Experimental demonstration of second-harmonic generation in high χ² metasurfaces

Metasurfaces able to concentrate light at various wavelengths are promising for enhancing nonlinear interactions. In this Letter, we experimentally demonstrate infrared second-harmonic generation (SHG) by a multi-resonant nanostructure. A 100 GaAs layer embedded in a metal-insulator-metal waveguide is shown to support various localized resonances. One resonance enhances the nonlinear polarization due to the transverse magnetic (TM)-polarized pump wavelength near 3.2 µm, while another is set near the TE-polarized generated wavelength (1.6 µm). The measured SHG efficiency is higher than 10⁻⁹ W⁻¹ for pump wavelengths ranging from 2.9 to 3.3 µm, which agrees with theoretical computations. This is typically 4 orders of magnitude higher than the equivalent GaAs membrane.