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.

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.

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.

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.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.