Light diffraction in all-optical time-varying metasurfaces

This thesis focuses on frequency control of optical waves using time-varying effects in nanophotonic media. Particularly, diffraction by a slit in time is demonstrated using ultrafast all-optical mirrors. We first demonstrate a large reflectivity change from an Indium Tin Oxide/Gold bilayer under optical pumping, at speeds high enough to observe time-varying effects. Then, treating the mirror as an interface, we show time-diffraction from the sample, corresponding to broadening and shifting of the spectrum of light reflected by the mirror during its modulation. This is clearly confirmed by a double slit diffraction experiment, replicating Young's experiment in time, with a generated spectrum exhibiting oscillations in frequency. Insights on the electron dynamics within Indium Tin Oxide is gained, with in particular a striking shortening of the permittivity modulation rise time unlocking the access to an even lower timescale for time-varying effects. Finally, time-varying effects on harmonic generation from Indium Tin Oxide and Gallium Phosphide thin films are demonstrated, exhibiting strong temporal refraction and new mechanisms for temporal modulation.Open Acces

Spectral and spatial shaping of Smith Purcell Radiation

The Smith Purcell effect, observed when an electron beam passes in the vicinity of a periodic structure, is a promising platform for the generation of electromagnetic radiation in previously-unreachable spectral ranges. However, most of the studies of this radiation were performed on simple periodic gratings, whose radiation spectrum exhibits a single peak and its higher harmonics predicted by a well-established dispersion relation. Here, we propose a method to shape the spatial and spectral far-field distribution of the radiation using complex periodic and aperiodic gratings. We show, theoretically and experimentally, that engineering multiple peak spectra with controlled widths located at desired wavelengths is achievable using Smith-Purcell radiation. Our method opens the way to free-electron driven sources with tailored angular and spectral response, and gives rise to focusing functionality for spectral ranges where lenses are unavailable or inefficient

Nonlinear dielectric epsilon near-zero hybrid nanogap antennas

High-index Mie-resonant dielectric nanostructures provide a new framework to manipulate light at the nanoscale. In particular their local field confinement together with their inherently low losses at frequencies below their band-gap energy allows to efficiently boost and control linear and nonlinear optical processes. Here, we investigate nanoantennas composed of a thin indium-tin oxide layer in the center of a dielectric Gallium Phosphide nanodisk. While the linear response is similar to that of a pure GaP nanodisk, we show that the second and third-harmonic signals of the nanogap antenna are boosted at resonance. Linear and nonlinear finite-difference time-domain simulations show that the high refractive index contrast leads to strong field confinement inside the antenna's ITO layer. Measurement of ITO and GaP nonlinear susceptibilities deliver insight on how to engineer nonlinear nanogap antennas for higher efficiencies for future nanoscale devices.Comment: main: 18 pages, 4 figues, supplemental: 8 pages, 4 figures, 1 tabl