|
Metal nanostructures are largely used to enhance light-matter interactions and the overarching optical response by means of plasmonic resonances and surface-enhanced spectroscopy. They can also be designed to improve efficiency of nonlinear optical processes like second and third harmonic generation, despite their high absorption in the visible and near infrared range. When the metallic layer thickness is reduced to a few nanometers in thickness, light-matter interactions can display new phenomena where conventional approximations may not always be applicable. While planar structures are generally the simplest to fabricate and easier to simulate, the efficiency of the harmonic signals are generally quite small but can be enhanced when resonant nanostructures are implemented. In this work we design a gold nanograting with resonant features around 800nm. When the fundamental beam is tuned around the resonance wavelength in the near-infrared region we obtain a second and third harmonic signal tuned in the opaque region of gold. We firstly measure TM and TE components of the SH signal at 400 nm as a function of the fundamental beam polarization, tuning the fundamental wavelength around the resonance. Then we experimentally estimate the relative enhancement induced by the grating with respect to SHG from planar gold, finding a maximum enhancement of 800 for the central resonant wavelength. We calculate the predicted harmonic conversion efficiencies of the grating employing our microscopic hydrodynamic approach to model light-matter interactions. This model relies on temporal and spatial derivatives and mere knowledge of the effective electron mass to determine the relative magnitudes of surface and volume. Our simulations predict an enhancement factor close to 1000, of the same order of magnitude as our measurements. The same model predicts a 500-fold enhancement for the THG with respect to the plain gold layer.
|
