High-harmonic generation from subwavelength silicon films

Recent years have witnessed significant developments in the study of nonlinear properties of various materials at the nanoscale. Often, experimental results on harmonic generation are reported without the benefit of suitable theoretical models that allow assessment of conversion efficiencies compared to the material’s intrinsic properties. Here, we report experimental observations ofeven and odd harmonics up to the 7th, generated from asuspended subwavelength silicon film resonant in the UVrange at 210 nm, the current limit of our detection system,using peak power densities of order 3 TW/cm2. We also highlight the time-varying properties of the dielectric functionof silicon, which exhibits large changes under intense illumination. We explain the experimental data with a timedomain, hydrodynamic-Maxwell approach broadly applicable to most optical materials. Our approach accounts simultaneously for surface and magnetic nonlinearities that generate even optical harmonics, as well as linear and nonlinearmaterial dispersions beyond the third order to account forodd optical harmonics, plasma formation, and a phase locking mechanism that makes the generation of high harmonics possible deep into the UV range, where semiconductorslike silicon start operating in a metallic regime.