UV and deep-UV ultrafast harmonic generation enhanced by resonant gold nano-pillar metasurface

We report experimental observations and full-vector numerical simulations of second and third-harmonic generation from a two-dimensional periodic gold nano-pillar metasurface, for efficient ultrafast light conversion from near-infrared to the UV and deep-UV spectral ranges. The metasurface is designed to support an angle-tunable plasmonic resonance in the near-infrared spectral range, with its resonance wavelength shifting as a function of the pump angle of incidence. Under resonant conditions, strong electromagnetic-field confinement near the nano-pillar corners enhances the nonlinear light-matter interactions. Compared with a flat gold mirror, the nanostructured metal exhibits enhanced nonlinear frequency conversion, with conversion efficiencies of second and third-harmonic generation increased by two and three orders of magnitude, respectively. Full-wave theoretical modeling of linear and nonlinear light-matter interaction in nanoscale structures validates the experimental results. We demonstrate that periodically arranged square gold pillars can be engineered to achieve enhanced second and third-harmonic generation at wavelengths that can be tailored through the structural design, without requiring complex geometries or hybrid structures that pose fabrication challenges. The close agreement between theoretical predictions and experimental measurements further advances our understanding of surface-mediated nonlinear optical processes in metals and highlights the potential of metallic nanostructures as ultrafast light sources in the UV and deep-UV.