Highly sensitive and actively tunable chiral metasurface enabled by bound states in the continuum

Chiroptical metasurfaces are novel materials capable of manipulating the polarization state of light. The active tuning of chiroptical metasurfaces further empowers the capability of real-time and wide-range manipulation of light. In this study, we propose and investigate a dielectric metasurface based on LiNbO₃ that enables active and efficient electro-optic tuning of chiroptical responses. We have observed a quasi-bound state in the continuum (q-BIC) in this metasurface, which exhibits a large circular dichroism (CD) of 0.89 and a high Q-factor of 9.3 × 10⁴ at 976.26 nm. Owing to the high Q-factor, the CD can be electro-optically modulated by up to 0.83 with a high sensitivity of 8 × 10^-5 nm/(V/m), which is orders of magnitude higher than that of bulk LiNbO₃ modulators. In addition, the polarization states of both linearly and circularly polarized light can be efficiently tuned through this metasurface, with tunable ranges of rotation angle and ellipticity up to 78° and 62°, respectively. We believe that the proposed chiroptical metasurface is an excellent candidate as a compact, versatile, and highly sensitive electro-optic modulator.