NanoPlasMeta 2020 Abstracts

Area 1 - Nanophotonics, Plasmonics and Metamaterials
Nr: 15
Title:

Insights into Surface-Enhanced Raman Scattering from Molecular Optomechanics
Authors:

Rubén Esteban
Abstract: We present a cavity Quantum Electrodynamics model that describes Surface-Enhanced Raman Scattering (SERS) as the result of an optomechanical process. We discuss how this framework reproduces the results of semi-classical models for typical configurations, and allows to study novel effects that could be accessible to experiments. For example, we study the correlations between the emitted photons, and collective effects in the presence of many molecules. We focus on off-resonant SERS, but also discuss how to extend the model to the resonant situation.
Nr: 17
Title:

Metasurfaces for Light Enhancement and Control of Spin States in Magnetic Compounds: Different Strategies to Enable Eeconfigurability
Authors:

Josep Canet-Ferrer
Abstract: Further and Emerging technologies are continuously demanding for the development of multifunctional devices capable of integrating components of different nature into a single platform. After being assembled in compact architectures for an easy portability and interconnection those devices are expected to manage information by responding to external stimuli e.g. optical, electrical or magnetic pulses. However, most of the proposed multifunctional systems do not show a clear interaction among their constituents and the synergic coupling among their components is observed only for specific applications and under limited conditions. With this concern, we are launching a new research line for the design of new class of hybrid devices, operating on at the nanoscale and conceived to optimize the coupling among excitons, phonons, plasmons, polaritons and magnons. The basis of our devices will be the silicon metasurface, fabricated by means of CMOS friendly processes, as a linker among the above mentioned resonances and to bust their interaction. In this talk I will review the state-of-art in this niche of application highlighting the main bone of contention in the current metasurface research, which is the reconfigurability strategy, and I wll give some insights for the design of reconfigurable metasurfaces for application in multifunctional devices.
Nr: 18
Title:

Graphene Nonlinear Optics
Authors:

Giancarlo Soavi
Abstract: The recent demonstration of gate tuneable third harmonic generation (THG) in single layer graphene (SLG) [1,2] has sparked renewed interest in the study of the nonlinear optical properties of 2D materials and Dirac semimetals. Interestingly, our results also suggest new routes towards the realization of on-chip integrated nonlinear devices based on SLG, such as broadband gate tuneable nonlinear optical switches. However, a major limiting factor in this regard, which heavily suppresses the efficiency of such devices, is the increase of the SLG electronic temperature that follows from interaction with ultrashort pulses. Indeed, photoexcitation of the electron gas with ultrashort (≈fs-ps) pulses leads to the creation of an out-of-equilibrium (non-thermal) regime, i.e. a condition where the electron population cannot be defined by a Fermi Dirac distribution, which rapidly (≈10fs) evolves through electron-electron (e-e) scattering into a hot-carrier distribution. Hot-electrons then transfer energy to the lattice via scattering with phonons (ph) on a ≈ps timescale. The study of the ultrafast hot electron dynamics in SLG is thus crucial both for the understanding of its nonlinear optical properties and for the realization of nonlinear optical devices such as frequency converters and saturable absorbers. In this talk I will discuss our current understanding of the interplay between hot electrons and nonlinear optics in SLG, focusing in particular on the process of THG. In SLG, the THG intensity can be tuned by over one order of magnitude by externally applied gate voltages. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear optical conductivity arising from multiphoton resonant transitions [1,2]. However, both the THG intensity and its power dependence are heavily affected by an increase in the electronic temperature. I will demonstrate that hot electrons are responsible of a two-orders of magnitude reduction of the THG intensity [1] and of a stark deviation from the cubic power law expected for THG [3]. Finally, I will discuss possible configurations to control the hot electron recombination dynamics. These include interlayer electron-phonon interactions [4] and gate tuning of the SLG chemical potential due to phase-space suppression of the hot electron scattering with optical phonons. References 1. G. Soavi et al., Nature Nanotechnology 13, 583 (2018) 2. T. Jiang et al., Nature Photonics 12, 430 (2018) 3. G. Soavi et al., ACS Photonics 6, 2841 (2019) 4. K.-J. Tielrooij et al., Nature Nanotechnology 13, 41 (2018).
Nr: 31
Title:

Thermal Effects: An Alternative Mechanism for Plasmonic-assisted Photo-catalysis
Authors:

Yonatan Sivan
Abstract: Recent experimental studies demonstrated that chemical reactions can be accelerated by adding plasmonic metal nanoparticles to the chemical reactants and illuminate them at their plasmon resonance. It was claimed that the enhanced reaction rate occurs via the reduction in the activation energy driven by the plasmon-induced non-thermal (“hot”) electrons. In this contribution, we show that these claims are extremely unlikely to be correct, and that instead, the faster chemical reactions are likely the result of mere heating [1]. To do that, we derive a self-consistent theory of the electron distribution in metal nanostructures under continuous wave illumination. We show that only about one billionth of the energy provided by the illumination goes to creating non-thermal (``hot'') electrons, and the rest goes to heating. Quite different from previous theoretical studies, we took account of the heat transfer from the illuminated nanoparticle to the environment via phonon-phonon coupling and ensured energy conservation in the electron-phonon-environment system (rather than just in the electron sub-system). This approach not only allows us to distinguish between the generation of high energy non-thermal (“hot”) electrons and the regular heating of the nanoparticles, but also enables the determination of electron and phonon temperatures in a unique and unambiguous way. The theory is then used to compute the rate and energy distribution of electrons that tunnel out of the metal and can participate in a chemical reaction or enable photodetection. Further, we develop a simple model based on the Fermi golden rule and the Arrhenius Law, which shows that the enhanced chemical reactions observed experimentally are highly unlikely to result from the generation of non-thermal non-thermal (“hot”) electrons in the metal; instead, it is more likely originate from a purely photo-thermal effect. Specifically, we focus on a few of the seminal papers on this field and identify experimental errors in the temperature measurements that led the authors of these papers to underestimate the photo-thermal effect. Then, we show that the alternative theory of illumination-induced heating can explain the experimental data to remarkable agreement, with minimal to no fit parameters. Comprehensive thermal calculations (whereby we sum properly the heat generated by all particles in the system) confirm the temperature extracted from the experimental data, thus, showing that any claim in these papers related with ``hot'' electron action is not supported by the data. Finally, we show that for sufficiently high temperature and/or illumination intensity, it is necessary to account for the thermo-optical nonlinearity due to the temperature dependence of the optical and thermal properties of the system. We discuss the dominant contributions to the nonlinearity and the sensitivity to the various parameters of the sample and illumination. Our results provide the first ever comprehensive theory of plasmon-assisted photocatalysis and should become the basis for analysis of future experiments; it also reveals various routes for optimization of the chemical reaction acceleration. Our theory is also instrumental in quantifying experiments aimed to enable efficient photodetection. [1] Sivan, Baraban, Un & Dubi, Science 364, eaaw9367 (2019).
Nr: 34
Title:

All-Dielectric Chiral Metasurfaces: Direct Application in Biosensing
Authors:

F. Reyes Gomez, J. R. Mejía-Salazar and Pablo Albella
Abstract: Circular dichroism spectroscopy is a technique used to discriminate molecular chirality, which is essential in fields like biology, chemistry, or pharmacology where different chiral agents often show different biological activities. However, molecular-chiroptical activity is inherently weak, and limits the use of this technique to high concentrations or large analyte volumes. Finding novel ways to enhance the circular dichroism would boost the performance of these techniques. So far, enhancing light–matter interaction with plasmonic nanoantennas is the most common way to develop chiral systems with extraordinarily strong chiroptical responses. Nevertheless, metals show absorptive losses at optical frequencies, hindering its practical use in many scenarios. Recently, the use of low-loss resonators made of high refractive index (HRI) dielectric materials (non-plasmonic) has shown to be also efficient in enhancing the interaction of light with matter [1]. HRI nanoparticles, show low-losses, strong confinement of electromagnetic energy and outstanding scattering efficiencies. Another key aspect of HRI nanoantennas is the presence of coherent effects between electric and magnetic resonances. Here, we will present a novel all-dielectric low-loss chiral metasurface with unit cells built by high-refractive-index nanoantennas. These unit cells, built of silicon, strongly increase the chiroptical effect through the simultaneous interaction of their electric and magnetic modes, which in contrast to other recent proposals show at the same time a high concentration of the electric field in its gap that leads to the presence of hotspots [2]. This property makes them potential candidates in chiral target sensing/biosensing. [1] A. I. Barreda, J. M. Saiz, F. González, F. Moreno and P. Albella. “Recent advances in high refractive index dielectric nanoantennas: Basics and applications”, AIP Advances 9, 040701 (2019) [2] F. Reyes-Gomez, J. Ricardo Mejía-Salazar and Pablo Albella. “All-Dielectric Chiral Metasurfaces Based on Crossed-Bowtie Nanoantennas”. ACS Omega 2019, 4, 25, 21041-21047
Nr: 38
Title:

Plasmon-exciton Coupling: Light-forbidden Transitions and Quasichiral Interactions
Authors:

Antonio Fernandez Dominguez
Abstract: We present two plasmon-exciton coupling phenomena emerging due to the deeply sub-wavelength nature of surface plasmon (SP) resonances in nanocavities. First, we will investigate the impact that light-forbidden exciton transitions [1] have in the population dynamics and far-field scattering spectrum of hybrid systems comprising nanoparticle-on-a-mirror SPs and three-level quantum emitters (QEs). We will show that the presence of quadrupolar transitions [2] in the QE leads to a strong modification of the usual Purcell enhancement and Rabi splitting phenomenology for dipolar excitons. Second, we will present a combined classical and quantum electrodynamics description of the interactions between two circularly-polarized QEs held above a SP waveguide [3]. We will establish the conditions required to achieve non-reciprocal, chiral, coupling between them [4]. Moreover, by relaxing the stringent requirements for chirality, we will reveal a quasichiral regime, in which the quantum optical properties of the system are governed by its subradiant state, giving rise to extremely sharp spectral features and strong photon correlations. [1] A. Cuartero-González and A. I. Fernández-Domínguez, “Light-Forbidden Transitions in Plasmon-Emitter Interactions beyond the Weak Coupling Regime”ACS Photonics 5, 3415 (2018). [2] A. Cuartero-González and A. I. Fernández-Domínguez, “Dipolar and quadrupolar excitons coupled to a nanoparticle-on-mirror nanocavity”, Phys. Rev. B. 117, 107401 (2016). [3] C. A. Downing, J. C. López Carreño, F. P. Laussy, E. del Valle, and A. I. Fernández-Domínguez, “Quasi-Chiral Interactions between Quantum Emitters at the Nanoscale”, Phys. Rev. Let. 101, 035403 (2020). [4] C. A. Downing, J. C. López Carreño, E. del Valle, and A. I. Fernández-Domínguez, in preparation (2020).

Area 1 - Nanophotonics, Plasmonics and Metamaterials

Nr: 1
Title:

Numerical Identification of Symmetries in Topological Photonics
Authors:

Samuel J. Palmer, Richard Craster and Vincenzo Giannini
Abstract: Typically, waves are scattered at crystal defects. Recently, however, it was discovered that backscattering immune surface modes can exist at the interfaces between crystals with different band topology, provided that the crystal defects, impurities, and surfaces do not break the symmetries responsible for the band topology. For non-degenerate bands, the band topology is measured by a unique Chern number obtained by integrating the Berry curvature of the band over the Brillouin zone. However, when there are band degeneracies, the typical definition of Berry curvature becomes non-analytic and instead we assign Berry curvature and Chern numbers to subspaces of the Hilbert space spanned by the set of degenerate bands [1]. In other words, we assign the topology to unitary combinations of modes, such as pseudo-spin states. When a combination of modes corresponds to one or more symmetries of the Hamiltonian, then non-zero Chern numbers of a subspace indicate a non-trivial ​ symmetry-protected ​ topological phase [1]. However, it is not always apparent which combination of modes will carry the Berry curvature, particularly in bosonic systems where non-trivial topology often arises from a combination of bosonic time-reversal symmetry and other point-group symmetries of the crystal. We present a numerical method to identify the unitary mixtures of modes that carry the Berry curvature at a given position in the Brillouin zone. This is done by probing the Hilbert space spanned by the set of bands using an infinitesimally small Wilson loop. Repeating this throughout the Brillouin zone produces a smooth splitting of the Hilbert space according to the local, non-Abelian Berry curvature. This allows symmetry-protected topological phases to be found even when the responsible symmetries are not known a-priori, which may be advantageous when considering systems with many degrees of freedom and/or many band crossings, such as perturbations to supercells of a crystal. [1] Z2Pack: Numerical implementation of hybrid Wannier centers for identifying topological materials, Gresch et al, 2017 (https://doi.org/10.1103/PhysRevB.95.075146).
Nr: 2
Title:

Chiral Nanophotonics with Atomically Thin Semiconductors
Authors:

Alberto G. Curto
Abstract: Chiral optical fields can address spins in condensed matter. In 2D semiconductors such as monolayer MoS2, the spin and momentum direction of carriers can be locked. As a result, using circularly polarized light, we can populate carriers moving in one direction or the opposite depending on the handedness of the optical excitation. This degree of freedom, known as valley polarization, could be exploited to add a new dimension to information processing, optoelectronics, and nanophotonics. In this presentation, we will describe our results on the exploitation of spin-valley polarization in 2D semiconductors for chiral nano-optics. We will show how transitions in the electronic band structure control spin-valley polarization in few-layer materials to achieve high degrees of circular polarization at the nanoscale. Second, in order to enhance spin-valley polarization, we design nanophotonic resonators that satisfy the conditions needed for improving chiral light emission with achiral resonators as a path towards efficient sources of spin-valley-polarized light.
Nr: 3
Title:

Spectral Tuning of Ultra-Low Loss Polaritons in a Natural van de Waals Crystal
Authors:

Pablo Alonso-González, Javier Taboada-Gutiérrez, Gonzalo Álvarez-Pérez, Jiahua Duan, Weiliang Ma, Kyle Crowley, Iván Prieto, Andrei Bylinkin, Marta Autore, Halyna Volkova, Kenta Kimura, Tsuyoshi Kimura, M.-H. Berger, Qiaoliang Bao, Xuan A. Gao, Ion Errea, Alexey Nikitin and Javier Martín-Sánchez
Abstract: Phonon polaritons (PhPs) – light coupled to lattice vibrations – hold great promises for an unprecedented control of the flow of energy at the nanoscale because of their strong field confinement and long propagation. Moreover, recent experiments in polar van der Waals (vdW) crystals such as h-BN and -MoO3, have demonstrated PhPs with anisotropic propagation, and ultra-long lifetime in the picosecond range. However, a main drawback of these PhPs is the lack of tunability of the narrow and material-specific spectral range where they exist – the so-called Reststrahlen Band (RB) –, which severely limits their implementation in nanophotonics technologies. Here, we demonstrate that intercalation allows for a broad spectral shift of RBs in a vdW crystal, and that the PhPs excited within them show ultra-low losses (lifetime of 5 ps) similar to PhPs in the non-intercalated crystal (lifetime of 8 ps). As a difference to previous attempts, which fail in keeping the polaritonic activity of the intercalated compound, our results are possible by employing an intercalation method based on single crystal growth, that we carried out in the vdW semiconductor -V2O5, thereby also adding a new member to the library of vdW materials supporting PhPs. We expect this intercalation method to be applied in other vdW materials, opening the door for the use of PhPs in broad spectral bands that eventually cover the whole mid-IR range, which seems to be elusive with currently known polaritonic materials.
Nr: 4
Title:

High-Q Polaritonic Nanoresonators for Dielectric Sensing
Authors:

J. Duan, F. J. Alfaro-Mozaz, J. Taboada-Gutiérrez, I. Dolado, G. Álvarez-Pérez, J. Martín-Sánchez, R. Hillenbrand, A. Y. Nikitin and P. Alonso-González
Abstract: Polaritons allow for squeezing light at the nanoscale enhancing light-matter interactions, thus showing potentials for applications in spectroscopy or (bio)-sensing. Particularly, phonon polaritons (PhPs) in van der Waals materials, such as hexagonal boron nitride (h-BN), have attracted much interest because of their ultra-long lifetimes in the picosecond range and hyperbolic propagation with extremely large density of optical states. Although propagating and dipolar-like PhPs modes have been studied in h-BN semi-infinite slabs and nanoresonators, respectively, high-order modes in h-BN nanoresonators, with presumably much higher quality factors (Q) and field confinement, remain unexplored. Here, by infrared nano-imaging, we study, for the first time, the excitation and field distribution of high-order Fabry-Perot modes in h-BN nanoresonators (Figure1) and realize in situ sensing of the local dielectric environment. For the latter, we either transfer the same h-BN nanoresonators on different polar substrates, such as SiO2 and SiC, or cover them with thin layers of another vdW material, such as high refractive-index transition metal dichalcogenides (e.g. WSe2). Our results provide insights into high order Fabry-Perot resonances in polaritonic nanostructures as well as their functionality for in situ local dielectric sensing, with applications in materials science and biosensing.
Nr: 5
Title:

The Morphological Characterisation of Surface Coatings using Surface Enhanced Raman Spectroscopy
Authors:

David A. De Souza, Andreas Poulos, David Bell and Colin R. Crick
Abstract: Confocal Raman microscopy (CRM) allows for the chemical mapping of a substrate’s compositional structure as it allows spatial resolution in three dimensions. The resolution of these measurements is limited due to the inherently weak Raman signals. Enhancement of the Raman signal (through Surface-enhanced Raman spectroscopy – SERS) can be induced by the inclusion of metallic nanoparticles capable of generating a surface plasmon. This increases the signal-to-noise of the Raman signal of analytes in close contact with the nanoparticles, as a result, this provides a greater resolution for CRM measurements.
Nr: 6
Title:

Lightning-fast Solution of Scattering Problems in Nanophotonics: An Effortless Modal Approach
Authors:

P. Y. Chen, E. Muljarov and Y. Sivan
Abstract: Modal expansion techniques have long been used as an efficient way to calculate radiation of sources in closed cavities. With one set of cavity modes, calculated once and for all, the solution for any arbitrary configu- ration of sources can be generated almost instantaneously, providing clear physical insight into the spatial variation of Greens function and thus the local density of states. Nanophotonics research has recently generated an explo- sion of interest in generalizing modal expansion methods to open systems, for example using quasinormal mode / resonant state expansion [1]. Yet one major practical obstacle remains: numerical generation of resonator modes is slow and unreliable, often requiring considerable skill and hand guiding. Here, we present a practical numerical method for generating suitable modes, possessing the trifecta of traits: speed, accuracy, and reliability. Our method is capable of handling arbitrarily-shaped lossy resonators in open systems. It extends existing methods that expand modes of the target struc- ture using modes of a simpler analytically solvable geometry as a basis [1]. This process is guaranteed to succeed due to completeness, but is ordinarily inefficient because optical structures are usually piecewise uniform, so the resulting field discontinuities cripple convergence rates. Our key innovation is use of a new minimal set of basis modes that are inherently discontinuous, yet remarkably simple. We choose to implement our method for the General- ized Normal Mode Expansion (GENOME) [2] which unlike its alternatives [1], is valid for any source configuration, including the important case of sources exterior to the scatterer. We achieve rapid exponential convergence, with 4 accurate digits after only 16 basis modes, far more than is necessary. This also means lightning-speed simulation results, faster by 2-3 orders of mag- nitude compared to mode generation using COMSOL. Finally, our method is extremely reliable, as it culminates in a small dense linear eigensystem. No modes go missing, nor are there spurious modes that need to be manually discarded, which is critical to the success of modal expansion methods. [1] M. B. Doost et al., Phys. Rev. A 90, 013834 (2014), C. Sauvan et al., Phys. Rev. Lett., 110 237401, (2013) [2] P. Chen, D. Bergman and Y. Sivan, Phys. Rev. Appl. 11, 044018 (2019).