Tunable continuously bound states for enhanced circular dichroism in graphene-enhanced metasurfaces

Abstract Metasurfaces are increasingly important for achieving optical chiral responses, especially those capable of spin-selective transmission and tunable circular dichroism (CD). In recent years, the introduction of asymmetry into structures, inspired by the concept of bound states in the continuum (BIC), has been found to enhance the chiral optical response. However, most of these chiral metasurfaces are either non-tunable or have low CD. In this paper, we present a planar metasurface that generates significant intrinsic chirality, while allowing for arbitrary control over the CD and transmittance of the two circular polarization states by adjusting the asymmetry parameters. By incorporating structured graphene, a successful transition between BIC and quasi-BIC states was achieved. The proposed metasurface exhibits dual quasi-BIC circular dichroism reaching −0.8 and 0.988, with the addition of graphene enabling a tunable CD range of 0.11 to 0.824. This range can be further improved by optimizing the geometric shape. The graphene all-dielectric metasurface opens up new possibilities for controlling polarization states in the terahertz frequency range. It demonstrates tremendous potential in practical applications such as dynamic displays, polarization-dependent optical isolators, modulators, and optical sensors.