Harmonic generation in the opaque region of semiconductors: the role of the surface and magnetic nonlinearities

The use of semiconductors such as GaAs, GaP or Si in the process of fabrication of actual nano devices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. At these scales the usual theory describing the nonlinear effects of electromagnetic fields are pushed to the limit of usual approximations and should be revisited and analyzed. Recently, we have studied in detail the generation of the second and third harmonic the opaque region of GaAs and Si, going beyond the previous studies and deeply analyzing the nonlinear process in order to infer which are the different mechanisms leading to the second and third harmonic generation at the surface of these materials. We demonstrate that the bulk nonlinearity is not the only one active term and that we have strong contributions coming from the surface and magnetic Lorentz terms, which usually are either hidden by the bulk contributions or assumed to be negligible. Experimental and theoretical simulations are contrasted, using a hydrodynamic model [1,2] that accounts for all salient aspects of the dynamics, including surface and bulk generated harmonic components. [3] The study, made in detail for GaAs is extended here to other semiconductors as Si and GaP. We also consider resonant structures as gratings and nanowires capable to strongly enhance the nonlinear efficiencies. Although the harmonic generation in this regime and materials still has low efficiency, these findings have significant repercussions and are consequential in nanoscale systems, which are usually investigated using only dispersion less bulk nonlinearities, with near-complete disregard of surface and magnetic contributions and their microscopic origins.