Nonlinear optical processes in resonant GaAs-based nanoantennas

Abstract

This PhD dissertation presents a systematic study in the linear and nonlinear optical properties of all-dielectric systems grounded on GaAs-based nanoantennas. First, I introduce a new nonlinear microscopy technique that determines the crystalline orientation of individual (100)-AlGaAs nanoantennas. The nonlinear microscopy technique, based in a point-scanning nonlinear microscope and structured light incidence, retrieves nonlinear intensity patterns which describe the crystalline orientation of the nanoantennas through the polarisation-dependency of the nonlinear process. The study of the nonlinear intensity maps indicate the transverse electric field components play a main role in the generation of harmonics. Also, the use of this nonlinear microscopy technique proved the generation of resonantly enhanced harmonic processes, driven by the presence of anapole-assisted modes.

Considering the main role of transverse electric field components in the harmonic generation of (100)-AlGaAs nanoantennas, a linearly polarised beam was used to study the properties of second-harmonic generation. A high conversion efficiency was demonstrated using (100)-AlGaAs nanoantennas, together with the generation of nonlinear nanoscale light sources emitting vector beams. In addition, continuous control in the transition between electric and magnetic second-harmonic generation was demonstrated. In all the studies using (100)-AlGaAs nanoantennas, zero normal second-harmonic generation was observed due to the symmetry of the second order nonlinear tensor. Normal second-harmonic generation can be attained when the symmetry of the system is reduced. This can be realised by extrinsic changes such as the design of asymmetric nanoantennas or the use of non-normal incidence. Another approach is to perform intrinsic changes such as changing the orientation of the nonlinear tensor through the fabrication of GaAs nanoantennas along different crystalline axis. Using the last approach, normal second-harmonic generation was realised as well as engineered second-harmonic emission from GaAs nanoantennas. Under the simple case of normal incidence, polarisation-independent conversion efficiencies and normal second-harmonic generation were demonstrated in (111)-GaAs nanoantennas. In addition to normal second-harmonic generation, control over the forward and backward second-harmonic emission was demonstrated in (110)-GaAs nanoantennas, including the case of nonlinear unidirectional emission.

While the previous studies were dedicated to the fundamentals of second-harmonic generation in GaAs-based nanoantennas, in the last part of this thesis I discuss the potential applications of GaAs metasurfaces to perform infrared up-conversion. Attractive nonlinear properties from (110)-GaAs metasurfaces are anticipated, due to the observed capabilities of (110)-GaAs nanoantennas. Enhancement of second-harmonic generation and sum-frequency generation was demonstrated in GaAs metasurfaces, through the excitation of electric and magnetic modes at the incident wavelengths. More interestingly, up-conversion of an invisible infrared image to the visible spectrum was demonstrated using (110)-GaAs metasurfaces, via sum-frequency generation. The sum-frequency generation process, originated in the metasurface, enabled the realisation of visible images which can be resolved in the femtosecond regime and can be detected by a conventional CCD camera.