The study of light-matter interaction at the nanoscale has become a growing field of research, facilitated by significant advancements in nanofabrication techniques. When the dimensions of nanoscale systems such as molecules, quantum dots and nanoparticles become comparable to the wavelength of the light, new physical phenomena arise. Plasmonic nanoantennas have shown control over the properties of the incoming light by the excitation of plasmonic resonances. However, plasmonic nanoantennas exhibit nonradiative losses and detrimental heating. High-index dielectric nanoantennas have emerged as a promising alternative to plasmonic counterparts, due to their low losses and the ability to excite electric and magnetic resonances. The development of dielectric nanoantennas with strong nonlinear optical properties is still in its infancy, therefore strategies to increase the conversion efficiency and control the directionality of nonlinear emissions remains to be explored.

In this seminar, I will present my studies on the nonlinear optical properties of GaAs-based nanoantennas. Firstly, I will introduce a new nonlinear microscopy technique which determines the crystalline orientation of (100)-AlGaAs nanoantennas and shows the generation of enhanced nonlinear processes driven by the presence of anapole states. Next, I will focus on the excitation of electric and magnetic resonant modes in (100)-AlGaAs nanoantennas to achieve high nonlinear conversion efficiencies, generation of nonlinear vector beams and transition between electric and magnetic second harmonic generation (SHG). Throughout these studies, zero normal SHG was observed which originates from the symmetry of the nonlinear tensor. To overcome this limitation, GaAs nanoantennas grown along two different crystallographic orientations were studied. In (111)-GaAs nanoantennas, normal SHG was observed together with polarisation-independent nonlinear conversion efficiencies. In addition to normal SHG, wide control over SHG directionality was obtained in (110)-GaAs nanoantennas. These results has lead us to explore potential applications of GaAs nanoantennas in arrays, also known as GaAs metasurfaces. In particular, we have been using (110)-GaAs metasurfaces to perform infrared imaging via sum-frequency generation, opening up new opportunities for bioimaging and compact night-vision devices.