Photonic-spin-controlled second-order nonlinear frequency conversion on permittivity-asymmetry lithium niobite dimer metasurfaces governed by chiral quasibound states in the continuum

The high-efficiency generation and dynamic control of nonlinear harmonics at the nanoscale are crucial for integrated nanophotonics. Herein, we theoretically propose a permittivity-asymmetry lithium niobite dimer metasurface and numerically investigate the photonic spin as a degree of freedom for tailoring high-efficiency second-order nonlinear frequency conversion, based on a chiral quasibound state in the continuum (quasi-BIC) with a quality factor approaching 7.5 \ifmmode\times\else\texttimes\fi{} ${10}^{3}$ and circular dichroism (CD) of 0.91. Finite-element method simulations exhibit that this chiral quasi-BIC enables the difference in second-harmonic generation efficiency exceeding three orders of magnitude under the illumination of left- and right-circularly polarized light, leading to a nonlinear CD approaching 1. Importantly, in combination with an achiral magnetic dipole resonance, the photonic-spin-controlled sum-frequency generation and photon-pair generation from spontaneous parametric downconversion are also observed. In this paper, we suggest that the chiral quasi-BICs open an avenue for controlling nonlinear frequency conversion by using the photonic spin, without reconfiguring structure using external stimuli. Our results provide insights for both linear and nonlinear photonic-spin-dependent applications such as chiral sensing, chiral light sources, and chiral filters.