Tunable multiband circular dichroism and asymmetric transmission enabled by chiral quasi-BICs in dielectric metasurfaces covered with graphene

High-Q chiral resonances have attracted great interest recently due to their practical photonic applications. For generating such resonances, chiral quasi-bound states in the continuum (quasi-BICs) have recently been proposed and experimentally demonstrated by many researchers, while their dynamic tunability has still not been discussed. In this work, we show multiband circular dichroism and asymmetric transmission based on high-Q chiral quasi-BICs in dielectric metasurfaces, which are dynamically tunable in the mid-infrared range by using graphene monolayers covering the metasurfaces. By changing the Fermi energy, the surface conductivity of graphene is tunable in a wide range, resulting in tunable chiroptical responses of the metasurfaces. Since the metasurfaces exhibit high-Q resonances with narrow linewidths, tuning the metasurfaces results in large modulation depths of circular dichroism and asymmetric transmission, which is of great importance for photonic applications, including tunable polarization manipulation and polarization-dependent detection.