Spin and Geometric Phase Control Four-Wave Mixing from Metasurfaces

The spin angular momentum of light plays an important role in nonlinear interactions in optical systems with rotational symmetries. Here, the existence of the nonlinear geometric Berry phase is demonstrated in the four-wave mixing process and applied to spin-controlled nonlinear light generation from plasmonic metasurfaces. The polarization state of four-wave mixing from the ultrathin metasurfaces, comprising gold meta-atoms with four-fold rotational symmetry, can be controlled by manipulating the spin of the excitation beams. The mutual orientation of the meta-atoms in the metasurface influences the intensity of four-wave mixing via the geometric phase effects. These findings provide novel solutions for designing metasurfaces for spin-controlled nonlinear optical processes with inbuilt all-optical switching.