Second Harmonic Generation in Geometric-Phase Resonant Dielectric Metasurfaces

We experimentally explore second harmonic generation (SHG), i.e. optical frequency doubling, in resonant dielectric metasurfaces.

Our metasurfaces are made of subwavelength arrays of amorphous silicon L-shaped nanoantennas which act as our meta-atom. These nanoantennas are designed to have an optical resonance at the principal wavelength of 800nm, and a good modal-fields overlap between the principal and second harmonic waves, which are the basic requirements for SHG.

When illuminated by circularly polarized light, the L-shaped nanoantenna may be rotated in space to apply a geometric phase to the incident light as well as to the SH, thus enable us to precisely control the optical phases at a length-scale smaller than the wavelength. The benefit of using an asymmetric nanoantenna for geometric phase is the ability to control the phase by rotating and reflecting the basic structure.

We use our basic nanoantenna and the concept of geometric phase to create nanoantenna-based geometric phase gratings, and measure their linear and nonlinear diffraction patterns, showing intriguing results such as SH diffraction from structures with no principal diffraction, and we analyze the nonlinear geometric phase created by our arrays.

Doing so, we verify the theory of nonlinear geometric phase for asymmetric structures, and show the correct polarization-dependency of SHG in geometric phase metasurfaces by measuring Stokes’-parameters of the second harmonic light.