Linear and Nonlinear Optics with Metal Oxides: From Single Nanoparticles to Metasurfaces

Nonlinear optics has become an important part of our technology related daily life since its rapid development right after the invention of laser in 1960. Indeed, it enabled the creation of high-intensity laser pulses, which are used in applications ranging from industrial manufacturing to microsurgery. The development of nonlinear materials allowed the production of very compact and affordable pocket laser pointers and enhanced the power of optical data transmission. Most applications using nonlinear optics are based on bulk materials. They profit from the large volume to build up nonlinear signals, which can be additionally increased by phase matching. In addition, the development of nonlinear optics at the nanoscale would bring further significant advances in bioimaging, integrated photonics circuits or nano-modulators. However, the generated nonlinear signals at the nanoscale are low because the nonlinear optical phenomena stem from the volume of a material. Furthermore, the nonlinear conversion efficiency of nanostructures cannot be improved by phase matching because their size is smaller than the coherence length. Therefore, we propose novel strategies and materials to increase the nonlinear optical signals at the subwavelength scale. We consider the second order nonlinearities, which are the lowest order nonlinearities, and we focus in particular on the second-harmonic generation (SHG). For SHG emission, a break of symmetry is required. This can be achieved in plasmonics, at the surface of metallic nanoparticles, or in materials with noncentrosymmetric crystal structures, where the SHG is generated from the volume. Promising materials with volume SHG are nanostuctures from noncentrosymmetric semiconductors. These materials have a dielectric-type behavior and confine light because of their high refractive indices. We use this confinement to show the formation of SHG enhancing anapole modes in aluminum gallium arsenide (AlGaAs) nanodisks. However, the applicability of most semiconductors is limited by the low energy band gap, which makes them highly absorbing below 500 nm. We propose two metal oxides, barium titanate (BaTiO3) and lithium niobate (LiNbO3), as alternative all-dielectric materials with an extended applicability range compared to semiconductors. We exploit the large band gaps and reasonably high second order susceptibility of BaTiO3 and LiNbO3 to show SHG at the nanoscale over a broad range from the near-ultraviolet (NUV) to the near-infrared (NIR) range. Both BaTiO3 and LiNbO3 nanostructures confine light at the Mie resonances, which appear because of the high refractive indices of these materials. We show that SHG is enhanced at these intrinsic Mie resonances. In particular, we prove that LiNbO3 nanostructures emit SHG most efficiently in the NUV range. Then, we exploit the strong field confinement outside the surfaces of plasmonic nanostructures by fabricating hybrid nanostructures to demonstrate the enhancement of SHG from BaTiO3 nanoparticles at the plasmonic resonances. Finally, we show the integration of BaTiO3 nanostructures in SHG emitting metasurfaces, which are nonlinear optical devices that exploit the optical properties of periodic arrays of single nanostructures to manipulate the wavefront of light over subwavelength scales. This work proposes two materials, BaTiO3 and LiNbO3, that can be successfully employed for subwavelength nonlinear photonics and that cover the NUV to the visible range. The full optical characterization and the fabrication methods proposed for these two materials can be used to develop new devices for a broad wavelength range, such as single photon emitters, nonlinear active metasurfaces and nonlinear flat optics or nano-modulators