Giant Nonlinear Optical Activity from Planar Metasurfaces

Second harmonic generation circular dichroism (CD) is more sensitive to the handedness of chiral materials than its linear optical counterpart. In this work, we show that 3D chiral structures are not necessary for introducing strong CD for harmonic generations. Specifically, we demonstrate giant CD for both second harmonic generation and third harmonic generation on suitably designed ultrathin plasmonic metasurfaces. It is experimentally and theoretically verified that the overwhelming contribution to this nonlinear CD is of achiral origin. The results shed new light on the origin of the nonlinear CD effect in achiral planar surfaces