Double chiral resonances in a quasi-BIC metasurface with both strong circular dichroism and high quality factor.

Chiral metamaterials with high quality factor resonance present an attractive platform for manipulating light-matter interactions for both fundamental and applied explorations. As limited by the nonintuitive design paradigm, it is challenging to obtain multiple chiral resonances with strong circular dichroism in one metamaterial structure, which could be useful for sensing applications with unprecedented performance. In this work, we propose a dielectric metasurface, which exhibits two quasi-bound states in the continuum (quasi-BICs) with both high quality (Q) factor and strong circular dichroism. In our designed metasurface made of asymmetric silicon ellipse pairs, two quasi-BICs are excited at 50.48 and 51.82 THz. Their chiral resonant responses lead to circular dichroism values of -0.87 and 0.91 and Q factors of 3873 and 7196, respectively. The two chiral resonances exhibit high sensitivities of 17.87 and 9.14 THz/RIU and figures of merit (FOMs) of 1371 and 1269 when they are used separately in refractive index sensing. For the case of using them jointly by tracking their frequency interval, a slightly smaller sensitivity of 8.73 THz/RIU is revealed in simulation. In addition, frequency shifts of the two chiral resonances are also suitable for distinguishing enantiomers in chiral sensing. These findings suggest that quasi-BIC-based metasurfaces offer a route to obtain multiple high-Q chiral resonant modes, which could boost the performance of metamaterial applications in sensing, light polarization control, spectroscopy, etc.