Mie-Resonant Nanophotonics and Metasurfaces
Yuri Kivshar
Nonlinear Physics Center, Research School of Physics, Australian National University, Canberra ACT 2601, Australia
E-mail: yuri.kivshar@anu.edu.au

Recent progress is metaphotonics is driven by the physics of Mie resonances excited in high-index dielectric
nanoparticles and voids created in dielectric media. This provides a novel platform for localization of light insubwavelength photonic structures and opens new horizons for metamaterial-enabled photonics, or metaphotonics.
Recently emerged field of Mie-resonant metaphotonics (also called "Mie-tronics") employs resonances in high-index
dielectric nanoparticles and dielectric metasurfaces and aiming for novel applications of the subwavelength optics andphotonics. High-index subwavelength resonant dielectric structures emerged recently as a new platform fornanophotonics [1]. They benefit from low material losses and provide a simple way to realize magnetic response whichenables efficient flat-optics devices reaching and even outperforming the capabilities of bulk components. Here we
aim to review the physics of Mie resonances and photonic bound states in the continuum (BICs) and their applications
in metaphotonics and metasurfaces, including enhancement of light-matter interaction for nonlinear and topological
metadevices, and the development of active nanophotonic devices and nanolasers.
Figure 1 display the major physical effects associate with the multipolar Mie resonances of individual dielectric
nanoparticles, and Fig. 2 shows examples of the Mie-resonant structures and some of their functionalities.

Figure 1. Mie resonances of individual dielectric nanoparticles and their interferences. (a) Schematic of a Si nanodisk and radiation patternsof its fundamental modes: electric dipole and quadrupole (ED and EQ, red), and magnetic dipole and quadrupole (MD and MQ, blue). (b)Characteristic scattering spectrum of a Si nanodisk excited by a normally incident plane wave. Each Mie mode is manifested with an asymmetricresonant peak with a Fano profile. The resonant wavelengths of electric and magnetic Mie resonances are shown with red and blue arrows,
respectively. (c) Mode interferences due to nonresonant mode overlap: the Kerker effect with increased forward directionality of the scattered field
and the anapole regime with the near-field enhancement and suppressed far-field scattered signal. (d) Formation of a quasi-BIC mode with increased
Q factor due to a strong coupling between two ED modes. Scattering spectra for different disk diameters. The spectra are shifted relatively by 0.35
μm2. The Q factor of neighbouring ED modes vs disk diameter.

Bound states in the continuum (BICs) are associated with the embedded eigenvalues, and they are of a considerable
scientific interest because they can provide an important physical mechanism for trapping electromagnetic energy inopen resonators for long time. In practice, BIC can be realized as quasi-BIC, being directly associated with the socalled supercavity mode, where both the Q factor and the resonance width become finite. BIC-inspired localization oflight makes it possible to realize high-Q modes in photonic structures such as nanoantennas and metasurfaces.
The author acknowledges a support from the Australian Research Council (grants DP200101168 and DP210101292).
Reference[1] K. Koshelev and Y. Kivshar, Dielectric resonant metaphotonics, ACS Photonics 8, 102-112 (2021).

Optical and Electronic Fourier Surfaces
Nolan Lassaline1, Raphael Brechbühler1, Deepankur Thureja2, Sander J. W. Vonk1,3, Korneel Ridderbeek1,
Martin Spieser4, Samuel Bisig4, Boris le Feber1, Thibault Chervy2,5, Daniel Petter1, Puneet A. Murthy2,
Armin W. Knoll6, Freddy T. Rabouw1,3, David J. Norris1
1. Optical Materials Engineering Laboratory, ETH Zurich, Zurich, Switzerland
2. Quantum Photonics Group, ETH Zurich, Zurich, Switzerland
3. Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, The Netherlands
4. Heidelberg Instruments Nano/SwissLitho, Zurich, Switzerland
5. NTT Research, Inc., Sunnyvale, California, USA
6. IBM Research – Zurich, Rueschlikon, Switzerland
E-mail: dnorris@ethz.ch

Gratings and holograms are patterned surfaces that tailor optical signals by diffraction. Despite the long history ofsuch structures, variants with remarkable functionalities continue to be discovered. Further advances could exploit
Fourier optics, which specifies the surface pattern that generates a desired diffracted output through its Fouriertransform. The required surface profile should contain a precise sum of sinusoidal waves, each with a well-defined
amplitude, spatial frequency, and phase, to shape the optical wavefront. However, because fabrication techniques
typically yield profiles with at most a few depth levels, complex “wavy” surfaces cannot be obtained, limiting thestraightforward mathematical design and implementation of sophisticated diffractive optics. In this presentation, we
will discuss a simple yet powerful approach to eliminate this design–fabrication mismatch by demonstrating opticalsurfaces that contain an arbitrary number of specified sinusoids [1]. Multicomponent linear gratings allow precise
manipulation of the dispersion, stopbands, and coupling of electromagnetic signals. More broadly, we analyticallydesign and accurately replicate intricate two-dimensional Moiré patterns, quasicrystals, and holograms, demonstratinga variety of previously impossible diffractive surfaces (Fig. 1a). Finally, we show that such patterns can be reduced tonanometer length scales, creating wavy Fourier surfaces for 2D electronics (Fig. 1b) [2]. Therefore, this approachprovides benefit for optical devices and emerging topics in photonics and 2D optoelectronics.

Fig. 1 Examples of optical and electronic Fourier surfaces. (a) Optical Fourier surface fabricated in silicon as a Moiré interference
pattern constructed from 12 overlapping sinusoids. (b) Electronic Fourier surface fabricated in a hexagonal boron nitride (hBN)flake as a Moiré interference pattern constructed from 9 overlapping sinusoids.

References[1] Lassaline, N., Brechbühler, R., Vonk, S. J. W., Ridderbeek, K., Spieser, M., Bisig, S., le Feber, B., Rabouw, F. T., Norris, D. J. 2020 Nature
582, 506–510.
[2] Lassaline, N., Thureja, D., Chervy, T., Petter, D., Murthy, P. A., Knoll, A. W., Norris, D. J. 2021 Nano Lett. 21, 8175–8181.

High Q-factor coupled Fabry-Perot plasmonic nanoresonator :description and applications
B. Fix1, J. Jaeck1,P. Bouchon1, N. Bardou2, B. Vest1 and R. Haïdar1
1

2

DOTA,ONERA,Université Paris Saclay F-91123 Palaiseau – FranceCenter for Nanoscience and Nanotechnology (C2N)-CNRS, Université Paris-Saclay, 91128 Palaiseau, France
baptiste.fix@onera.fr

Fabry-Perot (FP) like resonances have been widely described in nanoantennas. In the case of combination of nanoantennas, it has beenreported that each cavity behaves independently. Here, we evidence the interferences between two FP absorbing nanoantennas, which
has a strong impact on the optical behavior. While the resonance wavelength is only slightly shifted, the level of absorption reaches
nearly 100%. Moreover, the quality factor increases up to factor 7 and can be chosen by geometric design over a range from 11 to 75.
We first demonstrate thanks to a simple analytical model that this coupling can be ascribed to a double FP cavity resonance, with theunique feature that each cavity is separately coupled to the outer medium. Based on this principle, we experimentally illustrate theexistence of a high-Q factor resonance originating from the interference between two under-coupled ribbon-shaped nano Fabry-Perot.

The Fabry-Perot (FP) interferometer consists of two parallel highly reflecting mirrors leading dips in the reflection spectra [1].
The development of nanophotonics has given birth to FP-like nanoresonators [2]–[3], in which a guided mode in a metallic
cavity has a behavior that is well described by the FP formalism. The advantages of these nanoresonators are their compactness,and the possibility to combine them, usually with negligible influences on each other [4]–[6], to design broadband or multiband
resonances for applications ranging from thermal emission [7] to infrared detection[8]. Yet, they suffer from their low quality
factor.
In this letter, we introduce double cavity Fabry-Perot like nanoresonators in both vertical (slit) and horizontal (ribbon)
configurations. It gives birth to a spectacular effect where critical coupling, i.e. a zero of reflectivity, is obtained while,
independently, each of the two cavities is loosely-coupled to free space. Moreover, the quality factor of the double cavity
resonators is increased from 11 to 50, well beyond the limit for FP plasmonic nanoresonator [6].

Figure 1 Comparison between the reflectivity of two loosely-coupled single-nanogroove resonators and of a critically coupled resonatorcomposed of the two aforementioned cavities. The cavities are described in the insets (d=1µm, w=0.3µm, h1=0.62µm, h2=0.55µm). (2)Measured (dotted) and computed (plain) reflection spectra of (a) single-FP structures and (b) the coupled nano FP structure.

We developed an analytical model that confirms the multimirror FP behavior [9] and experimentally demonstrated the coupled
Fabry-Perot resonance on horizontal nanoresonators [10]. We extended the description to multi coupled Fabry-Perot resonant
structures with either 1D or 2D geometries and detailed some applications for non-linear emission, infrared photodetection and
THz filters.
The description of the coupled nano Fabry-Perot paves the way to the manipulation and the engineering of coupling in multiresonant nanostructures. Moreover, the resonators easily scaled geometry is also very promising for many practical applications
from the near-infrared to the mm wave domain.
REFERENCES
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X. Liu et al., Phys. Rev. Lett., vol. 104, no. 20, 2010.
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C. Koechlin et al., Opt. Express, vol. 21, no. 6, 2013.
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M. W. Knight et al., Science, 332, 6030, May 2011.

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Ultrafast infrared nano-imaging of far-from-equilibrium dynamics
Jun Nishida, Samuel C. Johnson, Peter T.S. Chang, Dylan M. Wharton, S.A. Dönges, O. Khatib,and Markus B. Raschke
Department of Physics and JILA, University of Colorado, Boulder, 2000 Colorado Ave, CO 80309, USA
E-mail: markus.raschke@colorado.edu

Quantum phase transitions, polaronic carrier transport, coherent vibrations driving singlet fission, or electronic energy
transfer in light-harvesting complexes emerge from the interplay of the elementary electronic, vibrational, andphononic quantum states. By exciting one of these degrees of freedom out of equilibrium and probing their dynamic
response, ultrafast spectroscopy disentangles mode-coupling and competing interactions that are otherwise convoluted in static spectroscopy. Ultrafast infrared nano-imaging has demonstrated access to ultrafast carrier dynamics on
the nanoscale in semiconductor, correlated-electron, or polaritonic materials. However, mostly limited to strong andshort-lived carrier and collective polaritonic excitations, the contrast obtained to date has remained insufficient toprobe with excited state specificity the broader range of long-lived excitations, which arise from many-bodyinteractions induced by strong perturbation among charge carriers, lattice phonons, or molecular vibrations.
Here, we demonstrate a generalizable approach to ultrafast nano-imaging based on non-degenerate heterodyne pumpprobe infrared scattering scanning near-field optical microscopy (HPP IR s-SNOM) with low repetition-rate
modulated excitation, which provides simultaneous space, time, frequency, and phase resolutions with high sensitivity(Fig. 1A) [1]. A modulated femtosecond pump pulse drives the system into an excited state, followed by infrared
heterodyne probing of the transient low-energy electronic and vibrational response. The induced third-order nanolocalized polarization is isolated by sideband lock-in detection and directly detected interferometrically in the timedomain, enabling ultrafast nano-imaging with high contrast even with low-repetition excitation. Excitation-modulated
HPP IR s-SNOM thus provides the time-resolved analogue of ground-state nano-FTIR spectroscopy, selectively
resolving the transient and nano-localized excited-state response.
As a representative application to quantum materials, we perform nano-imaging of the electron dynamics associatedwith the ultrafast photoinduced insulator-to-metal transition (IMT) in vanadium dioxide (VO2), resolving transientdomain dynamics (Fig. 1B) that is distinct from the established strain-induced heterogeneity in the thermally induced
transition. In another application to molecular materials, we directly resolve heterogeneity in polaron-cation couplingthat controls the photovoltaic response by probing the excited state vibrational dynamics in a triple cation perovskite(Fig. 1C-D). These examples show how heterodyne pump-probe nano-spectroscopy and -imaging with low-repetition
allows for ultrafast nano-imaging of elementary processes in quantum and molecular materials in space and time.

Fig. 1. (A) Ultrafast pump-probe nano-imaging with far-from equilibrium excitation and interferometric heterodyne near-field
detection (HPP IR s-SNOM) for quantitative excited state contrast. (B) Example of nano-imaging the ultrafast insulator-to-metaltransition in VO2. (C,D) Ultrafast local probe nano-spectroscopy of triple-cation perovskite resolving excited state vibrational
dynamics. (E) Ultrafast movie resolving associated transient heterogeneity of polaron-lattice dynamics.

References[1] J. Nishida et al. “Ultrafast infrared nano-imaging of far-from-equilibrium carrier and vibrational dynamics”, Nature Commun. (in press).

Nonlinear nano-imaging of few-fs coherent dynamics in 2D graphenesemiconductor heterostructures
Wenjin Luo1,2, Renkang Song2, Benjamin G. Whetten1, Xinbin Cheng2, Alexey Belyanin3, Tao Jiang2,*, andMarkus B. Raschke1,*

1. Department of Physics, and JILA, University of Colorado, Boulder, CO 80303, USA.
2. MOE Key Laboratory of Advanced Micro-Structured Materials, Shanghai Research Institute for Intelligent Autonomous Systems, and Schoolof Physics Science and Engineering, Tongji University, Shanghai, China.
3. Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA.
E-mail: tjiang@tongji.edu.cn; markus.raschke@colorado.edu

Layer-stacked two-dimensional (2D) heterostructures open new avenues for the design of a wide range of novel
photonic and quantum devices with properties determined by orientation-dependent interlayer coupling, band structureengineering, and sensitive external electric, magnetic, and optical control. However, the underlying mechanism ofcoupling, charge and energy transfer, coherent dynamics, population lifetimes, and their sensitivity with respect tospatial heterogeneities at defects, edges, and grain boundaries, are still poorly understood.
Here, we demonstrate tip-enhanced nonlinear nano-imaging with grating-coupled plasmonic nanofocusing, providing
access to the desired few-nanometer spatial and few-femtosecond temporal regimes to separate competing intra- andinterlayer processes, as illustrated in Fig. 1a [1-3]. In simultaneous nonlinear coherent broadband four-wave mixing(FWM) and incoherent 2-photon photoluminescence (2PPL) measurements, we probe graphene and WSe2 monolayers
and their heterostructures as shown in Fig. 1b-e. The observed nano-localized 2PPL and FWM spectra in WSe2 andWSe2/graphene contrast that of a pure FWM response for graphene with distinct underlying near-field-coupledenhancement and quenching of FWM and 2PPL, as well as associated effects on the coherent electron dynamics. Nearfield (NF) images (Fig. 1f-g) of monolayer (G1), bilayer (G2), and trilayer graphene (G3), and the corresponding
graphene-WSe2 heterostructures (W/G1, W/G2, W/G3) exhibit significant nanoscale heterogeneity not observed in
far-field (FF) imaging. In addition, the FF and NF FWM responses differ with respect to their scaling with increasinggraphene layer number as shown in Fig. 1h. Despite an appreciable FWM response from WSe2 itself, we consistently
observe a decrease in FWM intensity of the graphene-WSe2 heterostructure compared to the FWM signals fromcorresponding homogeneous materials which suggests interlayer coupling induces decoherence. Moreover, acorresponding decrease in the WSe2 2PPL signal with increasing graphene layer number suggests fast interlayerinduced non-radiative quenching that is further enhanced by the metallic tip-graphene pico-cavity.

Fig. 1 (a) WSe2/graphene nonlinear 2PPL and FWM nano-imaging, with corresponding pathways in WSe2 and graphene (b,c). (d) Nano-2PPL and-FWM spectra in WSe2, graphene, and their heterostructure. (e) Power dependence of 2PPL and FWM in monolayer WSe2. (f-g) FWM and 2PPL
images of the heterostructure. (h) Layer dependence of FWM intensities. (i-j) Coherence measurement of the inhomogeneous region (i) andhomogenous region (j), showing a fast decoherence time of ~10 fs and a slower decoherence time of > 80 fs, respectively.

In addition, in FWM nano-imaging of monolayer WSe2 itself, we observe distinct spatial heterogeneity with coherence
times ranging from ~10 fs (Fig. 1i) in regions of high intensity to values exceeding 80 fs in regions with lower intensity(Fig. 1j). In contrast, our previous study of electronic coherence in monolayer graphene showed ultrafast dephasingof 6 ± 1 fs [4]. We interpret this counterintuitive effect as the result of defect scattering in localized states with higher
nonlinear susceptibility yet faster dephasing time compared to structurally homogeneous WSe2.
In summary, we correlate strong interlayer coupling, which gives rise to many of the unique excitonic and coherence
properties of heterostructures, with nano-optical properties and their sensitivity to defects and heterogeneities. Thesefindings are particularly significant for the development of optoelectronic devices relying on quantum coherence.
References[1] M. A. May, et al. “Nanocavity clock spectroscopy: Resolving competing exciton dynamics in WSe2/MoSe2 heterobilayers,” Nano Lett. 21, 522(2020).
[2] V. Kravtsov, et al. “Plasmonic nanofocused four-wave mixing for femtosecond near-field imaging,” Nat. Nanotechnol. 11, 459 (2016).
[3] V. Kravtsov, et al. “Enhanced third-order optical nonlinearity driven by surface-plasmon field gradients,” Phys. Rev. Lett. 120, 203903 (2018).
[4] T. Jiang, et al. “Ultrafast coherent nonlinear nanooptics and nanoimaging of graphene,” Nat. Nanotechnol. 14, 838 (2019).

Diffusion and lifetime of optical phase singularities
Thijs van Gogh1, Kobus Kuipers1
1. Kavli Institute of Nanoscience Delft, Lorentzweg 1, 2628 CJ, Delft, The Netherlands
E-mail: m.a.vangogh@tudelft.nl

In general, the spatial structure of light fields contains exceptional points that exhibit singular behaviour. At thesepoints, a parameter of the describing field becomes undefined, resulting in a distinct topology of the surrounding
light field. Such singularities are ubiquitous in nature and occur in a plethora of systems, such as in black holes andat the bottom of a swimming pool. In light, these singularities can for instance manifest themselves as phasesingularities, where the phase becomes undefined, or as points where the in-plane Poynting vector vanishes [1].
Random waves are arguably the most generic composition of a light field and their investigation yields insight inthe basic behaviour of light itself. With our home-built phase- and polarization-resolving optical near-fieldmicroscope we experimentally study the temporal evolution of a random light field confined to 2D. We locate phasesingularities in space and track their motion as a function of time. As they are topological in nature, they can only becreated and annihilated in pairs of opposite topological charge. We can track their creation, movement and,ultimately, their annihilation in time and use concepts from statistical physics, such as the mean-square displacement,in order to study their diffusive properties. Additionally, the time-resolved data allows us to determine a mean
lifetime of the singularities. We show that there is a significant difference in behaviour between faithful singularities-those that are annihilated with their birth partner-, and unfaithful singularities, which annihilate with a different
partner. In Fig. 1a, we show the mean squared displacement for both faithful and unfaithful singularities. It is clearthat unfaithful singularities diffuse further on average before being annihilated. In Fig. 1b we show the decay inpopulation as a function of time. Again it is clear that unfaithful singularities live longer than their faithfulcounterpart. Furthermore, the decay shows a non-exponential decay for the faithful singularities, while the unfaithful
ones show a decay that more closely resembles exponential decay.
Fig. 1 Differences between faithful (black dots) and unfaithful (red dots) singularities. (a) Shows the mean squared displacement(MSD) as a function of time. The unfaithful singularities consistently show a higher MSD than the faithful ones. (b) Shows thedecline in population as a function of time. The unfaithful singularities show a much slower decline in population than the faithful
ones.

References[1] Van Gogh, M.A., Bauer, T., De Angelis, L,. and Kuipers, L. 2020, Opt. Lett., 45, 9: 2600-2603

Ultrafast electron probing of plasmon thermal dynamics
Eduardo Dias1, Vahagn Mkhitaryan1, Javier García de Abajo1,2
1. ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
2. ICREA - Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spain
E-mail: eduardo.dias@icfo.eu

Thermal engineering of plasmons and other forms of polaritons in nanomaterials offers an appealing way ofcontrolling light−matter interactions down to nanometer and femtosecond spatiotemporal scales. The study of ultrafast
thermal dynamics traditionally relies on optical pump-probe experiments which are, however, limited in spatial
resolution due to light diffraction. Electron energy-loss spectroscopy (EELS) performed on scanning transmission
electron microscopes overcomes the optical diffraction limit by using electrons rather than light to map the material
response with sub-Ångstrom spatial precision. In this work [1], we propose to study the ultrafast thermal dynamics ofnanomaterials using an optical-pump/electron-probe (OPEP) technique with sub-Ångstrom and femtosecond
spatiotemporal resolution.
Two-dimensional (2D) materials offer a splendid testbed for OPEP because they generally undergo substantialchanges in their electronic structure under optical pumping. We start by applying this technique to highly doped selfstanding graphene, which hosts electrically tunable plasmons. The graphene sample is optically pumped with anultrafast laser (Figure 1a), which creates an elevated electronic temperature in the material that decays over apicosecond time scale. When probed with a delayed electron pulse, the graphene plasmon dispersion can be mappedout from the energy- and angle-resolved inelastically scattered electron distribution (Figure 1b,c). The dynamics ofrapid heating and cooling of graphene electrons is traced through the delay-dependent variations observed in the
distribution of scattered electrons.
We further show that the OPEP technique can be applied to doped graphene and undoped graphite ribbon samples.
Interestingly, ribbons break translational invariance and produce lateral plasmon confinement, resulting in a
discretization in electron deflection. Finally, we also show that the described excitations and their temporal dynamics
can be revealed by integrating the inelastic electron signal over a broad energy range, thus avoiding the need for highly
monochromatic electron beams and precise spectrometers.
This previously unexplored approach to study the ultrafast dynamics of optical excitations can be of interest tounderstand and manipulate polaritons in 2D semiconductors and other materials exhibiting a strong thermo-opticalresponse.

Fig. 1 OPEP characterization of plasmons in highly doped extended graphene. (a) Schematic representation of a graphene layer
and OPEP configuration, with electrons impinging normal to the plane of the sample. (b, c) Momentum- and energy-resolved loss
probability ΓEELS (𝑘𝑘∥ , 𝜔𝜔) for 100 keV transmitted electrons, revealing features associated with plasmon excitation in the sample
for (b) low (300 K) and (c) high (3000 K) electronic temperature regimes. The graphene Fermi energy is 𝐸𝐸F = 0.2 eV and the
intrinsic damping is ℏ𝜏𝜏 −1 = 4 meV.

References[1] V. Mkhitaryan, E.J.C. Dias, F. Carbone, and F.J. García de Abajo, Ultrafast Momentum-Resolved Free-Electron Probing of OpticallyPumped Plasmon Thermal Dynamics, ACS Photonics 8, 614 (2021).

Phase mapping of the scattering signal from single plasmonic nanoparticles
Alfred J. Meixner*1, Frank Wackenhut1, Florian Laible2, Monika Fleischer2 and Kai Braun1
1. Institute of Physical and Theoretical Chemistry and Center LISA+, University of Tuebingen, Auf der Morgenstelle 18, 72076 Tuebingen
Germany
2. Institute for Applied Physics, University of Tuebingen and Center LISA+, Auf der Morgenstelle 10, 72076 Tuebingen Germany
E-mail: alfred.meixner@uni-tuebingen.de

The interest in gold nanoparticles (GNPs) optical sensors is unbroken due to their unique optical properties which
depend on their size, shape, and the interaction with their local chemical environment [1]. The distance dependent
electronic coupling of dimers or trimers leads to a spectral shift of their resonance [2] and can be controlled by a DNA
linker known as DNA-origami [3] or it is used to estimate nanoscale inter-particle, a concept known as plasmon ruler[4]. Single particle sensing is usually performed by recording white light spectra or luminescence spectra. The
scattering cross section scales as the square of the particle volume and decreases drastically for small particles, also
the luminescence quantum efficiency is low. An interesting alternative provides the interference of the elasticallyscattered signal with a strong reference which can easily be recorded by confocal interference microscopy [5]. For
particles much smaller than the focal volume the signal collected by the objective is the sum of the elastically scattered
background and the signal scattered by the particle and the contrast depends on their relative phase difference [6]. We
have set out to investigate the phase of the scattered signal of individual GNPs with an inverted confocal microscope
combined with a scanning Michelson interferometer and show that the phase shift caused by single gold nanorods withdifferent aspect ratios can be reliably determined and related to shift of the particles plasmon resonance [7].
Furthermore we show that the phase change of the signal scattered of a dimer can be observed very sensitively when
the gap distance is slightly changed [8]. Our work contributes to a better understanding of confocal scatteringinterference microscopy and provides new insight in single nanoparticle sensing.

Fig. 1 (a) Confocal interference microscope to measure the phase scattered by a single gold nanoparticle. (b, c) Spectral shift ofthe luminescence maximum together with the respective phase shift of the elastically scattered signal as a function of the gap
distance in a gold nanoparticle dimer.

References

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Link S, El-Sayed M A. 1999. J Phys Chem B. 103. 8410.
Jain P K, El_Sayed M A. 2008. J Phys Chem C. 112, 4954.
Dass M, Gür F N, Kołątaj K, Urban M J, Liedl T. 2021. J Phys Chem C. 125, 4969.
Laible F, Horneber A, Fleischer M. 2021. Adv Opt Mat. 9, 2100126.
Taylor R W, Sandogdar V. 2019. NanoLett. 19, 4827.
Failla A V, Qian H, Hartschuh A, Meixner A J. 2006. NanoLett. 6, 1374.
Hauler O, Wackenhut F, Jakob L A, Stuhl A, Laible F, et al. 2020. Nanoscale. 12, 1083.
Hauler O, Jakob L A, Braun K, Laible F et al. 2021. J Phys Chem C. 125, 6486.

Quantitative Phase Imaging for Nanophotonics
Guillaume Baffou

Institut Fresnel, CNRS, Aix Marseille University, Centrale Marseille, Marseille, France
guillaume.baffou@fresnel.fr

Quadriwave lateral shearing interferometry (QLSI) is a quantitative phase microscopy technique that enables the
mapping of the wavefront distortion of a light beam with both high spatial resolution and high sensitivity [1]. This
contribution is aimed at explaining how QLSI can benefit the field of Nanophotonics, namely, (i) how QLSI can beused as a temperature microscopy technique to map the temperature of gold nanoparticles under illumination [2],enabling applications in physics [3], chemistry [4] and biology [5] at the nano/microscale; (ii) how QLSI can map thecomplex optical conductivity and complex refractive index of 2D materials [6] ; (iii) how QLSI can retrieve thecomplex optical polarizability of nanoparticles, along with the extinction, scattering and absorption cross section froma single interferogram image [7,8], and finally (iv) how QLSI can fully characterize metasurfaces.

Fig. 1 Examples of intensity and optical-path-difference (OPD) images acquired using QLSI. From left to right: an assembly ofgold nanoparticles under illumination, creating a localized, radial temperature gradient ; a MoS2 flake ; a single 100-nm gold
nanosphere and a metalens.

References[1] Baffou, G. J. Phys D : Appl. Phys. J. Phys. D: Appl. Phys. 54, 294002 (2021)[2] Baffou, G. ; Bon, P. ; Savatier, J. ; Polleux, J. ; Zhu, M. ; Merlin, M. ; Rigneault, H. ; Monneret, S. 2012. ACS Nano 6, 2452-2458[3] Baffou, G. ; Berto, P. ; Bermúdez Ureña, E. ; Quidant, R. ; Monneret, S. ; Polleux, J. ; Rigneault , H. 2013. ACS Nano 7, 6478-6488 (2013)[4] Robert, H. M. L. ; Kundrat, F. ; Bermudez-Urena, E. ; Rigneault, H. ; Monneret, S. ; Quidant, R. ; Polleux, J. ; Baffou, G. 2016. ACS Omega 1,
2-8[5] Robert, H. M. L. ; Savatier, J. ; Vial, S. ; Verghese, J. ; Wattelier, B. ; Rigneault, H. ; Monneret, S. ; Polleux, J. ; Baffou, G. 2018. Small 14,
1801910[6] Khadir, S. ; Bon, P. ; Vignaud, D. ; Galopin, E. ; McEvoy, N. ; McCloskey, D. ; Monneret, S. ; Baffou, G. 2017 ACS Photonics 4, 3130-3139(2017)[7] Khadir, S. ; Chaumet, P. ; Baffou, G. ; Sentenac, A. 2019. J. Opt. Soc. Am. A 36, 478-484[8] Khadir, S. ; Andrén, D. ; Chaumet, P. C. ; Monneret, S. ; Bonod, N. ; Käll, M. ; Sentenac, A. ; Baffou, G. 2020. Optica 7, 243-248

Dielectric meta-optics for reconfigurable planar optics and biosensing
Romain Quidant1
1. Nanophotonic Systems Laboratory, ETH Zurich, Zurich, Switzerland
E-mail: rquidant@ethz.ch

In this talk, we present our latest advances in the development of novel nanophotonic platforms for both imaging
and biosensing.
In the first part, we introduce our most recent advances in the development of reconfigurable planar opticalelements, focusing on two original technologies. The first technology relies on dynamically controlling the distributionof refractive index by means of an engineered micro-resistor embedded in a thermo-optical material. We first showhow our approach can be used to create varifocal lenses with 100ms response time. When combined with machinelearning, this approach additionally enables to go beyond a simple lens and create complex phase fronts [1]. Our second
approach to reconfigurability relies on an optomechanical control. Upon illumination with a control beam, the metaatoms forming the lens mechanically rearrange, inducing a fast change of focus [2].
In the second part of the talk, we discuss the use of dielectric nanoresonators for biosensing and lab-on-a-chip
technology. In our approach, Si nanoresonators are integrated into a state-of-the-art PDMS microfluidic environment.
We first demonstrate that arrays of Si nanocylinders can be used for the specific detection of cancer markers in humanserum with sensitivity levels comparable to the one obtained with gold nanoantennas. We also show how dielectric
nanoresonators can benefit chiral molecular sensing, demonstrating enantio-selective differentiation with improved
performance over plasmonics. Finally, we discuss different novel directions toward improved sensing performanceand detection of emerging biomarkers [3].
References[1] R. Steffen et al, submitted (2022)[2] A. Afridi et al, submitted (2022)[3] J. Garcia Guirado et al, submitted (2022)

Hyperbolic Meta-Antennas for Light-Matter Interaction Engineering
Sema Ebrahimi 1,2,3, Ali Adawi 2,3, Anne-Laure Baudrion 1, Pierre-Michel Adam 1, Jean-Sebastien
Bouillard 2,3
1

Light, nanomaterials, nanotechnologies (L2n) Laboratory, CNRS EMR7004, University of Technology of Troyes, F-10004 Troyes Cedex,
France2
Department of Physics and Mathematics, University of Hull, Cottingham Road, HU6 7RX, UK
3
G.W.Gray Centre for Advanced Materials, University of Hull, Cottingham Road, HU6 7RX, UK
E-mail: sema.ebrahimi@utt.fr, s.ebrahimi-2019@hull.ac.uk

Hyperbolic Metamaterials are described as an emerging class of highly anisotropic plasmonic materials, providing
electromagnetic properties on-demand. Based on their hyperbolic optical dispersion curves, hyperbolic metamaterials
can support the propagation of high-k modes and represent a significant increase in the photonic density of states toengineer light-matter interactions [1, 2]. Here, we introduce a hyperbolic meta-antenna based on the multilayer Au/
TiO2 bowtie nanostructures, supporting a type Ⅱ hyperbolic dispersion. Effective medium theory (EMT) was used to
calculate the effective index and optical responses of the hyperbolic meta-antennas.
We provide a detailed study of the physical mechanisms underlying the meta-antenna optical response. Using
electric and magnetic field distributions, both in plane and out of plane, we identify two different modes supported by
the multilayer antennas. We find that one mode is purely scattering while the other has a purely absorptive nature (Fig.
1b). The main scattering mode originates from the bonding electric dipolar modes between the two elements of theantenna (Fig. 1e), similar to the pure gold corresponding antenna (Fig. 1d). On the other hand, the absorptive mode atlonger wavelengths is absent from the pure gold geometry and solely supported by the meta-antenna (Fig. 1f). Using
the field distributions, we propose that this second mode arises from the excitation of magnetic dipolar modes in themeta-antenna.
Additionally, by tuning the metal filling factor in the meta-antenna multilayer system, we obtain full control overthe intensity, spectral separation, and ratio of the scattering and absorption modes covering the visible and infrared
regions (Fig. 1c). The proposed hyperbolic meta-antennas are highly promising for applications in optical sensors, subwavelength meta-cavity lasers, photocatalysis, and hot-electron generation processes.

Fig. 1 Calculated Scattering and Absorption of (a) Single Au and (b) Multilayer Au/TiO2 bowtie nanoantennas. (c) Tunability ofthe scattering and absorption modes as a function of fill factor of Au in the visible and infrared regions. Near-field distribution of
the z-component of the electric field and related current density distributions of (d, e) the scattering and (f) absorption peaks related
to the Single Au and multilayer Au/TiO2 bowtie nanostructures, respectively.

References[1] Poddubny, A., Iorsh, I., Belov, P., & Kivshar, Y. (2013). Hyperbolic metamaterials. Nature photonics, 7(12), 948-957.
[2] Maccaferri, N., Zhao, Y., Isoniemi, T., Iarossi, M., Parracino, A., Strangi, G., & De Angelis, F. (2019). Hyperbolic meta-antennas enable fullcontrol of scattering and absorption of light. Nano letters, 19(3), 1851-1859.

Enhanced fluorescence emission in nano-imprinted rare-earth-doped photonic
metasurfaces
Michele Montanari1,2, Zeinab Chehadi3, Nicoletta Granchi1,2, Mehrnaz Modaresialam4, Mathieu Koudia3,
Mathieu Abel3, Magali Putero3, David Grosso3,4, Francesca Intonti1,2, Marco Abbarchi3,4
1. Department of Physics and Astronomy, University of Florence, Sesto Fiorentino 50019, Italy
2. LENS, University of Florence, Sesto Fiorentino 50019, Italy
3. Aix Marseille Univ, CNRS, Université de Toulon, IM2NP, UMR 7334, F-13397 Marseille, France
4. Solnil, Rue de la Republique, 13001 MarseilleE-mail: montanari@lens.unifi.it

Red light emission from Eu3+ doped phosphors is extensively used in lamp and display applications but theemployment of such materials in thin film applications is hampered by their small absorption cross-section. However,by fabricating a suitable two-dimensional photonic crystal (PC) on top of a doped thin layer, the PC dispersion can beengineered to exhibit Bloch resonances spatially and spectrally tuned to the absorbing medium, and, by boosting bothlight absorption and light extraction, the luminescence can be greatly enhanced [1-3].
Here, we will review on the experimental and numerical investigation of square arrays of ZrO2:Eu3+ nano-cylinders
fabricated by soft nano-imprinting lithography on a fused silica substrate [4] (Fig.1 a)). This technique is advantageous
for fast large-area production and for the fact that both the doped layer and the PC are realized with the same material
at the same time without any other fabrication steps. Arrays with different periodicity p have been investigated using
angle-resolved and angle-integrated measurements, supported by finite difference time domain (FDTD) simulations,to ensure the spectral and spatial coupling of Eu3+ emission with Bloch resonances (Fig. 1 b)). We experimentallydemonstrate that, by varying p, we can modify the absorption of the excitation light and the outcoupling of thephotoluminescence, achieving an enhancement of the luminescence intensity up to a factor ≈ 240 compared with theunpatterned flat surface (Fig. 1 c)) within the collection angle of a small numerical aperture lens (NA=0.28).

Fig. 1 a) Scanning electron microscopic (SEM) images of the ZrO2:Eu3+ nanocylinder array at different magnifications. b) Theoreticalsimulations of the angle-resolved reflection of the metastucture (p=370 nm) along the Γ-M and Γ-X directions for two different polarizations of
the incident light. c) Ange-integrated photoluminescence of the metastructure (p=370 nm, black curve) and of the unpatterned flat surface (redcurve).

References[1] Park, Y. et al., 2009, Opt. Express 17, 14312−14321.
[2] Noda, S. et al., 2007, Nat. Photonics, 1, 449−458.
[3] Hoang, N. V. et al., 2017, ACS Photonics 4, 1705–1712.
[4] Bottein, T. et al., 2018, Nanoscale 10, 1420–1431.

NFO16
Reconfigurable Hyperbolic Polaritonics with Correlated Oxide Metasurfaces
Yohannes Abate1, Neda Alsadat Aghamiri1, Guangwei Hu2,3, Alireza Fali1, Zhen Zhang4, Jiahan Li5, Sivacarendran
Balendhran6, Sumeet Walia6,7, Sharath Sriram7,8, James H. Edgar5, Shriram Ramanathan4, Andrea Alù2,9
1. Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, USA
2. Photonics Initiative, Advanced Science Research Center, City University of New York, NY 10031, USA
3. Department of Electrical Eng., National University of Singapore, Kent Ridge, Singapore 117583, Singapore
4. School of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA
5. Tim Taylor Department of Chemical Engineering, Kansas State University, Manhattan, KN 66506, USA
6. School of Engineering RMIT University Melbourne, Victoria, Australia
7. Functional Materials and Microsystems Research Group and the Micro Nano Research Facility RMIT University Melbourne, Australia
8. ARC Centre of Excellence for Transformative Meta-Optical Systems, RMIT University, Melbourne, Australia
9. Physics Program, Graduate Center, City University of New York, New York, NY 10016, USA
E-mail: Yohannes.abate@uga.edu

Polaritons enable subwavelength confinement and highly anisotropic flows of light over a wide spectral range, holding
the promise for applications in modern nanophotonic and optoelectronic devices. However, to fully realize theirpractical application potential, facile methods enabling nanoscale active control of polaritons are needed. Here, we
introduce a hybrid polaritonic-oxide heterostructure platform consisting of van der Waals crystals, such as hexagonalboron nitride (hBN) or alpha-phase molybdenum trioxide (α-MoO3), transferred on nanoscale oxygen vacancy patternson the surface of prototypical correlated perovskite oxide SmNiO3 (SNO). Using a combination of scanning probe
microscopy and infrared nanoimaging techniques, we demonstrate nanoscale real-time reconfigurability of complex
hyperbolic phonon polaritons patterned at the nanoscale with unmatched resolution. Hydrogenation and temperature
modulation allow spatially localized conductivity modulation of SNO nanoscale patterns, enabling robust real-time
modulation and nanoscale reconfiguration of hyperbolic polaritons. Our work paves the way towards nanoscale
programmable metasurface engineering as a new paradigm for reconfigurable nanophotonic applications facilitated bya hybrid material platform exploiting extreme light-matter interactions in polaritonic systems.

Fig. 1 | Tunable polaritonics with SNO-hBN architectures. a,Topographic image of pristine SNO, a lithography pattern is performed
by applying 10 V potential at the c-AFM tip and a flake of 60 nm hBN (~99% boron-10 enriched) is transferred on top. b, IR s-SNOM
second harmonic near field amplitude s2 image. c Measurement of the dispersion relation of HPhPs in 60 nm thick hBN on SNO. d,
Second-harmonic near-field amplitude s2 image of a 60 nm hBN flake transferred on a triangular cavity lithographically patterned byapplying a 10V potential c-AFM tip. e Corresponding KPFM surface potential images and f, simulation result (see Methods) for paneld. Second harmonic near field amplitude s2 images of a lithographic pattern on SNO written by applying a 10V potential at c-AFM tip
g, prior transferring a hBN flake and h, with a 50 nm thick hBN flake on top showing polaritons at 6.9 µm, and i, at 6.45 µm.

Plasmon-assisted lithium niobate metasurface with ultra-high-Q resonances
M. Saad Bin-Alam1, M. Zahirul Alam2, Ksenia Dolgaleva1,2, and Robert W. Boyd1,2,3

School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, ON, K1N 6N5, Canada
2
Department of Physics, University of Ottawa, Ottawa, ON, K1N 6N5, Canada
3
The Institute of Optics and Department of Physics and Astronomy, University of Rochester, Rochester,
New York 14627, USA
E-mail: msaad009@uottawa.ca

1

Lithium niobate (LiNbO3) is a material of choice in commercial implementations of second-order nonlinear opticaldevices [1-2]. Efficient LiNbO3-based metasurfaces may lead to devices with new functionalities for free-space
multimode applications such as mode-selective modulators, switches, beam shaping [3-5], etc. However, allnonlinear optical interactions scale with the propagation length and a sub-micron propagation distance typically does
not afford strong-enough nonlinear interactions to realize high-efficient nonlinear devices [6]. Here we show thatlattice-plasmons allows excitation of guided-mode resonances in LiNbO3 thin films with quality factor (Q-factor) on
the order of 104. Such high Q factors will allow efficient nonlinear interactions without phase matching concerns in ametasurface geometry.

Fig. 1 (a) A schematic of a plasmonic metasurface on lithium niobate (LiNbO3) thin film-on-insulator. The metasurface consistsof a plasmonic gold (Au) antenna array placed on a high refractive-index thin-film with fused silica (SiO2) substrate and cladding
layers. (b) Comparison between the simulated broadband transmittance spectra with and without the plasmonic rectangularnanostructure array shown in (a).

A collection of periodic, plasmonic antennas with a certain periodicity or lattice constant operated as weak
scatterers can direct normally incident light in various diffraction angles determined by the Bragg conditions [7-9].
When such a collection of antennas is placed on a sub-micron-thick film of LiNbO3 (see Fig. 1(a)), they allow theexcitation of many guided-wave modes (see Fig. 1(b)). Consequently, the transmission spectrum exhibits manycomb-like resonance dips with Q-factors as large as 104 over an octave of visible and near-IR frequencies. We
investigate the exact spectral location, the Q-factor, and nonlinear properties of the resonances as functions of the
thickness of LiNbO3, nature of the guided modes, and geometric properties of the plasmonic antennas (which
dictates the radiative and nonradiative loss).
We believe that our demonstrations lay the groundwork for high-efficient nonlinear metasurfaces exploiting secondorder nonlinear interactions.
References[1] Saleh, B. E., Teich, M. C. Fundamentals of Photonics, 2019, John Wiley & Sons.
[2] Boyd, R. W. Nonlinear Optics, 2020, Academic Press.
[3] Weigand, H., Volgler-Neuling, V. V., Escalé, M. R., Pohl, D., Richter, F. U., Karvounis, A., Timpu, F., Grange, R. 2021. ACS Photonics, 8,3004-3009.
[4] Klopfer, E., Dagli, S., Barton III, D., Lawrence, M., Dionne, J. A. 2022. Nano Lett., Articles ASAP (In Press).
[5] Barton III, D., Lawrence, M., Dionne, J. 2021. Appl. Phys. Lett., 118, 071104.
[6] Fedotova, A., Younesi, M., Sautter, J., Vaskin, A., Löchner, F. J., Steinert, M., Geiss, R., Pertsch, T., Staude, I., Setzpfandt, F. 2020. Nano
Lett., 20, 8608−8614.
[7] Kravets, V. G., Kabashin, A. V., Barnes, W. L., Grigorenko, A. N. 2018. Chem. Rev., 118, 5912-5951.
[8] Khlopin, D., Laux, F., Wardley, W.P., Martin, J., Wurtz, G.A., Plain, J., Bonod, N., Zayats, A.V., Dickson, W. and Gérard, D. 2017. JOSA B,
34, 691-700.
[9] Bin-Alam, M. S., Reshef, O., Mamchur, Y, Alam, M. Z., Carlow, G., Upham, J., Sullivan, B. T., Ménard, J-M, Huttunen, M. J., Boyd, R. W.,
Dolgaleva, K. 2021. Nat. Commun., 12, 1-8.

Invited: Plasmonic Forces at the Picoscale
Jeremy Baumberg
NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, United Kingdom
E-mail: jjb12@cam.ac.uk

For over 30 years, single molecule signatures in surface-enhanced Raman scattering (SERS) have suggested theextreme localisation of light, but the mechanisms involved remained elusive. Recently, this was finally explained as
due to the light concentration around single metal adatoms on a plasmonically-active surface [1]. In strongly-confining
nanocavities (constructed here from nanoparticle-on-mirror, NPoM, see Fig.1a), light is further confined on the subatomic scale by single atom asperities (Fig.1b) [1]. In this talk I will show how light is responsible for pulling out the
Au or Ag atoms involved, and show different ways that this can be controlled.
We measure millions of SERS spectra from a wide range of NPoMs with different molecular monolayers forming
their 1-2nm nanogaps, and automatically extract all picocavity events. We find a characteristic probability forformation, and for decay, which depends on light intensity, the molecule, and temperature. Traditional models basedon classical optical forces from the field gradients completely fail to explain this data (Fig.1c), as they are thousandfold too weak and would predict very long decay times. Instead we show a new effect is responsible, based on thelight-induced van-der-Waals forces from plasmonically-enhanced dipole-dipole interactions arising via thepolarizability of the tip atoms [2]. Shining light on a metal surface reduces the energy barrier required to pull out anatom from the facet.
Using two pump lasers simultaneously to track SERS from different spatial regions of the nanogaps allows lateral
localisation of the picocavities produced [3]. It also confirms that picocavities have spectral resonances, and thus showsthat they act as more than merely lightning rods. Using carboxylate molecules in the gap allows (de)protonation eventsat picocavities to be tracked in real time, for the first time giving optical measurements of pH at a single molecule [4].
We show that comprehensive analysis of picocavity SERS allows videos of individual molecules to be reconstructed[5]. Finally, we show that the optomechanical coupling of the vibrations with the plasmonic cavity allows efficient
upconversion to detect mid-IR light [6].

Fig. 1 (a,b) Nanoparticle-on-mirror cavity, trapping light in nanogap with molecules, which pull out adatoms to form
picocavities. (c) Formation rate of picocavities depends on light intensity and molecule, on a universal curve.

These results are important in many areas, particular for photocatalysis but also for broad impacts in molecular(opto)electronics, biosensing, photovoltaics, and single-atom switches.
References[1] F. Benz et al., “Single-molecule optomechanics in picocavities”, Science 354, 726 (2016)[2] Q. Lin et al., “Optical suppression of energy barriers in single molecule-metal binding”, submitted (2021)[3] J. Griffiths et al., “Locating Single-Atom Optical Picocavities ...”, ACS Photonics 8, 2868 (2021)[4] J. Huang et al., “Tracking interfacial single-molecule pH and binding dynamics ...”, Science Advances 7:eabg1790 (2021)[5] J. Griffiths et al, “Resolving sub-Å ambient motion through reconstructions from vibrational spectra”, Nature Comm 12:6759 (2021)[6] A Xomalis et al, Detecting mid-IR light by molecular frequency upconversion in dual-wavelength nanoantennas, Science 374, 1268 (2021)

Optical mode imaging via correlated free-electron cavity-photon pairs
Germaine Arend1,2, Armin Feist1,2, Guanhao Huang3, Yujia Yang3, Jan-Wilke Henke1,2, Arslan Sajid Raja3,
F. Jasmin Kappert1,2, Jiahe Pan3, Hugo Lourenco-Martins1,2, Zheru Qiu3, Junqiu Liu3, Ofer Kfir1,2,
Tobias J. Kippenberg3, Claus Ropers1,2
1. 4th Physical Institute, University of Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen
2. Max-Planck Institut for Multidisciplinary Sciences, Am Fassberg 11, D-37077 Göttingen
3. Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland
E-mail: germaine.arend@uni-goettingen.deFree electrons and photons are widely used as probes in atomic-scale imaging and high-resolution spectroscopy,
respectively. Applying electron microscopy to optical excitations is a powerful approach harnessing spontaneouselectron energy-loss and cathodoluminescence imaging [1], as well as stimulated near-field scattering [2]. Bringing
the inelastic electron-light interaction to the single-particle regime will open up new pathways for free-electron
quantum optics, including electron-photon entanglement [3]. This, however, requires the enhancement of thetypically weak coupling between free electrons and photonic modes.
Here, we establish fiber-coupled, photonic-chip-based silicon nitride microresonators as a versatile platform forinducing efficient coupling between free electrons and light [4]. Inelastic scattering of electrons at the vacuum statesof the microresonator leads to a quantized electron energy loss and correlated cavity photon generation.
Scanning the focussed electron beam, passing parallel to the chip surface, maps the interaction with the opticalresonator mode. The transmitted electrons are analysed in energy and arrival time using an event-based detector (cf.
Fig 1a). Simultaneously, the generated photons are detected with a fiber-coupled single-photon avalanche diode(SPAD), time-tagging each single-electron event. Correlating the arrival times of the two particles, a coincidence
peak at 0.8-eV electron energy loss (cf. Fig 1b) is observed, revealing the generation of electron-photon pairs.
Filtering the events using energy loss and time delay enables the exclusive study of the correlated particles. The
resonator modes are imaged by mapping the photon generation rate for variable electron beam position (Fig 1d, top).
Spectral filtering to a single mode leads to Ramsey-type interferences [5] at the empty cavity (Fig 1d, bot), as also
observed for stimulated scattering at an externally pumped resonator (cf. Fig 1c) [4]. Finally, we demonstrate astrong contrast enhancement in imaging of the photonic mode by using correlated events only.
In summary, we demonstrate the efficient coupling of single free electrons and cavity photons by inelastic electronlight scattering, which enables high-fidelity heralded single-electron or single-photon sources. This promises novel
experimental concepts, including the controlled generation of photon Fock states, as well as coincidence-enhanced
photonic mode imaging.

Fig. 1 (a) Schematic experimental setup and photon generation process, introducing a fiber-coupled high-Q microresonator in atransmission electron microscope; (b) Detected electrons as a function of energy loss and relative time delay to the nearest photon,
revealing a pronounced correlation peak; (c) electron-light coupling constant measured via stimulated electron scattering at an
optically pumped resonator (reproduced from [4]); (d) Rate of detected photons as a function of electron beam position along theresonator using the full optical spectrum (top) and filtering to a single mode (bottom).

References[1] Polman, A., Kociak, M., Garcia de Abajo, F.J.. 2019. Nature Mat 18 1158-1171[2] Barwick, B., Flannigan, D.J., Zewail, A.H.. 2009. Nature 462 902-906[3] Kfir, O.. 2019. Phys. Rev. Lett. 123, 103602[4] Henke, J.-W. et al.. 2021. Nature 521 200-203[5] Echternkamp, K. et al.. 2016. Nature Phys 12 1000-1004

Sub-diffractional cavity modes of terahertz hyperbolic phonon polaritons intin oxide nanobelts
Flávio H. Feres 1,2,9, Rafael A. Mayer1,2,9, Lukas Wehmeier3,4, Francisco C. B. Maia1, E. R. Viana5,Angelo Malachias6, Hans A. Bechtel7, J. Michael Klopf8, Lukas M. Eng3,4, Susanne C. Kehr3, J. C.
González6, Raul O. Freitas1 & Ingrid D. Barcelos1

1 Brazilian Synchrotron Light Laboratory (LNLS), Brazilian Center for Research in Energy and Materials (CNPEM), Campinas, SP,
Brazil.
2 Physics Department, Gleb Wataghin Physics Institute, University of Campinas (Unicamp), Campinas, SP, Brazil.
3 Institute of Applied Physics, Technische Universit t Dresden, Dresden, Germany.
4 ct.qmat, Dresden-Würzburg Cluster of Excellence-EXC 2147, Technische Universit t Dresden, Dresden, Germany.
5 Department of Physics, Universidade Tecnológica Federal do Paraná (UTFPR), Curitiba, PR, Brazil.
6 Department of Physics, Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, MG, Brazil.
7 Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
8 Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.
E-mail: raul.freitas@lnls.br

In the last few years, hyperbolic phonon polaritons have attracted considerable attention in nanophotonics mostly
due to their intrinsic strong electromagnetic field confinement, ultraslow polariton group velocities, and long lifetimes.
Here we present tin oxide (SnO2) nanobelts as a photonic platform for the transport of mid to far-IR surface and volume
phonon polaritons. This communication brings a comprehensive description of the polaritonic properties of SnO2 as ananometer-sized dielectric and as an engineered material in the form of a waveguide. By combining accelerator-based
IR-THz sources (synchrotron and free-electron laser) with s-SNOM, we employed nanoscale far-infrared
hyperspectral-imaging to uncover a Fabry–Perot cavity mechanism in SnO2 nanobelts via direct detection of phononpolariton standing waves. Our experimental findings are accurately supported by notable convergence between theory
and numerical simulations. After the publication of these results [1], SnO2 is now confirmed as a natural hyperbolic
material with unique photonic properties essential for future applications involving subdiffractional light traffic anddetection in the far-infrared range.

Fig. 1 (left) Experimental schematic showing the IR-THz beam illuminating a metallic AFM tip (nano-antenna) for the s-SNOM
experiment. The highly confined and vertically polarized electric fields (Ez) at the tip apex launch surface (SPhPs) and volume(HPhPs) polaritons waves in the SnO2-NB. (center) SINS amplitude S2(ω) point spectrum (red solid line) of the SnO2-NB1. Inset
shows 1x1 μm2 AFM topography of the SnO2-NB1 (t = 93 nm, w = 350 nm), and the red dot indicates the SINS point spectrum
location. FDTD-simulated spectrum (blue dashed line) obtained from |Ez| integration along out-of-plane axis. (right) SINS spectral
linescan along the white-dashed line in the inset in (center). Figure adapted from [1].

References[1] F. H. Feres, R. A. Mayer, L. Wehmeier, F. C. B. Maia, E. R. Viana, A. Malachias, H. A. Bechtel, J. M. Klopf, L. M. Eng, S. C. Kehr, J. C.
González, R. O. Freitas, I. D. Barcelos, Nature Communications 12, 1995 (2021).

Real-space nanoimaging of THz polaritons in the topological insulator Bi2Se3
Shu Chen1, Andrei Bylinkin1,2, Zhengtianye Wang3, Martin Schnell1,4, Greeshma Chandan3，Peining Li5,
Alexey Y. Nikitin2,4, Stephanie Law3, Rainer Hillenbrand4,6
1. CIC nanoGUNE BRTA, 20018 Donostia - San Sebastián, Spain
2. Donostia International Physics Center (DIPC), 20018 Donostia - San Sebastián, Spain
3. IKERBASQUE, Basque Foundation for Science, 48009 Bilbao, Spain
4. Wuhan National Laboratory for Optoelectronics & School of Optical and Electronic Information, Huazhong University of Scienceand Technology, Wuhan 430074, China
5. Department of Materials Science and Engineering, University of Delaware, Newark, Delaware, 19716 USA
6. CIC nanoGUNE BRTA and Department of Electricity and Electronics, UPV/EHU, 20018 Donostia-San Sebastián, Spain
E-mail: r.hillenbrand@nanogune.eu

Plasmon polaritons in van der Waals materials and topological insulators (TI) attract attention from a fundamental
perspective and for potential THz photonic applications. Although polaritons have been observed by THz far-field
spectroscopy on topological insulator microstructures, real-space imaging of propagating THz polaritons has been
elusive so far. Here, we show spectroscopic THz near-field images of thin Bi2Se3 layers (prototypical topologicalinsulators) revealing polaritons with up to 12 times increased momenta as compared to photons of the same energy
and decay times of about 0.48 ps, yet short propagation lengths.

Fig. 1 Complex-valued analysis of THz near-field line profiles of a 25 nm thick Bi2Se3 film. a Sketch of s-SNOM experiment.
b Topography line profile, showing the height h as measured by AFM. c Experimental s-SNOM amplitude and phase line profiles
recorded at 2.52 THz. d Amplitude and phase line profiles obtained from the data shown in panel c after subtraction of the complexvalued signal offset C at large distances 𝑥. e Representation of near-field line profiles in the complex plane. Data corresponding
to panel c are shown in red colour, data corresponding to panel d are shown in blue colour. c-e The black solid lines show thefitting of the experimental data by a radially and exponentially decaying wave.

We recorded real-space images of THz polaritons in the TI Bi2Se3. Despite the short polariton propagation, we
could measure the polariton dispersion and propagation length, owing to complex-valued analysis of near-field line
profiles, which we will discuss in detail (Fig. 1). The highly specific signature of polaritonic spatial signal oscillationsin the complex plane – representing a spiral – could be also applied in the future to distinguish polaritons from nonpolaritonic spatial signal oscillations that, for example, are caused by spatial variations of dielectric material properties
or by laser intensity fluctuations.
abFig. 2 Polaritons in Bi2Se3 films with d = 60 nm and 25 nm thickness. a Near-field phase images at different THz frequencies.
b Polariton dispersions. Red symbols in the diagrams show the polariton dispersions obtained by complex-valued fitting ofexperimental line profiles as demonstrated in Fig. 1. Solid lines show calculated dispersions based on various conductivity models.
We consider various optical conductivity contributions based on optical bulk phonons (OP), massive bulk carriers (BC), Diraccarriers (DC) at both Bi2Se3 surfaces, and massive two-dimensional electron gases (2DEG) at both Bi2Se3 surfaces.

Using dispersion calculations based on various optical conductivity models (Fig. 2), we found that the polaritonsin Bi2Se3 can be explained by simultaneous coupling of THz radiation to various combination of Dirac carriers, massive
2DEG carriers, massive bulk carriers and optical phonons. Our work paves the way for studying THz polaritons invarious conducting thin layers, as we will show with further examples including thin semiconductor layers.
References[1] S. Chen et al., arXiv:2107.10791

Tunable second harmonic generation in plasmonic nanogaps by local
symmetry breaking
Jessica Meier1, Luka Zurak1, Andrea Locatelli2, René Kullock1, Thorsten Feichtner1, Bert Hecht1
1. Nano-Optics and Biophotonics Group, Experimental Physics 5, University of Würzburg, Germany
2. Department of Information Engineering, University of Brescia, Italy
E-mail: jessica.meier@physik.uni-wuerzburg.de

Plasmonic nanostructures exhibit large field enhancement which is commonly exploited to boost nonlinear opticaleffects such as second harmonic (SH) generation [1]. Especially intense near-field hot spots emerge in the gap of
dimer antennas upon excitation of their antisymmetric (bonding) mode. For antennas with a centrosymmetric shape,
however, the strong SH sources created at the vicinal metal surfaces in the gap region oscillate out-of-phase and thus
destructively interfere in the farfield [2].
Here, we demonstrate that it is possible to systematically tune the SH yield by breaking the mirror symmetry in
the gap region while retaining the overall antisymmetric resonance at the fundamental frequency. To this end, we
employ gallium- and helium-ion beam milling of single-crystalline gold microplatelets [3], which allows us to
top-down fabricate nanostructures at a precision down to three nanometers. Due to the advanced control over thegeometric and thus optical properties of the antennas, we are able to compare the SH yield of asymmetric antennas
featuring only one tip with their respective symmetric counterparts (Fig. 1). For asymmetric antennas, SH intensity
increases dramatically with smaller tip opening angles corresponding to a higher degree of asymmetry, whereas only
a weak increase of SH intensity is observed for symmetric antennas. Our experimental results are supported by full
nonlinear simulations using a frequency-domain finite element method. The quantitative agreement between
experiment and simulation further confirms the high accuracy and reliability of our fabrication approach andcorroborates the underlying theoretical description.
Our results provide the basis for future experiments that will introduce further tunability of the SH response andwill make use of multiresonant structures and improved materials to optimize the SH efficiency.

Fig. 1 Measured and simulated SH intensity from asymmetric (red) and symmetric (blue) optical antennas dependent on the tip
opening angle. Error bars account for measurements on different antennas with comparable geometric and linear optical properties. Scanning electron micrographs show antennas with smallest (left) and largest (right) tip opening angle. Scale bars, 50 nm.

References[1] Celebrano, M., Wu, X., Baselli, M., Großmann, S., Biagioni, P., Locatelli, A., De Angelis, C., Cerullo, G., Osellame, R., Hecht, B., Duò, L.,
Ciccacci, F., Finazzi, M. 2015. Nature Nanotech, 10, 412-417.
[2] Berthelot, J., Bachelier, G., Song, M., Rai, P., Colas des Francs, G., Dereux, A., Bouhelier, A. 2012. Opt. Express, 20, 10498-10508.
[3] Krauss, E., Kullock, R., Wu, X., Geisler, P., Lundt, N., Kamp, M., Hecht, B. 2018. Cryst. Growth Des., 18, 1297-1302.

Near-arbitrary spectral placement of lattice resonances using Fourier analysis
Theng-Loo Lim1, Yaswant Vaddi1, M. Saad Bin-Alam2, Lin Cheng3, Rasoul Alaee1, Jeremy Upham1, Ksenia
Dolgaleva2, Orad Reshef2, Robert W. Boyd1,2,4
1. Department of Physics, University of Ottawa, 25 Templeton St, Ottawa, ON, K1N 6N5, Canada.
2. School of Electrical Engineering and Computer Science, University of Ottawa, 25 Templeton St, Ottawa, ON, K1N 6N5, Canada.
3. School of Instrument and Electronics, North University of China, Taiyuan, 030000, China.
4. Institute of Optics and Department of Physics and Astronomy, University of Rochester, 500 Wilson Blvd., Rochester, New York,
14627, USA
E-mail: tlim103@uottawa.ca

Resonances in optical systems are useful for many applications, such as frequency comb generation [1] and biosensing[2]. However, many of these applications are difficult to implement in optical metasurfaces because traditional
approaches for designing multi-resonant nanostructures require significant computational and fabrication efforts. To
address this challenge, we introduce Fourier lattice resonance (FLR) design technique, in which simply knowing thedesired multiple resonances is sufficient to determine the metasurface structure [3]. Because each resonance issupported by a distinct surface lattice mode, each can have a high-quality factor [4]. This flexible procedure requires
only the computation of a single Fourier transform for its design and then require only standard lithographic fabrication
methods, allowing one to design and fabricate a metasurface to fit any specific, optical-cavity-based application.
To demonstrate the FLRs concept, we first select a set of desired resonance wavelengths and create a virtual spectrum(Fig. 1a), where we deliberately place two SLRs at 1150 nm and 1220 nm. We label this set of desired wavelengths as
FLR1. Then, we extend the spectrum by adding replicas of these two desired resonances at integer harmonics in thisvirtual spectrum (i.e., λSLR/2, λSLR/3, etc.) for which higher-order diffraction terms will naturally appear for any latticeresonance. We then take the inverse FT (IFT) of the virtual spectrum (Fig. 1b), producing a series of peaks in thespatial domain that indicate the nanoparticle particle positions necessary to produce surface lattice resonances at thedesired wavelengths. We calculate the transmission spectra of this IFT-generated lattice arrangement using the lattice
sum approach (LSA) [5] and confirm peaks at the desired positions (Fig. 2 c, blue). We repeat this approach withanother set of desired wavelengths (1310 nm and 1550 nm, FLR2, Fig 1c red), and again producing the desiredtransmission spectrum.

Fig. 1 The demonstration of Fourier lattice resonance (FLR) concept. (a) The generated virtual spectrum. (b) The Fouriertransform (FT) of the virtual spectrum. (c) The calculated transmission spectra of two set of lattice arrangements, labelled as FLR1and FLR2, using the FT-simulated particle spacing and lattice sum approach (LSA). (d) The experimental transmission spectra of
FLR1 and FLR2.

To experimentally verify the FLR method, we fabricated these metasurfaces consisting of rectangular particles,
and their corresponding spacings according to the IFT-generated lattice arrangements. Their experimentally
characterized transmission spectra (Fig. 1d) confirm the desired resonances are obtained by the FLR technique.
Because this approach can easily generate metasurface solutions from only the desired spectra, it could provide designs
for applications with spectra too complex to fully simulate otherwise and may reveal insight into how these SLR canbe still further engineered.
References[1] Okawachi, Yoshitomo, et al. 2011. Opt. Lett 36.17, 3398-3400.
[2] Menezes, Jacson W., et al. 2010. Adv. Funct. Mater 20.22, 3918-3924.
[3] Lim, Theng-Loo, et al. 2021. arXiv preprint arXiv:2112.11625.
[4] Bin-Alam, M. Saad, et al. 2021. Nat. Commun, 12.1: 1-8.
[5] Draine, Bruce T., and Piotr J. Flatau. 1994. J. Opt. Soc. Am. A 11.4, 1491-1499.

Efficient Coupling of Visible Light into a Plasmonic Slit Waveguide
Marc Noordam1, Kobus Kuipers1
1. Kavli Institute of Nanoscience Delft, Department of Quantum Nanoscience, Delft University of Technology, Lorentzweg 1
E-mail: m.l.noordam@tudelft.nl

Confining guided light modes into dimensions below the diffraction limit has been one of the most interesting
challenges of the nanophotonic research community. Plasmonic waveguides are very promising for guiding light
below the diffraction limit, some recent demonstrations include the coupling of a plasmonic waveguide mode to an
excitonic mode in a few-layer transition metal dichalcogenide (TMDC) material [1] and nonlinear harmonic
generation along a plasmonic gold nanowire [2].
However, plasmonic waveguides, especially in the visible regime, suffer from two major challenges. Firstly,
plasmonic modes typically suffer from high losses in the metallic structure leading to short propagation lengths.
Secondly, the typically large wavevector mismatch between free-space light and the plasmonic mode gives rise to a
low coupling efficiency. An effective coupling mechanism should therefore ideally adiabatically transform adielectric light mode into a plasmonic mode over relatively short length scale, to prevent large losses during thecoupling.
Following a concept for telecom frequencies [3], here we demonstrate a nanofabricated device converting adielectric mode at visible frequencies into a plasmonic slit waveguide mode. A sketch of the intersection of ourdevice can be observed in fig. 1(a,b). A thin layer of silicon nitride is encapsulated in silicon dioxide where two gold
plates with a small gap in between are placed on top. When the gap between the metallic plates is large enough themode is mainly confined in the silicon nitride waveguide while as the gap closes the mode can be adiabaticallytransferred into the plasmonic slit mode. For the visible frequency regime silicon does not work as a dielectric layer
and another dielectric material must be used. Since only the even plasmonic mode couples to the dielectric mode andthe even mode increases when the gap closes, the dielectric mode can only couple into the plasmonic mode if its
effective index is larger than that of the plasmonic mode at large gap width. Therefore, we extensively simulate thishybrid model to find the precise device parameters that can transform the dielectric mode to the even plasmonicmode in the visible frequencies. The resulting simulated effective index of the hybrid mode as function of the gap
width is plotted in fig. 1(c).
Figure 1(d) depicts a scanning electron microscope image of our fabricated coupling device where we use a
specially designed high index silicon nitride layer. Here, a diffraction grating is used to couple incoming free-spacelight into the silicon nitride. Next, the silicon nitride light mode is adiabatically coupled to the slit plasmon mode by
reducing the gap between the gold plates. After propagation through the slit the plasmon mode is similarly coupledback to free-space light. We are currently preforming optical experiments to explore the transmission as a function of
the slit width and length.

Fig. 1 (a,b) Schematic representation of the mode coupler for slit opening widths of 1 µm (a) and 100 nm (b). The simulated local
electric field distribution of the corresponding hybrid mode is overlayed as a false-color map. (c) The simulated effective refractive
index of the hybrid mode as a function of the slit opening width. (d) Scanning electron microscope image of a fabricated hybrid
coupling device, scale bar is 5 µm.

References[1] Gong, S. H., et al., 2018, Science, 359.6374, 443-447.
[2] de Hoogh, A., et al., 2016, ACS Photonics, 3.8: 1446–1452[3] Nielsen, Michael P., et al. 2016. Nano Lett, 16.2, 1410-1414.

Nanoscale photodetectors based on graphene and graphene nanoribbons for
integrated plasmonic circuitry
Mo Lu1, Seyed Khalil Alavi1, Boris V. Senkovskiy2, Dirk Hertel1, Danny Haberer3,
Yoichi Ando2, Klaus Meerholz1, Felix R. Fischer3,4,5, Alexander Grüneis2, and Klas Lindfors1
1. Department für Chemie, Universität zu Köln, Greinstr. 4-6, 50939 Köln, Germany
2. II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
3. Department of Chemistry, University of California at Berkeley, 699 Tan Hall, Berkeley, CA 94720, USA
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
5. Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley
National Laboratory, Berkeley, California 94720, USA
E-mail: klas.lindfors@uni-koeln.de

Transmitting optical signals between a distant nanoscale transmitter and receiver in a wireless optical link could
enable low-loss large-distance communication between nano-objects [1,2]. For practical signal transmission a key
requirement is the conversion between electrical and optical signals at the transmitter and receiver in the link. In thiscontribution we present our progress in realizing integrated plasmonic circuitry making use of nanoscale
photodetectors based on graphene and graphene nanoribbons.
Armchair edge graphene nanoribbons (AGNRs) with tunable bandgap are a promising material for electronics andoptoelectronics. We have earlier investigated the photophysical properties of seven atom wide AGNRs (7-AGNRs) [3,4]. A particularly interesting feature of GNRs is their high absorption compared to graphene, which is beneficial forphotodetection. We have recently demonstrated a nanoscale photodetector based on 7-AGNRs [5]. Figure 1 shows anelectron micrograph of the device, which consists of a set of finger electrodes fabricated on top of a layer of aligned
7-AGNRs. Absorption of light results in photogenerated carriers in the semiconducting ribbons, which are transported
between the source and drain electrodes under a bias voltage. The responsivity of the detector at a wavelength of 635
nm is 0.04 mA/W and the dark current is below 30 pA for a bias voltage of 1.5 V. The aligned ribbons result in ahighly polarization dependent responsivity: Only light polarized along the ribbons is absorbed efficiently.

Fig. 1 Scanning electron micrograph of photodetector based on 7-AGNRs. The arrow shows the orientation of the GNRs.

In addition to 7-AGNR-based photodetectors, we have explored devices with graphene as the active material. In
such a detector the photogenerated carriers near electrodes can be harvested due to band-bending near the metalgraphene interface [6]. By incorporating plasmonic nanoantennas into the structure, the absorption efficiency can beengineered and locally enhanced. In combination with a nanoscale transmitter, we have taken the first steps to realize
wireless signal transmission using plasmonically enhanced graphene photodetectors.
Our simple proof-of-concept experiments highlight the promise of graphene-based photodetectors for integrated
plasmonic circuitry. In particular graphene nanoribbons offer a flexible material platform with engineerable material
properties making them a highly interesting option for nanoscale photodetectors. Graphene nanoribbons have thepotential of being a key material for future optoelectronics and electronics.
References[1] Alù, A.; Engheta, N. 2010. Phys. Rev. Lett., 104, 213902.
[2] Dregekely, D.; Lindfors, K.; Lippitz, M.; Engheta, N.; Totzeck, M.; Giessen, H. 2014. Nature Communications, 5, 4354.
[3] Senkovskiy, B. V.; Pfeiffer, M.; Alavi, S. K.; et al. 2017. Nano Lett., 17, 4029–4037.
[4] Alavi, S. K., et al. 2019. 2D Materials, 6, 035009.
[5] Alavi, S. K., et al. 2020. ACS Appl. Nano Mater., 3, 8343–8351.
[6] Liu, Y., et al. 2011. Nature Communications, 2, 579.

Atomistic Near-Field Optics
V. Ara Apkarian
Department of Chemistry, University of California at Irvine, Irvine, CA 92697, USAaapkaria@uci.edu

The pursuit of ever increasing resolution has driven the major advances in the long history of microscopy. Fromthe perspective of interrogating molecular matter, resolving atoms would be the sought natural limit, which in optical
microscopy has been attained through tip-enhanced Raman scattering (TERS) carried out in the atomistic near-field(ANF) [1-4]. The essential concept dates back to Synge’s proposal of near-field optics [5,6] to break the diffraction
limit 𝑑𝑑~𝜆𝜆0 /2𝑛𝑛 established by Abbe [7] and Helmholtz [8]. The modern solution is to use a plasmonic lens, 𝑅𝑅𝑅𝑅(𝑛𝑛2 ) ≡𝜀𝜀′ < 0, to focus light, following the seminal works of Takahara and Nerkararyan, who pointed out that there is no cutoff for light guided on a 1D wire [9], a 2D wedge [10] or 3D cone [11] structures of negative dielectric. On a conicalstructure, since the index of refraction scales as the inverse cone radius 𝑛𝑛 ∝ 1/𝑟𝑟, classically, there is no limit tofocusing of light [11]. In practice, the classical singularity of the cone at 𝑟𝑟 = 0 is eliminated by the atomic granularity(quantization) of matter, which explains the demonstrations of atomically confined light at the end of an atomicallyterminated silver needle [1-4]. While the description of the photon on a needle, namely the quantized electromagnetic
displacement, is readily obtained by solving the 1D scaled heterogeneous Helmholtz equation (SHHE), ∇2 + 𝑛𝑛2 (𝑟𝑟) =0, the implications are far-reaching. Quantization of the displacement, of entangled light field and polarization ofmatter, 𝐷𝐷 = 𝜀𝜀𝜀𝜀 = 𝐸𝐸 + 4𝜋𝜋𝜋𝜋, most naturally arises by the quantization of the dielectric, given by the eigenvalues of
the SHHE. The associated eigenmodes nicely describe the antenna, waveguide, and super-focusing functionalities ofa tapered plasmonic wire, as a paradigm that generalizes from TERS to SERS at plasmonic junctions. Notably, theapex mode on atomic asperities is atomically confined, which explains the physical volume of picocavities [12].
However, due to the dramatic increase in the dielectric 𝑛𝑛2 = (𝑐𝑐/𝑣𝑣𝐹𝐹 )2 , it consists entirely of polarization: the photon isself-trapped as it adiabatically converts to charge density 𝑒𝑒𝑒𝑒𝑒𝑒 and acquires mass √𝜀𝜀 − 1ℏ𝜔𝜔/𝑐𝑐 2 consistent with therelativistic wave equations. With regard to microscopy, ANF optics can be just as well regarded as near field scanningtunneling electron microscopy (STEM), which is demonstrated experimentally by approach curves that characterize
the tunneling conductance of the plasmon-polariton. Among the exciting applications of ANF optics is electrophotonics accessed by direct wiring of photons to intramolecular circuitry. More fundamentally, the extreme
confinement forces rethinking of paradoxes that arise in quantization of relativistic wave equations, required todescribe a photon in deep sub-wavelength plasmonic structures.
References[1] Crampton, K. T., Lee, J., Apkarian, V. A. 2019. ACS Nano 13, 6363.
[2] Lee, J ., Crampton, K. T., Tallarida, N., Apkarian, V. A. 2019. Nature 568, 7750.
[3] Lee, J., Tallarida, N., Chen, X., Jensen, L., Apkarian, V. A. 2018. Science Advances 4, eaat5472.
[4] Tallarida, N., Lee, J., Apkarian, V. A. 2017. ACS Nano 11, 11393.
[5] Synge, E. H. 1928. Phil. Mag. 6, 356-362.
[6] Synge, E. H. 1932. Phil. Mag. S7, 13, 297-300.
[7] Abbe, E. 1873. Archiv. F. Mikroskop. Anat, 9, 413.
[8] Helmholtz, H. 1874. Annal. Phys. 557-584.
[9] Tkahara, J., Yamagishi, S., Taki, H., Morimoto, A., Kobayashi, T. 1997. Optics Lett. 7, 475.
[10] Nerkararyan, Kh., V. 1997. Phys. Lett. A, 237, 103.
[11] Babadjanian, A. J., Margaryan, N. L., Nerkararyan, Kh., V. 2000. J. Appl. Phys. 87, 3785.
[12] Urbieta, M., Barby. M. Zhang, Y., Koval, P., Sánchez-Portal, D., Zabala, N., Aizpurua, J. 2008. ACS Nano, 12, 585.

Advanced hybrid plasmonic nano-emitters using smart photopolymer
Dandan Ge1, Ali Issa1, Safi Jradi1, Christophe Couteau1, Sylvie Marguet2 and Renaud Bachelot1
1.

Light, nanomaterials, nanotechnologies (L2n) Laboratory. CNRS EMR 7004. Université de Technologie de Troyes, 12 rue Marie
Curie, 10004 Troyes Cedex, France
2. CEA, CNRS, NIMBE, Université Paris Saclay, F-91191 Gif sur Yvette, France
E-mail: renaud.bachelot@utt.fr

The integration of quantum emitters onto plasmonic nano-devices with spatial control and nanometer precision has
become a great challenge, e. g. [1-6].
We report on the use of a smart polymer for selectively immobilizing nano-emitters on specific preselected sites ofgold nanocubes (GNC). The cunning use of the polymer is twofold. First, it records both the selected site and the future
emitters-GNC distance through plasmon-assisted photopolymerization [5-6]. Second, because the polymer ischemically functionalized [7], it makes it possible to attach the nano-emitters right at the preselected polymerized sites
which subsequently “recognize” the nano-emitters to get attached.
Since the resulting active medium is a spatial memory of specific plasmonic modes, it is anisotropic, making the
hybrid nanosources sensitive to light polarization.
By controlling the thickness of the nanopolymer, the ability to adjust the statistical average photoluminescence
lifetime of the hybrid emitters is demonstrated on two kinds of nano-emitters coupled to GNC: doped polystyrene
nanospheres and semiconductor colloidal quantum dots.
Fig. 1 shows an example of hybrid nano-emitter with CdSe/ZnS Quantum Dots (emitting at 623 nm) that were
integrated onto the surface of a functionalized polymer that has been patterned through plasmonic
photopolymerization.

Fig. 1 Use of the smart polymer for coupling spherical CdSe/ZnS quantum dots with a gold nanocube. (a) AFM image of thehybrid nanosource. Attached QDs are clearly visible. (b) Weighted photoluminescence lifetime (λ=620 nm) as a function of theaverage polymer thickness that depends on the fabrication condition.

References[1] Russell, K. J. et al. 2012. Nature Photon., 6, 459-462.
[2] Ming, T. et al. 2009. Nano Lett., 9, 3896–3903.
[3] Schell, A. W. et al. 2014. Nano Lett., 14, 2623–2627.
[4] Hübner, K. et al. 2019. Nano Lett., 19, 6629–6634.
[5] Ge, D. et al. 2020. Nat. Commun., 11, 3414, 2020.
[6] Zhou, X. et al. 2015, Nano Lett., 15, 7458–7466.
[7] Issa, A. et al. 2021. ACS Appl. Mater. Interfaces, 13, 41846–41856.

Nonlinear plasmonic response in atomically thin metal films
A. Rodríguez Echarri1, Joel D. Cox2,3 and F. Javier García de Abajo1,4
1. ICFO – Institut de ciències fotòniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain
2. Center for Nano Optics, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
3. Danish Institute for Advanced Study, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
4. ICREA – Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spainarodriguez@icfo.net

Crystalline metallic films are attracting significant attention within the nanophotonics community as amaterial platform for plasmonics that offers light concentration and manipulation on extreme subwavelength scaleswith lower intrinsic losses than for plasmons in amorphous metal films [1,2]. Recent advances in nanotechnology now
enable the fabrication of such crystalline films with nanometer-scale thickness, down to the level of only several
atomic monolayers, further boosting plasmon confinement for the miniaturization of next-generation electronic andphotonic devices. However, in very thin crystalline metallic films, quantum mechanical effects play a non-negligible
role in the dynamics of electrons and their interaction with external electromagnetic fields, manifesting in the spectralfeatures associated with its linear optical response that depend on film thickness and crystallographic orientation.
These effects are anticipated to become even more significant when dealing with nonlinear optical phenomena, which
necessitate intense optical fields that access additional transitions among quantized electronic states.
Here we reveal quantum and finite-size effects in the nonlinear optical response associated with plasmons supported
by few-atom-thick crystalline noble metal films (see Fig 1a) [3]. In particular, we employ a quantum-mechanicaldescription that captures the main features in the electronic band structure associated with the various crystal facetsof noble metals [4] to simulate their nonlinear optical response by extending the random-phase approximation tohigher orders. Our results demonstrate strong signatures of film thickness and crystallographic orientation in the linear
plasmonic response that are amplified in nonlinear optical processes such as harmonic generation and two-photon
absorption (see Fig. 1e), with additional effects attributed to surface states, electron spill out, and the directional band
gap associated with bulk atomic-layer corrugation (see Fig c-d). These findings are of key importance in thecharacterization of atomically-thin crystalline films and their applications for nonlinear plasmonic nanophotonic
devices.

Fig. 1: (a) Illustration of surface plasmons (SP) in a few-atom-thick silver (111) film. (b) SP dispersion relation calculated via the loss
function (imaginary part of the reflection coefficient) in momentum and energy space. Dispersion curves of SPs for various thicknesses
are superimposed on the dispersion map and indicated by the colored code in the legend. (c,d) Electronic density of states obtained by
Fourier-transforming the out-of-plane electronic wave functions for N = 100 and N = 5 Ag(111) atomic layers, respectively. (e) Figure of
merit quantifying the nonlinear plasmonic yield using quantum-mechanical (filled circles) or classical (hollow circles) models for selectedvalues of the in-plane wave vector Q.

References[1] D. N. Basov, M. M. Fogler and F. J. García de Abajo. Science 354, aag1992 (2016).
[2] D. Alcaraz Iranzo et al., Science 360, 291 (2018).
[3] A. Rodríguez Echarri, J. D. Cox, and F. J. García de Abajo, Optica 6, 630 (2019).
[4] E. V. Chulkov, V. M. Silkin and P. M. Echenique, Surf. Sci. 437, 330 (1999).

Surface-response functions and their role in emitter—plasmon coupling
Christos Tserkezis1, Fedor A. Shuklin1, Vasilios Karanikolas2, Ioannis Thanopulos3, P. André D. Gonçalves1,
Takashi Kuroda4, Jun-ichi Inoue4, Masakazu Ichikawa5, Emmanuel Paspalakis3, Christian Wolff1, Joel D.
Cox1,6, and N. Asger Mortensen1,6
1. Center for Nano Optics, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
2. International Center for Young Scientists, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
3. Materials Science Department, School of Natural Sciences, University of Patras, Patras 265 04, Greece
4. National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
5. Department of Applied Physics, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
6. Danish Institute for Advanced Study, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
ct@mci.sdu.dk

Surface-response functions (SRFs), in the spirit of the work of Feibelman [1], have been emerging as an efficientapproach for implementing quantum corrections in the description of metals for nanoscale light—matter interactions.
This appeal is owed to their ability to capture effects such us nonlocal screening, electron spill-in or spill-out, andsurface-enabled Landau damping [2], which cannot be disregarded when considering plasmonic nanoparticles (NPs)of less than ~10 nm sizes, or separations smaller than ~2 nm. But the aforementioned separations do not necessarilyrefer to the particle—particle distance; they can equally well concern the distance between a plasmonic system and aquantum emitter. Coupling to localised surface plasmons in small NPs provides an excellent mechanism for tailoring
the excitation and emission properties of such emitters, leading to significantly modified decay rates and the possibilityof strong coupling [3]. It is thus reasonable to expect that quantum effects will play a major role in the plasmon—
emitter interaction, and the implementation of SRFs in the theoretical description of this interaction should shed lighton the phenomena governing the coupling.
SRFs are typically calculated with ab initio quantum methods for solid-state physics that take the full electron
dynamics into account [1,2]. Here we show that much of the underlying quantum response can already be captured in
the context of the local response approximation, by just relaxing its usual step-function approximation for the electron
density n at the metal—dielectric interface, and assuming a smoothly varying density profile instead [4] (Fig. 1a). Oncethese functions have been obtained, they can be implemented in any otherwise classical model for computational
electrodynamics, by simply appropriately modifying the boundary conditions [5]. At a next step, we form a metal—
dielectric—metal cavity, with the metal described by actual calculated SRFs, and place a quantum emitter in the middle(Fig. 1b, top sketch). We show that the inclusion of quantum corrections in the response of the metal through SRFs
not only does not prevent the system from entering the strong coupling regime, but it can, in fact, lead to interesting
non-Markovian relaxation dynamics for the emitter [6] (Fig. 1b, bottom graph). We thus establish SRFs as a viable
and computationally efficient way to consider aspects of the microscopic response of the metal in studying emitter—plasmon coupling.

Fig. 1 (a) Schematic of a metal—dielectric interface with a sketch of an exact electron density (black line) and the approximation
used in [4] (red line). (b) A metal—dielectric—metal cavity with a dipolar emitter in the middle (top). For different emittertransition energies the system undergoes various interesting dynamics (bottom),References[1] Feibelman, P. J. 1982. Prog. Surf. Sci., 12, 287-407.
[2] Mortensen, N. A. 2021. Nanophotonics, 10, 2563-2616.
[3] Novotny, L. and Hecht, B. Principles of Nano-Optics. 2006, Cambridge University Press, Cambridge U.K.
[4] Mortensen, N. A., Gonçalves, P. A. D., Shuklin, F. A., Cox. J. D., Tserkezis, C., Ichikawa, M, and Wolff, C. 2021. Nanophotonics, 10, 36473657.
[5] Gonçalves, P. A. D, Christensen, T., Rivera, N., Jauho, A.-P., Mortensen, N. A., and Soljačić, M. 2020. Nat. Commun., 11, 366.
[6] Karanikolas, V., Thanopulos, I., Cox, J. D., Kuroda, T. Inoue J.-I., Mortensen, N. A., Paspalakis, E., and Tserkezis, C. 2021. Phys. Rev. B,
104, L201405.

Quantum purity of entangled states of light scattered by a nanostructureÁlvaro Nodar1, Ruben Esteban1,2, Carlos Maciel-Escudero1,3 , Jon Lasa-Alonso1,2, Javier Aizpurua1,2 , andGabriel Molina-Terriza1,2,3
1. Centro de Física de Materiales CFM (CSIC-UPV/EHU), Paseo Manuel de Lardizabal, 5, 20018, Donostia, Spain
2. Donostia International Physics Center DIPC, Paseo Manuel de Lardizabal, 4, 20018, Donostia, Spain
3. CIC NanoGUNE BRTA and Dept. of Electricity and Electronics UPV/EHU, Avda. Tolosa, 76, 20018, Donostia, Spain
4. IKERBASQUE, Basque Foundation for Science, Calle María Diaz de Haro, 48013, Bilbao, Spain
E-mail: alvaro_nodar001@ehu.eus

Quantum states of light are excellent carriers of quantum information, as they are very robust to propagation
decoherence. Furthermore, exploiting the angular momentum properties of these states makes it possible to encode
more information and increase their robustness to decoherence. Interestingly, nanostructures designed to enhance lightmatter interaction can enable the manipulation at the nanoscale of the quantum information. However, practicalapplications usually require that the purity of the quantum states is preserved in the interaction with the nanoparticle.
In this work, we provide a theoretical framework to analyze the preservation or destruction of the quantum purityof quantum states of light scattered by a nanostructure. Within this framework we first calculate the classical responseof nanostructures illuminated by beams with well-defined angular momentum [1, 2]. This calculation gives the input
parameters for a quantum description of the scattering process based on the quantum lossy beam splitter formalism[3]. We apply this methodology to a system of experimental relevance [4], the scattering by a rotationally symmetricnanostructure of two-photons states entangled in angular momentum. Specifically, we consider the input pulsed state�Ψ+𝑖𝑖𝑖𝑖 � = 𝜙𝜙(𝜔𝜔0 , 𝑡𝑡0 ) [|+⟩|+⟩ + |−⟩|−⟩] , where 𝜙𝜙 is the Gaussian envelope (with central frequency 𝜔𝜔0 and time 𝑡𝑡0 ),and |+⟩ and |−⟩ describe photons states with helicity Λ = +1 and Λ = −1, respectively (Λ is the spin projected in the
direction of propagation). The rotational symmetry of the nanostructure enables excellent control of the scattered statebecause it imposes the conservation of the total angular momentum. However, we find that the scattering of |Ψ+𝑖𝑖𝑖𝑖 ⟩ canlead to the loss of purity of the output state (see Fig. 1b) which can help understand recent experimental results [4].
We show that the loss of purity is due to the excitation of two (or more) optical resonances of the nanostructurethat are spectrally sharp. The exact calculations are complemented with an analytical expression that connects the lossof purity with the optical response of the nanoscatterer. This expression is obtained by using a simplified physicalpicture that describes the output state as a superposition of two pulsed states that are time-delayed and frequencyshifted due to the scattering by the nanostructure (see sketch in Fig. 1a). These time delays and frequency shifts are
the origins of the loss of purity. Understanding the relationship between the loss of purity and the excitation of opticalresonances of a nanostructure can be crucial for manipulating quantum information at the nanoscale.

Fig. 1 Loss of purity of a two-photon state interacting with a nanostructure. (a) Sketch of the interaction. A rotationally symmetric nanostructure isilluminated by an input two-photon state |Ψ+𝑖𝑖𝑖𝑖 ⟩ with a Gaussian pulse envelope 𝜙𝜙. The output state is given as a superposition of two states (blueand red arrows) that are frequency-shifted and time-delayed. (b) Loss of purity as a function of the central wavelength of the input |Ψ+𝑖𝑖𝑖𝑖 ⟩ pulse. The
loss of purity is obtained considering a spherical silicon nanoparticle of radius 𝑟𝑟 = 250 nm.

References[1] Zambrana-Puyalto, X., and Molina-Terriza, G. 2013, J. Quant. Spectrsc. Radiat. Transf., 126, 50[2] Molina-Terriza, G., 2008 Phys. Rev. A., 78, 053819[3] Barnett, S.M. et al. 1998, Phys. Rev. A, 57, 2134
[4] Büse,A. et. al., 2018, Phys. Rev. Lett., 121, 173901