Optical detection and manipulation of spontaneous gyrotropic electronic order
Qiong Ma
Boston College
Typically, geometrical chirality is predefined by the chirallattice structure of a material, which is fixed on formation of
the crystal. By contrast, in materials with gyrotropic order,
electrons spontaneously organize themselves to exhibit
macroscopic chirality in an originally achiral lattice.
Although such order—which has been proposed as thequantum analogue of cholesteric liquid crystals—hasattracted considerable interest, no clear observation ormanipulation of gyrotropic order has been achieved so far.
Here we report the realization of optical chiral induction and the observation of agyrotropically ordered phase in the transition-metal dichalcogenide semimetal 1T-TiSe2.
We show that shining mid-infrared circularly polarized light on 1T-TiSe2 while coolingit below the critical temperature leads to the preferential formation of one chiral domain.
The chirality of this state is confirmed by the measurement of an out-of-plane circular
photogalvanic current, the direction of which depends on the optical induction.

1Giant c-axis nonlinear anomalous Hall effect in Td-MoTe2 and WTe2
Adam W. Tsen
University of Waterloo
While the anomalous Hall effect can manifest even without an external magnetic field,
time reversal symmetry is nonetheless still broken by the internal magnetization of thesample. Recently, it has been shown that certain materials without an inversion center
allow for a nonlinear type of anomalous Hall effect (NLAHE) even when retaining time
reversal symmetry. First, I will discuss our observation of an extremely large c-axis
NLAHE in the non-centrosymmetric Td phase of MoTe2 (as well as WTe2) without
intrinsic magnetic order, which is driven by asymmetric scattering. Application of higher
bias yields a record Hall angle and anomalous Hall conductivity. Then, I will discuss
some possible consequences of the NLAHE in these materials and their experimental
signatures.

2Nonlinear Optical Response of Topological Semimetals and its Application forHigh Performance Photodetection
Dong Sun
Peking University
The experimental manifestation of topological effects in bulk materials under ambient conditions,especially those with practical applications, has attracted enormous research interest. Weyl semimetals
and chiral Fermions supply ideal material platforms for such endeavors. The Berry curvature in thesematerials becomes singular at the node points, creating an effective magnetic monopole in the k-space.
Such singular topology can have profound impact on the nonlinear optical response of these materials
and it is directly applicable to photodetection. In this talk, we started with our experimental work on
the revealing of the singular topology in a type-II Weyl semimetal TaIrTe4 in the photo responses,
which are shown to be directly related to the divergence of Berry curvature. As a result of thedivergence of Berry curvature at the Weyl nodes, TaIrTe4 exhibits unusually large photo responsivityof 130.2 mA/W with 4-µm excitation in an unbiased field effect transistor at room temperature arisingfrom the third-order nonlinear optical response. We further elucidate such effect is applicable to highperformance photodetectors based on topological semimetals, especially for long wavelength (lowphoton energy) range. Some recent progresses along this direction from our group will be discussed.
This project has been supported by the National Natural Science Foundation of China (NSFC GrantNos. 11674013, 91750109, 12034001).

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Nonlinear terahertz emission spectroscopy of topological chiral multifold
semimetals
Liang Wu
University of Pennsylvania
The absence of mirror symmetry, or chirality, is behind striking natural phenomena foundin systems as diverse as DNA and crystalline solids. A remarkable example occurs whenchiral semimetals with topologically protected band degeneracies are illuminated withcircularly polarized light. Under the right conditions, the part of the generated
photocurrent that switches sign upon reversal of the light's polarization, known as thecircular photogalvanic effect (CPGE), is predicted to depend only on fundamental
constants. The conditions to observe quantization are non-universal, and dependon material parameters and the incident frequency. In my talk, I will discuss nonlinear
terahertz emission spectroscopy with tunable photon energy from 0.2 eV - 1.1 eV in thechiral topological semimetals CoSi [1,2] and RhSi[3]. Particularly, we identify a large
longitudinal photocurrent peaked at 0.4 eV reaching ∼ 550 𝜇𝜇𝜇𝜇/𝑉𝑉2 in CoSi, which is muchlarger than the photocurrent in any chiral crystal reported in the literature. Using firstprinciples calculations we establish that the peak originates from topological bandcrossings, reaching 3.3±0.3 in units of the quantization constant. Our calculations indicatethat the quantized CPGE is within reach in CoSi upon doping and increase of the hotcarrier lifetime. If time permits, I will also briefly introduce our recent work on nonlinear
optical studies on 2D antiferromagnets [4,5].
References:[1] Ni, et al. Nat. Comm. 12, 154 (2021)[2] Xu, et al. PNAS, 117, 27104 (2020).
[3] Ni, et al. npj Quantum Materials, 5, 96 (2020)[4] Ni, et al. Nature Nanotechnology 16, 782-787 (2021)[5] Ni, et al. Phys. Rev. Lett. 127, 187201 (2021)

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Geometry and topology in nonlinear optical responses of quantum materials
Naoto Nagaosa1,2
1

RIKEN Center for Emergent Matter Science (CEMS)
Department of Applied Physics, University of Tokyo

2Studies on optical responses of solids have the long history, and have been considered
to be well established. However, a new development has been on-going recently, whichexplores the geometric and topological nature of the electronic states in solids and its
crucial role in optical processes including those in nonequilibrium states.
In this talk, I will discuss the geometry and topology in the nonlinear optical responses,
which includes (i) shift and injection currents in noncentrosymmetric quantum materials
related to Berry phase [1],[2],[3], and (ii) Riemannian geometry in nonlinear opticalresponses [4], and (iii) topological spin textures in nonequilibrium state [5].
References[1] T. Morimoto and N. Nagaosa, Sci. Adv. 2, e1501524 (2016).
[2] H. Ishizuka and N.Nagaosa, Proc. Nat. Acad. Sci. 118 (10) e2023642118 (2021).
[3] J. Ahn, G.Y, Guo and N.Nagaosa, Phys. Rev. X 10, 041041[4] J. Ahn, G.Y, Guo, N.Nagaosa, A. Vishwanath, Nature Phys. to appear.
[5] X.X.Zhang and N.Nagaosa, preprint

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Nonlinear electric and optical responses in van der Waals nanomaterials
Toshiya Ideue
Department of Applied Physics, University of Tokyo
Nonlinear electric and optical responses such as bulk rectification effect and bulk
photovoltaic effect are unique functionalities of noncentrosymmetric materials and also a
powerful probe of band geometry or anomalous scattering process [1].
In this talk, I will review our recent works on the nonlinear electric and optical responses
in van der Waals nanostructures. Large bulk rectification effect has been observed inlayered polar crystals or trigonal superconductors, which reflect the spin-splitting band
structure or superconducting fluctuation/vortex dynamics [2,3]. We also found the giant
enhancement of bulk photovoltaic effect in symmetry engineered van der Waals
nanostructures including WS2 nanotubes and WSe2/Black phosphorus heterointerfaces[4,5]. Observed photovoltaic response can be well interpreted by a quantum geometricshift current.
These results indicate that van der Waals nanomaterials are ideal platform for realizing
large nonlinear electric/optical responses and also for exploring new emergent nonlinear
responses in solids.
[1] T. Ideue and Y. Iwasa, Annu. Rev. Condens. Matter Phys, 12, 201 (2021).
[2] T. Ideue et al. Nat. Phys. 13 578 (2017).
[3] Y. Itahashi et al. submitted.
[4] Y. J. Zhang, T. Ideue et al. Nature 570 349 (2019).
[5] T. Akamatsu, T. Ideue et al., Science 372, 68 (2021).

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Nonlinear photocurrent generation in halide and organic polar semiconductors
Masao Nakamura
RIKEN Center for Emergent Matter Science (CEMS)
Photoexcitation of materials lacking the spatial inversion symmetry gives rise to anelectric current without external bias, known as the bulk photovoltaic effect. A primary
origin of the bulk photovoltaic effect is recently revealed to be the shift current that arises
from the geometric phase of electron wave functions. In contrast to the conventional driftdiffusion current, shift current is expected to be non-dissipative due to the topologicalorigin.
We have experimentally elucidated unique features of shift current in a representative
ferroelectric semiconductor SbSI. We have demonstrated that shift current is quite robust
to the localization of carriers associated with the temperature variation andimpurity/defect scattering by employing an optimum electrode material, which is ahallmark of the dissipation-less nature [1, 2]. We have also revealed by time-resolved
terahertz emission spectroscopy that the shift current shows an ultrafast response for pulse
excitation [3].In addition to inorganic compounds like SbSI, we have explored the shift
current in organic ferroelectrics. An organic charge-transfer complex TTF-CA is revealed
to exhibit a giant shift current [4]. I will also talk about the recent progress in themolecular beam epitaxy of high-quality halide thin films toward the device application ofshift current photovoltaics [5, 6].
[1] M. Nakamura et al., Appl. Phys. Lett. 113, 232901 (2018).
[2] H. Hatada, M. Nakamura et al, PNAS 117, 20411 (2020).
[3] M. Sotome, M. Nakamura et al., PNAS 116, 1929 (2019).
[4] M. Nakamura et al., Nat. Commun. 8, 281 (2017).
[5] S. Inagaki, M. Nakamura et al., Appl. Phys. Lett. 118, 012103 (2021).
[6] T. Yasunami, M. Nakamura et al., Appl. Phys. Lett. 119, 243101 (2021).

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Novel in-gap bulk current rectification mechanisms
Inti Sodemann
Max Plank InstituteWe develop a unified formalism for current rectification processes of electronic Blochbands in response to oscillatory electric fields that accounts microscopically forrelaxation processes by including the coupling to a physical heat bath. This allows us todescribe the impact of inelastic energy relaxation processes on intra-band phenomena
such as the non-linear Hall effect and on inter-band phenomena such as shift and injection
current effects. Moreover, we describe the conditions under which these effects survive
even when the frequency of the impinging radiation is inside the optical absorption bandgap and discuss when these effects are allowed to produce a rectified current andperform work on an external circuit without violating the fundamental laws ofthermodynamics. These considerations will allow us to uncover novel facet of the nonlinear Hall effect as an efficient amplifier of circularly polarized light.

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