Nonlinear optics at the nanoscale: experiment versus theory

Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. At this scale light-matter interaction displays completely new phenomena. Traditionally, second and third harmonic generation (SHG and THG) have been studied in highly nonlinear transparent materials and under phase matching conditions ensuring large conversion efficiencies. In this case, the dominant nonlinear polarization term is given by the bulk electric dipole contribution through the nonlinear susceptibilities. However, at nanoscale the scenario is totally different: the electric dipole contribution no longer predominates and other terms, usually neglected, become important. Understanding how light interacts at the nanoscale with metals, semiconductors or dielectrics is pivotal to engineer and implement nano-antennas, filters, etc. In this work, we present a collection of experimental results regarding the SHG and THG in a semiconductor (GaAs) a conductive oxide (Indium Tin Oxide (ITO) nanolayer) at epsilon-near-zero (ENZ) condition and a metal (Au nanolayer). We explain the different nonlinear contribution in each case comparing our measurements with numerical results predicted by our theoretical model which accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity, and changes in the effective mass of free electrons triggered by absorption [1] .