We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical nanoantennas. Using a selective oxidation technique, we fabricated epitaxial semiconductor nanocylinders on an aluminum oxide substrate. Second harmonic generation from AlGaAs nanocylinders of 400 nm height and varying radius pumped with femtosecond pulses delivered at 1554-nm wavelength has been measured, revealing a peak conversion efficiency exceeding 10⁻⁵ for nanocylinders with an optimized geometry.

Nonlinear metasurfaces constitute a key asset in meta-optics, given their ability to scale down nonlinear optics to sub-micrometer thicknesses. To date, nonlinear metasurfaces have been mainly realized using narrow band gap semiconductors, with operation limited to the near-infrared range. Nonlinear meta-optics in the visible range can be realized using transparent materials with high refractive index, such as lithium niobate (LiNbO₃). Yet, efficient operation in this strategic spectral window has been so far prevented by the nanofabrication challenges associated with LiNbO₃, which considerably limit the aspect ratio and minimum size of the nanostructures (i.e., meta-atoms). Here we demonstrate the first monolithic nonlinear periodic metasurface based on LiNbO₃ and operating in the visible range. Realized through ion beam milling, our metasurface features a second-harmonic (SH) conversion efficiency of 2.40 × 10^–8 at a pump intensity as low as 0.5 GW/cm². By tuning the pump polarization, we demonstrate efficient steering and polarization encoding into narrow SH diffraction orders, opening novel opportunities for polarization-encoded nonlinear meta-optics.

Two-photon photoluminescence (TPPL) from gold nanostructures is becoming one of the most relevant tools for plasmon-assisted biological imaging and photothermal therapy as well as for the investigation of plasmonic devices. Here we study the yield of TPPL as a function of the temporal width $\delta$ of the excitation laser pulses for a fixed average power. In the $\delta >1\text{ps}$ regime, the TPPL yield decreases as $\delta$ is increased, while for shorter pulse widths it becomes independent of $\delta$ and, consequently, of the laser-pulse peak power. This peculiar dynamics is understood and modeled by considering that two-photon absorption in Au is a two-step process governed by the lifetime of the metastable state populated by the first photon absorption.