoretically that it is crucial to access the full electron dynamics to reveal a non-Lorentz electronic
Weyl state in MoxW1−xTe2. Then, we experimentally explored MoxW1−xTe2 materials by a suite ofpump-probe time-resolved ARPES techniques. By comparing our pump-probe results with abinitio simulations, we concluded that the materials indeed features topological states associated with
non-trivial Berry curvature field which is the generator of the non-Lorentz Weyl topology and nontrivial Chern quantized numbers [4]. (2) In candidate topological semimetal class TaIr(Te/Se)4, theWeyl points and Fermi arcs live entirely above the chemical potential level, making theminaccessible to conventional ARPES. We utilized ultrafast ARPES to directly access the electronic
states above the Fermi level in TaIrTe4. We observe signatures of Berry curvature singularity nodes
and correspondingly coupled spin textured Fermi arcs states. A full map reveals, for the first time,that TaIrTe4 is a topological metal (doped semimetal) with the minimum number of four Weyl nodes
allowed by time-reversal symmetry [3]. (3) The third example would be on the possibility of novel
phenomena induced by light. Here, we used pump-probe photoemission spectroscopy to explore theoptically excited Dirac surface states in the bulk-insulating topological insulator Bi2Te2Se.

Our results revealed optical and ultrafast properties that are in sharp contrast to those of bulkmetallic (self-doped) topological insulators. We observed a gigantic optical lifetime
exceeding 4 μs for the surface states in Bi2Te2Se, whereas the lifetime in most topological insulators,
such as Bi2Se3, has been limited to a few picoseconds. Moreover, we discovered a surface
photovoltage, a shift of the chemical potential of the topological surface states, as large as 100 mV.
Our results demonstrate a rare quantum platform to explore charge excitation and relaxation inenergy and momentum space in two-dimensional systems [5]. (4) Finally, (time permitting) I will

talk about electronic structure and relaxation dynamics in a superconducting topological material.
Topological superconductors host new states of quantum matter which show a pairing gap in thebulk and gapless surface states providing a platform to realize Majorana zero modes. Recently,
alkaline-earth metal Sr intercalated Bi2Se3 has been reported to show superconductivity with a
Tc ~ 3 K and a large shielding fraction. We report systematic electronic structure studies of
Sr0.06Bi2Se3 (Tc ~ 2.5 K). Using ARPES, we observed a quantum well confined two-dimensional
state coexisting with a topological surface state in Sr0.06Bi2Se3. Furthermore, our time-resolved
ultrafast ARPES reveals the relaxation dynamics showing different decay mechanism between the
excited topological surface states and the two-dimensional states. Our experimental observation isunderstood by considering the intra-band scattering for topological surface states and an additional
electron phonon scattering for the 2D states, which is likely responsible for the superconductivity
and topological phenomena [5-7].These ultrafast studies and their future extensions will be helpful inunderstanding low temperature superconducting states realized in these topological materials [6, 7].
References[1] M. Z. Hasan, C. L. Kane, Review of Modern Physics 82, 3045 (2010).
[2] D. N. Basov, R. D. Averitt, D. Hsieh, Nature Materials 16, 1077 (2017).
[3] I. Belopolski, P. Yu, D. Sanchez, Y. Ishida, T. Chang, S. Zhang, S. Xu, H. Zheng, G. Chang, G. Bian, H.-T. Jeng, T. Kondo, H. Lin
Z. Liu, S. Shin, M. Z. Hasan, Nature Communications 8, 942 (2017).
[4] I. Belopolski, S. Xu, Y. Ishida, X. Pan, P. Yu, D. Sanchez, H. Zheng, M. Neupane, N. Alidoust, G. Chang, T. Chang, G. Bian, S. Huang,C. Lee
D. Mou, L. Huang, Y. Song, B. Wang, G. Wang, Y. Yeh, Nan Yao, J. Rault, P. Le Fèvre, F. Bertran, H. Jeng, T. Kondo, A. Kaminski, H. Lin
Z. Liu, F. Song , S. Shin, M. Z. Hasan, Physical Review B 94, 085127 (2016).
[5] M. Neupane, S.-Y. Xu, Y. Ishida, S. Jia, B. Fregoso, C. Liu, I. Belopolski, G. Bian, N. Alidoust, T. Durakiewicz, V. Galitski, S. Shin, R. J. Cava
M. Z. Hasan, Physical Review Letters 115, 116801 (2015).

K-dependent band gap renormalization in a
WS21-graphene
Heterostructure
2
3
3
3
4

N. Hofmann1, A. Steinhoff , J. Gradl , N. Mishra , G. Orlandini , S. Forti , C. Coletti , S. Refaely-Abramson5
T. Wehling6, I. Gierz1
1
University of Regensburg, 93053 Regensburg, Germany
2
University of Bremen, 28359 Bremen Germany
3
Istituto Italiano di Tecnologia, 56127 Pisa, Italy
4
Istituto Italiano di Tecnologia, 16163 Genova, Italy
5
Weizmann Institute of Science, Rehovot 7610001, Israel
6
Institute for Theoretical Physics, University of Hamburg, Germany

Monolayer transition-metal dichalcogenides show strong enhancement of Coulomb interactions dueto their reduced dimensionality with immediate effects on both the optical as well as the singleparticle band gap. Photogenerated electron-hole pairs have been shown to result in a giant band gap
renormalization that has been attributed to efficient screening of the Coulomb interaction [1]. The
corresponding band structure changes are predicted to show a pronounced momentum dependence[2] that we resolve using time- and angle-resolved photoemission spectroscopy (trARPES) on
monolayer WS2 supported by a graphene substrate (see Fig. 1).
Fig. 1 Momentum-dependence of
bandgap
renormalizationofmonolayer WS2 on epitaxial grapheneon SiC(0001) investigated by trARPES.
(a) Equilibrium band structure.
(b)
Pump-inducedchangesofthephotocurrent
240fs
after
photoexcitation with 2eV pump pulses.
(c) Momentum-dependent shift ofupper valence band of WS2 for
excitation at K (orange) and 𝑄 (blue),respectively.

We find good agreement with ab initio calculations for excitation across the direct band gap at the K
point. When excited close to the Q valley, however, we observe a strong k-dependence of the valenceband shift that is not reproduced by theory. This implies that correlations beyond the GW
approximation are at play.
References[1] M. M. Ugeda, A. J. Bradley, S.-F. Shi, F. H. da Jornada, Y. Zhang, D. Y. Qiu, W. Ruan, S.-K. Mo, Z. Hussain, Z.-X. Shen, F. Wang, S. G. Louie,
M. Crommie, Nature Materials 13, 1091 (2014).
[2] D. Erben, A. Steinhoff, C. Gies, G. Schönhoff, T. O. Wehling, F. Jahnke, Physical Review B 98, 035434 (2018).

Birth, rise, and collapse of Floquet-Bloch bands on
Subcycle time scales
U. Höfer
Philipps-University of Marburg, 35032 Marburg, Germany

Strong light fields offer spectacular opportunities to tailor novel functionalities of solids. FloquetBloch states can form under periodic driving of electrons and enable exotic quantum phases. On
subcycle time scales, lightwaves can simultaneously drive intraband currents and interband
transitions and pave the way towards ultrafast electronics. Yet, the interplay of intra- and interband
excitations as well as their relation with Floquet physics have been the key to open questions as
dynamical aspects of Floquet states that have remained elusive. By monitoring the ultrafast build-up
of Floquet-Bloch bands with time- and angle-resolved photoemission spectroscopy, we succeeded inproviding this pivotal link.For this purpose, we extend our subcycle ARPES experiment [1] to thestrong-field regime. We drive surface states of the topological insulator Bi2Te3 with MIR pumppulses of 25-40 THz and electric field strengths in the MV/cm range. Under these conditions, thecarrier wave of the light ballistically accelerates the electrons in the Dirac cone back and forth, which
gives rise to high-harmonic generation [2]. Our subcycle ARPES experiment reveals how FloquetBloch sidebands build up in the Dirac state in this regime. Starting with strong intraband currents, weobserve that Floquet replicas emerge already in the second optical cycle, and that electrons in highorder sidebands scatter into bulk states and dissipation destroys the Floquet bands. Quantum
nonequilibrium calculations confirm this behavior and explain the simultaneous occurrence of
Floquet states with intra- and interband dynamics [3].On the experimental side, several challenges
have to be overcome in order to obtain results as the ones depicted in Fig. 1. Firstly, the oscillating
electric field of the lightwave does not only accelerate electrons in the material but also interacts withphotoelectrons and results in momentum streaking of up to 0.1 A-1. Analysis of the photoelectron
streaking enables in situ reconstruction of the MIR electric field (Fig. 1b). Streaking compensated
ARPES spectra then reveal the intrinsic electron dynamics of the Dirac state. Secondly, the driving
frequency corresponds to optical cycles between 25 and 40 fs; thus ultrashort probe pulses which
inevitably broaden the ARPES spectra are necessary to achieve subcycle resolution. By utilizing
curvature-based image processing, we have succeeded in extracting the fine signatures induced bylight under these conditions.
Fig. 1. a: Schematics of subcycle ARPES
with the MIR pump field and the ultrashort
3-eV 17-fs 2PPE probe pulse. b:
Photoelectron momentum-streaking trace ofthe 25 THz driving field; thin vertical lines
indicate temporal positions of the ARPES
maps displayed in c. c: Curvature-filtered
lightwave ARPES maps recorded before thearrival of the MIR field (t = -100 fs) and atselected delay times during the first half ofthe driving pulse; the images arecompensated for the streaking shifts in b.
The electronic distributions are initiallyaccelerated along the ground-state band
dispersion, but subsequently divert from it,
splitting into multiple branches that follow
approximately the TSS band structure shiftedby integer multiples of the driving frequency(thin grey curves).Up to 100 fs before themaximum field crest, the effect of the MIR
lightwave on the TSS is negligible. The
ARPES image shows a partially filled Diraccone (Fig. 1c). 40 fs later (t = -60 fs), theelectric field has induced a strong
asymmetry in the electronic distribution
along the momentum axis, the hallmark oflightwave-driven currents. Most remarkably,a distinct splitting of electronic populations
into multiple branches appears only one halfof an oscillation cycle later (t = -40 fs). Simultaneously, the electron population oscillates back and forth along
the momentum axis while the driving field evolves. The split bands notably follow the ground-state band structure
shifted by integer multiples of ℏΩMIR= 0.1 eV (Fig. 1c, thin grey curves). As the field increases, the band splitting
becomes more prominent and the probability to find the electrons shifts entirely to the upper branch near the field
crest (Fig. 1c, t = -18 fs).

Our observation is surprising because the conventional Floquet picture assumes multi-cycle
lightwaves to form sidebands by coherent superposition of electronic states, whereas our sub-cycle
experiments identify Floquet-Bloch states already within the first two cycles of the MIR pulse.
Quantum nonequilibrium calculations are in excellent agreement with this behavior and describe thesimultaneous occurrence of Floquet states with intraband dynamics very well [3].Close to the
maximum of the field crest (t = -12 fs, not shown) and beyond, we observe transition of electrons
into the bulk conduction band via its hybridization with split-off bands. The interplay between theintraband currents and the non-perturbative interband transition underlies various strong-field
phenomena such as HHG. Our subcycle ARPES in the strong-field regime marks the firstexperimental visualization of the microscopic processes in momentum space.In conclusion, we truly
visualized the birth, rise, and collapse of Floquet-Bloch states in a topological insulator. By
introducing subcycle ARPES in the MIR, we directly monitor in momentum space how thelightwave-driven surface state of Bi2Te3 crosses over from an initial regime of deterministic quantum
trajectories to a Floquet-Bloch band structure resulting from multi-path quantum interference. The
dynamics unify the particle aspect of electrons describing subcycle acceleration with the wave-likeinterference of electrons forming light-induced sidebands.
References[1] J. Reimann, S. Schlauderer, C. P. Schmid, F. Langer, S. Baierl, K. A. Kokh, O. E. Tereshchenko, A. Kimura, C. Lange, J. Güdde, U. Höfer
R. Huber, Nature 562, 396 (2018).
[2] C. P. Schmid, L. Weigl, P. Grössing, V. Junk, C. Gorini, S. Schlauderer, S. Ito, M. Meierhofer, N. Hofmann, D. Afanasiev, J. Crewse, K. A. Kokh
O. E. Tereshchenko, J. Güdde, F. Evers, J. Wilhelm, K. Richter, U. Höfer, R. Huber, Nature 593, 385 (2021).
[3] S. Ito, M. Schüler, M. Meierhofer, S. Schlauderer, J. Freudenstein, J. Reimann, D. Afanasiev, K. A. Kokh, O. E. Tereshchenko, J. Güdde
M. A. Sentef, U. Höfer, R. Huber, “Build-up and dephasing of Floquet-Bloch bands on subcycle timescales”, Nature (in print), (2023)
DOI: 10.1038/s41586-023-05850-x.

Ultrafast X-ray photoelectron spectroscopy
P. Hofmann
Aarhus University, 8000 Aarhus C, Denmark

X-ray photoelectron spectroscopy (XPS) is an experimental tool capable of accurately determining
the core level binding energies of atoms. This energy is not only element-specific but also provides
detailed information on the atoms' oxidation state and chemical environment, making XPS an
essential tool for studying catalytic processes. Indeed, the technique is so important that recent
experimental developments have pushed it into regimes for which it was long deemed unsuitable,e.g., the presence of near-ambient-pressure gases or liquids. Due to the relative simplicity of the corelevel photoemission process, XPS line shapes have attracted considerable theoretical interest andmany-body theories to study the solid’s electronic and vibrational many-body response to theremoval of the photoelectron have been developed since the 1970s [1]. Finally, the core level
photoemission intensity can be interpreted as a diffraction pattern that gives access to the emitting
atom’s local geometrical environment in an experimental approach called X-ray photoelectron
diffraction. The increasing availability of ultrafast X-ray sources at free electron lasers now opens
the opportunity to take XPS into the ultrafast time domain and this talk will give recent examples:
The extension of the conventional line shape model to account for the electronic temperature inlaser-excited graphene [2], the excitonic Mott transition in WSe2 [3], and preliminary data for excited
WTe2. The graphene case is briefly presented in this abstract for illustrative purposes.
Fig. 1: (a) C 1s photoemission intensity as
a function of pump-probe delay andbinding energy. (b) Same as in (a) but withthe average spectrum before excitation
subtracted. Red and blue indicate anincrease and decrease in electron counts,
respectively. (c) Selected spectra (cuts atspecific time delays) along with best fit to amodel taking into account the electronic
temperature (blue line). The electronic
temperatures resulting from the fit are
noted close to the spectra. [2].

For graphene, it is shown that the time-dependent XPS line shape can reveal detailed insight into theexcitation of the system, directly giving access to parameters such as the electronic temperature [2].
After excitation with an optical pump pulse, the electronic temperature can rise to several thousand
Kelvin and this is usually probed directly by the broadening of the Fermi edge in the valence band[4]. In the core level spectra, an ultrafast laser excitation gives rise to a time-dependent broadening,as shown in Fig. 1. It turns out that the origin of this broadening is an increased scattering with the
hot electron gas and, upon expanding the conventional linewidth expression for XPS, data from theC 1s peak can be used to track the time-dependent electronic temperature. This is illustrated in Fig. 2
which shows a comparison of the electronic temperature determined by XPS and by the Fermi edgein the valence band.
Fig.2:Electronic temperature in graphene as afunction of delay time between an optical pump pulse
and an X-ray probe pulse. The markers show theresults from the C 1s XPS line shape. The grey line
shows the temperature obtained from the width of the
Fermi edge in the valence band [2].

Temperature-induced broadening effects are very common in core level spectra and usually causedby phonon scattering [1]. Broadening and line shape change due to a high electronic temperature, on
the other hand, have been predicted a long time ago [5] but never observed because for samples inthermal equilibrium, the phonon-induced effects are usually much larger than the electronic effects.
The ultrafast time scale, however, now offers a window on completely new mechanisms affecting
XPS line shapes.
References[1] P. H. Citrin, G. K. Wertheim, Y. Baer, Physical Review B 16, 4256 (1977).
[2] D. Curcio, S. Pakdel, K. Volckaert, J. A. Miwa, S. Ulstrup, N. Lanatà, M. Bianchi, D. Kutnyakhov, F.Pressacco, G. Brenner, S. Dziarzhytski
H. Redlin, S. Agustsson, K. Medjanik, D. Vasilyev, H.-J. Elmers, G. Schönhense, C. Tusche, Y.-J. Chen, F. Speck, T. Seyller, K. Bühlmann
R. Gort, F. Diekmann, K. Rossnagel, Y. Acremann, J. Demsar, W. Wurth, D. Lizzit, L. Bignardi, P. Lacovig, S. Lizzit, C. E. Sanders,P. Hofmann, Physical Review B 104, L161104 (2021).
[3] M. Dendzik, R. P. Xian, E. Perfetto, D. Sangalli, D. Kutnyakhov, S. Dong, S. Beaulieu, T. Pincelli, F. Pressacco, D. Curcio, S. Y. Agustsson
M. Heber, J. Hauer, W. Wurth, G. Brenner, Y. Acremann, P. Hofmann, M. Wolf, A. Marini, G. Stefanucci, L. Rettig, Ralph Ernstorfer
Physical Review Letters 125, 096401 (2020).
[4] J. C. Johannsen, S. Ulstrup, F. Cilento, A. Crepaldi, M. Zacchigna, C. Cacho, I. C. E. Turcu, E. Springate, F. Fromm, C. Raidel, T. Seyller
F. Parmigiani, M. Grioni. P. Hofmann, Physical Review Letters 111, 027403 (2013).
[5] S. Satpathy, J. D. Dow, Solid State Communications 42, 637 (1982).

Cavity-enhanced optical single-shot readout of a quantum dot
Spinstate onnanosecond
timescales
1
1
1
1
1
2
2

M.R. Hogg , N.O. Antoniadis , W.F. Stehl , A. Javadi , N. Tomm , R. Schott , S.R. Valentin , A.D. Wieck2
A. Ludwig2, R.J. Warburton1
1
University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
2
Ruhr-Universität Bochum, 44780 Bochum, Germany

The ability to perform a projective measurement of a quantum state in a single measurement iteration(single-shot readout) is an enabling technique in quantum technologies. Single-shot readout isnecessary in quantum computation protocols for extracting information at the end of thecomputation, as well as error detection and correction as the quantum processor runs [1]. Single-shot
qubit readout is also essential to close the fair sampling loophole in fundamental tests of quantum
non-locality [2]. Achieving single-shot readout within the qubit dephasing time enables experiments
such as measurement-based quantum feedback [3]. The spin states of optically active III-V
semiconductor quantum dots (QDs) are a promising resource for quantum technologies; however
single-shot readout has proven challenging due to low photon collection efficiencies and back-action
induced by the readout laser. Of the small number of previous experiments to achieve single-shotreadout of InAs QD spin states, the most rapid to date achieved a fidelity of 82% in a readout time of
800 ns [4], significantly longer than the maximum spin T2* times achieved to date (125 ns [5]).

Fig. 1. Tunable microcavity and singleshot spin readout.
(a) Experimental setup. Fast opticalreadout pulses are created using anelectro-optic modulator (EOM) driven by
an arbitrary waveform generator (AWG).
(b) Spin-selective cavity enhancement of
quantum dot emission.
(c) Repeated single-shot measurements of
the quantum dot spin state, showing
bright and dark periods due to quantum
jumps in the QD spin state.

Here we achieve single-shot readout of an InAs quantum dot spin state with a fidelity of 95.2% in
only 3 nanoseconds [6], an improvement of more than two orders of magnitude and well within theachievable T2* time. Our approach uses a miniaturised Fabry-Perot microcavity to significantly
enhance the spin readout signal. To the best of our knowledge, this is the fastest single-shot readoutof a matter qubit ever achieved. Together with the recent demonstration of an on-demand singlephoton source with a record 57% efficiency [7], our work provides a promising pathway towards thegeneration of high-rate spin-photon entanglement and photonic cluster states.
References[1] A. G. Fowler, M. Mariantoni, J.M. Martinis, A.N. Cleland, Physical Review A 86, 032324 (2012).
[2] B. Hensen, H. Bernien, A. Dréau, Nature 526, 682 (2015).
[3] P. Campagne-Ibarcq, E. Flurin, N. Roch, D. Darson, P. Morfin, M. Mirrahimi, M. H. Devoret, F. Mallet, B. Huard, Physical Review X 3, (2013).
[4] A. Delteil, W. Gao, P. Fallahi, J. Miguel-Sanchez, A. Imamoğlu, Physical Review Letters 112, 116802 (2014).
[5] D. M. Jackson, U. Haeusler, L. Zaporski, J. H. Bodey, N. Shofer, E. Clarke, M. Hugues, M. Atatüre, C. Le Gall, D. A. Gangloff, Physical Review
X 12, 031014 (2022)[6] N. O. Antoniadis, M. R. Hogg, W. F. Stehl, A. Javadi, N. Tomm, R. Schott, S. R. Valentin, A. D. Wieck, A. Ludwig, R. J. Warburton,arXiv:2210.13870 (2022)[7] N Tomm, A. Javadi, N. O. Antoniadis, Nature Nanotechnology 16, 399 (2021)* Acknowledgement(s) : We acknowledge financial support from Horizon-2020 FET-Open Project QLUSTER, Swiss National Science
Foundation project 200020 204069, and NCCR QSIT. A.J. acknowledges support from the European Union Horizon 2020 Research andInnovation Programme un- der Marie Sklodowska-Curie grant agreement no. 840453 (HiFig). S.R.V., R.S., A.L. and A.D.W. gratefully
acknowledge support from DFH/UFA CDFA05-06, DFGTRR160, DFG project 383065199 and BMBF Q.Link.X.

Nonthermal melting of a charge density wave in atomic wires at the
Quantum limit
M. Horn von Hoegen
Duisburg-Essen University,47057 Duisburg, Germany

In this talk, we present experimental results on the structural dynamics in the prototypical indium
atomic wire system formed by self-assembly on Si(111). This surface system under-goes a Peierlslike symmetry breaking at Tc = 130 K resulting in the formation of a surface charge density wave.
During this thermally driven first-order structural phase transition the system doubles the periodicity
along and perpendicular to the wires resulting into a (8×2) reconstructed ground state [1]. The
symmetry breaking is accompanied by the opening of a band gap of 0.2 eV and thus constitutes ametal to insulator transition. The insulating (8×2) ground state is composed of isolated distorted In
hexagons which transform in parallel zig-zag chains of In atoms in the high temperature (4×1) state
as is sketched in Fig. 1(a, c).

Fig. 1: (a,c) RHEED patterns at negative time delay prior to and 6 ps after excitation, respectively. Diffraction
spots indicative for the ground (82) and (41) excited state are marked. Spot profiles show the intensity loss
and gain for (82) and (41) spots, respectively. (b) The difference pattern clearly depicts the systematic
intensity losses (blue) for the features indicative for the (8 2) ground state, i.e., spots and streaks, and gains(red) for the features indicative for the (41) excited state. (d) The intensity evolution of the (00) spot for thetransition from (82) ground to (41) excited state occurring in only 0.7 ps. No ringing of the intensity isobserved.

The non-equilibrium structural dynamics of these arrays of In atom wires upon impulsive excitation
by fs-IR laser pulses is probed through ultra-fast time resolved reflection high energy electron
diffraction (RHEED) under grazing incidence at a temporal resolution of 350 fs [2]. The sample isheld in the (8×2) ground state at 30 K, i.e., way below Tc . The optical excitation through the 80 fs
laser pulse at an incident laser fluence higher than  > 2 mJ/cm2 results in a non-equilibrium
population of the electronic surface states which causes transient changes of the potential energy
surface [3] as is sketched in Fig. 2. As a result, an accelerated displacive structural transition to the(4×1) excited state is driven. Since we do not observe any oscillatory behavior of the diffracted
intensity we assume a critically damped behavior which is completed in only 700 fs [3,4]. The large
surface unit cell together with the dynamically changing phononic band structure lead to an
extremely effective coupling of the degrees of freedom for the kinetic energy of the In atoms towards
the unexcited and cold Si substrate.

Fig. 2: (a,b) Transient change of 3-valley potential energy landscape upon optical excitation through fs-laser
pulse. (b,c) Accelerated displacive transition to excited state in less than a ps. (d) Energy dissipation throughphonon emission into the substrate at 6 ps. (e) Trapped in metastable supercooled state for ns.

Transient heating of the In atoms from 30 to 60...80 K occurs delayed at 6 ps [5]. Thus, the phasetransition is driven by electronic entropy and not thermally. A weak energy barrier of 60 meV for theatoms collective motion from the excited (4×1) state to the (8×2) state hinders the immediate
recovery of the ground state: the In layer remains for many nanoseconds in a super cooled metastable(4×1) state, which is not accessible under equilibrium conditions. The relaxation into the (8×2)
ground state happens through the nucleation of the (8×2) at pre-existing defects like adsorbates [6] or
step edges [7] which act as seeds and trigger a recrystallization front. This front propagates 1dimensionally at a speed of ~100 m/s as determined from a correlation of the step morphology torecovery time constant [7].

Fig. 3: (a) Intensity drop of the (00) diffraction spot at various fluences. (b) The residuum after subtraction ofan exponential decay from (a) clearly exhibits one oscillation maxima at t (8×2) = 1.7 ps. (c) Under weakexcitation conditions  < 2 mJ/cm2 the amplitudon mode of the CDW is excited. (d) Ab initio molecular
dynamics simulations show, that the system does overcome the reaction barrier and consequently not reach theexcited (4×1) state. (e,f) Sketch of the two soft phonon modes transferring the surface system from (8×2) to(4×1), i.e., the hexagonal rotation mode at 0.82 THz and the shear mode at 0.54 THz.

Under weak excitation conditions at a fluence lower than  < 2 mJ/cm2 only part of the surface istransformed to the excited high temperature (4×1) state. The recovery to the ground state then occurs
in ~50 ps independent of density of adsorbates or step edges. From this and transient broadening ofthe (8×2) diffraction spots we can conclude, that the surface is composed of small patches of excited(4×1) units with a length of ~10 nm embedded in the (8×2) reconstructed groundstate surface.
From the untransformed (8×2) patches we observe one single intensity oscillation at t(8×2) ~ 1.7 ps ofthe (00)-spot which we attribute to the excitation of the amplitudon mode of the surface charge
density wave at the characteristic frequencies 0.82 THz and 0.54 THz of the surface localized
hexagonal rotational and shear soft-phonon modes facilitating the transformation from (8×2) to (4×1)[8,9].
References[1] F. Klasing, T. Frigge, B. Hafke, S. Wall, A. Hanisch-Blicharski, M. Horn-von Hoegen, Phyical Review B 89, 121107(R) (2014)[2] T. Frigge, B. Hafke, T. Witte, C. Brand, M. Horn-von Hoegen, Review of Scientific Instruments 90, 045119 (2019).
[3] T. Frigge, B. Hafke, T. Witte, B. Krenzer, C. Streubühr, A. Samad Syed, V. Mikšić Trontl, I. Avigo, P. Zhou, M. Ligges, D. von der Linde
U. Bovensiepen, M. Horn-von Hoegen, S. Wippermann, A. Lücke, S. Sanna, U. Gerstmann, W. G. Schmidt, Nature 544, 207 (2017).
[4] C.W. Nicholson A Lücke, W G Schmidt, M Puppin, L Rettig, R Ernstorfer, M Wolf, Science 362, 821 (2018).
[5] T. Frigge, B. Hafke, T. Witte, M. Horn-von Hoegen, Structural Dynamics 5, 025101 (2018)[6] S. Wall, B. Krenzer, S. Wippermann, S. Sanna, F. Klasing, A. Hanisch-Blicharski, M. Kammler, W.G. Schmidt, M. Horn-von Hoegen,Physical Review Letters 109, 186101 (2012)[7] B. Hafke, T. Witte, T Frigge, D. Janoschka, P. Dreher, M. Horn von Hoegen, Structural Dynamics 6, 045101 (2019)[8] E. Speiser, N. Esser, S. Wippermann, W. G. Schmidt, Physical Review B 94, 075417 (2016)[9] S. Wippermann, W.G. Schmidt, Physical Review Letters 105, 126102 (2010).
* Acknowledgement: M. Horn von Hoegen acknowledges financial support from DFG through project B02 of Collaborative Research Center
SFB616 "Energy dissipation at surfaces” and project C03 of Collaborative Research Center SFB1242 "Nonequilibrium dynamics of condensed
matter in the time domain" (Project-ID 278162697).

Transient gap generation in BaFe2As2 driven by
Coherentlatticevibrations
1
1
2
3
1

J. A. Warshauer , D. A. B. Lopez , Q. Dong , G. Chen , W. Hu
1
Boston University, Boston, MA 02215, USA
2
Institute of Physics Chinese Academy of Sciences, Beijing 100190, China
3
University of Chinese Academy of Sciences, Beijing 101408, China

Iron-based superconductors provide a rich platform to investigate the interplay between
unconventional superconductivity, nematicity and magnetism. The electronic structure and themagnetic properties of iron-based superconductors are highly sensitive to the pnictogen height.
Coherent excitation of the A1g phonon by femtosecond laser directly modulates the pnictogen height,
which has been used to control the physical properties of iron-based superconductors. Previous
studies show that the driven A1g phonon resulted in a transient increase of the pnictogen height in
BaFe2As2, favoring an enhanced Fe magnetic moment. However, there are no direct observations oneither the enhanced Fe magnetic moments or the enhanced spin-density wave (SDW) gap. Here, we
use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFe2As2 in the
A1g phonon driven state. Below the SDW transition temperature, we observe a transient gap
generation at early time delays. A similar transient feature is observed in the normal state up to room
temperature.

Fig. 1. (a): The real part of optical conductivity σ1(ω) at equilibrium (grey) and the photo-excited state (red) at
T = 75 K. The transient σ1(ω) shows a broadened Drude component and a gap formation near 50 meV, whichtakes the spectral weight from the equilibrium SDW gap at 45 meV. (b) Coherent excitation of the A1g phonon
modulates the pnictogen height hAs. (c): Transient change in optical conductivity ∆σ1(ω) at T = 75 K. The
transient gap is seen as the blue region at around 40 meV at early time delays. (d): Different time scales for thelight-induced Drude component (solid squares) and the transient gap (open circles) are evidenced by the time
evolution of the transient change in spectral weight. Solid lines are exponential fits. Figures are from Ref. [1].

Below the SDW transition temperature, we observe a transient gap generation at early time delays.
A similar transient feature is observed in the normal state up to room temperature.
References[1] J. A. Warshauer, D. A. B. Lopez, Q. Dong, G. Chen, W. Hu, arXiv:2212.03337 (2022).
* Acknowledgement(s) : authors J. A. W. and W. H. acknowledge support from the National Science Foundation under Grant No. 1944957.
D. A. B. L. was supported by the US Department of Energy under Award Number DE-SC-0021305. Q. D. and G. C. were supported by the
National Natural Science Foundation of China (Grant Nos. 11874417, 12274440), the Strategic Priority Research Program (B) of Chinese
Academy of Sciences (Grant No. XDB33010100) and the Ministry of Science and Technology of China (Grant No. 2022YFA1403903).

Linking high-harmonic generation and strong-field
Ionizationin bulk crystals
1
1
1
2
1

A. Husakou , P. Jürgens S. D.C. R. Abbing , M. Mero , M. J.J. Vrakking
A. Mermillod-Blondin1, P. M. Kraus3
1
Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, 12489 Berlin, Germany
2
Advanced Research Center for Nanolithography (ARCNL, 1098XG Amsterdam, the Netherlands
3
Vrĳe Universiteit, 1081 HV Amsterdam, the Netherland

High-order harmonic generation in gases marked the birth of attosecond science. Transferring theconcept of HHG to solid-state systems has opened up several research avenues ranging fromfundamental investigations of strong-field-driven carrier dynamics to petahertz electronics. In solids,
three main sources of nonlinearity have been identified with regards to harmonic generation. First is
the traditional Kerr-type nonlinearity [1] which is based on the anharmonic motion of boundelectrons in the valence band. Second, high-order harmonics have been associated with three-steplike [2] interband recollisions and intraband currents where the nonlinearity enters through the nonparabolic shape of the bands [3]. Third, the nonlinearity that is inherent to the process ofphotoionization was proposed as a possible origin for harmonic generation [4] and was recently
identified as the dominant source of low-order wave-mixing in amorphous fused silica [5].

Fig. 1. Experimentally determined high harmonic generation and absorption in MgO. (a) Experimental
setup, (b) dependence of the emitted harmonics strength and pump absorption on the crystal orientationfor 200-µm-thick MgO crystal and intensity of 18 TW/cm2, (c) dependence of the diatomic and monoatomic
harmonic emission strength on the intensity, (d) dependence of the average transmission and harmonic
modulation amplitude on intensity.
For crystals, the connection between carrier excitation and harmonic emission and its impact on theanisotropic angular dependence has not yet been fully understood. While photoionization is often a
key factor, there has been no investigation of its angular dependence with regards to harmonic
generation. In this contribution we combine measurements of the angular dependence of nonlinear
absorption and high-order harmonic generation to identify the relation between high-harmonic
emission and laser-induced ionization. An unambiguous correlation between the emission ofharmonics and laser-induced ionization is found, which is supported by numerical solutions of thesemiconductor Bloch equations and calculations of orientation-dependent ionization rates usingmaximally localized Wannier functions.In Fig. 1(a), the experimental setup is shown along withobtained results for the MgO crystal. For low intensities the nonlinearity is dominated by thirdorder response with fourfold symmetry (not shown). With higher intensity, third harmonics
response starts to show eight-fold symmetry, as shown in Fig. 1(c), simultaneously, 5th and 7th
harmonics start to appear in the signal, characterized by almost perfect eight-fold symmetry, as
can be seen in Fig. 1(b). The preferred directions of the emission are associated with the crystal
orientation, with maximum emission occurring when the field is along the monoatomic (O-O) or
diatomic (O-Mg) bond. The eight-fold anisotropic harmonic emission is accompanied by, andstrongly correlated to, the anisotropy in absorption, as shown in Fig. 1(d). This establishes adefinite link between the harmonic emission in the considered regime and the photoionization
process, which leads to harmonic emission predominantly through injection current [5]. We havealso performed similar measurements using Al 2 O3 crystal (not shown). While the clear
correlation between the photoionization and harmonic emission was also established, for Al2 O3 weobserve a surprising change of the preferred direction from diatomic at lower intensities tomonoatomic at higher intensities. To gain a deeper understanding of the photoionization andemission processes, we have performed numerical simulations using semiconductor Blochequations as well as simulated ionization using maximally localized Wannier functions.
In Fig. 2(a) we illustrate the possible transitions from the valence band to conduction band in MgO.
While same-site transitions and transitions to interatomic space are not expected to show a strong
degree of orientation dependence, the Wannier jumping to the nearest-neighbor (and next-nearestneighbor) atoms do show significant orientation dependence, as indicated in Fig. 2(b).

Fig. 2. Results of numerical simulations. (a) Scheme of the possible transition from the valence zone to the
conduction zone, (b) orientational dependence of the ionization rate for MgO crystal and different
intensities, (c) contribution of different Wannier jumpings to the ionization rate for the intensity of 20 TW/cm2.

In addition, this dependence changes with intensity, illustrated by transition from fourfold symmetry
to eight-fold symmetry in Fig. 2(b), in agreement with the experiment and in the intensity range
corresponding to the experimental values. In addition, for still higher intensities the strong
electric field starts to “dress” the states in the valence and conduction bands, inducing
further modifications to the orientation dependence of the photoionization, as shown in Fig. 2(c),where at higher intensities 12- fold symmetry starts to emerge.
References[1] P. A. Franken, A. E. Hill, C. W. Peters, G. Weinreich, Physical Review Letters 7, 118 (1961).
[2] M. Lewenstein, P. Balcou, M. Y. Ivanov, A. L’Huillier, P. B. Corkum, Physical Review A 49, 2117 (1994).
[3] E. Goulielmakis, T. Brabec, Nature Photonics 16, 411 (2022).
[4] F. Brunel, JOSA B 7, 521 (1990).
[5] P. Jürgens, B. Liewehr, B. Kruse, C. Peltz, D. Engel, A. Husakou, T. Witting, M. Ivanov, M. Vrakking, T. Fennel, A. Mermillod-Blondin,
Nature Physics 16, 1035 (2020).
* Acknowledgement(s) : It is our pleasant duty to thank M. Ivanov and T. Fennel for discussion of our experimental results and the numericalmethods. A.H. acknowledges support from 823897 (ATLANTIC) RISE project funded by European Commission.

Highly efficient stimulated Raman conversion in gas-filled photonic
Crystal fibers
A. A. Ishaaya
Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel

Gas-filled hollow-core optical fibers provide an excellent platform for stimulated Raman scattering(SRS), thanks to their long interaction lengths and inherent field confinement. Here, we review ourinvestigations on SRS in SF6- and CF4-filled photonic bandgap fibers using ns-pulsed 1 μm laser
sources. With relatively short fiber lengths, we were able to achieve ~70% conversion efficiency to
the first Stokes order, with low peak power and low gas pressure. Moreover, we observed
polarization-dependent Raman conversion, which we related to slight asymmetry in the fiber
structure that affected wavelengths on the "red" side of the bandgap. Currently, we are pursuingsimilar investigations with ps sources. Wavelength conversion in gas-filled hollow-core fibers(HCFs) has been an active research field over the past two decades. The tight light confinement, thelong interaction length, and the ability to guide very high peak powers make them an ideal platform
for nonlinear light-matter interactions. First, efficient wavelength conversions via stimulated Raman
scattering (SRS) were performed in H2-filled hollow-core Kagome' photonic crystal fibers [1]. These
Kagome' fibers have a relatively wide transmission band and can transmit both the pump and Stokes
wavelengths for various gases. In this feature, they are similar to the relatively new class of hollow
core fibers, the anti-resonant (ARF) or negative curvature fiber. These new-generation fibers have asimple microstructure cladding and, thus, are relatively easy to fabricate. The unique cladding
facilitates low-loss guidance over a broad spectrum (100’s of nm) and is lossy for high-order modes
resulting in single-mode guidance. The typical mode field diameters are relatively large, 20-50 µm,
and hence can sustain high power transmission. In hollow-core photonic bandgap fibers (PBGFs),
where the low-loss transmission bandgap is limited to only ~100 nm, only gases with a small
vibrational or rotational Raman shift can be exploited. In this case, a self-filtering effect of the higher
Stokes lines will facilitate higher conversion efficiency to the first Stokes line [2]. As evident, withthe above-noted HCFs, one can reach great conversion efficiencies with low pump peak powers
relative to free-space large gas cells. In the PBGFs, the conversion is limited to wavelengths close to
the pump wavelength, while in Kagomé and ARFs, the energy is typically transformed to multiple
Stokes orders. This general principle is elegantly demonstrated in [3], with two hydrogen-filled
hollow-core photonic crystal fibers, a broad, phase-locked, purely rotational frequency comb was
established. A clean first Stokes seed pulse was generated using a narrowband photonic bandgap
fiber via SRS coupled resonantly with a pump in a second broadband guiding kagomé- fiber. Using aspectral interferometric technique based on sum frequency generation, they show that the comb
components are phase-locked. Finally, in both Kagomé and PBG fibers, only relatively low averagepower pulsed pump lasers were used. In this regard, ARFs potentially pave the way for higher
average-power conversions. Recently, the ARFs surpassed the gold standard of SMF in terms oflower transmission losses [4] and are expected to be further developed and incorporated into
practical applications. We have been pursuing wavelength conversions in gas-filled hollow-core
PBGFs for almost a decade and a half. We began by investigating fs pulse propagation in PBGF
filled with Tetrafluoromethane (CF4) and Sulphur Hexafluoride (SF6) gases and observed self-phasemodulation broadening [5]. Next, we obtained relatively high SRS conversion efficiency in CF4filled PBG fibers demonstrating 36% conversion from 532 nm to 559 nm in a 35 cm long fiber at
low gas pressures and with low input peak power ns pulses [6,7]. Recently [8], we experimentallystudied the SRS conversion process of ns-duration pulses as a function of gas, fiber length, input
power, and gas pressure. For SF6 pumped at 1030 nm and 1065 nm wavelengths, the first Stokes wasobserved at 1119 nm and 1161 nm, respectively. Fiber lengths of 5 m, 6 m, and 15 m produced
maximum conversion efficiencies of 11.9%, 34.3%, and 55.7%, respectively, with the 1030 nm
pump. The highest conversion efficiency for the 1065 nm pump reached 52.2% with a 15 m fiber,
though it required double the peak power of the 1030 nm pump. Lastly, we optimized the SRS
conversion, and the conversion efficiency to the first Stokes reached up to 70% in a 6 m fiber with a1065 nm pump [9]. This indicates twice as much enhancement over the previous study. The increaseis mainly due to a non-trivial correlation between the vibrational Raman conversion efficiency andthe input pump beam's angle of linear polarization (AoLP). Also, the pump pulse dynamics (i.e., therise and fall time relative to the pulse duration) were better adjusted for topping the conversion
efficiencies. The experimental setup for the optimized SRS conversion is portrayed in Fig. 1a. The
pump source is a high peak power Q-switched Ytterbium-doped PCF laser with a pulse duration of~25 ns, a wavelength of 1065 nm, and a bandwidth of 0.2 nm. The pump beam is attenuated andlinearly polarized via two perpendicularly oriented linear polarizers and a half-wave plate placed in
between. These are followed by an additional rotatable half-wave plate to control the AoLP of the .

Fig. 1. (a) The experimental setup. (b) SEM of the transverse microstructure of the NKT Photonics HC-1060-02
fiber and its measured attenuation in the relevant spectrum.

beam. The beam is focused using a short focal length aspherical lens into the bare end of the HCPBG fiber, which is open to the air. The opposite end of the HC-PBG fiber is mounted inside a gas
cell, pointing at an antireflection-coated window. The cell is first vacuum-pumped, except for aminor leakage of the open-aired fiber tip, and then pressurized up to 12 bar, measured by a pressuregauge. The fiber is gradually filled with the gas to the state of a steady differential pressure
distribution throughout the fiber (with a very slow flow of gas out of the fiber). The output beam,comprising both the pump and the converted Stokes signal, is collimated after the cell window andreflected off two dichroic mirrors to separate the first Stokes signal from the pump. The pump andStokes beams are measured at the transmission of the first and the reflection of the second dichroic
mirrors, respectively. The HC fiber is a commercially available NKT Photonics HC-1060-02
designed for single-mode operation in the transmission range. Fig. 1b displays a scanning electron
micrograph of the transverse microstructure of the fiber and its measured attenuation at the relevant
spectrum. We visualize in Fig. 2a the average powers of the pump and Stokes signal measured at theexit of a 6 m long SF6-filled fiber versus the AoLP of the input pump beam.

Fig. 2. The average powers of the pump and Stokes signal (a) and the conversion efficiency (b), measured at theexit of a 6 m long SF6-filled fiber, versus the AoLP of the input pump beam.

The Stokes signal power experience significant variations of ~±68.5%, whereas the pump shows~±17.5%. We experimentally verified that the input AoLP does not affect the transmitted pump
power for peak powers below the SRS threshold. Consequently, the pump variations in Fig. 2a areprimarily due to corresponding stimulated Raman losses. The conversion efficiency, measured ratioof first Stokes to total coupled power, is plotted in Fig. 2b. The efficiency strongly depends on the
AoLP with up to ~±70.6% peak variation, minima are clearly π/2 periodic, and the maxima seem
similar though less sharp. Vibrational SRS in a perfectly circular symmetric fiber with a seedlessconfiguration is independent of the AoLP of the pump beam. Thus, we hypothesized andexperimentally verified that the significant variations of the conversion efficiency stem from thepolarization sensitivity of the fiber and that the fiber's imperfection is more significant at the Stokes
wavelength as it is out of the fiber's bandgap and has a larger mode field diameter and deeper
penetration into the microstructured cladding than the pump. Due to the nonlinear nature of SRS andthe long interaction length inside the fiber, slight asymmetries which result in variations in the field
distributions can yield significant SRS polarization sensitivity. Consequently, with the optimized
AoLP of the input pump beam, we reached ~ 70% conversion efficiency in a 6 m SF6-filled HCPCF. This is more than double the efficiency reported in [8]. To the best of our knowledge, it also
shows a record conversion efficiency to a single Stokes order in SF6 gas.
References[1] F. Benabid, J. C. Knight, G. Antonopoulos, P. St. J. Russell, Science 298, 399 (2002).
[2] F. Benabid, G. Bouwmans, J. C. Knight, P. St. J. Russell, F. Couny, Physical. Review Letters 93, 123903 (2004).
[3] A. Abdolvand, A. M. Walser, M. Ziemienczuk, T. Nguyen, P. St. J. Russell, Optics Letters 37, 4362 (2012).
[4] H. Sakr, Y.Chen, G.T.Jasion, T.D.Bradley, J.R.Hayes, H.C.H.Mulvad, I.A.Davidson, E.N.Fokoua, F. Poletti, Nature Communications 11, (2020).
[5] A. Gilad, A. A. Ishaaya, OSA Annual Meeting, Rochester, NY, USA (Oct. 2010).
[6] L. Ben Yehud, D. Belker, G. Ravnitzki, A. A. Ishaaya, Optics Letters 39, 1026 (2014).
[7] L. Ben-Yehud, A. A. Ishaaya, CLEO/QELS 08, San Jose, CA, USA (June 2014).
[8] S. Edelstein, A. A. Ishaaya, Optics Letters 44, 5856(2019).
[9] R. Avrahamy, D. Belker, A. Halstuch, A. A. Ishaaya, “Polarization controlled, ultra-efficient Raman conversion to the first Stokes in gases-filled
hollow-core photonic crystal fibres,” (to be submitted), (2023).

Ultrafast coherent charge dynamics inStrongly
correlated
systems
1
1
1
1
2

S. Iwai , T. Amano , Y. Kawakami , H. Itoh and K. Yonemitsu
1
Tohoku University, 980-8578, Sendai, Japan
2
Chuo University, 112-8551, Tokyo, Japan

Recent advances of ultrafast laser technologies on attosecond time scale and quantum materials open
coherent manipulations of correlated electron systems [1-3]. Thus, a strong light-field enables us to
expect realization of a new class of photoinduced symmetry breaking which is different from theconventional regime on slower time scales, i.e., breaking of space inversion symmetry inferroelectric compounds and that of time reversal symmetry in antiferromagnets. Here, we would
like to discuss i) second harmonic generation (SHG) induced by non-scattering light-currents in acentrosymmetric organic superconductor [4, 5] and ii) ultrafast and large helicity-dependent
polarization rotation in a Kitaev-type spin liquid candidate α-RuCl3[6] as new examples ofphotoinduced symmetry breaking. i) An electromagnetic oscillation of light cannot directly accessspace inversion symmetry breaking because of its symmetric nature on the time axis (i. e., the time
average of the oscillation is zero). However, recent developments of ultrashort laser technologies
enable us to control the direction of charge motion by carrier-envelope phase (CEP) control of astrong light field. Considering non-scattering light-induced current, we can expect petahertz controlof the space inversion symmetry in solids. Here, in a layered organic superconductor ( κ (ET)2Cu[N(CN)2]Br), SHG is observed by using a single-cycle 6 femtosecond near infrared pulse,which is in contrast to the perturbation theory where even harmonics are forbidden incentrosymmetric systems. The SHG shows a CEP sensitive nature and an enhancement near thesuperconducting temperature. The result and its quantum many-body analysis indicate that apolarized current is induced by non-dissipative acceleration of charge, which is amplified bysuperconducting fluctuations. ii) In a honeycomb-lattice spin-orbit assisted Mott insulator α-RuCl3,
an ultrafast magnetization is induced by circularly polarized excitation below the Mott gap. A
helicity dependent light-induced polarization rotation in spin-liquid phase is 400 times [7] (20 times[8]) larger than that of a conventional paramagnet TGG (antiferromagnet NiO). Photo-carriers play
an important role, which are generated by turning down the synergy of the on-site Coulombinteraction and the spin-orbit interaction realizing the insulator state. An ultrafast 6- fs measurementof photo-carrier dynamics and a quantum mechanical analysis clarify the mechanism, according towhich the magnetization emerges from a coherent charge motion between different t2g orbitals (dyzdxz-dxy) of Ru3+ ions. This ultrafast magnetization is weakened in the antiferromagnetic (AF) phase,
which is opposite to the general tendency that the inverse Faraday effect is larger in AF compounds
than in paramagnetic ones.

Fig. 1 Schematic illustrations of (a)SHG induced by a light-induced nonscattering current in an organic
superconductor and(b)light-induced magnetization in a
Kitaev-type spin-liquid system.

This temperature dependence indicates that the interorbital charge motion is affected by pseudo-spinrotational symmetry breaking in the AF phase.
References[1] T. Ishikawa, Y. Sagae, Y. Naitoh, Y. Kawakami, H. Itoh, K. Yamamoto, K. Yakushi, H. Kishida, T. Sasaki, S. Ishihara, Y. Tanaka, K. Yonemitsu
S. Iwai, Nature Communications 5, 5528(2014).
[2] Y. Naitoh, Y. Kawakami, T. Ishikawa, Y. Sagae, H. Itoh, K. Yamamoto, T. Sasaki, M. Dressel, S. Ishihara, Y. Tanaka, K. Yonemitsu, S. Iwai
Physical Review B93, 165126(2016).
[3] Y. Kawakami, T. Amano, Y. Yoneyama, Y. Akamine, H. Itoh, G. Kawaguchi, H. M. Yamamoto, H. Kishida, K. Itoh, T. Sasaki, S. Ishihara
Y. Tanaka, K. Yonemitsu, S. Iwai, Nature Photonics 12, 474 (2018).
[4] Y. Kawakami, T. Amano, H. Ohashi, H. Itoh, Y. Nakamura, H. Kishida, T. Sasaki, G. Kawaguchi, H. M. Yamamoto, K. Yamaomto, S. Ishihara
K. Yonemitsu S. Iwai, Nature Communications 11, 4138(2020).
[5] S. Iwai, Y. Kawakami, H. Itoh, and K. Yonemitsu, Faraday Discussions 237, 353(2022).
[6] T. Amano, Y. Kawakami, H. Itoh, K. Konno, Y. Hasegawa, T. Aoyama, Y. Imai, K. Ohgushi, Y.Takeuchi, Y.Wakabayashi, K. Goto, Y. Nakamura
H. Kishida, K. Yonemitsu, S. Iwai, Physical Review Research 4, L032032(2022).
[7] R. V. Mikhaylovskiy, E. Hendry, V. V. Kruglyak, Physical Review B86, 100405(R)(2012).
[8] T. Satoh, S-J. Cho, R. Iida, T. Shimura, K. Kuroda, H. Ueda, Y.Ueda, B. A.Ivanov, F. Nori, M. Fiebig., Physical Review Letters 105, 077402(2010).
* Acknowledgement(s) :These studies have been performed in collaboration with T. Sasaki (IMR, Tohoku Univ.), Y. Nakamura, H. Kishida (Nagoya
Univ.), G. Kawaguchi, H.M. Yamamoto (IMS), K. Yamamoto (Okayama Univ. Sci.).
These works are supported by JPMXS0118067426(Q-Leap) and JP19198318(JST-CREST).

Ultrafast loss of lattice coherence in the photoinduced
Structural2 phase transition
of1 V2O3
1
3
4

A. S. Johnson , D. Moreno-Mencía , E. B. Amuah , M. Menghini , J.-P. Locquet , C. Giannetti5
E. Pastor6, S. E. Wall3
1
IMDEA Nanoscience, 28049 Madrid, Spain
2
ICFO—Institut de Ciencies Fotòniques, 08860 Castelldefels, Barcelona, Spain
3
Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
4
Katholieke Universiteit Leuven, 3001 Leuven, Belgium
5
Universita Cattolica, 25121 Brescia, Italy
6
Université de Rennes, 35000 Rennes, France

Coherent phononic motion is one of the most distinctive behaviour of solids when photoexcited on
the ultrafast timescale. Characteristic phonon oscillations have been used extensively to track when amaterial is driven through a phase transition [1, 2], and have been manipulated in multiple-pulse
sequences to enable coherent control of structural phase transitions [3]. However, the question ofwhether and how this coherent motion, especially in zone centre modes, is sustained across a
structural phase transition remains largely unexplored. This is because in many systems only the
initial low-symmetry phase has appreciable Raman active phonon modes which can be observed
using ultrafast spectroscopy [4], preventing direct measurement of coherent lattice motion in the final
high-symmetry phase. Here, we present coherent phonon spectroscopy measurements of the lightinduced phase transition in V2O3 [5]. V2O3 is notable as it hosts Raman-active modes readily
observable through coherent phonon generation in both its low-symmetry monoclinic phase andhigh-symmetry corundum phase, thus making it an ideal testing ground for tracking the transfer ofcoherence across a light-induced phase transition. The low-symmetry phase has three characteristics
phonon modes at 7, 8.3 and 9.8 THz, unfold through the structural phase transition leaving a single
Raman-active mode in the high-symmetry phase at 7.5 THz (Fig. 1).

Fig. 1. Representative phononic band structure exhibiting multiple Raman active modes in the low
symmetry phase (a) and a single Raman active mode in the high symmetry phase (b).

When exciting across the phase transition, if coherence were preserved, we would expect to see thecharacteristic mode of the high symmetry phase emerge. Instead we observe a complete loss of oscillations
and lattice coherence. This is particularly interesting as, by focusing on Raman-active modes, we areobserving modes which do not undergo a significant change in momentum through the zone-folding in thephase transition, and which may be expected to be more resilient than modes which change from zone-centerto zone-edge as have been studied in previous phonon spectroscopy experiments. Our results suggest that
V2O3 undergoes an order-disorder style transition when photoexcited, adding it to the growing family ofidentified photoinduced order-disorder transitions [4,6]. In contrast to previous systems however, we canuniquely associate the disordering to the transition itself, rather than to significant disorder in the highsymmetry phase. We speculate this disordering may be due to mixing of the low-symmetry modes across thetransition or due to anomalously strong phonon-anharmonicity due to the effective compression of the highsymmetry phase immediately following photoexcitation [7].
References[1] S. Wall, D. Wegkamp, L. Foglia, K. Appavoo, J. Nag, R.F. Haglund Jr, J. Stähler M. Wolf , Nature Communications 3, 721 (2012).
[2] A. Tomeljak, H. Schäfer, D. Städter, M. Beyer, K. Biljakovic, J. Demsar,. Physical Review Letters 102, 1 (2009).
[3] J. G. Horstmann, H. Böckmann, B. Wit, F. Kurtz, G. Storeck, C. Ropers, Nature 583, 232(2020).
[4] D. Perez-Salinas, A. S. Johnson, D. Prabhakaran, S.Wall, Nature Communications 13, 8 (2022).
[5] A. S. Johnson, D. Moreno-Mencía, E. B. Amuah, M. Menghini, J.-P. Locquet, C. Giannetti, E. Pastor, S. E. Wall, Physical Review Letters 129,255701 (2022).
[6] S.W. Wall, S. Yang, L. Vidas, M. Cholett, J.M. Glownia, M. Kozina, T. Katayama, T. Henighan, M. Jiang, T.A. Miller D.A. Reis, L.A. Boatner
O. Delaire, , M. Trigo, Science 362, 572(2018).
[7] A. S. Johnson,D. Perez-Salinas,K.M. Siddiqui,S. Kim,S. Choi,K.Volckaert, P.E. Majchrzak, S. Ulstrup, N. Agarwal, K. Hallman
R. F. Haglund Jr, C.M. Günther, B.Pfau, S. Eisebitt, D.Backes, F.Maccherozzi, A. Fitzpatrick, S. S. Dhesi, P.Gargiani, M.Valvidares, N.Artrith
F.de Groot, H. Choi, D. Jang, A. Katoch, S. Kwon, S.H. Park, H. Kim, S. E. Wall, Nature Physics 19, 215 (2023).

Ultrafast dynamics and control of structure in
Charge-density-waves
1
1
1
2
2
2

R. Winkler1, L. Boie , M. Savoini , V. Ovuka , Y. Deng , S. Zerdane , A. Nag , S. Gurung1, E. Abreu1
S. Biasco1, D. Soranzio1, T. Suter1, J. Dössegger1, M. Bosch1, B. Strudwick2, M. Sander2, E. Divall2
P. Beaud2, R. Mankowsky2, A. Oggenfuss2, C. Arrell2, D. Babich2, H. Lemke2, M. Chollet4, T. Sato4
V. Esposito4, B. Liu2, V. Krapivin4, G. Orenstein4, Y. Huang4, P. Bhattacharyya7, D. Zhu4, P. Zalden5
F. Ardana-Lamas5, Y. Uemura5, H. Yousef5, M. Biednov5, C. Milne6, U. Staub3
S. Teitelbaum6, J. Demsar7, M. Trigo4
S. L. Johnson1
1
ETH Zurich, 8093 Zurich, Switzerland
2
Paul Scherrer Institute, 5232 Villigen, Switzerland
3
Swiss Light Source, 5232 Villigen, Switzerland
4
SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
5European XFEL GmbH, 22761 Hamburg, Germany
6
Arizona State University, Tempe, AZ 8528, USA
7
Johannes Gutenberg-University Mainz, 51099 Mainz, Germany
With strong connections to BCS-type superconductivity, the phenomenon of charge-density-waves (CDWs) in
low-dimensional metallic systems is an important example of a second-order structural phase transition driven
by electron-phonon coupling. In this talk I will present the current status of our work on K0.3MoO3, a material
with a highly anisotropic structure (see Fig. 1) that results in a nearly 1-D conductivity. Below 183 K, thisstructure becomes unstable with respect to an incommensurate lattice distortion with wavevector qCDW = (1 qb0.5), where 𝑞𝑏 ≈ 0.748 is a weak function of temperature. Time-resolved x-ray diffraction measurements
have directly measured coherent structural dynamics of the charge density wave distortion in this material
over a range of excitation levels driven by a femtosecond laser [1, 2]. The sudden laser-driven suppression of
the interatomic potential that stabilizes the CDW is thought to cause coherent dynamics that transiently invert
the phase of the CDW, a finding that is at least thematically consistent with optically based studies whichshow evidence of CDW domain formation with a phase that depends on the local excitation level of thematerial [3].

Fig. 1. Left: High-temperature structure of K0.3MoO3. The conductivity along the b-axis is significantly higher
than in other directions, making this system a quasi-1D conductor that undergoes a charge-density-wave
transition below approximately 183 K. Right: Proposed scenarios for femtosecond laser-driven melting anddomain reformation, where the domain phase is determined by the relaxation time of the excitation relative tothe coherent dynamics [3].

Here I discuss recent results using x-ray free electron lasers to both better characterize the structuraldynamics of the coherent phonon driven at low excitation levels, and to investigate the excitation
regime where transient CDW melting and phase reversal are expected. I will also discuss future
perspectives for making such control more general for second-order-like phase transitions.
References[1] T. Huber, S. O. Mariager, A. Ferrer, H. Schäfer, J. A. Johnson, S. Grübel, A. Lübke, L. Huber, T. Kubacka, C. Dornes, C. Lauhe, S. Ravy
G. Ingold, P. Beaud, J. Demsar, S. L. Johnson, Physical Review Letters 113, 026401 (2014).
[2] M. Neugebauer, T. Huber, M. Savoini, E. Abreu, V. Esposito, M. Kubli, L. Rettig, E. M. Bothschafter, S. Grübel, T. Kubacka, J. Rittmann
G. Ingold, P. Beaud, D. Dominko, J. Demsar, S. L. Johnson, Physical Review B 99, 220302 (2019).
[3] R. Yusupov, T. Mertelj, V. V. Kabanov, S. Brazovskii, P. Kusar, J.-H. Chu, I. R. Fischer, D. Mihailovic, Nature Physics 6, 681 (2010).
* This work was supported by the Swiss National Science Foundation under grant 200020_192337.

Infrared spectroscopy of 2D moiré
Superlattice materials
1
2
1
1
1

J. Yang1, T. Han1, T. Han , G. Chen , D. Rodan-Legrain , J. M. Park , Q. Zhang , L. Jiang2, B. Lyu4 , H.Li2
K. Watanabe5, T. Taniguchi5, Z. Shi4, Y. Zhang6, P. Jarillo-Herrero1, F.Wang2 , L. Ju1
1
Massachusetts Institute of Technology, Cambridge, MA 02135, USA
2
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
3
University of California at Berkeley, Berkeley, CA 94720, USA
4
Shanghai Jiao Tong University, Shanghai, 200231China
5
National Institute for Materials Science, Tsukuba 305-0047, Japan
6
Fudan University, Shanghai, 200437 China

Moiré superlattices of two-dimensional materials have emerged as a new material platform toengineer and study electron correlation and topology. So far, experimental progresses have been
dominated by electron transport measurements which do not provide spectroscopy evidence ofcorrelation or the relevant energy scales directly. Due to their ~micron lateral size, atomic thickness,and complicated device structures involving encapsulating layers and metallic gates, spectroscopy
measurements of moiré superlattices have been very challenging.
Fig. 1. Device structure and optical transitions in
ABC TLG/hBN moire superlattice. A. Illustration ofdual-gated TLG device with a moiré wavelength of~15 nm. B. Interband optical transitions between
moiré bands at various displacement fields.
Flatband formation can be visualized as the sharptransition peak emerges in the photocurrent
spectrum. C. Photocurrent spectrum when the flatmoire band is half-doped, featuring a strong
resonance at ~18 meV that corresponds to opticaltransition across the correlated insulating gap.

In this talk, I will introduce our recent efforts on FTIR photocurrent spectroscopy measurements ofdual-gated ABC trilayer graphene/hBN moiré superlattice, which is a representative moiré
superlattice material to study correlated and topological electron physics. We observed strong gatetunable optical transitions that originated from the moire flat band. At half-filling of the valence flat
band, a broad absorption peak emerges at ~18 meV, indicating direct optical excitation across anemerging Mott gap. Furthermore, I will talk about a unique moire-enabled interlayer electron-phonon
coupling phenomenon in this system.
Fig. 2. Moiré-enabled interlayer electron-phonon
coupling in ABC trilayer graphene/hBN. a. Optical
micrograph of the ABC TLG/hBN device. Scale bar: 3
micro-meters. b. Illustrattion of the interlayer EPC in
TLG/hBN with a moiré wavelength M. The hBN ZO
phonon hybridizes with electron-hole pairs in graphene
to gain electrical dipole. c. Photocurrent spectrum of
TLG. The ZO phonon appears as a peak while the E2g
phonon appears as a dip due to light absorption. d.
Photocurrent spectra at different gate displacement fields
D. The asymmetric line-shape is due to interference ofthe renormalized ZO phonon with electron-hole pairs at
the immediate vicinity in spectrum, which can be welldescribed by a Fano line-shape represented byt thedashed curves.

The ZO phonon in hBN hybridizes with electron-hole excitations in trilayer graphene and appear as
an asymmetric peak in the photocurrent spectrum. This Fano line-shaped peak evolves continuously
as we tune the displacement field. I'll talk about the implications of this phenomenon to engineering
the physics of moire quantum matters.
References[1] J. Yang, G. Chen, T. Han, Q. Zhang, Y.H. Zhang, L. Jiang, B. Lyu, H. Li, K. Watanabe, T. Taniguchi, Z. Shi, S. Todadri, Y. Zhang, F. Wang,
L. Ju, Science 375, 1295 (2022).
[2] T. Han, J. Yang, Q. Zhang, L.Wang, K. Watanabe, T. Taniguchi, P.L. McEuen, L. Ju, Physical Review Letters 126, 146402(2021).
[3] L. Ju, L. Wang, T. Cao, T. Taniguchi, K. Watanabe, S.G. Louie, F. Rana, J. Park, J Hone, F. Wang, P.L McEuen, Science 358, 907(2017).
[4] L. Ju, L.Wang, X.Li, S.Moon, M.Ozerov, Z.Lu, T.Taniguchi, K.Watanabe, E.Mueller, F. Zhang, D. Smirnov, Nature Communications 11, (2020).
* This work was supported by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319. K.W. and T.T. acknowledge
support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHI Grant
Number JP20H00354 and the CREST(JPMJCR15F3), JST. B.L. and Z.S. acknowledges support from the National Key R & D Program of China(2016YFA0302001) and the National Natural Science Foundation of China (11774224 and 12074244). The
electron-phonon coupling experiment was supported by NSF Grant No. DMR-2225925. L. J acknowledge the support from Sloan Fellowship.

Magnetic resonances1 of chiral2 phonons
1
2

D. M. Juraschek, M. Fechner

Tel Aviv University, Tel Aviv 6997801, Israel

Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany

Circularly polarized lattice vibrations, also known as chiral phonons, produce orbital motions of theatoms around their equilibrium positions in a solid. Due to this orbital motion, chiral phonons carry
angular momentum (real or pseudo) and can therefore interact with the angular momentum of lightand with the spin and orbital angular momentum of electrons [1,2]. The investigation of angularmomentum coupling between chiral phonons and electrons has seen a tremendous increase ofactivity in recent years, including demonstrations of the existence of phonon analogs to the Hall
Einstein-de Haas effects [3,4]. In dielectric materials, where the atoms possess ionic charges, an
intriguing implication arises: circularly and elliptically polarized phonons can carry magnetic
moments, induced by the orbital charge currents that the ions produce when moving along the
eigenvectors of chiral phonon modes [5,6]. These charge currents are proportional to the ratio of theionic charges and the ionic masses of the ions in the material, and the resulting magnetic moments
usually lie on the order 1-10% of a nuclear magneton (μN) per phonon quantum. During the past
year, theoretical predictions and experimental measurements have indicated that the magnitude of thephonon magnetic moment can be drastically enhanced by the means of electron-phonon, spinphonon, and orbit-phonon coupling, where the phonon “borrows” part of the magnetic moment fromthe electron [7-10]. Through these mechanisms, much larger effective magnetic moments on theorder fractions of or even several Bohr magnetons (μB) can be achieved, orders of magnitude larger
than those expected from purely ionic charge currents. Here, we demonstrate that the presence of a(effective) phonon magnetic moment leads to resonances in the second-order nonlinear
magnetoelectric and linear magnetic susceptibilities [11]. We show that the second-order nonlinear
magnetoelectric susceptibility enables the excitation of chiral phonons through circularly polarized
infrared absorption, as illustrated in Fig. 1(a), in agreement with recent experimental and theoreticalstudies [7,9,10].
Fig.1: Chiral phonon excitation. (a)The electric field component of a circularly
polarized THz pulse, E(t), can couple to theelectric dipole moment of the two
orthogonal components of a chiral phononmode, Pph. This leads to a contribution to thesecond-order nonlinear
magnetoelectricsusceptibility, χme(2). (b) The magnetic fieldcomponent of a linearly polarized THz
pulse, B(t), can couple to the magnetic
moment of a chiral phonon mode, Mph. This
leads to a contribution(1) to the linear
magnetic susceptibility, χm .

The linear magnetic susceptibility, in contrast, comes from a Zeeman-type coupling of the magnetic
field component of light with the phonon magnetic moment, as shown in Fig. 1(b)We demonstrate that the mechanism described by the linear magnetic susceptibility leads to a
multitude of novel phenomena: it enables the excitation of circularly polarized phonons with linearlypolarized light, it can be utilized for phononic frequency up-conversion covering several orders ofmagnitude, and it makes the excitation of Raman-active phonons possible without Raman scattering.
We use a combination of first-principles methods and phenomenological modeling to describe thechiral phonon dynamics that arise from the excitations by ultrashort laser pulses, and we propose
candidate materials in which the mechanism can potentially be detected.
References[1] L. Zhang, Q. Niu, Physical Review Letters 112, 085503 (2014).
[2] H. Zhu, J. Yi, M.-Y. Li, J. Xiao, L. Zhang, C.-W. Yang, R. A. Kaindl, L.-J. Li, Y. Wang, X. Zhang, Science 359,
579 (2018).
[3] G. Grissonnanche, S. Thériault, A. Gourgout, M.-E. Boulanger, E. Lefrançois, A. Ataei, F. Laliberté, M. Dion
J.- S. Zhou, S.Pyon, T.Takayama, H. Takagi, N. Doiron-Leyraud and L. Taillefer, Nature Physics. 16, 1108 (2020).
[4] S. R. Tauchert, M. Volkov, D. Ehberger, D. Kazenwadel, M. Evers, H. Lange, A. Donges, A. Book
W. Kreuzpaintner, U. Nowak, P. Baum, Nature 602, 73 (2022).
[5] T. F. Nova, A. Cartella, A. Cantaluppi, M. Först, D. Bossini, R. V. Mikhaylovskiy, A. V. Kimel
R. Merlin, A. Cavalleri, Nature Physics 13, 132 (2017).
[6] D. M. Juraschek, M. Fechner, A. V. Balatsky, N. A. Spaldin, Physical Review Materials 1, 014401 (2017).
[7] D. M. Juraschek, T. Neuman, P. Narang, Physical Review Research 4, 013129 (2022).
[8] A. Baydin, F. G. G. Hernandez, M. Rodriguez-Vega, A. K. Okazaki, F. Tay, G. T. Noe II, I. Katayama, J. Takeda
H. Nojiri, P. H. O. Rappl, E. Abramof, G. A. Fiete, J. Kono, Physical Review Letters 128, 075901 (2022).
[9] M. Basini, M. Pancaldi, B. Wehinger, M. Udina, T. Tadano, M. C. Hoffmann, A. V. Balatsky, S. Bonetti,
arXiv:2210.01690 (2022).
[10] C. S. Davies, N. Fennema, A. Tsukamoto, A. Kirilyuk, IRMMW-THz 43035 (2022).
[11] D. M. Juraschek, M. Fechner, in preparation (2023).

Field-resolved metrology for
Attosecond-scale carrier dynamics
N. Karpowicz
Max Planck Institute of Quantum Optics, 85748 Garching, Germany

For decades, attosecond science has been giving us new insight into light-matter interaction by
allowing time-resolved access to events lasting only a fraction of an optical cycle. The first
generation of attosecond experiments made use of the high harmonic generation process to produce
short bursts of extreme ultraviolet radiation, which were then used in a variety of experimental
geometries to excite and observe laser-induced dynamics. As we learned about these physicalprocesses, one thing became clear: the electric field of light often leaves its own imprint behind in a
way that makes it possible to extract it directly from the measurement. We will show that this has
surprisingly opened a door to a new generation of attosecond experiments, where charge carriers
inscribe their motion directly into an electromagnetic wave that we can record with sub-fs resolution.
Fig. 1. The framework of field-resolved attosecond
metrology. Left: a single-cycle pulse of light enters amedium. Center: The field induces time-dependent
dynamics in the system (center). Right: The dynamics are
encoded into subtle modifications of the electromagnetic
wave. Field metrology allows us to obtain these traces of
the interaction and relate them back to the underlying
microscopic dynamics.

There are a few things that need to be established for this class of experiment, sketched in Fig. 1, towork: 1) What are the physical origins of our “gate” that allows us to turn a rapidly-oscillating
optical field into a nearly-constant signal suitable for electronics? 2) Can we model this gate
precisely enough to invert its influence on the recorded waveform? 3) How is information embedded
into the electric field, and how can we get it out?There are several solutions to the first question,
which we can divide into two categories: electronic and photonic. Electronic methods make use ofcarrier-excitation dynamics in the detector to, for example, rapidly change the conductivity of a solid[1, 2, 3], resulting in a relatively long-lived current that will, as its time delay relative to the fieldunder study is changed, allowing the waveform to be recorded electronically. Alternatively, photonic
techniques [4], such as electro-optic sampling [5], translate the electric field present in a nonlinear
medium into a modification of a measurable aspect of a second pulse.Once an electric fieldmeasurement has been implemented, there is still an important question to answer: what is therelationship between this measured trace and the true electric field within or impinging on thedetector? And how does it relate to the electric field inside of a sample under study? This question
can be highly non-trivial, and one must not forget that the electric field is not only a function of time,
but also of space; sometimes untangling it will require techniques that capture the spatiotemporal
evolution of the waveform [6]. Addressing this not only from the experimental side, we havedeveloped a robust open-source nonlinear optics simulation platform that will be briefly discussed,with the goal of lowering barriers between experimentalists and deep and detailed views of theevolution of electric fields inside optical media [7]. Finally, how does the signal from the carriers getembedded into the electric field? There are a few experimental geometries where changes in electricfield waveforms may be related back to the microscopic nonlinear polarization in the sample [8],which is an observable closely related to many other physical properties of interest, such as theenergy exchanged between the medium and the sample.Finally we will provide an overview of theunderlying technology that drives these experiments and of new ultrashort sources that are coming
online. Optimally performing field driven metrology requires a rethinking of the ideal source,
favoring phase and amplitude stability over the high intensities required to drive high harmonic
generation.In summary, the field of attosecond metrology is undergoing a revolution in how data is
acquired and insight is obtained, based on recording and interpreting the waveform of light.
Understanding and controlling all aspects of the dynamics responsible for giving this unprecedented
access to the electromagnetic field will be an essential component of the future of this ever-evolving
field of study.
References[1] A. Schiffrin, T. Paasch-Colberg, N. Karpowicz, V. Apalkov, D. Gerster, S. Mühlbrandt, M. Korbman, J. Reichert, M. Schultze, S. Holzner,
J. V. Barth, R. Kienberger, R. Ernstorfer, M. I. Stockman, F. Krausz, Nature 493, 70 (2013).
[2] S. Sederberg, D. Zimin, S. Keiber, F. Siegrist, M.S. Wismer, V.S. Yakovlev, I. Floss, C. Lemell, J. Burgdoerfer, M. Schultze, F. Krausz
N. Karpowicz, Nature Communications 11, 430 (2020).
[3] M. Ossiander, K. Golyari, K. Scharl, L. Lehnert, F. Siegrist, J.P. Bürger, D. Zimin, J.A. Gessner, M. Weidman, I. Floss, V. Smejkal, S. Donsa,
C. Lemell, F. Libisch, N. Karpowicz, J. Burgdoerfer, F. Krausz, M. Schultze, Nature Communications 13, 1620 (2022).
[4] D. Zimin, V.S. Yakovlev, and N. Karpowicz, Science Advances 8, eade1029 (2022).
[5] S. Keiber, S. Sederberg, A. Schwarz, M. Trubetskov, V. Pervak, F. Krausz, and N. Karpowicz, Nature Photonics 10, 159 (2016).
[6] M. Mamaikin, Y. Li, E. Ridente, W.Chen, J. Park, A. Y.Zhu, F.Capasso, M.Weidman, M.Schultze, F.Krausz, N.Karpowicz, Optica 9, 616 (2022).
[7] https://www.github.com/NickKarpowicz/LightwaveExplorer[8] A. Sommer, E. M. Bothschafter, S. A. Sato, C. Jakubeit, T. Latka, O. Razskazovskaya, H. Fattahi, M. Jobst, W. Schweinberger, V. Shirvanyan, V.
S. Yakovlev, R. Kienberger, K. Yabana, N. Karpowicz, M. Schultze, F. Krausz, Nature 534, 86 (2016).

New pathways for ultrafast structural control via
Nonlinear phononics
G. Khalsa, J. Zheng, J. Moses
Cornell University, Ithaca, New York, 14853, USA

The strong coupling between structure, symmetry, and function is a broad reaching paradigm guiding
all levels of science and technology from the discovery of new fundamental physical phenomena to
the design of new technologies. The development of bright, tunable, mid- and far-infrared lightsources has enabled transient ultrafast control of crystal structure and symmetry, unveiling new andhidden physics and functionality in crystalline materials [1]. Through the resonant excitation of
infrared-active phonons with contemporary light sources the optical energy in the laser field isquickly converted to mechanical energy in the crystal, pushing it to a far-from-equilibrium state
where symmetry and the local chemical environment can be dramatically altered. To enable access to
new symmetries and new far-from-equilibrium and hidden phenomena in crystalline materials, newroutes for structural and symmetry control on ultrafast timescales are sought.
To explain existing ultrafast structural and functional changes in this field, termed nonlinear
phononics, the anharmonic lattice energy coupling between the resonantly excited infrared-active
phonons and indirectly driven Raman-active phonons has been invoked and investigated. Recent
theoretical work has suggested that nonlinear contributions to the lattice polarizability should be heldon equal footing with the anharmonic lattice potential energy, predicting an induced broad frequency
dielectric response in the infrared 7-orders-of-magnitude larger than silica[2]. Can the combinationof the conventional anharmonic lattice potential pathway and the nonlinear lattice polarizability
enable new pathways for control of symmetry, structure, and function on ultrafast timescales?

Fig. 1.: Coherent structural control through a two-THz-pulse scheme (left) enabled by the combination of the anharmonic lattice potential(ALP) and the nonlinear lattice polarizability (NLP). (Top-right) The ALP rectifies a Raman active phonon with a direction defined byintrinsic microscopic materials physics (red), while the NLP is sensitive to the relative phase between the ringing IR phonon excited by the
first pulse and the electric field of the second pulse (red and blue). The combination of the ALP and NLP leads to control of the direction
and duration of the rectified Raman response (Combined Effect) with pulse separation. (Bottom-right) Simulated dynamics of Raman
phonon rectification through the ALP and NLP pathways for two-IR-pulse excitation along with schematic structural changes.

Here we show theoretically that careful preparation of infrared-resonant light through, for example,
the control of frequency, ellipticity, pulse shape, and pulse sequencing (Fig. 1), can enable control ofstructure and symmetry that are inaccessible by other means when the anharmonic lattice potential
energy and nonlinear lattice polarizability are treated on equal footing.
References[1] A. S. Disa, T. F. Nova, A. Cavalleri, Nature Physics 17, 1087 (2021).
[2] G. Khalsa, N. A. Benedek, J. Moses, Physical Review X 11, 021067 (2021).
* Acknowledgement(s): his work is supported by the Cornell Center for Materials Research with funding from the NSF MRSEC program (Grant
No. DMR-1719875).

Ultra low loss silicon nitride integrated photonics:
Frequency agile low noise lasers
T. J. Kippenberg
Swiss Federal Institute of Technology, 1015Lausanne, Switzerland

Recent advances in attaining ultralow loss, highly confining silicon nitride waveguides with loss in thedB/meter range [1] have opened up novel applications that benefit not only from scalable manufacturing,compact form factor and low power, but crucially have now reached a point where the performance is on
par and even exceeding that of legacy optical systems. However, Si3N4 is a purely passive optical material
with only a thermal and Kerr nonlinearities and hence must be endowed with pivotal capabilities such as
light amplification, fast modulation, etc. through advanced integration techniques. I will describe a range
of novel advances based on ultra-low loss Si3N4 and advanced photonic integration, including photonic
integrated circuit based frequency agile lasers with fiber laser phase noise[2], parametric traveling wave
amplifiers[3], Erbium amplifiers on chip[4] as well as soliton frequency combs[5], with applications fromcoherent communications[6], LiDAR [7,8] to cryogenic quantum interconnects[9] (cf Fig. 1) Erbium
doped amplifiers are pivotal components for modern communication networks. Their unique property toachieve high gain, continuous, low-noise and high-power amplification is unmatched in long haulcommunication systems. Yet, their photonic integration has been compounded by high waveguide
propagation losses. We demonstrate a hi waveguide amplifier based on erbium ion implantation into ultralow loss Si3N4 with more than 30 dB net-gain and output power up to 142 mW[4]. Another paradigm for
amplification are parametric amplifiers based on the optical Kerr effect. Having been pioneered in silicon
photonics more than a decade ago [10], it was not possible to achieve time-continuous operation due to two
photon absorption in silicon. Here we overcome this outstanding challenge and demonstrate timecontinuous net gain fiber-to-fiber in a 2 m long photonic Damascene Si3N4 waveguide spiral [3].
Fig.1.Applications of ultra-low
loss silcon nitride based
photonic integrated circuits.

Using monolithic
integration of piezoelectric
actuator ubiquitous in
MEMS foundries [11] we
can
endow
photonic
Damascene Si3N4 circuitswith fast, low power andhysteresis free non-thermal
tuning capability without
impairingpropagation
loss. Based on thisplatform we demonstrated
the first ultra-fast tunable hybrid integrated laser, that combines the low noise of a fiber laser with
unprecedented frequency modulation speed and linearity. We demonstrate proof-of principle applicationsin frequency modulated continuous wave LiDAR[2]. Electro-optical tuning via the optical Pockels effect isthe fastest and most powerful modulation technique. Here we present a hybrid silicon nitride (Si3N4)–
LiNbO3 photonic platform and demonstrate its use for coherent laser ranging. Our platform is based onheterogeneous integration of ultralow-loss Si3N4 photonic integrated circuits with thin-film LiNbO3
through direct bonding at the wafer level, in contrast to previously demonstrated chiplet-level integration,
featuring low propagation loss of 8.5 dB/m[12], enabling narrow-linewidth lasing (intrinsic linewidth of
3 kilohertz) by self-injection locking to a laser diode[13].
References[1] J. Liu, G. Huang, R. N. Wang, J. He, A. S. Raja, T. Liu, N. J. Engelsen, T. J. Kippenberg. Nature Communications 12, 2236 (2021).
[2] G. Lihachev, J. Riemensberger, W. Weng, J. Liu, H. Tan. A. Siddharth, V. Snigirev, V. Shadymov, A. Voloshin, R. N. Wang, J. He, S. A. Bhave,
T. J. Kippenberg. Nature Communications 13, 3522 (2022).
[3] J. Riemensberger, N. Kuznetsov, J. Liu, J. He, R. N. Wang, T. J. Kippenberg. Nature 612, 56 (2022).
[4] Y. Liu, Z.Qiu, X. Ji, A.Lukashchuk, J. J.He, J.Riemensberger, M.Hafermann, R. N.Wang, J.Liu, C.Ronning, T. J.Kippenberg.Science 376, (2022)[5] T. J. Kippenberg, A. L. Gaeta, M. Lipson, M. L. Gorodetsky. Dissipative Kerr solitons in optical microresonators. Science 361, eaan8083 (2018).
[6] P. Marin-Palomo, J. N. Kemal, M. Karpov, A.Kordts, J.Pfeifle, M. H. P. Pfeiffer, P.Trocha, S. Wolf, V. Brasch, M. H. Anderson, R. Rosenberger,
K. Vijayan, W. Freude, T. J. Kippenberg, C. Koos. Nature 546, 274 (2017).
[7] P. Trocha, M. Karpov, D. Ganin, M. H. P. Pfeiffer, A. Kordts, S. Wolf, J. Krockenberger, P. Marin-Palomo, C.Weimann, S. Randel, W. Freude,
T. J. Kippenberg, C. Koos. Science 359, 887 (2018).
[8] J. Riemensberger, A. Lukashchuk, M. Karpov, W. Weng, E. Lucas, J. Liu, T. J. Kippenberg. Nature 581, 164 (2020).
[9] A. Youssefi, I. Shomroni, Y. J. Joshi, N. R. Bernier, A. Lukashchuk, P. Uhrich, L. Qiu, T. J. Kippenberg. Nature Electronics 4, 326 (2021).
[10] M. A. Foster, A. C. Turner, J. E. Sharping, B. S. Schmidt, M. Lipson, A. L. Gaeta. Nature 441, 960 (2006).
[11] H. Tian, J. Liu, B. Dong, J. C. Skehan, M. Zervas, T. J. Kippenberg, S. A. Bhave. Nature Communications 11, 3073 (2020).
[12] M. Churaev, R. N. Wang, V. Snigirev, A. Riedhauser, T. Blésin, C. Möhl, M. A. Anderson, A. Siddharth, Y. Popoff, D. Caimi, S. Hönl,
J. Riemensberger, J. Liu, P. Seidler, T. J. Kippenberg. arXiv:2112.02018 (2021).
[13] V. Snigirev, A.Riedhauser, G.Lihachev, M.Churaev, J.Riemensberger, R. N. Wang, A. Siddharth, G. Huang, C. Moehl, Y. Popoff, U. Drechsler,
D. Caimi, S. Hoenl, J. Liu, P. Seidler, T. J. Kippenberg. Nature 615, 411 (2023).

Floquet topological superconductivity induced by
Chiral many-bodyinteraction
1
2

S. Kitamura , H. Aoki
The University of Tokyo, Tokyo 113-8656, Japan
2
Advanced Industrial Science and Technology (AIST), Ibaraki 305-8568, Japan
1

Light irradiation is a new means of controlling the quantum phase and properties of materials, withwhich various novel properties including nontrivial band topology can be achieved in a dynamicalmanner. Floquet theory enables us to describe such nontrivial modulation from the viewpoint of theeffective static Hamiltonian, nevertheless the original system irradiated with an intense laser light isfar from equilibrium. In particular, when the circularly-polarized laser light is applied, the effective
static Hamiltonian acquires interaction terms with broken time-reversal symmetry (TRS), which isdifficult to realize in conventional approaches [1,2].
While this idea triggered various attempts to control topology of superconductivity, especially withbroken TRS, it turns out to be much more nontrivial than controlling band topology of normal states,
because the superconducting gap does not couple to electromagnetic field directly due to its inherent
particle-hole symmetry. Previous studies mainly focused on the topological superconductivity via themodulation of the normal band structure without changing the pairing symmetry [3].
As a new perspective, in this study, we focus on strongly-correlated superconductors, and propose anapproach that combines control of topology and control many-body interaction [4]. We show that
topologically-nontrivial chiral (d+id wave) superconductivity can be achieved in the doped Hubbard
model on the square lattice, due to the modulation of the effective pairing interaction induced by thecircularly-polarized light, as sketched in Fig. 1.

Fig. 1. Left: Sketch of the present proposal. The d-wave superconductivity on cuprates is depicted with red andblue bonds, which have positive and negative pairing amplitude, respectively. Illuminating an intense
circularly-polarized light induces imaginary pairing amplitude (colored by magenta and green) in diagonal
directions, which makes superconductivity chiral (d+id-wave). Right: Numerical results on the ground state of
Floquet effective Hamiltonian of the t-J form. (a,b) Amplitude and phase of gap function F(k). The dashed lines
represent the Fermi surface (at the critical temperature). (c) The density of states in the unit of hopping
amplitude t0, showing the full-gap nature of the d+id-wave.

We first derive the Floquet t–J model by combining the Floquet theory and the Schrieffer-Wolff
transformation. The obtained Hamiltonian has chiral many-body interactions with broken timereversal symmetry, i.e., the scalar spin chirality term and so-called three-site terms with complex
amplitudes. Then we show that these emergent interactions indeed modulate the pairing symmetry
within the framework of Gutzwiller approximation, and demonstrate it by numerically calculating
the phase diagram and transient dynamics.
References[1] T. Oka, H. Aoki, Physical Review B 79, 081406, (2009).
[2] T. Kitagawa, T. Oka, A. Brataas, L. Fu, E. Demler, Physical Review B 84, 235108, (2011).
[3] K. Takasan, A. Daido, N. Kawakami, Y. Yanase, Physical Review B 95, 134508, (2017).
[4] S. Kitamura, H. Aoki, Communications Physics 5, 174, (2022).
* This work was supported by JSPS KAKENHI Grant (20K14407 and JP17H06138), and CREST (Core Research for Evolutional Science andTechnology; Grant number JPMJCR19T3 and JPMJCR18T4).

Spectroscopy of Cavity Vacuum-Driven Materials
J. KonoRice University, Houston, TX 77005, U.S.A.

There is currently much interest in studying solids placed in cavities to uncover exotic new phases
and phenomena in “strongly driven” materials in the complete absence of any external fields otherthan the fluctuating vacuum, or zero-point, electromagnetic fields. Judicious engineering of thequantum vacuum surrounding the matter inside the cavity can lead to significant and nonintuitive
modifications of electronic states, producing a vacuum-dressed material with novel properties.
Recent stimulating theoretical predictions include cavity-enhanced, cavity-induced, and cavitymediated enhancement of electron-phonon coupling and superconductivity, electron pairing,anomalous Hall effect, ferroelectric phase transitions, quantum spin liquids, and photon
condensation. Figure 1 highlights the opportunities in the cavity–matter interactions leading toseveral exotic phenomena ranging from the formation of polaritonics to nonlinear optics.

Fig. 1. Various solid-state cavity quantum electrodynamics (QED) systems have recently emerged,
opening up exciting new opportunities for studying the interaction of quantized vacuum fields withquantum many-body systems. Driven by some pioneering experimental developments, there have been
many theoretical proposals for manipulating material properties and inducing new phases of matterwith cavity-enhanced quantum vacuum, or zero-point, electromagnetic fields.

The so-called ultrastrong coupling (USC) regime arises when the interaction energy becomes asignificant fraction of the bare frequencies of light and matter [1]. Most intriguingly, when a material
is ultrastrongly coupled with cavity-enhanced vacuum electromagnetic fields (or zero-point fields),
its ground-state properties can be considerably modified. This nonperturbative virtual driving
without any external field can lead to novel equilibrium phases with exotic properties.
This talk will describe our recent studies of USC phenomena in various solid-state systems in searchof such vacuum-induced phases of matter [2–9]. We employ the quantum optics concept of Dicke
cooperativity [10], i.e., many-body enhancement of light–matter interaction, to explore new statesand phenomena in condensed matter systems in the USC regime. These results provide quantum
optical strategies for creating, controlling, and utilizing novel phases in condensed matter systems
enabled by the quantum vacuum.
References[1] N. Marquez Peraca, A. Baydin, W. Gao, M. Bamba, Semiconductors and Semimetals 105 , 89 (2020).
[2] Q. Zhang, M. Lou, X. Li, J. L. Reno, W. Pan, J. D. Watson, M. J. Manfra, J. Kono, Nature Physics 12, 1005 (2016).
[3] X. Li, M. Bamba, Q. Zhang, M. Lou, J. D. Watson, K. Yoshioka, M. J. Manfra, J. Kono, Nature Photonics 12, 324 (2018).
[4] W. Gao, X. Li, M. Bamba, J. Kono, Natures Photonics 12, 362 (2018).
[5] X. Li, M.Bamba, N. Yuan, Q. Zhang, Y. Zhao, M. Xiang, K. Xu, Z. Jin, W. Ren, G.Ma, S.Cao, D.Turchinovich, J. Kono, Science 361, 794 (2018).
[6] T. Makihara, K. Hayashida, G. T. Noe II, X. Li, N. Marquez Peraca, X. Ma, Z. Jin, W. Ren, G. Ma, I. Katayama, J. Takeda, H. Nojiri,
D. Turchinovich, S. Cao, M. Bamba, J. Kono, Nature Communications 12, 3115 (2021).
[7] M. Bamba, X. Li, N. Marquez Peraca, J. Kono, Communications Physics 5, 3 (2022).
[8] K. Hayashida, T. Makihara, N. Marquez Peraca, D. Fallas Padilla, H. Pu, J. Kono, M. Bamba, Scientific Reports 13, 2526 (2023).
[9] N. Marquez Peraca, X. Li, J. M. Moya, K. Hayashida, X. Ma, K. J. Neubauer, D. Fallas Padilla, C.-L. Huang, P. Dai, A. H. Nevidomskyy, H. Pu
E. Morosan, S. Cao, M. Bamba, J. Kono, arXiv:2302.06028.
[10] K. Cong, Q. Zhang, Y. Wang, G. T. Noe II, A. Belyanin, J. Kono, Journal of Optical Society of America B 33, C80 (2016).
* Acknowledgements: The author acknowledges support from the U.S. Army Research Office (Grant No. W911NF2110157), the Gordon and Betty
Moore Foundation (Award No. 11520), and the W. M. Keck Foundation (Award No. 995764).

Unraveling optically induced ultrafast dynamics ofNanoscale magnetic textures
R. Kukreja
University of California, Davis, CA 95616, USA

Ultrafast optical control of magnetization has emerged as a new paradigm for the next generation
memory and data storage devices. Numerous studies have been performed to understand themechanism of transfer of angular momentum at such fast timescales. However, it has been recently
recognized that spatial domain pattern and nanoscale heterogeneities can play a critical role indictating the ultrafast behavior [1-4]. These experimental findings have been possible due to recent
advances in x-ray and extreme ultraviolet sources which combine the power of coherent x-rays withfemtosecond (fs) temporal resolution [5]. I will discuss some of the key studies performed at x-ray
free electron lasers (XFELs) sources in the recent years which have shed light on the magnetic
behavior at ultrafast and ultrasmall frontier. I will also describe some of our recent experimental
results at European XFEL and FERMI where we uncovered symmetry-dependent behavior of theultrafast response [6, 7]. Our results on the same sample with the same experimental conditions but
different ground state (isotropic labyrinth domains vs anisotropic stripe domains) shows that thesymmetry of magnetic texture dictates the magnitude and timescale of the ultrafast response (seeFig. 1).
Fig. 1. Laser pump fluence dependence of magnetic scatteringobtained from isotropic labyrinth and anisotropic stripe domains:(a) Amplitude, (b) ring position, and (c) ring width for magnetic
scattering diffraction pattern obtained from isotropic labyrinth(A0, q0, Γ0) and anisotropic stripe domains (A2, q2, Γ2). The
magnetization quench amplitude for both of the domain pattern
behave similarly, and shows linear dependence with the pumpfluence. Ultrafast distortions of diffraction pattern (ring width andring position) behave strikingly different for labyrinth and stripe
domain pattern. A fluence threshold is observed for the ultrafast distortions of diffraction pattern.

These results clarify the previous controversy in the literature for time-resolved magnetic studies fordifferent samples showing distinct responses. We also utilized time-resolved magnetic scattering totest recent predictions of >10 km/s domain wall speeds. We observed fluence threshold dependence
for distortions of diffraction pattern which are not seen for magnetization quenching, consistent witha picture of domain wall motion with pinning sites (see Fig.1). Supported by simulations, we showthat a speed of 66 km/s for highly
curved domain walls can explain theexperimental data (see Fig. 2) [7].
Fig. 2. Simulated modification of domain
pattern and calculated domain wall velocity:(a) Simulated modified domain pattern (black
and white domains) and initial state (coloredoutline). The comparison clearly shows that
regions with high curvature (dark red or blue)
undergo noticeable domain wall motion, (b) Fluence dependence of calculated domain wall velocity for curved domain
walls for labyrinth domain pattern.

We thus show that these intriguing observations suggest preferential texture-dependent paths for the
spin transport and provide us with a unique way to manipulate spin degrees of freedom.
References[1] D. Z. E. Iacocca, L. Le Guyader, A. H. Reid, W.F. Schlotter, T.-M. Liu, D. J. Higley, G. Coslovich, S. F. Wandel, P. M. Tengdin, S.K. K. Patel,
A. Shabalin, N.Hua, S.B. Hrkac, H.T. Nembach, J. M. Shaw, S.A. Montoya, A. Blonsky, C. Gentry, M. A. Hoefer, M.M. Murnane, H. C.
KapteynE. E. Fullerton, O. Shpyrko, H. A. Dürr, T. J. Silva, Physical Review B 106, 144422 (2022).
[2] M. Hennes, A. Merhe, X. Liu, D. Weder, C. Von Korff Schmising, M. Schneider, C. Günther, Mahieu, B., Malinowski, G., Hehn, M., Lacour,
D. Capotondi, F., Pedersoli, E., Nikolov, I. P., Chardonnet, V., Jal, E., Lüning, J., & Vodungbo, B, Physical Review B 102,174437 (2020).
[3] B. Vodungbo, J.Gautier,G. Lambert,A.B, Sardinha, M. Lozano, S. Sebban, M. Ducousso, W.Boutu, K.Li, B. Tudu, M. Tortarolo, R. Hawaldar
R. Delaunay, V. López-Flores, J. Arabski, C. Boeglin, H. Merdji, P. Zeitoun, J. Lüning, Nature Communications 3, 999 (2012).
[5] B. Pfau, S.Schaffert, L. Müller,C. Gutt, A. Al-Shemmary, F.Büttner, R.Delaunay, S Düsterer, S.Flewett, R.Frömter, J. Geilhufe, E.Guehrs, C.
Günther,R. Hawaldar,M. Hille,N. Jaouen, A. Kobs, K. Li, J. Mohanty, S. Eisebitt, Nature Communications. 3, 1100 (2012).
[6] J. Spencer, R. Kukreja. APL Materials 9, 100702 (2021).
[7] N.Z. Hagström, R. Jangid, T.D. Meera, J. Brock, E.S. Lamb, B. Stoychev, J. Schlappa, N. Gerasimova, B.Van Kuiken, R. Gort, L. Mercadier,L. Le.Guyader, A. Samartsev, A. Scherz, G. Mercurio, H.A. Dürr, A.H. Reid, M. Arora, H.T. Nembach, J.Shaw, E. Jal, E. Fullerton, M.W.
Keller,R. Kukreja, S. Bonetti, T.J. Silva. E. Iacocca, Physical Review B 106, 224424(2022).

Time resolved studies of ultrafast dynamics inWideband-gap
materials
1
1
2
2
2

P. Lample , M. Weis , S. G. Mizrahi , E. Peronne , D. Boschetto , R. Geneaux1
S Guizard1
1
Université Paris-Saclay, 91191 Gif-sur-Yvette, France.
2
Ecole Polytechnique, 91120 Palaiseau, France.

The interaction of intense light pulses with transparent materials is a domain of research with a long
history which is however more active than ever, and this revival concerns both applied science andfundamental research. In this work, our aim is to explore the relaxation dynamics in wide band gap
dielectrics, namely SiO2 and Al2O3. Recently we have shown that time resolved studies are ideal
tools to get new insights in the different electronic relaxation, trapping and avalanche mechanismthat occur during and just after the interaction [1].
We intend now to investigate the interaction of excited carriers with the lattice, and how electronic
excitation can influence the relaxation and lifetime of phonons. For this purpose, we have built at
LOA an experimental set-up, which allows measuring simultaneously the change in transmission andreflectivity, induced by a pump pulse. This exciting pulse is the second harmonic of a Ti-Sa laser
which delivers 40 fs pulse at a repetition rate of 1 KHz, and the probe pulse is the fundamental at
800 nm. In this poster, we wish to present the first set of results.
In SiO2, (see Figure 1), we observe different mechanism depending on the pump intensity: first Kerr
effect, followed by the coherent excitation of phonon, and finally photoexcitation of carriers in the
conduction band, which is followed by a fast trapping of electron hole pairs.

Fig. 1. Variation of reflectivity (left) and transmission (right), for various pump intensity, as a function of time.

It can be seen that a modulation, due to coherent excitation of phonon, is visible in both reflectivity
and transmission change. For the highest pump intensity, this modulation vanishes, indicating thatcarrier excitation and the following electron-phonon collisions lead to a much faster damping of thephonons. A numerical model currently under development will be presented, which allows
simulating these different mechanisms and the change of optical properties that we measure.
In Al2O3, we measure a much longer lifetime of excited carriers, in the range of tens of picosecond,showing that no self-trapping mechanism is occurring in this material. We have also studied thevariation of both signal as a function of the orientation of the crystalline sample. Surprisingly astrong modulation is observed in the reflectivity signal, while the transmission is only weakly
influenced by the orientation of the crystal. We give a possible interpretation of this unexpectedbehavior, in terms of surface phonons. This first set of experiments will be completed by measuring
the polarization modulation induced for instance by coherent phonon excitation, and also by using a
different probe wavelength domain, in the VUV in the Attolab facility at CEA- LIDYL.
References[1] A. Bildé, K. Redeckas, A. Melninkaitis, M. Vengris, S. Guizard, Journal of Physics. Condensed Matter 33, 315402 (2021).
* Acknowledgements: this work is supported by the ANR program TOCYDYS, contract ref. ANR-19-CE30-0015.

Electron-phonon coupling and electronic thermal transport across a
Metal-semiconductor
interface:a structural
study
1
3
1
1
2
E. C. Landahl , W. Jo , B. Griffiths , J. Grammich , S. Lee
1
DePaul University, Chicago, IL 60614 USA
2
Korean Research Institute of Standards and Science, 305-340 Daejon, South Korea
3
European X-ray FEL GmbH, 22869 Shenfield, Germany

Understanding the interactions between energetic charge carriers and lattice vibrations (phonons) is
crucial for optimizing the performance of optoelectronic devices, photovoltaics, and photocatalysts.
The electron-phonon coupling coefficient, which characterizes the strength of these energy
exchanges, is challenging to measure due to the involved microscopic time and length scales. In thisstudy, we present an alternative approach to determine the coupling coefficient, thermal boundary
resistance, and electron conductivity. We employ high angular-resolution time-resolved X-ray
diffraction to examine lattice deformation after laser excitation of a nanoscale polycrystalline metal
film on a semiconductor substrate.

Fig. 1. (a) Schematic of the TRXD experiment of metal layered semiconductor, and (b) time- and angle-resolved
intensity profiles of the obtained GaAs (0 0 4) Bragg diffraction peak. The red dashed box indicates the regionof interest where the intensity modulation along the angular axis is detected due to X-ray Brillouin scattering.

Our results provide direct experimental evidence for differentiating between ballistic and diffusivetransport components at the interface, with only the latter participating in thermal diffusion. This
novel technique offers a robust method for investigating microscopic energy transport processes invarious solid-state materials, advancing our understanding of their underlying mechanisms andpotential applicationsLight-matter interactions involving energetic electrons and quasiparticles are offundamental interest in condensed matter physics [1,2] and are also critical to the development of
next-generation microelectronics. In particular, the interaction between electronic states and thelattice in excited solids stands out as one of the dominant scattering processes that determine various
material properties, ranging from charge carrier and thermal transport, high-temperature
superconductivity [3,4] and charge density waves [5]. Despite such importance, characterization ofelectron-phonon coupling (EPC) is challenging, especially when the system is driven out ofthermodynamic equilibrium. The essence of EPC lies in the fact that perturbation of the electronic
state leads to an exchange of energy and momentum between electrons and phonons, and therefore
results in elastic stress. Ultrafast optical excitation can induce such transient states, in which a large
population of hot carriers is generated and relaxes its excess energy to the cold lattice. Therefore,
atomic-scale measurements of structural changes in relevant timescales potentially allow us to
explore fine details of the coupling between the electronic states and the lattice.

Fig. 2. (a) Si (004)rocking curve (black
circle) and calculation
result of dynamicaldiffraction theory (bluedashed and red solidlines based on theliterature values). The
two temperature modelfor electron temperatureand lattice temperature
following
laser
excitation are shown forthe literature values (b)and the current work (c).
The calculated strain
profiles for both theliterature values (dashedblue) and current work
(red solid) were solved
at Δt = 5 ps (d) and at Δt= 0.34 ps (e).

The capability to deliver femtosecond laser pulses on metallic surfaces provides a systematic means
to induce strong non-equilibrium between electron and phonon subsystems. Typically, the EPC
coefficients can be measured by using time-domain thermo-reflectance methods [6-9] where thesample’s surface reflectively is monitored after the incidence of femtosecond laser pulses. The timedependent reflectance is compared with the two-step thermal relaxation model [10] that assumes thelinear relation between the reflectivity and temperature.

Fig. 3. Strain profile of GaAs substrate at Δ𝑡=1.8 ns, and corresponding Bragg diffraction curve together with a
fit result (inset). The non-homogeneous temperature distribution near the interface gives rise to the asymmetryof the curve to the part of the lower angle.

However, the experimentally acquired values have often shown considerable variation likely due tothe complex interplay between electrons and phonons as well as multiple scattering mechanisms. To
mitigate such experimental challenges, first principle calculations have been conducted on metals
and semiconductors to confirm the EPC coefficients [11]. Nevertheless, the availability of such
results is still limited and their ability to account for discrepancies between the simulation and theexperiments remains questionable [12].
References[1] C. W. Siders, A. Cavalleri, K. Sokolowski-Tinten, Cs. Tóth, T. Guo, M. Kammler, M. Horn von Hoegen, K. R. Wilson, D. von der Linde
C. P. J. BartySiders, Science 286, 1340 (1999).
[2] A. Rousse, C. Rischel, S. Fourmaux,I. Uschmann, S. Sebban, G. Grillon, Ph. Balcou, E. Förster, J.P. Geindre, P. Audebert, J.C. Gauthier
D. Hulin, Nature 410, 65 (2001).
[3] J. Bardeen, L.N. Cooper, J.R. Schrieffer, Physical Review 108, 1175 (1957).
[4] A. Lanzara, P. V. Bogdanov, X. J. Zhou, S. A. Kellar, D. L. Feng, E. D. Lu, T. Yoshida, H. Eisaki, A. Fujimori, K. Kishio, J.-I. Shimoyama,
T. Noda, S. Uchida, Z. Hussain Z.-X. Shen, Nature 412, 510 (2001).
[5] H. Luo, Q. Gao, H.Liu, Y.Gu, D.Wu, C.Yi, J. Jia, S.Wu, X.Luo, Y.Xu,L.Zhao, Q.Wang, H.Mao, G.Liu, Z.Zhu, Y.Shi, K.Jiang, J.Hu, Z.Xu
X. J. Zhou, Nature Communications 13, 273 (2022).
[6] A. Giri, J. T.Gaskins, B.M. Foley, R. Cheaito, P.E. Hopkins, Journal of Applied Physics 117, 044305 (2015).
[7] Y. Hu, L. Zeng, A.J. Minnich, M.S. Dresselhaus, G. Chen, Nature Nanotechnology 10, 701 (2015).
[8] S. M. Oommen, S. Pisana, Journal of Physics: Condensed Matter 33, 085702 (2020).
[9] K. Kang, G.-M. Choi, Materials 14, 2755 (2021).
[10] K. Katayama, Y. Inagaki, T. Sawada, Physical Review B 61, 7332 (2000).
[11] Z. Lin, L.V. Zhigilei, V. Celli, Physical Review B 77, 075133 (2008).
[12] U. De Giovannini, H. Hübener, S.A. Sato, A. Rubio, Physical Review Letters 125, 136401 (2020).
* Acknowledgements: This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User
Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. E.C.L. was
supported by a DePaul CSH FSRG and URC SRG. This research was supported by the National Research Foundation of Korea under contract
No. NRF-2019K1A3A7A09033397, NRF-2020R1A2C1009597, NRF-2021R1A6A1A10044154, and NRF-2019K1A3A7A09033387.
Preliminary sample characterization at 1C PAL-KRISS beamline at PLS-II were supported in part by MSIT and POSTECH.

Ultrasmall and tunable THz surface plasmon cavities in the deep
Plasmonic
regime
1
1
1
1
2
3
1
4
I. Aupiais , R.Grasset , T. Guo , D. Daineka , J. Briatico , S. Houver , L. Perfetti , J.-P. Hugonin
J.-J. Greffet4, Y. Laplace1
1
École Polytechnique -Institut Polytechnique de Paris, 91120 Palaiseau, France
2
Unité Mixte de Physique CNRS/Thales, 91767 Palaiseau, France
3
Université Paris Cité, 75006, Paris, France
4
Université Paris-Saclay, 91127 Palaiseau, France

The ability to confine THz photons into highly subwavelength volumes is a major goal towards therealization of ultrastrong light-matter coupling at low energy in quantum materials, with possible far
reaching implications for the cavity control of material properties [1]. In optics, surface plasmons canprovide an efficient confinement of light at visible/infrared frequencies, i.e. in the range of theplasma frequency of noble metals. On the other hand, the use of noble metals in most of the THz
cavity architectures realized to date does not harness the potential of THz surface plasmons forphoton confinement. In this talk, I will present our approach for making THz cavities which exploit
the resonance of localized THz surface plasmon modes [2]. These THz plasmonic cavities achieve
extreme plasmonic confinement of the EM-field in mode volumes 𝑉𝑐𝑎𝑣 as small as 𝑉𝑐𝑎𝑣 /𝜆30 ~10−8(𝜆0 = free space wavelength) together with a large tunability as compared to more conventionalarchitectures. Deep inside the plasmonic regime of light-matter interaction, the THz cavities exhibit
strong signatures of electromagnetic nonlocality as the length scales involved become comparable tothat of the microscopic degrees of freedom of the plasma.

Fig. 1. Sketch of THz photons entering a deeply sub-wavelength THz plasmonic cavity

These results demonstrate a new path towards the realization of ultrastrong, tunable and potentially
exotic forms of light-matter coupling with cavities in the THz frequency range.
References[1] F. Schlawin, D. M. Kennes, M. A. Sentef, Applied Physics Reviews, 9, 011312 ( 2022).
[2] I. Aupiais, R. Grasset, T. Guo, D. Daineka, J. Briatico, S. Houver, L. Perfetti, J.-P. Hugonin, J.-J. Greffet, Y. Laplace(in preparation), (2023)* Acknowledgement(s): this work was supported by Agence Nationale de la Recherche (grant n° ANR- 17-CE30-0011-11) andLabex PALM (grant n° ANR-10-LABX-0039-PALM).

Distilling the nonlinearity of a cuprate superconductor using 2-D
Terahertz1 Spectroscopy
1
1
1,2
A. Liu , D. Pavicevic , M. Fechner , A. Cavalleri
Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany
2
University of Oxford, Oxford OX1 3PU, UK

1

Quantum materials, systems in which quantum effects lead to unique macroscopic phenomena withtremendous technological potential, comprise the forefront of condensed matter physics research. In
particular, collective excitations associated with broken-symmetry phases have attracted tremendous
attention as powerful windows into their microscopic physics and dynamics. However, spectroscopyof these collective excitations has been hindered by the so-called ‘terahertz gap’, which refers todifficulties in generation and detection of radiation in the terahertz frequency range, where many
relevant modes of quantum materials are found. In response to this challenge, we translate atechnique known as 2-D spectroscopy [1], an optical analogue of multi-dimensional NMR
spectroscopy, into the terahertz frequency range. We implement, for the first time, 2-D Terahertz
Spectroscopy in a non-collinear, reflection geometry, enabling study of opaque materials andisolation of their constituent terahertz nonlinearities. We apply this technique to the Josephson
plasma resonance [2] in La2-xSrxCuO4, a layered high-temperature superconductor, to distill theunderlying plasmon correlations.

Fig. 1. a: Linear reflectivity experiment, involving an incident field Ein and a reflected field Erefl. b: Terahertz
pulse reflected from optimally-doped LSCO0.16, which exhibits long-lived oscillations fromtheJosephson plasma resonance at temperatures below Tc. c: Schematic of 2-D terahertz spectroscopy in a noncollinear and reflection geometry. Exemplary nonlinear terahertz emission ENL is shown inset for 𝜏𝜏 = 0 and
below Tc.

Measurements of the superconducting transition provide evidence of an unconventional phasedisordering transition without pair breaking.
References[1] S. T. Cundiff, S. Mukamel, Physics Today 66, 44 (2013).
[2] Y. Laplace, A. Cavalleri, Advances in Physics X 1, 3(2016).
* A. Liu acknowledges support from the Alexander von Humboldt Foundation

Artificial quantum states of light fromSinglephotons1
1
2
2

P. Steindl , H. Snijders , J. A. Frey , J. Norman , J. E. Bowers2
D. Bouwmeester1, W. Löffler1
1
Leiden University, 2333 CA Leiden, the Netherlands
2
University of California, Santa Barbara, CA 93106, USA

Multi-photon entanglement is an important resource for applications in quantum information, frommeasurement-based quantum information processing [1] to photonic quantum repeaters [2].
Recently, high-quality and high-rate single photon sources have been developed [3-6] which enable
the production of multi-photon entanglement directly from single photons – from photon number
superpositions [7] over GHZ states [8] to linear cluster states [9].Here we show engineering ofartificial states of light based on single photons, in particular we show that light which reproduces
the Poissonian statistics of coherent laser light can be synthesized [10]. We use a free-space optical
Sagnac loop [11] to manipulate a continuous random stream of true single photons stream. The
photons are produced using a single self-assembled InGaAs quantum dot in an optical micropillar
cavity [4] using resonant laser excitation and cross-polarization laser filtering [12]. The photons aresent into the delay loop where photons in the loop act as a quantum memory leading to quantum
correlations in the photon stream, by inducing quantum interference of photons produced at different
times. We demonstrate experimentally that this leads to light with a second-order photon correlation
function g(2)(0) → 0 as expected for coherent light, and from a model fit we derive indeed a
Poissonian photon-number distribution. In contrast to text-book coherent states, photons in our
artificial coherent states are quantum entangled in the form of linear cluster states – possibly
shedding new light on an old discussion about realistic coherent states [13].

Fig. 1 Left: Sketch of the experimental setup. Right: Photon number distribution of the output light, thestatistics of thermal, coherent, and our artificial coherent light is compared.

This is manifested by a certain amount of squeezing – which might possibly find use in continuousvariable quantum information.
References[1] R. Raussendorf, H. J. Briegel, Physical Review Letters 86, 5188 (2001).
[2] K. Azuma, K. Tamaki, H.-K. Lo, Nature Communications 6, 6787 (2015).
[3] N. Somaschi, V. Giesz, L. De Santis, J. C. Loredo, M. P. Almeida, G. Hornecker, S. L. Portalupi, T. Grange, C. Antón, J. Demory, C. Gómez
I. Sagnes, N. D. Lanzillotti-Kimura, A. Lemaítre, A. Auffeves, A. G. White, L. Lanco, P. Senellart, Nature Photonics 10, 340 (2016).
[4] H. Snijders, J. A. Frey, J. Norman, V. P. Post, A. C. Gossard, J. E. Bowers, M. P. van Exter, W. Löffler, D. Bouwmeester, Physical Review
Applied 9, 031002 (2018)[5] N. Tomm, A. Javadi, N. O. Antoniadis, D. Najer, M. C. Löbl, A.R. Korsch, R. Schott, S. R. Valentin, A. D. Wieck, A. Ludwig, R. J Warburton
Nature Nanotechnology 16, 399 (2021)[6] S. E. Thomas, M. Billard, N. Coste, S. C. Wein, Priya, H. Ollivier, O. Krebs, L. Tazaïrt, A. Harouri, A. Lemaitre, I. Sagnes, C. Anton, L. Lanco
N. Somaschi, J. C. Loredo, P. Senellart, Physical Review Letters 126, 233601 (2021)[7] S. C. Wein, J. C. Loredo, M. Maffei, P. Hilaire, A. Harouri, N. Somaschi, A. Lemaître, I. Sagnes, L. Lanco, O. Krebs, A. Auffèves, C.Simon
P. Senellart, C. Antón-Solanas, Nature Photonics 16, 374 (2022).
[8] M. Pont, G. Corrielli, A. Fyrillas, I. Agresti, G. Carvacho, N. Maring, P.-E. Emeriau, F. Ceccarelli, R. Albiero, P. H. D. Ferreira, N. Somaschi
J. Senellart, I. Sagnes, M. Morassi, A. Lemaitre, P. Senellart, F.Sciarrino, M. Liscidini, N. Belabas, R. Osellame, arXiv:2211.15626 (2022).
[9] D. Istrati, Y. Pilnyak, J. C. Loredo, C. Antón, N. Somaschi, P. Hilaire, H. Ollivier, M. Esmann, L. Cohen, L. Vidro, C. Millet, A. Lemaître
I. Sagnes, A. Harouri, L. Lanco, P. Senellart, H. S. Eisenberg, Nature Communications 11, 5501 (2020).
[10] P. Steindl, H. Snijders, G. Westra, E. Hissink, K. Iakovlev, S. Polla, J. A. Frey, J. Norman, A. C. Gossard, J. E. Bowers, D. Bouwmeester
W. Löffler, Physical Review Letters 126, 143601 (2021).
[11] E. Megidish, T. Shacham, A. Halevy, L. Dovrat, H. S. Eisenberg, Physical Review Letters 109, 080504 (2012).
[12] P. Steindl, J.A. Frey, J. Norman, J.E. Bowers, D. Bouwmeester, W. Löffler, arXiv:2302.05359 (2023).
[13] C. Noh, H. J. Carmichael, Physical Review Letters 100, 120405 (2008).

Long-lived valley polarization in dichalcogenide monolayer andHeterostructure systems
P. H.M. van Loosdrecht
University of Cologne, 50937 Cologne, Germany

The two-dimensionality of monolayers and heterostructures of transition-metal dichalcogenides leadto a reduced dielectric screening and consequently to relatively strongly bound excitons. Not
surprisingly, these systems show a wealth of interesting exciton and optical physics phenomena,
including the formation of trions, strong luminescence, and multi-exciton phenomena. One of themost intriguing phenomena is that the lack of inversion symmetry in these systems, combined with astrong spin-obit coupling yields a novel quantum mechanical degree of freedom, the valley
pseudospin, which is interesting from a fundamental point of view and even has some application
potential.

Fig. 1. Principles of polarization selective heterodyned transient grating spectroscopy.
(a) Illustration of the population grating with parallel polarized pump excitation beams;(b) Illustration of the spin or valley grating with cross polarized pump excitation beams;(c) Optical scheme for heterodyne detection of both population and grating signals.
(d) Intervalley scattering dynamics in a low-pressure CVD grown WSe2 monolayer at T=150 K.
Open circles: data; red solid lines: bi-exponential fits.
The inset shows the dynamics on the short time scales.
Adopted from [1].

The current contribution will highlight some recent work on the nature and dynamics of excitons inthese systems, with some focus on the valley degree of freedom as probed using transient absorption
and 4-wave mixing (transient grating) spectroscopies.
References[1] J. Wagner , H. Kuhn, R. Bernhardt, J. Zhu, P.H M van Loosdrecht, 2D Materials 8, 035018 (2021).

How to activate and detect the Higgs mode in superconductors
D. Manske
Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany

Higgs spectroscopy is a new and emergent field [1-3] that allows to classify and determine thesuperconducting order parameter by means of ultra-fast optical spectroscopy. There are two
established ways to activate the Higgs mode in superconductors, namely a single-cycle ‘quench’ oran adiabatic, multicycle ‘drive’ pulse, both illustrated in Figure 1. In the talk I will review and reporton the latest progress on Higgs spectroscopy, in particular on the role of the third-harmonicgeneration (THG) [4-6] and the possible IR-activation of the Higgs mode by impurities or external
dc current [7,8]. I also provide new predictions for time-resolved ARPES experiments in which, after
a quench, a continuum of Higgs mode is observable and a phase information of the superconducting
gap function would be possible to extract [9].

Fig. 1. Two ways to activate the Higgs mode in superconductors: ‘Driving’ or ‘Quenching’. (left) A driven
superconductor in steady-state non-equilibrium becomes resonant at the eigen-frequency of the Mexican hat
andleads to third harmonic generation (THG).
(right) In the non-adiabatic case, in which a single-cycle THz pulse shrinks suddenly the Mexican hat, one canobserve Higgs oscillations directly in various quantities.

Interestingly, I argue that the Higgs mode can solve the 25-years-old A1g-puzzle in equilibrium
Raman scattering on high-Tc cuprates [10]. Finally, I review all possibilities for activating the Higgs
mode in superconductors known so far.
References[1] L. Schwarz, L. Schwarz, B. Fauseweh, N. Tsuji, N. Cheng, N. Bittner, H. Krull, M. Berciu, G. S. Uhrig, A. P. Schnyder, S. Kaiser, Nature
Communications 11, 287 (2020).
[2] L.Schwarz, D. Manske, Physical Review B 101, 184519 (2020).
[3] H.Chu, M.-J.Kim, K.Katsumi,S. Kovalev, R.D. Dawson, L.Schwarz, N.Yoshikawa, G.Kim, D Putzky, Z. Z. Li, H.Raffy, S.Germanskiy,
J.-C. Deinert, N.Awari, I.Ilyakov, B.Green, M.Chen, M. Bawatna, G.Cristiani, G. Logvenov, Y.Gallais, A. V. Boris, B.Keimer, A. P. Schnyder
D. Manske, M.Gensch, Z.Wang, R. Shimano, S. KaiserS.,Nature Communications 11, 1793 (2020).
[4] L. Schwarz. R. Haenel, D. Manske, Physical Review B 104, 174508 (2021).
[5] H.Chu, S. Kovalev, Z. X. Wang, L.Schwarz, T.Dong, L. Feng, R.Haenel, M.-J. Kim, P. Shabestrari, H.L. Phuong, K. Honasoge
R. D. Dawson, D.Putzky, G.Kim, M.Puviani, M. Chen, N. Awari, A.N. Ponomaryov, I.Ilyakov, M. Blushke, F. Boshini, M. Zonno
S. Zhdanovich, M. Na, G. Christiani, G. Logvenov, D. J. Jones, A. Damascelli, M.Minola, B. Keimer, D. Manske, N. Wang, J.-C. Deinert
S. Kaiser, Nature Communications 14, 1343 (2023).
[6] M.-J. Kim, S. Kaiser, D. Manske, (submitted to Nature Materials) (2023).
[7] M. Puviani, L. Schwarz, X.-X. Zhang, S. Kaiser, D. Manske, Physical Review B 101, 220507 (2020).
[8] R. Haenel, P. Froese, D. Manske, L. Schwarz, Physical Review B 104, 134504 (2021).
[9] L.Schwarz, B. Fauseweh, D. Manske, Physical Review B 101, 224510 (2020).
[10] M. Puviani, A. Baum, S. Ono, Y. Ando, R. Hackl, D. Manske , Physical Review Letters 127, 197001 (2021).

Spin-driven optical nonlinearities for detection and generation of
Spin currents
M. Matsubara
Tohoku University, Sendai 980-8578, Japan

Future spintronics will require a portfolio of techniques for manipulating electron spins in functional
nanodevices. Especially, the establishment of the methods to control spin current is the key
ingredient essential for the transfer and processing of information, enabling faster and low-energy
operation. In this talk, I will show that spin-driven optical nonlinearities are useful to detect andgenerate spin currents [1-3]. (1) I will show that photo-driven ultrafast spin currents can be generated
in a magnetic metamaterial (MM) at room temperature via the magneto-photogalvanic effect (Fig. 1)[1]. By tuning the polarization state of the excitation light, spin currents can be directed with tunablemagnitude along an arbitrary direction in the two-dimensional plane of the MM.
Fig. 1. (a): Schematic of theexperimental setup for generating
spin-driven photocurrents. A
normally incident light with anarbitrarily
polarized
state(polarization angle , ellipticity
angle) excites a magnetic
metamaterial (MM) with threefold
rotational symmetry and out-ofplane magnetization. (b):
Dynamical responses of spindriven photocurrent for. (c):
Magnetic-field dependence ofspin-driven photocurrent. (d): The
flow directionof spin-driven
photocurrent as a function of .
(e): The magnitude | | of spindriven photocurrent as a functionof .

(2) The manipulation of domains by external fields in ferroic materials is of major interest for
applications. In multiferroics with strongly coupled magnetic and electric order, however, themagnetoelectric coupling on the level of the domains is largely unexplored. By using spin-driven
optical second harmonic generation (SHG) microscopy we visualized what happened to themultiferroic domains in the archetypal spin-spiral TbMnO3 under the application of electric,
magnetic, and optical fields (Fig. 2) [2,3]. The results show the deterministic ferroelectric
polarization flop by magnetic field, which leads to the transformation of neutral into nominallycharged domain walls, and the reversible optical switching of multiferroic domains.
(a)

(b)

(c)

Fig. 2. Control of multiferroic domains in TbMnO3 revealed by spin-driven SHG microscopy. (a): Electric-field
control. (b): Magnetic-field flop. (c): Optical switching.
References[1] M. Matsubara, T. Kobayashi, H. Watanabe, Y. Yanase, S. Iwata, T. Kato, Nature Communications 13, 6708 (2022).
[2] M. Matsubara, S. Manz, M. Mochizuki, T. Kubacka, A. Iyama, N. Aliouane, T. Kimura, S. Johnson, D. Meier, M. Fiebig, Science 348, 1112 (2015).
[3] S. Manz, M. Matsubara, T. Lottermoser, J. Büchi, A. Iyama, T. Kimura, D. Meier, M. Fiebig, Nature Photonics 10, 653 (2016).

Challenging space-time-energy limits of magnetism byHarnessing fluctuations
J.H. Mentink
Radboud University, 6525 Nijmegen, the Netherlands
Findings ways to switch between magnetically ordered states at the smallest possible length and time scale,
while simultaneously dissipating the least amount of energy is a major challenge in magnetism. One of the mostappealing routes to achieve this goal is by bringing a magnetic system strongly out of equilibrium, after which
the dynamics is driven by exchange interaction, the strongest force in magnetism, which due to its short-rage
nature also directly acts on nanometer length scales. Ferromagnets multilayers are very interesting model
systems to explore exchange driven switching at the nanoscale, since they host magnetic skyrmions as (meta-)
stable states, which can have nanoscale dimensions and feature an energy barrier for nucleation that is definedby the exchange interaction itself. Moreover, ultrafast optical skyrmion nucleation has been recently
demonstrated in ferromagnetic multilayers [1-3]. Interestingly, despite the high energy barrier in equilibrium,
X-ray studies [3] revealed that the switching was completed in less than 300ps, which is significantly faster thanthe characteristic timescale of skyrmions dynamics.In this contribution, we will present our efforts to gain
understanding of the mechanism of ultrafast skyrmion nucleation. Based on atomistic spin dynamics
simulations, we show that that the nucleation mechanism can stem from a transient topological fluctuation state[3]. By simulating the relaxation at fixed temperature and magnetic field, we demonstrate that the properties ofthe fluctuation state are governed by thermal fluctuations giving rise to spontaneous nucleation and decay ofskyrmions (see figure 1), consistent with Arrhenius-like activation over a potential barrier. Stable textures canfreeze out of such fluctuations when the effective temperature of the system relaxes faster than the decay timeof the skyrmion textures [4].

Fig.1: Time evolution of the topological charge Q (number of skyrmions) and selected snapshots of the out-ofplane magnetization at different constant temperatures and fields in the atomistic simulations. The timestampsof the snapshots are shown as dashed vertical lines and encoded 1-4. Row (a) corresponds to a regime withspontaneous formation of skyrmions. The starting configuration was the ferromagnetic state. Row (b) shows aregime where skyrmions spontaneously decay, starting from completely randomly aligned spins. Row (c)represents the regime featuring both nucleation and decay, with an initial ferromagnetic state.

Hence, our results show that fluctuations can be harnessed instead of considering them as a nuisance,suggesting a new route towards ultimately fast, small and more energy-efficient writing of stable magnetic bits.
References[1] G. Berruto, I. Madan, Y. Murooka, G. M. Vanacore, E. Pomarico, J. Rajeswari, R. Lamb, P. Huang, A. J. Kruchkov, Y. Togawa, T. LaGrange,
D. McGrouther, H. M. Rønnow, F. Carbone, Physical Review Letters 120, 117201 (2018).
[2] S. -G. Je, P. Vallobra, T. Srivastava, J.-C. Rojas-Sańchez, T.H. Pham, M. Hehn, G. Malinowski, C. Baraduc, S. Auffret, G. Gaudin, S. Mangin
H. Beá, O. Boulle, Nano Letters 18, 7362 (2018).
[3]
e,
a ,
che ,
Sch e e ,
ec
,
he ,
e
,
e,
mann, K. Gerlinger, L.-M. Kern
S
e,
Sch
,
ch ,
e,
h
a, S
a , S z ,e e h,
a , . Caretta, D. Weder,a a,e,a e ,S a ,a ch ,a e ,e ca e , Sch a a, A. Yaroslavtsev, L. Le Guyarder
N. Gerasimova, A. Scherz, C. Deiter, R. Gort, D. Hickin, J. Zhu, M. Turcato, D. Lomidze, F. Erdinger, A. Castoldi, S. Maffessanti, M. Porro,
A. Samartsev, J. Sinova, C. Ropers, J.H. Ment ,, S
each, S. Eisebitt, Nature Materials 20, 30 (2021).
[4] K. Gerlinger, R. Liefferink, M. Schneider, L.-M. Kern, C. Klose, D. Metternich, D. Engel, F. Capotondi, D. De Angelis, M. Pancaldi, E. Pedersoli,
F. Büttner, S. Eisebitt, J.H. Mentink, B. Pfau, arXiv 2212.03143 (2022).

Optical manipulation of Rashba-split
2-dimensional
electron
gas 1
3
1
1

M. Michiardi2, F. Boschini , H.-H. Kung , M. X. Na , S. K. Y. Dufresne , A. Currie1, G. Levy1
S. Zhdanovich1, A. K. Mills1, D. J. Jones1, J. L. Mi4, B. B. Iversen4, Ph. Hofmann4, A. Damascelli1
1
University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
2
Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
3Institut National de la Recherche Scientifique, Varennes, QC, J3X 1S2, Canada
4
Aarhus University, 8000 Aarhus C, Denmark

In spintronic devices, the two main approaches to actively control the electrons' spin degree of
freedom involve either static magnetic or electric fields. An alternative avenue relies on the use of
optical fields to generate spin currents, which promises to bolster spin-device performance allowing
for significantly faster and more efficient logic. To date, research has mainly focused on the opticalinjection of spin currents through the photogalvanic effect, and little is known about the direct opticalcontrol of the intrinsic spin splitting. Here, to explore the all-optical manipulation of a material's spin
properties: we consider the Rashba effect at a semiconductor interface. The Rashba effect has long
been a staple in the field of spintronics owing to its superior tunability, which allows the observationof fully spin-dependent phenomena, such as the spin-Hall effect, spin-charge conversion, and spintorque. In this work, by means of time and angle-resolved photoemission spectroscopy (TR-ARPES),
we demonstrate that an ultrafast optical excitation can be used to manipulate the Rashba-induced
spin splitting of a two-dimensional electron gas (2DEG) engineered at the surface of the topologicalinsulator Bi2Se3.

Fig. 1. TR-ARPES experimental setup and all-optical switch of the Rashba splitting in a 2DEG. An infrared
pulse impinges on an engineered 2DEG, inducing a reduction of its spin-splitting on a picosecond timescale,
which is directly observed via photoemission.

We establish that light-induced photovoltage and charge carrier redistribution modulate the spinorbit coupling strength of the 2DEG on a sub-picosecond timescale. The charge diffusion governs
the return to equilibrium on a nanosecond timescale. The all-optical switching of the Rashba states
can offer an unprecedented platform for achieving THz spin logic devices.
References[1] M.Michiardi, F. Boschini,H.-H. Kung, M.X. Na, S.K.Y, Dufresne , A. Currie , G. Levy, S. Zhdanovic, A.K. Mills, D.J. Jones, J.L Mi, B.B Iversen
P. Hofmann, A. Damascelli, Nature Communications 13, 3096 (2022).
* Authors acknowledge support from: the Max Planck-UBC-UTokyo Centre for Quantum Materials and the Canada First Research Excellence Fund,
Quantum Materials and Future Technologies Program; the Gordon and Betty Moore Foundation's EPiQS Initiative, Grant GMBF4779 to A.D. and
D.J.J.; the Killam, Alfred P. Sloan, and Natural Sciences and Engineering Research Council of Canada's (NSERC's) Steacie Memorial Fellowships(A.D.); the Alexander von Humboldt Foundation (A.D.); the Canada Research Chairs Program (A.D.); NSERC, Canada Foundation for Innovation(CFI); the British Columbia Knowledge Development Fund (BCKDF); the Department of National Defence (DND), the VILLUM FONDEN via
the Centre of Excellence for Dirac Materials (Grant No. 11744); and the CIFAR Quantum Materials Program.

Fluctuation dynamics in the fractionally charged spatial fabric of a
Polaronic
Wigner
crystal 1lattice 2
1
1
1
1
D. Mihailovic , J. Vodeb , V. Kabanov , I. Vaskivskyi , E. Božin , J. Ravnik
1
Jozef Stefan Institute, 1000 Ljubljana, Slovenia
2
Brookhaven National Laboratory, Upton, NY 11973, USA

Ultrafast techniques give us useful a wealth of information on the behavior of single particle andcollective excitations in systems undergoing transitions on the timescales from femtoseconds topicoseconds. However, the dynamics of many-body systems evolves over a much larger span oftimescales. In particular, in the aftermath of a phase transition many-body systems display very richmesoscopic self-assembly into metastable states that requires new techniques, beyond current
optical, x-ray and electron diffraction techniques. Of particular interest are materials close to the
Wigner crystal limit can exhibit very unusual metastable orders. A good example are the layered
dichalcogenides which exhibit complex chiral domain states [1] and an amorphous Wigner glass [2],both of which appear after an external perturbation that forces the system through a symmetry
breaking transition. New experimental evidence of characteristic polaronic local structural distortions
well above CDW ordering temperatur es from X-ray local structure analysis of 1T-TaS2 (Fig. 1 left),
implies that the complex charge ordering in this material is better described in terms of a polaronic
Wigner crystal than a conventional CDW [3].
The application of the Wigner crystal paradigm leads to significant progress in understanding its
complex metastable state dynamics. In particular, optical charge injection is shown to lead to thecharge fractionalization and entanglement with strong non-local effects that explains the remarkablerobustness of the metastable state to local perturbations (Fig.1, middle and right).

Fig 1. Left: Local displacement of atoms within a polaron measured in the different ordered phases of 1T-TaS2.
Chiral atomic distortions appear in the ordered phases. Middle: a schematic representation of the domains ofpolarons in the discommensurate phase. Right: Experimental STM data (left panel) and modeling (right panel)of the electronic charge density in the after charge injection shows fractionally charged sites on domain
walls and their junctions within the charged lattice gas model, calculated with Monte-Carlo simulations.

The entangled fractionally charged network that results upon charge injection presents a
topologically robust gapless space-time fabric for the propagation of single particle excitations thatare measured by conventional transport measurements. On the other hand, new fast STM scanningtechniques reveal unusual and hitherto unobserved mesoscopic polaron dynamics. Remarkably, at
low temperatures, the topological protection is overcome by incoherent tunneling between domain
configurations which is very slow, but observable with STM. The domain dynamics is confirmed bysimulations on a noisy superconducting quantum processor (NSQP) which uses direct mapping of
NSQP qubits onto the crystal lattice sites of 1T-TaS2, and dynamics within the transverse field Isingmodel. The noise in the superconducting qubit system and the electronic crystal both have similar
spectral characteristics, enabling quantitative simulations of the domain dynamics based solely on
microscopic interactions without need for error correction.
References[1] Y. A. Gerasimenko, P. Karpov, I. Vaskivskyi, I.Brazovskii, D. Mihailovic, Npj Quantum Materials 4, 1(2019).
[2] Y. A. Gerasimenko, I. Vaskivskyi, M. Litskevich, J. Ravnik, J. Vodeb, M. Diego, V. Kabanov, D. Mihailovic, Naure Materials 317, 505 (2019).
[3] E. S. Bozin M. Abeykoon, S. Conradson, G. Baldinozzi, P. Sutar, D. Mihailovic, arxiv 2301.05670 (2023).

Terahertz and infrared-driven spin dynamics in iron oxides
R.V. Mikhaylovskiy
Lancaster University, Lancaster LA1 4YB, United Kingdom

Understanding the physics of ultrafast angular momentum transfer between light and magnetic
materials is a one of the key unresolved problems in magnetism. Its importance arises from the factthat light-driven control of magnetization may lead to revolutionary all-optical magnetic recording
technology. Recently we demonstrated that in the iron oxides the terahertz and mid-infrared electricfield triggers large amplitude deflection of the spins [1] that leads to transient switching of the net
magnetization [2,3]. Revealing the microscopic mechanisms of THz-driven spin response to theterahertz pumping will allow to optimize the discovered spin switching mechanism and thereby open
unforeseen routes to enable ultrafast magnetic recording.

Fig. 1. The intricate interplay between the spins, the lattice and the orbitals in transition metal oxides enables
different pathways for manipulation of the material properties. As the spin-orbit, spin-lattice, electron-phonon
and magnetic exchange interactions have meV energy scales, the THz-driven strong perturbation of themenables different pathways for spin control.

In my talk I shall discuss most recent experiments performed in my group on antiferromagnetic ironoxides (e.g. iron borate and orthoferrites), which represent a very large group of magnetic materials
at the heart of current condensed matter and materials science. While some of the most importantfundamental processes of light-spin interactions on ultrafast time scales have been demonstrated inthese compounds in the last decade they are particularly interesting for THz photonics, thanks totheir high-frequency spin dynamics and wide bandgap. The intricate low-energy (~1-10 meV)interactions between lattice, orbital and spin degrees of freedom offer a rich playground for the THz
manipulation of their properties (Fig. 1).
References[1] S. Baierl, M. Hohenleutner, T. Kampfrath, A. K. Zvezdin, A. V. Kimel, R. Huber, and R. V. Mikhaylovskiy, Nature Photonics 10, 715 (2016)[2] S. Schlauderer, C. Lange, S. Baierl, T. Ebnet, C. P. Schmid, D. C. Valovcin, A. K. Zvezdin, A. V. Kimel, R. V. Mikhaylovskiy, R. Huber, Nature
569, 383 (2019).
[3] D. Afanasiev, J. R. Hortensius, B. A. Ivanov, A. Sasani, E. Bousquet, Y. M. Blanter, R. V. Mikhaylovskiy, A. V. Kimel, A. D. Caviglia, Nature
Materials 20, 607 (2021).
* Acknowledgement(s): this work is supported by ERC Grant Starting Grant 852050 MAGSHKE and Horizon 2020 ITN 861300 COMRAD.

Ultrafast lasers based on transition
metal
doped chalcogenides
1
2
1

S. Vasilyev , S. Mirov
IPG Photonics Southeast Technology Center, Birmingham, AL 35211, USA
2
University of Alabama at Birmingham, Birmingham AL 35294, USA

Femtosecond (fs) middle-IR (MIR, 2–20 µm, 500–5000 cm–1) lasers have many applications inscience, technology, industry, medicine. In particular, fs MIR frequency combs combining highbrightness of laser beams with high temporal coherence of mode-locked laser oscillators and fewoptical-cycle pulses are appealing because they significantly improve precision and sensitivity of
conventional Fourier-transform spectroscopy [1] and enable various regimes of dual-comb
spectroscopy [2]. Ultrafast MIR lasers based on transition-metal-doped II-VI chalcogenides (TM:IIVI, [3]) attract growing attention because they provide direct access to peak- and average-power
scalable few-optical-cycle pulses in the 2 – 6 µm range. The progress in TM:II-VI lasers has beenfurther accelerated by the development of new methods of laser material fabrication. Specifically, thetechnology based on a post-grown thermal diffusion doping of ZnS and ZnSe [3] has enabled largescale production of large-size polycrystalline gain elements with high optical quality and high dopant
concentration. This, in turn, has dramatically simplified the development of ultrafast amplifiers andFig.1 (a) Design of Kerr-lens mode-locked polycrystalline Cr:ZnS(Se) oscillator (not to scale). P, pumplaser at 1.5–2.1 µm; L, pump focusing lens; G, AR coated polycrystalline gain element at normal incidence;HR, high reflectors; HRCC, concave high reflectors; OC, output coupler; f, main output of the oscillator andthe fundamental wavelength; 2f, 3f, … optical harmonics of the main signal; *, optionally, the oscillator
canbe equipped with a dispersive prism (P) for the wavelength tuning. For simplicity, only the end mirrors
HRand OC are shown in the resonator; additional HR mirrors can used to fold the laser beams for footprint
reduction. (b) Output of a Kerr-lens mode-locked polycrystalline oscillator. Mid-IR fundamental band
andoptical harmonics, generated directly in the laser medium via RQPM process, are dispersed with a prism andvisualized with a laser viewing card.

enabled the development of fs oscillators with unique output parameters.Cr:ZnS is a typical cavity isrepresentative of a large TM:II-VI family. It combines superb ultrafast laser capabilities with broad
IR transparency and high nonlinearity of wide-bandgap II-VI semiconductors, and supports all
mode-locking regimes from active, to passive, to Kerr-lens mode-locking. Extensive experimental
studies of Kerr-lens mode-locking in polycrystalline Cr:ZnS(Se) have resulted in the development of
a dedicated laser design, which is illustrated in Fig. 1. A prominent distinctive feature of this designis the normal incidence mounting of the polycrystalline gain element in an X-folded asymmetric
optical cavity formed by two curved mirrors (HRCC) and two plane mirrors (HR and OC). Dependingon the desired fundamental repetition frequency fR, and, hence, the required cavity length, the radiiof the curved mirrors can be as large as 200 mm or as small as 10 mm. The curved mirrors are
typically set at small incidence angles (2 – 10°, depending on the fR). The asymmetry of the cavity isdefined by its unequal legs with the typical ratio of 2:5, while the radii of the curved mirrors are
typically equal. Thus, in contrast to classical designs of Kerr-lens mode-locked laser oscillators, thenon-polarizing and astigmatic. The described unconventional design of polycrystalline oscillator
allows to: -operate the laser at increased pump power; -obtain shorter output pulses with higher
energy at a given repetition rate fR; - utilize gain elements with extended length and, hence, enhance
the non-linear interactions inside the optical cavity; - generate few-cycle vector solitons directly inthe mid-IR [4]. Other distinctive features of the developed design are the utilization of the output

couplers with high transmission (up to 60%) and exclusive use of multilayer mirrors for dispersion
control. Further, a part of the fundamental mid-IR laser radiation (f) is converted to second harmonic(2f) and higher harmonics (3f, 4f, 5f) via random-quasi-phase-matched (RQPM) process inpolycrystal. Usually, the harmonics are out-coupled together with the residual pump light via a
curved mirror with high near IR transmission, as illustrated in Fig.1. In most cases, thepolycrystalline Cr:ZnS(Se) oscillators are optically pumped by the radiation of Er-doped fiber lasers
at the wavelength 1.56 µm. However, the pumping by the radiations of Tm-doped fiber lasers (1.9µm) and laser diodes (1.65 µm,) is possible too. The pump sources with low-intensity noise areutilized for demanding applications related to the generation and stabilization of the mid-IR
frequency combs [5]. The Kerr-lens mode-locking with high temporal purity of the pulse train can beobtained in a wide range of relative air humidifies (from 0 to 60%, at temperatures up to 22 C). The
spectra of pulses of selected Kerr-lens mode-locked polycrystalline oscillators hold the world records
in terms of (i) instantaneous spectral span [6], (ii) average power, and (iii) pulse repetition frequency[7]. In general, few-cycle Kerr-lens mode-locked polycrystalline Cr:ZnS(Se) oscillators developed
during the past years are efficient, compact, and reliable. The typical optical-to-optical conversion
from CW pump light to few-cycle mid-IR pulses is 20–25%, and the typical size of the oscillator's
package is 75×100×400 mm3 (for the fundamental pulse repetition frequency fR=80 MHz andexcluding the pump laser). The initiation of mode-locked regime is now fully automated. The
oscillators can operate for thousands of hours without maintenance, including continuous laser
operation in mode-locked regime for hundreds of hours. More details about fs polycrystalline
Cr:ZnS(Se) oscillators can be found in [3].Advanced applications of ultrafast mid-IR lasers impose
stringent requirements on the parameters of output pulses. For instance, dual-comb spectroscopy
requires fs pulse trains with stabilized carrier-envelope offset frequency and ultra-low, attosecondlevel, timing jitter [8]. On the other hand, the experiments on the generation of isolated attosecond
pulses via high harmonic generation require CEP-stable single-cycle mid-IR sources that combine
high energy and high repetition rate. A fruitful approach to those very demanding laser regimes isprovided by a combination of a few-cycle polycrystalline oscillator described above with simple, andhence robust, single-pass polycrystalline Cr:ZnS(Se) amplifier, as illustrated in Fig. 2(a) anddescribed in detail in [6, 9, 10].

Fig. 2. (a) Single-pass polycrystalline amplifier and supercontinuum generator. L, lenses; DM, dichroic mirror.
For simplicity, only the transmissive imaging components (L) are shown; actual setups may also include reflective
imaging components. (b) Supercontinuum spectrum and frequency comb referencing scheme.

In short, the output of the oscillator is superimposed with CW or pulsed radiation from a pump laserusing a dichroic mirror. Two co-propagating radiations are then coupled to a specially tailoredpolycrystalline Cr:ZnS(Se). During their propagation through the laser and non-linear medium, fs
pulses simultaneously experience amplification and non-linear broadening to an opticaloctave.Further, RQPM process with ultra-broad bandwidth results in a multitude of intrapulse threewave mixings. Thus, pulses' spectrum is extended to a continuum spanning many octaves from thebandgap edge of the material to its phonon cut-off. The structure of the continuum is illustrated inFig. 2 (b). It consists of the specially broadened fundamental (f), optical harmonics (2f, 3f, …), and along-wave component (0f) generated in the polycrystalline material via the optical rectification [11].
It consists of several superimposed frequency combs (0f, f, 2f, 3f, …) with different offset
frequencies (0, f0, 2f0, 3f0) but precisely the same tooth spacing (which is equal to the pulse repetition
rate fR = 80 MHz). An offset-free longwave IR comb 0f spanning from 5 µm to 12 µm and beyond isgenerated either directly in the polycrystalline material [6] or in an additional χ(2) medium (e.g. ZGP

or GaSe, [12]) via optical rectification.Single-pass amplifiers combine simplicity of implementation
with the complexity of laser and non-linear interactions. Therefore, they provide a lot of flexibility interms of the achievable output laser parameters. Simple and robust single-pass Cr2+:ZnS(Se) fs
amplifiers allow for power scaling of the few-cycle pulse trains up to 30 W, while the kHz CPAs
provide access to fs pulses with up to 68 GW peak power. Ultrafast Cr2+:ZnS(Se) lasers are uniquely
suited for generation of the optical frequency combs with ultra-low, attosecond-level, timing jitter ofthe pulse trains.The available optical spectra of the Cr:ZnS-based frequency combs are illustrated inFig. 3. Depending on the application, one can utilize a direct output of a single-pass polycrystalline
Cr:ZnS amplifier (Fig. 3 (a)).

Fig. 3 (a, b, c) Spectra of Cr:ZnS-based frequency combs. Numbers near spectra show optical power in theoffset-free 0f band. (d) A photo of Cr:ZnS frequency comb with a business card shown for scale (excluding the
EDFL pump and locking electronics).

In this case, the optical power will be primarily concentrated in the fundamental comb f (2-cycle
pulses with 4-W power spanning 60 THz at -20 dB level with respect to the main peak).
Alternatively, one can equip the system with an additional optical rectification stage to enhance theoptical power in the offset free longwave IR spectral band (Fig. 3 (b, c)). For instance, ZGP crystal
allows the record-breaking efficiency of optical rectification. It generates super-octave longwave IR
combs with Watt-level power. Remarkably, optical rectification of 2-cycle MIR pulses at 2.4 µm
initiates secondary three-wave mixings between 0f and f combs that, in turn, results in a red-shift of
the fundamental comb. Therefore, f and 0f combs merge in a continuum. The additional red-shifted
spectral components of the fundamental comb are highlighted in Fig. 2 as f*.
The key advantage of Cr:ZnS ultrafast laser and frequency comb technology is high efficiency: 20 –25% optical-to-optical conversion from low-cost CW EDFL light to 2-cycle MIR pulses and 10 –12% conversion of 2-cycle pulses at 2.4 µm to single-cycle electromagnetic transients in the
longwave IR. Further, the system's complexity is significantly reduced because all the optical signals
required for stabilizing the MIR comb with the large lever arm are generated directly inside thepolycrystalline Cr:ZnS laser and nonlinear medium. Those advantages, in turn, allowed us to
implement shoe-box-sized, light-weight, fs frequency combs with the brightness exceeding thebrightness of a synchrotron by orders of magnitude in a broad spectral range. The developed MIR
frequency combs open new avenues in imaging, sensing, and spectroscopy. Our preliminary
evaluations confirm the applicability ultrafast Cr:ZnS lasers for dual-comb spectroscopy with theoptical-to-rf mapping factors in excess of 106 and the adequate time of coherent averaging time fromseconds to tens of seconds, to minutes.
References[1] L. Rutkowski, P.Masłowski, A.C.Johansson, A.Khodabakhsh, A.Foltynowicz, J. Quantitative Spectroscopy& Radiative Transfer 204, 63 (2018).
[2] A. S. Kowligy, H. Timmers, A. J. Lind, U. Elu, F. C. Cruz, P. G. Schunemann, J. Biegert, S. A. Diddams, Sciience Advances 5, eaaw8794 (2019).
[3] S. Mirov, I. Moskalev, S. Vasilyev, V. Smolski, V. Fedorov, D. Martyshkin, J. Peppers, M. Mirov, A. Dergachev, V. Gapontsev, IEEE Journal
Selective Topics. Quantum Electronics 24, 1601829 (2018).
[4] S. Vasilyev, M. Y. Sander, J. Gu, V. Smolski, I. Moskalev, M. Mirov, Y. Barnakov, J. Peppers, M. Kolesik, S. Mirov, V. Gapontsev, Journal
Optical Society of America -JOSA B 38, 1625 (2021).
[5] S. Vasilyev, V. Smolski, J. Peppers, I. Moskalev, M. Mirov, Y. Barnakov, S. Mirov, V. Gapontsev, Optical Express 27, 35079 (2019).
[6] S. Vasilyev, I. Moskalev, V. Smolski, J. Peppers, M. Mirov, Y. Barnakov, V. Fedorov, D. Martyshkin, S. Mirov, V. Gapontsev, Optical Express
29, 2458 (2021).
[7] V. Smolski, S. Vasilyev, I. Moskalev, M. Mirov, Q. Ru, A. Muraviev, P. Schunemann, S. Mirov, V. Gapontsev, K. Vodopyanov, Applied Physics
B 124, 101 (2018).
[8] I. Coddington, N. Newbury,W. Swann, Optica 3, 414 (2016).
[9] S. Vasilyev, I. Moskalev, M. Mirov, S. Mirov, V. Gapontsev, Optical Express 24, 1616 (2016).
[10] S. Vasilyev, I. Moskalev, V. Smolski, J. Peppers, M. Mirov, V. Fedorov, D. Martyshkin, S. Mirov, and V. Gapontsev, Optica 6(2), 126(2019).
[11] J. Zhang, K. Fritsch, Q. Wang, F. Krausz, K.-F. Mak, O. Pronin, Optics Letters 44, 2986 (2019).
[12] S. Vasilyev, I.S. Moskalev, V.O. Smolski, J.M. Peppers, M. Mirov, A.V. Muraviev, K. Zawilski, P.G. Schunemann, S.B. Mirov,
K.L. Vodopyanov, V.P. Gapontsev, Optica, 6(1), 111 (2019).
* Acknowledgement(s) : authors acknowledge support from IPG Photonics Corporation, S.M. also acknowledges support from DARPA (grant
W31P4Q-15-1-0008), DOE (grant DE-SC0018378) and NIEHS (grant)P42ES027723.

Ultrafast x-ray
spectroscopyof light-driven
quantummaterials
1
2
3
1
1
4
1

D. Baykusheva , I-T. Lu , H. Jang , S. F. R. TenHuisen , F. Glerean , C. Weber , T. Meng
H. Padmanabhan1, W. He5, Y. Shen5, G. D. Gu5, D. M. Kennes2,4, M. Claassen6, M. P. M. Dean5, A. Rubio2
M. Mitrano1
1
Harvard University, Cambridge, MA 02138, USA
2
Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany
3
Pohang Accelerator Laboratory, Gyeongbuk 37673, Republic of Korea
4
Aachen University, 52056 Aachen, Germany
5
Brookhaven National Laboratory (BNL), Upton, NY 11973 USA
6
University of Pennsylvania, Philadelphia, PA 19104
7

The last two decades have witnessed a remarkable progress in the use of ultrafast lasers pulses as atool to realize and control nontrivial dynamical states of matter, e.g., transient superconductivity and
Floquet topological phases. Among a variety of nonthermal excitation pathways, a particularly
intriguing route is represented by the direct light-engineering of effective many-body interactions,such as electron hopping amplitudes and electron-electron repulsion. Achieving control over theseparameters in strongly correlated electron systems gives us the opportunity to tune their microscopic
physics in ways akin to what achieved in quantum simulators. Here, I will discuss recent
developments in our capability to manipulate electronic interactions in light-driven quantum
materials and to accurately quantify these effects by using time-resolved x-ray spectroscopy. I will
focus on quasi-one-dimensional and two-dimensional copper oxides as model systems to investigate
the light-matter interaction in presence of intense electromagnetic fields.

Fig. 1. Crystal structure of quasi-1D cuprate Sr2CuO3. Cu atoms are shown in blue, O in red, Sr in white. The
compound is driven by an intense pump field, and probed with delayed soft x-ray pulses.

In the quasi-2D La2-xBaxCuO4, MV/cm laser fields are shown to dynamically renormalize thescreened onsite Coulomb repulsion (“Hubbard U”) [1]. In the quasi-1D ott insulator Sr2CuO3, adifferent picture emerges. Combining time-resolved optical and x-ray spectroscopy measurementswith advanced theoretical modeling, we map out the transient electronic states giving rise to thelight-dressed band structure as well as the renormalized electronic interactions [2].
We discuss the role of dimensionality in the light-matter interaction and the implications for thecontrol of superconductivity, magnetism, as well as to the realization of other long-range-orderedphases such as spin liquidity [3,4] and η-pairing correlations [5,6] in light-driven quantum materials.

References[1] D. R. Baykusheva, H. Jang, A. A. Husain, S. Lee, S. F. R. TenHuisen, P. Zhou, S. Park, H. Kim, J.-K. Kim, H. D. Kim, M. Kim, S.-Y. Park,
P. Abbamonte, B. J. Kim, G. D. Gu, Y. Wang, M. Mitrano, Physical Review X 12, 011013 (2022).
[2] D. R. Baykusheva, I-T. Lu, H. Jang, S. F. R. TenHuisen, F. Glerean, C. Weber, T. Meng, H. Padmanabhan, W. He, Y. Shen, G. D. Gu,
D. M. Kennes, M. Claassen, M. P. M. Dean, A. Rubio, M. Mitrano, (submitted) (2023).
[3] M. Claassen, H.-C. Jiang, B. Moritz, T. P. Devereaux Nature Communications 8, 1192 (2017).
[4] A. Szasz, J. Motruk, M. P. Zaletel, J. E. Moore, Physical Review X 10, 021042 (2020).
[5] T. Kaneko, T. Shirakawa, S. Sorella, S. Yunoki, Physical Review Letters 122, 077002 (2019).
[6] F. Peronaci, O. Parcollet, M. Schiró, Physical Review B 101, 161101 (2020).
* Acknowledgement(s): This work is primarily supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Early Career Award
Program, under Award No. DE-SC0022883. M. M. further acknowledges support by the Aramont Fellowship Fund for Emerging Science
Research at Harvard University. D. R. B. was supported by the Swiss National Science Foundation through Project No. P400P2_194343. M. C.
acknowledges support by NSF Grant No. DMR-2132591. D. M. K. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) via Germany’s Excellence Strategy—Cluster of Excellence Matter and Light for Quantum Computing (ML4Q)
EXC 2004/1—390534769 and within the RTG 1995. We also acknowledge support from the Max Planck-New York City Center for NonEquilibrium Quantum Phenomena.

Transient Enhancement of the Ferroelectricity in the Rashba
Semiconductor
α-GeTe
1
2
3
1
2
1
1
4
G. Kremer , J. Maklar , L. Nicolaï , C.W. Nicholson , C. Silva , C. Yue , P. Werner , J.H. Dil
J. Krempaský5, G. Springholz6, R. Ernstorfer2, J. Minár3, L. Rettig2, C. Monney11
Université de Fribourg, CH-1700 Fribourg, Switzerland
2
Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
3
University of West Bohemia, 306 14Plzen, Czech Republic
4
École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
5
Photon Science Division, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
6
Johannes Kepler Universität, A-4040 Linz, Austria

α-GeTe(111) is a bulk ferroelectric Rashba semiconductor that exhibits the largest known Rashba-typespin splitting of so-far known materials. It has been recently demonstrated that it is possible toreversibly manipulate the spin polarization of its low-energy electronic structure by an external electricfield in α-GeTe(111) [1], a promising behavior for spintronics applications. A stimulating direction ofresearch is to investigate whether it is possible to coherently modify the ferroelectric properties of GeTewith light pulses.Using a 800 nm photoexcitation, we drive α-GeTe(111) out-of-equilibrium and probe
its transient low-energy electronic structure with time-resolved angle-resolved photoemission
spectroscopy (ARPES). We reveal that the Rashba splitting of its bulk states is enhanced after 200 fs.
We show that this change of the electronic structure is driven by a shift of the Ge atomic layer towards
the Te atomic layer, meaning a transient increase of the ferroelectric distortion [2].

Fig. 1. Left: we performed time-resolved ARPES on the ferroelectric Rashba semiconductor GeTe. Right:
temporal evolution of the Rashba splitting (blue curve) showing enhancement and a coherent modulation,
following the photoexcitation populating the conduction band (red curve). See Ref. [2] for more details.

We identify a surface photovoltage effect as the mechanism responsible for this transient enhancementof the ferroelectricity at the surface of GeTe and link it to a delayed displacive excitation of the coherent
phonon of the ferroelectric distortion. Our work potentially opens the route for the control of theferroelectric polarization in α-GeTe(111).
References[1] J. Krempaský, S. Muff, J. Minár, N. Pilet, M. Fanciulli, A.P. Weber, E.B. Guedes, M. Caputo, E. Müller, V.V. Volobuev, M. Gmitra, C.A.F. Vaz
V. Scagnoli, G. Springholz, J.H. Dil, Physics Review X 8, 021067 (2018).
[2] G. Kremer, J. Maklar, L. Nicolaï, C.W. Nicholson, C. Yue, C. Silva, P. Werner, J.H. Dil, J. Krempaský, G. Springholz, R. Ernstorfer, J. Minár,
L. Rettig, C. Monney, Nature Communications 13, 6396 (2022).

Reliability and performance improvement in femtosecond lasers andAmplifiers, and their components
J. Henrich, M. Arrigoni, L. Moog, P. Vogt, S. Mosquera
Coherent Corporation, Santa Clara, CA95054, USA

In the last decades, femtosecond laser-based experiments have become increasingly complex.
Multidimensional spectroscopy or Attosecond physics are examples of this evolution where the laser
source is just the starting point of an extremely sophisticated optical train leading to the final
experiment and data production. With these stringent boundary conditions, the main added value ofan ultrafast laser source is the production of consistent, reliable and high-performance pulses of light.
While ultimate laser performance in term of pulse duration, energy per pulse and mode quality is andhas been the enabling point of experimental success, the reliability of the laser source has a dramatic
impact on data productivity and ultimately technical paper throughput. Following this demand for
performance and reliability, we took a series of steps towards the design and manufacturing of
femtosecond laser sources that match performance (“technical specification”) improvement withdemanding reliability and environmental testing based on rugged industrial protocols. Many
applications in advanced spectroscopy require pulses shorter than 100 femtoseconds and withenergies achievable only with Chirped Pulsed Amplification (CPA) of the laser pulses. This process
requires one or multiple stages of amplification of the initial pulses provided by the “seed” laser, as
shown in the figure below. Multiple active and passive optical components are needed, andmanaging their interplay is critical for safe and reliable operation, often close to the limit of opticaldamage.

Fig.1 Key optical elements in a fiber or bulk ultrafast amplifier

We determined that an important pathway to improved performance and reliability is to control thedesign and manufacturing of as many critical elements of the supply chain as possible. This control
is exercised by assessing the ultimate limitations of these components using testing tools notavailable to outside manufacturers or by provide extensive and proprietary feedback to the in-housecomponent design and manufacturing teams [1]. To this end, we provide a review of recent
improvements in component technology and performance that enable the scaling of ultrafast
amplified systems in reliability, performance and efficiency.
Finally, we will present some examples of applications that are significant for this audience and thatused ultrafast amplifiers from Coherent to produce high-quality data.
References[1] M. Arrigoni, T. McComb, Presentation at NATO SET-308 workshop at Photonics West (2022)

Multi-mode-mixing
Deep-strong
light-matterinteraction
1
1
1
1

J. Mornhinweg , L. Diebel , M. Halbhuber , M. Prager , J. Riepl1
T. Inzenhofer1, D. Bougeard1, R. Huber1, C. Lange2
1
Department of Physics, University of Regensburg, 93040 Regensburg, Germany
2
Department of Physics, TU Dortmund University, 44227 Dortmund, Germany

Dressing quantum states of matter with virtual photons can drive non-perturbative light-matter
dynamics on time scales even faster than the oscillation cycle of light. Ultrastrong coupling of lightand matter features an exotic vacuum ground states hosting a significant number virtual excitations.
Their fluctuating vacuum fields allows for spectacular cavity-QED effects ranging from vacuumfield induced charge transport [1], polaritonic chemistry [2], strong nonlinearities [3] and coherent
polariton scattering [4]. With deep-strong coupling (DSC), the coupling strength defined as the ratioof the vacuum Rabi frequency ΩR and the carrier frequency of light ωc, even exceeds unity,
ΩR/ωc ≥ 1 [5,6]. Non-adiabatic switching of ΩR [7] is expected to release the virtual ground state
population in the form of vacuum radiation – a scenario which is becoming more realistic ascoupling strengths are further increased. Despite impressive progress made in this direction, newresearch has found fundamental constraints caused by light-matter decoupling [8] or dissipation [9].
Moreover, the commonly used principle of resonant interaction of two modes [10] is intrinsicallylimited by the oscillator strength of a single electronic excitation.
Here [11], we overcome this limitation and present a conceptually novel regime of deep-strong lightmatter coupling, making use of the fact that even strongly detuned electronic modes can significantly
boost the vacuum ground state population of an optical mode. Going beyond single pairs of coupledmodes (Fig. 1a), we cooperatively couple multiple, highly non-resonant modes (Fig. 1b) –magnetoplasmons specifically tailored by our maximally compact metasurface (Fig. 2a,b). We test
our concept with several GaAs multi-quantum well (QW) heterostructures which host Landauquantized two-dimensional (2D) electron gases, featuring a cyclotron resonance (CR) with a tunablefrequency νc. Multiple discrete plasmon resonances formed by our metasurface hybridize with the
CR and couple to the optical modes j of the resonator structure with an individual vacuum Rabi
frequency ΩR,j,α. We investigate 6 samples with nQW = 1, 3, 6, 12, 24 and 48 QWs (Fig. 1c-h),
displaying an evolution of the fan of upper polaritons, UPj,β from a tightly packed ensemble tomultiple, clearly separated resonances. This ultrabroadband spectrum of more than 20 cavity
polaritons spans up to 6 optical octaves, clearly evidencing the multi-mode character and the extreme
coupling. We created a multi-mode theory of DSC based on the Hopfield Hamiltonian to properly
characterize these spectra. Since the notion of the anti-crossing where the coupling strength ismaximized becomes irrelevant for off-resonant, multi-mode coupling, we highlight the number ofvirtual photons in the vacuum ground state 〈Nj〉 of each photonic mode j as the figure of merit.
Applying our theory to our experimental results, we reach up to 〈N1〉 = 1.00 and 〈N2〉 = 0.18,
corresponding to ΩR,j=1/ωj=1 = 2.83 and ΩR,j=2/ωj=2 = 0.93 for the more than 20 unique, strongly
coupled resonances of the sample with nQW = 48. The combined vacuum ground state population ismore than 2 times higher than previously achieved [5,6], and, for the first time, exceeds unity.

Fig. 1. a, Resonant ultrastrong coupling of a single cavity mode (upper parabola) to a single matter excitation(bottom parabola) with a vacuum Rabi frequency ΩR. The weak population by virtual excitations in the vacuum
ground state is indicated by semi-transparent spheres. b, Non-resonant DSC of one light mode (upper parabola)
to multiple matter excitations (bottom parabolas) with vacuum Rabi frequencies ΩR,j,α under off-resonantconditions. Owing to the extremely large light-matter coupling, a significant number of virtual excitations arepresent.

The extreme interaction results in a strongly subcycle exchange of vacuum energy between themultiple bosonic modes akin to high-order nonlinearities otherwise reserved to strong-field physics.
This is also evident in the properties of the extreme vacuum ground state – equivalent to a record
coupling strength of ΩR/ωc = 3.19 – resulting in strong two-mode squeezing, a highly non-classicalFock-state probability distribution and the emission of correlated photon pairs, which is expected for
non-adiabatic modulation [7].

Fig. 2. a, Conventional metasurface (gold) and its electric near-field E ̃, for the fundamental mode at νc =0.52 THz. The unit cell (size: 60 µm × 60 µm) is indicated by the dashed line. QW: quantum well stack. b,
Electric field E ã nd unit cell for the maximally compact metasurface. c-h, THz transmission for 1, 3, 6, 12, 24,
48-QWs. White curves: polariton modes for the first resonator mode. Dashed curves: polaritons linked to thehigher mode ν2. c, 1 QW, coupling strength: 𝛺𝑅,1 /𝜔1 = 0.55.d, 3-QWs (𝛺𝑅,1 /𝜔1 = 0.76). e, 6-QWs (𝛺𝑅,1 /𝜔1 =1.34). f, 12-QWs (𝛺𝑅,1 /𝜔1 = 2.32). g, 24-QWs (𝛺𝑅,1 /𝜔1 = 2.65, 𝛺𝑅,2 /𝜔2 = 0.87). h, 48-QWs (𝛺𝑅,1 /𝜔1 = 2.67,
𝛺𝑅,2 /𝜔2 = 0.88).

In conclusion, our work [11] represents a leap forward in c-QED by overcoming the limitations ofresonant light-matter coupling with our novel concept of cooperative, multi-mode hybridization,
allowing to increase the total vacuum photon population almost arbitrarily by adding electronic
oscillator strength within a wide spectral window. This even facilitates hybridization of otherwise
orthogonal matter states purely through vacuum fields of a common cavity mode and can be applied
to interactions between a variety of systems such as magnons, phonons or Dirac electrons, including
the mixing of entirely different excitations. Combined with the resulting sizeable virtual populationsof light and matter modes, our concept offers new flexibility and an unprecedented level of controlof, for example, electronic transport [1], light-induced phase transitions [12] or chemical reactions[2], by merely shaping the dielectric environment with vacuum fields.
References[1] G. L.Paravicini-Bagliani, F. Appugliese, E. Richter, F. Valmorra, J. Keller, M. Beck, N. Bartolo, C. Rössler, T. Ihn, K. Ensslin, C.Ciuti, G. Scalari
J. Faist. Nature Physics 15, 186 (2019).
[2] A. D. Dunkelberger, B. S. Simpkins, I. Vurgaftman, J. C. Owrutsky. Annual Review of Physical Chemistry 73, 429 (2022).
[3] J. Mornhinweg, M. Halbhuber, C. Ciuti, D. Bougeard, R. Huber, C. Lange. Physical Review Letters 126, 177404 (2021).
[4] M. Knorr, J. M. Manceau, J. Mornhinweg, J. Nespolo, G. Biasiol, N. L. Tran, M. Malerba, P. Goulain, X. Lafosse, M. Jeannin, M. Stefinger
I. Carusotto, C. Lange, R. Colombelli, R. Huber. Physical Review Letters 128, 247401 (2022).
[5] A. Bayer, M. Pozimski, S. Schambeck, D. Schuh, R. Huber, D. Bougeard, C. Lange. Nano Letters 17, 6340 (2017).
[6] N. S. Mueller, Y. Okamura, B. G. M. Vieira, S. Juergensen, H. Lange, E. B. Barros, F. Schulz, S. Reich. Nature 583, 780 (2020).
[7] M. Halbhuber, J. Mornhinweg, V. Zeller, C. Ciuti, D. Bougeard, R. Huber, C. Lange. Nature Photonics 14, 675 (2020).
[8] S. De Liberato, Physical Review Letters 112, 16401 (2014).
[9] S. Rajabali, E. Cortese, M. Beck, S. De Liberato, J. Faist, G. Scalari. Nature Photonics 15, 690 (2021).
[10] E. Cortese, J. Mornhinweg, R. Huber, C. Lange, S. De Liberato. Optica, 10, 11 (2023).
[11] J. Mornhinweg, L. Diebel, M. Halbhuber, M. Prager, J. Riepl, T. Inzenhofer, D. Bougeard, R. Huber, C. Lange, (submitted) (2023).
[12] F. Schlawin, A. Cavalleri, D. Jaksch., Physical Review Letters 122, 133602 (2019).
* Acknowledgements: The authors thank Dieter Schuh and Imke Gronwald for valuable discussions and technical support. We gratefully
acknowledge support by the Deutsche Forschungsgemeinschaft through Project IDs 231111959 and 422 31469 5032-SFB1277 (Subproject A01),
grants no. LA 3307/1-2, and HU 1598/2, as well as by the European Research Council (ERC) through Future and Emerging Technologies (FET)
grant no. 737017 (MIR-BOSE).

Subcycle dynamics of quantum light
A. S. Moskalenko
Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea

Quantum properties of light play one of the central roles for emerging quantum technologies. Usually
they are characterized in the frequency domain in a vicinity of a well-defined carrier frequency. In thispicture the Wigner function is frequently used for a phase-space visualization of the states as well as
for accessing their physical properties via the classical averaging of the corresponding quantities overthe phase space. Extending this approach to a pulsed ultrabroadband quantum light would lead to aquite involved description in terms of a large set of single-frequency or shaped temporal modes, where
each mode has to be characterized separately while keeping the intermode phase relation fixed, or
would require an a priori knowledge about the shape of a smaller number of the localized temporalmodes in which the state of the light can be expanded. An alternative approach is to consider thequantum fields directly in the time domain [1-3]. For example, we have shown that it is possible toformulate a time-domain theory of the generation and time-resolved detection of ultrabroadband
pulsed squeezed vacuum states [4] and that it should be possible to get access to arbitrary rotatedgeneralized quadratures of the field [5,6]. In this contribution, we discuss how to define and retrievethen the corresponding subcycle-resolved Wigner function.

Fig.1. Scheme of the statistical readout capable to trace the quantum statistics of the signal, containing the(quantum) shot-noise contribution and the part originated from the sampled quantum field .

In the corresponding setup a few-femtosecond near-infrared (NIR) probe pulse is sent through a thinelectro-optic crystal where it interacts with a quantum field in the mid-infrared (MIR) range and thenanalyzed via a statistical readout (see Fig. 1). We show that a slightly modified version of the setup
enables the detection of the probability distribution of the analyzed probe field for arbitrary phaseshifts, thus enabling a full quantum tomography of its state [5]. In order to extract phase-space
distributions of the sampled MIR field, a procedure to disentangle the contributions of the MIR fieldfrom the NIR quantum shot noise has still to be found.
Assuming that this procedure is accomplished, we can demonstrate a way to reconstruct thephase-space distributions of the MIR field with an extreme (subcycle) temporal resolution. In order toavoid the complications connected with the shot noise, in this contribution we also show how toretrieve the subcycle-resolved Wigner function based on the homodyne detection withphase-controlled half-cycle pulses.

We illustrate the obtained theoretical results for the case of the pulsed squeezed vacuum states andderived from them photon-subtracted states (see Fig. 2).We have developed a new tomography scheme
to completely resolve the instantaneous structure of quantum ultrabroadband light in the time domain.
The function reconstructed from the tomography protocol represents a joint quasi-probability
distribution of the sampled instantaneous electric field and its conjugated generalized quadrature. It iscapable to visualize the dynamics of the field state, providing direct access to the ultrafast evolution of
its characteristic features, such as photon content, squeezing and negativity.

Fig. 2. Snapshots of the subcycle-resolved Wigner function are shown for several values of the delay time td
for the pulsed squeezed vacuum state (corresponding photon-subtracted state) in the upper (lower) row.

This achievement paves the ways to further advances towards ultrafast quantum spectroscopy capableto probe femtosecond dynamics of correlations and entanglement in many-body systems, hidden to
probing by methods of classical ultrafast photonics.
References[1] C. Riek, D.V. Seletskiy, A.S. Moskalenko, J.F. Schmidt, P. Krauspe, S. Eckart, S. Eggert, G. Burkard, A. Leitenstorfer, Science 350, 420 (2015).
[2] A. S. Moskalenko, C. Riek, D. V. Seletskiy, G. Burkard, A. Leitenstorfer, Physical Review Letters 115, 263601 (2015).
[3] C. Riek, P. Sulzer, M. Seeger, A.S. Moskalenko, G. Burkard, D.V. Seletskiy, A. Leitenstorfer, Nature 541, 376 (2017).
[4] M. Kizmann, T.L.M. Guedes, D.V. Seletskiy, A.S. Moskalenko, A. Leitenstorfer, G. Burkard., Nature Physics 15, 960 (2019).
[5] M. Kizmann, A.S. Moskalenko, A. Leitenstorfer, G. Burkard, S. Mukamel, Laser and Photonics Reviews 16, 2100423 (2022).
[6] S. Gündoğdu, S. Virally, M. Scaglia, D.V. Seletskiy, A.S. Moskalenko, Laserand Photonics Reviews, DOI:10.1002/lpor.202200706 (2023).
* Acknowledgement(s): the author acknowledges support from the National Research Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (2020R1A2C1008500) and from the Deutsche Forschungsgemeinschaft (DFG), Project No. 425217212 - SFB 1432, via the
Mercator Fellowship.

Ultrafast spectroscopy of optical and electrical properties of
Dirac semimetal driven by periodic light fieldY. Murotani, R. Matsunaga
The University of Tokyo, Chiba 277-8581, Japan

Floquet engineering provides an efficient protocol to manipulate the states and properties of matter
using external fields periodic in time. Interesting examples include the Floquet-Weyl semimetal(FWSM), which has been predicted to emerge in three-dimensional Dirac semimetals (DSMs)
irradiated by circularly polarized light [1]. However, manifestations of such Floquet-engineered
states to experimental probes remain largely unclear. We have studied optical and electricalproperties of a three-dimensional DSM, Cd3As2, exposed to an intense multi-terahertz light field [2,3]. Figure 1(a) shows the transient broadband absorption spectrum under the driving field at 120
meV (30 THz) [2].

Fig. 1. (a) Transient optical conductivity during irradiation by linearly polarized pump pulse.
(b) Schematic of the formation of Floquet states and the resulting stimulated Rayleigh scattering.

The spectrum is characterized by a large dispersive structure with net optical gain around the driving
frequency. Microscopic calculations revealed this drastic change to arise from optical transitions
among the Floquet subbands (Fig. 1(b)), which take the form of stimulated Rayleigh scatteringenhanced by the longitudinal plasma mode [2]. We next examined the anomalous Hall effect (AHE)of Cd3As2 under circularly polarized light, using time-resolved terahertz Faraday rotation
spectroscopy [3]. Though a light-induced AHE has been proposed as an indication of the FWSM, weobserved an anomalous Hall conductivity larger than, and with a sign opposite to, the theoreticalprediction for a FWSM (Fig. 2(a)).

Fig. 2. (a) Anomalous Hall conductivity during irradiation by left circularly polarized pump pulse.
(b) Schematic of the field-induced injection current, allowed by the terahertz electric field, E0.

Microscopic analysis shows that this phenomenon originates from “field-induced injection current,”
i.e., a transverse photocurrent generation in the presence of an electric field (Fig. 2(b)). This result
clarifies the essential role of photoexcited carriers under a bias field, and suggests that materials
transparent to the driving field are better to observe the AHE by a FWSM.
References[1] H. Hübener, M. A. Sentef, U. De Giovannini, A. F. Kemper, A. Rubio, Nature Communications 8, 13940 (2017).
[2] Y. Murotani, N. Kanda, T. N. Ikeda, T. Matsuda, M. Goyal, J. Yoshinobu, Y. Kobayashi, S. Stemmer, R. Matsunaga, Physical Review Letters
129, 207402 (2022).
[3] Y.Murotani, N.Kanda, T.Fujimoto, T.Matsuda, M.Goyal, J.Yoshinobu, Y.Kobayashi, T.Oka, S.Stemmer, R. Matsunaga, arXiv:2211.02229 (2022).

ALBATROSS – a single-cycle infrared light source with unique
Waveform Stability
N. Nagl3, P. Steinleitner1, M. Kowalczyk,4, N. Karpowicz1, A. Sebesta4, A. Paudel2
D. Potamianos4, M. Stadter1, S. Gröbmeyer2, A. Głuszek5, A. Hudzikowski5
V. Pervak2, J. Sotor5, K. F. Mak1, F. Krausz1, A. Weigel1
1
Max Planck Institute of Quantum Optics, 85748 Garching, Germany
2
Ludwig Maximilian University Munich, 85748 Garching, Germany
3
PULSED GmbH, 85748 Garching, Germany
4
Center for Molecular Fingerprinting, 1093 Budapest, Hungary
5
Wrocław University of Science and Technology, 50-370 Wrocław, Poland

Waveform-controlled single-cycle light sources lay the foundation for manipulating the nonlinear
polarization of matter on a sub-femtosecond time scale. Due to their precisely controllable carrierenvelope phase (CEP), they have paved the way for ground-breaking research in the past, including
the ultrafast manipulation of electric currents in solids [1] and the generation of isolated attosecond
pulses [2]. In particular, strong-field processes in condensed matter, such as the tunneling throughnanoscopic gaps [3], can already be induced at moderate, sub-nano-joule-level pulse energies.
Until recently, Ti:sapphire systems have been the gold standard for providing laser pulses with highwaveform stability [4] and pulse durations approaching a single optical cycle [5]. Yet, theirapplication has been constrained by their near-infrared emission spectrum at around 800 nm,
requiring complex and inefficient frequency down-conversion to directly address light wave-driven
carrier dynamics in low-bandgap semiconductors. Laser oscillators based on Cr2+-doped II-VI gain
media have evolved as promising alternatives, providing emission wavelengths around 2-3 µm —ideal for direct frequency-comb spectroscopy [6] and efficient down-conversion to the mid-infrared
region [7]. In this contribution we present a unique CEP-stable single-cycle source at 2.2 µm with
unprecedented waveform stability. The system is based on a diode-pumped Cr:ZnS laser oscillator
combined with subsequent extreme nonlinear spectral broadening in a single thin TiO2 bulk plate.
Efficient frequency down-conversion can further extend the spectral coverage towards longer
wavelengths and allows to generate single-cycle mid-infrared pulses with CEP-controlled
waveforms [7]. For the wider proliferation of this new technology, a compact and monolithic
platform has been developed: ALBATROSS. The heart of the experimental system, i.e. the precursorof ALBATROSS, constitutes a diode-pumped Cr:ZnS oscillator, delivering 28-fs short pulses at arepetition rate of 22.9 MHz (Fig. 1a).

Fig. 1. a: CEP-stable single-cycle pulse generation. Schematic of the CEP-stabilized Cr:ZnS
laser system. b: Temporal profile of the single-cycle output pulses and beam profile (inset). The
pulse duration was measured with the SHG-FROG technique. c: CEP noise power spectral density
and the corresponding integrated phase noise of the CEP-stabilized laser system d: Measured(orange) and simulated (black, dashed) supercontinuum spectrum after spectral broadening in
TiO2.

The output of the laser is focused into a single 0.5-mm-thick TiO2 plate and broadened to asupercontinuum spanning from 1.2 to 3.0 µm at -20 dB and 360 mW of output power (Fig. 1d). By
using custom dispersive optics, the super-octave-spanning pulses are compressed to pulse durations
down to 6.9 fs, which corresponds to a single oscillation cycle of the electric field at the spectral
centroid of 2.2 µm (Fig. 1b). Detailed insights into the broadening mechanism in TiO2 are obtained
through numerical simulations, which closely reproduce the output spectrum of the broadening stage(Fig. 1d). To generate a feedback signal for active CEP stabilization, a fraction of the broadened
output is sent into a highly compact common-path f-2f interferometer. Absolute CEP stability, i.e.
with the carrier-envelope-offset frequency fceo set to zero, has been achieved with a residual phasenoise of only 5.9 mrad in the range between 0.1 Hz to 11.45 MHz (Fig. 1c). This corresponds to thelowest value ever shown for single or few-cycle laser sources.Efficient down-conversion by cascaded
intra-pulse difference frequency generation (IPDFG) in a ZnGeP2 (ZGP) crystal allows to create
CEP-controlled mid-IR waveforms across more than three optical octaves from 0.9 µm to 12 μm at 30 dB (Fig. 2a) [7].

Fig. 2. CEP-controlled single-cycle mid-infrared-pulse generation. a: Mid-infrared spectrum
from cascaded intra-pulse difference frequency generation (IPDFG), shown for the extreme casesof constructive (black) and destructive (orange) interference between the even and odd cascaded
IPDFG orders. The dashed line indicates the detection limit of the EOS measurement techniquewhen using a 7-fs gate pulse. b: EOS time-domain data for the two different CEP-settings of thedriver laser pulse. For better comparison, the two traces are offset by a constant factor (0.2 V).
The pulse durations correspond to the full-width at half-maxima (FWHM) of the intensity
envelopes. For calculating the number of optical cycles, the spectral centroids of the waveforms
are considered.

The adjustment of the waveforms is achieved by tuning the CEP of the driver laser pulse. In the two
extreme cases shown, this leads to fully constructive (black curve in Fig. 2a) or destructive (orange
curve in Fig. 2a) interference between the even and odd cascaded IPDFG orders, correspondingly
modulating the spectral intensity around 5 μm. The average power of the mid-infrared part of thesupercontinuum (>3.6 µm) for the case of constructive interference is 31 mW, corresponding to an
IPDFG power conversion efficiency of 14%. To observe changes of the mid-infrared waveform
induced by varying the CEP of the driving laser, the technique of electro-optic sampling (EOS) wasused. It confirmed excellent temporal confinement of the waveforms to single-cycle durations (Fig.
2b). In conclusion, we demonstrate a compact and coherent infrared laser source, ALBATROSS, fordelivering single-cycle pulses with ultra-high waveform stability – the ideal frontend for future fieldresolved femtosecond experiments including light wave-driven control of charge carriers in lowbandgap semiconductor materials, and for generating controllable mid-infrared waveforms spanning
multiple octaves. With regard to infrared-based biosensing and life science applications, we expect toachieve a new level in specificity and sensitivity, when combined with field-resolved detection(EOS) and dual-oscillator scanning techniques [8].
References[1] S. Sederberg, D. Zimin, S. Keiber, F. Siegrist, M. S. Wismer, V. S. Yakovlev, I. Floss, C. Lemell, J. Burgdörfer, M. Schultze, F. Krausz
N. Karpowicz, Nature Communications 11, 430 (2020).
[2] M. Hentschel, R. Kienberger, C. Spielmann, G. A. Reider, N. Milosevic, T. Brabec, P. Corkum, U. Heinzmann, M. Drescher, F. Krausz, Nature
414, 509 (2001).
[3] T. Rybka, M. Ludwig, M. F. Schmalz, V. Knittel, D. Brida, A. Leitenstorfer, Nature Photonics 10, 667 (2016).
[4] R. Liao, H. Tian, T. Feng, Y. Song, M. Hu, G. Steinmeyer, Optics Letters. 44, 1060 (2019).
[5] S. Rausch, T. Binhammer, A. Harth, J. Kim, R. Ell, F. X. Kärtner, U. Morgner, Optics Express 16, 9739 (2008).
[6] S. Vasilyev, V. Smolski, J. Peppers, I. Moskalev, M. Mirov, Y. Barnakov, S. Mirov, V. Gapontsev, Optics Express 27, 35079 (2019).
[7] P. Steinleitner, N. Nagl, M. Kowalczyk, J. Zhang, V. Pervak, C. Hofer, A. Hudzikowski, J. Sotor, A. Weigel, F. Krausz, K. F. Mak, Nature
Photonics 16, 512 (2022).
[8] A. Weigel, T. Buberl, P. Jacob, T. Amotchkina, C. Hofer, M. Trubetskov, P. Sulzer, S. A. Hussain, W. Schweinberger, V. Pervak, F. Krausz,
I. Pupeza, 2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, Paper Cf_9_5, (2021).

New methods for driving and monitoring quantum materials

Z. Zhang1, Z.-J. Liu1, F. Y. Gao2, E. Baldini2, Y.-C. Chien1, J. Deschamps1, J. Lem1, Y. Kai1, I. Chaban1
T. Pezeril3, K. A. Nelson1
1
Massachusetts Institute of Technology, Cambridge, MA 02135
2
The University of Texas at Austin, Austin, TX 78705
3
Institut de Physique de Rennes, 35042 Rennes, France
There have been striking advances in recent years in our understanding of quantum materials, theirtransformations across complex multiphase landscapes, and the underlying interactions among theircoupled modes. There also remain experimental challenges that must be overcome in order toprovide further key insights and to guide progress from fundamental understanding to practicalapplications. Here we discuss recent methodological progress and results.
The first advance is aimed at using both optical and terahertz-frequency (THz) probes for timeresolved measurements of ultrafast material transformations that are irreversible or slowly reversible.
Since the samples under study do not return to their initial states after photoexcitation, suchtransformations cannot be monitored through conventional methods that require many repetitions ofa pump-probe pulse sequence with variable temporal delays between the pump and probe pulses. We
have developed [1] and refined [2,3] a method for single-shot, real-time monitoring of ultrafast
dynamics in which the usual variably delayed probe pulse is replaced by hundreds of probe pulsesthat all arrive at the pumped sample location with different time delays, covering a total temporal
range of about 10 ps with approximately 25-fs intervals between successive pulses. For opticalmonitoring of the photoinduced response, the pulses are all measured in different regions of a camera
after reflection from or transmission through the sample. We recently have extended the method topermit measurement at THz probe frequencies as well [4]. In this case, the sample is irradiated by a
THz probe pulse and the resulting time-dependent THz field that is either transmitted through or
reflected from the sample is measured through electro-optic (EO) sampling, i.e. through the change
in optical polarization that the THz field induces. This is ordinarily measured with an optical“readout” pulse that is variably delayed to overlap temporally with different parts of the THz field
profile. We have instead used several hundred readout pulses to measure the entire THz signal field
profile in a single laser shot. We recently studied ultrafast metastable phase formation in the quasi2D charge-density wave (CDW) crystal TaS2 using both optical reflection and THz transmission
measurements to probe ultrafast lattice and electronic responses [5]. Because the photoinduced
collective transformation is irreversible at low temperatures (the initial commensurate CDW phasecan be recovered by temperature cycling, which is extremely slow), conventional ultrafast
measurement methods with many pump-probe repetitions cannot be used. Single-shot measurements
clearly revealed ultrafast nonthermal collapse of the initial CDW ordering and formation of a far
more highly conducting (H) state which showed its own CDW ordering signature (amplitude-mode
oscillations) on subsequent shots after the first. See Fig. 1. The results also showed further collective
evolution and conductivity increase when the excitation pulse fluence was raised further onsubsequent shots. The results yielded greatly improved understanding of the nature of thephotoinduced metastable state and the dynamics of its formation.

Fig. 1. Dynamics of the emergent hidden phase formed after photoexcitation in 1T-TaS2. Left: Single-shottransient reflectivity measurements (dots) at 7.8 K following irradiation with 2.5 mJ/cm2 1.55-eV pump pulses
are shown along with the corresponding fits to biexponential decays and oscillatory responses (solid lines). Ten
shots were recorded at this fluence but only the 1st, 2nd, and 10th are reproduced here for clarity. Each
irradiation was separated by more than 10 s. Right: The corresponding shot-to-shot change in steady-state
reflectivity (red) and 8-ps reflectivity (blue) over the full sequence of irradiations. Note that n = 1 (left))corresponds to the 11th shot (right).

In samples that do not undergo long-lived photoinduced transformations, the single-shotmeasurement approach may still provide great advantages in data acquisition times and signal/noise(S/N) ratios, especially in experiments that are conducted using amplified laser systems withrepetition rates that are comparable to the rate at which several hundred multi-time signals can be
downloaded from the camera. We have realized these advantages in two-dimensional THz
spectroscopy, which involves two distinct time delays – the time between two THz excitation pulses
and the time at which the nonlinear THz signal is read out through EO sampling – that must be
scanned. We earlier reported 2D THz electron spin resonance (ESR) spectra from magnon modes [6]as well as 2D THz rotational spectra from polar molecules in the gas phase [7,8]. In this work, each
2D spectrum required several days of uninterrupted data acquisition. We have recently adopted thesingle-shot EO sampling measurement approach described above to 2D THz spectroscopy conducted
at a 1-kHz repetition rate [9]. This has resulted in dramatic improvement in both data acquisition
times and S/N ratios, enabling systematic ESR study of antiferromagnetic crystals with slightly
canted sublattice spin orientations that yield a nonzero net magnetization M [10,11]. In crystals
among this broad class, there are two collective spin wave (magnon) branches formed from in-phaseor out-of-phase precessions of the sublattice magnetic moments. In the ferromagnetic (F) magnon
mode, the net magnetization direction undergoes precession about its unperturbed orientation, whilein the antiferromagnetic (AF) mode, the magnetization direction remains unchanged but its
magnitude is modulated. Linear ESR spectra recorded using weak THz fields polarized
perpendicular or parallel to M reveal the F or AF mode respectively. We used THz pump pulse pairs
to excite erbium orthoferrite (ErFeO3, EFO) and yttrium orthoferrite (YFeO3, YFO) with the crystal
orientations adjusted so the incident THz polarization could drive either the F mode, the AF mode, orboth modes. 2D ESR spectra were recorded from each crystal with different orientations relative tothe incident THz polarization, rotating a full 360 degrees in 5-degree increments while detecting thecoherent nonlinear THz signal with polarizations either parallel or perpendicular to the incident
polarization. These 2D THz polarimetry studies, with 144 2D spectra from each sample, would havebeen inconceivable earlier since each one would have required more than one year of uninterrupted
data acquisition. The fully automated polarimetry measurements from each sample were completed
in less than 24 hours! Additional measurements were conducted as a function of sample temperature
and THz field strength. The results showed second-order signals including 2-quantum coherences(radiating at magnon sum and difference frequencies) that revealed mode-mode couplings that had
never before been observed for different magnon modes. In one unusual example [10], an offdiagonal peak was observed due to unidirectional nonlinear coupling in which pumping only theferromagnetic magnon mode yielded signals (with polarization perpendicular to that of the incident
THz fields) from the antiferromagnetic mode. It occurs because driving the precessional F mode
nonlinearly reduces the projection of the magnetization along the original M direction, thereby
coupling to the AF mode. Driving the AF mode alone does not similarly couple to the F mode
(although there may be other coupling mechanisms) because even a large-amplitude modulation of
the amplitude |M| does not induce any precessional (F mode) response.
Many quantum material (and other crystalline) transformations involve changes in the lattice
parameters. In such cases, strain can play a key role in controlling which phase is present. We havedeveloped a facile method for laser generation of compressional or surface acoustic wave (SAW)shocks [12] that focus in the plane of a sample to reach multi-GPa stresses sufficient to causematerial fracture, chemical decomposition, phase transitions, and other effects [13-17]. In that work,
as in other laser shock generation methods, the sample is damaged by the optical excitation pulse
even if it is not damaged by the shock. Thus is it not practical to repeat shock generation and delivery
to the same sample region many times. This is also not practical in conventional shock generation bymechanical impact. In very recent work, we have developed a method for non-destructive generationof large-amplitude acoustic waves [18]. The approach permits repeated shocks to be delivered to thesame sample region, and has been used to observe cumulative effects (i.e. fatigue) caused bythousands of shocks. See Fig. 2. The shocks also may be used to drive phase transitions bythemselves or in conjunction with THz or optical excitation. More generally, shock can be used as acomponent of multimodal excitation of samples that could be probed in any spectral range from THz
to x-ray at kHz repetition rates. A duration of >1 ns makes the shock quasi-static on the time scale ofmany fs-ps spectroscopy measurements, permitting such measurements to be conducted on a single
sample as a function of strain. The possibility for repeated shock excitation in pump-probe
measurements greatly broadens the parameter space across which searches for hidden phases
including metastable phases may be conducted. This development offers novel prospects for
fundamental study and potential practical applications of collective quantum dynamics.

Fig. 2. Generation of large-amplitude acoustic waves without optical damage to the sample. (a): Build-up of
SAW amplitude by a sequence of cylindrically focused excitation pulses that irradiate parallel linear regions ofa SrTiO3 (STO) sample that is doped with 0.7% Nb in order to absorb the 800-nm light. The image wasrecorded in reflection mode. The left-most pulse arrives first, then the next one to the right, and so on, withsuccessive pulses shifted spatially and temporally to match the SAW velocity. The thermoelastic responses to all
the pulses superpose coherently to reach a large amplitude. (b): Displacement of a Nb:STO sample surface
measured interferometrically after excitation with 1.0 J/cm2 laser fluence. The superposed SAW was measured
at the “Probe 2” location in (a). The SAW responses that propagated leftward from each excitation pulse did
not superpose, giving rise to the small individual signals measured at the “Probe 1” location. The superposed
SAW has a longitudinal strain of 1.0% at the free surface, corresponding to a stress of about 3 GPa. SAW
amplitudes up to 3x larger than that shown were generated without optical damage to the irradiated region of
the sample. (c): Schematic illustration of multimodal excitation involving acoustic/shock and THz, IR, or opticalpump pulses. The SAW (or bulk compressional wave) propagates to a pristine sample region at which a fs-pstime-resolved pump-probe (or 2D) measurement takes place.

The doped SrTiO3 sample absorbed the pump light in a penetration depth of only 15 µm, yielding athermoelastic response that matched the SAW strain profile. In a more lightly doped material with alonger optical penetration depth, a bulk-like compressional wave will be generated. Either wave canbe transduced into a sample of interest, including SAW propagation into a thin sample deposited
onto the substrate surface or bulk shock propagation into a thicker sample.
The combination of strain and THz, IR, optical, or UV pumping makes possible the excitation ofmultiple, coupled degrees of freedom that play central roles in material transformations. This will
advance our fundamental understanding and our ability to control material traversal across complex
multiphase free energy landscapes.
References[1] P. R. Poulin, K. A. Nelson, Science 313, 1756 (2006).
[2] T. Shin, J.W. Wolfson, S.W. Teitelbaum, M. Kandyla, K.A. Nelson, Review of Scientific Instruments 85, 083115 (2014).
[3] S. W. Teitelbaum, T. Shin, J. W. Wolfson, Y.-H. Cheng, I. J. Porter, M. Kandyla, K. A. Nelson, Physical Review X 8, 031081 (2018).
[4] S.M. Teo, B.K. Ofori-Okai, C.A. Werley, K.A. Nelson, Review of Scientific Instruments 86, 051301 (2015).
[5] F. Y. Gao, Z. Zhang, Z. Sun, L. Ye, Y.-H. Cheng, Z.-J. Liu, J. G. Checkelsky, E. Baldini, K. A. Nelson, Science Advances 8, eabp9076 (2022).
[6] J. Lu, X. Li, H. Y. Hwang, B. K. Ofori-Okai, T. Kurihara, T. Suemoto, K. A. Nelson, Physical Review Letters 118, 207204 (2017).
[7] J. Lu, Y. Zhang, H.Y. Hwang, B.K. Ofori-Okai, S. Fleischer, K.A. Nelson, PNAS 113, 11800 (2016).
[8] Y. Zhang, J. Shi, X. Li, S. L. Coy, R. W. Field, K. A. Nelson, PNAS 118, e2020941118 (2021).
[9] F. Y. Gao, Z. Zhang, Z.-J. Liu, and K. A. Nelson, Optics Letters 47, 3479 (2022).
[10] Z. Zhang, F. Y. Gao, Y.-C. Chien, Z.-J. Liu, J. B. Curtis, E. Sung, X. Ma, W. Ren, S. Cao, P. Narang, A. von Hoegen, E. Baldini, K. A. Nelson,
arXiv:2207.07103 (2022).
[11] Z. Zhang, F. Y. Gao, J. B. Curtis, Z.-J. Liu, Y.-C. Chien, A. von Hoegen, T. Kurihara, T. Suemoto, E. Sung, P. Narang, E. Baldini, K. A. Nelson,
arXiv:2301.12555 (2023).
[12] T. Pezeril, G. Saini, D. Veysset, S. Kooi, R. Radovitzky, K.A. Nelson, Physical Review Letters 106, 214503 (2011).
[13] D. Veysset, T. Pezeril, S. Kooi, A. Bulou, K.A. Nelson, Applied Physics Letters 106, 161902 (2015).
[14] D. Veysset, U. Gutiérrez-Hernández, L. Dresselhaus-Cooper, F. De Colle, S. Kooi, K. A. Nelson, P. A. Quinto-Su, T. Pezeril, Physical Review
E 97, 053112 (2018).
[15] D. Veysset, A.A. Maznev, I.A. Veres, T. Pezeril, S. Kooi, A. Lomonosov, K.A. Nelson, Applied Physics Letters 111, 031901 (2017).
[16] D. Veysset, S. E. Kooi, R. Haferssas, M. Hassani-Gangaraj, M. Islam, A. A. Maznev, Y. Chernukha, X. Zhao, K. Nakagawa, D. Martynowych
X. Zhang, A. M. Lomonosov, C. A. Schuh, R. Radovitzky, T. Pezeril, K. A. Nelson, Scripta Materialia 158, 42 (2019).
[17] L. Dresselhaus-Cooper, D. J. Martynowych, F. Zhang, C. Tsay, J. Ilavsky, S. G. Wang, Y.-S. Chen, L. Leninger, K. A. Nelson, Journal of Physical
Chemistry A 124, 3301 (2020).
[18] J. Deschamps, Y. Kai, J. Lem, I. Chaban, A. Lomonosov, A. Anane, S. E. Kooi, K. A. Nelson, T. Pezeril, arXiv:2209.13897 (2022).
* Acknowledgement: Work at MIT was supported by the DOE Office of Basic Energy Sciences, under Award Number DE-SC0019126.

Sub-diffusive topological defects of a charge density wave
Probed by an x-ray
laser11
1
1
1G. Orenstein , R.A. Duncan , G. A. de la Peña Muñoz , Y. Huang , V. Krapivin , Q. L. Nguyen1
S. Teitelbaum2, A. G. Singh3, R. Mankowsky4, H. Lemke4, M.Sander4, Y. Deng4, C.Arrell4, I. R. Fisher1
D.A. Reis1, M. Trigo1
1
SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
2
Arizona State University, Tempe, AZ 85281, USA
3
Stanford University, Stanford, CA 94305, USA
4Paul Scherrer Institut, 5232 Villigen, Switzerland

Excitation of materials with ultrafast light pulses has led to the discovery of novel states of matterout of equilibrium and represents a promising avenue to control their functional properties (1).
Understanding and manipulating these emergent properties requires a deeper insight into how thephotoinduced states evolve away from and towards equilibrium. In many photoexcited systems theevolution of these transient states is characterized by the formation of metastable, inhomogeneous
regions and self-organization on mesoscopic length-scales (2–5). However, most ultrafast probes are
only sensitive to the average structure and consequentially afford only limited insight into thetransient emergent properties of the photoexcited materials (6). In this work we use the highwavevector and time resolution afforded by an x-ray free electron laser (XFEL), combined with a
new scaling analysis, to gain unprecedented access to the dynamics on the relevant mesoscopic
length-scales. We study the ultrafast dynamics of the charge density wave (CDW) in LaTe3, aquantum material which has attracted attention for its competing charge ordered phases and exotic
collective excitations (4,7-9). Our findings reveal unconventional subdiffusive dynamics which isgoverned by topological vortex strings of the CDW.In a material undergoing a symmetry breaking
phase transition under nonequilibrium conditions, the low-symmetry phase develops independently
in separate regions of space, leaving behind disconnected domains. This leads to the formation of
topological defects (TDs) by the Kibble-Zurek mechanism (10, 11). As the system evolves towards
equilibrium, these defects annihilate and the independently ordered domains coalesce. Generally, thiscoarsening process is characterized by a single length scale,, whereis the growth
exponent (12,13). Herecorresponds to a mean domain size, or an average distance between
TDs.

Fig. 1. (a): X-ray intensity of the G = (2/a, 2/b, (1 − qCDW)/c) CDW Bragg peak of LaTe3 on a logarithmic scaleversus wavevector along G+ k, for k ≈ (0, k, 0), where a, b and c are the respective standardized cellparameters and qCDW ≈ 2/7. The different colors represent different pump-probe delays starting from 6.5 ps. The
dashed black line is the equilibrium intensity. The narrowing of the peak is the hallmark of coarsening of
spatially inhomogeneous regions as the system recovers towards equilibrium. (b): Log of the typical scattering
wavevector, ktyp , versus log of the pump-probe delay, where ktyp
. The red solid straight line is a
fit to the data giving β = 0.29.

We probed the dynamics of the lattice component of the CDW following an ultrafast opticalexcitation by using femtosecond total x-ray scattering at the Bernina instrument (14) of the
SwissFEL facility (15). The high brightness of the XFEL, together with its exceptional temporal andwavevector resolution, allow subtle details of the dynamics of the diffuse intensity on the tails of the
CDW Bragg peak to be captured. This enabled us to directly observe the coarsening process andextract the growth exponent of(Fig. 1).A growth exponent ofis an intriguing
result. Extensive studies (13,16,17) concluded that in systems with a non-conserved order parameter,
the recovery of long range order at long times follows a diffusive process with. Subdiffusive

behavior, indicated by, is a manifestation of restricted microscopic motion of the system andsuggests that the dynamics are constrained by additional conserved quantities.
Fig. 2. (a): Data collapse of the LaTe3
CDW Bragg peak intensity at various
delays to the universal function
Ig(t)=F[kL(t)] (12,13), for kL(t)>0.02Å1. The data shows an asymptotic powerlaw of ~k-5 (blue solid line), a clear
signature of vortex strings (18). At
larger kL(t) the power law changes to~k-2 (purple solid line) which is typicalto phase fluctuations of the CDW (16).
(b): Ginzburg-Landau simulation (12)of a complex order parameter, ψ(x, t),
in three spatial dimensions following asudden quench to the low symmetry
potential energy. The isosurface plot of|ψ(x, t)|2 = 0.5, 4 ps after the quench,shows that the initial random
configuration quickly breaks down into
topologicalvortexstrings.
(c):
Illustration of a CDW lattice distortion
on a single plane at a delay of 22 ps.
Two vortices with opposite winding
numbers
correspondtotwodislocations of the CDW with opposite
Burgers vectors (19), indicated by blue
arrows.
1

Our scaling analysis of the diffuse scattering's asymptotic wavevector behavior reveals compelling
evidence that photoinduced topological vortex strings (18), which are dislocations of the CDW,
dominate the coarsening process (Fig. 2). The subdiffusive behavior is likely a signature of therestricted mobility of these dislocations (20,21).Our results constitute a clear measurement ofphotoinduced topological defects of the prototypical CDW in LaTe3 on ultrafast time scales. We
show that shortly after photoexcitation the evolution of the highly nonequilibrium state is dominatedby vortex strings. The observed subdiffusive dynamics is indicative of the strings’ restrictedmobility, which crucially impacts the system’s thermalization and broken ergodicity. Our methods aswell as the implications of our findings extend well beyond LaTe3, as formation of topologicaldefects is expected to be ubiquitous in symmetry breaking phase transitions under nonequilibrium
conditions (10, 11). Thus, the high resolution XFEL measurements, combined with scaling and datacollapse analysis, establish an incisive approach to probe the mesoscopic, out-of-equilibrium
behavior of intertwined orders in quantum materials.
References[1] D. N. Basov, R. D. Averitt, D. Hsieh, Nature Materials 16, 1077 (2017).
[2] J. Ravnik, Nature Communications 12, 2323 (2021).
[3] L. Stojchevska, Science 344, 177 (2014).
[4] A. Kogar, Nature Physics 16, 159 (2020).
[5] S. Wandel, Science 376, 860 (2022).
[6] S. Wall, Science 362, 572 (2018).
[7] A. Zong, Nature Physics 15, 27 (2019).
[8] F. Schmitt, Science 321, 1649 (2008).
[9] Y. Wang, Nature 606, 896 (2022).
[10] T. W. B. Kibble, Journal of Physics A: Mathematical and General 9, 1387 (1976).
[11] W. H. Zurek, Nature 317, 505 (1985).
[12] M. Mondello, N. Goldenfeld, Physical Review A 45, 657 (1992).
[13] A. J. Bray, A. D. Rutenberg, Physical Review E 49, R27 (1994).
[14] G. Ingold, Journal of Synchrotron Radiation 26, 874 (2019).
[15] E. Prat, Nature Photonics 14, 748 (2020).
[16] G. F. Mazenko, M. Zannetti, Physical Review B 32, 4565 (1985).
[17] S. Vogelgesang, Nature Physics 14, 184 (2018).
[18] A. J. Bray, K. Humayun, Physical Review E 47, R9 (1993).
[19] F. Nabarro, Oxford University Press (1967).
[20] M. Pretko, L. Radzihovsky, Physical Review Letters 120, 195301 (2018).
[21] A. Gromov, A. Lucas, R. M. Nandkishore, Physical Review. Research 2, 033124 (2020).
* Acknowledgement(s): We acknowledge the Paul Scherrer Institute, Villigen, Switzerland for provision of free-electron laser beamtime at theBernina instrument of the SwissFEL ARAMIS branch. Funding: G.O., R.A.D, G.A.P.M., Y.H., V.K., D.A.R., and M.T. were supported by the US
Department of Energy, Office of Science, Office of Basic Energy Sciences through the Division of Materials Sciences and Engineering underContract No. DE-AC02-76SF00515. For crystal growth and characterization, A.G.S. and I.R.F. were supported by the Department of Energy,
Office of Basic Energy Sciences, under contract DE-AC02-76SF00515. G.O. acknowledges support from the Koret Foundation. R.A.D.
acknowledges support through the Bloch Postdoctoral Fellowship in Quantum Science and Engineering from the Stanford University Quantum
Fundamentals, Architectures, and Machines initiative (Q-FARM), and the Marvin Chodorow Postdoctoral Fellowship from the Stanford
University Department of Applied Physics.

Extreme ultraviolet metaoptics

M. Ossiander1, H. K. Hampel2, M. L. Meretska1, S. W. D. Lim1
N. Knefz2, T. Jauk2, F. Capasso1, M. Schultze2
1
Harvard University, Cambridge, MA 02138 USA
2
Graz University of Technology, 8010 Graz, Austria

Modern attosecond science and state-of-the-art semiconductor lithography require extreme
ultraviolet (EUV) light. However, the unavailability of optics for this spectral regime halts progress.
Here, we realize the first metalenses for EUV radiation with 50 nm vacuum wavelength, which isless than a quarter of the current low wavelength cutoff of dielectric metasurfaces. The technology
constitutes the first universal approach for achieving EUV optics, and devices can be manufactured
using only semiconductor-foundry-compatible processes. To achieve the EUV metaoptics, we
exploit that holes etched in a Silicon membrane have a considerably higher index (n = 1) than thesurrounding Silicon (n = 0.77), allowing us to guide light and shift the phase of transmitted EUV
radiation by varying the diameter of the holes. To design a metalens, we first digitize the phaseprofile of an aspheric lens and then match the required position-dependent transmission phase withthe correct hole diameter.Via electron beam lithography and reactive ion etching, we fabricated ametalens with 10 mm focal length and 1 mm diameter from the device layer of a Silicon on insulator
wafer. Fig. 1a shows a light image of the final optics. To characterize this metalens, we generated
EUV radiation using high-harmonic generation and focused the 21st harmonic of the driving laser(49 nm wavelength). Fig. 1b presents the waist evolution of the focused EUV light beam using aknife-edge measurement. Focusing by the metalens is proved by the clear beam size minimum along
the propagation direction.

Fig. 1. Experimental knife-edge characterization of a meta-lens focusing extreme ultraviolet light. a) Light microscope
picture of a fabricated metalens (ML, focal length f = 10 mm, optics diameter 1 mm, λvac = 49 nm).
b) Position-dependent waist size (green dots) of the metalens-focused light beam around the focus located at z =0mm. The error bars mark 95% confidence.

The meta-lens achieves a minimum beam waist of w0metalens = (0.7 ± 0.3) um, which is only 1.6 times
the diffraction limit determined by the nominal lens parameters and the incoming beam profile.
References[1] M. L. Tseng, M. Semmlinger, M. Zhang, C. Arndt, T.-T. Huang, J. Yang, H. Y. Kuo, V.-C. Su, M. K. Chen, C. H. Chu, B. Cerjan , D. P. Tsai
P. Nordlander, N. J. Halas, Science Advances 8, 5644 (2022).
[2] Y. Deng, X. Wang, Z. Gong, K. Dong, S. Lou, N. Pégard, K. B. Tom, F. Yang, Z. You, L. Waller, J. Yao, Advanced Materials 30, 1802632(2018).
[3] K. Huang, J. Deng, H. S. Leong, S. L. K. Yap, R. B. Yang, J. Teng, H. Liu, Laser Photonics Review 13, 1800289 (2019).
[4] C. Zhang, S. Divitt, Q. Fan, W. Zhu, A. Agrawal, Y. Lu, T. Xu, H. J. Lezec, Light Science Applications 9, 55 (2020).

Using heterostructure engineering to tailor
Two-photon absorption in nanomaterials
A. Alo, C.A. Sousa, J.C. Lemus, L.A. PadilhaInstituto de Fisica “Gleb Wataghin”, Campinas, SP 13083, Brazil
Significant advances on synthesis methods and on the understanding of the photo-physics incolloidal semiconductor nanocrystals (NCs) have been achieved in the last decades. By controllingshape, composition, dimensions, and even by heterostructure engineering, now a days, it is possible
to control multi-exciton, electron-phonon, and light-matter interactions to unprecedented levels. All
these advances have allowed these NCs to move from laboratories bench top to real world
applications, including luminescent solar concentrators, LEDs, among others. Within the several
interesting properties presented by these quantum-confined materials, their nonlinear opticalresponse has been subject of intense study in the last two decades. Much of the initial studies ofnonlinear absorption and nonlinear refraction in NCs has been fueled by predictions of quantum
confinement induced enhancement of the oscillator strength for nonlinear optical transitions insemiconductors [1-2]. Nevertheless, as it has been shown for a number of different nanomaterials,[35] quantum confinement does not enhance the overall nonlinear optical response of semiconductors
because, despite the fact that the oscillator strength of a given nonlinear optical transition is enhanced
due to quantum confinement, this results also on a drastic reduction on the density of states, spoilingthe expected enhancement. [3, 6] In fact, considering NCs with confinement on all three directions(called here quantum dots), it has been shown that two-photon absorption (2PA) cross-section grows
linearly with the QD volume and, at least for visible emitting nanomaterials, the trend seems to be
independent of the composition, forming a universal volume scaling. [5] A consequence of this direct
dependence of the 2PA cross-section on the QD volume (and radius) is that increasing the 2PA
cross-section is directly associated with increasing the QD size, resulting on a red-shift of the NC
bandgap. One way to reduce the coupling between 2PA cross-section and the NC bandgap energy is
to change the nanomaterial shape. For example, in nanorods type of NCs, one can increase the 2PA
cross-section by changing their length, while the bandgap energy is defined by the NC diameter. [6]Nevertheless, elongated nanostructures are typically less stable than their spherical counterparts, andthe questions that arises is how much one can decouple the magnitude of the 2PA cross-section andthe emission wavelength for spherically shaped NCs, by band structure engineering. Motivated bythis, here we investigate the influence of different heterostructures on the 2PA cross-section of CdSebased core/shell quantum dots. To do so, we have studied the 2PA cross-section for series of type-I
CdSe/CdZnS, quasi-type-II CdSe/CdS, and core only CdSe QDs. Our results indicate that the volume
dependence of the 2PA cross-section in core/shell heterostructures differs from that of core only QDs
and follow a sublinear dependence on the total nanoparticle volume. Fig. 1a shows how the 2PA
cross-section depends on the NC volume for a series of type-I CdSe/CdZnS NCs and two series ofquasi-type-II CdSe/CdS.

σ2PA 800nm (GM)

a)
105

b)

CdSe/CdS core 3.0 nm
CdSe/CdS core 4.0 nm
CdSe/ZnCdS core 4.0 nm

104
100

1000

Total NC volume (nm3)Fig. 1. a) 2PA cross section measured at 800 nm for three series of heterostructured CdSe-based NCs. The line
represents the volume dependence of the 2PA cross-section at 800 nm for core only CdSe QDs. (b) 2PA crosssection as a function of the emission wavelength for quasi-type-II CdSe/CdS NCs compared to core only CdSe
QDs.

For each series, the core diameter is kept constant and the shell thickness is changed. It is interesting
to note that for all series, for a given total volume, the 2PA cross-section is smaller for theheterostructures than for core only. This is a result of the reduced density of final states in core/shell
nanoparticles when compared to core only systems. Furthermore, when comparing different types ofcore/shell structures with thick shell, one can see that the 2PA cross section reaches the largest
values for the quasi-type-II system with smaller core. Reducing the core size favors the NC’s band
structure to be aligned as a type-II system. Consequently, our data suggests that, for NCs containing
thick shells, the more the type-II aspect of the band alignment the larger the 2PA cross section.
Finally, Figure 1b compares the 2PA cross section as a function of the wavelength of the peakemission for core only and the series of quasi-type-II with 3.0 nm core. One can see that, byemploying engineered NCs it is possible to enhance the 2PA cross section by almost one order ofmagnitude without drastically shifting the emission wavelength.
In conclusion, our results indicate a clear path to decouple the magnitude of 2PA cross-section fromthe emission properties of semiconductor NCs. Despite the bandgap engineered NCs do not show
2PA cross-section larger than for core only systems, they offer a solution for two-photon absorption
on demand, in which one is able to obtain high two-photon absorbing NCs without paying the priceof drastically changing the emission spectra.
References[1] S. Schmitt-Rink, D. Miller, D.S. Chemla, Physical Review B 35, 8113 (1987)[2] P. Roussignol, D. Ricard, C. Flytzanis, Applied Physics B 51, 437 (1990).
[3] L.A. Padilha, J.Fu, D.J.Hagan, E.W.Van Stryland, C.L.Cesar, L.C.Barbosa, C.H.B.Cruz, D.Buso, AMartucci, Physical Review B 75 075325 (2007)[4] N.S. Makarov, P.C. Lau, C. Olson, K.A. Velizhanin, K. M. Solntsev, K. Kieu, S. Kilina, S. Tretiak, R.A. Norwood, N. Peyghambarian, ACS Nano
8, 12572 (2014)[5] G. Nagamine, J.O. Rocha, L.G. Bonato, A.F. Nogueira, Z. Zaharieva, A.A. Watt, C.H. de Brito Cruz, L.A. Padilha, Journal Physical Chemistry
Letters 9, 3478 (2018)[6] M. Allione, A. Ballester, H. Li, A. Comin, J.L. Movilla, J.I. Climente, L. Manna, I. Moreels, ACS Nano 7, 2443 (2013).
* Acknowledgement: This work has been funded by the Sao Paulo Research Foundation (FAPESP)under the grants 2018/15574-6, 2018/25339-4,
2019/22823-5.

Spin and charge current dynamics in
Spintronic THz
emitters2
1
1
2

E. Th. 1Papaioannou , B. Das-Mohapatra , N. Kanistras ,T. Kehagias , G Schmidt1
Martin-Luther University Halle-Wittenberg, 06120 Halle, Germany
2
Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

Ultrafast spin-to-charge conversion in heterostructures composed of ferromagnetic (FM)/nonmagnetic (NM) thin films can give rise to the emission of THz electromagnetic waves [1]. The
experimental scheme involves the use of femtosecond (fs) laser pulses to trigger ultrafast spin andcharge dynamics in FM/NM bilayers, where the NM layer features a strong spin-orbit coupling. Via
the inverse spin Hall effect (ISHE), the spin current generated in the FM layer by the fs-laser pulse isconverted to an ultrashort charge current burst that gives rise to the THz radiation. In thispresentation, we explore the potential of spintronic THz emitters (STE) for stronger THz radiation
and adjustable bandwidth by studying the spin and the subsequent charge current dynamics after thelaser illumination. We first show the influence of the FM/NM interface engineering on the strengthof the spin current, Figure 1a). We grow Fe/Pt bilayers and we induce at the interface the L10-FePt
alloyed phase. Our findings show that the presence of 1-2 nm thick L10-FePt interlayer promotes theinterface transmission of the spin current and amplifies the THz emission [2]. The unique result of
the increased THz emission in Fe/L10-FePt/Pt structure opens new perspectives in the direction of
application of spintronic THz emitters. Next, we reveal that not only the spin current dynamics but
also the charge redistribution is important for the properties of the THz signal. The charge relaxation
leads to a current backflow with a delay and a time constant that mainly depends on the conductivity
and the dielectric properties of the emitter Figure b) [3]. We show that to extract and quantify thespin current from the detected THz signal we need to take into consideration the charge
redistribution in the spintronic layers [3].

Fig. 1. a): Cross-sectional high-resolution transmission electron microscopy of a Fe/L10-FePt/Pt trilayer witha few monolayers thickness. The structure promotes the THz emission after fs-laser illumination. b): Current
over time for the original excitation pulse I0 (dashed red), the total current Itot (solid black) and the response of
the system Ires (dotted blue) for a 10 nm thick spintronic THz emitter. Insets show the current and charge
distribution at two different times, respectively, marked a and b in the black curve.

We further discuss how the ultrafast charge redistribution accounts for significantly changes of
profile in time of the THz pulse. We indicate that the time constant of the system reduces the lower
frequency part of the spectrum but not the higher frequencies and the corresponding cutoff frequency
becomes higher with increasing conductivity of the emitter.
References[1] E. Th. Papaioannou, R. Beigang, Nanophotonics 10, 1243(2021).
[2] L. Scheuer, M. Ruhwedel, D. Karfaridis, I. Vasileiadis, D. Sokoluk, G. Torosyan, G. Vourlias, G. Dimitrakopoulos, M. Rahm, B. Hillebrands
T. Kehagias, R. Beigang, E.Th. Papaioannou, iScience 25, 104319, (2022).
[3] G. Schmidt, B. Das-Mohapatra, E. Th. Papaioannou, Physical Review Applied 19,L041001 (2023).

Revealing the pseudospin texture of Dirac states using
Circular dichroism in photoemission
E. Papalazarou, J. Zhang, Z. Chen, N. Nilforoushan, M. Marsi
Université Paris-Saclay , 91405 Orsay, France

Angle-resolved photoemission spectroscopy (ARPES) is an indisputable tool to have an insight into
the quantum properties of materials using polarized light. For instance, using circularly polarized
light (CPL) one can retrieve dichroic photoelectron intensity maps that reflect the material’s electronchirality and the pseudospin properties. Moreover, when the latter is combined with time-resolution
it can provide unprecedented insights of out-of-equilibrium states. In the present work, we focus ourattention on quasi two-dimensional materials bounded to Dirac surface states. We perform acomparative study of the momentum transfer from polarized low-energy photons to Dirac surface
states in various Dirac compounds such as Bi2Se3, Bi2Te2Se and BaNiS2. Analysis of the circulardichroism (CD) in the photoemission yield of photo-excited Dirac states reveals the spin-vector state.
Our experimental approach based on out-of-equilibrium time-of-flight multidimensional ARPES isable to disentangle experimental geometry and matrix element effects allowing extracting
qualitatively the pseudospin texture of excited surface states [1]. In the case of the topologicalinsulators Bi2Se3 and Bi2Te2Se, the analysis of the CD-ARPES evolution as a function of theelectron's binding energy from the Dirac point to the bulk conduction band indicates (Fig 1a) thatthe Dirac surface states of Bi2Te2Se present a strong out-of-plane spin polarization compared to thoseof Bi2Se3 [2]. BaNiS2 has generated increasing attention because of its peculiar properties related tothe combined effects of crystal field and strong spin-orbit coupling [3, 4], resulting in hidden
Rashba-split spin-polarized bands Dirac states. Here, we reveal the orbital character and the spintexture of the various bulk and surface states (Fig 1b). Moreover, we show that Dirac states can betuned in momentum space by acting on the charge transfer from hybridized orbitals [5].
a)b)BaNiS2

Fig. 1.a. Isoenergy contours of Bi2Se3 (left) and Bi2Te2Se (right) for energies close to Dirac point using CD andb: CD (left) and p-polarized (right) isoenergy contours of BaNiS2 around Γ.

Moreover, we show that Dirac states can be tuned in momentum space by acting on the chargetransfer from hybridized orbitals [5].
References[1] Y.H. Wang, D. Hsieh, D. Pilon, L. Fu, D.R. Gardner, Y.S. Lee, N. Gedik, Physical Review Letters, 107, 207602 (2011).
[2] J. Zhang, J. Caillaux, Z. Chen, M. Konczykowski, A. Hruban A. Wołoś, A. Materna, L. Perfetti E. Papalazarou, M. Marsi, Journal of Electron
Spectroscopy and Related Phenomena, 253, 147125 (2021).
[3] D. Santos-Cottin, M. Casula, G. Lantz, Y. Klein, L. Petaccia, P. Le Fèvre, F. Bertran, E. Papalazarou, M. Marsi, A. Gauzzi, Nature
Communications 7, 11258 (2016).
[4] N. Nilforoushan, M. Casula, A. Amaricci, M. Caputo, J. Caillaux, L. Khalil, E. Papalazarou, P. Simon, L. Perfetti, I. Vobornik, P.K. Das, J. Fujii,
A. Barinov, D. Santos-Cottin, Y. Klein, M. Fabrizio, A. Gauzzi, M. Marsi, Proceedings National Academy of Sciience. 118, e2108617118, (2021).
[5] J. Zhang, Z. Chen, J. Caillaux, Y. Klein, A. Gauzzi, A. Bendounan, A. Taleb-Ibrahimi, L. Perfetti, E. Papalazarou, M. Marsi. Europen Phyical
Journal Special Topics (2022). https://doi.org/10.1140/epjs/s11734-022-00746-9.
* Acknowledgement(s): authors (E. Papalazarou, M. Marsi) acknowledge support from "Investissement d'avenir Labex Palm" (Grant No. ANR-10LABX-0039-PALM.

Imaging the constituent electron and hole of a
Moiré localized interlayer exciton
O. Karni1, E. Barré2,, V. Pareek3, J. D. Georgaras1, M. K. L. Man3, C. Sahoo3, D. R. Bacon3, X. Zhu3
H. B. Ribeiro1, A. L. O’Beirne1, J. Hu1, A. Al-Mahboob3, M. M. M. Abdelrasoul3, N S. Chan3
A. Karmakar3, A. J. Winchester3, B. Kim4, K. Watanabe5, T. Taniguchi5, K. Barmak4, J. Madéo3
F. H. da Jornada1, T F. Heinz1, K. M. Dani3
1
Stanford University, Stanford, CA 94305, USA
2
SLAC, Menlo Park, CA 94025, USA
3
Okinawa Institute of Science and Technology, Onna-son 904-0495, Japan
4
Columbia University, New York, NY 10027, USA
5
Naional Institute for Material Science, Tsukuba 305-0044, Japan

Heterostructure made by stacking two distinct van der Waals monolayer (1L) semiconductors, suchas transition metal dichalcogenides (TMDCs, e.g., MoS2), has attracted attention owing to their novel
and tunable optoelectronic properties[1]. One of these properties includes hosting interlayer excitons(ILXs) such that electrons in one layer are bound to holes in another layer of the heterostructure bycoulomb interaction. This results in the ILXs having long lifetimes, making them an attractive
platform to study many-body exciton physics and various device applications. On top of that, the two
1L semiconductors in the heterostructure can be put together with a small twist angle between them,giving rise to moiré superlattices that modulate its electronic structure and various novel phenomena
such as superconductivity. However, a deeper understanding of the properties of ILXs, such as theirsize, valley configuration, and the effect of the moiré confinement, is still largely lacking. Several
optical spectroscopic studies on various heterostructure systems provide only partial information
regarding the nature of ILXs[2–4]. For instance, optical studies can access only a limited portion of
the momentum distribution of the ILX and thus cannot provide complete information about its sizeor valley configuration reliably. One must access the full momentum space distribution of its
constituent-bound electrons and holes for a complete understanding.
Recently, our group was able to directly image the size of the excitons in 1L WSe2 by capturing the
momentum distribution of the exciton-bound electrons through time-resolved and angle-resolved
photoemission spectroscopy (Tr-ARPES)[5,6].

Fig. 1. Left: Unexcited
band structure of WSe2/MoS2
heterostructure.
Right: Interlayer exciton at
25ps time delay following
1.67eV excitation.

However, capturing the momentum distribution of the exciton-bound holes has not been possible sofar. In this talk, I will discuss our recent study on ILXs in WSe2/MoS2 heterostructure using TrARPES[7]. I will discuss the advancement made by our group in resolving the momentum
distribution of the ILX's electrons and its constituent holes. I will also discuss the impact of moiré
potential on the ILX and show that one does not require a large moiré lattice to confine ILXs,
contrary to the current assumptions.
References[1] P. Rivera, H. Yu, K. L. Seyler, N. P. Wilson, W. Yao, X. Xu, Nature Nanotechnology 13, 1004 (2018).
[2] K. Tran, G. Moody, F. Wu, X. Lu, J. Choi, K. Kim, A. Rai, D. A. Sanchez, J. Quan, A. Singh, J. Embley, A. Zepeda, M. Campbell, T. Autry
T. Taniguchi, K. Watanabe, N. Lu, S. K. Banerjee, K. L. Silverman, S. Kim, E. Tutuc, L. Yang, A. H. MacDonald, X. Li, Nature 567, 71 (2019).
[3] K. L. Seyler, P. Rivera, H. Yu, N. P. Wilson, E. L. Ray, D. G. Mandrus, J. Yan, W. Yao, X. Xu, Nature 567, 66 (2019).
[4] O. Karni, E. Barré, S. C. Lau, R. Gillen, E. Y. Ma, B. Kim, K. Watanabe, T. Taniguchi, J. Maultzsch, K. Barmak, R. H. Page, T. F. Heinz
Physical Review Letters. 123, 247402 (2019).
[5] M. K. L. Man, J. Madéo, C. Sahoo, K. Xie, M. Campbell, V. Pareek, A. Karmakar, E. L. Wong, A. Al-Mahboob, N. S. Chan, D. R. Bacon,
X. Zhu, M. M. M. Abdelrasoul, X. Li, T. F. Heinz, F. H. da Jornada, T. Cao, K. M. Dani, Science Advances 7, eabg0192 (2021).
[6] J. Madéo, M. K. L. Man, C. Sahoo, M. Campbell, V. Pareek, E. L. Wong, A. Al-Mahboob, N. S. Chan, A. Karmakar, B. M. K. Mariserla, X. Li
T. F. Heinz, T. Cao, K. M. Dani, Science 370, 1199 (2020).
[7] O. Karni, E. Barré, V. Pareek, J. D. Georgaras, M. K. L. Man, C. Sahoo, D. R. Bacon, X. Zhu, H. B. Ribeiro, A. L. O’Beirne, J. Hu
A. Al-Mahboob, M. M. M. Abdelrasoul, N. S. Chan, A. Karmakar, A. J. Winchester, B. Kim, K. Watanabe, T. Taniguchi, K. Barmak, J. Madéo
F. H. da Jornada, T. F. Heinz, and K. M. Dani, Nature 603, 247 (2022).

The role of current fluctuations in the creation ofLight-matterentanglement
12
3

G. Passetti , C. J. Eckhardt , C. S. Weber
M. Claassen3, M. A. Sentef4, D. M. Kennes1
1
Aachen University, 52056 Aachen, Germany
2
Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany
3
University of Pennsylvania, Philadelphia, PA 19104, USA
4
University of Bristol, Bristol BS8 1TL, United Kingdom

Driving quantum materials is a promising route for potential future applications in the engineering ofquantum properties, such as superconductivity and topologically protected edge states [1].
First, we are going to discuss how to control superconducting properties in ultrafast transientphenomena [2, 3] by the transient steering of symmetries. Then we will switch gears and discuss
potential routes to manipulate quantum properties in the emerging field of cavity quantum
electrodynamics. The latter aims at controlling material properties via the coupling with thequantized photon modes in a cavity. Through the hybridization of light and matter we explore towhich extent it is possible to tailor properties of extended solids, realizing what was coined “cavity
quantum materials” [4, 5]. The entanglement entropy between light and matter is a direct measure of
the degree of hybridization, but a clear understanding on what are the necessary conditions for this tobe generated in the emergent field of cavity coupled band electrons [6, 7, 8] is still missing. We
consider a paradigmatic model of interacting spinless fermions coupled with the first resonant modeof a cavity. We derive an analytic expression, exact in the cavity high frequency limit, that relates thecavity-matter entanglement to the quantum fluctuations of the current operator.
This relation establishes that the presence of such fluctuations is a necessary condition for non-zerolight-matter entanglement. Furthermore, we solve numerically exactly the model, showing that thequalitative behavior predicted by our analytic expansion holds in a wide range of parameters.
We also discuss problematic aspects of mean field approaches to this model- Fig. 1.

Fig. 1. Cartoon picture of the mechanism generating light-matter entanglement, a superposition of Blochstates is responsible for the insurgence of entanglement between light and matter degrees of freedom.

The results presented in this work are expected to hold in more general settings than the particularmodel discussed, providing a first fundamental condition for the realization of true light-matterhybridized band electrons in a cavity [9].
References[1] A. de la Torre, D. M. Kennes, M. Claassen, S. Gerber, J. W. McIver, M. A. Sentef, Review of Modern Physics 93, 041002 (2021).
[2] M. Claassen, D. M. Kennes, M. Zingl, M. A. Sentef, A. Rubio, Nature Physics. 15, 766-770 (2019).
[3] C. S. Weber, S. Gassner, D. M. Kennes, M. Claassen, (to be submitted).
[4] F. Schlawin, D. M. Kennes, M. A. Sentef, Applied Physics Reviews 9, 011312 (2022).
[5] A. Frisk Kockum, A. Miranowicz, S. De Liberato, S. Savasta, F. Nori, Nature Review Physics 1, 19 (2019).
[6] F. Schlawin, A. Cavalleri, D. Jaksch, Physical Review. Letters 122, 133602 (2019).
[7] H. Gao, F. Schlawin, M. Buzzi, A. Cavalleri, D. Jaksch, Phyical Review Letters 125, 053602 (2020).
[8] A. Chakraborty, F. Piazza, Physical Review Letters 127, 177002 (2021).
[9] G. Passetti, C. J. Eckhardt, M. A. Sentef, D. M. Kennes, arxXiv: 2212.03011 (2022).

Using ultrafast disorder to control a
Photo-induced
phase transition
2
3
4
5

A. S. Johnson1, E. Pastor , S. B. Porro , H. Benzidi , T. Katayama , G. A. de la Peña Muñoz6, V. Krapivin6
N. López4, M. Trigo6, S. E. Wall7
1
IMDEA Nanoscience, 28049 Madrid, Spain
2
Université de Rennes, 35000 Rennes, France
3
ICFO—Institut de Ciencies Fotòniques, 08860 Castelldefels, Barcelona, Spain
4
Institute of Chemical Research of Catalonia, 43007 Tarragona, Spain
5
Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan.
6
SLAC - National Accelerator Laboratory,Menlo Park, CA 94025, USA.
7
Aarhus University, 8000 Aarhus, Denmark

A solid can exist in different crystal forms with unique chemical and physical properties. Interchange
between such forms or polymorphs is known as a solid-solid phase transition and is a ubiquitous
process in nature that can be induced by changes in temperate and pressure. From a technological
viewpoint, the capacity to stabilise solid phases is paramount to harness their properties and generate
new applications. Indeed, control over how solids transform is central for the development of energy
conversion or high-speed data storage technologies. For example, phase control is essential toguarantee the stability of high performing solar cells and is an integral part of the mechanism ofdifferent non-volatile electronics [1]. Light excitation offers an alternative way to pressure andtemperature to control a phase transition by exploring non-equilibrium dynamics. In this framework,
the excitation of the solid in one of the phases can launch coherent motion of atoms in multiple unit
cells [2, 3]. Re-excitation of the sample alongside the period of the relevant vibrational modes thatconnect the two phases can potentially modulate the energy required to drive the transition. Recently
ground-breaking studies have shown that phase transitions in low-dimensional systems can becoherently controlled [4,5]. Inspired by the demonstration of coherent control of solids, herein we
explore routes to control the solid-solid phase transition in (3D) VO2, a prototypical material that
undergoes an insulator to metal transition accompanied by a dramatic change in structure frommonoclinic to rutile. We perform double pump experiments in which one laser beam weaklyinteracts with the sample to generate an excited monoclinic state and launch coherent phonons.
Subsequently a second laser beam re-excites the sample (Fig 1). We assess how the dividing the totalenergy between the two pulses affects the energy required to initiate the phase transition, namely thetransition’s fluence threshold.
Fig.1.(a)Representation of
the VO2 structure and thetransformation pathways
we monitor.
(b) Double pumpexperiment and (c) the
reduction in the fluence
threshold as a function of
the delay between the two
excitation pumps.

We find that we are unable to exert coherent control over the sample and observe that, despite ameasurable coherent phonon signal, the fluence threshold does not oscillate with the phonon
response. However, our data reveals a large incoherent decrease in the fluence threshold at earlytimes. No effect is observed at long times. To understand this effect, we conducted time-resolved
diffraction measurements at the SACLA XFEL to monitor Bragg and diffuse scatter simultaneously.
Our results, indicate that excitation of the monoclinic structure results in the population of incoherent
phonons within 50fs that manifest as specific 2D patterns of the diffuse scatter. These data,
alongside DFT calculations, suggest that photoinduced correlated disorder can provide a path tocontrol the phase transition. In this presentation I will discuss the supporting data and observations
and its possible implications in our ability to control solids on demand.
References[1] T. Y. Teo, X. Ma, E. Pastor,H. Wang, J.K. George, J.K.W. Yang, S. Wall, M.Miscuglio, R.E. Simpson, V.J.Sorger, Nanophotonic 11, 4073 (2022)[2] A.S. Johnson, D. Moreno-Mencía E.B. Amuah, M. Menghini, J.-P. Locquet, C. Giannetti, E. Pastor, S.E. Wall,. Physical Review Letters 129
255701 (2022).
[3] E. Pastor, D. Moreno-Mencía, M. Monti, A.S. Johnson, N. Fleischmann, C. Wang, Y. Shi, X. Liu, D.G. Mazzone, M.P.M Dean, S. Wal,
Physical. Review. B, 105, 064409 (2022).
[4] J.G. Horstmann, H. Böckmann, B. Wit, F. Kurtz, G. Storeck, C. Ropers, Nature 583, 232 (2020).
[5] J. Maklar, S. Dong, J. Sarkar,Y.A. Gerasimenko, T. Pincelli, S.Beaulieu, P.S.Kirchmann, J.A. Sobota, S.-L. Yang, D. Leuenberger, R.G Moore
Z.-X.Shen, M. Wolf, ,D. Mihailovic, R. Ernstorfer,L. Rettig, arxiv.org/abs/2206.03788 (2022)..

Tomography of light–controlled
Superconductivity by using
Phase and amplitude of two THz E&M pulses
I. E. Perakis
University of Alabama at Birmingham, Birmingham, AL 35294-1170, USA

Quantum Science relies on correlations between elementary excitations. These correlations aremeasured imperfectly with existing experimental tools. Multi-dimensional Coherent Terahertz(THz-MDCS) spectroscopy is a new tool when studying quantum materials. It allows sensing of
the changes in the collective modes arising from nonlinear couplings and THz driving ofcoherence and high-order correlation in ways not seen in conventional materials. However,establishing this experimental technique in correlated condensed matter systems such as
superconductors and topological materials requires solving a non-equilibrium many-bodyproblem that spans across several fields of current interest, including light-induced
superconductivity, parametric driving of metastable phases, and quantum entanglement ofsupercurrent qubits. In this talk, I will review the recent theoretical and experimental
advancements [1-6] that establish THz multi-dimensional coherent spectroscopy as a quantum
correlation tomography tool achieving “super” resolution of highly driven quantum states. In
particular, I will show how THz-MDCS monitors directly the field-induced changes in thecollective modes that uniquely characterize strongly driven non-equilibrium quantum states notrealized close to equilibrium. While conventional THz spectroscopy enables ultrafast timeresolved studies of quantum states of matter, with intense electromagnetic field pulses, several
nonlinear excitation pathways can occur. This complicates the interpretation of the conventional
one-dimensional pump-probe spectroscopies applied so far in superconductors, as different
processes contribute at the same frequencies. By taking advantage of both amplitude and relative
phase of two phase-locked THz field pulses to excite a superconductor, I will discuss how toseparate in two-dimensional frequency space various light-induced higher order signals fromthose arising from previously studied pump–probe, four-wave–mixing, high-harmonic
generation and Raman processes. The level of imaging and visualization of higher correlationin driven states achieved with THz-MDCS is not possible with conventional single-particle or
pump–probe spectroscopy measurements or seen in previously studied conventional materials.
I will demonstrate how to create and visualize in this way long-lived light-induced
superconductivity states with finite momentum and order parameter phases not seen in equilibrium.
Superconducting materials are being considered for quantum computing, and superconducting
quantum bits, or qubits, are the heart of the new technology. One strategy to control supercurrent
flows in qubits is to use strong light-wave pulses to accelerate the electrons. I will discuss how thiscan be achieved via nonlinear quantum transport phenomena resulting from acceleration ofsuperconducting condensates in thin superconducting films by the effective field resulting from theinterplay between electromagnetic propagation and nonlinearity. In this way, persistent dynamical
symmetry breaking is achieved by THz driving of long-lived finite-momentum-pairing statesstabilized well after the THz pulse. I will also discuss how the interplay between such long-lived
Cooper pairs with finite momentum with parametric driving of superconductivity by persistentoscillations of the relative phase of the order parameters between electron and hole bands in ironbased superconductors leads to new quantum states characterized by amplitude-phase collectivemodes that differ drastically from the Higgs and Leggett collective modes. In particular, thisparametric driving results in Floquet-like sidebands at twice the Higgs frequency, which have been
observed experimentally in iron pnictide superconductors. I will compare iron pnictides with strong
inter-band Coulomb interaction with those of BCS multi-band superconductors where intraband
Coulomb interactions dominate over inter-band ones to demonstrate the crucial role of the interband
interaction. I will present direct experimental evidence of transitions between different nonequilibrium states achieved above a critical THz field that show the breakdown of the conventional
susceptibility description of nonlinear responses and Raman processes. I will interpret such
experiments in terms of parametric driving of superconductivity by coherent modulation of orderparameter amplitude and phase that is controlled experimentally by THz pulse-pairs. The emergenceof parametrically-driven superconductivity is witnessed directly by the drastic transition from THzMDCS spectra dominated by the nonlinear peaks predicted by third-order responses and Ramanprocesses to THz-MDCS spectra dominated by the Higgs peak to a drastically different 2Dfrequency profile dominated by high-order nonlinear peaks located at high frequencies. In particular,
I will show the emergence of Floquet-like superconducting states witnessed directly through thedevelopment of new THz-MDCS peaks above critical driving. These new high-frequency peaks,
absent at low excitation, are located at twice the Higgs mode frequency and dominate over the Higgs
mode peaks which are suppressed. Finally, I will discuss the THz-control of soliton non-equilibrium
states achieved by tuning THz pulse-pair superconductor excitation.

Fig. 1. Transition between different driven superconductivity states in iron-based superconductors,
characterized by different collective modes controlled by phase coherent THz-pulse-pair excitation(from reference [1])In conclusion, the recent application of THz-MDCS to quantum materials provides the basis of amore general quantum tomography tool useful for quantum information science and engineering.
This multi-dimensional phase coherent spectroscopy technique enables quantum sensing and control
of complex superconducting and topological order parameters controlled by utilizing both the phaseand the amplitude of a pair of intense phase-locked THz electromagnetic fields.
References[1] L. Luo, M. Mootz, J. H. Kang, C. Huang, K. Eom, J. W. Lee, C. Vaswani, Y. G. Collantes, E. E. Hellstrom, I. E. Perakis, C. B. Eom, J. Wang
Nature Physics 19, 201(2023) https://doi.org/10.1038/s41567-022-01827-1[2] M. Mootz, L. Luo, J. Wang, I. E. Perakis Communications Physics 5, (2022).
[3] C Vaswani, J. H Kang, M Mootz, L. Luo, X Yang, C Sundahl, D. Cheng, C. Huang, R. H.J Kim, Z. Liu, Y.G. Collantes, E.E. Hellstrom
I. E Perakis, C.B Eom, J. Wang, Nature Communications 12, 258(2021).
[4] C Vaswani, M Mootz, C. Sundahl, D. H. Mudiyanselage, J. H. Kang, X. Yang, D. Cheng, C. Huang, R. H. J. Kim, Z. Liu, L. Luo,I. E Perakis
J. Wang, Physical Review Letters 124, 207003 (2020).
[5] M. Mootz, J. Wang, I. E. Perakis, Physical Review B 102, 054517 (2020).
[6] X. Yang, C. Vaswani, C. Sundahl, M. Mootz, L. Luo, J. H. Kang, I. E. Perakis, C. B. Eom, J. Wang, Nature Photonics 13, 707 (2019).

Theoretical and experimental results in ultrafast dynamics andUltrafast bandgap photonics
J. Dong1, D. Shin2, E. Pastor3, T. Ritsche4, L. Cario5, Z. Chen6, W. Qi1, R. Grasset1, M. Marsi6, A. TalebIbrahimi7, N. Park8, A. Rubio9, E. Papalazarou6, L. Perfetti1
1
Institut Polytechnique de Paris, 91128 Palaiseau, France
2
Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea
3
Universitat Jaume I, 12006 Castello, Spain
4
Technische Universitat Dresden, 01069 Dresden, Germany
5
Université de Nantes, 44000 Nantes , France
6
Université Paris-Saclay, 91405 Orsay, France
7
France Société Civile Synchrotron SOLEIL, 91192 Gif-sur-Yvette, France
8
Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea
9
Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany
The transition metal 1T-TaS2 is a layered insulator with a rich phase diagram aSoci\'{e}t\'{e} civile
Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin - BP 48, 91192 GIF-sur-YVETTE,Frances a function of pressure and temperature. Its broken symmetry phases include
incommensurate, nearly commensurate, and commensurate Charge Density Waves (CDWs). Withineach layer, the Ta lattice undergoes a periodic distortion in which 13 Ta ions form clusters with the
motif of a Star-of-David (SD). These clusters have an odd filling and lock-in to a Commensurate
CDW (C-CDW) below 180 K. The observed insulating behavior of the C-CDW phase is generally
attributed to the Mott localization of the electron in the highest occupied state of SDs. Recent
calculations revised the strength of Coulomb repulsion in this family of compounds and highlighted
the strong effects that electronic interactions have on the band structure of 1T-TaS2.Although widelybelieved to be a Mott insulator, the commensurate CDW phase also features an interlayer stacking
with SDs dimerization. The stacking of two adjacent layers can be of three different kinds: Top
Aligned (A) and Laterally displaced (L) with a vector of magnitude 2a. The dimerized geometry of
1T-TaS2 is formed by alternating stacking between A and L configurations, called AL stacking. By
hosting an even number of electrons, the dimerized unit cell of the commensurate CDW cannot be apure Mott phase. This duality gave origin to several works, addressing the Slater-towards-Mottcharacter of the ground state.This work reports time-resolved ARPES measurement on high-quality
single crystals of 1T-TaS2 in the insulating C-CDW phase. By making use of different polarizationsof the probe pulse, we are able to visualize the dispersion of electronic states below and above thechemical potential. The experimental data are compared with state-of-the-art Density Functional
Theory calculations with the Generalized Orbital U (DFT+GOU). The Coulomb U of a SD cluster isself-consistently calculated via the ACBN0 method.
Fig.1.A-E):Photoelectron
intensity map acquired with Sprobe polarization along theΓ-M direction for different
pump-probe delays. The
intensities above and below
the Fermi level have been
multipliedbyrescaling
factors in order to better
visualize the electronic stateswith respect to a fixed color
scale. The blue line in panel
A stands for the electronic
gap size.

Our results indicate that both stacking order and electronic correlations are essential to reproduce thecorrect gap size. Moreover, time-resolved ARPES data acquired with S polarized probe disclose
novel aspects of the photoinduced phase transition. The pump pulse erases the band dispersion andhalves the gap magnitude within half a period of the coherent CDW motion. Besides the oscillationsof CDW amplitude,we propose that photoexcitation also engenders local variations of dimerization,
orbital filling, and U potential. The combination of these effects triggers the melting of the MottPeierls gap.
References[1] D. Shin, N. Tancogne-Dejean, J. Zhang, M. S. Okyay, A. Rubio, N. Park, Physical Review Letters 126, 196406 (2021).
[2] J. Dong, D. Shin, E. Pastor, T. Ritschel, L. Cario, Z. Chen, W. Qi, R.Grasset, M.Marsi, A.Taleb, N. Park, A. Rubio, L. Perfetti, E. Papalazarou
arXiv:2210.11052.

Terahertz lightwave-driven control of magnetism
I. RaduEuropean X-ray Free-Electron Laser, 22869 Schenefeld, Germany

Harnessing order parameters in solids, as e.g. magnetism, ferroelectricity, superconductivity etc,
using ultrashort light pulses is a key science driver in condensed matter research. This is particularly
true for the field of magnetism and spintronics [1-4] where besides a fundamental scientific interest
there is an exciting potential for technological applications in, e.g., high-speed magnetic logic andmagnetic storage devices. A long sought-after and yet to be realized phenomenon is the coherent anddeterministic control of a macroscopically ordered spin ensemble on the sub-cycle timescales of thephoto-exciting light field [4].
Here, I will showcase the latest developments in our projects on ultrafast magnetism by employing
strong-field THz and mid-IR excitations on ferrimagnetic materials to (i) demonstrate an ultrafast
and fully deterministic magnetization switching process upon single-shot THz pulse exposure and(ii) photo-drive the generation of a coupled spin-lattice quasiparticle at THz frequencies withultralong coherence lifetimes – see Fig. 1.

Fig. 1. Left: Time-resolved magneto-optical Faraday signal measured on a ferrimagnetic Ho3Fe5O12 sample
upon flipping the B field component of the pumping THz pulse. The narrowband THz pulse centered at 0.83 THz
resonantly excites an IR-allowed spin-lattice mode generating a long-lived precession of the macroscopic
magnetization. Right: FTIR spectrum measured at 10 K on the Ho3Fe5O12 sample showing the spectral
signature of the coupled spin-lattice mode below 1 THz.

I will conclude with our future plans on highly brilliant THz light sources to be implemented at theEuropean XFEL.
References[1] A. Kirilyuk, A.V. Kimel, Th. Rasing, Review Modern Physics 82, 2731 (2010).
[2] K. Carva, P. Balasz, I. Radu, Handbook of Magnetic Materials 26, 29 (2017).
[3] I. Radu, K. Vahaplar, C. Stamm, T. Kachel, N. Pontius, H. A. Dürr, T. A. Ostler, J. Barker, R. F. L. Evans, R. W. Chantrell, A. Tsukamoto
A. Itoh, A. Kirilyuk, Th. Rasing, A. V. Kimel, Nature 472, 205 (2011).
[4] I. Radu, Journal Physics Condensed Matter 33, 353001 (2021).

All optical control of magnetism for energy efficient andBrain inspired computing
Th. Rasing
Radboud University, 6525 Nijmegen, the Netherlands

The ability to switch magnets between two stable bit states is the main principle of digital data
storage technologies since the early days of the computer. However, the explosive growth of digital
data and its related energy consumption is pushing the need to develop fundamentally new physicalprinciples and materials for faster and more energy-efficient processing and storage of data [1]. Since
our demonstration of magnetization reversal by a single 40 femtosecond laser pulse, themanipulation of spins by ultra-short laser pulses has developed into an alternative and energy
efficient approach to magnetic recording [2.3]. Plasmonic antennas have allowed us to push this
optical control even down to nanometer length scales [4], while photonic networks allow thedevelopment of an optically switchable MRAM [5].

Fig. 1. (Left): Magneto-optical image of two physically-separated laser-written synaptic weights, w1, w2 (darkspots); the bottom shows eight background subtracted images illustrating evolution of the magnetization
changes due to the right (top row) and left (bottom row) circularly-polarized fs laser pulses irradiating the
Co/Pt sample. (Right): Extracted intensity changes as function of the number of laser pulses, demonstrating
continuously-controllable weights, shown in black (blue) for w1 (w2). Black solid disks correspond to the images
in the bottom rows of (a). The inset shows the pulse packets used for learning.

However, new ICT technologies, such as Artificial Intelligence push the exponentially increasingenergy requirement of data manipulation even more [6]. Therefore, the development of radically new
physical principles that combine energy-efficiency with high speeds and high densities is crucial fora sustainable future. One of those is neuromorphic computing, that is inspired by the notion that ourbrain uses a million times less energy than a supercomputer while, at least for some tasks, it evenoutperforms the latter [7]. Ultimately, future brain-inspired technology should provide room
temperature operation down to picosecond timescales, nanoscale dimensions and at an energy
dissipation as low as the Landauer limit (~zJ). In this talk, I will discuss the state of the art inultrafast manipulation of magnetic bits and present some first results [8,9,10] to implement braininspired computing concepts in magnetic materials that operate close to these ultimate limits.
References[1] T. Becker, R. Haas, J. Schemmel, S. Furber, S. Dolas, S,https://www.etp4hpc.eu/white-papers.html#unconv (2022).
[2] A. Kirilyuk, A. V. Kimel, Th. Rasing, Review Modern Physics 82, 2731(2010).
[3] A. V. Kimel, M. Li, Nature Reviews Materials 4, 189 (2019).
[4] T.-M. Liu, T. Wang, A. H. Reid, M. Savoini, B. Koene, P. Granitzka, C.E. Graves, D.J. Higley, Z. Chen
G. Razinskas, M. Hantschmann, A. Scherz, J. Stohr, A. Tsukamoto, B. Hecht, A.V. Kimel, A. Kirilyuk, Th. Rasing
H. Durr, Nano Letters, 15, 6862 (2015).
[5] L. Avilés-Félix, L. Alvaro-Gomez, G. Li, C.S. Davies, A. Olivier, M. Rubio-Roy, S. Auffret, A. Kirilyuk,
A.V. Kimel, Th. Rasing, L.D. Buda-Prejbeanu, R.C. Sousa, B. Dieny, I.L. Prejbeanu, AIP Advances 9, 125328 (2019).
[6] A. Mehonic, A.J. Kenyon, Nature 604, 255 (2022).
[7] D. J. Kösters, B. A. Kortman, I.Boybat, E. Ferro, S.Dolas, R.Ruiz de Austri, J.Kwisthout, H.Hilgenkamp, T. Rasing, H.Riel,A.Sebastian S. Caron
J.H. Mentink, APL Machine Learning 1, 016101 (2023)[8] A. Chakravarty, J.H. Mentink, C. S. Davies, K. Yamada, A.V. Kimel, Th. Rasing, Applied Physics Letters 114 192407 (2019).
[9] A. Chakravarty, J.H. Mentink, S. Semin, Th. Rasing, Applied Physics Letters 120, 022403 (2022).
[10] K. Raab, M.A. Brems, G. Beneke, T. Dohi, J. Rothörl, F.Kammerbauer, J.H. Mentink, M. Kläui, Nature Communications 13, 6982 (2022).
* Acknowledgement(s): Support from the Dutch Research Council (NWO) and the European Research Council ERC grant agreement no.856538(3D-MAGiC) is acknowledged.

Towards imaging optically induced charge density with atomic
resolution
1
3
4
2
2
5
5

C. Ornelas-Skarin , T. Bezriadina , M. Fuchs , S. Ghimire , J. B. Hastings , N. N. Hua , L. Leroy
Q. Nguyen2, G. de la Peña2, D. Popova-Gorelova3,S. Shwartz6, M. Trigo2, T. Sato2, D. Zhu2
D. A. Reis1
1
Stanford University, Stanford, CA 94305, USA
2
SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
3
University of Hamburg, D-20355 Hamburg, Germany
4
University of Nebraska, Lincoln, NE, 68588, USA
5Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
6
Bar-Ilan University, Ramat Gan 5290002, Israel

The nonlinear interaction of x-ray and optical photons in materials can be used to image the atomicscale local optically induced charge density [1, 2]. This method can be thought of as x-ray andoptical sum and difference frequency generation, or equivalently x-ray diffraction from the opticallymodulated charge density, 𝛿𝜌(𝑟⃗ , 𝑡). By measuring energy and momentum sidebands to ordinary
Bragg diffraction gives direct information about the local induced charges (and thus currents) thatwould otherwise be invisible because of the long wavelength at optical frequencies. Similar toconventional crystallography which ideally measures 𝜌(𝑟⃗) information about 𝛿𝜌(𝑟⃗, 𝑡). is obtained
through the nonlinear diffracted intensity. This intensity depends on the spatial and temporal Fouriercomponents, δρ(ω)
since
�G
�⃗
(𝜔)

IG��⃗

(𝜔) 2

∝ �δρ𝐺⃗ � = |∫ δρ(r⃗, t)e𝑖𝐺⃗ ⋅𝑟⃗ e−𝑖𝜔𝑡 dr⃗ dt |2 ,where 𝐺⃗ is a reciprocal lattice vector. The feasibility of imaging the linear optical response with atomic

resolution was first demonstrated in [3]. In that experiment, the mixing of single photons from an xray free-electron laser (FEL) and a Ti:sapphire laser was reported in single crystal diamond. Higherorder wave-mixing, involving more than one optical photon with one x-ray photon is expected due tothe nonlinear interaction of the optical beam with the material. The higher-order Fourier components)of the nonlinear induced charge density, δρ(ω=nω(where |n| > 1 ), can have nontrivial dependence on�G�⃗polarization compared to the first-order optical response,opt.

(ω=nωo.)δρG��⃗∝ |𝜖⃗ ⋅ 𝐺⃗ |,where 𝜖⃗ is the laser polarization and 𝜔𝑜 is the optical frequency. An atomic-scale image of the linear
much less nonlinear response to optical excitation would be an enormous advance. Measurements of
the nonlinear response would be useful for Floquet engineering of novel states in quantum materials[4] and imaging the nonperturbative currents in solid-state high-harmonic generation [5, 6]. Here we
discuss recent measurements of XOM for 𝑛 = ±1 and 𝑛 =2, corresponding to oscillations at 𝜔𝑜 and2𝜔𝑜 inside silicon. Particularly the 2𝜔𝑜 component is an example of how XOM can be used toobserve details of the electron dynamics invisible to longer wavelength probes, such as purely opticalmeasurements, due to the inversion symmetry in bulk silicon. The experimental challenges ofmeasuring XOM stem from its relatively small efficiencies and large elastic back-grounds. The
mixing signal is concentrated in an extremely small solid-angle and phase-matching conditions about
the elastic Bragg peaks. This requires high precision and stable mechanics as well as a relatively
narrow monochromator and analyzer with high rejection in angle and energy. We have designed andconstructed a suitable multi-channel cut (CC) Si 311 dispersive monochromator (CCM) and analyzer(CCA). The proposed setup is shown in Fig. 1.

Fig. 1. a) Schematic of the setup used to measure the sum frequency generation signal, 𝜔𝑆𝐹𝐺 . The x rays
(𝜔𝑥 = 9.5 keV) from the LCLS FEL then pass through a beryllium lens and the monochromator, which reduces
the bandwidth and collimates the x rays. The x rays then diffract from the optically pumped sample (𝜔𝑜 = 1eV).
The sum frequency photons are collected while the elastically diffracted photons are rejected by the analyzer.
The diodes act as intensity monitors to track any drift in the optics. In b) and c) we show the polarization
dependence of the signal corresponding to oscillations at b) 𝜔𝑆𝐹𝐺 = 𝜔𝑥 + 𝜔𝑜 1ωoptical and c) 2ωoptical in theplane containing [100] and [010] as pictured in the inset image of the sample in c). The red dashed line is a fit
to the form given the text.
(±𝜔 )We measured the sum and difference frequency signals about the (220) Bragg peak, I(220), in a 40 μm(2𝜔 )thick silicon crystal, as well as the second-order sum frequency (I(220) ), at LCLS using our double
CCM/CCA pairs. Even though silicon is inversion symmetric at the macroscopic scale, both thediffraction geometry and the local bonding environment break in- version symmetry; this allows us
to measure sidebands associated with the anharmonic charge density oscillations at laser second-6
-9
(𝜔 )
(0)
(2𝜔 ) (0)harmonic of the laser. We measured a relative efficiency I(220)/I(220) ∼ 10 and I(220) /I(220) ∼ 10 for an
122
incident intensity of a few 10 W/cm at a 1eV optical photon energy. The polarization dependenceof the 2nd-order sideband for the symmetric 220 peak of the (001) cut crystal is shown in Fig. 1 c).
The 2nd-order charge density is proportional to the product of two-optical fields,
(2𝜔 )
𝛿𝜌𝐺⃗
∝ 𝐺𝑖 𝜒� (2)𝐺,𝑖𝑗𝑘 𝐸𝑗 𝐸𝑘 ≡ 𝐹𝐺,𝑗𝑘 𝐸𝑗 𝐸𝑘 . Due to the crystal cut, we were constrained to measuring two of the four2(𝜔 )independent components of 𝜒�(2)(220), such that I(220)∝ �𝐹(220),𝑥𝑥 + 𝐹(220),𝑥𝑦 cos 2𝜃𝜖�⃗⋅𝐺⃗ � as shown in Fig. 1 c). We-6
find 𝐹(220),𝑥𝑦 ≈ 2𝐹(220),𝑥𝑥 , and the measured background is less than 5 10 photons/x-ray pulse, orders ofmagnitude below the measured signal. We will discuss future directions including crystallographic
reconstruction of optically induced charge density, including by direct phasing, extension to lower
quality crystalline samples and extension to sub-cycle time-domain measurements at FELs.
𝑜

0

𝑜

𝑜

𝑜

𝑜

References[1] P. Eisenberger, S. McCall. Mixing of x-ray and optical photons. Physical Review A 3, 1145 (1971).
[2] I. Freund, B. F. Levine. Optically modulated x-ray diffraction. Physical Review Letters 25, 1241 (1970).
[3] T.E. Glover, D. M. Fritz, M. Cammarata, T.K. Alison, S. Coh, J. M. Feldcamp, H. Lemke, D. Zhu, Y. Feng, R. N. Coffee, M. Fuchs
S. Ghimire, J. Chen, S. Shwartz, D A. Reis, S. E. Harris, J. B. Hastings, Nature 488, 603(2012).
[4] D. Basov, R. Averitt, D. Hsieh, Nature Materials 16, 1077 (2017).
[5] D. Popova-Gorelova, D. A. Reis, R. Santra, Physical Review B 98 , 224302 ( 2018).
[6] S. Ghimire, D. A. Reis, Nature Physics 15, 10(2011).
* This work is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences,and Biosciences Division through the AMOS program and performed on the LCLS. We thank X. Huang for the fabrication and advice on thedesign of the Si 311 channel cut crystals. We thank S. Gerber, H. Lemke, R. Mankowsky, M. Sander and C. Svetina for help with preliminary
experiments on SwissFEL.

Observing quantum materials in real time by
Pump-probe Raman scattering
D. Reznik
University of Colorado-Boulder, Boulder, CO 80309, USA
In this talk I describe the time-resolved Raman scattering work at the University of Colorado. The

Raman lab shares an ultrafast laser system with an angle-resolved photoemission (ARPES) setup,
which allows investigation of bosonic and fermionic excitations under the same driven
nonequilibrium conditions. I will discuss new physics that was uncovered in three recent projects.
We combined ultrafast pump-probe Raman scattering and ARPES to directly follow electron-holeexcitations as well as the G-phonon in graphite after an excitation by an intense laser pulse. This
phonon is known to couple relatively strongly to electrons. Cross-correlating effective electronic andphonon temperatures places new constraints on model-based fits. The accepted two-temperaturemodel predicts that G-phonon population should start to increase as soon as excited electron-holepairs are created and that the rate of increase should not depend strongly on the pump fluence.
Instead we found that the increase of the G-phonon population occurs with a delay of ~65 fs. This
time-delay is also evidenced by the absence of the so-called self-pumping for G phonons. The timedelay decreases with increased pump fluence. These observations imply a new relaxation pathway:
Instead of hot carriers transferring energy to G-phonons directly, the energy is first transferred to
optical phonons near the zone boundary K-points, which then decay into G-phonons via phononphonon scattering. Our work demonstrates that phonon-phonon interactions must be included in any
calculations of hot carrier relaxation in optical absorbers even when only short timescales are
considered [1].

Fig. 1. (a): Anti-Stokes Raman scattering in graphite as a function of the time delay between the pump and theprobe. G-phonon at 1580 cm-1 emerges at positive times [1]. (b): Stokes 2-magnon Raman spectrum of
antiferromagnetic YBCO with intensity suppression at short delay times clearly visible [2]. (c): Anti-Stokes
Raman scattering in nearly optimally-doped YBCO (superconducting Tc=90K) dominated by the apical oxygen
phonon at 500 cm-1 as a function of the pump-probe time delay [3].

The second project focused on the time-evolution of the two-magnon Raman scattering in an
insulating antiferromagnetic sample of YBa2Cu3O6+x (YBCO). These experiments demonstrated thattime-resolved 2M Raman scattering is a powerful probe of ultrafast demagnetization in antiferromagnetic
Mott/charge transfer insulators. It can be applied to a variety of materials where 2M Raman scattering has
been observed. Electron-spin coupling plays an important role in photo-carrier relaxation as evidenced by aradical disturbance of the magnon spectrum. In YBCO the maximum effect occurred within our experimental
resolution of 90 fs of photoexcitation, which is much faster than demagnetization timescales in itinerantferromagnets. Slower timescales characterizing subsequent relaxation to thermal equilibrium are of the sameorder of magnitude or greater than phonon-driven relaxation in conventional materials (e.g., graphite). We
have proposed a simple explanation of these results in terms of slow relaxation of the charge sector and fast
relaxation of the magnetic sector. Our results demonstrate strong coupling between charge and spin degrees of
freedom, which potentially accounts for high-Tc superconductivity. The coupling between spin and phonon
degrees of freedom remains to be understood. In the future tuning pump laser energy to resonate withparticular dipole-active phonons may allow using these measurements to provide new insights [2].We also
used pump-probe Raman scattering to investigate the apical oxygen vibration in strongly doped
superconducting YBa2Cu3O6+x under nonequilibrium conditions. We have demonstrated quantitative
agreement with theory of the time-varying apical phonon frequency as the electronic system loses its excessenergy to the broader phonon bath. This results highlights that the dynamics of energy transfer are responsible
for the temporal behavior of the electrons and phonons, as well as the disparity between interactions in and outof equilibrium. Although in strongly correlated materials such as YBCO there are strong Coulomb processes
and impurity scattering that dominate the electronic spectra, when it comes to time domain these processes
rapidly come to an internal equilibrium and effectively shut off. Our results provide a new, phonon-centered,perspective on previous experiment in Bi2Sr2CaCu2O8 with both time-resolved ARPES and ultrafast electron
diffraction that both observed a similar quantitative agreement [3].This work provides new insights into
photoinduced superconductivity. Its signatures were recently reported in the optical spectra of underdoped
YBCO up to time-delays of about 1ps when pumping with 790 nm near-IR pulses as well as with pulses that
resonated with IR-active apical oxygen phonons. Our experiments reproduced the former pumping conditionand showed that optimally-doped and underdoped superconducting YBCO behave similarly. We found that
hot and cold phonons were out of thermal equilibrium, but electrons and hot phonons were at or near thermal
equilibrium at time delays below 1ps. At these time-delays electronic temperatures were always well above
room temperature. We plan to determine transient heating while pumping the IR-active phonons in future
experiments [3].
References[1] J.A. Yang, S. Parham, D. Dessau, and D. Reznik, Scientific Reports 7, 40876 (2017).
[2] J.A. Yang, N. Pellatz, T. Wolf, R. Nandkishore, D. Reznik, Nature Communications 11, 2548 (2020).
[3] N. Pellatz, S. Roy, J.W. Lee, J.L. Schad, H. Kandel, N. Arndt, C.-B. Eom, D. Reznik, Physical Review B 104, L180505 (2021).
* Raman experiments at the University of Colorado were supported by the NSF under Grant No. DMR-1709946 and laboratory upgrade that madethese experiments possible by DARPA through the DRINQS program. The time-resolved ARPES work was supported by NSF grant DMR1508785, with the equipment purchased in part through NSF MRI-1546961. The work at University of Wisconsin-Madison (thin film synthesis andstructural and electrical characterizations) was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy
Sciences (BES), under award number DE-FG02-06ER46327 as well as by the National Science Foundation under Grant No. DMR-1752713. The
work at the University of Wisconsin-Parkside was supported by WiSys and UW System Applied Research Grant award number 102-4-812000AAH1775.

Optically induced avoided crossing in Graphene studied by
MIR pump/ Raman probe spectroscopy
M. Rübhausen
University of Hamburg, 22761 Hamburg, Germany

Spontaneous time-resolved Raman scattering in a pump-probe configuration is a novel tool that hasessentially four important parameters that can be tuned to be used for different types of research.[1]Firstly, the incident photon energy of the probe can be tuned into resonance with the desired
quasiparticle excitation of interest.[2] Resonant Raman scattering has been used to amplify
excitations such as phonons, magnons, and inelastic scattering from order parameters in condensed
matter. Secondly, in pump-probe configuration, the pump wavelength and corresponding energy canaddress different degrees of freedom by pumping resonantly phonons, charge-transfer excitations ortransitions between lower and upper Hubbard bands. Both photon pulses, thus, benefit from asubstantial tunability of pump and probe energies. Thirdly, we have time resolution that allows todisentangle the impact of the pump on the spontaneous probe spectra as a function of time. And,
finally, fluence is important, in particular, for scenarios where coherent pumping is required. With
this in mind, we designed, constructed, and put into operation a pump-probe setup for spontaneous
Raman scattering that allows to tune the incident energy of the probe laser beams between 1 eV and
6 eV utilizing a fully reflective coupling of the scattered photons combined with a frequency and also
spatially filtered bandpass operation of a custom-made spectrometer as shown in Fig. 1[3]. We haveembedded this setup with a laser system that allows pumping of various degrees of freedom
continuously from the mid-infrared spectral range up to the deep UV (0.1 eV to 6 eV) – see Fig. 1.

Fig. 1.: General outline of the time-resolved spontaneous Raman setup at the CFEL. Three main laser sources(two diode-pumped TiSa lasers, one fiber amplified laser) serve as general sources for the following steps of flexible
harmonic generation, optical parametric amplification followed by second and third harmonic generation, as well as
difference frequency generation. By these means laser pulses with pulse durations between 300 fs and 5 ps can beobtained. Focusing and signal collection is done by a fully reflective Cassegrain type objective with a numerical apertureof 0.52 featuring parabolic on-axis mirrors for diffraction limited imaging of the source into the UT-3 Raman
spectrometer. Straylight rejection of the instrument is amplified by a beam block implemented in the objective and thepremonochromator stage made of off-axis parabolic mirrors with embedded spatial filtering to suppress edge diffraction
effects occurring at the blades of the slits. The spectral range of the detection system covers 1 eV – 6 eV. The spectral
range of the pump covers 0.1 eV to 6 eV.

The temporal width of the probe pulse can vary between 800 fs to 5 ps and the temporal width of thepump can vary between 300 fs to 1.2 ps. Delays between pump and probe can reach up to the ns
range. We have utilized this setup for studies of the superconducting order parameter in cuprates in
the past revealing the response of the pair-breaking peak after a charger-transfer pump.[1] However,
the extension into the mid-infrared spectral range allows to study the coherent anharmonic coupling
between two orthogonal phonon modes generating a new vibronic state. Furthermore, only few
theories to calculate the Raman response function under non-equilibrium conditions exist.
Frequently, the observed line shape is dominated by the pulse broadening of the probe. However,upon proper tuning of pump and probe pulse durations it is possible to derive a spontaneous Raman
signal that is dominated in its spectral response by the intrinsic behavior of the sample making it
useful to analyze the changes of the line shape of the transient Raman results with varying pumpconditions. I am going to show the application of this approach to the degenerate Raman and IR
active modes in bi- and multilayer graphene and discuss how to analyze the changes in line shapeupon tuning the pump wavelength through the IR mode.[4] Due to the strong anharmonic coupling ofthese degenerate modes a new vibronic state appears with distinct spectral features and properties
that I am going to discuss in my presentation. Most notable is a line shape sharpening of the transient
spontaneous Raman scattering compared to the equilibrium, a frequency change upon tuning of theIR pump and an ultra-sharp resonance of the transient Raman response that matches the width of thepumped IR phonon. I will be utilizing an equation of motion approach using a Greens-function
method calculating the transient Raman response of two degenerate harmonic oscillators [5]. Overalla good agreement with the experimental observation can be found. In an outlook I will briefly
address the possibility to study in-gap excitations of high TC superconductors that will be in contrast
to our previous work studying the temporal evolution of the pair-breaking excitation insuperconductors [6,7].
References[1] R. P. Saichu, I. Mahns, A. Goos, S. Binder, P. May, S. G. Singer, B. Schulz, A. Rusydi, J. Unterhinninghofen, D. Manske, P. Guptasarma
M. S. Williamsen, M. Rübhausen, Physical Review Letters 102, 177004 (2009).
[2] D. Budelmann, B. Schulz, M. Rübhausen, M. V. Klein, M. S. Williamsen, P. Guptasarma, Physical Review. Letters 95, 057003 (2005).
[3] B. Schulz, J. Bäckström, D. Budelmann, R. Maeser, M. Rübhausen, M. V. Klein, E. Schoeffel, A. Mihill, S. Yoon, Review of Scientific
Instruments 76, 073107 (2005).
[4] S.Buchenau, J. Reichstetter, B.Grimm-Lebsanft, F Biebl, M.Fechner, S.Herres-Pawlis, D.Manske, A. Cavalleri, M. Rübhausen,(submitted) 2023.
[5] M. Rübhausen, J. Reichstettet, D. Manske, (submitted) 2023.
[6] L. Schwarz, B. Fauseweh, N. Tsuji,D. Manske, S. Kaiser, Nature Communications 11, 287 ( 2020).
[7] M. Puviani, A. Baum, S. Ono, Y. Ando, R. Hackl, D. Manske, Phys. Rev. Lett. 127, 197001 – 2021.
* Acknowledgement(s): I would like to thank the numerous researchers who have helped to set up this unique instrument over the past years. For
the new particular research that I am addressing in my presentation I would like to thank S. Buchenau, T. Glier, B. Grimm-Lebsanft, F. Biebl
L. Westphal, M. Rerrer, M. v. Heek. For many discussions on the physics I would also like to acknowledge D. Manske, J. Reichstetter,
M. Fechner, A. Cavalleri. S. Kaiser. This work was financially support by the BMBF (05K19GU5) and the DFG (RU773/7, RU773/8).

Time-resolved photoelectron diffraction: mapping atomic motion in
Bi2Se23 Phonon Oscillations
1
2
3
2
5
3

C. E. Sanders , D. Curcio , K. Volckaert , D. Kutnyakhov , S. Y. Agustsson , K. Bühlmann , F. Pressacco
M. Heber3, S. Dziarzhytski3, Y. Acremann5, J. Demsar4, W. Wurt,6, Ph. Hofmann2
1
STFC Central Laser Facility, Didcot OX11 0QX, UK
2
Aarhus University, 8000 Aarhus-C, Denmark3
Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany
4
Johannes Gutenberg-Universität, 55099 Mainz, Germany
5
ETH Zürich, 8093 Zürich, Switzerland
6
Hamburg University, 22761 Hamburg, Germany

X-ray photoelectron diffraction (XPD) is a powerful technique, based on the tools of X-ray photoelectron
spectroscopy (XPS). It combines chemical sensitivity with the power to elucidate local geometric order[1]. It can be used to determine the global surface lattice of a crystal [2,3], and can also reveal the local
structural order around individual adsorbates on crystalline surfaces [4]. Free-electron lasers (FELs) candeliver ultra-short X-ray pulses in an energy range suitable for XPD study. The fact that FEL pulse trains
can be synchronized with those of secondary, lower-photon-energy laser sources means that XPD canpotentially be adapted to pump-probe, time-resolved studies. This would expand the technique’s scope insome of the most important research areas in the chemistry and physics of out-of-equilibrium systems.
One can imagine using it to probe the excitation and detailed evolution of complex phonon modes incrystals, or to visualize surface catalysis in action.The fundamental principle of XPD is thatphotoelectrons travel to a photoelectron detector both by direct paths and by paths that involve multiple
scattering from surrounding atoms. The intensity at the detector is the coherent sum of all these partial
waves, and a diffraction pattern arises as a function of emission angle (or parallel momentum), as
measured at the detector. An example of such a pattern is shown in Fig. 1(b).

Fig. 1. (a) Schematic of experimental setup. (b) Measured

static (unpumped) XPD pattern. (c) Simulated XPD patternobtained from the optimized structural model. Figure
reproduced from [5].

In post-analysis, one simulates diffraction patterns for assumed structures, and then refines the structuralparameters of the models, so as to cause the simulated diffraction pattern to converge to themeasurement. Such a simulation is shown in Fig. 1(c).We have now used pump-probe techniques at thePG2 beamline of the FLASH free-electron laser (FEL) to demonstrate time-resolved XPD measurementsof the “simple” model system Bi2Se3 [5]. Key to the success of this experiment is the HEXTOF
experimental station, which is based on a time-of-flight (ToF) momentum microscope (MM). The ToFMM allows the entire XPD pattern to be probed simultaneously, without the need to rotate or tilt the
sample. The experimental setup is shown in Fig. 1(a). The symmetric 𝐴1𝑔 coherent optical phononmodes can be excited with ultrashort laser pulses [6,7]. In the present experiment, we pumped the 𝐴11𝑔
mode with a tabletop laser system. The pumping power was 4.2 mJ cm-2 with 1.55-eV photon energy.
The pump beam was synchronized with the pulse train generated by the FEL. FEL photon energy of 113
eV was chosen, so as to photoemit from the Se 3d core levels. With our approach, we can successfully
observe—with temporal and spatial resolution of 140 fs and 1 pm, respectively—the pumping ofoscillations in the first interlayer spacings d1 and d2 (see Fig. 1[a]). The results are consistent withprevious reports based on other techniques [8,9], and pave the way for studies of more complex
dynamical systems by XPD, with improved statistics at higher repetition rates and at time scales down totens of fs.
References[1] D. P Woodruff, Journal of Electron Spectroscopy and Related Phenomena 126, 55, (2002).
[2] W. Auwärter, T. J. Kreutz, T. Greber, J. Osterwalder, Surface Science 429, 229 (1999).
[3] H. Bana, E. Travaglia, L. Bignardi, P. Lacovig, C. E. Sanders, M. Dendzik, M. Michiardi, M. Bianchi, D. Lizzit, F. Presel, D. De Angelis
N. Apostol, P. K. Das, J. Fujii, I. Vobornik, R. Larciprete, A. Baraldi, P. Hofmann, S. Lizzit, 2D Materials 5, 035012 (2018).
[4] Ph. Hofmann, K.-M. Schindler, S. Bao, A. M. Bradshaw, D. P. Woodruff, Nature 368, 131 (1994).
[5] D. Curcio, K. Volckaert, D. Kutnyakhov, S. Y. Agustsson, K. Bühlmann, F. Pressacco, M. Heber, S. Dziarzhytski, Y. Acremann, J. Demsar
W. Wurth, C. E. Sanders, P. Hofmann, Physical Review B 106, L201409, (2022).
[6] J. Qi, X. Chen, W. Yu, P. Cadden-Zimansky, D. Smirnov, N. H. Tolk, I. Miotkowski, H. Cao, Y. P. Chen, Y. Wu, S. Qiao, Z., Jiang, Applied
Physics Letters 97, 182102, (2010).
[7] N. Kumar, B. A. Ruzicka, N. P. Butch, P. Syers, K. Kirshenbaum, J. Paglione, H. Zhao, Physical Review B 83, 235306, (2011).
[8] J. Hu, K. Igarashi, T. Sasagawa, K. G. Nakamura, O. V. Misochko, Applied Physics Letters 112, 031901, (2018).
[9] J. A. Sobota, S.-L. Yang, D. Leuenberger, A. F. Kemper, J. G. Analytis, I. R. Fisher, P. S. Kirchmann, T. P. Devereaux, Z.-X. Shen, PhysicalReview Letters 113, 157401, (2014).
* Acknowledgements: the authors acknowledge funding from VILLUM FONDEN via the Centre of Excellence for Dirac Materials (Grant No.11744) andthe Independent Research Fund Denmark (Grant No. 1026-00089B). This research was carried out at FLASH at DESY. The research has been supported by
the project CALIPSOplus under Grant Agreement No.30872 by the Deutsche Forschungsgemeinschaft (DFG)within project B2), and within TRR 173268565370 (Project No.A05). We thank Holger Meyer, Sven Gieschen, and Harald Redlin for experimental support, as well as Gerd Schönhense, Davide
Campi,Anton Tamögl, and Wolfgang Ernst for helpful discussions.

Extremely high average power THZ sources for
Nonlinear THz spectroscopy at high repetition rate
T. Vogel, C. Millon,M. Khalili, Y. Wang, S. Mansourzadeh C. J. Saraceno
Ruhr University Bochum, 44801, Bochum, Germany

Terahertz Time Domain Spectroscopy (THz-TDS) has become a ubiquitous tool in many scientificfields and is also increasingly deployed in industrial settings. While these systems become more andmore mature, efficient and lab-based THz generation methods combining broad bandwidth and highdynamic range (e.g., as provided by high THz average power and correspondingly high repetitionrate) remain rare. Most industrial THz-TDS systems make use of semiconductor-based
photoconductive emitters and receivers; these offer high dynamic range operation at very highrepetition rates of hundreds of MHz, and the corresponding emitters provide high conversion
efficiencies with low power excitation, driven by compact ultrafast fiber-lasers. For applications
where strong-fields are desired, for example for nonlinear THz spectroscopy, the most commonlyused technique is optical rectification in nonlinear crystals, for instance zinc telluride (ZnTe),
gallium phosphide (GaP), lithium niobate (LiNbO3) or organic crystals (e.g., BNA, DAST,DSTMS). Whereas all these techniques have seen continuous performance progress in the last few
years driven by different application fields, their average power has mostly remained low. A
promising route of current great interest is simply to increase the average power of the driving lasers
using state-of-the-art, high-average power ultrafast Yb-gain based lasers providing multi-100-W tokilowatt average-power levels as excitation sources, which are increasingly available both inlaboratory setting and commercially. This new excitation regime for THz generation in various
schemes has become an active area of research in the last few years, and current results have allowed
to reach power levels in the THz domain in the multi-ten to multi-hundred milliwatts in different
repetition rate regions [1,2,3] – which was previously restricted to accelerator facility-type THz
sources. This progress opens the door to a multiplicity of new and old research areas to be re-visited.

Fig. 1. Left Overview of lab-based THz-TDS sources showing current trend towards higher average powers –both increasing the pulse energy at very high repetition rates but also making strong-field sources available atvery high repetition rate.

In this presentation, we will review recent progress in the generation of high-average power THzTDS. We will present a record-holding THz-TDS with 643 mW of average power at 40 kHz
repetition rate. We will present the state-of-the-art of high-power ultrafast laser sources withpotential for driving THz sources, current technological challenges in scaling THz average power,
and applications areas that could potentially benefit from these novel lab-based sources; in particularareas in ultrafast spectroscopy that could be greatly advanced by the availability of these uniquesources.
References[1] F. Meyer, T. Vogel, S. Ahmed, C. Saraceno, Optics Letters 45,2494 ( 2020).
[2] P. L. Kramer, M. K. R. Windeler, K. Mecseki, E. G. Champenois, M. C. Hoffmann, F. Tavella, Optics Express 28, 169512 (2020).
[3] J. Buldt H. Stark, M. Müller, C. Grebing, C.Jauregui,J. Limpert Optics Letters 46, 5256 ( 2021),

Polariton condensate lattices: novel
Quantum Simulator Platform
P.G. Savvidis
Westlake University, Hangzhou, Zhejiang 310030, China

Exciton-polaritons, are mixed light-matter quasiparticles resulting from the strong coupling ofphotons confined in a microcavity and quantum well excitons. Being bosons, polaritons cancondense into macroscopically coherent many-body state and have thus emerged as prime candidates
for the study of non-equilibrium systems of interacting bosons. Our recent studies, exploit nonequilibrium nature of polariton condensates, showing that polariton condensates can spontaneously
magnetize [1,2], and how their spin can be controlled both optically and electrically[3]. Direct
coupling of polaritons to leaking microcavity photons provides on-the-fly information of all
characteristics of the polariton condensates such as energy, momentum, spin, and their phase. We
employ spatially patterned external laser excitation to create arbitrary potential landscapes forpolaritons and demonstrate ferromagnetic and antiferromagnetic coupling between neighbouringcondensates[4].

Fig.1. Optically controlled magnetized polariton lattices.

Furthermore, using such techniques, polariton condensates can now be imprinted into arbitrary twodimensional lattices with tunable intra[5]- and inter-site interactions providing exciting opportunities
for devising novel and versatile quantum simulation platforms[6].
References[1] H. Ohadi, A. Dreismann, Y. G. Rubo, F. Pinsker, Y. del Valle-Inclan Redondo, S. I. Tsintzos, Z. Hatzopoulos, P. G. Savvidis, J. J. Baumberg
Physical Review X 5, 031002 (2015).
[2] Y. C. Balas, E. S. Sedov, G. G. Paschos, Z. Hatzopoulos, H. Ohadi, A. V. Kavokin, P. G. Savvidis, Physical Review Letters 128, 117401 (2022).
[3] A. Dreismann, H. Ohadi, Y. del Valle-Inclan Redondo, R.Balili, Y. G. Rubo, S.I. Tsintzos, G.Deligeorgis, Z.Hatzopoulos, P.G. Savvidis
J. J. Baumberg, Nature Materials 15, 10741078 (2016).
[4] A. Dreismann, H.Ohadi, Y. del Valle-Inclan Redondo, R.Balili, Y. G. Rubo, S.I. Tsintzos, G. Deligeorgis, Z.Hatzopoulos, P. G. Savvidis
J.J. Baumberg, Physical Review Letters 116, 106403 (2016).
[5] S. I. Tsintzos, A. Tzimis, G. Stavrinidis, A. Trifonov, Z. Hatzopoulos, J. J. Baumberg, H. Ohadi, P. G. Savvidis, Physical Review Letters 121
037401 (2018)[6] H. Ohadi, Y. del Valle-Inclan Redondo, A. J. Ramsay, Z. Hatzopoulos, T. C. H. Liew, P. R. Eastham, P. G. Savvidis, J. J. Baumberg, PhysicalReview B 97, 195109 (2018).

Giant photoelasticity of polaritons for detecting
Coherent
phononswith quantum
sensitivity
1
1
1
2

A. V. Scherbakov , M. Kobecki , D. D. Yaremkevich , T. Henksmeier , A. Trapp2
D. Reuter2, S. M. Kukhtaruk3, V. E. Gusev4, A. V. Akimov5, M. Bayer1
1Technische Universität Dortmund, 44227 Dortmund, Germany
2
Universität Paderborn, 33098 Paderborn, Germany
3
V.E. Lashkaryov Institute, 03028 Kyiv, Ukraine
4
Le Mans Universite, 72085 Le Mans, France
5
University of Nottingham, Nottingham NG7 2RD, UK

Coherent acoustic phonons of sub-THz frequencies are attractive information carriers for quantum
communications and nanoscopic imaging due to their nanometer wavelengths [1]. Up to now, thegeneration and detection of coherent acoustic phonons in this frequency range exploit ultrashort laser
pulses [2]. Generation is based on ultrafast optical excitation of an opaque material with thefollowing energy transfer to the lattice on the sub-picosecond time scale. The detection operates withthe ultrafast phonon-induced modulation of the optical properties of a media. The spectroscopic
effects; which accompany this modulation, are fascinating in structures with narrow opticalresonances. We have demonstrated chirping of exciton resonances in semiconductor quantum well[3], THz modulation of light emission from a microcavity [4], picosecond switching of polariton
condensate [5]. Here we focus on our recent experiments on detecting coherent acoustic phonons in
GaAs/AlAs superlattice [6]. We exploit the giant photoelasticity of polaritons [7] to detect phonons
in the transient reflectivity signal with extremely high sensitivity. The demonstrated reflectivity
modulation by the phonon-associated dynamical strain owned by the polariton resonance is sufficientfor detecting single phonon quanta in pump-probe setups. Fig. 1(a) shows the experimental scheme.

(d)Fig. 1. (a) Experimental scheme. (b) Reflectivity spectrum in the vicinity of the polariton resonance(blue curve). Dashed red curve shows the spectrum of the spectrally narrow probe pulse centered at thepolariton resonance. (c) Simulated temporal profile of the strain pulse injected into the GaAs substratefrom the Al optoacoustic transducer. (d) Spectral-spatial density of phonons in the coherent phonon
wave packet for the pump power density J = 0.5 mJ/cm2. Shaded area shows the spectral range of
detected phonons.

The studied structure is a short-period superlattice with 30 pairs of GaAs(12 nm)/AlAs(14.2 nm)
grown on a GaAs substrate. The SL polariton resonance shown in Fig. 1(b) is centered at ℏ𝜔0 =1.55
eV and has a spectral width of 0.7 meV. For generating and detecting coherent phonons, we use
200-fs pulses of a Ti:Sapphire regenerative amplifier (100-kHz repetition rate, central photon energyof 1.55 eV). The pump pulses hit the Al film deposited on the substrate’s backside. The film serves
as an optoacoustic transducer: it expands due to the optically induced heating and generates acoherent phonon wave packet in the form of a bipolar strain pulse with ∼10 ps duration andamplitude η0, as shown in Fig 1(c). Its spectrum with a maximum at 𝑓 = 20 GHz is shown in Fig.
1(d). The wave packet propagates in the structure with the velocity of longitudinal sound, 𝜐 =4.8
km/s. The coherent phonons are detected in the SL by measuring the transient reflectivity changes,∆𝑅(𝑡), using the probe pulses split from the same laser source, and delayed on the propagation time

of the phonon wave packet through the substrate. Scanning the time delay between the probe andpump pulses by a mechanical delay line provides the sub-ps time resolution. To study the effect of
the SL polariton resonance on the efficiency of phonon detection, we extend the duration of theprobe pulse up to 1.35 ps with a corresponding narrowing of its spectral width down to 1.4 meV byusing a tunable filter and measure the signal ∆𝑅(𝑡) for different central photon energy ℏ𝜔.
Fig. 2(a) shows the transient reflectivity signal measured at the probe photon energyℏ𝜔 = ℏ𝜔0 =1.55 eV It possesses the oscillatory behavior known as time domain Brillouin scattering(TDBS) [8]. The oscillations begin at t = −150 ps and last until t = +150 ps. During this time interval,
coherent phonons propagate through the SL toward the free surface and after reflection at t = 0 ps, inthe opposite direction toward the substrate. The fast Fourier transform of ∆𝑅(𝑡) shown in Fig. 2(b)demonstrates an intense line at 𝑓𝐵 = 42 GHz. The oscillation frequency at the normal incidence of theprobe beam is set by the selection rule:𝑓𝐵 = 𝜐𝑞𝐵 ⁄2𝜋 ; 𝑞𝐵 = 2𝑘1

where 𝑞𝐵 is the wave vector of detected phonons and 𝑘1 is the wave vector of the probe light in the
media. The main parameters of the transient reflectivity signal, i.e., its amplitude, frequency, phase,and decay rates, depend on the probe photon energy. Fig. 2(c) demonstrates the dependence of the
TDBS amplitude 𝐴𝐵 on ℏω. It is seen that 𝐴𝐵 (ℏ𝜔) has a maximum at the spectral position of thepolariton resonance and rapidly decreases with the increase of the detuning of ℏ𝜔 from ℏ𝜔0 . It leads
us to the conclusion that the TDBS signal is governed by the polariton resonance when the probe
photon energy ℏ𝜔 is close to ℏ𝜔0 . The most spectacular observation is the huge amplitude 𝐴𝐵 ~ 10−2
for the used pump fluence J ∼ 0.1 mJ/cm2 at ℏ𝜔 = ℏ𝜔0 . The amplitude of the TDBS signal in amaterial without a narrow optical resonance for a similar phonon wave packet would be 3 orders ofmagnitude smaller [9,10].

ckward(d)Fig. 2. (a) TDBS signals measured at ℏ𝜔 = ℏ𝜔0 and its FFT spectrum (b). (c) Dependence of the
TDBS signal amplitude 𝐴𝐵 on the probe photon energy. (d) (a) Dependence of the TDBS signal
amplitude on the optical excitation. The dashed lines indicate the estimated number of detected phonons
per pulse per square micron 𝑁𝐵 for the corresponding strain amplitudes. In (c) and (d) the experimental
data is shown by symbols, and solid lines show the results of numerical modelling.

We propose a simplified model for a qualitative explanation of the TDBS signal and its dependence
on the probe photon energy. With an assumption that the spectral width of the probe pulse is muchsmaller than the spectral width of the polariton resonance, we show analytically that the detection
� on the strain. It has
sensitivity is determined by the derivative of the complex refractive index 𝑛�(ℏ𝜔)a sharp extremum at the spectral position of the polariton resonance, and the spectral shift of thepolariton resonance due to the phonon-induced strain [11] results in strong reflectivity modulation.
The qualitative conclusions are supported by comprehensive numerical modelling, in which we take
into account the finite spectral width of the probe pulse and its Stokes/anti-Stokes spectral shifts dueto the scattering process. The numerical modelling demonstrates perfect agreement with theexperiment. The large amplitude of the transient reflectivity signal allows us to achieve quantum
sensitivity of phonon detection. The number of phonons responsible for the TDBS is determined bytheir spectral density in the phonon wave packet in the frequency interval 𝑓𝐵 ± ∆𝑓𝐵 The finite
spectral width 2∆𝑓𝐵 = 4 GHz shown by dashed rectangular in Fig. 1(d) is due to the finite size of the
SL and the limited penetration of the probe light, which determine the rise-decay rate of the TDBS
signal. The number of phonon in the detected frequency interval is determined by the spatialtemporal profile of the generated phonon wave packet. Figure 2(d) shows the power dependence of
the TDBS amplitude measured at ℏ𝜔 = ℏ𝜔0. The minimal density of detected phonons, proportional
to J2, is 𝑁𝐵 ≈102 μm−2 and corresponds to the pump fluence, at which the TDBS amplitude exceeds
the noise level. In the experiments with a high repetition rate laser, the sensitivity limit can be easily
lowered to 𝑁𝐵 ≈10-2 μm−2 (𝐴𝐵 ~10-5). This enables the detection of phonon wave packets with one
phonon quantum at the Brillouin frequency. At the same time, increasing the number of phonons in
the wavepacket results in decreased detection sensitivity. The decrease of 𝐴𝐵 (𝐽) at large pumpexcitation density is due to the large phonon-induced shift of the polariton resonance, which becomescomparable with the polariton spectral width. In this case, 𝑛�(ℏ𝜔) becomes dependent on thecoordinate and time. It is equivalent to the spectral broadening of the polariton resonance and, thus,
decreases the TDBS sensitivity. The demonstrate quantum sensitivity of the optical detection ofcoherent acoustic phonons paves the way for applications in phononics with extremely low-density
phonon fluxes. For instance, this technique could be used for the detection of phonons emitted fromsingle nanoobjects and the nanoscopy of such delicate nanoobjects as biological cells [12].
References[1] P. Delsing, A. N. Cleland, M. J. A. Schuetz, J. Knörzer, G. Giedke, J. I. Cirac, K. Srinivasan, M. Wu, K. C. Balram, C. Bäuerle, Journal of
Physics D 52, 353001 (2019).
[2] O. Matsuda, M. C. Larciprete, R. Li. Voti, O. B. Wright, Ultrasonics 56, 3 (2015).
[3] A. V. Scherbakov, A.S. Salasyuk, A.V. Akimov, X. Liu, M. Bombeck, C. Bruggemann, D.R. Yakovlev, V.F. Sapega, J.K. Furdyna
M. Bayer, Physical Review Letters 105, 117204 (2010).
[4] C. Brüggemann, A. V. Akimov, A. V. Scherbakov, M. Bombeck, C. Schneider, S. Höfling, A. Forchel, D. R. Yakovlev, M. Bayer, Nature
Photonics 6, 30 (2012).
[5] A. A. Demenev, D. D. Yaremkevich, A. V. Scherbakov, S. M. Kukhtaruk, S. S. Gavrilov, D. R. Yakovlev, V. D. Kulakovskii, M. Bayer
Physical. Review B 100, 100301(R) 2019.
[6] M. Kobecki, A. V. Scherbakov ,S. M. Kukhtaruk, D. D. Yaremkevich, T. Henksmeier, A. Trapp, D. Reuter, V. E. Gusev, A. V. Akimov
M. Bayer, Physical Review Letters 128, 157401(2022).
[7] A. N. Poddubny, A. V. Poshakinskiy, B. Jusserand, A. Lemaître, Physical Review B 89, 235313 (2014).
[8] C. Thomsen, H. T. Grahn, H. J. Maris, J. Tauc, Optical Communications 60, 55 (1986).
[9] A. Huynh, B. Perrin, N. D. Lanzillotti-Kimura, B. Jusserand, A. Fainstein, A. Lemaître, Physical Review B 78, 233302 (2008).
[10] A. V. Scherbakov, M. Bombeck, J. V. Jäger, A. S. Salasyuk, T. L. Linnik, V. E. Gusev, D. R. Yakovlev, A. V. Akimov, M. Bayer, Optics Express
21, 16473 (2013).
[11] A. V. Akimov, A. V. Scherbakov, D. R. Yakovlev, C. T. Foxon, M. Bayer, Physical Review Letters 97, 037401 (2006).
[12] F. Prez-Cota1, R. Fuentes-Domnguez, S. La Cavera, W. Hardiman, M. Yao, K. Setchfield, E. Moradi, S. Naznin, A. Wright, K. F. Webb
A. Huett, C. Friel, V. Sottile, H. M. Elsheikha, R. J. Smith, M. Clark, Journal Appled. Physics 128, 160902 (2020).
* Acknowledgement(s) : authors acknowledge support by the Deutsche Forschungsgemeinschaft (Grant No. TRR 142 Project A06 and Grant No.
TRR160 Project A01) and the Volkswagen Foundation (Grant No. 97758).

Ultrafast lattice dynamics during singlet
Exciton fission
in2 pentacene
single 4crystals1
1
2

1

H. Schwoerer , H. Seiler , R. Ernstorfer , S. Hammer , M. Rossi
Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany
2
Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
3
Julius-Maximilian University, 97074 Würzburg, Germany

Singlet exciton fission SEF is an electronic process occurring in certain photoexcited organic
molecular solids, where one exciton (bound electron hole pair) of spin zero spontaneously decays
into two excitons of spin 1 each. In the context of organic photovoltaics, this provides the possibilityof generating two free electrons and two holes (after separation at a boundary) per one absorbed
photon, rather than one, of course of maximum half the energy each. The (not immediately obvious)
efficiency benefit for photovoltaics sparks the continuous attention to SEF since its first observation
more than half a centure ago, and will be explained in the introductory part of the talk. As a processof high yield in an electronically excited state, it has to be ultrafast; and since fission of an exciton
eventually leads to two spin and electronically independent and spatially separated excitons, it
requires strong correlation in the crystal beyond the molecular entities, mediated by overlap of the(intramolecularly delocalized)  molecular orbitals. Hence, it is intriguing to learn, whether andwhich intra- and/or intermolcular structural motions accompany SEF or even positively or negatively
affect its yield. The current understanding assumes singlet exciton fission to be a three step process,with an initial ultrafast fission into two electronically and spin coupled triplet excitons, a subsequentloss of their electronic coherence, and eventually loss of their spin coherence and spatial separationof the two triplets. For recent reviews see [1,2]. So far, fission dynamics have almost exclusively
been investigated by femtosecond laser spectroscopic means (transient absorption or twodimensional spectroscopy etc.), observing electronic populations and interactions thereof andindirectly inferring atomic and molecular structural dynamics.

Fig. 1.: Ploted are changes of diffraction peak intensity of different orders as a function of time after pahoto
excitation of pentacene single crystals. These changes are manifestation of incoherent and coherent structuraldynamics of photoexcited during singlet exciton fission, such as the slip sliding motion of the two pentacenemolecules in the unit cell or the lattice reorganisation due to the final plaronic nature of the triplet excitons [3].

Here, we apply ultrafast electron diffraction being sensitive to structural dynamics of periodc lattices
with a high spatial resolution and a temporal resolution sufficient to detect coherent motion andreorganization on a subpicosecond scale. We will discuss the present state of the art of UED
experiments, its current limitations and promising directions of improvements. We investigate single
crystalline pentacene with the initial (exothermic) fission shorter than 100 fs and a 100% fission
yield. Pentacene is one of the most thoroughly investigated SEF materials, also due to its substantialbalance of complexity and simplicity on both, the molecular and the crystalline level. Despite
pentacene, as pure hydrocarbon material, being a bad scatterer, we could identify oscillatory coherent
and incoherent dynamics with three distinct and characteristic time constants, present in various
diffraction orders, see Fig, 1. By combining analysis of diffraction symmetries, time dependent
density function theory and molecular dynamics simulations, we assign the observations to coherent
intermolecular motion and lattice reorganisations during the exciton fission evolution towards thefree triplets [3].
References[1] M. Smith, J. Michl, Annual Review of Physical Chemistry 64, 361, (2019).
[2] K. Miyata, F. Conrad-Burton, F. Geyer, X.-Y. Zhu, Chemical Reviews 119, 4261, (2019).
[3] H. Seiler, M. Krynski, D. Zahn, S. Hammer, Y. Windsor, T. Vasileiadis, J. Pflaum, R. Ernstorfer, M. Rossi, H. Schwoerer, Science Advances 7,
eabg0869, (2021).

Light-induced coherent c-axis charge carrier responses in stripeordered cuprate superconductors
R. Shimano
The University of Tokyo, 113-0033 Tokyo, Japan

Light-induced superconductivity in cuprate superconductors has been reported in stripe-orderedphases of La1.675Eu0.2Sr0.125CuO4 [1] and La2-xBaxCuO4 [2], and also in underdoped YBa2Cu3Oy[3]. In
particular for the stripe-ordered system, the mechanism is interpreted as the revival of inter-layer
Josephson coupling which is vanished in equilibrium because of the  phase shift of superconducting
order between neighbouring stripes within a plane [4]. To have a deeper understanding on thecompetition between the charge/spin stripes and the superconductivity, we investigated anotherarchetypical stripe-ordered superconductor, La2-x-yNdySrxCuO4 (x=0.12) where the correlation lengthof stripes is shown to be short-ranged compared to that of La2-xBaxCuO4 [5]. Upon the irradiation ofultrafast near-infrared (800 nm) light pulse, a clear plasma edge and the corresponding loss function
peak were identified in the c-axis terahertz reflectivity above Tc(=3.2 K) until the onset temperatureof the charge stripe Tco (=67 K) as represented in Fig.1. On the other hand, the 1/  divergence of theimaginary part of the optical conductivity, which has been identified as an optical fingerprint of thelight-induced superconductivity, was not observed and the spectrum was mostly reproduced by theDrude model with unusually small scattering rate [6].

Fig. 1. (a): Schematic setup of optical pump-terahertz probe spectroscopy in La2-x-yNdySrxCuO4 (LNSCO)(x=0.12) with
Tc=3.2 K. (b): Reflectivity and (c) loss function spectrum at 1 ps after the photoexcitation at the indicatedtemperatures.

To reconcile these results, we discuss the possibility of short-ranged (or fluctuating)superconductivity in the NIR pump-induced states and its relevance to the charge stripes. We will
also compare the results with that of YBa2Cu3O7- under the NIR pumping studied by the observationof THz nonlinear responses [7].
References[1] D. Fausti, R. I. Tobey, N. Dean, S. Kaiser, A. Dienst, M. C. Hoffmann, S. Pyon, T. Takayama, H. Takagi, A. Cavalleri, Science 331, 189 (2011).
[2] D. Nicoletti, E. Casandruc, Y. Laplace, V. Khanna, C. R. Hunt, S. Kaiser, S. S. Dhesi, G. D. Gu, J. P. Hill, and A. Cavalleri, Physical Review
B 90, 100503(R) (2014).
[3] S. Kaiser, C. R. Hunt, D. Nicoletti, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, A. Cavalleri, Physical Review B 89
184516 (2014).
[4] E. Berg, E. Fradkin, E.-A. Kim, S. A. Kivelson, V. Oganesyan, J. M. Tranquada, S. C. Zhang, Physical Review Letters 99, 127003 (2007).
[5] S. B. Wilkins, M. P. M. Dean, Jörg Fink, Markus Hücker, J. Geck, V. Soltwisch, E. Schierle, E. Weschke, G. Gu, S. Uchida, N. Ichikawa
J. M. Tranquada, J. P. Hill, Physical Review B 84, 195101 (2011).
[6] M. Nishida, K. Katsumi, D. Song, H.i Eisaki, R. Shimano, arXiv:2303.01961 (2023).
[7] K. Katsumi, M. Nishida, S. Kaiser, S. Miyasaka, S. Tajima, R. Shimano, arXiv:2209.01633 (2022).

Ultrafast surface melting of electronic phase transition
K.Siddiqui, M. Monti, S.Wall
Aarhus University, 8000 Aarhus-C, Denmark

Our understanding of ultrafast phase transitions has been shaped by the assumption that the orderedbulk order properties are prevalent and represent a homogeneous system as a whole. However, recent
studies have unveiled the role of disorder and heterogeneity with the latter being a ubiquitous featureof many considered quantum materials [1,2]. Moreover, the equilibrium surface properties can bevery different from that of the bulk [3,4], but studies that clarify how surface affects the nonequilibrium properties during a phase transition are lacking but are important in improving ourunderstanding of the ultrafast phase transitions. In this talk, we will report on our results in which
we probe the ultrafast dynamics of the layered manganite, L0.5Sr1.5MO4 which displays an orbital
ordering (OO) phase transition below TOO = 220 K accompanied by a structural phase transition
leading to quadrupling of the unit cell. Using grazing incidence x-ray diffraction at a femtosecond xray free-electron laser facility (SwissFEL), we measure the surface orbital ordering peak (orbital
truncation rod, OTR) and the bulk orbital order Bragg peak and compare their dynamics induced byan intense femtosecond near-infrared laser pulse at 800 nm. We find that the melting threshold for
OTR peak is significantly lower (Fig 1) than the suppression of the structural distortions. In addition,
we perform reciprocal space mapping of both peaks to clarify how disorder contributes to the phasetransition by analysis of diffuse scattering and via extraction of the corresponding correlation
lengths. In the talk, we will reveal the details of the analysis and discuss the implication of ourobservations for the description of phase transition in quantum materials.

Fig. 1. Left: Temporal response of ORT (blue) and Bragg peak (red) following 800 nm photoexcitation.
Right: Plot comparing the magnitude of reduction of the respective peaks as a function of fluence.
These plots reflect that the melting threshold for the surface order is much lower than the bulk order.

In addition, we perform reciprocal space mapping of both peaks to clarify how disorder contributes
to the phase transition by analysis of diffuse scattering and via extraction of the corresponding
correlation lengths. In the talk, we will reveal the details of the analysis and discuss the implicationof our observations for the description of phase transition in quantum materials.
References[1] S. Wall, S.Yang, L.Vidas, M. Chollet, J. M. Glownia, M. Kozina, T. Katayama, T. Henighan, M. Jiang, T. A. Miller, D. A. Reis, L. A. Boatner,
O. Delaire M. Trigo, Science 362, 572 (2018).
[2] D. Perez-Salina, A. S. Johnson, D. Prabhakaran and S. Wall, Nature Communications 13, 238 (2022).
[3] S. B. Wilkins, X. Liu, Y. Wakabayashi, J.-W. Kim, P. J. Ryan, H. Zheng, J. F. Mitchell, and J. P. Hill, Physical Review B 84, 165103 (2011).
[4]. Y. Wakabayashi, M. H. Upton, S. Grenier, J. P. Hill, C. S. Nelson, J.-W. Kim, P. J. Ryan, A. I. Goldman, H. Zheng , J. F. Mitchell, Nature
Materials 6, 972 (2007).

Picosecond Volume Expansion in a
Nano-textured Mott Insulator
A. Singer
Cornell University, Ithaca, New York, 14853, USA

Technology is moving towards ever faster switching between different electronic and magnetic statesof a material. Manipulating properties at terahertz rates requires accessing the natural timescales ofthe electrons (fs) and phonons (sub-ps to few ps) in condensed matter, possible with short-pulse
photoexcitation. Yet, in Mott insulators featuring abrupt, volume-changing insulator-to-metal (IMT)phase transitions, such as in VO2 and V2O3, the nucleation and growth of percolating domains often
hamper the ultrafast dynamics [1, 2]. Here, using time-resolved x-ray diffraction, we show a ~1.2 ps
photoinduced volume expansion in an epitaxially strained Mott insulator Ca2RuO4 (see Figure). The
diffraction data from the strained thin films display a transient nano-texture in the low-temperatureground state. X-ray imaging elucidates that the satellite peaks in diffraction form due to straininduced ferroelastic domains emerging transiently during the metal-insulator transition (the nanotexture disappears when heated above the transition). Instead of homogeneously transforming into a
low-temperature structure (like in bulk), the strained Mott insulator splits into nanodomains withalternating lattice constants, as confirmed by cryogenic scanning transmission electron
microscopy[3]. Time-resolved x-ray reciprocal space mapping reveals dynamic nano-texture,
proposing a mechanism where the photoinduced phase heterogeneously nucleates at inclusions of theincipient long-bond phase stabilized through epitaxial strain. The structure factor and lattice spacing
analysis reveals a structure absent in equilibrium. Photoexcitation also launches lateral domain
dynamics centered at a frequency of ~ 0.2 THz, consistent with the strain-induced heterogeneity.

Fig.1 . (a) Ca2RuO4 unit cell diagram for insulating (S-Pbca) and metallic (L-Pbca) phases where calcium ions
are shown in red, ruthenium ions in blue, and oxygen ions in peach color, (b) Time-resolved normalized
scattering intensity of 008 Bragg peak plotted against the time-delay. (c) Normalized scattering intensity plot
with a series of time delays at fix fluence, where (c) 5.4 mJcm-2, (d) 10.4 mJcm-2, and (e) 48.4 mJcm-2 andextracted characteristics parameters of the peak at various pump fluence and time delays, these are fitted by theGauss function, where (f) Peak height as a function of time-delay at pump fluences 5.4 mJcm-2, 10.4 mJcm-2,
and 48.4 mJcm-2

This work highlights the use of strain for tuning the timescales in photoinduced transitions,potentially offering opportunities for terahertz technologies.
References[1] E. Abreu, S. Wang, J.G. Ramírez, M. Liu,J. Zhang,K. Geng,I.K. Schuller,R.D. Averitt, Physical Review B 92 085130 (2015).
[2] A. Singer, J.G. Ramirez, I. Valmianski, D. Cela, N. Hua, R. Kukreja, J. Wingert, O. Kovalchuk, J.M. Glownia
M. Sikorski, M. Chollet,M. Holt,I.K. Schuller,O.G. Shpyrko, Physical Review Letters 120, 207601 (2018).
[3] Z. Shao, N. Schnitzer, J. Ruf, O.Y. Gorobtsov, C. Dai, B.H. Goodge, T. Yang, H. Nair, V.A. Stoica, J.W. Freeland, J. Ruff, L.-Q. Chen
D.G. Schlom,K.M. Shen,L.F. Kourkoutis,A. Singer arXiv 2211.01506 (2022).

En route to next generation high energy high repetition rate
Ultrafastlaser sources
in 1the mid-IR
1
1
1
1
1

I. T. Sorokina , V. L. Kalashnikov , A.Rudenkov , M.Demesh , C. Grivas , N. Gusakova , E. Sorokin2
1
Norwegian University of Science and Technology, 7034 Trondheim, Norway
3
Technische Universität Wien, A-1040 Vienna, Austria

The solid-state lasers based on the Cr2+-doped crystals of the II-VI family [1-3] operate at room
temperature, possess the widest among existing solid-state lasers bandwidth of up to 45% of thecentral wavelength, provide tens of Watts power levels, and deliver few-cycle femtosecond pulses[2,4], recently also in diode-pumped setups [5]. The output peak powers reaching 1 MW and pulse
durations as short as 28 fs, direct generation of CEP-stable mid-infrared pulses became a reality. The
works on power scaling have enabled the generation, via nonlinear frequency conversion, of mid-IR
light that spans the entire “molecular fingerprint” region between 4.5 and 14 µm [4,5]. Despite therapid development of ultrafast lasers based on Cr2+- crystals during the last two decades, both withrespect to applications and sources, the interaction of these ultra-short pulses with matter are mostly
limited by the relatively low nanojoule pulse energies obtainable directly from the oscillator.
Currently, the highest pulse energy was obtained from the Cr:ZnS chirped-pulse oscillator (CPO),
amounting to about 50 nJ [6]. In turn, development of energetic 0.1-10 µJ ultrafast mid-IR sources
would pave the way for additional interesting industrial applications such as e.g. sub-wavelength 3-D
sub-surface processing of silicon and other semiconductor materials [7,8].
In this talk we review the state-of-the art in the field with a particular emphasis on our recent
advances towards the challenging goal of development of compact high-energy ultrashort-pulsed
laser systems in the 2–3 µm wavelength range. We will discuss approaches for generating high laser
peak power and energy scaling in a hybrid mid-IR chirped pulse oscillator-amplifier (CPO-CPA)
system and discuss ways to achieve stable operation in fluctuating environment. We also present theresults of our feasibility study of spatiotemporal mode-locking in a mid-IR Cr:ZnS waveguide laser
based on the nonlinear spatial mode coupling, providing a route to the energy-scalable waveguidelasers. In our latest report in Optics Express we proposed a novel technique to provide hybrid CPOCPA pulse energy scaling, preserving the output spectrum’s high-fidelity compression. In thefollowing, we describe the first steps in this direction, both experimentally and theoretically,
demonstrating the viability of the proposed concept.

Fig. 1. Chirped pulse oscillator (CPO), single pass amplifier (AMP), prism compressor (CMP), concave mirror(CM), chirped mirror (ChM), output coupler (OC), dichroic mirrors (DM), erbium doped fiber laser (EDFL),
fused silica prisms (FS), highly reflective mirrors (HR).

The classical route of obtaining the high-energy pulses incorporates a small-power ultrashort seedoscillator, pulse picker, stretcher, amplifier and compressor, which might be too complex for many
possible uses. Another way is to employ a travelling-wave amplifier. This approach has been
demonstrated with ultrashort pulses [4], showing that the obtainable energy is limited by the strong
nonlinear interactions in the gain medium if no stretching is used. We propose and demonstrate usinga hybrid reduced-nonlinearity scheme, which incorporates a reduced repetition rate chirped-pulse
oscillator (CPO) directly followed by an amplifier, which thus operates in a chirped pulse regime aswell (Fig. 1) [9]. The system produces pulses of 1.5-3 ps duration which can be compressed below
200 fs using a simple prism compressor (Fig. 2).

Fig. 2. Amplified pulse spectrum (left) and autocorrelation trace (right) after compression from 3 ps.

There are more advantages in using such a scheme than just saving on pulse picker and stretcher.
First, the CPO oscillator has a well-confined spectrum with sharp edges (Fig. 2), which in the case of
Cr:ZnS helps to stay away from air humidity (see Section 3). Another CPO advantege is thepossibility of pulse energy scalability, which allows reducing the oscillator repetition rate evenfurther. In this regime, called dissipative resonance, the higher pulse energy increase isaccommodated by chirp growth without spectral broadening. Reaching this regime [9] allows
anticipating higher energies for low repetition rate systems in basically the same simple setup.
For practical applications of ultrashort pulses in the spectral regions with significant atmosphericabsorption and dispersion it is important to provide a reliable operation of the system which would
not be subject to humidity, pressure or other fluctuations. To avoid cumbersome evacuation orcontinuous purging, one would need a way to compensate for the atmospheric changes. Using the
Cr:ZnS ultrashort-pulsed laser as a test-bed, we demonstrated the air influence on a femtosecond andchirped-pulse oscillator performance and showed that the humidity GVD is the main factor [10]. The
GVD due to the molecular absorbers can be readily calculated to provide wildly oscillating by ps2
features even in the transparency windows (Fig. 3a). We have shown that it is the slowly varying
average GVD that matters (Fig. 3b) and provided methods to calculate it for given humidity,
temperature and pressure.

(a)

(b)

Fig. 3. (a) Transmission of the atmosphere and corresponding GDD for the 10 cm optical path at 40% relative
humidity (r.h.) atK andatm. Note the scale of the GDD, which is in picoseconds squared. (b)Slowly varying GVD due to water vapour using different spectral windowing.

.

Fig. 4. (a) CPO oscillator spectra under nitrogen purging. (b) CPO oscillator spectra at open air with different
YAG wedge insertions. (c) Approximate compensation of higher humidity by increased YAG wedge insertion.
For all graphs, the lower plot shows the calculated round-trip GDD.

Knowing the dispersion associated with the humidity, CO2 or other atmospheric absorbers allows
compensating for its changes. We have demonstrated the feasibility of this approach using a Cr:ZnS
CPO with a four-meter long cavity round-trip. The CPO is a good test-bed as it is very sensitive todispersion changes (Fig. 4a). As it is shown in Fig. 4b, by changing the wedge insertion by mere 2
mm it is possible to nearly completely compensate for the humidity change of 20% r.h. This result

demonstrates the possibility to operate mid-IR oscillators in a wide range of laboratory conditions
without evacuation or purging the whole setup. The high nonlinearity of the II-VI materials opens theway to realizing the concept of spatio-temporal mode-locking [11]. We demonstrate the feasibility of
implementing this concept in a Cr:ZnS waveguide laser operating in the regime of distributed Kerrlens mode-locking [12] when the nonlinear coupling between the spatial modes causes the formationof an ultrashort pulse by only a transversely profiled pump beam.

Fig. 5. (left) Evolution of the beam power with waveguide rounf-trip for wavegude radius 30 m and pumpbeam radius 40 m. The left inset shows the dimensionless power profile near the power peak. The right inset
demonstrates the corresponding relative gain profile. (right) Spatiotemporal contour plots of the dimesionlessdissipative soliton intensity | | on the dimensionless (radial coordinate, time)-plane at the noted waveguideroundtrip for the anomalous group-delay dispersion.

Below the mode-locking threshold defined by a pump power, waveguide, and pump beam radii,
the field consists of many spatial modes (Fig. 5, left), which compete through the gain saturation (seeinsets). As a result, the laser operates in a Q-switched regime. When the nonlinear coupling between
the modes sets on, the modes of the most intensive field spike relax to the ground mode, which
experiences the maximum gain. Thus, a spatio-temporal dissipative soliton develops (Fig. 5, right).
Our most recent experiments and numerical simulations [13] demonstrate that pump beam forminga soft aperture could provide an effective spatial mode control in a nonlinear multimode waveguidewhich, through nonlinear spatial inter-mode coupling, enables formation of a spatiotemporaldissipative soliton - a ‘light bullet’. This novel approach paves the way to the next generationcompact MIR laser sources - distributed Kerr-Lens Mode-locked energy-scalable solid-state
waveguide lasers. Our most recent experiments and numerical simulations [13] demonstrate thatpump beam forming a soft aperture could provide an effective spatial mode control in a nonlinear
multimode waveguide which, through nonlinear spatial inter-mode coupling, enables formation of a
spatiotemporal dissipative soliton - a ‘light bullet’. This novel approach paves the way to the next
generation compact MIR laser sources - distributed Kerr-Lens Mode-locked energy-scalable solidstate waveguide lasers
References[1] I. T. Sorokina, Optical Materials, 26, 395 (2004).
[2] I. T. Sorokina, E. Sorokin, IEEE Journal of Selected Topics in Quantum Electronics 21, 1601519 (2015).
[3] . S. B. Mirov, D. Martyshkin; I. S. Moskalev; M.Mirov; S. Vasilyev, IEEE Journal Selective Topics. Quantum Electronics 21, 292 (2015).
[4] S. Mirov, I. Moskalev, S. Vasilyev, V. Smolski, V. Fedorov, D. Martyshkin, J. Peppers, M. Mirov, A. Dergachev, V. Gapontsev, IEEE Journal
Selective Topics. Quantum Electronics 24, 1601829 (2018).S. Mirov, et al., IEEE JSTQE 24, 1601829 (2018).
[5] N. Nagl, “A New Generation of Ultrafast Oscillators for Mid-Infrared Applications”, PhD thesis, Springer (2021).
[6] N. Tolstik, C. S. J. Lee, E. Sorokin, I.T. Sorokina, Proceedings: Conference on Lasers and Electro- Optics", CLEO SF1N.2(2018) .
[7] N. Tolstik, E. Sorokin, J.C. Mac-Cragh, R. Richter, I. T. Sorokina Proceedings: Conference Lasers and Electro-Optics", CLEO AM41.8 (2022).
[8] I. T. Sorokina, OPIC’2022, Japan (2022).
[9] A. Rudenkov V. L. Kalashnikov, E.Sorokin, M. Demesh, I.T. Sorokina , arxiv.org/abs/2212.00626.
[10] E. Sorokin, Alexander Rudenkov, Vladimir Kalashnikov, Irina Sorokina, arxiv.org/abs/2212.00909.(2022).
[11] W. Fu, L. G. Wright, P. Sidorenko, S. Backus, F.W. Wise, Optics. Express 268, 9432 (2018).
[12] J. Zhang, . Pӧtzlberger, Q. Wang, J. Brons, M. Seidel D.Bauer, D Sutter, V. Pervak, A. Apolonski, K.F. Mak, V.Kalashnikov, Z. Wei, F. Krausz
O. Pronin, Ultrafast Science 2022, 9837892 (2022).
[13] M. Demesh, (submitted to JOSA B) (2023).
* Acknowledgement(s) : authors ITS, VLK, AR, MD, CG, and NG acknowledge support from the Norwegian Research Council projects #303347(UNLOCK), 326503 (MIR) and #326241(LAMMO-3D).

Ultrafast dynamics in quantum materials

P. Padmanabhan1, Y. Huang1, N. S. Sirica1, F. L. Buessen2, R. V. Aguilar3, R. Tutchton1
K. W. C. Kwoc4, S. Gilinsky1, M. C. Lee1, M. A. McGuire 5, S. R. Singamaneni6, A. Paramekanti2
J.-X. Zhu1, S. A. Trugman1,S.-W. Cheong7, L. Yuan7, D. A. Yarotski1, A. Azad1, R. P. Prasankumar1
A. J. Taylor1
1
Los Alamos National Laboratory, Los Alamos, NM 87545 , USA
2
University of Toronto, Toronto, ON M5S Canada
3
Ohio State University, Columbus, OH43210, USA
4
Columbia University, New York, NY 10027, USA
5
Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
6
The University of Texas at El Paso, El Paso, TX 79968
7
Rutgers University, Piscataway, NJ 08854, USA

TRPR

Quantum materials exhibit a wide range of properties resulting from the intricate interplay of thecharge, spin, lattice, and orbital degrees of freedom. Unraveling their underlying interactions enables
the development of an understanding of emergent properties in quantum materials. In this context,
ultrafast optical spectroscopy has provided many valuable insights into the study of competing
many-body interactions. Quantum material systems hosting novel intrinsic magnetic orderrepresent an exciting new playground to study the transient interactions between photons and spin
excitations in ordered phases, uncovering the nature of the mechanisms that underlie these processes
and exploiting them to harness new magnetic phenomena. In this talk, we focus on ultrafast
dynamics in emerging magnetic materials.The enormous potential of van der Waals (vdW) magnets
in nanoscale information storage and spintronic technologies has stimulated a great deal of research
into their equilibrium magnetic and structural properties. Nevertheless, comparatively little is known
about their behavior under non-equilibrium conditions or the nature of magneto-structural couplingin this material class on its intrinsic ultrafast timescale. This talk will discuss our recent studies ofnon-equilibrium lattice and spin dynamics in vdW magnets using ultrafast spectroscopy. In the
prototypical 2D magnet, CrI3, we observe strong light-driven coherent lattice oscillations,commensurate with phonon modes of A1g symmetry. Intriguingly, the amplitude of the c-axis A1g
mode shows a strong pump helicity dependence below the magnetic ordering temperature. The
correlation between magnetic order and vibrational amplitude of this mode is indicative of a strong
coherent coupling between the spin and lattice degrees of freedom [1]. Simulations suggest that thiscoupling originates from the modulation of exchange interactions through the distortion of the lattice(Fig. 1).

Tc
simulation

PSDS (arb. Units)

Delay (ps)Frequency (THz)

Fig. 1. Left: Oscillations in the time-resolved polarization signal at T=15K in the ferromagnetic phase, after
subtraction of the demagnetization background, for left circularly polarized σ+ (blue) and right circularly
polarized σ− (red) pump helicities (upper panel) and their corresponding power spectral densities (lower panel)
Middle: Schematic of the eigenvectors of the two phonon modes, A11g (top) and A11g (bottom). Right: The ratioof σ+ /σ− of the integrated Fourier transform peaks of the measured signal at the A11g (orange and A21g green)mode frequencies, and the simulated helicity-dependent ratio at the A21g frequency (pink) versus normalized
temperature. The solid green line is a fit using an FM order-parameter-like function and the solid orange line isa guide to the eye.

This work highlights the strong interplay between the lattice and spin structure in layered magnetic
materials, providing crucial insight for the realization of the next generation of high-speed nanoscale
magneto-optic technologies.
Understanding and controlling the antiferromagnetic order in multiferroic materials on an ultrafast
time scale underpins potential applications of these materials in magnetic data storage and ultrafast
magnetoelectric switching. In the second half of the talk, we present an optical pump-THz probe
study of the multiferroic Eu0.75Y0.25MnO3. The pump energy of 1.55eV was chosen to match thedominant d-d transitions of the Mn3+ ions and the temporal evolution of the pump-induced transient
conductivity was recorded via electro-optic sampling of the THz probe [2]. Our measurement found
two clear relaxation times due to: (1) spin-lattice thermalization and (2) magnetic order-related
electron-hole recombination. While the spin-lattice thermalization shows a power law relationshipwith temperature from the melting of spin order, the temperature dependence of the electron-hole
recombination suggests a channel opening below the Néel temperature. Suppression ofelectromagnon modes was observed within the spin-lattice thermalization time and potentially resultsfrom the melting of the magnetic order. These measurements reveal the dynamic functionality ofquantum materials by directly probing low-energy excitations such as electromagnons, offering a
powerful approach to understanding the coupling between their different degrees of freedom.
References[1] P. Padmanabhan, F. Buessen, R. Tutchton, K.W.C. Kwock, S. Gilinsky, M.C. Lee, M.A. McGuire, S.R Singamaneni, D.A. Yarotski
A. Paramekanti, J-X. Zhu, R.P. Prasankumar, Nature Communications 13, 4473 (2022).
[2] Y. Huang, R. V. Aguilar, S.A. Trugman, S.-W. Cheong, L. Yuan, M.-C. Lee, A. Azad, A.J. Taylor, N.S. Sirica
R. P. Prasankumar, D.A.Yarotski, (in preparation) (2023).
* Acknowledgement: This work was supported, in part, by the Center for Integrated Nanotechnologies, an Office of Science User Facility operated
for the U.S. Department of Energy.

Mesoscale structure in nonequilibrium
Chargedensity3 wave systems
visualizedwith
X-ray freeelectronlasers2
1
3
2
1
1
3

A. Miller , G. Orenstein , V. Krapivin , Y. Huang , Jade Stanton , Matthew Hurley , Ryan Duncan , Q. Nguyen
G. De la Pena3 , M. Sander4, R. Mankowsky4, Y. Deng4, H. Lemke4, M. Trigo2, S. W. Teitelbaum1
1

Arizona State University, Tempe AZ 85287, USA
SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
3
Stanford University, Stanford, CA 94305, USA
4Paul Scherrer Institut, 5232 Villigen, Switzerland

2

Heterogeneous dynamics are both a well-known to mediate phase competition in systems withcomplex phase diagrams near equilibrium, and heterogeneity is a well-known consequence ofsystems entering an ordered phase far from equilibrium (e.g. the Kibble-Zurek mechanism forformation of topological defects) [1]. And yet, in light-driven systems far from equilibrium on very
fast timescales, we are just beginning to use tools such as ultrafast electron microscopy and x-ray
free electron lasers to reveal the role intrinsic heterogeneity plays at very fast timescales. These
early studies reveal the role of topological defects in mediating the superconductivity/CDW ordercompetition in cuprates [2,3], stabilization of the so-called “hidden” charge density (CDW) phase in
1T-TaS2 [4] to the importance of phase competition in the rare-earth tritellurides [5,6].X-ray freeelectron lasers (XFELs) are particularly incisive tools to probe these critical yet complex heterogeneous
dynamics due to their combination of high brightness, high momentum resolution, and high temporal
resolution. All three parameters key for observing the relatively weak, rapidly evolving scattering signals
from mesoscale (~10 nm) optically generated domains and defects. In two distinct quasi-two dimensional
systems with CDW order, Pd-intercalated ErTe3, a prototypical charge density wave (CDW) system, and 1TTiSe2, an excitonic insulator candidate, we observe complex dynamics near the CDW ordering wavevector
due to the intrinsic heterogeneity of phase transitions system. In the rare-earth tritelluride CDW system
Pd:ErTe3, we observe scattering along the b-axis (surface normal) direction after optical excitation with 800
nm light strong enough to quench the CDW order near the surface. This scattering is due to the formation ofan antiphase domain wall [7,8]. In metal-intercalated tritellruides, these defects are observed to persist for
hundreds of picoseconds, much longer than the lifetime of the excited electronic state. We attribute themechanism of domain wall relaxation to its decay into vortex strings, which are topologically protected
defects in the material. The slow relaxation of these defects is key to the long-lived suppression of long-range
CDW order. We model the melting and recovery of the CDW order with a time and spatially dependent
Ginzburg-Landau model that accounts for both thermal fluctuations and impulsive optical excitation, andproduces the expected topologically protected states. The Hamiltonian of the system can be
described by a complex order parameter 𝜓 with the effective (local) free energy U given by
1

𝑈(𝒓, 𝑡) = �1 − 𝜂(𝒓, 𝑡)�|𝜓|2 + 2 |𝜓|4 + 𝜉 2 |∇𝜓|2 ,where 𝜂(𝒓, 𝑡) represents a temporally and spatially dependent optical excitation density that decays awayfrom the surface, and over time as hot electrons thermalize with the lattice. The finite lifetime of the hot
carriers generates a fast quench of the potential on the timescale of the CDW response.

The finite penetration depth of the laser leads to a quench that inverts the sign of the CDW at thesurface, generating domains throughout the material.

Fig. 1: Time-domain x-ray scattering of charge density wavesreveals dislocation formation after a quench.
(a) Differential intensity I(q) near the CDW peak of ErTe3 10 ps following photoexcitation by 800 nm light. The
bright region near the top is a satellite peak from domain wall scattering. The blue line shows the linocut along
the [0 1 0] out of plane direction. (b) Intensity I(q,t) along the linecut as a function of time. The time axis islogarithmically scaled to show the range of timescales observed in the experiment. The blue dots represent the
maximum scatted intensity of the satellite peak.
(c) Position of the satellite peak maximum as a function of time, fit to a power law, which reveals a power-law
scaling of 𝑞 ∝ 𝑡 −1/3 , indicating the presence of a conserved quantity in the relaxation. (d) The phase of theorder parameter of a time-dependent Ginzburg-Landau model following a quench. Singularities in the phase(topological defects) are shown in white.

Our simulations reproduce the production of topological defects after a light-driven quench at finite
temperature, and produce the power-law scaling of the fundamental length scale that we observe
experimentally.
References[1] T. W. B. Kibble, Journal of Physics A: Mathematical and General Topology of cosmic domains and strings 9, 1387 (1976).
[2] S. Wandel, F Boschini , E H da Silva Neto, L Shen, M X Na, S Zohar, Y Wang, S B Welch, M H Seaberg, J D Koralek, G L Dakovski, W Hettel
M-F Lin, S P Moeller, W F Schlotter, A H Reid, M P Minitti, T Boyle, F He, R Sutarto, R Liang, D Bonn, W Hardy, R A Kaindl, D G Hawthorn
J.-S. Lee, A F Kemper, A Damascelli, C Giannetti, J J Turner, G Coslovich, Science 376, 860 (2022).
[3] M. Mitrano, S. Lee, A. A. Husain, L. Delacretaz, M. Zhu, G. de la Peña Munoz, S. X.-L. Sun, Y. I. Joe, A. H. Reid, S. F. Wandel,
Science Advances 5 , 3346 (2019).
[4] Y. A. Gerasimenko, P. Karpov, I. Vaskivskyi, S. Brazovskii, D. Mihailovic, Npj Quantum Materials 4, 1 (2019).
[5] A. Kogar A. Zong, P. E. Dolgirev, X.Shen, J.Straquadine, Y.-Q. Bie,X. Wang, T.Rohwer, I-C. Tung, Y.Yang, Li, J.Yang, S.Weathersby, S.Park
M. E. Kozina, E. J. Sie, H.Wen, P. Jarillo-Herrero, I. R. Fisher, X.Wang, N. Gedik , Nature Physics 16, 159 (2020).
[6] A. Zong, M. Huber, Y. Lin, N. G Dale, R.Sailus, S.Tongay, R. A. Kaindl, A. Lanzara, Nature Physics 15, 27 (2019).
[7] M. Trigo, P. Giraldo-Gallo, M. E. Kozina, T. Henighan, M. P. Jiang, H. Liu, J. N. Clark, M. Chollet, J. M. Glownia, D. Zhu, T. Katayama
D. Leuenberger, P. S. Kirchmann, I. R. Fisher, Z. X. Shen, D. A. Reis, Physical Review B 99, 104111 (2019).
[8] M. Trigo, P. Giraldo-Gallo, J. N. Clark, M. E. Kozina, T. Henighan, M. P. Jiang, M. Chollet, I. R. Fisher, J. M. Glownia, T. Katayama
P. S. Kirchmann, D. Leuenberger, H. Liu, D. A. Reis, Z. X. Shen, D. Zhu, Physical. Review B 103, 054109 (2021).

In-gap spectral weight of the optical conductivity induced by a strong
Subcycle pulse in low-dimensional Mott insulators
T. Tohyama
Tokyo University of Science, Tokyo 125-0051, Japan

Electric pulse applied to the Mott insulators induces insulator-to-metal transitions. Not only lowenergy Drude component but also in-gap excitations emerge in the optical conductivity after thetransitions. Strong subcycle (mono- or half-cycle) pulses with zero frequency induce quantum
tunneling in the low-dimensional Mott insulators. It is interesting to know how in-gap spectral
weights behave when quantum tunneling occurs. We calculate time-resolved optical conductivity for
one-dimensional and two-leg ladder Mott insulators driven by strong subcycle pulses. The timedependent Lanczos method and time-dependent density-matrix renormalization group are used forthe calculation of the optical conductivity in the half-filled extended Hubbard model. In a onedimensional Mott insulator, subcycle pulses with zero frequency suppress spectral weights
contributing to the Drude weight and enhance in-gap spectral weights with increasing the strength ofthe pulses (see Fig. 1).

Fig.1. Time-dependent optical conductivity excited bya zero-frequency mono-cycle pulse for a half-filled
one-dimensional extended Hubbard lattice with 32 sites
and on-site (nearest-neighbor) Coulomb interaction
U=10 (V=3) in the unit of hopping energy. (a):electric-field strength E0=1.5. (b): E0=1.8. (c): E0=2.1.
Black, red, and blue-dashed lines are for time τ <0
(before pumping), τ =12, and τ =14, respectively.

This is in contrast to a metallic behavior induced by photon absorption and chemical doping. The
strong suppression of the Drude weight and the enhancement of in-gap weights in the quantum
tunneling regime are a result of the emergence of the Hilbert-space fragmentation, which makes
pulse-excited states glassy [1]. The glassy state is accompanied by electric polarization that breaks
inversion symmetry [2]. We also demonstrate that, using an ultrashort subcycle pulse, one cangenerate a steady electric current because of an Aharonov-Bohm flux instantaneously introduced
through the phase of an electric field. Consequently, time-reversal symmetry is broken [2]. Bothsymmetry breakings can be monitored by second harmonic generation. These findings propose a new
methodology for designing the symmetries of electronic states and open up a new field of subcyclepulse engineering. In a two-leg ladder, on the other hand, in-gap spectral weights are strongly
suppressed and show negative values [3]. A similar behavior is obtained in pumping an absorption
peak just above on-site Coulomb energy. Since the absorption peak is related to the presence ofmagnetic dimer in the ground state, the negative in-gap weights are due to a magnetic origin.
These works were done in collaboration with Kazuya Shinjo, Shigetoshi Sota, and Seiji Yunoki.
References[1] K. Shinjo, S. Sota, and T. Tohyama, Physical Review Research 4, L032019 (2022).
[2] K. Shinjo, S. Sota, S. Yunoki, T. Tohyama, arXiv:2211. 08694, (submitted to Physical Review B), (2022).
[3] K. Shinjo, S. Sota, S. Yunoki, T. Tohyama, (to be submitted), (2023).

Measuring everything you’ve ever wanted to know about an
Ultrashortlaser1 pulse 1
1
2
1
1
R. Trebino , E.Grace , R.Jafari , P.Zhu , Z. Guang , P.Bowlan
1Georgia Institute of Technology, Atlanta, GA 30332 USA
2Lawrence Livermore National Lab, Livermore, CA 94550 USA

Because ultrashort laser pulses are the shortest events ever created, their measurement has beenchallenging, and, as a result, mismeasurements have been common and remain so even today[1].
Many poorly developed pulse-measurement techniques have been introduced and even widely
implemented, often yielding anomalously short pulse lengths and anomalously simple pulse
shapes[2]. Worse, they have even masked the presence of instability of pulse shapes in trains ofpulses. Fortunately, a few techniques and devices (and only a few!) have been shown, over many
years, to yield essentially unambiguous results and that are quite reliable[3,4]. The main issue,
widely misunderstood for decades, has been the effect of pulse-shape instability in trains of pulses
measured using techniques that scan a delay or another parameter and so yield measurements that
average over many, potentially quite different pulses, a very common case. In early measurements
using autocorrelation techniques, this yielded the infamous coherent artifact—basically, a measure of
only the shortest spike of an often much more complex pulse. This resulted in numerous claims oferroneously short pulses. Even today, this problem remains common (See Fig. 1).

Fig. 1. Intensity autocorrelations of
trains of stable (top row) and unstable(middle and bottom rows) pulses. Notethat the unstable, moderately complex
pulse train appears to yield a smoothshort pulse, but this assumption yields
an erroneously short pulse length andno indication of the presence of thepulse-shape
instability.
Figurereprinted from frog.gatech.edu.

This problem is particularly severe in many recently introduced interferometric techniques, most ofwhich operate in the frequency domain, where the coherent artifact appears in a less familiar form:
specifically, a linear spectral phase[2]. This is because a complex spectral phase, which contains the
specific complex temporal pulse structure, washes out when averaging over many different pulses.
Some of these methods, including the most popular ones, measure only the coherent artifact and,worse, provide no information about the presence or absence of the pulse train’s pulse-shape instability(See Fig. 2). As a result, they cannot distinguish a train of short, simple, stable pulses from thediametrically opposite case of a train of long, complex, unstable pulses. They have likely achieved
popularity due to a combination of a desire to claim the shortest pulses and humankind’s penchant for
self-deception. On the other hand, spectrographic methods, the first class of methods shown to be able
to measure the complete pulse intensity and phase vs. time, do not suffer from these issues. Theirreliability and power have now been convincingly demonstrated for a wide range of pulse intensities,
rep rates, wavelengths, and even complexities[3, 4] (See Fig. 2). Unfortunately, their adoption has
been hampered by their reporting of longer—albeit accurate—pulse lengths and also the resulting
discrepancies between measured and retrieved traces when pulse-shape instability is present. In
addition, their use of novel iterative algorithms for pulse retrieval, which has often been blamed forsuch discrepancies, when in fact such discrepancies indicate pulse-shape instability, have also slowed
their adoption. Finally, a poor understanding by many researchers of the role of unstable pulse trains,in general, has played a role, as well. And finally, admittedly, until recently, iterative pulse-retrieval
algorithms have not been reliable, having had a tendency to stagnate, that is, not converge. But theselatter problems have been solved with the recent introduction of a modified algorithm that has beenshown, not only to be fast, but also 100% reliable, even for extremely complex pulses [5]. This latter
algorithm reliably retrieves even extremely complex pulses with time-bandwidth products as large as
100 and likely even higher even in the presence of significant noise.

Fig. 2. The same trains of stable (top row) and unstable (middle and bottom rows) pulses and their
SPIDER and FROG traces and retrieved pulses. Note that SPIDER yields the same measured trace andonly the coherent artifact for all three pulse trains, even the highly unstable train of long comple pulses,
severely under-estimating the pulse length when pulse-shape instability is present. This occurs for
SPIDER measurements for all types of pulse-shape instability. Also, there is no indication of instability
in SPIDER. The various versions of FROG, on the other hand, yield more realistic pulses and pulse
lengths, and their rertieved traces reveal the presence of instability via obvious discrepancies between
the measured and retrieved traces. Figure reprinted from frog.gatech.edu.

It also definitively distinguishes between stable and unstable pulse trains, finally also solving this longstanding problem[6] (Fig. 3).In addition, once a reliable measurement has been made of a pulse
directly from a laser, such a measured pulse can act as a reference pulse to assist additional (eveninterferometric) methods to measure even more complex quantities, such as the complete
spatiotemporal intensity and phase—that is, the complete electric field—of an arbitrary ultrashortlaser pulse[7-10]. We recently used such a method to measure even a single extremely complex
terawatt laser pulse completely in space and time at Lawrence Livermore National Lab[11]. This
was the first ever single-shot complete spatiotemporal pulse measurement of any pulse.
Fig.3. Plots of the rms trace discrepancy between simulated measured
and retrieved SHG FROG traces for different initial guesses for an
unstable pulse train. Because the measured trace is the sum of manydifferent traces due to the many different pulses, it cannot correspondto a single pulse, and so there will necessarily be discrepancies
between it and the retrieved trace. Plotted here are the rms trace
discrepancies for 100 different initial guesses for the same trace (of amoderately complex pulse) using the standard GP pulse-retrieval
algorithm and also for the new, ultra-reliable RANA approach. Notethat the RANA approach converges to the lowest possible trace
discrepancy for all initial guesses (yielding a true estimate of thepulse-shape instability), whereas the GP algorithm often stagnatesand so over-estimates the discrepancy. As a result, the RANA
approach reliably indicates the presence of stability or instability.
Figure reprinted from frog.gatech.edu.

Our conclusion is that the field of pulse measurement is essentially solved—unless we count theneed to eradicate the remaining misconceptions and misleading methods. The next step is to use
these existing reliable methods to measure important pulses.
References[1] R. Trebino, Optics and Photonics News 31, 46 (2020).
[2] M. Rhodes, G. Steinmeyer, J. Ratner, R. Trebino, Laser & Photonics Reviews 7, 557 (2013).
[3] R. Trebino, R. Jafari, S. A. Akturk, P. Bowlan, Z. Guang, P. Zhu, E. Escoto, G. Steinmeyer, SRC Handbook of Laser Technology and Applications
1, 487 (2021).
[4] R. Trebino, R. Jafari, S. A. Akturk, P. Bowlan, Z. Guang, P. Zhu, E. Escoto, G. Steinmeyer, Journal Applied Physics 128, 171103 (2020).
[5] R. Jafari, R. Trebino, IEEE Journal. of Quantum Electronics 56, 1(2020).
[6] S. D. K. R. Jafari, R. Trebino, Nature Scientific Reports 12, (2022).
[7] P. Zhu, R. Jafari, T. Jones, R. Trebino, Optics Express 25, 24015 (2017).
[8] Z. Guang, M. Rhodes, R. Trebino, Applied Optics 56, 3319 (2017).
[9] Z. Guang, M. Rhodes, R. Trebino, Journal Optical Society of. America-JOSA B 33, 1955 (2016).
[10] P. Bowlan, U. Fuchs, R. Trebino, U. D. Zeitner, Optics Express 16, 13663 (2008).
[11] E. Grace, T. Ma, Z. Guang, R. Jafari, J. Park, J. Clark, E. Kemp, J. Moody, M. Rhodes, Y. Ping, R. Shepherd, B. Stuart, R. Trebino, Journal of
Optics 23, 075505 (2021).
* Rick Trebino owns a company that sells pulse-measurement devices, and Rana Jafari consults for it.

Ionisation dynamics and surface plasmon excitation inDielectrics irradiated with Mid-Infrared
femtosecond pulses
1,2
G. D. Tsibidis , E. Stratakis
Institute of Electronic Structure and Laser (IESL), 70013 Heraklion, Greece
2
University of Crete, 71003 Heraklion, Greece

1

The employment of ultrashort laser sources at the mid-IR spectral region for dielectrics is expected
to open innovative routes for laser patterning and a wealth of exciting applications in optics andphotonics. To elucidate the material response to irradiation with mid-IR laser sources, a consistentanalysis of the interaction of long wavelength femtosecond pulses with dielectric materials is presented(Fig.1a). The influence of the pulse duration is particularly emphasized in specifying the laserparameters for which photoionization and impact ionization are important [1]. Simulation results
using pulses at 2.2 μm, 3.2 μm (Fig.1b) and 5 μm are conducted to illustrate the optimum conditions
for the onset of damage on the solid that is related to the occurrence of the optical breakdown.
Results predict that the damage threshold scales as ~() at all laser
wavelengths (Fig.1c). Given the significant effect of the induced excitation level on the excitationof Surface Plasmons (SP) which account for the formation of laser-induced periodic structures(LIPSS) [2] oriented perpendicular to the laser polarization, a correlation of the produced electron
densities with SP and the threshold of SP excitation (~ ,) are also discussed inthis as yet unexplored spectral region (Fig.1d). These periodic structures are just one category of
LIPSS in addition to other topographies predicted and observed at this spectral region [2].

Fig. 1 Photoionization rates WPI as a function of (a) laser wavelength and (peak) intensity, (b) laser (peak)fluence and pulse duration (μm). (c) Minimum fluence to reach OBT as a function of laser pulse
duration (forμm,
μm, μm); Brown coloured filled circle corresponds to experimental data
at 2 μm for 150 fs, (d) Threshold for SP excitation as a function of laser pulse duration (forμm,
μm, μm).

Results are expected to guide development of an innovative approach to surface patterning using
strong mid-IR pulses for advanced applications.
References[1] G. D. Tsibidis, E. Stratakis, Applied Physics Letters 122, 04350 (2023).
2 G. D. Tsibidis, E. Stratakis, Scientific Reports 10, 8675 (2020).

Two-mode excitations with terahertz light pulses: from phonons to
Superconducting
plasmons
1
2
1
1
3
1
2
4

M. Udina , M. Basini , J. Fiore , F. Gabriele , M. Pancaldi , N. Sellati , V. Unikandanunni , S. Bonetti
L. Benfatto1
1"
Sapienza” University of Rome, 00185 Rome, Italy
2
Stockholm University, 10691 Stockholm, Sweden
3
Elettra-Sincrotrone Trieste S.C.p.A, 34149 Basovizza, Trieste, Italy
4
Ca’ Foscari University of Venice, 30172 Venice, Italy

The technical improvements in the generation of intense and phase-stable terahertz (THz) pulses
considerably enlarged the possibilities to design experiments aimed at coherently driving low-energy
excitations of complex systems on ultrafast time scales. The general idea is that one can takeadvantage of the quadratic coupling of light to Raman-active modes to drive large amplitude
fluctuations of the corresponding degrees of freedom, ranging from lattice vibrations [1] tocollective-modes of broken-symmetry states, as for magnetic or superconducting (SC) transitions [2].
Within this picture, the largest response occurs when twice the pump frequency of the THz lightmatches the mode resonance, i.e. 2Ω = 𝜔𝑟𝑒𝑠 , highlighting the underlying two-photon sum-frequency
nature of the excitation process [3-4]. On the other hand, a related but different possibility is thatstrong pulses can drive simultaneously two modes. In this case, one would expect the response to belarger when the pump frequency directly matches the mode resonance at 𝜔𝑟𝑒𝑠 .
So far, such a possibility has been mainly discussed within the context of the so-called non-linear
phononics [5]. Indeed, thanks to anharmonic couplings among lattice vibrations, the simultaneous
excitation of two infra-red (IR) active phonon modes acts as a driving process for exciting Ramanactive phonons [3], opening in principle a pathway to drive materials towards metastable states
which may not be accessible at thermal equilibrium and leading to interesting effects like e.g. theobservation of silent phonon modes [6]. On the other hand, the very same mechanism underlies also
the non-linear driving of plasma waves in superconductors [7]. Below 𝑇𝑐 the emergence of acomplex SC order parameter leads to two collective electronic modes related to its amplitude (Higgs)
and phase fluctuations, respectively, with the latter carrying information on the spectrum of SC
plasma modes. While in ordinary, isotropic superconductors this is usually a large energy scale,around hundreds of THz, in layered superconductors, as e.g. high-𝑇𝑐 cuprates, the weak coupling
among layers leads to a soft out-of-plane plasma mode at few THz, depending on the cuprate family,
well described by a discrete Josephson-like model for inter-layer phase fluctuations. Due to the large
value of the SC gap, such a soft Josephson plasma mode (JPM) is pushed below the optical gap andleads to the appearance of a well-defined plasma edge in the out-of-plane reflectivity at THz
frequencies.In this talk I will discuss the possibility of driving simultaneously multiple collectivemodes using intense THz pulses, starting from the more conventional case of IR active phonons inwide-band insulators, to the resonant excitation of JPMs in superconducting cuprates. In particular,
we have recently shown that two-phonon excitations give a sizable contribution to the THz Kerr
response in insulating SrTiO3 [8]. While the conventional electronic Kerr effect, probing a change inthe refractive index proportional to the square of the applied electric field, is associated with offresonant electronic transitions, such a ionic contribution, named ionic Kerr effect (IKE), relies on theresonant excitations of multiple infrared modes as an intermediate step (see Fig. 1a), thus providing
an alternative mechanism to modulate the refractive index on ultrashort time-scales. Given thecapability of terahertz-driven phonons to couple to different order parameters and to drive materials
towards metastable states which may not be accessible at thermal equilibrium [5], the IKE can beused to investigate the electron-phonon coupling across various phase transitions.A similar
theoretical description can be extended to describe the excitation of SC Josephson plasmons, which
has been intensively studied in cuprate families with both one and two layers per unit cell. In the firstcase, experiments have shown that the nonlinear response at THz frequencies has a well-definedresonance at 2𝜔𝑝 [9-10], where 𝜔𝑝 is the energy of the soft plasmon, supporting the idea that THz
pulses can simultaneously excite two JPMs with opposite momentum (see Fig. 1b).

Fig. 1. (a): Schematic representation of the ionic Kerr effect in a THz pump-optical probe experiment. When thepump pulse is resonant with an IR-active phonon at 𝜔𝐼𝑅 , two modes can be excited to reach a virtual electronic
state. The time-delayed probe field then couples to this virtual excitation, leading to a modulation of the emitted
light at 2𝜔𝐼𝑅 . (b): Schematic view of the mexican-hat potential for the free energy 𝐹(𝜓), with 𝜓 the complex
order parameter of a superconductor below 𝑇𝑐 . A phase-gradient excitation corresponds to a longitudinal shift
along the minima. An intense light pulse with almost zero momentum can excite simultaneously two
plasma waves with frequency 𝜔𝑝 and opposite momenta.

The resonant excitation occurs when the pump frequency matches the energy of the soft out-of-plane
plasmon measured by linear reflectivity [7]. In contrast to the single-layer case, recent experiments[11] suggest instead that no resonances are present when the pumping frequency matches the valueof the reflectivity plasma edge in double-layer YBCO. In a recent work [12], we have provided acomprehensive derivation of the nonlinear optical response in layered superconductors, accounting
for the full momentum dispersion of the two plasma modes simultaneously excited by the THz pumppulse. We have shown that the unavoidable entanglement between in-plane and out-of-plane plasma
waves in a layered system represents a crucial ingredient to understand what marks the difference
between the single-layer and the bilayer case, and to fully reproduce the existing experimental
results.
References[1] S. Maehrlein, A, Paarmann, M.Wolf, T.Kampfrath, Physical Review Letters 119, 127402, (2017).
[2] R. Matsunaga, Y. I. Hamada, K.Makise, Y. Uzawa, H.Terai, Z.Wang, R.Shimano, Physical Review Letters 111, 057002, (2013).
[3] D. M. Juraschek, S. Maehrlein, Physical Review B 97, 174302, (2018).
[4] M. Udina, T. Cea, L. Benfatto, Physical Review B 100, 165131, (2019).
[5] D. Nicoletti, A. Cavalleri, Advances in Optics and Photonics 8, 401-464 (2016).
[6] M. Kozina, M. Fechner,P. Marsik, T. van Driel, J. M. Glownia, C. Bernhard, M. Radovic, D. Zhu, S. Bonetti,U. Staub ,M. C. Hoffmann, Nature
Physics 15, 387 (2019).
[7] F. Gabriele, M. Udina, L. Benfatto, Nature Communications 12, 752 (2021).
[8] M.Basini,M. Udina, M. Pancaldi, V. Unikandanunni, S. Bonetti, arXiv:2210.14053 (2022).
[9] S. Rajasekaran, S. Rajasekaran, E. Casandruc, Y. Laplace, D. Nicoletti, G. D. Gu, S. R. Clark, D. Jaksch, A. Cavalleri, Nature Physics 12, 1012
(2016).
[10] K. Kaj,K.A. Cremin, I.Hammock, J.Schalch, D. N. Basov, R. D. Averitt, arXiv:2211.17184 (2022)[11] K. Katsumi,, M.Nishida, S.Kaiser, S.Miyasaka, S. Tajima, R. Shimano, arXiv:2209.01633 (2022).
[12] J. Fiore, M. Udina, (to be submitted) (2023).
* Acknowledgement(s): M. Udina acknowledge support from EU under program MORE-TEM ERC-SYN (grant agreement No 951215).

High-precision high-resolution spectroscopy with frequency combs:
from Mid-IR
to THz1
2
1
1
D. Konnov , S. Vasilyev , A. Muraviev , K. Vodopyanov
1

University of Central Florida, Orlando, FL 32816, USA
2
IPG Photonics Corporation, Oxford, MA 01752, USA

We present our new approach to the creation of ultra-broadband frequency combs in the whole midIR to terahertz range (1-100 THz) enabled by a new laser technology: Kerr-lens mode-locked solidstate Cr:ZnS lasers producing sub-3-cycle pulses at λ=2.4 µm as a driving source. This allowed
conducting dual-comb spectroscopy of molecular gases in a wide spectral range, and at a record highresolution and speed according to the following 3 scenarios for different spectral bands:
In our first approach, the dual-comb spectroscopy (DCS) system is based on a pair of subharmonic
optical parametric oscillators (OPOs) pumped by either a pair of phase-locked 1.93-µm Tm-laser
combs that produce mid-IR combs spanning 3.1-5.4 µm [1], or by two 2.4-µm Cr:ZnS laser combs
that produce combs spanning two octaves, 3-12 µm [2]. The beam from the first OPO comb passes
through a multipass gas cell with a mixture of gases and serves as a ‘sensing’ comb, while the outputof the second OPO serves as a ‘sampling’ comb. The beams are coherently combined and sent to afast mid-IR detector to produce DCS interferograms. This approach allowed massively parallel andultrasensitive detection of molecules and their isotopologues in a mixture of gases with sub-part-perbillion sensitivity [1]. Also, the line list produced by our high-resolution DCS measurements ofcarbon disulfide (CS2) at 4-5 µm [3] is now part of the HITRAN database.

(a)

(b)
Fig. 1. (a): Comb-mode-resolved spectrum of the longwave IDFG output achieved in ZGP crystal. (b): IDFG
comb spectrum achieved in GaSe crystal (the dip at ~15 THz is due to 2-phonon absorption in GaSe).

In our second approach, both ‘sensing’ and ‘sampling’ combs are produced in the long-wave
infrared (LWIR) region (4–20 μm) via intrapulse difference frequency generation (IDFG) in ZGP or
GaSe nonlinear crystals with 2.4-µm pulses serving as a pump. A comparatively long-wavelength
pump at 2.4 µm allows generating IDFG outputs with a record high conversion efficiency reaching
10% in ZGP crystal. Thanks to the high average power of the combs (300 mW per each comb, span
6.6–11.4 μm, >1 µW per comb mode, Fig 1a), we were able to acquire DCS data in this wavelength
range corresponding to 240,000 comb-mode resolved spectral points at 80-MHz spacing, withacquisition speeds up to video rate (10 Hz). Also, metrological-grade LWIR absorption spectra ofnitrous oxide (N2O) and methanol (Figs. 2-3) were obtained in just 12 seconds [4].
In our third approach, Cr:ZnS pumping creates a long wavelength output via IDFG in a GaSe
crystal that can cover the entire range from 1 to 50 THz (6-300 µm) with an instantaneous comb spanthat can reach 2 octaves (e.g. 10-40 THz, 7.5-30 µm). Another Cr:ZnS laser comb generates thesecond harmonic pulse at 1.2 µm serving as a probe for electrooptic sampling (EOS); here theelectric field of the mid-IR/THz transient is detected via induced change of the polarization state of
the probe pulse inside an electrooptic crystal using ellipsometry and a balanced InGaAs detector. Fig.
1b shows the span of the comb we used to obtain the high resolution long wavelength spectrum of
the low pressure mixture of carbon dioxide (CO2) and acetylene (C2H2) shown in Fig.3.

a)b)
Fig. 2. (a) High-resolution DCS spectrum of low pressure nitrous oxide (N2O) and (b) methanol (CH3OH) –both obtained in just 12 seconds.

The expanded views of separately CO2 and C2H2 spectra and their comparison with the HITRAN
simulation are depicted in Figs. 3c-d. We will also discuss our DCS measurements with tuning thecombs’ repetition rate that provides spectral resolution well below the spacing between the comblines (80 MHz in our case), as well as our pilot long wavelength EOS measurements at 2-4 THz.

Fig. 3. (a,b): High-resolution LWIR spectrum of a mixture of two molecules (CO2 and C2H2) with buffer gas N2
at few mbar) pressure. (c) Expanded view of the CO2 spectrum and its comparison with the HITRAN simulation.
(d) Expanded view of the C2H2 (acetylene) spectrum and its comparison with the HITRAN simulation.

In summary, we report a novel approach to dual-comb spectroscopy based on a 2.4-µm Cr:ZnS laser
platform. To our knowledge, we have demonstrated for the first time all the advantages of the DCS
technique achievable simultaneously in the longwave IR region (λ>8 µm): broadband coverage (>500 cm-1), Doppler spectral resolution, absolute frequency referencing and data acquisition speeds up
to video rate with 240,000 comb lines resolved.
References[1] A. V. Muraviev, V. O. Smolski, Z. E. Loparo, K. L. Vodopyanov, Nature Photon. 12, 209 (2018).
[2] Q. Ru, T. Kawamori, P. G. Schunemann, S. Vasilyev, S. B. Mirov, K. L. Vodopyanov, Optics Letters 46, 709 (2021)[3] A. V. Muraviev, D. Konnov, K. L. Vodopyanov, Scientific Report 10, 18700 (2020).
[4] S. Vasilyev, A. Muraviev, D. Konnov, M. Mirov, V. Smolski, I. Moskalev, S. Mirov, K.L. Vodopyanov, Optics Letters 48, (in print) (2023)* We acknowledge support from the ONR, DARPA, DOE and AFOSR grants.

Light induced1 metastable
magnetization
ina 2D 2antiferromagnet
2
1
1
1
B. Ilyas , T. Luo , A. von Hoegen , E. V. Bostr m , A. Rubio , N. Gedik
1
Massachusetts Institute of Technology, 02139 Cambridge, United States
2
Max Planck for the Structure and Dynamics of Matter, 22607 Hamburg, Germany

The XPS3 (X=Mn, Ni, Fe, Co) family of van der Waals antiferromagnets has recently attracted a lotof attention due to their strongly coupled spin, lattice and electronic degrees of freedom1-4. They
offer a rich testing ground for 2D magnetism and provide an opportunity to explore its interplay withother correlated order parameters and their ultrafast control. In particular, the perturbation of theelectronic degrees of freedom via resonant pumping of specific electronic transitions in thesematerials was explored to find new ways to control spins and lattice at an ultrafast timescale5. In ourwork we go along a different avenue, by using intense THz light pulses, we resonantly drive thefundamental eigenmodes of the spin and lattice degrees of freedom, i.e., magnons and phonons, in
FePS3 directly. This way, the electronic degrees of freedom remain unperturbed and we can study the
low energy physics associated with the magnetism along non-thermal pathways. Our THz excitation
launches spin and lattice dynamics in the form of coherent magnons and phonons, which we follow
as a function of temperature by recording the polarization rotation (dichroism) and ellipticity(birefringence) of a transmitted 800-nm wavelength ultra-short laser pulse (see Fig.1,left). Besides
these fast dynamics, we also observe the emergence of a slow component close to the Neeltemperature TN = 118 K (see Fig. 1, right).

Fig. 1. Left: Schematic of the ultrafast linear dichroism measurement with THz pump (orange) and 800-nm
probe pulse (red) together with an example time-trace and fast Fourier transform featuring coherent magnon(green) and phonon (red) oscillations. Right: Schematic of the ultrafast circular dichroism measurement withan example time-trace at 118K (red) and 140K (grey) and the full temperature dependence close to thetransition temperature.

These slow dynamics are concomitant with the appearance of a long-lived circular dichroism (CD),strongly suggesting a finite out-of-plane magnetization inside the sample. This CD signal shows anonlinear dependence on the excitation field strength, appears only when the THz pump spectrally
covers the relevant long-energy modes and has a lifetime of about 1.4 ms. To understand our results,
we employed first principles calculations to evaluate the spin-lattice coupling. We found that one
lattice vibrational mode, which is strongly hybridized with the AF-magnon at zero field2, distorts thelattice in such a way that the exchange coupling between first-, second-, and third-nearest neighbor
Fe atoms favors a ground state with a finite magnetization. These findings demonstrate how themagnetic ground state in 2D van der Waals magnets can be efficiently manipulated along nonthermal pathways using THz light. The long lifetime of the light-induced magnetization in FePS3
promises possible applications in micro- and optoelectronic devices and impacts the field of
spintronics in general.
References[1] E. Ergeçen, B. Ilyas, Ju. Kim, J. Park, M. B. Yilmaz, T. Luo, D. Xiao, S. Okamoto, J.-G., N. Gedik, PNAS 120 (12), (2023)[2] D. Vaclavkova, M. Palit, J. Wyzula, S. Ghosh, A. Delhomme, S. Maity, P. Kapuscinski, A. Ghosh, M. Veis, M. Grzeszczyk, C. Faugeras, M. Orlita,
S. Datta, M. Potemski, Physical Review B 104, 134437 (2021)[3] K. Hwangbo, Q. Zhang, Q. Jiang, Y. Wang, J. Fonseca, C. Wang, F. M. Diederich, D. R. Gamelin, D. Xiao, J.-H. Chu, W. Yao,X. Xu, Nature
Nanotechnology 16, 655 (2021).
[4] C. A. Belvin, E. Baldini, I. O. Ozel, D. Mao, H. C. Po, C. J. Allington, S. Son, B. H. Kim, J. Kim, I. Hwang, J. Hoon Kim, J.-G. Park, T. Senthil
N. Gedik , Nature Communications 12, 4837 (2021).
[5] D. Afanasiev, J. R. Hortensius, M. Matthiesen, S. Mañas-Valero, M. Šiškins, M. Lee, E. Lesne, H. S. J. van der Zant, P. G. Steeneken,
B. A. Ivanov, E. Coronado, A. D. Caviglia, Science Advances 7, 23, (2021)* Acknowledgement(s) : Alexander von Hoegen acknowledges support from the Alexander von Humboldt foundation. We acknowledge support
from the US Department of Energy and Gordon and Betty Moore Foundation’s EPiQS Initiative grant GBMF9459.

Probing nonlinear transport in a non-equilibrium
Superconductor
E. Wang, J. D. Adelinia, M. Chavez-Cervantes, T. Matsuyama, M. Fechner, M. Buzzi
G. Meier, A. Cavalleri
Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany

Optically driven quantum materials exhibit a variety of non-equilibrium functional phenomena [112], which to date have been primarily studied with ultrafast optical, photoemission, X-Ray
scattering and spectroscopy methods. However, little has been done to characterize their transient
electrical responses, which are directly associated with the functionality of these materials.
Especially interesting are linear and nonlinear current-voltage characteristics at frequencies below 1
THz, which are not easily measured at picosecond temporal resolution. Here, we report on ultrafast
transport measurements in photo-excited K3C60, in which a transient phase with superconducting like
optical properties has been recently documented [13-17]. Nonlinear transport measurements are
executed by connecting thin films of this compound to photo-conductive switches with co-planar
waveguides. We observe characteristic signatures of a photo-induced granular superconductivity,
including characteristic nonlinear current-voltage responses that reveal the presence of excited weaklinks between transiently superconducting grains.
Fig. 1. Conventional DC four-point transportmeasurement and on-chip ultrafast transportmeasurement. a, Upper: Schematic of a DC fourpoint transport measurement on a K3C60 thin film.
Lower: Measured resistance versus temperatureof the K3C60 thin film with 1 µA current bias.
b, Upper: Schematic of on-chip ultrafast transport
device. The MBE grown K3C60 thin film (cyan) andthree pairs of photo-conductive switches (white)were incorporated within a coplanar waveguide(yellow)
on a sapphire substrate (purple).
Ultrashort electrical pulses were launched byilluminating laser pulses onto the left pair ofphoto - conductive switches,
which were
simultaneously biased by a voltage source.
The launched pulse V1(t) was sampled byilluminating one switch of the middle pair and bychanging the mutual delay between the launchingand sampling laser pulses. Similarly, the transmitted
pulse V2(t) was sampled with one switch of the rightpair. Lower: V1(t) and V2(t) measured at 8 K and 20
K. Both measurements were normalized by the peakvalue of V1(t).

Ultrafast nonlinear transport provides access to the physics of driven quantum materials and will
enable integration of non-equilibrium functionalities into ultrafast optoelectronic platforms.
References[1] I. Radu, K Vahaplar, C Stamm, T Kachel, N Pontius, H A Dürr, T A Ostler, J Barker, R F L Evans, R W Chantrell, A Tsukamoto, A Itoh
A Kirilyuk, Th Rasing, A V Kimel,. Nature 472, 205 (2011).
[2] T F Nova, A S Disa, M Fechner, A Cavalleri, Science 364, 1075 (2019).
[3] X.Li T.Qiu, J.Zhang,, E.Baldini, J.Lu, A. M. Rappe, K.A. Nelson, Science 364,1079 (2019)[4] A. S. Disa, M. Fechner, T.F. Nova, B.Liu, M.Först, D.Prabhakaran, P.G. Radaelli,A.Cavalleri, Nature Physics 16, 937 (2020).
[5] Y.H. Wang, Y.H., Y. H. Wang, H. Steinberg, P. Jarillo-Herrero, N. Gedik, Science 342, 453 (2013).
[6] J. W. McIver,B. Schulte, F.-U. Stein, T. Matsuyama,G. Jotzu, G. Meier,A. Cavalleri, Nature Physics 16, 38 (2020).
[7] S.Zhou, C. Bao, B. Fan, H. Zhou, Q. Gao, H. Zhong, T.Lin, H.Liu, P. Yu, P. Tang, S. Meng, W. Duan, S.Zhou, Nature 614, 75 ( 2023).
[8] X Wang, C.Xiao, H.Park, J. Zhu, C.Wang, T. Taniguchi, K. Watanabe, J.Yan, D.Xiao, D. R Gamelin, W.Yao, X. Xu, Nature 604, 468 (2022).
[9] D. Fausti , R. I. Tobey, N. Dean, S. Kaiser, A. Dienst, M. C. Hoffmann, S. Pyon, T. Takayama, H. TakagI, A. Cavalleri, Science 331,189 (2011).
[10] W. Hu, S. Kaiser, D. Nicoletti, C. R. Hunt, I. Gierz, M. Hoffmann, M. Le Tacon,T.Loew, B.Keimer,A.Cavalleri, Nature Materials 13, 705 (2014).
[11] K.A. Cremin, J. Zhang, C. C. Homes, G. D. Gu, Z.Sun, M. M. Fogler, A.J. Millis, D. N. Basov, R. D. Averitt, Proceedings of the National
Academy of Sciences 116, 19875 (2019).
[12] K.Isoyama, N.Yoshikawa, K.Katsumi, J.Wong, N.Shikama, Y.Sakishita, F.Nabeshima, A.Maeda, R.Shimano, Communications Physics 4
160(2021).
13] M.Mitrano, A.Cantaluppi, D.Nicoletti, S Kaiser, A,Perucchi, S Lupi, P.Di Pietro, D.Pontiroli, M Riccò, S. Clark, D.Jaksch, A.Cavalleri, Nature
530,461(2016)[14] A.Cantaluppi,M. Buzzi, G. Jotzu, D. Nicoletti, M. Mitrano, D. Pontiroli,M. Riccò, A. Perucchi, P. Di Pietro,A. Cavalleri, Nature Physics 14, 837( 2018)[15] M.Buzzi, D. Nicoletti, M. Fechner, N. Tancogne-Dejean, M. A. Sentef, A. Georges, T. Biesner, E. Uykur, M. Dressel, A. Henderson, T. Siegrist,
J. A. Schlueter, K. Miyagawa, K. Kanoda, M.-S. Nam, A. Ardavan, J. Coulthard, J. Tindall, F. Schlawin, D. Jaksch, A. Cavalleri, PhysicalReview X 10, 031028 (2020).
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A.Cavalleri, Nature Physics 17, 611 (2021).
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A.Cavalleri, Physical Review Letters 127, 197002 (2021).

Spontaneous symmetry breaking by geometric motive force at
Topological instability
J. Wang
Iowa State University, Ames, IA 50011, USA

An emerging light-induced symmetry and topology switch is breaking new ground in quantum
science and technology [1-5]. Symmetry is at the heart of defining order parameters and classifying
matter phases. Topology provides a beyond-symmetry classification and reveals a class of matterstates which are symmetrically identical but topologically distinguishable. For a long time, topologysymmetry interplay has been studied “one-way”, i.e., topological states are examined subject to a
specific fixed symmetry. This work demonstrates that their interplay could be mutual. We show thatcrystalline symmetries (e.g., inversion symmetry) and time-reversal symmetry may turn fragile when
topological phase transitions are triggered, which are manifested by the excitation of IR phononmode off-resonantly, the generation of strong non-field-driven photocurrents and geometric charge
pumping [6]. Such observations apply broadly and can establish a generic link between symmetry
and topological transitions.

Fig. 1: Light-induced of symmetry breaking at topological instability (left) in a Dirac material of ZrTe5.
Inversion symmetry and time-reversal symmetry breaking are manifested by the excitation of IR phonon mode
off-resonantly (right), the generation of strong non-field-driven photocurrents and geometric charge pumping.

On the other side, light engineering of correlation gaps in broken-symmetry ground states of
topological materials provides a new avenue of achieving exotic topological phases inaccessible byconventional tuning methods. Here we demonstrate a light-controllable topological phase switchingin a model CDW and polaron insulator [7]. Our ultrafast terahertz photocurrent spectroscopy reveals
a topological phase switching from an axion insulator phase to a hidden Weyl phase with chirality
controlled by light helicity.
References[1] L. Luo, D.Cheng, B. Song, L.-L. Wang, C.Vaswani, P. M. Lozano, G. Gu, C. Huang, R. H. J. Kim, Z.Liu, J.-M. Park, Y. Yao, K.Ho,
I. E. Perakis, Q.Li, J,Wang , Nature Materials 20, 329 (2021).
[2] C. Vaswani, C. Vaswani, L.-L. Wang, D. H. Mudiyanselage, Q. Li, P. M. Lozano, G. D. Gu, D. Cheng, B. Song, L. Luo, R. H. J. Kim, C. Huang
Z. Liu, M. Mootz, I. E. Perakis, Y. Yao, K. M. Ho, J. Wang, Physical Review X 10, 021013 (2020).
[3] R. H. J. Kim, C.Huang,Y.Luan,L.-L. Wang,Z. Liu, J.-M. Park,L.Luo,P. M. Lozano,G. Gu,D. Turan,N. T. Yardimci,M.Jarrahi,I.E. Perakis
Z. Fei,Q. Li,J. Wang, ACS Photonics 8, 1873 (2021).
[4] X. Yang X. Yang, L.Luo, C. Vaswani, X.Zhao, Y.Yao, D.Cheng, Z.Liu, R. H. J. Kim, X.Liu, M. Dobrowolska-Furdyna, J.K. Furdyna,
I. E. Perakis, C. Wang, K. Ho,J. Wang, Quantum Materials 5, 13 (2020).
[5] C. Vaswani, C. Vaswani, J. H. Kang, M. Mootz, L. Luo, X. Yang, C. Sundahl, D. Cheng, C. Huang, R. H. J. Kim, Z. Liu, Y. G. Collantes
E. E. Hellstrom, I. E. Perakis, C. B. Eom, J. Wang, Nature Communications 12, 258 (2021).
[6] L. Luo, preprint(2023).
[7] B. Cheng, preprint (2023)

Nonlinear terahertz spectroscopy study on the interplay between
Superconductivity and pseudogap in cuprate superconductor
N.-L. Wang
Peking University, Beijing 100871, China

We present nonlinear terahertz third harmonic generation (THG) measurement on different doping
YBa2Cu3O6+x thin films and electron-doped LCCO thin films. Different from conventional
superconductors [1], the THG signal starts to appear in the normal state, which is consistent with thecrossover temperature T* of pseudogap over broad doping levels. Upon lowering temperature, theTHG signal shows anomaly just below Tc. Strikingly, we observe a beat pattern directly in themeasured real time waveform of THG signal, as shown below in Fig. 1 [2]. We elaborate that the
Higgs mode, which develops below Tc, couples to the mode already developed below T*, resulting inan energy level splitting. The strong coupling effect offers new insight into the interplay between
superconductivity and pseudogap. The result suggests that the pseudogap phase is not likely aprecursor of superconductivity but represents a distinct order.

Fig. 1: (a)-(d), THG intensity continuous wavelet transformation chronograms in optimal doped YBCO thinfilm driven by 0.5 THz pulse for (a) 89 K, (b) 86 K, (c) 85 K and (d) 84 K. (e)-(h) The real time waveforms of THG
signal after a digital 1THz high pass filter. The beating pattern in the waveform is corresponding to the dip in
CWT chronogram. (i)-(i) The THG spectrum for 89 K, 86 K, 85 K and 84 K after the global FFT. The splitting is
caused by the dynamical strong coupling between Higgs mode and pseudogap collective mode.

Work done in collaboration with Tao Dong, Jia-Yu Yuan and other team members in Peking
University.
References:[1] Z.-X. Wang, J.-R. Xue, H.-K. Shi, X.-Q. Jia, T. Lin, L.-Y. Shi, T. Dong, F. Wang, N.-L. Wang, Physical Review B 105, L100508 (2022).
[2] J. Y. Yuan, L. Y. Shi, L. Yue, B. H. Li, Z. X. Wang, S. X. Xu, T. Q. Xu, Y. Wang, Z. Z. Gan, F. C. Chen, Z. F. Lin, X. Wang, K. Jin, X. B. Wang,
J. L. Luo, S. J. Zhang, Q. Wu, Q. M. Liu, T. C. Hu, R. S. Li, X. Y. Zhou, D. Wu, T. Dong, N. L. Wang, arXiv:2211.06961.
*
Acknowledgement: This work was supported by the National Science Foundation of China and the National Key Research and Development
Program of China.

Controlling magnetic excitations and entanglement using
Ultrafast laser
Y. Wang

Clemson University, Clemson, SC 29634
The rapidly evolving quantum science calls for precise and predictive control of collective electronic
properties beyond the classical realm. Among various control knobs, an ultrafast laser pump is apromising approach due to its rich degrees of freedom. Together with the laser's capability of
influencing electronic structure comes the necessity to track the instantaneous status of a timedependent nonequilibrium material. Novel pump-probe spectral techniques play a critical role inbridging experiments and theory and quantifying the nonequilibrium state of materials.

Fig. 1. (a): Collective spin excitations measured by trRIXS in a 2D doped Mott insulator, reflecting the lightinduced softening of paramagnons. The dashed line tracks the evolution of the peak position in time, following
the pump pulse (white line). (b) The comparison of the transient paramagnon energy (circles) at the center ofpump pulse with the Floquet-linear-spin wave theory (solid line). (c) The trRIXS measurement of a quarterly
filled 1D cuprate chain. (d) Dynamics of quantum Fisher information revealed from trRIXS, witnessing abipartite-entangled state induced by laser.
In this talk, I will discuss the application of time-resolved resonant inelastic x-ray scattering (trRIXS) inmagnetic materials [see Fig.1(a)]. By comparing the trRIXS for antiferromagnetic materials, we find that the
instantaneous paramagnon excitations can be manipulated by pulsed laser in a predictive manner, following
the Floquet theory in the center of the pulse [see Fig.1(b)]. Such a Floquet engineering works only at doped
Mott insulators without long-range order, while the trRIXS study of an undoped Mott insulator violates the
Floquet approximation. Leveraging the light-engineered magnetic excitations, we further study thenonequilibrium dynamics of a 1D cuprate chain [see Fig.1(c)]. Through a self-consistent iteration, trRIXS canprobe the transient entanglement of wavefunctions in nonequilibrium materials via the quantum Fisher
information. Via this approach, we reveal the possibility of enhancing entanglement in a cuprate chain using
an ultrafast laser pulse [see Fig.1(d)].
References[1] M. Mitrano, Y. Wang. Communications Physics 3, 184 (2020).
[2] Y. Wang, Y. Chen, T.P. Devereaux, B. Moritz, M. Mitrano, Communications Physics 4, 212 (2021)[3] J. Hales, U. Bajpai, T. Liu, D.R. Baykusheva, M. Li, M. Mitrano, Y. Wang, arXiv:2209.02283 (2022)* Acknowledgement(s) : Y.W. acknowledges support from U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Early
Career Award No. DE-SC0022874.

Ultrafast spin excitations in 3d and 4f metals
M. Weinelt
Freie Universität Berlin, 14195 Berlin, Germany

Understanding ultrafast spin dynamics is not only a complex and fascinating challenge infundamental physics, but carries the potential for magnetic recording based on all-optical switching(AOS) of the magnetic order [1]. AOS triggered by a single fs laser pulse was first observed in theferrimagnetic alloy FeCoGd and more recently in Pt/Co/Gd stacks [2-4]. The key to AOS isexchange of angular momentum between the oppositely aligned magnetic moments of the transition
metal and rare earth sublattices [5,6]. Spin transport and exchange scattering are discussed as
microscopic processes responsible for spin dynamics and all-optical switching. However, it remains
debated to what extent ultrafast magnetization dynamics generates spin currents and vice versa. We
will highlight a few examples of the signature of spin dynamics in the electronic structure in 3d and
4f metals and the role of spin transport. In particular, we use time- and spin-resolved photoemission
spectroscopy to study an antiferromagnetically coupled Gd/Fe bilayer, a prototype system for alloptical switching. Spin transport leads to an ultrafast drop of the spin polarization at the Gd surface
demonstrating angular-momentum transfer over several nanometers. Thereby Fe acts as spin filter,
absorbing spin majority but reflecting spin minority electrons. Spin transport from Gd to Fe wascorroborated by an ultrafast increase of the Fe spin polarization in a reversed Fe/Gd bilayer. In
contrast, for a pure Gd film spin transport into the tungsten substrate can be neglected as spinpolarization stays constant.

Fig. 1. A) Spin transport in Gd|Fe on W(110) and Fe|Gd on W(110) revealed by spin- and time-resolved
photoelectron spectroscopy. B) Spin-filter effect.

Our results suggest that ultrafast spin transport drives the magnetization dynamics in Gd/Fe andreveal microscopic insights into ultrafast spin dynamics [8].
References[1] V. Kimel, M.Li, Nature Review Materials 4, 189 (2019).
[2] C. D. Stanciu F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, Th. Rasing, Physical Review Letters 99, 047601(2007);[3] T. A. Ostler, J. Barker, R. F. L. Evans, R. W. Chantrell, U. Atxitia, O. Chubykalo-Fesenko, S. El Moussaoui, L. Le Guyader
E. Mengotti, L. J. Heyderman, F. Nolting, A. Tsukamoto, A. Itoh D. Afanasiev, B. A. Ivanov, A. M. Kalashnikova
K. Vahaplar, J. Mentink, A. Kirilyuk, Th. Rasing, and A. V. Kimel, Nature Communications 3, 666 (2012).
[4] M. Beens M. Beens, M. L. M. Lalieu, A. J. M. Deenen, R. A. Duine, B. Koopmans, Physical Review B 100, 220409 (2019).
[5] I. Radu, K. Vahaplar, C. Stamm, T. Kachel, N. Pontius, H. A. Dürr, T. A. Ostler, J. Barker, R. F. L. Evans, R. W. Chantrell
A. Tsukamoto, A. Itoh, A. Kirilyuk, Th. Rasing, A. V. Kimel, Nature 472, 205 (2011).
[6] C. E. Graves, A. H. Reid, T. Wang, B. Wu, S. de Jong, K. Vahaplar, I. Radu, D. P. Bernstein, M. Messerschmidt, L. Müller
R. Coffee, M. Bionta, S. W. Epp, R. Hartmann, N. Kimmel, G. Hauser, A. Hartmann, P. Holl, H. Gorke, J. H. Mentink
A. Tsukamoto, A. Fognini, J. J. Turner, W. F. Schlotter, D. Rolles, H. Soltau, L. Strüder, Y. Acremann, A. V. Kimel
A. Kirilyuk, Th. Rasing, J. Stöhr, A. O. Scherz, H. A. Dürr, Nature Materials 12, 293 (2013).
[7] B. Frietsch, A. Donges, R. Carley, M. Teichmann, J. Bowlan, K. Döbrich, K. Carva, D. Legut, P. M. Oppeneer, U. Nowak
M. Weinelt, Science Advances 6, eabb 1601 (2020).
[8] B. Liu, H. Xiao, M. Weinelt, Science Advances, (submitted) (2023).
* Acknowledgement: author acknowledges support from Deutsche Forschungsgemeinschaft, CRC/TRR 227 - Ultrafast Spin Dynamics, ProjectA01, and from Bundesministerium für Bildung und Forschung, grant: 05K22KE2

Ultrafast electron and spin dynamics in NiO upon above
Bandgap excitation
K. Gillmeister1, D. Golez2, Y. Pavlyukh3, J. Berakdar1, P. Werner2, W. Widdra1
1
Martin-Luther-Universität Halle-Wittenberg, Halle, Germany
2
University of Fribourg, Fribourg, Switzerland
3
Technische Universität Kaiserslautern, Kaiserslautern, Germany

Charge excitation across an electronic band gap plays an important role in optoelectronics and lightharvesting. In strongly correlated materials, such as NiO, studies of above-band-gap photoexcitation
are still in their infancy, despite their use in spintronics. Here we present a combined scanningtunnelling microscopy (STM), scanning tunnelling spectroscopy (STS), and time-resolved twophoton photoemission (2PPE) study for ultrathin films of NiO(001) to address the electronic
response upon optical excitation [1-4].

Fig. 1. Left: NiO band structure and excitation scheme. Right: Time-resolved 2PPE intensity of the in-gap
states upon excitation across the bandgap. The frequency of the oscillations corresponds to the Ni-O-Ni superexchange energy.

At the surface of epitaxial NiO(001) films grown on Ag(001), we find series of well-defined image
potential states below the vacuum level with film thickness dependent lifetimes in the range from 30
to 120 fs [2]. In contrast to these rather long lifetimes, we find an ultrafast (<10 fs) relaxation forelectrons that are excited just across the charge-transfer gap into the conduction band. The latterforms the upper Hubbard band of a charge-transfer insulator NiO as sketched in Fig. 1 [3]. We
identified the initial optical excitation into the upper Hubbard band which is followed by an ultrafast
relaxation into long-lived many-body in-gap states. Remarkably, the spectral weight of these in-gap
states displays coherent THz oscillations up to 2 ps at low temperature as shown in the right part of
Fig.1 [3]. The frequency of these oscillations corresponds to the strength of the antiferromagnetic
superexchange interaction in NiO and their lifetime vanishes slightly above the Néel temperature.
These observations indicate a strong coupling of the excited states to the antiferromagnetic spin
system and pave the way for addressing antiferromagnetic spin-spin correlation in oxides on theultrafast time scale [3].
References[1] K. L. Kostov, F. O. Schumann, S. Polzin, D. Sander, W. Widdra, Physical Review B 94, 075438 (2016).
[2] K. Gillmeister, M. Kiel, W. Widdra, Physical Review B 97, 085424 (2018).
[3] K. Gillmeister, D. Golež, C.-T. Chiang, N. Bittner, Y. Pavlyukh, J. Berakdar, P. Werner, W. Widdra, Nature Communications 11, 4095 (2020).
[4] Y. W. Windsor, D. Zahn, R. Kamrla, J. Feldl, H. Seiler, C.-T. Chiang, M. Ramsteiner, W. Widdra, R. Ernstorfer, L. Rettig, Physical Review
Letters 126, 147202 (2021).
* Support by the Sonderforschungsbereich SFB/TRR-227 “Ultrafast spin dynamics” is gratefully acknowledged

Ultrafast dynamics probed by time-resolved and local
Spectroscopy at the nanoscale
M. Wolf
Fritz-Haber Institut der Max-Plank-Gesellschaft, 14195 Berlin, Germany

Interaction of many-body systems with ultrashort light pulses may lead to emergent phenomena
away from equilibrium. Recent advances in time-resolved photoemission spectroscopy (trARPES)
combined with momentum microscopy allow direct probing of excited states throughout thecomplete Brillouin zone. 2D materials and their heterostructures provide a vast playground toobtain a mechanistic understanding of exciton dynamics and energy transfer into nuclear motions.
Here we extend these studies and investigate the primary step of singlet fission in crystalline
pentacene using orbital tomography. Knowledge about the localization and orbital character of theexciton wave functions allows to decompose energetically overlapping states on the basis of theirorbital character [1]. Our results indicate a charge-transfer mediated mechanism with ahybridization of Frenkel and charge transfer states in the lowest bright singlet exciton.
Using scanning probe microscopy, extreme spatial resolution can be obtained by atomic-scale light
confinement in plasmonic “picocavities”. We show that inelastic light scattering is dramaticallyenhanced by forming a quantum point contact (QPC) on a single adatom in a well-defined
picocavity controlled precisely by low-temperature scanning tunneling microscopy (STM). Tip
enhanced Raman spectra from a single silver atom on Ag(111) in the QPC regime exhibit spectralfeatures originating from the interaction with both vibrations and electrons in the STM junction.
The atomic-scale light confinement is reproduced by TDDFT simulations including the vibrational
dynamics of the atomistic structures. Coherent phonons can provide microscopic insight into
ultrafast lattice dynamics and coupling to other degrees of freedom. We implement ultrafast laserinduced STM applied to coherent phonon spectroscopy (see Fig.1). Excitation and relaxation ofcoherent phonons may be susceptible to the local nanoscale environment, calling for real-spaceobservation of ultrafast lattice dynamics. We demonstrate nanoscale local coherent phonon
spectroscopy employing time-resolved STM in a plasmonic junction, and unveil spatial inhomogeneities of coherent phonon dynamics in ultrathin zinc oxide (ZnO) films with few nanometerspatial and femtosecond temporal resolution. The coherent phonons in ZnO are locally excited by
the tightly-confined gap plasmon, and are probed via the photoinduced tunneling current through anelectronic resonance of the ZnO film.

Fig. 1. Ultrashort pulsed laser-induced STM in a plasmonic junction on Ag(111), 2-ML and 3-ML ZnO,
respectively. Main panel: Interferrometric autocorrelation (IAC) trace of the photocurrent for a Ag tip–
Ag(111) junction. The right inset shows FT spectra of the IAC traces at different sample positions on ZnO(with 2 nm steps).

In combination with tip-enhanced Raman spectroscopy and scanning tunneling spectroscopy, we
reveal the involved phonon modes and a correlation of the local electronic structure with thecoherent phonon dynamics [2].
References[1] A. Neef, S. Beaulieu, S. Hammer, S. Dong, J. Maklar, T. Pincelli, P. Xian, M. Wolf, L. Rettig, J. Pflaum, R. Ernstorfer, Nature 616, 275 (2023).
[2] S. Liu, A. Hammud, I. Hamada, M. Wolf, M. Müller, T. Kumagai, Science Advances 8, eabq5682 (2022).
* Acknowledgements: This work was performed in collaboration with Alexander Neef, Samuel Beaulieu, Sebastian Hammer, Shuo Dong, Julian
Maklar, Tommaso Pincelli, R. Patrick Xian, Laurenz Rettig, Jens Pflaum, Ralph Ernstorfer and Shuyi Liu, Takashi Kumagai, Melanie Müller,
Franco Bonafe, Heiko Appel, Angel Rubio

Propagation of extreme pulsed light:
First-principles computational study
K. Yabana
University of Tsukuba, Tsukuba 305-8577, Japan

When describing the interaction between intense ultrashort laser pulses and solids theoretically, two
aspects need to be considered: The strong electric field of the light pulse induces highly nonlinear
and transient electron dynamics in the solid that cannot be described by perturbation theory. Thisnonlinear electron motion then manifests itself as nonlinear polarization on a macroscopic scale anddominates the propagation of intense pulsed light. Therefore, to understand the interaction between
significantly intense pulsed light and matter, especially its propagation, it is necessary to consider the
coupling between the dynamics of the optical electromagnetic field and the nonlinear dynamics ofthe electrons. The spatial scale of the two dynamics also needs to be taken into account: forinteractions with two-dimensional atomic layer materials and very thin films, microscopic
electromagnetics that varies on the atomic scale is effective, while for sufficiently thick films andbulk surfaces, macroscopic electromagnetics with coarse-graining is required. Based on the firstprinciples
time-dependent
densityfunctional
theory(TDDFT), we have developeda theoretical and computational
method for the interaction of
intense ultrashort pulsed lightwith solids [1,2], which we callthe Maxwell - TDDFT
calculation
method.
Thismethod has been implemented
in the open-source software
SALMON developed by theauthors' group [3].
Figure 1(a) summarizes theMaxwell-TDDFT calculation
method with coarse-graining
for the case of pulsed lightpropagating through a thinfilm. One-dimensional lightpropagation in the thin film is
described by the wave
equation for a vector potential
using uniform grid points intheZ-axiscoordinates;
electron motion is considered
at each grid point in Z anddescribed by a time-dependent
Bloch orbital function. The
time evolution of that Blochorbital is described by thetime-dependent
Kohn-Shamequation,
which
is
the
fundamental
equationof
TDDFT. Thus, by solving theMaxwell equations and thefundamental equations of
TDDFT
jointlyandsimultaneously,
lightpropagation
andelectron
dynamics can be describedsimultaneously. This methodwith coarse-graining is called
the Maxwell-TDDFT multiscaleFig. 1. Overview of the theoretical methodmethod because it uses two lattice systems with different resolutions [1]. We have also developed amethod for the microscopic Maxwell equations using a single lattice, which we call the MaxwellTDDFT single-scale method [2]. Figures 1(b) and 1(c) show typical calculations of pulsed lightpropagating through a 3m-thick silicon (Si) thin film. Figure 1(b) shows the electric field of theinitial pulse located in front of the thin film; the thin film is shown as a light gray area. The averagefrequency of the pulse was set to 1.55 eV, which is below the direct band gap of Si.
The maximum intensity of the incident pulse was set to 5x1012 W/cm2. Figure 1(c) shows a snapshotof the electric field at 150 fs. The results corresponding to the initial pulses of two different
maximum intensities are shown here: the 5x1012 W/cm2 is shown by the solid red line, and the much
weaker pulse is shown by the dotted blue line. The latter is multiplied by a factor so that thedifference between the two lines manifests the nonlinear effect of the stronger pulse. It is observedthat the nonlinear effects are more pronounced in the transmitted pulse than in the reflected pulse.
The stronger transmitted pulse undergoes strong nonlinear effects with an almost flat envelope. Fromthese calculations, various physical information can be extracted. For example, as shown in the inset
of Fig. 1(c), the energy transfer from the light field to the electrons in the unit cell as a function of Z
provides useful information for understanding the initial stages of non-thermal laser processing [4].
Fourier spectra of reflected and transmitted pulses (RHHG and THHG) for strong incident pulses areshown in the inset of Figure 1(c). These show spectra of higher-order harmonic generation, a typicalThe light response of a material varies significantly with lightnonlinear phenomenon [5,6].
intensity. The response to weak light is described by the dielectric constant, which varies greatly
depending on whether the material is a dielectric or a metal, and in the case of dielectrics, on theband gap and the frequency of light. On the other hand, for very intense pulsed light, it is well known
that all materials exhibit plasma reflections. Using the Maxwell-TDDFT multiscale method, we
present the results of a systematic analysis of how the optical response of various materials changes
as the maximum intensity of the pulsed light increases. Figure 2 shows the calculated reflectance,
absorption, and transmittance of a 200 nm thick Si thin film normally irradiated with linearlypolarized pulsed light of various intensities, with the maximum intensity of the pulse on thehorizontal axis. At sufficiently low intensities, the optical response is linear and is described by adielectric function, as expected.
1.0
Reflection
0.8
0.6
0.4

Absorption

0.2
0Transmission
9

10

11

12

13
14
log(Imax)

15

16

17

Fig. 2. Reflection, absorption, and transmission rates of a Si thin film of 200nm thickness are plotted against
the maximum intensity of the pulsed light.

As the intensity increases, multiphoton absorption occurs, with the absorption showing a maximum
at about 1013 W/cm2. Above 1014 W/cm2, more electrons are excited at the leading edge of the pulse
and plasma reflection becomes dominant. However, the process from multiphoton absorption toplasma reflection is not monotonous, and complex changes are observed at intensities around 1015
W/cm2. At this intensity, the plasma frequency of the excited electrons coincides with thefundamental frequency of the incident pulse, resulting in the appearance of a minimum in thereflection. Such an analysis can be performed for any material understood by the band structure.
References[1] K. Yabana, T. Sugiyama, Y. Shinohara, T. Otobe, G.F. Bertsch, Physical Review B 85, 045134 (2012).
[2] S. Yamada, M. Noda, K. Nobusada, K. Yabana, Physical Review B98, 245147 (2018).
[3] M. Noda, S.A. Sato, Y. Hirokawa, M. Uemoto, T. Takeuchi, S. Yamada, A. Yamada, Y. Shinohara, M. Yamaguchi, K. Iida, I. Floss, T. Otobe
K.-M. Lee, K. Ishimura, T. Boku, G.F. Bertsch, K. Nobusada, K. Yabana, Computer Physics Communications 235, 356 (2019).
[4] A. Yamada, K. Yabana, The European Physical Journal D73, 1 (2019).
[5] S. Yamada, K. Yabana, Physical Review B103, 155426 (2021).
[6] S. Yamada, T. Otobe, D. Freeman, A. Kheifets, K. Yabana, Physical Review B107, 035132 (2023).

Ultrafast intrinsic optical-to-electrical conversion dynamics inGraphene investigated using on-chip THz spectroscopy
K. Yoshioka
NTT Corporation, 243-0198 Atsugi, Japan

With the anticipated exponential growth of data traffic, there is an imminent need for opticalreceivers that enable the conversion of optical signals into electrical signals while maintaining
ultrahigh bandwidth and low power consumption. Graphene-based photodetectors (PDs) utilizing thephotothermoelectric (PTE) effect have gained significant attention due to their potential tooutperform existing semiconductor devices [1]. However, despite their promising potential, thedemonstrated operational speed of these devices is limited to approximately 70 GHz [2], which falls
far short of the theoretical expectation of 200 GHz. Despite the crucial role of optical-to-electrical(O-E) conversion on the intrinsic timescale for the design of ultrafast graphene optoelectronic
devices, achieving this capability and understanding its underlying mechanism have yet to berealized.
Here, we showcase the ability to perform ultrafast electrical readout of PTE current in high-quality
graphene that is encapsulated in hexagonal boron nitride (hBN). By employing on-chip terahertz(THz) spectroscopy [3] and a zinc oxide (ZnO) gate structure that eliminates high-frequency
capacitive coupling [4], we successfully overcome the limitations imposed by the bandwidth ofreadout electronics and the large RC time constant of the device. Fig. 1(a) shows a schematic of our
on-chip THz spectroscopy setup

Fig. 1. a: Schematic of on-chip THz spectroscopy with ZnO gate structure to investigate
intrinsic O-E conversion dynamics in graphene. b: Fourier spectrum of obtained photocurrent
with the 3 dB bandwidth of 220 GHz. Inset shows a time-domain waveform.

Our graphene FET is connected to a photoconductive switch with the terahertz (THz) waveguide toread out photocurrent with sub-picosecond time resolution. To exclude the dominant RC time
constant from the large gate capacitance, we used the ZnO gate, which is transparent above the GHz
frequency range [4]. This enables us to operate graphene PD with its intrinsic timescale. As shown in
Fig. 1(b), the 3 dB bandwidth of our graphene PD reaches 220 GHz, which indicates that our
methodology is suitable for tracking the O–E conversion mechanisms in graphene.Furthermore, we
comprehensively explore the nonlocal ultrafast dynamics of the PTE current by adjusting parameterssuch as VGate, pump position, channel length, and graphene mobility. As a result, we have achieved aquantitative understanding of the O-E conversion processes [5]. We are convinced that our on-chip
ultrafast electrical readout approach removes the obstacle between device engineering and ultrafast
optical science by showing how to bring out the functionality based on a fundamental understandingof carrier dynamics in quantum materials.
References[1] M. Romagnoli, V. Sorianello, M. Midrio, F. H. L. Koppens, C. Huyghebaert, D. Neumaier, P. Galli, W. Templ, A. D’Errico, A. C. Ferrari, Nature
Review Materials 3, 392 (2018).
[2] S. Marconi, M. A. Giambra, A. Montanaro, V. Mišeikis, S. Soresi, S. Tirelli, P. Galli, F. Buchali, W. Templ, C. Coletti, V. Sorianello
M. Romagnoli, Nature Communications 12, 806 (2021).
[3] K. Yoshioka, N. Kumada, K. Muraki, M. Hashisaka, Applied Physics Letters 117, 161103 (2020).
[4] N. H. Tu, K. Yoshioka, S. Sasaki, M. Takamura, K. Muraki, N. Kumada, Communications Materials 1, 7 (2020).
[5] K. Yoshioka, T. Wakamura, M. Hashisaka, K. Watanabe, T. Taniguchi, N. Kumada, Nature Photonics 16, 718 (2022).

MILESTONES 2016–2023

Ultrafast Dynamics and Metastability
Ultrafast Bandgap Photonics

Physica Scripta – 2017

Focus: Ultrafast Bandgap Photonics
Guest Editors: Michael K. Rafailov and Luca Perfetti, 2017
http://iopscience.iop.org/journal/1402-4896/page/Focus-issue-on-Ultrafast-Bandgap-Photonics

Ultrafast Bandgap Photonics

Baltimore, 2016
http://spie.org/Publications/Proceedings/Volume/9835
Anaheim, 2017
http://spie.org/Publications/Proceedings/Volume/10193
Orlando, 2018https://spie.org/Publications/Proceedings/Volume/10638
Ultrafast Dynamics and Metastability 2017 Workshop
Washington, DC, 2018https://sites.google.com/a/georgetown.edu/ultrafast-dynamics-and-metastability-archive-2017/

Ultrafast Dynamics and Metastability & Ultrafast Bandgap Photonics 2019
V International Workshop
Washington, DC, April 2019

Ultrafast Bandgap Photonics: Dynamics and Metastability of Transient States
VI International Conference
St. Petersburg, 2019

Ultrafast Bandgap Photonics: Dynamics and Metastability of Transient States 2020
VII International Symposium
Hersonissos, Crete, Greece June 2022
Ultrafast Dynamics and Metastability & Ultrafast Bandgap Photonics 2021
VIII International Workshop

Washington, DC, November 2021

Ultrafast Dynamics and Metastability & Ultrafast Bandgap Photonics 2022
IX International Workshop
Washington, DC, November 2022

Ultrafast Bandgap Photonics: Dynamics and Metastability of Transient States 2023
X International Symposium
Hersonissos,Crete, June 04-10 2023
Ultrafast Bandgap Photonics & Ultrafast Dynamics 2023
XI International Workshop 2023