Thermal Unfolding and Refolding Trajectory of Single Calmodulin Proteins
by Plasmonic Optical Tweezers
Cuifeng Ying,1 Edona Karakaçi,2 Esteban Bermúdez-Ureña,2 Reuven Gordon,3 Michael Mayer 2
1. Advanced Optics and Photonics Lab, Department of Engineering, Nottingham Trent University, United Kingdom
2. Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland.
3. Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia V8P 5C2, Canada
Email: cuifeng.ying@ntu.ac.uk

Conformational dynamics are critical for the function of many proteins. Studying conformational changes ofproteins in response to external stimuli is therefore essential for determining their function in human cells, and theircontribution to both health and disease. Established approaches to monitor conformational changes of proteins require
modification of the target proteins, either by attaching fluorophores (or other labels) to conduct single-molecule
fluorescence resonance energy transfer (smFRET) experiments or by attaching molecular tethers to conduct singlemolecule force spectroscopy (smFS) experiments. A recently developed approach in the nanophotonics field,
plasmonic optical tweezers, has demonstrated the potential to trap unlabeled, nanoscale objects and monitor the
behavior of these objects over time [1, 2]. This work reports the first thermal unfolding and refolding trajectory of asingle protein by using the double-nanohole (DNH) plasmonic optical tweezer (Fig. 1a-d) [3]. We trapped single
calmodulin proteins from solution and monitored the conformational changes of proteins from changes in transmission,at the same time tuning the temperature in the trapping site by adjusting the laser power (Fig.1e,f). We demonstrated
that single apo-calmodulin thermally unfolds and refolds in steps by conformational fluctuations of individual protein
domains (Fig. 1g, h). The conformational changes of individual calmodulin domains result in readily detectablechanges in transmission for as long as two hours without possible limitations by photobleaching of fluorescent labelsor contamination of scanning probe tips for force spectroscopy. The results agree with smFS and suggest general utility
for single-molecule protein folding studies [4]. We expect that this approach will rapidly establish itself in the field ofconformational protein dynamics because research groups without expertise in site-directed mutagenesis and chemical
modification of single proteins will be able to carry out detailed biophysical studies on conformational dynamics.

Fig. 1 a, Schematic of the plasmonic optical tweezer platform. The scanning electron microscopy (SEM) image shows a plasmonic
double-nanohole (DNH) structure in a 100 nm thick film of gold. b, Experimental transmission time trace for a protein being
trapped in a DNH structure. Here T0 is the baseline from the light transmission of the empty DNH. The inset cartoon illustrates thepolymer coating of the DNH samples with a protein trapped in the gap. c, Simulated change in transmission, ΔT/T0, as a functionof wavelength after trapping a particle with a diameter, d, of 6 nm and a refractive index, n, of 1.8. d, Simulated dependence of
ΔT/T0 (at λ = 852 nm) on particle size (blue, n = 1.8) and refractive index (red, d = 6). e, f Transmission through a DNH with either
a trapped e, apo-CaM or f, Ca2+-CaM in response to stepwise increases in temperature. g, h, magnified transmission trace for g,
apo-CaM or h, Ca2+-CaM at different temperatures from panels e and f. Four dashed lines represent three folding states of CaM
that are identified in the transmission signal. The corresponding structures are illustrated as cartoons on the right. The cartoons are
adapted from Ref [4].

References[1] Juan, M. L.; Gordon, R.; Pang, Y.; Eftekhari, F.; Quidant, R. 2009, Nat. Phys. 5, 915–919[2] Pang, Y.; Gordon, R. 2012, Nano Lett. 12, 402–406.
[3] Ying, C.; Karakaci, E.; Bermudez-Urena, E.; Ianiro, A.; Foster, C.; Awasthi, S.; Guha, A.; Bryan, L.; List, J.; Balog, S. 2021, arXiv Prepr,
arXiv2107.06407.
[4] Stigler, J.; Ziegler, F.; Gieseke, A.; Gebhardt, J. C. M.; Rief, M. 2011, Science 334, 512–516.

Polarization dependence of double nanohole tweezers for localization andorientation
Ghazal Hajisalem1,2, Michael Dobinson1,2, Zohreh Sharifi1,2, Jon Eby1,2, and Reuven Gordon1,2
1. Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia
V8P 5C2, Canada
2. Centre for Advanced Materials and Related Technology (CAMTEC), University of Victoria, Victoria, British Columbia
V8P 5C2, Canada
E-mail: ghazalh@uvic.ca

Double nanohole (DNH) optical tweezers localize the high field enhancement in the gap regions as small as
10 nm allowing for trapping and analyzing proteins [1]. However, using a DNH as a trapping site requires localizing
it on the substrate by complicated methods such as scanning electron microscope (SEM). Here we investigate thepolarization dependence of DNHs using the optical trapping setup and show that can be used to identify a DNH in
the trapping site. We also show that the polarization response can determine orientation of DNHs on a substrate. The
polarization dependence analyzes of DNHs allows for using DNH optical tweezers without the need for SEM orother expensive and complicated methods and make it more accessible without access to such facilities.
Figure 1(a) shows a schematic of an optical tweezer setup [2] we used in this measurement. Here a 980 nm continuous
wave diode laser beam was collimated, expanded, and focused through a 100× microscope objective into a
nanoaperture (in the sample substrate). The transmitted signal was collected by a 10× microscope objective andmeasured by an avalanche photodiode (APD – Thorlabs, APD120A). The polarization direction of the laser beam wasrotated in steps by a half-wave plate (HWP) in front of the laser. Figure 1(b) shows SEM images of nanoapertures ina substrate: two DNHs apertures (labeled as “DNH 1” and “DNH 2”) with their axes normal in respect to each otherand a single nanohole (labeled as “SNH 1”). Nanoapertures in gold on glass substrate were fabricated by using thecolloidal lithography method [3]. Figure 1(c) shows the polarization dependence of the laser transmission signal
through the DNHs and the SNH apertures shown in (b). Sinus lines were fitted to the measured data in each plot. The
DNHs with normal axes in respect to each other showed opposite transmission response of the laser beam, allowing
for determining the orientation of the DNHs in respect to each other and without having their SEM images. The
polarization response of DNHs was much stronger than the SNH, allowing for identifying the DNHs from the SNH.
The SNH is expected to not show any polarization dependence. Our measurements with SNHs and without a sample
showed similar slight polarization dependence that we believe this was mainly coming from the polarization
dependence of the setup. We showed that using the polarization dependence analysis allows for identifying DNHs insite and without using complicated imaging methods such as SEM. Also, it can be used to determine their orientation,
making the high quality optical trapping using DNH structures accessible to a broad scientific community.

Fig. 1 (a) Schematic of an optical tweezer setup: linear polarizer (LP), half-wave plate (HWP), beam expander (BE), dichroic
mirror (D), 100 × oil immersion microscope objective (100 × OI MO), piezo stage (stage), 10 × microscope objective (10 ×MO),
lens (L), and avalanche photodetector (APD). (b) SEM images of DNHs with their axes normal in respect to each other and an
SEM image of a SNH. (c) Polarization dependence of the laser transmission of the apertures shown in (b). The DNHs with normal
axes in respect to each other show opposite transmission response of the laser beam. The SNH polarization dependent response
was much weaker than the DNHs responses.

References[1] S. Wheaton and R. Gordon, “Single molecule protein sizing in double nano-hole optical tweezers,” in Optical Trapping Applications, (OpticalSociety of America, 2015), pp. OtM3E–5.
[2] G. Hajisalem, E. Babaei, M. Dobinson, S. Iwamoto, Z. Sharifi, J. Eby, M. Synakewicz, L. S. Itzhaki, and R. Gordon, “Accessible highperformance double nanohole tweezers,” Opt. Express 30(3), 3760–3769 (2022).
[3] A. L. Ravindranath, M. Seyed Shariatdoust, S. Mathew, and R. Gordon, “Colloidal lithography double-nanohole optical trapping of
nanoparticles and proteins,” Opt. Express 27(11), 16184–16194 (2019).

Manipulating and characterizing individual bio-particles in nanochannels
Gabriel Schnoeringa, Christian Höllera, Hadi Eghlidia, Maarit Suomalainenb, Urs F. Greberb and Dimos
Poulikakosa
aLaboratory of Thermodynamics in Emerging Technologies, ETH Zurich, Sonneggstrasse 3, Zurich,
SwitzerlandbInstitute of Molecular Life Sciences, University of Zurich, Zurich, Switzerlande-mail: schngabr@ethz.ch

Manipulation of matter at the nanoscale and at the single entity level is of utmost importance in a large
variety of fields ranging from basic sciences to biology and medicine. It is particularly appealing in order todevelop an understanding of complex entities such as viruses, characterize them and assert their
biological/chemical function when interacting with other entities in biological fluids. Modern and successful
approaches mostly exploit configurations where a strong gradient of field provides a stable attracting
potential in space, generated by tightly focused propagating fields or in the near field of antennas. However,
due to the volume dependence of their polarizability, sub-100 nm nano-objects require strong and potentiallyharmful field intensities.
In this work, we transport individual nano-objects in biological level ionic solutions to select locations in onchip nanochannels using dielectrophoretic nanovalves [1]. In-between two such nanovalves the nano-object
is confined and we study its judiciously restricted thermal motion [2]. The confined particle dynamics isanalyzed and provides important properties at the individual nano-object level such as the diffusion
coefficient, hydrodynamic diameter, trap stiffness and electrical conductivity of the individual particles. We
show the versatility of our system by assessing the properties of polystyrene nanospheres, conjugatedpolymer nanoparticles and adenoviruses.
The collaborative effect of the applied alternating current (AC) electric field between nanoelectrodes and adesigned and fabricated nanochannel topography locally amplifies the near-field gradient of a RF field,
forming together a harmonic trap between a nanovalves system with no-moving-parts effective for various
dielectric nanoparticles. This work is an important advancement in manipulating and characterizing
individual nano-objects in biologically relevant ionic solutions, including novel medical nanomaterials, or
specific viruses.

References[1] Eberle, P.; Höller, C.; Müller, P.; Suomalainen, M.; Greber, U. F.; Eghlidi, H.; Poulikakos, D. Single Entity
Resolution Valving of Nanoscopic Species in Liquids. Nat. Nanotechnol. 2018, 13 (7), 578–582.
[2] Höller, C; Schnoering, G; Eghlidi, H; Suomalainen, M; Greber, U; Poulikakos, D. On-chip transporting
arresting and characterizing individual nano-objects in biological ionic liquids. Science Advances, In pressFigure 1. Individual nano-objects are transported through an on-chip nanochannel and arrested between two
nanovalves. The restricted motion is recorded and analyzed via its spectral density and from which the radiusand conductivity of individual particles are obtained at different electrodes potential. This is performed forindividual nanoparticles such as conjugated polymer nanoparticles and adenoviruses in high-ionicenvironments.

Photothermal expansion nanoscopy of the strong coupling between
a patch nanoantenna and a semiconductor quantum well
Simone Sotgiu1, Mario Malerba2, Leonetta Baldassarre1,3, Valeria Giliberti3, Lianhe Li4, A. Giles Davies4,
Edmund H. Linfield4, Andrea Schirato5,6, Alessandro Alabastri7, Mathieu Jeannin2, Jean-Michel Manceau2,
Raffaele Colombelli2 and Michele Ortolani 1,3
1. Department of Physics, Sapienza University of Rome, 00185 Rome, Italy
2. Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, 91405 Orsay, France
3. Istituto Italiano di Tecnologia, Center for Life Nano- & Neuro-Science, I-00161 Rome, Italy
4. School of Electronic and Electrical Engineering, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom
5. Department of Physics - Politecnico di Milano, I-20133 Milan, Italy
6. Istituto Italiano di Tecnologia, I-16163, Genoa, Italy
7. Rice University, Houston (TX), U.S.A.
E-mail: simone.sotgiu@uniroma1.it

The strong light-matter coupling is an exotic phenomenon of interest not only for basic physics, but also because of its
potential for improving optoelectronic devices. Conduction-band electrons in semiconductor quantum wells (QWs)
display narrow intersubband (ISB) transitions in the far- and mid-IR that can couple to the optical resonances ofmetallic nanoantennas. Strong light-matter coupling appears in the form of new mixed light-matter eigenmodes, calledintersubband cavity polaritons [1]. The signature of this regime is the characteristic anti-crossing of the “light” and“matter” excitations when the two resonances spectrally overlap.
In the mid-IR, cavity electrodynamics phenomena are best explored using metallic antennas, one of the simplest layout
being the metal-insulator-metal (MIM) patch antenna, as metal losses are acceptable and optical confinement can berecord-high. Mid-IR spectroscopy measurements in the far-field however require arrays made up of hundreds ofresonators to obtain a good signal-to-noise ratio. Furthermore, the core of the architecture – the active region where
the light-matter interaction takes part – is concealed by the metal and mostly accessible to direct near-field nanoscopy
with scanning probes (e.g. scattering SNOM).
Here we present a novel approach to single resonator spectroscopy based on photothermal induced resonance (PTIR)
expansion nanoscopy with a contact-mode scanning probe, alternative to SNOM techniques [2], capable not only ofdetecting the spectral response of a single MIM cavity in the mid-IR range, but also of mapping the electromagnetic
fields buried within with deep-subwavelength resolution.
We achieve this by inserting in a MIM resonator a 100-nm-thick “probe” layer of (IR-transparent) polyethylene alongwith a single heavily-doped epitaxially-grown QW (Fig. 1a). By shining a tunable mid-IR quantum cascade laser on
the system, the cavity absorbs energy at its resonance wavelengths.

Fig. 1 (a) Scheme of the sample and experimental set-up. (b) Color plot of the spectra acquired as a function of the patch lateral side. (c) IR absorption
maps acquired at the first and second resonance order.

In contrast to usual PTIR nanoscopy, the polymer here is not the object of investigation. It operates instead as a thermal
expansion transducer transforming the light absorbed by the cavity into a vertical displacement, measured by thescanning probe. It permits to “probe” the light-matter interaction directly inside the resonator, where light absorption
takes place. Measuring the PTIR spectra of single cavities with varying patch size, the energy anticrossing signatureof the strong coupling regime is reconstructed (Fig. 1b), with a good correlation with FTIR measurements over large
arrays and with electromagnetic+thermal simulations. At last, by collecting PTIR maps of the patch surface at theresonance frequencies, the near-field coupling between the probe tip and the antenna can be imaged. The extracted
profiles clearly show a single lobe for the (1,0) mode and a double lobe for the (2,0) mode (Fig. 1c), as qualitatively
expected for the sinusoidal current distribution calculated from a simplified electromagnetic solution.
In conclusion we used PTIR nanoscopy to study strong light-matter coupling in single mid-IR resonators withintersubband transitions in semiconductor quantum wells. Both nanoimaging, nanospectroscopy and electromagnetic
and thermal simulation results are reported.
References[1] Todorov, Y., Andrews, A. M., Colombelli, R., De Liberato, S., Ciuti, C., Klang, P., ... & Sirtori, C. (2010). Phys. Rev. lett, 105(19), 196402.
[2 Gillibert, R., Malerba, M., Spirito, D., Giliberti, V., Li, L., Davies, A. G., ... & Ortolani, M. (2020). Appl. Phys. Lett, 117(10), 101104.

Revealing the Local Band Structures of 2D Materials by Near-Field Optical
Imaging: Sharp WS2/MoS2 Heterojunction and Graded WxMo1-xS2 Alloy
Po-Wen Tang1, He-Chun Chou1, Shiue-Yuan Shiau2, Xin-Quan Zhang3, Yi-Hsien Lee3, and Chi Chen1*
1. Research Center for Applied Sciences, Academia Sinica, Taipei,115, Taiwan
2. Physics Division, National Center for Theoretical Sciences, Taipei, 106, Taiwan
3. Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu, 300, Taiwan
E-mail: chenchi@gate.sinica.edu.tw

Two-dimensional (2D) materials are promising for next-generation semiconductor devices and open thepossibilities toward mesoscopic physics. With the development of various chemical vapor deposition (CVD) methods[1, 2], many artificial 2D semiconductors have been synthesized, which increases the chances of forming disrupted
interfaces, non-periodic, or small-size systems (defects and grain boundaries). All such systems create local electronic
band structures within a finite scale, which cannot be readily explained by the solid-state band theory nor be probed
easily by confocal microscopes and macroscopic transport. Local probes, including structure (AFM), electronic states(STM or conductive AFM), and optical transition (SNOM), would be significant for mesoscopic, non-k-space physics.
Here, we investigated the abrupt heterojunction and the graded alloy between two transition metal dichalcogenides(TMD), which both involve with non-periodic band structures and require high spatial resolution. A homemade
horizontal-type scanning near-field optical microscopy (SNOM) was developed to ensure high stability scanning overa long spectral acquisition time [3]. We employed near-field photoluminescence (NF-PL) imaging to study theatomically sharp 1D interfaces between the WS2 and the MoS2 (Fig. 1b). With an optical resolution of 68 nm, a 105
nm-wide region for quenched PL is confirmed with the NF-PL imaging, which is so far the best SNOM resolution in
studying TMDs. Our NF-PL imaging resolved the narrowest quenching width and sharpest strain mapping due to thesuperior spatial resolution and stability of our home-built SNOM.
We further developed the near-field broadband absorption (or transmittance, NF-tr) imaging method to overcome
the limitation of NF-PL in the case of low-quantum-yield materials. NF-tr technique provides the abbreviation-freeand nanoscale-resolution imaging capability of the whole conduction bands over highly lateral inhomogeneity. We
utilized the NF-tr microscopy to investigate the varying bandgap and the bowing factor of the single-layered WxMo1xS2 alloy. High-spatial-resolution spectral capability is essential for analyzing such compositional and locationdependent bandgap evolution. For bilayer WxMo1-xS2 alloy (Fig. 1c), the energy contour maps present the bandgap
evolution in the alloy and reveal the bilayer coupling between the top and bottom layers. We can conclude the alloy
nature of the bottom layer, but the top layer is pure WS2.

Fig. 1 Schematics of the near-field absorption/transmittance (NF-abs/tr) and near-field photoluminescence (NF-PL)
measurements. (b) NF-tr spectroscopic imaging and band structure of the graded alloy. (c) NF-PL imaging and spectra of thelateral heterojunction.

References(1)

Zhang, X.-Q.; Lin, C.-H.; Tseng, Y.-W.; Huang, K.-H.; Lee, Y.-H. Synthesis of Lateral Heterostructures of Semiconducting Atomic Layers.
Nano Lett. 2015, 15 (1), 410–415.

(2)Chiu, K.-C.; Huang, K.-H.; Chen, C.-A.; Lai, Y.-Y.; Zhang, X.-Q.; Lin, E.-C.; Chuang, M.-H.; Wu J.-M.; Lee, Y.-H. Synthesis of In-Plane
Artificial Lattices of Monolayer Multijunctions. Adv. Mater., 2018, 30, 1704796.

(3)Yu, J.-R.; Chou, H.-C.; Yang, C.-W.; Liao, W.-S.; Hwang, I.-S.; Chen, C. A Horizontal-Type Scanning Near-Field Optical Microscope with
Torsional Mode Operation toward High-Resolution and Non-Destructive Imaging of Soft Materials. Rev. Sci. Instrum. 2020, 91 (7), 073703.

Infrared correlation nanoscopy
with unprecedented spectral coverage
Andreas J. Huber1, Tobias Gokus1, Artem Danilov1, Alexander A. Govyadinov1
1. attocube systems AG (neaspec), Eglfinger Weg 2, 85540 Munich-Haar, Germany.
E-mail: andreas.huber@attocube.com

Nanoscale resolved imaging and spectroscopy using scattering-type Scanning Near-field Optical Microscopy (sSNOM) or tapping AFM-IR (local detection of photothermal expansion) enables bypassing the ubiquitous diffraction
limit of light and to achieve a wavelength-independent spatial resolution of <20 nm in the infrared (IR) frequency
range [1,2]. Measurements have successfully demonstrated a wide range of analytical capabilities for e.g. nanoscalechemical mapping and material identification, conductivity profiling, determination of secondary structure ofindividual proteins and vector field mapping, making them a trusted tool for surface analysis in many branches ofsciences and technology.
While s-SNOM and tapping AFM-IR provide nanoscale spatial resolution, applications are often limited by limited
availability of illumination source, which prevents studies of low energy phonons, polaritons and molecular vibrations.
This problem is partially circumvented by nanoscale Fourier transform infrared spectroscopy (nano-FTIR) – a potent
s-SNOM technique – which can achieve large spectral coverage down to 320cm-1 (ca. 31µm wavelength) using
ultrabroadband infrared radiation from synchrotrons [3]. However, nanoscale resolved imaging in the far-IR spectral
range at wavelengths of e.g. >12 µm that provide rapid mapping of field or materials properties remains challenging
due to lack of suitable table-top light sources with sufficiently small bandwidth (e.g. < 10 cm-1).
Here we demonstrate s-SNOM and tapping AFM-IR imaging and spectroscopy based on a fully integrated andautomated commercial OPO laser source covering the spectral range from 1.5 – 18.2µm (ca. 7100 – 540cm-1) withnarrow linewidth <4 cm-1 in the entire tuning range. Figure 1 shows a ca. 34nm thin hBN flake on a SiO substrate
imaged by s-SNOM with interferometric detection in order to obtain nanoscale resolved amplitude and phase imagesof the propagating surface polariton mode. Clearly, the amplitude image at 810cm-1 reveals the characteristic fringe
pattern at the edge of the flake stemming from propagating Surface Phonon-Polaritons (SPhP) launched by the AFM
probing tip, which was not accessible previously.

Fig. 1 hBN Surface Phonon-Polariton near-field imaging at the lower Reststrahlen band revealing characteristic fringes of
propagating polaritons (amplitude left, phase center). Analysis of fringe spacing enables analysis of polariton dispersion as
established for upper Reststrahlen band (right) [4].

Further, the methodology was successfully applied to selectively map the nanoscale spatial distribution of PVAC
in a PS polymer matrix based on the 603cm-1 absorption line, allowing to study characteristic material heterogeneityand interfaces in the far-IR spectral range. Obtained results have been verified by correlative AFM-IR imaging with
operating the laser at selected repetition rates. It is very important to mention that sweeping the laser frequency also
allows to measure spectroscopic signatures of materials and other nanostructures with unprecedented spectral
coverage, enabling studies of fundamental molecular resonances and quantum states in the long wavelength IR spectral
range, which until now was not possible.

References[1] F. Keilmann, R. Hillenbrand. 2004. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences,362, 787–805.
[2] J. Jahng, A. O. Potma, E. S. Lee. PNAS, 116, 26359-26366.
[3] O. Khatib, H. A. Bechtel, M. C. Martin, M. B. Raschke, G. L. Carr. 2018. ACS Phot., 5, 2773-2779.
[4] Q. Zhang, G. Hu, W. Ma, P. Li, A. Krasnok, R. Hillenbrand, A. Alú, C.W. Qiu. 2021. Nature, 597, 187-195.

High-fidelity nano-FTIR spectroscopy by on-pixel normalization of signal
harmonics
Lars Mester1,2, Alexander A. Govyadinov2 und Rainer Hillenbrand3,4
1. CIC nanoGUNE BRTA, Tolosa Hiribidea 76, 20018 Donostia-San Sebastian, Spain
2. attocube systems AG, Eglfinger Weg 2, 85540 Munich-Haar, Germany
3. CIC nanoGUNE BRTA and Department of Electricity and Electronics, EHU/UPV, Donostia-San Sebastian 20018
4. IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
E-mail: lars.mester@attocube.com

Scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared nanospectroscopy(nano-FTIR) are emerging tools for physical and chemical nanocharacterization of organic and inorganic composite
materials. Being based on (i) diffraction-limited illumination of a scanning probe tip for nanofocusing of light and (ii)recording of the tip-scattered radiation, the efficient suppression of background scattering has been critical for theirsuccess. Here, we show that indirect tip illumination via far-field reflection and scattering at the sample can produce
s-SNOM and nano-FTIR signals of materials that are not present at the tip position – despite full backgroundsuppression. Although these artefacts occur primarily on or near large sample structures, their understanding andrecognition are of utmost importance to ensure correct interpretation of images and spectra. Detailed experimental andtheoretical results show how such artefacts can be identified and eliminated by a simple signal normalization step, thus
critically strengthening the analytical capabilities of s-SNOM and nano-FTIR spectroscopy.

Fig. 1 Illustration of typical s-SNOM and nano-FTIR experiments. A metallized AFM tip near a known reference material (left)or unknown sample (right, generic sample composed of materials a, b, c, d) is illuminated by the electric field Einc of a diffractionlimited infrared laser beam (red area). Near-field interaction between tip and sample modifies the tip-scattered light, Esca,
depending on the electrostatic reflection coefficient βj of the material j below the tip apex. The tip is illuminated (and the tipscattered light is detected) directly (red arrows) and indirectly via far-field reflection at the sample surface (blue arrows), dependingon the far-field reflection coefficient rj.

Near-field optical mapping of Dark excitons at room temperature based on
Nano-imprinted pyramid probe
Junze Zhou1, Edward Barnard1, John Thomas1, Elyse Barre1, Archana Raja1, Keiko Munechika2, Adam
Schwartzberg1, Alexander Weber-Bargioni1, Stefano Cabrini1
1. The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
2. HighRI Optics, Inc. Oakland, CA 94618.
E-mail: junzezhou@lbl.gov

Two-dimensional transition metal dichalcogenide (TMDCs) are direct bandgap materials with strong light-matterinteractions in the visible light wavelength range. The light-induced exciton has binding energies as large as a few
hundreds of meV at room temperature, which offers promising applications in optoelectronic devices[1, 2]. The
presence of strong spin-orbit coupling in the energy bands of the monolayers results in the lowest energy exciton in
WX2 (X=S, Se) being spin-forbidden[3-6]. The theoretical study predicted the spin-dark excitonic emission is ~103
times lower than the bright one induced by this optical selection rule[7]. In addition, the optical access of dark exciton(XD) is hampered by the out-of-plane polarization, which is difficult to excite and detect with the normally incident
excitation photoluminescence measurement scheme[4]. Previously, several far-field approaches such as in-planemagnetic field[5], polarization-selective edge detection[4] have been proposed to observe the dark exciton. In nearfield optics, Zhou et al.[8] used the silver thin film as the active substrate with an out-of-plane Purcell effect andcoupled the XD emission out at the edge of the substrate at cryogenic temperature. Park et al.[6] has applied a nearfield probe to increase the radiative decay rate of the XD at room temperature. So far, however, spatially resolved
hyperspectral nano-PL mapping of XD is still missing to explore how material heterogeneities impact the emission
since reproducibly lifting the spin-forbidden selection rule with the scanning probe is challenging.
Here, we employ a reliable and mass-producible gap-mode geometry consisting of the ultra-flat Au thin film
covered with a 2nm dielectric layer and a high-performance near-field probe to spectrally map out the XD emission in
2d WSe2. As shown in the right figure, we designed this gap-mode plasmonic configuration, based on the nanoimprinted scanning probe, for selectively enhancing the out-of-plane dipole emission, and enabling the near-field
optical mapping of the dark exciton in a WSe2 monolayer with the correlated topographic imaging. We demonstrated
that the gap mode nanocavity could effectively enhance the out-of-plane XD, which was observed at ~50meV blow thebright exciton (X0), through a Purcell enhancement of more than 3 orders. The difference in spatial resolution of X0
and XD mappings indicate that the enhancement of XD emission is localized within the nanogap. The correlation of
gap-mode plasmonic enhancement and XD was further confirmed by actively controlling the emission by changing thetip-sample distance. An optical resolution of ~30nm has been achieved through spatially resolved photoluminescence
mapping, which is consistent with the curvature size of the nano-imprinted pyramid tip. This study demonstrates theoutstanding correlated topographic and optical imaging performances of the nanoimprinted pyramid probe, providing
a low-cost and high throughput technique in studying the optical properties of the new advanced quantum materials.

Fig. 1 Gap-mode plasmonic enhancement of PL emission based on the pyramid tip. (a) Schematic of the optical measurement
configuration, which selectively enhanced the out-of-plane dipole emission. (b) PL spectrum of the WSe2 monolayer collected
through the gold pyramid tip shows both bright and dark excitonic emission features. The dark exciton (XD) state is observed at~50meV below the bright exciton (XO). Integrated PL map (2×2µm2) of XD (c) and XO (d).

References[1] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, Emerging Photoluminescence in Monolayer MoS2, Nano Letters
10 (2010) 1271-1275.
[2] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nature Nanotechnology 6 (2011) 147-150.
[3] R.J. Gelly, D. Renaud, X. Liao, B. Pingault, S. Bogdanovic, G. Scuri, K. Watanabe, T. Taniguchi, B. Urbaszek, H. Park, M. Loncar, Probing
dark exciton navigation through a local strain landscape in a WSe2 monolayer, Nat Commun 13 (2022) 232.
[4] G. Wang, C. Robert, M.M. Glazov, F. Cadiz, E. Courtade, T. Amand, D. Lagarde, T. Taniguchi, K. Watanabe, B. Urbaszek, X. Marie, In-Plane
Propagation of Light in Transition Metal Dichalcogenide Monolayers: Optical Selection Rules, Physical Review Letters 119 (2017) 047401.
[5] X.X. Zhang, T. Cao, Z. Lu, Y.C. Lin, F. Zhang, Y. Wang, Z. Li, J.C. Hone, J.A. Robinson, D. Smirnov, S.G. Louie, T.F. Heinz, Magnetic
brightening and control of dark excitons in monolayer WSe2, Nat Nanotechnol 12 (2017) 883-888.
[6] K.-D. Park, T. Jiang, G. Clark, X. Xu, M.B. Raschke, Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell
effect, Nature Nanotechnology 13 (2018) 59-64.
[7] J.P. Echeverry, B. Urbaszek, T. Amand, X. Marie, I.C. Gerber, Splitting between bright and dark excitons in transition metal dichalcogenide
monolayers, Physical Review B 93 (2016) 121107.
[8] Y. Zhou, G. Scuri, D.S. Wild, A.A. High, A. Dibos, L.A. Jauregui, C. Shu, K. De Greve, K. Pistunova, A.Y. Joe, T. Taniguchi, K. Watanabe, P.
Kim, M.D. Lukin, H. Park, Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons, Nature
Nanotechnology 12 (2017) 856-860.

Nano-optics of Epsilon Near Zero materials on metallic substrates
Flávio H. Feres1,2,3, Ingrid Barcelos1, Rafael A. Mayer1,2, Raul de O. Freitas1, Maximilian Obst3, Tobias
Nörenberg3,4, Lukas M. Eng3,4, Susanne C. Kehr3, and Francisco C. B. Maia1
1. Institute of Physics “Gleb Wataghin”, State University of Campinas (UNICAMP), Campinas, Sao Paulo, Brazil
2. Brazilian Synchrotron Light Laboratory (LNLS), Campinas, Sao Paulo, Brazil
3. Institute of Applied Physics, Technische Universität Dresden, 01062 Dresden, Germany
4. Dresden-Würzburg Cluster of Excellence-EXC 2147 (ct.qmat), 01062 Dresden, Germany
E-mail: flavio.feres@lnls.br

The manipulation of light at the nanometer length scale by means of photonic and quantum materials empowers
the development of new optoelectronic devices and technologies [1]. In the kernel of light-matter interaction at nanometer length scales, high attention is directed towards low-dimensional materials, as are van der Waals (vdW) crystals[2,3], that have shown striking importance since providing strong light-matter interactions in the form of polariton
waves. Some vdW materials such as the hyperbolic types, may intrinsically present anisotropic permittivities, where
the real parts of the in-plane (𝜀𝜀𝑥𝑥𝑥𝑥 and 𝜀𝜀𝑦𝑦𝑦𝑦 ) and the out-of-plane (𝜀𝜀𝑧𝑧𝑧𝑧 ) tensor components have opposite signs, i.e.
𝑅𝑅𝑅𝑅�𝜀𝜀𝑥𝑥𝑥𝑥,𝑦𝑦𝑦𝑦 � ∙ 𝑅𝑅𝑅𝑅[𝜀𝜀𝑧𝑧𝑧𝑧 ] < 0, for certain excitation frequencies 𝜔𝜔 located between the transverse 𝜔𝜔 𝑇𝑇𝑇𝑇 and longitudinal𝜔𝜔𝐿𝐿𝐿𝐿 optical phonon branches [4]. Interesting phenomena arise whenever the excitation frequency meticulously approaches permittivity values near zero (𝜀𝜀𝑥𝑥𝑥𝑥,𝑦𝑦𝑦𝑦,𝑧𝑧𝑧𝑧 ~ 0) which exactly defines the epsilon near-zero (ENZ) zones. Within this
scenario, intriguing optical effects are observed, including decoupling of electromagnetic fields [1], infinite impedance[5], slow-light [6], super transmission/reflection [7], and/or ultra-strong coupling to plasmonic structures [8]. Natural
hyperbolic materials, such as hexagonal Boron Nitride (hBN) and alpha-Molybdenum Trioxide (𝛼𝛼-MoO3) possess thisout-of-plane ENZ condition (𝜀𝜀𝑧𝑧𝑧𝑧 ~ 0), spanning from the mid-infrared (Mid-IR) to the terahertz (THz) regime.

In order to explore the influence of the ENZ condition of hyperbolic vdW crystals, we performed synchrotron
infrared nano-spectroscopy (SINS) on 𝛼𝛼-MoO3/Au samples. Fig 1a) shows the interference fringes (highlighted by thewhite dashed-line box) formed by standing waves between the tip and the crystal edge, propagating on the Au surface.
These waves have both long wavelengths (up to 𝜆𝜆𝑝𝑝 = 6 𝜇𝜇𝜇𝜇) and long propagation lengths (up to 𝐿𝐿𝑝𝑝 = 15 µ𝑚𝑚), due tothe low light-confinement at the Au/air interface. Extracting the amplitude profiles for different frequencies, weobserve that such waves only appear for frequencies near 1002 𝑐𝑐𝑐𝑐−1 , which exactly matches with the out-of-plane𝜔𝜔𝐿𝐿𝐿𝐿,𝑧𝑧𝑧𝑧 that corresponds to the ENZ (𝜀𝜀𝑧𝑧𝑧𝑧 ~ 0) mode. Note that the oscillation amplitude vanishes already for detuned
frequencies 𝜔𝜔𝐿𝐿𝐿𝐿,𝑧𝑧𝑧𝑧 of Δ𝜔𝜔 = 10 𝑐𝑐𝑐𝑐 −1 only, hence showing a highly narrow-band response. We interpret thephenomenon as the superposition of the far-field coming from the source reflected by the crystal and the surface
plasmons polaritons (SPP) launched by the tip and reflected by the edges of the slab, where the reflection massively
increases due to the ENZ, producing the interference pattern.
In conclusion, we understand that the out-of-plane hyperbolic crystal, concerning a general effect, can behave as aperfect mirror either for far-field or near-field optics at the ENZ frequencies, most specifically at 𝜔𝜔𝐿𝐿𝐿𝐿,𝑧𝑧𝑧𝑧 frequencies.

Fig. 1: a) Amplitude signal of the 3rd harmonic (S3) of broadband spectral linescan performed by SINS on a 100-nm-thick 𝛼𝛼-MoO3 crystal placedon a gold substrate; b) amplitude profiles extracted from the spectral linescan along the gold substrate, for different excitation frequencies 𝜔𝜔.

References:[1] Liberal, I. et al., 2017. Nat. Phot., 11, 149-158.
[2] Low, T. et al. 2017. Nat. Mat., 16, 182-194.
[3] Ferrari, L. et al., 2015. Prog. Quantum Electron. 16, 1-40.
[4] Narimanov, E. et al., 2015. Nat. Photonics 9, 214-216.
[5] Alù, A. et al., 2007. 75, 155410.
[6] Bello, F. et al., 2017. Sci. Rep. 7, 8702.
[7] Passler, N. C. et al., 2018. Nano Lett. 18, 4285-4292.
[8] Yoo, D. et al., Nat. Photonics 15, 125-130.

Focusing of In-plane Hyperbolic Polaritons in Van der Waals Crystals with
Tailored Infrared Nanoantennas
Javier Martín-Sánchez1, Jiahua Duan1, Javier Taboada-Gutiérrez1, Gonzalo Álvarez-González1, Kirill V.
Voronin2, Iván Prieto3, Weiliang Ma4, Qiaoliang Bao5, Valentyn S. Volkov2, Rainer Hillenbrand6, Alexey Y.
Nikitin7, Pablo Alonso-González1
1. University of Oviedo, Department of Physics c/ García Lorca, Oviedo, Spain
2. Moscow Institute of Physics and Technology, Russia
3. Institute of Science and Technology Austria IST, Am Campus 1, 3400 Klosterneuburg, Austria
4. Huazhong University of Science and Technology, Wuhan, China
5. Department of Materials Science and Engineering and ARC Centre of Excellence in Future Low-Energy Electronics Technologies(FLEET), Monash University, Clayton, Victoria 3800, Australia.
6. nanoGUNE BRTA and Department of Electricity and Electronics, UPV/EHU, Spain
7. Donostia International Physics Center (DIPC), Donostia-San Sebastián, Spain.
E-mail: javiermartin@uniovi.es

Polaritons -hybrid light-matter excitations- play a crucial role in fundamental and applied sciences, as they enable
control of light on the nanoscale [1]. The recent emergence of low-loss van der Waals (vdW) materials opens the door
to achieving anisotropic optical phenomena owing to their layered crystal structure, which leads to an intrinsic andstrong out-of-plane (perpendicular to the layers) optical anisotropy. A prominent example is given by hyperbolic
phonon polaritons (PhPs) -infrared light coupled to lattice vibrations in layered polar materials- in hexagonal boron
nitride (h-BN) [2], which exhibit long lifetimes, ultra-slow propagation and hyper-lensing effects. Only recently, PhPs
with in-plane hyperbolic dispersion, a key requirement for on-chip planar optical circuitry, have been demonstrated innatural slabs of α-phase molybdenum trioxide (α-MoO3) [3-5] and vanadium pentaoxide (α-V2O5) [6].
In this work, we demonstrate focusing of infrared ray-like hyperbolic PhPs into deep subwavelength focal spots
along the surface of α-MoO3 crystals by using metal antennas with an optimized design. Specifically, field confinement
is achieved in focal spots with a size of λp/4.5=λ0/50 (λp is the polariton wavelength and λ0 is the photon wavelengthin free space). Moreover, the achievable focal distance in in-plane hyperbolic α-MoO3 can be tuned to values well
below the diffraction limit in in-plane isotropic materials, along with a better performance in terms of near field
confinement and optical absorption. Our findings set the grounds for planar polaritonic technologies at the nanoscale[7].

Fig. 1 Nanofocusing of phonon polaritons along the surface of a α-MoO3 flake into deep-subwavelength focal spots (λ0/50, being
λ0 the illuminating wavelength) using rod-like trapezoidal metal nanoantennas. The comparison between the experiment (a) andnumerical simulations (b) are shown.

References[1] Low, T., Chaves, A., Caldwell, J.D., Kumar, A., Fang, N.X., Avouris, P., Heinz, T.F., Guinea, F., Martin-Moreno, L., Koppens, F. 2017. Nat.
Mater., 16, 182.
[2] Dai,S., Fei, Z., Ma, Q., Rodin, A.S., Wagner, M., McLeod, A.S., Liu, M.K., Gannett, W., Regan, W., Watanabe, K, Taniguchi, T., Thiemens,M., Dominguez, G., Castro Neto, A.H., Zettl, A., Keilmann, F., Jarillo-Herrero, P., Fogler, M.M., Basov, D.N. 2014. Science, 343, 1125.
[3] Ma, W., Alonso-González, P., Li, S., Nikitin, A.Y., Yuan, J., Martín-Sánchez, J., Taboada-Gutiérrez, J., Amenabar, I., Li, P., Vélez, S., Tollan,C., Dai, Z., Zhang, Y., Sriram, S., Kalantar-Zadeh, K., Lee, S.T., Hillenbrand, R., Bao, Q. 2018. Nature, 562, 557.
[4] Duan, J., Capote-Robayna, N., Taboada-Gutierrez, J., Álvarez-Perez, G., Prieto, I., Martín-Sánchez, J., Nikitin, A.Y., Alonso-González, P.
2020. Nano Lett., 20, 5323.
[5] Duan, J., Álvarez-Pérez, G., Voronin, K.V., Prieto, I., Taboada-Gutiérrez, J., Volkov, V.S., Martín-Sánchez, J., Nikitin, A.Y., AlonsoGonzález, P. 2021. Sci. Adv., 7, eabf2690[6] Taboada-Gutiérrez, J., Álvarez-Pérez, G., Duan, J., Ma, W., Crowley, K., Prieto, I., Bylinkin, A., Autore, M., Volkova, H., Kimura, K.,
Kimura , T., Berger, M.-H., Li, S., Bao, Q., Gao, X.P.A., Errea, I., Nikitin, A.Y., Hillenbrand, R., Martín-Sánchez, J., Alonso-González , P. 2020.
Nat. Mater., 19, 964.
[7] Martín-Sánchez, J., Duan, J., Taboada-Gutiérrez, J., Álvarez-Pérez, G., Voronin, K.V., Prieto, I., Ma, W., Bao, Q., Volkov, V.S., Hillenbrand,R., Nikitin, A.Y., Alonso-González, P. 2021. Sci. Adv., 7, eabj012.

Observation of phonon polaritons in multilayer hexagonal boron nitride films
grown by chemical vapor deposition
Calandrini Eugenio1, Voronin Kirill2, Balci Osman3, Shinde Maruti Shinde3, Sharma Subash3, Barra Burillo
Maria1, Bylinkin Andrei1, 2, Casanova Felix1, 4, Hueso Arroyo Luis1, 4, McAleese Clifford5, Conran R. Ben5,
Wang Xiaochen5, Kenneth Teo5, Ferrari Andrea C.3, Nikitin Alexey2 and Hillenbrand Rainer1, 4
1. CIC nanoGUNE BRTA, Donostia - San Sebastián, Spain
2. Donostia International Physics Center (DIPC), Donostia - San Sebastián, Spain
3. Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, United Kingdom
4. IKERBASQUE, Basque Foundation for Science, Bilbao, Spain
5. AIXTRON Ltd. Buckingway Business Park Anderson Rd, Swavesey, Cambridge CB24 4FQ, UK
Email: e.calandrini@nanogune.eu

Hexagonal boron nitride (hBN) is a heavily studied van der Waals (vdW) material that is able to support highly
confined phonon polaritons (PhP) in the mid-infrared spectral range [1,2]. It inspired novel concepts in the field ofnanoscale infrared imaging [3], field-enhanced infrared sensing [4] and vibrational strong coupling [5,6]. However,
the typical approach for fabricating h-BN PhP devices relies on mechanical exfoliation of bulk crystals, viable only
for proof- of- concept experiments due to its low yield. Although the interest in producing hBN and other vdW
materials by other means is raising, PhPs on synthetized hBN were only reported on mono- and bilayers [7]. PhP
nanoresonators and other devices based on such thin layers, however, may be unpractical for real-world applications
due to very small extinction cross sections.
Here we present a combined scattering-type scanning near-field optical microscopy (s-SNOM) and far-field
infrared spectroscopy study of wafer-scale hBN films of several nanometer thickness, which were grown by chemical
vapor deposition (CVD) and transferred onto a Si/SiO2 substrate.
We performed far- field reflection spectroscopy in order to extract the dielectric permittivity of the CVD grown
hBN films and calculated the expected dispersion and lifetime of the PhPs. Interestingly, and in stark contrast to
exfoliated hBN, s-SNOM imaging of the CVD grown material revealed random interference patterns all over the film
(see Fig. 1) that we can attributed to polaritonPhP scattering at randomly distributed defects in the hBN layer. To
analyze these patterns and determine the polariton dispersion and lifetime, we developed a theoretical analysis modeland an experimental method that result to confirm our predictions. I don’t understand. Does not seem to make sense.
Finally, we designed and etched hBN nanoribbons displaying phonon polaritonthat exhibit transverse PhP resonanceswith quality factors ~50, which is still competitive if compared to resonators made of natural hBN flakesare only afactor of about two smaller than that of PhP resoantors made of exfoliated hBN flakes [5]. Our study shows the potentialof CVD- grown hBN layers to be used for large- scale fabrication of PhP based resonators and devices, e.g. for infraredsensing appliations.

Fig. 1. a, Illustration of infrared s-SNOM imaging of PhPs in CVD-hBN. b. Topography image of a 24 nm thick CVD hBN film on SiO2. c, Infrareds-SNOM phase images, revealing PhP interference fringes parallel to the hBN edge (analog to what is observed in s-SNOM images of exfoliatedhBN flakes [6] but also random interference patterns far away from the edge (whidhwhich can be explained by polariton scattering at defects), d,
Averaged s-SNOM profiles (showing the real part of the s-SNOM signal) extracted from panel d perpendicular to the hBN edge. The period of thesignal oscillations period (corresponding to half of the polariton wavelength insert p) is clearly shrinking with decreases with increasing
frequency, indicating PhPs propagating. revealing propagating PhPs.

References[1] Basov, D N. 2016. Science, 354, 195.
[2] Dai S. 2014. Science, 343, 1125-1129.
[3] Li P. 2015. Nat. Comm., 6, 1-9.
[4] Bareza N. 2022. ACS Photonics, 9, 34–42.

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[5] Autore M. 2018. Light: Science & Applications, 7, e17172.
[6] Bylinkin A. 2021. Nat. Phot., 15, 197–202.
[7] Dai S. 2019. Adv. Mater., 31, 1806603.

Anomalous coupling in hyperbolic medias
Lorenzo Orsini1, Hanan Herzig-Sheinfux1, Iacopo Torre1, Matteo Ceccanti1 and Frank H. L. Koppens1
1. ICFO, Av. Carl Friedrich Gauss 3, Castelldefelds, Spain
E-mail: lorenzo.orsini@icfo.eu

In optics, photonic crystals are widely used to manipulate the dispersion of light and control its propagation. The
possibility of creating metamaterials with optical properties exceeding the capabilities of natural materials has been
investigated with great success in Silicon-based devices [1]. It has been shown that bringing this control to the nanoscale is extremely interesting to enhance light-matter interaction [2]. However, the Silicon platform has limited
capabilities of confining light to subwavelength volumes. The alternative are polaritonic based metamaterial [3, 4], but
the detrimental effect of absorption is still the main limiting factor. Recently, a new concept for confining light to thenanoscale using indirectly patterned hexagonal boron nitride (hBN) has been demonstrated [5]. In this material,
multimodal nano-rays are the natural excitations, and the detrimental effect of absorption are neglected by multimodal
effects (Fig. 1). Moreover, the coupling between these resonators is expected to inherit the multi-modal character ofthe resonance. This property can be used to produce very complex and rich metamaterials.
(a)
(b)
(c)
(d)
50nm
hBN
Gold film

hBNGold film

Fig. 1. (a) Illustration of the indirect patterned hyperbolic nano-cavity measured with sSNOM. (b) Top panel: illustrationof the cross-section of the nano-cavity presented in (a). The hBN flake is suspended over a hole milled through a thingold film. Bottom panel: color-plot showing the simulated electric field intensity |E|2 at resonance of the structureillustrated above. The mode is well confined in the center of the cavity and the multi-modal character of this resonatoris visible as nano-rays propagating inside the hBN slab at a fixed angle. (c) Quality factor measured with infrared nearfield optical spectroscopy. (d) SEM image of the milled gold film showing the capability of reducing the size of a single
nano-resonator down to ∼ 10𝑛𝑚 x 10𝑛𝑚.

Spacing, nm

Here, we study this multimodal coupling mechanism, experimentally and theoretically. An array of coupledresonators is designed to study how the coupling strength changes with respect to the separation distance (Fig. 2a).
Using near-field microscopy we measure the resonance frequency of the coupled resonators. Notably, we observe a
non-monotonic shift as the separation distance is increased (Fig. 2b). All in all, this is indicating the uniqueness of thecoupling mechanism, since the most typical evanescent coupling would have produced a monotonically decreasing
resonance shift. In conclusion, this peculiar coupling mechanism represents an alternative path to manipulate light on
the extreme nanoscale, allowing the design of nanostructures with optical properties even richer than the traditional
metamaterials. For example, photonic lattices hosting multimodal band-structures, enhanced next-nearest-neighborcoupling and second-order topological phases.
(a)(b)

Reference
film

1 μm
Wavenumber, cm-1
Fig. 2. (a) Top panel: illustration of the coupled resonators array of pairs having different separation distances. Bottom
panel: near-field scan of the array shown above at 1380cm-1. The different brightness along the array indicates thateach pair has different resonance frequency. At this specific illumination frequency only the pair in the middle isresonating. (b) Color-map of the spectral response of many coupled resonators. Blue color indicates a weak response
whereas the strong one is indicated in red. The black dashed line represents the resonance frequency of the isolated
resonator used as a reference. The white dashed arrow is a guide to the eye, highlighting the non-monotonic resonanceshifts.

References[1] Staude, I., Schilling, J. 2017. Nature Photon., 11, 274-284.
[2] Rider, M.S., Palmer, S.J., Pocock et al. 2019. J. Appl. Phys., 125, 120901.
[3] Alfaro-Mozaz, F.J., Rodrigo, S.G., Alonso-González, P. et al. 2019. Nat. Commun., 10, 42.
[4] Xiong, L., Forsythe, C., Jung, M. et al. 2019. Nat. Commun., 10, 4780.
[5] Herzig-Sheinfux, H., Orsini, L., Jung, M. et al. 2022. arXiv preprint, 08611v1

Ordered and Disordered Porous Nanostructures
for Passive Radiative Cooling
Azadeh Didari-Bader , M. Pinar Mengüç
12

1. School of Basic Sciences, Hanbat National University, Yuseong-gu, Daejeon 34158, South Korea
2. Center for Energy, Environment and Economy, Ozyegin University, Cekmekoy, Istanbul 34794, Turkey
E-mail: azadeh@hanbat.ac.kr, pinar.menguc@ozyegin.edu.tr

The atmosphere’s first transparency window (between 8-13 μm) coincides with the peak of the blackbody radiation
spectrum for temperatures around 300 K. Within this window, nanophotonic systems can be designed to achieve
radiative cooling even under direct sunlight. To realize this, designer systems should enhance thermal emission within
the atmospheric transparency window and suppress it everywhere else outside of this window [1]. This allows theuniverse to act like a heat sink and lets the heat through the atmospheric window which greatly reduces the absorptionof incoming atmospheric radiation. In order to design such selective emitters careful selection of materials andgeometrical properties is important [2]. These designer nanophotonic systems could be used in radiative coolingapplications such as in buildings, electronics and wearable technologies.
In this work, we present a systematic numerical analysis of various porous nanophotonic structures for radiative
cooling applications. As shown in Fig. 1, ordered (left) and disordered (i.e., randomly distributed; right) are considered.
Using a finite difference time domain (FDTD) method, we present the results for porous and mesoporous photonic
systems made of materials such as TiO2, SiO2, ITO, W and phase change materials including VO2 [3-4]. We analyze
the impact of the frequency-dependent material properties and geometrical characteristics in order to identify the
optimum selective emitters. We further perform optimization analysis on the geometrical characteristics such as radiusof the pores, pore spacing and combined effects of pores of different sizes in these systems and identify the geometricproperties that provide the optimum results for enhanced radiative cooling. The findings of this work can help thedesign of innovative structures to be used as selective thermal emitters.

Fig. 1. Schematics of (left) ordered and (right) disordered nanophotonic systems considered for radiative cooling applications.

References[1] Ono, M., Chen, K., Li, W., Fan, S. 2018. Opt. Express 26, A777-A787.
[2] Didari, A., Menguc, P. M. 2018. Sci. Rep 8 (1), 1-9.
[3] Wu, S. R., et al. 2018. Sci. Rep 8, 1-6.
[4] Gentle, A., et al. 2018. Proc. SPIE 10759, 107590L.

SQUEEZED STATES FROM PHOTONIC COOPER PAIRS
Sanker Timsina,1,3 Filomeno S. de Aguiar Júnor,2,3 Sahar Gholami Milani,1,3 Alexandre Brolo,2,3 and Rogériode Sousa1,21Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada
2
Department of Chemistry, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada
3
Centre for Advanced Materials and Related Technology,
University of Victoria, Victoria, British Columbia V8W 2Y2, Canada
Email: stimsina@uvic.ca

Raman scattering may generate entangled photon pairs when the same excitation created during a Stokes process isannihilated by another incoming photon. This correlated Stokes-antiStokes (SaS) process can also occur due to theexchange of virtual phonons, leading to a Hamiltonian that is identical to the one used in the Bardeen-CooperSchrieffer theory of superconductivity. Entangled photon pairs generated in this way are called photonic cooper pairs[1].
So far the SaS experiments are focused on coincidence measurements yielding large zero-time second order correlationg(2)(0), which proves that the photons are in a robust quantum state. While this demonstrates that the SaS process is aquantum phenomenon, it does not allow quantification of entanglement for quantum optics applications. Here we use
the theory of photonic Cooper pairs to show that the ensemble of photonic Cooper pairs are actually in a multimodesqueezed state. We calculate the degree of squeezing as a function of laser intensity and material properties, anddiscuss whether photonic Cooper pairs can be a useful resource in quantum sensing.
References1.

Saraiva, A., Júnior, F. S. D. A., De Melo E Souza, R., Pena, A. P., Monken, C. H., Santos, M. F., Jorio, A. (2017). Physical Review
Letters, 119(19).

Electro-optical control of atomic bistability with graphene
Mikkel Have Eriksen1, Jakob E. Olsen1, Christian Wolff1, and Joel D. Cox1,2

1. Center for Nano Optics, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
2. Danish Institute for Advanced Study, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, DenmarkE-mail: cox@mci.sdu.dk

The intrinsically strong light-matter interactions available in graphene—the atomically-thin carbon layer—giverise to a broadband 2.3% light absorption that can be actively modulated by electrostatic gating; recent experiments
confirm that the optoelectronic tunability of graphene can enable fast dynamical control of strong near-fieldinteractions, leading to over 1000-fold decay enhancement of erbium emitters [1]. Highly-doped graphene also
supports long-lived and actively-tunable plasmons that can dramatically enhance near-field coupling, but are limit to
THz and IR parts of the electromagnetic spectrum, well-below the operational frequencies of robust quantum lightsources [2,3]. Here we explore the optical bistability emerging from the self-interaction of an atom mediated by anextended sheet of highly-doped graphene, which we demonstrate can be harnessed to actively drive the atom into
different quantum states (see Fig. 1). The strong electro-optical response of the graphene-atom hybrid system can beobserved in the resonance fluorescence spectrum, which we predict to exhibit Mollow triplets and single Rayleighpeaks when electrically tuned through hysteresis loops. More specifically, we show that the Mollow triplets can be
abruptly created or quenched in a manner reminiscent of a first-order phase transition in thermodynamics. Our findings
motivate further experimental studies of driven atomic light emitters, while offering a prescription for active and insitu manipulation of quantum optical states in integrated nanophotonic platforms.

Fig. 1 Atom-field coupling of an optically-driven two-level atom interacting with a graphene sheet. (a) Schematic of the hybrid
system comprised of a two-level atom above an extended graphene sheet driven by monochromatic light of frequency ω. (b)Atomic Rabi frequency Ω and (c) self-interaction G normalized to the intrinsic decay rate Γ0 as functions of the graphene Fermi
energy. Population difference in the two-level atom obtained by adiabatically sweeping the (d) light intensity, (e) frequency, and(f) graphene Fermi energy.

References[1] Cano, D. et al. Fast electrical modulation of strong near-field interactions between erbium emitters and graphene. Nat. Commun. 2020, 11,4094.
[2] Gonçalves, P. A. D., Stenger, N., Cox, J. D., Mortensen, N. A., and Xiao, S. Strong light-matter interactions enabled by polaritons inatomically-thin materials. Adv. Opt. Mater. 2020, 8, 1901473.
[3] Cox, J. D. and García de Abajo, F. J. Nonlinear atom-plasmon interactions enabled by nanostructured graphene. Phys. Rev. Lett. 2018, 121,
257403.

Study of correlated Stokes and anti-Stokes components in Surface-enhanced
Raman Scattering
Filomeno Soares de Aguiar Junior, Sahar Gholami Milani, Sanker Timsina, Stanislav Konorov,
Rogerio de Sousa, Alexandre Brolo.
1. University of Victoria - UVic
E-mail: filomenojunior@uvic.ca

The Raman scattering has two components: the Stokes (S) and anti-Stokes (aS). In the S process, one photon fromthe laser loses energy, creating one phonon in the sample, while in the aS process one photon gains energy byabsorption of one phonon. When the same phonon created in the anti-Stokes process participates in the aS scattering,
we have the correlated Stokes-anti-Stokes Raman Scattering, denominated SaS process, proposed by Klysko in 1976
[1]. The SaS is characterized by the scattering of a correlated S-aS photon pair, that was also observed out of resonancewith the phonon energy in a process denominated virtual SaS. The virtual SaS is described by a formalism analogous
to the BCS theory of superconductivity, putting these photons as photonic counterparts of Cooper pairs [2,3].
The SaS real and virtual processes were studied in many transparent materials, including diamond [4], water [5,2],
and hydrocarbons [2]. The ability to generate correlated S-aS photon pair in Raman scattering has been explored inquantum optics and quantum information [6,7], used in the implementation of the quantum memory [8]. However,
because of the low efficiency of SaS scattering, the observation of this phenomenon requires high laser power being
achieved using pulsed lasers or electronic resonances [9]. Besides, the number of observed S-aS photon pairs is still
low in comparison with other sources of correlated photon pairs, such as parametric conversion [4]. However, is
expected to increase the S-aS intensity by increasing the quantum efficiency of conventional Raman scattering [3].
Thus, the Surface-enhanced Raman scattering (SERS) arises as a possible source of correlates SaS photon pairgeneration, once this technique can increase the Raman cross-sections values on the order of 108 or large [10]. In thiswork, we propose to investigate the SaS photon pair production from single-molecule, using SERS, looking for theenhanced intensity of the correlated Raman scattering.

References[1] Klyshko, D. N. (1977). Correlation between the Stokes and anti-Stokes components in inelastic scattering of light. Sov. J. Quantum Electron.
Kvantovaya Elektron. (Moscow), 7(4), 755–1341.
[2] Saraiva, A., Júnior, F. S. D. A., De Melo E Souza, R., Pena, A. P., Monken, C. H., Santos, M. F., Jorio, A. (2017). Photonic Counterparts of
Cooper Pairs. Physical Review Letters, 119(19)[3] Júnior, F. S. de A., Saraiva, A., Santos, M. F., Koiller, B., Souza, R. de M. e, Pena, A. P., Jorio, A. (2019). Stokes–anti-Stokes correlated
photon properties akin to photonic Cooper pairs. Physical Review B, 99(10), 100503.
[4]Kasperczyk, M., Jorio, A., Neu, E., Maletinsky, P., & Novotny, L. (2015). Stokes-anti-Stokes correlations in diamond. Opt Lett, 40(10), 2393–
2396.
[5]Kasperczyk, M., De Aguiar Júnior, F. S., Rabelo, C., Saraiva, A., Santos, M. F., Novotny, L., & Jorio, A. (2016). Temporal Quantum
Correlations in Inelastic Light Scattering from Water. Physical Review Letters, 117(24).
[6] Duan, L. M., Lukin, M. D., Cirac, J. I., & Zoller, P. (2001). Long-distance quantum communication with atomic ensembles and linear optics.
Nature, 414(6862), 413–418.
[7] Kuzmich, A., Bowen, W. P., Boozer, A. D., Boca, A., Chou, C. W., Duan, L.-M., & Kimble, H. J. (2003). Generation of nonclassical photon
pairs for scalable quantum communication with atomic ensembles. Nature, 423(6941), 731.
[8] Lee, K. C., Sussman, B. J., Sprague, M. R., Michelberger, P., Reim, K. F., Nunn, J., … Walmsley, I. a. (2011). Macroscopic non-classicalstates and terahertz quantum processing in room-temperature diamond. Nature Photonics, 6(1), 41–44.
[9] Jorio, A., Kasperczyk, M., Clark, N., Neu, E., Maletinsky, P., Vijayaraghavan, A., & Novotny, L. (2014). Optical-phonon resonances withsaddle-point excitons in twisted-Bilayer graphene. Nano Letters, 14(10), 5687–5692.
[10] Langer, J., Jimenez de Aberasturi, D., Aizpurua, J., Alvarez-Puebla, R. A., Auguié, B., Baumberg, J. J., ... & Liz-Marzán, L. M. (2019).
Present and future of surface-enhanced Raman scattering. ACS nano, 14(1), 28-117.

Sensing Angular Momentum of Light using Cavity Optomechanical Coupled
Oscillator System
Bishnupada Behera1, Hamidreza Kaviani1, David Lake1, Ghazal Hajisalem1, Gustavo Luiz2 and Paul Barclay1
1. Institute for Quantum Science and Technology, University of Calgary,2500 University Drive NW, Calgary, AB, T2N 1N4, Canada
2. nanoFAB Centre, University of Alberta, 116 St NW, Edmonton, AB, T6G 2V4, Canada
E-mail: bishnupada.behera@ucalgary.ca

It is well known that light has both linear as well as angular momentum. The circular polarization of light gives
rise to spin angular momentum, and light’s helical phase structure gives rise to orbital angular momentum, which is
discrete and, in principle, unbounded [1]. This spatial structure of light enables access to a large state space which canbe used to encode a large amount of information per photon. This characteristic feature of light has the potential for awide range of applications in technologies such as in high bandwidth data transfer, optical tweezers, LIDAR, quantum
communication, etc. Moreover, it is also known that light carrying spin or orbital angular momentum, upon interaction
with any object can exert a mechanical torque on that object, although miniscule.
In a nanoscale structure consisting of an optical cavity, the strong interaction between light and mechanicaloscillations enables very high precision optomechanical measurement [2] of force or torque. Here, as shown in Fig. 1,
we present a novel experimental scheme involving an on-chip cavity optomechanical coupled oscillator system. This
coupled oscillator system serves as a torque-sensing device with a sensitivity of τmin ~ 10-22 – 10-19 Nm/√Hz formechanical frequencies in the MHz-GHz regime. In this system, we have a nanomechanical torsional oscillator, i.e.,a microdisk (md) suspended using two support beams (of length, ls). An intensity-modulated (red) laser beam carrying
angular momentum upon incidence on the microdisk actuates/excites its in-plane torsional modes. The mechanicaltorsional motion of the microdisk is transduced to in-plane vibrational modes of an orthogonally connected suspended
nanobeam (mb). This nanobeam (mb) along with another suspended nanobeam (rb) in parallel create an
optomechanical two-beam photonic crystal ‘zipper’ cavity [3], to readout/transduce these hybridized torsional motion(excited in the mechanically coupled microdisk by light carrying angular momentum), onto the cavity laser field
coupled into and out of the photonic crystal cavity using a tapered optical fiber.

Fig. 1. (a) Scanning electron micrograph of the suspended microdisk-zipper cavity optomechanical device in Silicon, along withthe optical field simulations of the two-beam photonic crystal ‘zipper’ cavity. (b) Schematic of actuation of the torsional motion.
(c) Finite element simulation of an in-plane hybridized torsional mode of the device. (d) Coupled oscillator model for the torquesensing device. [ΩD: Intensity modulation frequency of the drive (red) laser carrying angular momentum, γxx: Dissipation rates ofthe mechanical oscillators/modes, κ: Decay rate of the optical cavity field, gm: Mechanical coupling coefficient, gom:
Optomechanical coupling coefficient, Ωxx: Mechanical mode(s) frequencies, ωc: Optical cavity resonance frequency]

Our scheme/device provides a platform for interfacing free-space angular momentum carrying optical fields to onchip nanophotonic components. This experiment not only opens the door for the study of interesting new dynamics ofcoupled nanomechanical torsional oscillators but also provides a foundation for ultra-sensitive torque/optical angularmomentum measurement of single photon in the quantum regime.
References[1] Allen, L., Beijersbergen, M.W., Spreeuw, R.J.C. and Woerdman, J.P., 1992. Orbital angular momentum of light and the transformation of
Laguerre-Gaussian laser modes. Physical review A, 45(11), p.8185.
[2] Aspelmeyer, M., Kippenberg, T.J. and Marquardt, F., 2014. Cavity optomechanics. Reviews of Modern Physics, 86(4), p.1391.
[3] Eichenfield, M., Camacho, R., Chan, J., Vahala, K.J. and Painter, O., 2009. A picogram-and nanometre-scale photonic-crystal optomechanicalcavity. nature, 459(7246), pp.550-555.

Circularly polarized light emission by incandescent metasurfaces
Anne Nguyen1, Anne-Lise Coutrot1, Jean-Paul Hugonin1, Enrique Garcia-Caurel2,
Benjamin Vest1, Jean-Jacques Greffet1
1. Université Paris-Saclay, Institut d'Optique Graduate School, CNRS, Laboratoire Charles Fabry, 91127 Palaiseau, FRANCE
2. LPICM, CNRS, Ecole polytechnique, Institut Polytechnique de Paris, 91127 Palaiseau FRANCE
E-mail: benjamin.vest@institutoptique.fr

Thermal emission is an intrinsically incoherent process, delivering broadband, isotropic and unpolarized radiation.
Therefore, generation of a circularly polarized beam from an incandescent source generally requires the use of several
components, such as filters and waveplates, in order to provide the adequate quarter-wave retardance within a limited
range of frequencies. After these manipulation steps, a significant fraction of the beam energy is lost. Moreover, thisapproach limits the compactness of the whole light-emitting system. In our presentation, we report about the design,
fabrication and characterization of a single ultrathin component, a thermal metasurface [1], designed to provide
circularly polarized light in the mid-wave infrared domain.
The device consists of a three-layer structure: a back-reflecting gold mirror supporting a dielectric SiNx spacer
whose top surface is paved with a subwavelength metallic grating. This grating is fabricated using thin platinum wires
following a chiral pattern (see Fig. 1a)). The geometry of the system is optimized using a generalized Kirchhoff’s law
approach [2]. In other words, we design our system and estimate quantitatively its performances in emission using
numerical calculations of its absorptivity. Our approach aims at maximizing both the overall efficiency of the device
and its ability to generate light with a high degree of circular polarization.

Fig. 1 a) Schematic of the device. b) Experimental determination of the Stokes parameters, based on emissivity measurements.
Stokes(1) is proportional to the contrast in emissivity for polarization emitted at 0° and 90° (along and orthogonal to the grating
period). Stokes(2) is proportional to the contrast in linear polarization emitted at 45° and -45° with respect to the same axis.
Stokes(3) is proportional to the contrast between right-handed and left-handed circular polarization. This contrast is maximal aroud
4.5 um with negligible values for Stokes(1) and Stokes(2) on the same spectral domain. All values are normalized to the value of
Stokes(0), the total emitted intensity.

The device operate in a Salisbury screen configuration. The thickness and permittivity of the spacer layer ischosen in order to maximize the absorption of an incoming light beam within the platinum grating [3]. Besides, thegrating pattern and dimensions are chosen in order to make the grating a good chiral absorber: we maximize thecontrast in absorption of the system when illuminated respectively by right-handed circularly polarized light (RCP)
and left-handed circularly polarized light (LCP). The system is then turned into an incandescent metasurface byheating the top metallic grating by Joule effect. The contrast in absorptivity for LCP and RCP leads to a contrast inemissivity between LCP and RCP (see Fig 1.b)).
We report about the experimental performances of our devices. Our devices generate partially polarized lightbeams, with a near-zero degree of linear polarization (DLP) and a degree of circular polarization (DCP) ranging
between 0.3 and 0.35 between 4.3 um and 4.8 um. The total degree of polarization increases up to 0.5 for longer
wavelengths but is associated to more elliptical states of polarization.
Our system paves the way towards cheap and compact devices to generate infrared light with a decent DCP using
only one single component, for applications in communication, sensing, security and counterfeiting.
References[1] Overvig, A. C., Mann, S. A., & Alù, A. (2021). Phys. Rev. X, 11(2), 021050. https://doi.org/10.1103/PhysRevX.11.021050[2] Greffet, J.-J., Bouchon, P., Brucoli, G., & Marquier, F. (2018). Phys. Rev. X, 8(2), 021008. https://doi.org/10.1103/PhysRevX.8.02100[3] Wojszvzyk, L., Nguyen, A., Coutrot, A. L., Zhang, C., Vest, B., & Greffet, J. J. (2021).. Nat. Comm., 12(1), 1–8.
https://doi.org/10.1038/s41467-021-21752-w

Multipoles zoology made simple
Marco Riccardi, Andrei Kiselev, Karim Achouri and Olivier J.F. Martin
Nanophotonics and Metrology Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland
E-mail: olivier.martin@epfl.ch

Most publications that study the response of nanophotonic structures – from an isolated nanostructure to the mostcomplex metasurface – resort at one stage or the other to a multipoles analysis. Such analysis is very helpful tounderstand the spectral response of the system or the variations of its near-field distribution. The interplay between
different multipoles leads to specific optical responses in the far-field, either very strong or very weak, depending on
the multipoles that are interacting.
In spite of this ubiquity, multipoles can appear at first sight quite intricate since different multipoles families are needed
to describe specific physical situations. Sometimes, a pair of electric and magnetic dipoles of vectorial nature that canbe easily computed will suffice to obtain the response of the system. Often, higher order multipoles with a tensorial
nature and concomitant complications are required. There are even situations that necessitate a seemingly completely
different collection of so-called toroidal multipoles.
In this work, we clarify the multipoles zoology by reviewing three different approaches to calculate electromagnetic
multipoles: the Cartesian primitive multipoles, the Cartesian irreducible multipoles and the spherical multipoles. We
show that toroidal moments stem from the residuals that remain after expanding primitive multipoles into irreducible
multipoles. We also study the influence of substrates and surfaces on the multipolar response of scattering systems andexplain which multipoles family is the best suited to describe electromagnetic scattering by specific geometries andmake a clear link with experiments.

Solar steam generation on scalable ultrathin thermoplasmonic TiN nanocavity
arraysLuca Mascaretti1,#, Andrea Schirato2,3,#, Radek Zbořil1,4, Štěpán Kment1,4, Patrik Schmuki5, Alessandro
Alabastri6, Alberto Naldoni1,7
1. Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University, Šlechtitelů 27, 78371 Olomouc, Czech
Republic
2. Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
3. Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genoa, Italy
4. Nanotechnology Centre, VŠB–Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic
5. Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany
6. Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, 77005 Houston, TX, United States
7. Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China# These authors contributed equally to this work
E-mail: luca.mascaretti@upol.cz

Solar-driven steam generation aims at exploiting the sunlight energy to vaporize water by means of a solar absorber.
Among various materials, plasmonic-based absorbers can efficiently convert light to heat upon dissipation ofplasmonic modes; furthermore, they can be fabricated in form of ordered quasi-two dimensional arrays, i.e., plasmonic
metasurfaces, which exhibit broadband light absorption and promote a substantial heat generation thanks to collective
photothermal effects.
In this work, we introduce a solar absorber based on a periodic array of titanium nitride (TiN) nanocavities. The
absorber is enclosed within a custom-made PTFE cell that provides both thermal insulation as well as a water container
in its top part, which is then illuminated by solar-simulated light (Fig. 1A). Unlike previously reported periodic
structures, the present one features an ultrathin (~250 nm) nanocavity layer prepared by a scalable method consistingof electrochemical anodization of a Ti foil followed by nitridation in NH3 atmosphere. In particular, the TiNnanocavities feature a self-ordered hexagonal array as a result of the anodization process (Fig. 1B) and, together witha Ti2N underlayer, provide a broadband light absorption within ~300 nm from the sample surface. Numericalsimulations reveal that the physical origin of such enhanced absorption is due to different resonant phenomena: purecavity modes in the visible range (λ = 500 nm) and localized surface plasmon resonances at the cavity corners in thenear-infrared (λ = 1700 nm). Water evaporation experiments performed with the custom PTFE cell under 1.4–14 Suns
irradiation show a non-linear increase of the evaporation rate with light intensity, going from ~0.3-0.4 kg h–1 m–2(comparable to water only and to the Ti plate substrate) up to ~14.9 kg h–1 m–2 for the TiN-based solar absorber (Fig.
1C), corresponding to ~76% efficiency. Numerical simulations further suggest the acceleration of the evaporation
process in case of sub-µm thickness of both the TiN absorber and the water layer, paving the way for additional
applications including nano-optofluidics and fast phase separation.

Fig. 1 (A) Schematic of water evaporation and steam generation by the TiN nanocavity sample under moderate light concentration
in a custom-made PTFE cell. (B) Top-view scanning electron microscope image of the TiN nanocavities. (C) Evaporation rate
under different light intensities (1 Sun = 1 kW m–2) evaluated after 25 minutes for water only, a bare titanium substrate and theTiN nanocavities (the latter both experimentally and numerically). Reproduced from ref. [1]. Copyright 2021, Elsevier.

References[1] Mascaretti L., Schirato A., Zbořil R., Kment Š., Schmuki P., Alabastri A., and Naldoni A. Nano Energy 2021, 83, 105828.

Influence of plasmonic substrate on the whispering-gallery modes in a πconjugated polymer microsphere
Zhan-Hong Lin1, Soh Kushida2, Ching-Hang Chien3, Jhih-Yuan Chen4, Ankit Kumar Singh1, Fan-Cheng
Lin3, Yohei Yamamoto2, Yia-Chung Chang3, and Jer-Shing Huang1,3,4,5,6
1. Leibniz Institute of Photonic Technology, Albert-Einstein Straße 9, 07745 Jena, Germany
2. Department of Materials Science and Tsukuba Research Center for Energy Materials Science (TREMS), Faculty of Pure andApplied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
3. Research Center for Applied Sciences, Academia Sinica, 128 Sec. 2, Academia Road, Nankang District, 11529 Taipei, Taiwan
4. Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
5. Abbe Center of Photonics, Friedrich-Schiller University Jena, Jena, Germany
6. Department of Electrophysics, National Chiao Tung University, 1001 University Road, 30010 Hsinchu, Taiwan
E-mail: jer-shing.huang@leibniz-ipht.de

We theoretically and experimentally study the influence of atomically flat gold substrate [1] on the whispering
gallery modes (WGMs) in perfect fluorescent π-conjugated polymer microspheres [2] (Fig. 1a). Due to the large
absorption coefficient of the polymer, excitation beam does not go into the sphere and the fluorescence only comesfrom the localized spot of laser focus. Therefore, we were able to select the emission position and thereby control the
excitation of WGMs with different polarization states and orbitals. This allows us to study the effect of substrate on
the WGMs. By analyzing excitation position dependent emission spectra, we observed different spectral shift andintensity change of the WGM spectrum, depending on the symmetry of the WGM and the material of the substrate.
For intensity change, we observed clear symmetry selective damping of WGMs on gold substrate due to leakage into
surface plasmons (Fig. 1b). For the spectral shift, we found peculiar and subtle spectral shift for the transverse-electric(TE) and transverse-magnetic (TM) WGMs depending on the mode symmetry and the substrate material (Fig. 1c).

Fig. 1 (a) Illustration of the concept of this work. The left inset shows the SEM image of the sphere and the right one shows thesimulated field distribution of the spheres in air. (b) Emission spectra of sphere from center (upper, black) and edge (bottom,
green). The blue arrow indicates the peak leakage. (c) Spectral shift of TE and TM obtained from glass and gold substrate.

To understand the spectral shift, we constructed an analytical method based on full Green function within the Miebasis [4] and the Dyson equation including the substrate effect is solved numerically within the Mie basis. We foundthat the substrate effect produces a strong mixing of the resonant TE and TM modes that gives rise to significant mode
shift and broadening. This work gains insight into the influence of gold substrate on the WGMs.
References[1] J.-S. Huang et al., 2010 Nat Commun. 1, 150.
[2] Adachi, T., Tong, L., Kuwabara, J., Kanbara, T., Saeki, A., Seki, S., Yamamoto, Y., 2013 J. Am. Chem. Soc. 135, 870-876.
[3] Biagioni, P., Huang, J.-S., Hecht, B., 2012 Rep. Prog. Phys. 75, 024402.
[4] Mie, G., 1908 Ann. Phys. 25, 377.

Giant mid-IR resonant coupling to molecular vibrations
in plasmonic nanogaps
Rakesh Arul1, David Benjamin-Grys1, Rohit Chikkaraddy1, Niclas S Mueller1, Ermanno Miele1, Angelos
Xomalis1, and Jeremy Baumberg1
1. NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, United Kingdom
E-mail: ra554@cam.ac.uk

Nanomaterials capable of confining light across the visible to infrared (IR) spectrum are desirable for SERS andSEIRA spectroscopies, enhancement of nonlinear upconversion [1], and control of chemical reactions [2]. Here, we
demonstrate how short-range-ordered Au nanoparticle multilayers (NP𝑛𝑛ML with 𝑛𝑛 layers, Fig.1a), self-assembled bya precise sub-nm molecular spacer, support collective plasmon-polariton resonances in the IR region [3]. These
resonances are continuously tunable beyond 11 µm (Fig.1c) by simply varying the nanoparticle size, number of layers,and by placing the NP 𝑛𝑛 ML on a mirror (Fig.1b). The collective plasmon-polariton mode is highly robust to
nanoparticle vacancy disorder and is sustained by the consistent gap size defined by the cucurbit-5-uril (CB[5])molecular spacer. We show these multilayers are easily fabricated and scalable, giving IR absorption enhancement forvarious molecular analytes.
The resulting molecule-plasmon system approaches the vibrational strong coupling regime [2], and displays giant
Fano resonant dip strengths (Fig.1d) with SEIRA enhancement factors ~106, exceeding the enhancement of most stateof-the-art resonant plasmonic antennae fabricated through lithography [4]. Molecules within NP 𝑛𝑛 ML structures
experience molecule-plasmon coupling strengths of 𝑔𝑔 ~100 cm-1 and Purcell factors ~106 due to the compression of
IR light into mode volumes 𝑉𝑉𝑚𝑚 ~10-8 𝜆𝜆3 , resulting in strongly enhanced vibrational signatures. The small mode volume
is a result of localization of the plasmon propagation due to disorder, and corresponds to 7 nanoparticle gaps, which isthe average nanoparticle arrangement in a random close-packed structure.
The simplicity of self-assembly enables the creation of robust amorphous metasurfaces. Rather than being a burden,
disorder is here a strength as it aids diffusion of molecules into the gaps. Structural disorder more efficiently couples
light into the gaps between the multilayers and mirror, enabling real time SERS and SEIRA measurements of subpicolitre sample volumes. We show how NP𝑛𝑛ML films allow sensing of various molecular analytes in a continuous
flow geometry, and how the extraordinarily strong light-matter interaction within these structures supports collective
light emission of quantum emitters.

Fig. 1 Optical response of amorphous AuNP multilayers on gold (NP𝒏𝒏ML-on-mirror). (a) SEM of NP2ML film. (b) Modefrom 𝑛𝑛 = 3 layers of AuNP lattice-on-mirror. Red line depicts compression of the effective IR wavelength 𝜆𝜆 within the NP𝑛𝑛MLon-mirror. (c) IR extinction spectrum of NP𝑛𝑛ML-on-mirror films for 𝑛𝑛 = 1 to 9 with AuNP diameter 𝐷𝐷 = 100 nm. Red shaded
represents spectral region with molecular spacer cucurbit-5-uril (CB[5]) vibrational absorption peaks. (d) Fano lineshape of CB[5]vibration in coupled oscillator model (black line) with measured reflectance spectrum (blue line) of NP7ML-on-mirror films. Fano
dip strength (𝑅𝑅𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 ), molecular resonance (𝜔𝜔𝑚𝑚 ) and plasmon resonance (𝜔𝜔BPP ) labelled.

References[1] Xomalis, A., Zheng, X., Chikkaraddy, R., Koczor-Benda, Z., Miele, E., Rosta, E., & Baumberg, J. J. Science 374, 1268 (2021)[2] Garcia-Vidal, F. J., Ciuti, C., & Ebbesen, T. W. Science, 373, 6551 (2021)[3] Arul, R.; Benjamin-Grys, D.; Chikkaraddy R.; Mueller, N.S.; Xomalis, A.; & Baumberg, J.J. submitted (2022)[4] Neubrech, F.; Huck, C.; Weber, K.; Pucci, A.; & Giessen, H.. Chemical Reviews 117, 5110 (2017)

A nanoscale plasmonic Su-Schrieffer-Heeger chain
Benedikt Schurr1,5, Philipp Grimm1, Tobias Helbig2, Tobias Hofmann2, Lukas Wehmeier3, Thorsten
Feichtner1,4, Monika Emmerling1, Susanne C. Kehr3, Lukas Eng3,5, Ronny Thomale2,5 and Bert Hecht1,5
1. NanoOptics & Biophotonics Group, Experimental Physics 5, University of Würzburg, Am Hubland, 97074 Würzburg, Germany
2. Institute for Theoretical Physics and Astrophysics, University of Würzburg, Am Hubland, 97074 Würzburg, Germany
3. Institut für Angewandte Physik, Technische Universität Dresden, 01062 Dresden, Germany
4. Dipartimento di Fisica, Politecnico Milano, Piazza Leonardo da Vinci, 20133 Milano, Italy
5. Würzburg-Dresden Cluster of Excellence ct.qmat, University of Würzburg, Am Hubland, 97074 Würzburg, Germany
E-mail: benedikt.schurr@physik.uni-wuerzburg.de

The tight-binding Su-Schrieffer-Heeger (SSH) model describes 1D periodic chains of resonators with alternating
coupling strengths. For certain configurations, topologically protected localized collective states occur at the edges,
so called edge modes (Fig. 1 (a)). Such modes have recently been observed in low-frequency electric circuits [1].
Based on this concept, we propose a collinear plasmonic particle chain SSH-system with alternating gaps of a few
nanometers in size. Simulations based on an efficient semi-analytical transfer matrix approach prove the existence ofedge modes which manifest themselves by a deep reflection minimum within the plasmonic bandgap (Fig. 1 (b)) [2].
Furthermore, full FDTD simulations show the occurrence of edge modes for such geometries in real space. By focusedHe-ion beam milling, we are able to fabricate stable and nanometer precise plasmonic chains from single-crystalline
gold micro-platelets (Fig. 1 (c)). Providing extraordinary control over particle size and coupling across the gaps, thefrequency at which edge states occur can be fully controlled. Scattering scanning near-field optical microscopy(sSNOM) experiments indeed indicate the presence of topologically protected edge states (Fig. 1 (d)).

Fig. 1 (a) Artistic sketch of a nanoscale plasmonic Su-Schrieffer-Heeger (SSH) chain showing edge states. Below: Simulated electric fielddistribution of trivial- and topological chain configuration. (b) Calculated and simulated reflection amplitude spectra of a chain system
revealing a clear reflection minimum inside the energy gap for the topological phase. (c) Scanning electron microscope (SEM) image of afabricated SSH-chain showing measured gap distances. (d) Experimental results of SNOM imaging showing topography (top), opticalamplitude (middle) and optical phase (bottom).

References[1] Lee, C.H., Imhof, S., Berger, C., Bayer, F., Brehm, J., Molenkamp, L.W., Kiessling, T., Thomale, R. 2018. Commun Phys 1, 39[2] Grimm P, Razinskas G, Huang J, Hecht B. 2021. Nanophotonics 10 (7). p. 1879-1887

Light-driven microdrones
Xiaofei Wu, Raphael Ehehalt, Gary Razinskas, Thorsten Feichtner, Jin Qin, and Bert Hecht
Nano-Optics and Biophotonics Group, Experimentelle Physik 5, Universität Würzburg, 97074 Würzburg, Germany
E-mail: xiaofei.wu@physik.uni-wuerzburg.de

Optical forces due to scattering or absorption of photons can be employed for remote manipulation of microscopic
objects, especially for electrically and magnetically neutral ones, such as dielectric particles, biological cells, andatoms. However, until now, using light to control object motion in 2D/3D in all three/six degrees of freedom remains
an unsolved challenge.
Here we present light-driven microdrones and demonstrate their systematic remote control in aqueous environment
in 2D with all three independent degrees of freedom (forward-backward and right-left translations and clockwisecounterclockwise rotation), as seen in Fig. 1. The devices contain up to four specially designed plasmonicnanoantennas acting as thrust motors based on resonant directional light scattering, i.e., photon recoil. The nanomotors
can be addressed individually by respective circular polarization components of unfocused light fields of two
wavelengths, and therefore react independently of the in-plane orientation and position of the drone within theillumination. The microdrones can be steered in arbitrary paths by only adjusting the optical power for each motor(the power of each circular polarization component of each wavelength), which is very similar to the working principleof macroscopic quadcopters. The same concept can also be extended to 3D with all six degrees of freedom, without
having to increase the number of light sources. Steering in all independent degrees of freedom would allow
stabilization of position and orientation as well as the motion of the microdrones through automated feedback control.
Owing to their unique properties, light-driven microdrones may serve as a novel experimental platform for many
intriguing applications, such as transport and release of cargos, nanomanipulation, and local probing and sensing ofnano and mesoscale objects.

Fig. 1. Illustration of the setup and microdrone actuation concept. a, Two unfocused light beams of different wavelengths (orange,
830 nm and red, 980 nm) are used to steer two microdrones by adjusting the laser power and the polarization state, i.e., S0 and S3of the Stokes parameters, of each beam. Gold nanoantennas integrated into the transparent body of the microdrone are resonantly
excited by either of the two wavelengths and a certain circular polarization and then scatter light into defined lateral directions(indicated by the small wavy arrows), which leads to corresponding effective optical recoil forces. b, Top-view of the 4-motor
microdrone (top) and comparison to a macroscopic quadcopter drone (bottom). Despite the tremendous difference in size, themotor configuration (two pairs of motors with opposite chirality) and steering principles (by only adjusting the power of eachcontrol channel) are very similar.

Reference[1] Wu, X., Ehehalt, R., Razinskas, G., Feichtner, T., Qin, J., and Hecht, B. Light-driven microdrones. Nat. Nanotech. 2022, accepted.

Nonlinear Pumping of Molecular Vibrations in Plasmonic Nanocavities
Lukas A. Jakob1, William M. Deacon1, Yuan Zhang2, Elena Pavlenko1, Rakesh Arul1, Bart de Nijs1,
Niclas S. Müller1, Tomas Neumann3, Ruben Esteban3, Javier Aizpurua3, Jeremy J. Baumberg1
1. NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK.
2. Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, Ministry of Education, School of Physics and Microelectronics,
Zhengzhou University, Zhengzhou 450052, China.
3. Center for Material Physics (CSIC - UPV/EHU and DIPC), Paseo Manuel de Lardizabal 5, Donostia-San Sebastian Gipuzkoa 20018 Spain.
E-mail: lj366@cam.ac.uk

When bound to metals, molecular vibrations play a key role in sensing, catalysis, molecular electronics, and beyond,but investigating their coherence and dynamics is difficult as pulsed experiments prove very challenging. Here we
study vibrations of 1-100 molecules in a plasmonic nanocavity [1] (Fig.1a) when driven by ps-pulsed lasers out of thelinear regime [2]. This unravels new nonlinear effects such as room-temperature vibrational pumping, giant
optomechanical spring shifts [3], accelerated decay of vibrational energy [4], and SERS quantum correlations.
In plasmonic nanocavities, surface-enhanced Stokes scattering of a pulsed laser can significantly increase the phonon
population above thermal equilibrium. This vibrational pumping leads to superlinear anti-Stokes scattering above athreshold laser power (Fig.1b). For different vibrational modes this threshold depends on their Raman cross sectionand energy. These molecular vibrations then interact with the optical cavity resonance via optomechanical coupling[5]. Due to Raman dipole image charges in the metal, this leads to a dramatic redshift of the vibrational energy by>100 cm-1 (optomechanical spring shift) and broadening of the Raman line at high peak laser powers [3].
To study the dynamics of these vibrations, we use time-resolved coherent anti-Stokes Raman scattering. Using a newly
developed single-photon lockin-detection technique [6], we simultaneously record the decay of the vibrational
population and the vibrational dephasing for each of >100 nanocavities. The vibrational decay is found to strongly
accelerate at higher laser powers due to vibrational energy exchange with other modes [4]. Despite this accelerated
decay, further experiments show that Stokes-induced anti-Stokes scattering exhibits strong quantum mechanical crossfrequency correlations (Fig.1c). These correlated Stokes/anti-Stokes photon pairs show non-classical breaking of the
Cauchy-Schwarz inequality and can be used for applications in quantum computing and communication.

Fig. 1. (a) Monolayer of organic molecules in a plasmonic nanocavity under pulsed laser excitation. (b) Power dependence of
Stokes and anti-Stokes scattering from nanocavity showing threshold of vibrational pumping. (c) Histogram of Stokes and antiStokes photon coincidences at single-photon detectors. Both photons are strongly correlated in a non-classical photon pair.

References[1] Baumberg, J.J., Aizpurua, J., Mikkelsen, M.H., Smith, D.R. Nat. Mater., 18, 668 (2019).
[2] Lombardi, A., Schmidt, M.K., Weller, L., Deacon, W.M., Benz, F., de Nijs, B., Aizpurua, J., Baumberg, J.J. Phys. Rev. X, 8, 011016 (2018).
[3] Jakob, L.A., Deacon, W.M., Baumberg, J.J. et al. Submitted to Nat. Commun. (2022).
[4] Deacon, W.M., Jakob, L.A., Baumberg, J.J. et al. In preparation (2022).
[5] Roelli, P., Galland, C., Piro, N., Kippenberg, T.J. Nat. Nanotechnol., 11, 164 (2016).
[6] Jakob, L.A., Deacon, W.M., Hicks, O., Manyakin, I., Ojambati, O.S., Traxler, M., Baumberg, J.J. Optica, 8, 1646 (2021).

NFO16 Template
Strong Er radiative emission enhancement by quasi-BIC modes coupling in
all-dielectric slot nanoantenna arrays
3+

Boris Kalinic1, Tiziana Cesca1, Ionut Gabriel Balasa1, Andrea Jacassi2, Riccardo Sapienza2, Giovanni Mattei1
1. Department of Physics and Astronomy, University of Padova, Via Marzolo 8, I-35131 Padova, Italy
2. The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2BW, United Kingdom
E-mail: boris.kalinic@unipd.it

In the quest for new and increasingly efficient photon sources, the design of the photonic environment at the
subwavelength scale is fundamental for controlling the emitter optical properties. It is well known, indeed, that the
spontaneous emission rate of an emitter can be modified acting on its electric and magnetic local density of opticalstates (LDOS). Recently, all-dielectric high refractive index nanostructures have attracted increasing interest due totheir unique optical properties (i.e., low absorption, optical magnetism, and multipolar responses) that can be exploited
to enhance the optical properties of a nearby emitter without decreasing its quantum efficiency. However, the relatively
modest Q-factors exhibited by electric and magnetic Mie resonances (Q∼5-10) have limited the application of highindex nanoparticles in the enhancement of the LDOS. A possible way for obtaining orders of magnitude higher Qfactors in all-dielectric nanostructures is based on optical bound states in the continuum (BICs). Although true BIC
can exist in structures that are infinitely extended at least in one direction, finite-size systems can support their analogue
in the form of quasi-BICs, with the resonance quality factor that increases rapidly before reaching a maximum value
limited by the finite-size effects. Despite the novelty of the field, quasi-BICs have already demonstrated their ability
to outperform traditional photonic nanostructures for many photophysical processes usually limited either by losses orby low Q-factor resonances, such as second-and third-harmonic generation, light guiding, beam shaping, and sensing.
Nonetheless, the realization of nanoantennas able to boost the emitter optical properties through an efficient couplingwith q-BIC modes remains still an open issue [1].
In this framework, we propose a novel design for the all-dielectric nanoantennas supporting quasi-BICs, with anultrathin silicon oxide layer doped with erbium ions placed inside slotted silicon nanopillars arranged in a square array(a sketch of the nanoantenna is shown in Fig. 1 (a)) [2,3]. Fig. 1(b) shows that a giant field intensity enhancement withan extremely narrow spectral width (Q~2x105) can be obtained at the resonance condition in the nanopillar region
corresponding to the emitting layer. The reflectance has the asymmetric Fano-like line-shape of quasi-BIC modes
where destructive interference of the scattered field outside the nanostructure brings to a strong enhancement of thenear-field inside the nanoantenna. We demonstrate that by coupling the Er3+ radiative emission at about λ=1540 nmwith quasi-BIC resonances, up to 3 orders of magnitude photoluminescence intensity increment and 2 orders ofmagnitude decay rate enhancement have been measured at room temperature (Fig. 1 (c)). Furthermore, acting on the
nanoantenna aspect ratio, the emitter magnetic branching ratio can be tailored from 10% to 90%, keeping the quantum
efficiency almost unitary. Finally, exploiting the reciprocity principle we were able also to design and control theemission directivity from the nanoslot, focusing more than 90% of the Er3+ emitted radiation at λ=1540 nm in a lobe
normal to the sample surface with an angular width of Δθ<15. Hence, we have designed and realized CMOS compatible
lossless nanoantennas able to boost the photon generation at telecom wavelength.

Fig. 1 (a) Sketch of the all-dielectric nanoantenna and a SEM image of the square array of slotted nanopillars. (b) The average electric field intensity
enhancement in the SiO2 slot and far-field reflectance for a plane wave impinging on the square array of slotted silicon nanoantennas with AR=1.5.
(c) The Er3+ PL intensity enhancement in the λ=1450-1650 nm wavelength range for slotted nanopillars with increasing aspect ratio (AR=d/h).
Upper panels report the magnetic field configuration (|H|/|H0|) at quasi-BIC resonances.

References[1] Kuznetsov, A. I., Miroshnichenko, A. E., Brongersma, M. L., Kivshar, Y. S., Luk’yanchuk, B. 2016. Science, 354, 6314.
[2] Mignuzzi, S., Vezzoli, S., Horsley, S. A., Barnes, W. L., Maier, S. A., Sapienza, R. 2019. Nano Letters, 19, 1613–1617.
[3] Kalinic, B., Cesca, T., Mignuzzi, S., Jacassi, A., Balasa, I. G., Maier, S. A., Sapienza, R., Mattei, G. 2020. Phys. Rev. Appl., 14, 14086.

Invited: Plasmonic hot carriers and their application in catalysis
Pierre Berini
School of EECS, University of Ottawa, 25 Templeton St, Ottawa, Canada, K1N 6N5
Department of Physics, University of Ottawa, Ottawa, Canada
E-mail: berini@eecs.uottawa.ca

Plasmonics has evolved explosively over the last two decades to become a hallmark of nanophotonics. Applications
are abundant, including, e.g., biosensors, optoelectronics, devices for telecom, spectroscopy, decorative texturing,
anticounterfeit markings, and catalysis. A recent and exciting direction of investigation concerns the ability ofsurface plasmon polaritons (SPPs) to generate energetic carriers in metals, and the subsequent harvesting of suchcarriers to alter fundamental processes and open new applications [1].
SPPs are TM-polarised surface waves guided by a metal-dielectric interface at optical wavelengths. They aresupported as propagating modes on metal waveguides or as resonant modes on metal nanoparticles, nanoantennas or
gratings. They are strongly localized to the metal surface, and under certain conditions are strongly absorbed therein,
leading to the effective creation of energetic carriers - “hot electrons” and/or “hot holes” - also localized along thesurface. These energetic carriers can be harvested and used for a broad range of applications, e.g., to enhance theperformance of optoelectronic and photovoltaic devices, or to open new reaction pathways in chemistry. Recent andongoing progress along these threads will be discussed, including the reduction of CO2 to solid light-emitting carbon
dots [2], and the opening of new redox channels in electrochemistry [3].
The left panel of Fig. 1 shows a series of SERS spectra evolving over time as a Ag grating under SPP excitation
is excited by green light (532 nm) under CO2 exposure in a gas flow cell [2]. The spectra indicate growth ofphotoluminescent graphitic carbon on the surface, taking the form of spherical deposits that are visible under helium
microscope (Fig. 1, middle panel). The observed luminescence consists of yellow light and persists as long as CO2
exposure is maintained (Fig. 1, bottom left panel).
The right panel of Fig. 1 shows cyclic voltammograms obtained under SPP excitation of the working electrode,shaped as a Au stripe waveguide, as a function of incident optical power (λ0 ~1350 nm) [3]. Processes involving
energetic holes are separated from processes involving energetic electrons by investigating oxidation and reduction
reactions separately. Redox current densities increase by 10× under SPP excitation. The oxidation, reduction andequilibrium potentials decrease by as much as 2× and split in correlation with the photon energy beyond a clear
threshold with SPP power. Electrochemical impedance spectroscopy reveals a drop of almost 2× in charge transfer
resistance under SPP excitation.

Fig. 1. Left panel: SERS spectra evolving over time showing the growth of solid carbon deposits in CO2. Middle panel: helium
ion microscope image of carbon nanodots on a Ag grating. Left bottom panel: increase and decrease of yellow light emission as
carbon nanodots form then disappear. Right panel: cyclic voltammogram of 𝐾𝐾4 [𝐹𝐹𝐹𝐹(𝐶𝐶𝐶𝐶)6 ] ↔ 𝐾𝐾3 [𝐹𝐹𝐹𝐹(𝐶𝐶𝐶𝐶)6 ] + 𝑒𝑒 − in KNO3
electrolyte with increasing SPP power (legend). Left panels: [2], right panels: [3].

References[1] Brongersma, M. L., Halas, N. J., Nordlander, P., “Plasmon-induced hot carrier science and technology,” Nature Nanotechnol. 10, 25 (2015)[2] Walia, J., Rashid, S., Killaire, G., Weck, A., Berini, P., “Reconfigurable carbon quantum emitters from CO2 gas reduced via surface
plasmons,” Optica 8, 708 (2021)[3] Hirbodvash, Z., Krupin, O., Northfield, H., Olivieri, A., Baranova, E. A., Berini, P., “Infrared surface plasmons on a Au waveguide electrodeopen new redox channels associated with the transfer of energetic carriers,” Science Advances, in press

Hot electrons in metal nanostructures – “reality” or “fake news”?
Yonatan Sivan1, Ieng Wai Un1, Joshua Baraban2, Yonatan Dubi2

1. School of Electrical Engineering, Ben-Gurion University of the Negev, Be’er-Sheva 8410501 Israel
2. Department of Chemistry, Ben-Gurion University of the Negev, Be’er-Sheva 8410501 Israel
E-mail: jdubi@bgu.ac.il

We present a self-consistent theory of the steady-state electron distribution in metals under continuous-wave
illumination which treats, for the first time, both thermal and non-thermal effects on the same footing [1]. We show
the number of non-thermal electrons (i.e., the deviation from thermal Fermi-Dirac equilibrium) is very small, so thatthe power that ends up generating these non-thermal electrons is many orders of magnitude smaller than the amountof power that leads to regular heating. We will review briefly an experimental confirmation of our prediction and its
relevance to plasmonic nanostructures of various sizes and shapes.
Using this theory, we re-examine the exciting claims on the possibility to enhance chemical reactions with these
non-thermal electrons, claims which have been pushed forward by various experimental groups, and published (withessentially no theoretical confirmation) in top journals. We identify a series of rather astounding errors in thetemperature measurements in some of the most famous papers on the topic which led their authors to under-estimate
regular heating effects. As an alternative, we show that a very simple 19th century theory, based on just simple heating,can explain the published experimental data with excellent accuracy [2,3].

Fig. 1 Experimental data (symbols) and fits using the classical Arrhenius theory (lines), showing remarkable agreement. Data collected fromhigh-impact papers that claimed (incorrectly) for dominance of non-thermal effects.

References[1] Y. Dubi, Y. Sivan, “Hot electrons in metallic nanostructures - nonthermal carriers or heating?,” Light: Science & Applications 8, 89 (2019). Y.
Sivan, I.W. Un, Y. Dubi, “Assistance of plasmonic nanostructures to photocatalysis – just a regular heat source,” Faraday Discussionss 214, 215 –
233 (2019).
[2].Y. Sivan, J. Baraban, I.W. Un, Y. Dubi, “Comment on “Quantifying hot carrier and thermal contributions in plasmonic photocatalysis””. Science
364 (6439), eaaw9367 (2019). Dubi, I.W. Un, Y. Sivan, “Thermal effects – an alternative mechanism for plasmon-assisted photocatalysis”. Chemical
Science 11, 5017-5027 (2020). Y. Sivan, J. Baraban, Y. Dubi, “Experimental practices required to isolate thermal effects in plasmonic photocatalysis – lessons from recent experiments”. OSA Continuum 3, 483-497 (2020). Y. Dubi, I.E. Un, J. Baraban, , Y. Sivan, “Matters Arising in“Plasmon-driven carbon–fluorine (C(sp3)–F) bond activation with mechanistic insights into hot-carrier-mediated pathways”. Nature Catalysis,
accepted.
[3] Y. Dubi, I.W. Un, J. Baraban & Y. Sivan, “Distinguishing thermal from non-thermal contributions toplasmonic hydrodefluorination”, Nature
Catalysis (2022, accepted).

Theory of metallic nanostructures photoluminescence under continuous
pumping
Aurelian Loirette--Pelous1, Mondher Besbes1, Jean-Jacques Greffet1
1. Université Paris-Saclay, Institut d’Optique Graduate School, CNRS, Laboratoire Charles Fabry,91127 Palaiseau, France
E-mail: aurelian.loirette-pelous@institutoptique.fr

In the past decades, metallic nanostructures have received a great attention due to their ability to confine electricfields in the nanoscale. For long, it has been remarked that such structures emit light over a broad spectral range [1],akin to fluorescence of molecules, both under pulsed and continuous pumping. While under pulsed pumping, electrons
can reach very high temperatures leading to blackbody emission in the visible range, this emission turns negligible
under continuous pumping. Instead, intraband recombination of high energy (“hot”) electrons induced by pumping has
been pointed out, and the corresponding non-equilibrium electron distribution calculated [2]. However, quantitative
predictions of the emitted light still lack from a unified formalism considering the modal landscape in which theemission occurs.
Here, we show that the spectrum, the angular dependence and polarization of the photoluminescence from arbitrary
metallic nanostructures can be calculated easily using a generalization of the Kirchhoff law. In its usual version, theKirchhoff law relates the emitted power in a given direction, polarization and frequency to the absorptivity and theblackbody radiance. The derivation of Kirchhoff law is based on the Fluctuation-Dissipation theorem which is valid
for local thermodynamic equilibrium [3]. Under continuous pumping, it has been shown that the distribution is nolonger a Fermi-Dirac distribution [2], so that the validity of Fluctuation-Dissipation relation is under question. Here,
we first derive a non-equilibrium Fluctuation-Dissipation relation which connects the electronic current density
fluctuations in a metal with the non-equilibrium distribution. This enables us to derive a generalized form of theKirchhoff law valid for this non-equilibrium situation. We will also provide a simple physical picture associated withthis model.
To conclude, we show that this model allows to explain the main experimental features of photoluminescence. In
particular, we recover the spectral modification of the signal as a function of the incident frequency and thenanostructure plasmonic resonance [4]. The model also accounts for the temperature dependence of thephotoluminescence which has been used as a temperature sensor in [5]. Finally, we show that the signal is proportional
to the pump intensity, in contrast to blackbody radiation. Beyond these theoretical considerations, we confront ourmodel to experimental results presented in detail in a separate contribution at this conference. All in all, these results
make a general framework to quantitatively analyze photoluminescence of metal in the cw regime.
References[1] Mooradian, A. (1969). Physical Review Letters, 22(5), 185.
[2] Sivan, Y., & Dubi, Y. (2021). ACS nano, 15(5), 8724-8732.
[3] Greffet, J. J., Bouchon, P., Brucoli, G., & Marquier, F. (2018). Physical Review X, 8(2), 021008.
[4] Lin, K. Q., Yi, J., Hu, S., Sun, J. J., Zheng, J. T., Wang, X., & Ren, B. (2016). Acs Photonics, 3(7), 1248-1255.
[5] Carattino, A., Caldarola, M., & Orrit, M. (2018). Nano letters, 18(2), 874-880.

Distinguishing thermal from non-thermal (“hot”) carriers in illuminatedmolecular junctions
Yonatan Dubi1, Ieng Wai Un2, Yonatan Sivan2
1. Department of Chemistry, Ben-Gurion University; 2. School of Electrical and Computer Engineering, Ben-Gurion University
E-mail: sivanyon@bgu.ac.il

When a plasmonic nanostructure is continuously illuminated, the system unavoidably heats up, and nonthermal (so-called “hot”) carriers are being generated. Under continuous wave illumination, the information about
heating is characterized by a Fermi-like distribution of carriers close to the Fermi level, whereas the non-thermal
carriers reside further away from the Fermi level in nearly-flat “shoulders” [1]. The separation between these different
carriers is very challenging, because the number of the high excess-energy non-thermal electrons is many orders ofmagnitude smaller compared to the number of thermal (i.e., low excess energy) carriers. In an attempt to circumvent
this problem, Reddy, Wang and co-authors [2] suggested measuring directly the non-thermal carrier distribution bycoupling a plasmonic Au film to a molecular junction. By measuring the I-V curves through the molecular junction(MJ) under illumination and in the dark, they assessed the effect of illumination on the electronic distribution in theilluminated Au electrode.
Here [3], we interpret the results of [2]. Specifically, we extend the theory of transport through a MJ coupled
to two electrodes, by combining the standard Landauer theory of transport through MJs and the analytic form for theelectron non-equilibrium distribution of an illuminated metal [1]. Using this formulation, we demonstrate that the main
results in [2] have a satisfactory explanation as mere heating of the bottom electrode, whereas additional measurementsof a different molecule (discussed in the SI of [2]) provides (probably for the first time) a direct observation of nonthermal electrons, but not in the way interpreted in the original manuscript. This finding confirms the original
theoretical prediction [1] in a reasonable quantitative manner, and thus confirms the claims on the extremely small role
played by non-thermal electron in plasmon-assisted photocatalysis experiments.

Fig. 1 (a) Schematic depiction of the experimental setup, comprising an Au slab with surface plasmons ecited at the nanofabricated gratings, and amolecular junction formed between the slab and an STM tip. (b) Differential current as a function of bias voltage (log scale) for a molecular junction
with resonance close to the Fermi level. The dashed line shows the differential current without illumination, thus showing only the thermal response.
Near zero bias (dashed black square), the dominant feature is the thermal contribution to the current, marking that nonthermal effects are negligible
there. Inset: same plot in linear scale. (c−d) Fits to experimental data of [2] (c) 6 nm and (d) 13 nm thick slabs, assuming that the tip is not heated
but the slab right under it is. Red and blue circles are the experimental data, and black squares are the fits.

References[1] Dubi, Y., Sivan, Y., (2019) Light: Science & Applications, 8, 89-95.
[2] Reddy, H. et al. (2020) Science, 369, 423–426.
[3] Dubi, Y., Un, I.W., Sivan, Y., (2022) Nano Letters, in Print.

Far-field wavefront optimization of the near-field of disordered
plasmonic metasurfaces
Gauthier Roubaud1, Sébastien Bidault1, Sylvain Gigan2, Samuel Gresillon1
1.
2.

Institut Langevin, ESPCI Paris, Université PSL, CNRS, 75005 Paris, France

Laboratoire Kastler Brossel, ENS, CNRS, Sorbonne Université, PSL University, 24 rue Lhomond, 75005 Paris
E-mail : samuel.gresillon@espci.fr

Plasmonic nanoantennas featuring nanoscale gaps can exhibit strongly enhanced optical near-fields thathave been extensively used in surface enhanced spectroscopy (Raman and Fluorescence) and in biosensing.
However, deterministic nanostructures do not provide numerous degrees of freedom to control optically these local
field enhancements. By comparison, wavefront shaping techniques in disordered scattering media provide numerous
degrees of freedom to control light focusing in space and time [1]. To associate local field enhancements and farfield wavefront control, we use disordered plasmonic surfaces close to the percolation threshold that feature both
hotspots [2] and delocalized plasmon modes that can be controlled using a spatial light modulator [3].
In this presentation, we demonstrate how controlling the phase of an incoming femtosecond pulsed laser on
a disordered gold surface allows us to optimize the two-photon induced luminescence (TPL) at a chosen position.
Importantly, the TPL signal has been shown to provide a far-field image of local field enhancements in nanoantennas[4]. Our results therefore indicate a far-field optimization of the optical near-field in disordered plasmonic
metasurfaces.

Fig. 1 (a) Widefield Two Photon Luminescence images of the gold metasurface before (left) and after (right) the
optimization process; with a strong increase of the TPL signal in the centre of the image after optimization. (b)Distributions of TPL signals before (orange data points) and after optimization (blue) for 30 different optimization
experiments.

In practice, the surface of a spatial light modulator is conjugated with the sample plane using a highnumerical aperture inverted microscope in order to spatially modulate the phase of the incoming femtosecond source
while preserving a homogeneous intensity. The emitted TPL is then imaged in parallel on a CMOS camera. A
random optimization process of the incoming wavefront is then performed in order to enhance the TPL signal at thecentre of the image (see Fig. 1-a).
Fig. 1-b presents the distribution of TPL signals before and after optimization for 30 different realizations
with a typical two orders of magnitude enhancement of the measured signal. These results demonstrate a strong
enhancement of the nonlinear luminescence at any chosen position of the sample, at the diffraction limit; indicating
an optimization of the near-field in disordered plasmonic metasurfaces by far-field wavefront shaping[5,6].
References[1] M. Mounaix, D. Andreoli, H. Defienne, G. Volpe, O. Katz, S. Grésillon, and S. Gigan, “Spatiotemporal Coherent Control of Light through aMultiple Scattering Medium with the Multispectral Transmission Matrix”, Phys. Rev. Lett. 116, 253901 (2016).
[2] S. Grésillon L. Aigouy, A. C. Boccara, J. C. Rivoal, X. Quelin, C. Desmarest, P. Gadenne, V. A. Shubin, A. K. Sarychev, and V. M. Shalaev,“Experimental Observation of Localized Optical Excitation in Random Metal-Dielectric Films”, Phys. Rev. Lett. 82, 4520 (1999).
[3] P. Bondareff, G. Volpe, S. Gigan and S. Gresillon, “Probing Extended Modes on Disordered Plasmonic Networks by Wavefront Shaping”,
ACS Photonics 2, 12, 1658 (2015).
[4] P. Ghenuche, S. Cherukulappurath, T. H. Taminiau, N. F. van Hulst, and R. Quidant, “Spectroscopic Mode Mapping of Resonant Plasmon
Nanoantennas”, Phys. Rev. Lett. 101, 116805 (2008).
[5] G. Roubaud, P. Bondareff, G. Volpe, S. Gigan, S Bidault and S. Gresillon, “Far-Field Wavefront Control of Nonlinear Luminescence inDisordered Gold Metasurfaces“, Nanoletters 20, 2291 (2020).
[6] G. Roubaud, S. Bidault, S. Gigan and S. Gresillon, “Statistical Nonlinear Optical Mapping of Localized and Delocalized Plasmonic Modes inDisordered Gold Metasurfaces”, ACS Photonics 8, 1973 (2021).

Tuning surface lattice resonance energy levels and their degeneracy bybreaking isotropy of nanoparticles arrays
Florian Lamaze1, William D’Orsonnens1, Julien Proust1, Louis Giraudet2
1. Light, nanomaterials, nanotechnologies EMR 7004, Université de Technologie de Troyes, 10004 Troyes
2. Laboratoire De Recherche en Nanosciences (LRN) - EA 4682, Université de Reims Champagne-Ardenne, 51687 Reims
E-mail: florian.lamaze@utt.fr

The study of plasmonic nanoparticles arrays has led to the observation of the well-known Surface Lattice
Resonance (SLR). They appear from the diffraction coupling at specific wavelengths, associated with the appearance
or disappearance of a diffraction order [1]. The resulting wave is able to propagate parallel to the array, at a grazing
angle. This phenomenon is known as the Rayleigh anomaly and is linearly dependent on the period. This propagating
wave thus provides long-distance coupling between the individual nanoantennas along the propagation direction of thediffracted order. In these conditions, a hybridization between the resonance associated with the Rayleigh anomaly andthe localized plasmon resonance is supported by the nanoantennas. As such, these period dependent anomalies can be
modulated by acting on the geometry of the structure.
In this talk, we will describe and study these resonances for various shapes of particles, both experimentally andwith Finite Difference Time Domain (FDTD) simulations, to exploit their specific properties and different responses
in polarization. These variety of shape will be investigated for different configuration of arrays. We used electron beamlithography to fabricate the structure on solid substrate, as shown on the SEM image on Fig.1 (b), and then transferred
on a flexible substrate with a homemade process.
In particular, it is possible to remove the degeneracy existing between the diffracted orders (±1.0) and (0.±1) in thecase of a square array by making it asymmetrical [2], for example by applying a deformation. We then obtain twoRayleigh anomalies, each depending on a distinct period and moving respectively towards the lowest and highest
energies levels as shown on Fig 1 (c,d).

(a)

(c)

(d)

(0.±1)(±1.0)(b)

Fig. 1 (a) Schematic of the structure consisting of an array of gold nanorods. (b) SEM image of the structure. (c) Experimental
extinction spectra of the array for different deformation state. (d) Experimental extinction map for different deformation statewith the associated Rayleigh anomalies.

References[1] Khlopin, D. Laux, F. Wardley, W.P. Martin, J, Wurtz, G.A., Plain, J. Bonod, N. Zayats, A.V. Dickson, W and Gerard, D. 2017. J Opt Soc Am
B, 3(34), 691-700.
[2] Guo, R. Hakala, T.K. and P. Törmä. 2017. Phys. Rev. B, 95(15), 155423.

Optical antennas driven by inelastic tunnel junctions
Kai Braun*1, Michel Rebmann1 and Alfred J. Meixner1

1. Institute of Physical and Theoretical Chemistry, University of Tuebingen, Auf der Morgenstelle 18, 72076 Tuebingen and LISA+, Germany
E-mail: kai.braun@uni-tuebingen.de

The conversion of an electrical signal to an optical signal with maximum bandwidth and practical on-chip
integration is of fundamental interest. Therefore, light emission by inelastic tunneling has gained considerable
interest, since it allows to construct an electrically driven plasmonic nano gap antenna. Accurate control of the shape,
aspect ratio, and gap size of the structures determines the spectral shape[1], position, and width of the plasmonicresonances[2]. Many emerging nano-photonic technologies depend on the careful control of such localized
resonances, including optical nano antennas for high-sensitivity sensors, nanoscale control of active devices[3], andimproved photovoltaic devices.
However, achieving reproducible and stable experimental conditions with nanometer sized gaps in tunnelingrange remains a challenging task in high demand. We have investigated different approaches to establish these
conditions. Ether, by an gold tip positioned in tunneling range above an gold samples, to produce a gap mode
antenna, which can be driven by an applied bias voltage, or by fabricated arrays of vertical coupled antennas inmetal-insulator-metal arrangements. These IET devices are based on electrically connected gold nano rods (GNR)
and hexagonal boron nitride as insulating layer. By tuning the aspect ratio of the GNRs we shift the localized
plasmon resonance of the antennas and adjust the emission wavelength. Secondly, we used
The results discussed here show solutions of antenna modulated light emission from a tunneling junction
approaching the ultimate size limits of an opto-electronic device, while the operating speed is only limited by theelectron tunneling time. The reviewed concept represents a novel platform for ultra-small, fast, optically, andelectronically switchable devices.
(a)

(b)

Fig. 1 Different approaches to establish plasmonic tunnel junctions. (a) A STM tunneling junction on a plasmonic particle and (b)a stacking device, starting with an ITO bottom electrode on glass, followed by an array of GNRs. Afterwards a few layers sheetof h-BN is transferred, and an evaporated Au top electrode complete the device. Electrons tunnel from the Au top electrode to the
GNRs, where inelastic tunneling occurs and transfers energy to the plasmon, which later determines the emission wavelength

References[1] Mühlschlegel, P., Eisler, H.J., Martin, O. J. F. et al. „Resonant Optical Antennas“ Science 308, 1607-1609, 2005
[2] Kern J., Grossmann J., B. Hecht et al. „Atomic-Scale Confinement of Resonant Optical Fields” Nano Letters, 12, 5504, 2012[3] Braun K., Laible F., Wang X., et al. “Active optical antennas driven by inelastic electron tunneling” Nanophotonics, 7 (9), 1503, 2018

Generating ultrafast stationary magnetic fields with light in a plasmonicnanostructure
Mathieu Mivelle1, Xingyu Yang1, Ye Mou1, Bruno Gallas1, Agnès Maitre1, Laurent Coolen1
1. Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75005 Paris, France
E-mail: mathieu.mivelle@sorbonne-universite.fr

The inverse Faraday effect allows the generation of stationary magnetic fields through optical excitation only [1].
This light-matter interaction in metals results from creating drift currents via non-linear forces that light applies to the
conduction electrons. Here, we theoretically demonstrate that a gold photonic nano-antenna, optimized by a genetic
algorithm, allows, under high excitation power, to maximize the drift currents and generate a pulse of stationary
magnetic fields in the tesla range [2] (fig. 1). This intense magnetic field, confined at the nanoscale and for a few
femtoseconds, results from annular optical confinement and not from the creation of a single optical hot spot.
Moreover, by controlling the incident polarization state, we demonstrate the orientation control of the created magnetic
field and its reversal on demand. Finally, the stationary magnetic field's temporal behavior and the drift currents
associated with it reveal the sub-cycle nature of this light-matter interaction. The manipulation of drift currents by aplasmonic nanostructure for the generation of stationary magnetic field pulses find applications in the ultra-fast controlof magnetic domains with applications in data storage technologies, but also in research fields such as magnetic
trapping, magnetic skyrmion, magnetic circular dichroism, to spin control, spin precession, spin currents, and spinwaves, among others.

Fig. 1. a) A genetically-designed plasmonic nanostructure excited by a femtosecond pulse of light (green oscillations) sets theelectrons of the metal (symbolized by the blue sparks) in a drift motion, generating a stationary magnetic field of a few Tesla at
the nanoscale (red halo) and b) of only a few femtoseconds [2].

References[1] Van Der Ziel, J.; Pershan, P. S.; Malmstrom, L. 1965. Phys. Rev. Lett., 15, 190.
[2] Yang, X.; Mou, Y.; Gallas, B.; Maitre, A.; Coolen, L.; Mivelle, M.; 2021. ACS nano.,16, 386-393.

Spectroscopic Mapping of Plasmonic Dynamics on the Surface of Nanoparticle
Plasmonic Particles
Derek B. Nowak1, Padraic O’Reilly1
1. Molecular Vista Inc, 6840 Via Del Oro Suite 110, San Jose CA
E-mail: derek@molecularvista.com

The study of plasmonic particle absorption and scattering behavior has been ongoing for centuries. In more recent
times the role that localized surface plasmon resonances (LSPR) play in this understanding has been demonstrated
mathematically through analytical and numerical modeling, and experimentally through the far-field response ofplasmonic nanoparticles either individually isolated or in ensembles using a variety of spectroscopic and microscopic
techniques. Moreover, near-field studies have further confirmed our understanding of the optical response of nanoscaleparticles using scattering scanning near-field optical microscopy (sSNOM), two photon-induced luminescence (TPL)
microscopy and electron energy loss spectroscopy (EELS). Where these techniques have been successful is allowing
researchers to map the field distributions at the nanoscale, there are still some challenges. For s-SNOM the strong
background scattered light component limits the signal to noise ratio of the system [1], it is also challenging to change
the wavelength of excitation in real-time and provide a spectrum of the plasmonic response of the particles. For TPL
the spatial resolution tends to be lower in comparison to tip based techniques [2]. For EELS, while it fares better for
spatial resolution, the limitations in sample compatibility, sample charging, and sample alterations require
experimental consideration [3]. Ideally a technique that probes the absorption behavior at the intra-particle level willlead to information about how these nanoparticles interact with their local environment, which would have far-reaching
impact in the areas of biomedical technologies, renewable energy, plasmon-enhanced spectroscopies, andoptoelectronic devices, to name a few. An emerging new near-field detection method based on optical force, photoinduced force microscopy (PiFM) was used first by Halas group at Rice University to investigate LSPRs and showedsome initial promise in this direction [4]. A critical element that is needed is the capability to tune the excitation visible
light source to provide a local spectroscopic measurement. Recent developments with the combination of PiFM withsupercontinuum light sources have now made this type of spectroscopic measurement routinely possible [5].
To demonstrate the spectroscopic capabilities of the PiFM detection mechanism we use the output of asupercontinuum laser source focused onto the imaging apex of a metal coated AFM cantilever. An acoustic opticaltunable filter is used to spectrally filter the output of the of this white light source, controlling both the linewidth andcenter output wavelength electrotonically. We can observe local intra-particle variation in absorption spectrum present
on the surface of nanoparticles as shown in Fig 1. These results correspond to the expected results from absorption
experiments [6] and mathematical modeling. PiFM demonstrates a promising method to study intraparticle absorption
dynamics and how these particles interact in their local environments. Moreover, these spectroscopic measurements
are acquired simultaneously with topography from the AFM probe, providing a direct correlation with morphology ofthe nanoparticle and local environment.

Fig. 1 A single Au nanorod is imaged using a metallic AFM cantilever (left) while a simultaneous PiFM image is acquired with a fixed
excitation wavelength of 680 nm (center). A set of spectra acquired across the nanorod shows clearly the longitudinal (690 nm) andtransverse modes (525 nm) of the nanorod as measured near the surface of the particle.

References[1] Xiao, L.; Schultz, Z. D. Anal. Chem. 2018, 90 (1), 440-458[2] Chen, L.; Li, G.; Liu, G.; Dai, Q.; Lan, S.; Tie, S.; Deng, H. J. Phys. Chem. C 2013, 117(39), 20146-20153.
[3] Liang, H.; et.al. J. Am. Chem. Soc. 2013, 135(26), 9616-9619.
[4] Tumkur, T. U.; Yang, X.; Cerjan, B.; Halas, N. J.; Nordlander, P.; Thomann, I. Nano Lett. 2016, 16 (12), 7942-7949.
[5] D. Nowak, (2022) Spectroscopic and High-Resolution Microscopic Characterization of Intra-Nanoparticle Plasmonic Absorption Dynamics at
Visible Wavelengths (Manuscript in preparation), Molecular Vista Inc.
[6] Muskens, O. L.; et. al. Appl Phys Lett 2006, 88, 063109.

Acousto-Plasmonic Coupling: The Raman Energy Density (RED)
Nicolas Large1, Jose Luis Montaño-Priede1, Adnen Mlayah2,3
1. Department of Physics & Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States
2. Centre d'Elaboration de Matériaux et d'Etudes Structurales, CNRS – Université de Toulouse, 29 Rue J. Marvig, 31055 Toulouse, France
3. Laboratoire d'Analyse et d'Architecture des Systèmes, CNRS – Université de Toulouse, 7 avenue du Colonel Roche, 31031 Toulouse, France
E-mail: Nicolas.Large@utsa.edu

Interactions between elementary excitations, such as plasmon-exciton and plasmon-phonon, are of great interest
from a fundamental point of view and for applications. While plasmon-exciton interactions have been extensively
studied both experimentally and theoretically [1], the interaction mechanisms between acoustic vibrations (phonons)
and localized surface plasmons (LSPs) remain largely unexplored [2]. Here we present a theoretical investigation of
the interactions between confined acoustic vibrations and LSPs in the framework of resonant acoustic Raman
scattering. We express the Raman scattering process in the framework of Fermi golden rule and introduce for the firsttime the concept of Raman energy density (RED). Similarly to the Raman-Brillouin electronic density (RBED)
introduced for semiconductors [3–4], this new physical quantity is used as a theoretical tool for the interpretation ofresonant Raman scattering mediated by LSPs in metallic nanoparticles. The RED represents the electromagnetic
energy density excited by the Raman probe and modulated by the acoustic vibrations of the nanoparticle. We show
that, similarly to the local density of optical states (LDOS) and the RBED, the RED is a local physical quantity thatcan be mapped in the near-field region. It provides a clear picture of the interaction between LSPs and acoustic
vibrations which give rise to inelastic scattering measurable in the far-field. Here, we use the newly introduced RED
concept to investigate elastic (an)isotropy effects and calculate the Raman selection rules of spherical nanoparticles
in a dielectric environment.

Fig. 1 Left: Isotropic (l=0) and Anisotropic (A1g) breathing acoustic mode of a AuNP modulating the near electric field induced
par the dipolar localized surface plasmon (LSP). Center: Raman energy density (RED) resulting from the interaction of the l=0
and A1g vibration mode with the dipole LSP. Right: Calculated acoustic Raman spectra for the isotropic and anisotropic
nanoparticles.

References[1] Schlather, A. E.; Large, N.; Urban, A. S.; Nordlander, P. and Halas, N. J. “Near-Field Mediated Plexcitonic Coupling and Giant Rabi Splitting
in Individual Metallic Dimers,” Nano Lett., 2013, 13, 3281
[2] Large, N.; Saviot, L.; Margueritat, J.; Gonzalo, J.; Afonso, C. N.; Arbouet, A.; Langot, P.; Mlayah, A. and Aizpurua, J. ”Acousto-plasmonic
Hot Spots in Metallic Nano-Objects,” Nano Lett., 2009, 9, 3732
[3] Large, N.; Huntzinger, J. R..; Aizpurua, J.; Jusserand, B. and Mlayah, A. “Raman-Brillouin electronic density in short period superlattices,”
Phys. Rev. B, 2010, 82, 075310[4] Mlayah, A.; Huntzinger, J.-R. and Large, N. “Raman-Brillouin light scattering in low-dimensional systems: Photoelastic model versus
quantum model,” Phys. Rev. B, 2007, 75, 245303