Nonlinear chiroptical response in lithium niobate metasurface driven by quasi-bound states in the continuum

Quasi-bound states in the continuum (QBIC) can significantly localize the light field and enhance light–matter interactions at the nanoscale, providing a platform for high-Q chiral light response and promoting nonlinear effects of materials. In this work, we numerically study the chiral linear and nonlinear light responses of the QBIC lithium niobate (LN) metasurface and achieve chirality modulation. The designed metasurface consists of LN nanobar dimers, and the chiral QBIC mode is excited by breaking the in-plane and out-of-plane symmetries of the structure, with the circular dichroism (CD) value and Q-factor reaching 0.92 and 1.24×104, respectively. Then, we investigate the second harmonic generation (SHG) of this device. The conversion efficiency of SHG under right circularly polarized pumping reaches 7.3×10−3, which is more than three orders of magnitude higher than that under the left circularly polarized pumping. The corresponding CD value of SHG reaches 0.99. In addition, by introducing phase change materials, we study the active modulation of the chiroptical response. Our results provide a crucial route for high-quality chiral light sources.