Resonant Chiral Effects in Nonlinear Dielectric Metasurfaces

We study the resonant enhancement of linear and nonlinear chiroptical effects in planar silicon metasurfaces with an in-plane asymmetry supporting multipolar Mie resonances and quasi-bound states in the continuum (quasi-BICs). We demonstrate theoretically and observe in experiment the pronounced linear circular dichroism at the quasi-BIC resonances originating from the interaction of modes with the substrate. We further find that both local field enhancement and third-harmonic signal are large for Mie resonances and some quasi-BIC modes due to the critical coupling. We demonstrate experimentally a strong nonlinear chiroptical response associated with high efficiency of the third-harmonic generation and large nonlinear circular dichroism varying from +0.918 ± 0.049 to −0.771 ± 0.004 for the samples with different asymmetries. We reveal the nonreciprocal nature of nonlinear chirality governed by the microscopic symmetry of nonlinearities and macroscopic symmetries of the meta-atom and metasurface lattice. We believe our results suggest a general strategy for engineering nonlinear chiroptical response in dielectric resonant metasurfaces

Beam Steering of Nonlinear Optical Vortices with Phase Gradient Plasmonic Metasurfaces

The generation of photons with spin and orbital angular momentum is of great importance in the fields of classical and quantum optical communications. Recent studies show that optical vortices with on-demand angular momentum can be realized with geometric phase-controlled metasurfaces. However, such optical vortices have two spin-locked orbital angular momentum states, which are difficult to distinguish in the same propagating direction. While the beam steering of the optical vortices can be easily realized in the linear optical regime, it remains elusive in the nonlinear optical counterpart. Here, we propose to generate and spatially separate the spin-locked second-harmonic vortex beams through phase gradient plasmonic metasurfaces. Based on the concept of the nonlinear geometric phase, the fork-type phase distributions are encoded onto the metasurfaces by using gold meta-atoms with a threefold rotational symmetry. Under the pumping of fundamental waves in the near-infrared regime, the spin-locked optical vortices at second-harmonic frequency are generated and then projected to different diffraction orders. The proposed strategy may have important applications in high-dimensional optical information processing