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Abstract
Different from conventional two-dimensional photonic metasurfaces, chiral photonic metasurfaces, which are composed of a chiral pattern with subwavelength dimensions, exhibit different chiroptical physical phenomena, such as spin-preserving, nonlinear chiroptical effects, and optical vortex generation. However, the effect of symmetry in the vertical direction on the linear and nonlinear response of the chiral quasi-two-dimensional metasurface has been seldom reported. In this study, we design and simulate a chiral metasurface using topological patterns in lithium niobate (LN) thin film to obtain the optimal circular dichroism and its corresponding nonlinear effect. Triangular holes with a certain rotation constitute a quasi-two-dimensional metasurface, which possesses outstanding chirality for the spin state of light, thus allowing it to control linear and nonlinear chiralities. The nonlinear conversion efficiency ${\eta _{\rm{SHG}}}$ reaches $2 \times {10^{- 4}}$ and $4 \times {10^{- 3}}$ on the LN structure with substrate and without substrate, respectively. Nonlinear circular dichroism ${\rm CD_{\rm{SHG}}}$ is 5.8 dB at 783 nm on a ${{\rm SiO}_2}$ substrate. This tunable chiral metasurface is expected to be applicable to nonlinear chiroptical responses and chiral light modulation.
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