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Strong light-matter coupling has garnered significant attention for its potential to optimize optoelectronic responses. In this study, we designed open cavities featuring non-local metasurfaces composed of aluminum nanoparticle arrays. The surface lattice resonances in these metasurfaces exhibit electronic strong coupling with the boron difluoride curcuminoid derivative, known for its highly efficient thermally activated delayed f luorescence in the near-infrared. Our results show that delayed fluorescence induced by triplet-triplet annihilation can be enhanced by a factor of 2.0-2.6 in metasurfaces that are either tuned or detuned to the molecular electronic transition. We demonstrate that delayed fluorescence enhancements in these systems primarily stem from increased absorption in the organic layer caused by the nanoparticle array, while strong coupling has negligible effects on reverse intersystem crossing rates, aligning with previous studies. We support these findings with finite-difference-time-domain simulations. This study elucidates how light-matter interactions affect delayed fluorescence, highlighting the potential applications in optoelectronic devices.

Vibrational strong coupling can modify chemical reaction pathways in unconventional ways. Thus far, Fabry-Perot cavities formed by pairs of facing mirrors have been mostly utilized to achieve vibrational strong coupling. In this study, we demonstrate the application of plasmonic microparticle arrays defining non-local metasurfaces that can sustain surface lattice resonances as a novel tool to enable chemical reactions under vibrational strong coupling. We show that the solvolysis kinetics of \textit{para}-nitrophenyl acetate can be accelerated by a factor of 2.7 by strong coupling to the carbonyl bond of the solvent and the solute with a surface lattice resonance. Our work introduces a new platform to investigate and control polaritonic chemical reactions. In contrast to Fabry-Perot cavities, metasurfaces define open optical cavities with single surfaces, which removes alignment hurdles, facilitating polaritonic chemistry across large areas.

We show the first experimental demonstration of room-temperature exciton-polariton (EP) condensation from a bound state in the continuum (BIC). This demonstration is achieved by strongly coupling stable excitons in an organic perylene dye with the extremely long-lived BIC in a dielectric metasurface of silicon nanoparticles. The long lifetime of the BIC, mainly due to the suppression of radiation leakage, allows for EP thermalization to the ground state before decaying. This property results in a condensation threshold of less than 5 \mu J cm^{-2}, one order of magnitude lower that the lasing threshold reported in similar systems in the weak coupling limit.

Various optical phenomena can be induced in periodic arrays of nanoparticles by the radiative coupling of the local dipoles in each particle. Probably the most impressive example is bound states in the continuum (BICs), which are electromagnetic modes with a dispersion inside the light cone but infinite lifetime, i.e., modes that cannot leak to the continuum. Symmetry-protected BICs appear at highly symmetric points in the dispersion of periodic systems. Although the addition of nonequivalent lattice points in a unit cell is an easy and straightforward way of tuning the symmetry, BICs in such particle lattice, i.e., non-Bravais lattice, are less explored among periodic systems. Starting from a periodic square lattice of Si nanodisks, we have prepared three non-Bravais lattices by detuning size and position of the second disk in the unit cell. Diffraction-induced coupling excites magnetic/electric dipoles in each nanodisk, producing two surface lattice resonances at the $\Gamma$ point with a band gap in between. %of $\sim$ 41 meV. The high/low energy branch becomes a BIC for the size/position-detuned array, respectively, while both branches are bright (or leaky) when both size and position are detuned simultaneously. The role of magnetic and electric resonances in dielectric nanoparticles and the change of BIC to bright character of the modes is explained by the two different origins of BICs in the detuned arrays, which is further discussed with the aid of a coupled electric and magnetic dipole model. This study gives a simple way of tuning BICs at telecom wavelengths in non-Bravais lattices, including both plasmonic and dielectric systems, thus scalable to a wide range of frequencies., Comment: 26 pages, 5 figures

Bound states in the continuum (BICs) represent a new paradigm in photonics due to the full suppression of radiation losses. However, this suppression has also hampered their direct observation. By using a double terahertz (THz) near-field technique that allows the local excitation and detection of the THz amplitude, we are able to map for the first time the electromagnetic field of BICs over extended areas, unveiling the field-symmetry protection that suppresses far-field radiation. This investigation, done for metasurfaces of dimer rod resonators, reveals the in-plane extension and formation of BICs with anti-symmetric phases, in agreement with coupled-dipole calculations. By displacing the rods, we demonstrate that mirror symmetry is not a necessary condition for BIC formation. Only $\pi$-rotation symmetry is required, making BICs exceptionally robust to structural changes. This work makes the local field of BICs experimentally accessible, which is crucial for the engineering of cavities with infinite lifetimes., Comment: 7 pages and 7 figures plus supplemental material (6 pages and 5 figures)

Optically induced Mie resonances in dielectric nanoantennas feature low dissipative losses and large resonant enhancement of both electric and magnetic fields. They offer an alternative platform to plasmonic resonances to study light-matter interactions from the weak to the strong coupling regimes. Here, we experimentally demonstrate the strong coupling of bright excitons in monolayer WS$_2$ with Mie surface lattice resonances (Mie-SLRs). We resolve both electric and magnetic Mie-SLRs of a Si nanoparticle array in angular dispersion measurements. At the zero detuning condition, the dispersion of electric Mie-SLRs (e-SLRs) exhibits a clear anti-crossing and a Rabi-splitting of 32 meV between the upper and lower polariton bands. The magnetic Mie-SLRs (m-SLRs) nearly cross the energy band of excitons. These results suggest that the field of m-SLRs is dominated by out-of-plane components that do not efficiently couple with the in-plane excitonic dipoles of the monolayer WS$_2$. In contrast, e-SLRs in dielectric nanoparticle arrays with relatively high quality factors (Q $\sim$ 120) facilitate the formation of collective Mie exciton-polaritons, and may allow the development of novel polaritonic devices which can tailor the optoelectronic properties of atomically thin two-dimensional semiconductors., Comment: 27 pages, 7 figures

Excitons spread through diffusion and interact through exciton-exciton annihilation. Nanophotonics can counteract the resulting decrease in light emission. However, conventional enhancement treats emitters as immobile and noninteracting. Here, we go beyond the localized Purcell effect to exploit exciton dynamics. As interacting excitons diffuse through optical hotspots, the balance of excitonic and nanophotonic properties leads to either enhanced or suppressed photoluminescence. We identify the dominant enhancement mechanisms in the limits of high and low diffusion and annihilation to turn their detrimental impact into additional emission. Our guidelines are relevant for efficient and high-power light-emitting diodes and lasers based on monolayer semiconductors, perovskites, or organic crystals.

Controlling the momentum of carriers in semiconductors, known as valley polarization, is a new resource for optoelectronics and information technologies. Materials exhibiting high polarization are needed for valley-based devices. Few-layer WS$_2$ shows a remarkable spin-valley polarization above 90%, even at room temperature. In stark contrast, polarization is absent for few-layer WSe$_2$ despite the expected material similarities. Here, we explain the origin of valley polarization in both materials based on the interplay between two indirect optical transitions. We show that the relative energy minima at the $\Lambda$- and K-valleys in the conduction band determine the spin-valley polarization of the direct K-K transition. Polarization appears as the energy of the K-valley rises above the $\Lambda$-valley as a function of temperature and number of layers. Our results advance the understanding of the high spin-valley polarization in WS$_2$. This insight will impact the design of both passive and tunable valleytronic devices operating at room temperature., Comment: 36 pages, including supplementary information

We demonstrate an enhanced emission of high quantum yield molecules coupled to dielectric metasurfaces formed by periodic arrays of polycrystalline silicon nanoparticles. Radiative coupling of the nanoparticles, mediated by in-plane diffraction, leads to the formation of collective Mie scattering resonances or Mie surface lattice resonances (M-SLRs), with remarkable narrow line widths. These narrow line widths and the intrinsic electric and magnetic dipole moments of the individual Si nanoparticles allow to resolve electric and magnetic M-SLRs. Incidence angle- and polarization-dependent extinction measurements and high-accuracy surface integral simulations show unambiguously that magnetic M-SLRs arise from in- and out-of-plane magnetic dipoles, while electric M-SLRs are due to in-plane electric dipoles. Pronounced changes in the emission spectrum of the molecules are observed, with almost a 20-fold enhancement of the emission in defined directions of molecules coupled to electric M-SLRs, and a 5-fold enhancement of the emission of molecules coupled to magnetic M-SLRs. These measurements demonstrate the potential of dielectric metasurfaces for emission control and enhancement, and open new opportunities to induce asymmetric scattering and emission using collective electric and magnetic resonances., Comment: 27 pages with 9 figures

We demonstrate experimentally an enhanced delayed fluorescence in tetracene single crystals strongly coupled to optical modes in open cavities formed by arrays of plasmonic nanoparticles. Hybridization of singlet excitons with collective plasmonic resonances in the arrays leads to the splitting of the material dispersion into a lower and an upper polariton band. This splitting significantly modifies the dynamics of the photo-excited tetracene crystal, resulting in an increase of the delayed fluorescence by a factor of four. The enhanced delayed fluorescence is attributed to the emergence of an additional radiative decay channel, where the lower polariton band harvests long-lived triplet states. There is also an increase in total emission, which is wavelength dependent, and can be explained by the direct emission from the lower polariton band, the more effcient light out-coupling and the enhancement of the excitation intensity. The observed enhanced fluorescence opens the possibility of effcient radiative triplet harvesting in open optical cavities, to improve the performance of organic light emitting diodes.

The light-matter coupling between electromagnetic modes guided by a semiconductor nanowire and excitonic states of molecules localized in its surrounding media is studied from both classical and quantum perspectives, with the aim of describing the strong coupling regime. Weakly guided modes (bare photonic modes) are found through a classical analysis, identifying those lowest-order modes presenting large electromagnetic fields spreading outside the nanowire, while preserving their robust guided behavior. Experimental fits of the dielectric permittivity of an organic dye that exhibits excitonic states are used for realistic scenarios. A quantum model properly confirms through an avoided mode crossing that the strong coupling regime can be achieved for this configuration, leading to Rabi splitting values above 100 meV. In addition, it is shown that the coupling strength depends on the fraction of energy spread outside the nanowire, rather than on the mode field localization. These results open up a new avenue towards strong coupling phenomenology involving propagating modes in non-absorbing media., Comment: 10 pages, 7 figures

Fano resonances and bound states in the continuum (BICs) exhibit a rich phenomenology stemming from, respectively, their asymmetric line shapes and infinite quality factors. Here, we show experimentally and theoretically that rod dimer metasurfaces exhibit narrow (high-Q) Fano resonances at THz frequencies. These resonances evolve continuously into a BIC as the rods in each dimer become identical. We demonstrate analytically that this is a universal behavior occurring in arrays of dimers consisting of detuned resonant dipoles. Fano resonances arise as a result of the interference between broad and narrow lattice dipole resonances, with high-Q factors tending to infinity in the detuning parameter space as the narrow lattice resonance becomes a BIC for identical resonant dipoles. Similar configurations can be straightforwardly envisioned throughout the electromagnetic spectrum leading to ultrahigh-Q Fano resonances and BICs of interest in photonics applications such as sensing and lasing., Comment: 6 pages, 4 figures

We demonstrate the strong coupling of direct transition excitons in tungsten disulfide (WS2) with collective plasmonic resonances at room temperature. We use open plasmonic cavities formed by periodic arrays of metallic nanoparticles. We show clear anti-crossings with monolayer, bilayer and thicker multilayer WS2 on top of the nanoparticle array. The Rabi energy of such hybrid system varies from 50 to 110 meV from monolayer to sixteen layers, while it does not scale with the square root of the number of layers as expected for collective strong coupling. We prove that out-of-plane coupling components can be disregarded since the normal field is screened due to the high refractive index contrast of the dielectric layers. Even though the in-plane dipole moments of the excitons decrease beyond monolayers, the strong in-plane field distributed in the flake can still enhance the coupling strength with multilayers. However, the screened out-of-plane field leads to the saturation of the Rabi energy. The achieved coherent coupling of TMD multilayers with open cavities could be exploited for manipulating the dynamics and transport of excitons in 2D semiconductors and developing ultrafast valley/spintronic devices.

Polaritons are quasiparticles arising from the strong coupling of electromagnetic waves in cavities and dipolar oscillations in a material medium. In this framework, localized surface plasmon in metallic nanoparticles defining optical nanocavities have attracted increasing interests in the last decade. This interest results from their sub-diffraction mode volume, which offers access to extremely high photonic densities by exploiting strong scattering cross-sections. However, high absorption losses in metals have hindered the observation of collective coherent phenomena, such as condensation. In this work we demonstrate the formation of a non-equilibrium room temperature plasmon-exciton-polariton condensate with a long range spatial coherence, extending a hundred of microns, well over the excitation area, by coupling Frenkel excitons in organic molecules to a multipolar mode in a lattice of plasmonic nanoparticles. Time-resolved experiments evidence the picosecond dynamics of the condensate and a sizeable blueshift, thus measuring for the first time the effect of polariton interactions in plasmonic cavities. Our results pave the way to the observation of room temperature superfluidity and novel nonlinear phenomena in plasmonic systems, challenging the common belief that absorption losses in metals prevent the realization of macroscopic quantum states., Comment: 23 pages, 5 figures, SI 7 pages, 5 figures

We experimentally demonstrate the coherent control, i.e., phase-dependent enhancement and suppression, of the optical absorption in an array of metallic nanoantennas covered by a thin lu- minescent layer. The coherent control is achieved by using two collinear, counter-propagating and phase-controlled incident waves with wavelength matching the absorption spectrum of dye molecules coupled to the array. Symmetry arguments shed light on the relation between the relative phase of the incident waves and the excitation efficiency of the optical resonances of the system. This coherent control is associated with a phase-dependent distribution of the electromagnetic near-fields in the structure which enables a significant reduction of the unwanted dissipation in the metallic structures., Comment: 4 pages, 4 figures

Controlling the dispersion and directionality of the emission of nanosources is one of the major goals of nanophotonics research. This control will allow the development of highly efficient nanosources even at the single photon level. One of the ways to achieve this goal is to couple the emission to Bloch modes of periodic structures. Here, we present the first measurements of the directional emission from nanowire photonic crystals by using Fourier microscopy. With this technique we efficiently collect and resolve the directional emission of nanowires within the numerical aperture of a microscope objective. The light emission from a heterostructure grown in each nanowire is governed by the photonic (Bloch) modes of the photonic crystal. We also demonstrate that the directionality of the emission can be easily controlled by infiltrating the photonic crystal with a high refractive index liquid. This work opens new possibilities for the control of the emission of sources in nanowires.

We present an experimental and theoretical study of the angle- and polarization-dependent light extinction in random arrays of polydisperse semiconductor nanowires epitaxially grown on substrates. The specular reflectance is described by averaging the scattering properties of individual nanowires obtained from Lorenz-Mie theory over the diameter distribution. The complex effective refractive index describing the propagation and attenuation of the coherent beam scattered in forward direction is determined in the independent scattering approximation and used to calculate the angle- and polarization-dependent reflectance. Our measurements demonstrate the highly anisotropic scattering in ensembles of aligned nanowires.

Localized surface plasmon polaritons in conducting structures give rise to enhancements of electromagnetic local fields and extinction efficiencies. Resonant conducting structures are conventionally fabricated with a fixed geometry that determines their plasmonic response. Here, we challenge this conventional approach by demonstrating the photo-generation of plasmonic materials (THz plasmonic antennas) on a flat semiconductor layer by the structured optical illumination through a spatial light modulator. Free charge carriers are photo-excited only on selected areas, which enables the definition of different plasmonic antennas on the same sample by simply changing the illumination pattern, thus without the need of physically structuring the sample. These results open a wide range of possibilities for an all-optical spatial and temporal control of resonances on plasmonic surfaces and the concomitant control of THz extinction and local field enhancements., Comment: 17 pages, 5 figures

We demonstrate a large light absorptance (80%) in a nanometric layer of quantum dots in rods (QRs) with a thickness of 23 nm. This behavior is explained in terms of the coherent absorption by interference of the light incident at a certain angle onto the very thin QR layer. We exploit this coherent light absorption to enhance the photoluminescent emission from the QRs. Up to a seven- and fivefold enhancement of the photoluminescence is observed for p- and s-polarized incident light, respectively., Comment: Physical Review B 2012

We report a novel graded refractive index antireflection coating for III/V quadruple solar cells based on bottom-up grown tapered GaP nanowires. We have calculated the photocurrent density of an InGaP-GaAs-InGaAsP-InGaAs solar cell with a MgF2/ZnS double layer antireflection coating and with a graded refractive index coating. The photocurrent density can be increased by 5.9 % when the solar cell is coated with a graded refractive index layer with a thickness of 1\mu m. We propose to realize such a graded refractive index layer by growing tapered GaP nanowires on III/V solar cells. For a first demonstration of the feasibility of the growth of tapered nanowires on III/V solar cells, we have grown tapered GaP nanowires on AlInP/GaAs substrates. We show experimentally that the reflection from the nanowire coated substrate is reduced and that the transmission into the substrate is increased for a broad spectral and angular range., Comment: 5 Figures

Plasmonic bowtie antennas made of doped silicon can operate as plasmonic resonators at terahertz (THz) frequencies and provide large field enhancement close to their gap. We demonstrate both experimentally and theoretically that the field confinement close to the surface of the antenna enables the detection of ultrathin (100 nm) inorganic films, about 3750 times thinner than the free space wavelength. Based on model calculations, we conclude that the detection sensitivity and its variation with the thickness of the deposited layer are related to both the decay of the local THz field profile around the antenna and the local field enhancement in the gap of the bowtie antenna. This large field enhancement has the potential to improve the detection limits of plasmon-based biological and chemical sensors.

We describe both theoretically and experimentally the existence and excitation of confined modes in planar arrays of gold nanodisks. Ordered 2D lattices of monodispersive nanoparticles are manufactured, embedded in a silica matrix, and exposed to evanescent prism-coupling illumination, leading to dark features in the reflectivity, which signal the presence of confined modes guided along the arrays. We find remarkable agreement between theory and experiment in the frequency-momentum dispersion of the resonances. Direct excitation of these modes reveals long propagation distances and deep extinction features. This characterization of guided modes shows the great potential of metallic particle arrays for optical signal processing and distant sensing applications.

Current detection and identification of micro-organisms is based on either rather unspecific rapid microscopy or on more accurate complex, time-consuming procedures. In a medical context, the determination of the bacteria Gram type is of significant interest. The diagnostic of microbial infection often requires the identification of the microbiological agent responsible for the infection, or at least the identification of its family (Gram type), in a matter of minutes. In this work, we propose to use terahertz frequency range antennas for the enhanced selective detection of bacteria types. Several microorganisms are investigated by terahertz time-domain spectroscopy: a fast, contactless and damage-free investigation method to gain information on the presence and the nature of the microorganisms. We demonstrate that plasmonic antennas enhance the detection sensitivity for bacterial layers and allow the selective recognition of the Gram type of the bacteria.

Semiconductor nanowires exhibit large polarization anisotropy for the absorption and emission of light, making them ideal building blocks for novel photonic metamaterials. Here, we demonstrate that a high density of aligned nanowires exhibits giant optical birefringence, a collective phenomenon observable uniquely for collections of wires. The nanowire material was grown on gallium phosphide (GaP) (111) in the form of vertically standing GaP nanowires. We obtain the largest optical birefringence to date, with a difference between the in-plane and out-of-plane refractive indices of 0.80 and a relative birefringence of 43%. These values exceed by a factor of 75 the natural birefringence of quartz and a by more than a factor of two the highest values reported so far in other artificial materials. By exploiting the specific crystallographic growth directions of the nanowires on the substrate, we further demonstrate full control over the orientation of the optical birefringence effect in the metamaterial., Comment: 10 pages, 4 figures