Nonlinear chiral light generation from resonant metasurfaces.

Chiral nonlinear response has been explored for decades due to its extreme sensitivity to molecular and structural dissymmetry. Conventional approaches often require bulky systems and produce only static nonlinear chirality. Here, we report on a generic mechanism for the generation and control of nonlinear chiral light in resonant optical systems. We reveal that nonlinear resonant generation of circularly polarized light from achiral dielectric metasurfaces is extremely sensitive to the polarization state of the fundamental wave, and a resonant metasurface can produce light with arbitrary degree of nonlinear chirality (DNC). Experimentally, we demonstrate that the chirality of nonlinear radiation from one metasurface can be continuously tuned from DNC = -0.86 to DNC = 0.94 by simply varying the polarization angle of the incident wave. By further exploiting the instantaneous polarization state, nonlinear chirality has been switched in a delay time step of 3.2 fs, which is orders of magnitude more sensitive than the current state-of-the-art polarization modulation. These results promise to enrich our understanding of nonlinear processes in chiral structures and their manipulation with resonant photonic structures.