Refractive index sensing using quasi-bound states in the continuum in silicon metasurfaces.

This work presents a bulk refractive index sensor based on quasi-bound states in the continuum (BICs) induced by broken symmetries in metasurfaces. The symmetry is broken by detuning the size and position of silicon particles periodically arranged in an array, resulting in multiple quasi-BIC resonances. We investigate the sensing characteristics of each of the resonances by measuring the spectral shift in response to changes in the refractive index of the surrounding medium. In addition, we reveal the sensing range of the different resonances through simulations involving a layer of deviating refractive index of increasing thickness. Interestingly, the resonances show very different responses, which we describe via the analysis of the near-field. This work contributes to the development of highly sensitive and selective BIC-based sensors that can be used for a wide range of applications.

A dynamic passive thermoregulation fabric using metallic microparticles.

Maintaining comfort using photonic thermal management textiles has a large potential to decrease the energy cost for heating and cooling in residential and office buildings. We propose a thermoregulating fabric using metallic microparticles, which provides a dynamic and passive control of the infrared transmission, by adapting to the ambient temperature and humidity. The fabric is composed of tailored metal microparticles and a stimuli-responsive polymer actuator matrix, in order to benefit from strong scattering effects to control the wideband transmission of thermal radiation and to provide a sharp, dynamic response. The detailed numerical design demonstrates a wide dynamic ambient setpoint temperature window of ∼8 °C, with the wearer staying comfortable in the range between 18 and 26 °C. Its compatibility for large-scale manufacturing, with a safe and strong thermoregulating performance indicates a vital energy-saving potential and paves the way to a more sustainable society.

Unveiling the nonlinear optical response of Trictenotoma childreni longhorn beetle.

The wings of some insect species are known to fluoresce under illumination by ultraviolet light. Their fluorescence properties are however, not comprehensively documented. In this article, the optical properties of one specific insect, the Trictenotoma childreni yellow longhorn beetle, were investigated using both linear and nonlinear optical (NLO) methods, including one- and two-photon fluorescence and second harmonic generation (SHG). These three distinct optical signals discovered in this beetle are attributed to the presence of fluorophores embedded within the scales covering their elytra. Experimental evidence collected in this study indicates that the fluorophores are non-centrosymmetric, a fundamental requirement for SHG. This study is the first reported optical behavior of this type in insects. We described how NLO techniques can complement other more convenient approaches to achieve a more comprehensive understanding of insect scales and integument properties.

Nonlinear optical spectroscopy and two-photon excited fluorescence spectroscopy reveal the excited states of fluorophores embedded in a beetle's elytra.

Upon illumination by ultraviolet light, many animal species emit light through fluorescence processes arising from fluorophores embedded within their biological tissues. Fluorescence studies in living organisms are however relatively scarce and so far limited to the linear regime. Multiphoton excitation fluorescence analyses as well as nonlinear optical techniques offer unique possibilities to investigate the effects of the local environment on the excited states of fluorophores. Herein, these techniques are applied for the first time to study of the naturally controlled fluorescence in insects. The case of the male Hoplia coerulea beetle is investigated because the scales covering the beetle's elytra are known to possess an internal photonic structure with embedded fluorophores, which controls both the beetle's coloration and the fluorescence emission. An intense two-photon excitation fluorescence signal is observed, the intensity of which changes upon contact with water. A third-harmonic generation signal is also detected, the intensity of which depends on the light polarization state. The analysis of these nonlinear optical and fluorescent responses unveils the multi-excited states character of the fluorophore molecules embedded in the beetle's elytra. The role of form anisotropy in the photonic structure, which causes additional tailoring of the beetle's optical responses, is demonstrated by circularly polarized light and nonlinear optical measurements.

Metasurface-based multi-harmonic free-electron light source.

Metasurfaces are subwavelength spatial variations in geometry and material where the structures are of negligible thickness compared to the wavelength of light and are optimized for far-field applications, such as controlling the wavefronts of electromagnetic waves. Here, we investigate the potential of the metasurface near-field profile, generated by an incident few-cycle pulse laser, to facilitate the generation of high-frequency light from free electrons. In particular, the metasurface near-field contains higher-order spatial harmonics that can be leveraged to generate multiple higher-harmonic X-ray frequency peaks. We show that the X-ray spectral profile can be arbitrarily shaped by controlling the metasurface geometry, the electron energy, and the incidence angle of the laser input. Using ab initio simulations, we predict bright and monoenergetic X-rays, achieving energies of 30 keV (with harmonics spaced by 3 keV) from 5-MeV electrons using 3.4-eV plasmon polaritons on a metasurface with a period of 85 nm. As an example, we present the design of a four-color X-ray source, a potential candidate for tabletop multicolor hard X-ray spectroscopy. Our developments could help pave the way for compact multi-harmonic sources of high-energy photons, which have potential applications in industry, medicine, and the fundamental sciences.

Time reflection and refraction of graphene plasmons at a temporal discontinuity.

A dynamical change of material properties induces a special type of reflection and refraction at a temporal discontinuity. Here, we study the interaction of graphene plasmons with single and double temporal discontinuities or shocks, leading to controlled in-plane scattering. We analytically determine the Fresnel-like coefficients for graphene plasmons at these boundaries, and validate our results by rigorous numerical simulations. Temporally controlled doping of two-dimensional materials such as graphene thus leads to a new mechanism for planar and compact plasmonic devices.

Insights into directional scattering: from coupled dipoles to asymmetric dimer nanoantennas.

Strong and directionally specific forward scattering from optical nanoantennas is of utmost importance for various applications in the broader context of photovoltaics and integrated light sources. Here, we outline a simple yet powerful design principle to perceive a nanoantenna that provides directional scattering into a higher index substrate based on the interference of multiple electric dipoles. A structural implementation of the electric dipole distribution is possible using plasmonic nanoparticles with a fairly simple geometry, i.e. two coupled rectangular nanoparticles, forming a dimer, on top of a substrate. The key to achieve directionality is to choose a sufficiently large size for the nanoparticles. This promotes the excitation of vertical electric dipole moments due to the bi-anisotropy of the nanoantenna. In turn, asymmetric scattering is obtained by ensuring the appropriate phase relation between the vertical electric dipole moments. The scattering strength and angular spread for an optimized nanoantenna can be shown to be broadband and robust against changes in the incidence angle. The scattering directionality is maintained even for an array configuration of the dimer. It only requires the preferred scattering direction of the isolated nanoantenna not to be prohibited by interference.

Symmetry recovery for coupled photonic modes with transversal PT symmetry.

Typical parity-time (PT) symmetric structures switch from the unbroken to the broken phase when gain increases through an exceptional point. In contrast, we report on systems with the unusual, reverse behavior, where the symmetric phase is recovered after a broken phase. We study this phenomenon analytically and numerically in the simplest possible system, consisting of four coupled modes, and we present potential dielectric and plasmonic implementations. The complex mode merging scheme, with two distinct unbroken PT phases encompassing a broken one, appears for a specific proportion range of the coupling constants. This regime with "inverse" exceptional points is interesting for the design of novel PT devices.

Patterned graphene edges for tailored reflection of plasmonic modes.

Combining graphene with plasmonics is expected to lead to new nanoscale applications such as sensors, photodetectors, and optical circuits, since graphene plasmons in the infrared have relatively low losses and are easily tunable. It was shown that the edges of a graphene sheet completely reflect these plasmons with negligible radiation losses. Here, however, we examine structured graphene edges, which provide the ability to tailor and even completely cancel the reflection. These properties depend on the suitable dimensions of the edge grating. We explain the reflection modulation via the appearance of longitudinal Fabry-Perot type modes. Interesting phase changes and resonances appear when the longitudinal modes interact with lateral modes mediated by edge plasmons.

Active liquid crystal tuning of metallic nanoantenna enhanced light emission from colloidal quantum dots.

A system comprising an aluminum nanoantenna array on top of a luminescent colloidal quantum dot waveguide and covered by a thermotropic liquid crystal (LC) is introduced. By heating the LC above its critical temperature, we demonstrate that the concomitant refractive index change modifies the hybrid plasmonic-photonic resonances in the system. This enables active control of the spectrum and directionality of the narrow-band (∼6 nm) enhancement of quantum dot photoluminescence by the metallic nanoantennas.

Simulations of high-Q optical nanocavities with a gradual 1D bandgap.

High-quality cavities in hybrid material systems have various interesting applications. We perform a comprehensive modeling comparison on such a design, where confinement in the III-V material is provided by gradual photonic crystal tuning, a recently proposed method offering strong resonances. The III-V cavity couples to an underlying silicon waveguide. We report on the device properties using four simulation methods: finite-difference time-domain (FDTD), finite-element method (FEM), bidirectional eigenmode propagation (BEP) and aperiodic rigorous coupled wave analysis (aRCWA). We explain the major confinement and coupling effects, consistent with the simulation results. E.g. for strong waveguide coupling, we find quantitative discrepancies between the methods, which establishes the proposed high-index-contrast, lossy, 3D structure as a challenging modeling benchmark.

Comparing plasmonic and dielectric gratings for absorption enhancement in thin-film organic solar cells.

We theoretically investigate and compare the influence of square silver gratings and one-dimensional photonic crystal (1D PC) based nanostructures on the light absorption of organic solar cells with a thin active layer. We show that, by integrating the grating inside the active layer, excited localized surface plasmon modes may cause strong field enhancement at the interface between the grating and the active layer, which results in broadband absorption enhancement of up to 23.4%. Apart from using silver gratings, we show that patterning a 1D PC on top of the device may also result in a comparable broadband absorption enhancement of 18.9%. The enhancement is due to light scattering of the 1D PC, coupling the incoming light into 1D PC Bloch and surface plasmon resonance modes.

Combined plasmonic gratings in organic solar cells.

We propose an organic solar cell structure with combined silver gratings consisting of both a front and a back grating. This combination provides multiple, semi-independent enhancement mechanisms which act additively, so that a broadband absorption is obtained. Both gratings couple the incident light into various plasmonic modes, showing a more localized or propagating character respectively. In addition, some modes only appear for tilted incident light, and therefore present a complex angle-dependent behavior. We provide extensive numerical simulations, resulting in an optimized period of 490 nm, with front grating elements of 60 by 10nm and back elements of 60 by 30 nm. With these parameters an integrated absorption enhancement factor around 1.35 is observed, with absorption increasing from 48% to 65% under TM polarized light. In addition, the solar cell with combined gratings is much less sensitive to the angle of incident light than the single grating cases. Furthermore, the grating structure does not have a large influence on the TE polarized light absorption.

Excitation of multiple dipole surface plasmon resonances in spherical silver nanoparticles.

We observe the appearance of multiple dipole surface plasmon resonances in spherical Ag nanoparticles when embedded in an organic semiconductor that exhibits a highly dispersive permittivity. Comparing the absorption spectra of thin-films with and without Ag nanoparticles reveals the presence of two plasmon peaks. Numerical simulations and calculations based on an electrostatic model allow us to attribute both peaks to dipole resonances, and show that the strong dispersion of the organic permittivity is responsible for this behavior. The presence of these two plasmon resonances was found to enhance the absorption of the organic semiconductor over a broad wavelength range.

Tunable optical forces between nanophotonic waveguides.

The confinement of light in components with nanoscale cross-sections in nanophotonic circuits significantly enhances the magnitude of the optical forces experienced by these components. Here we demonstrate optical gradient forces between two nanophotonic waveguides, and show that the sign of the force can be tuned from attractive to repulsive by controlling the relative phase of the optical fields injected into the waveguides. The optical gradient force could have applications in optically tunable microphotonic devices and nanomechanical systems.

Nonlinear photonic crystals near the supercollimation point.

We uncover a strong coupling between nonlinearity and diffraction in a photonic crystal at the supercollimation point. We show that this is modeled by a nonlinear diffraction term in a nonlinear-Schrödinger-type equation in which the properties of solitons are investigated. Linear stability analysis shows solitons are stable in an existence domain that obeys the Vakhitov-Kolokolov criterium. In addition, we investigate the influence of the nonlinear diffraction on soliton collision scenarios.

Symmetry breaking with coupled Fano resonances.

We describe the effect of symmetry breaking in a system with two coupled Fano resonators. A general criterion is derived and optimal parameter regions for switching are identified. By extending the single resonator behavior we show that one achieves the effect at very low input powers.

Microcavities based on multimodal interference.

We describe intricate cavity mode structures, that are possible in waveguide devices with two or more guided modes. The main element is interference between the scattered fields of two modes at the facets, resulting in multipole or mode cancelations. Therefore, strong coupling between the modes, such as around zero group velocity points, is advantageous to obtain high quality factors. We discuss the mechanism in three different settings: a cylindrical structure with and without negative group velocity mode, and a surface plasmon device. A general semi-analytical expression for the cavity parameters describes the phenomenon, and it is validated with extensive numerical calculations.

Switching through symmetry breaking in coupled nonlinear micro-cavities.

We describe stable symmetry-breaking states in systems with two coupled nonlinear cavities, using coupled-mode theory and rigorous simulations. Above a threshold input level the symmetric state of the passive Kerr system becomes unstable, and we show how this phenomenon can be employed for switching and flip-flop purposes, using positive pulses only. A device with compact photonic crystal microcavities is proposed by which we numerically demonstrate the principle.

Nonlinear lattice model for spatially guided solitons in nonlinear photonic crystals.

Numerical simulations have shown the existence of transversely localized guided modes in nonlinear two-dimensional photonic crystals. These soliton-like Bloch waves induce their own waveguide in a photonic crystal without the presence of a linear defect. By applying a Green's function method which is limited to within a strip perpendicular to the propagation direction, we are able to describe these Bloch modes by a nonlinear lattice model that includes the long-range site-to-site interaction between the scattered fields and the non-local nonlinear response of the photonic crystal. The advantages of this semi-analytical approach are discussed and a comparison with a rigorous numerical analysis is given in different configurations. Both monoatomic and diatomic nonlinear photonic crystals are considered.