Controllable second harmonic generation based on topological spin-dependent edge states.

Spin-dependent nonlinear harmonic generations from nanostructures may provide a new class of design for future applications such as nonlinear optical switching and modulation and nonlinear information processing. Most reported works are based on the utilization of metasurfaces with specific symmetry (such as C₃ and C₄) and the application of the Pancharatnam–Berry phase approach, making them sensitive to local defects. In this work, we investigated a controllable second harmonic generation (SHG) based on the topological spin-dependent edge state, which is robust to a broad class of perturbations. Simulation results demonstrated that spin-dependent edge states with strong field localization can be obtained through spin–orbit coupling, resulting in SHG with increased efficiency. In addition, the propagation direction of the enhanced SHG could be dynamically controlled using an excitation source with left/right circular polarizations, exhibiting negligible backscattering and topological protection against local defects. The proposed approach may open a new avenue for manipulating light–light interactions. [ABSTRACT FROM AUTHOR]