Exceptional points in chiral metasurface based on graphene strip arrays

We investigate the exceptional points (EPs) in chiral metasurfaces composed of periodic arrays of two orthogonally orientated graphene strips in the terahertz frequency. By tuning the geometric parameters and incident wavelengths, the EPs emerge as two eigenvalues of the transmission matrix coalesce. The EPs possess a certain chirality as light at the output is completely left- or right-handed circularly polarized (LCP or RCP). The chirality of the EPs is determined by the sign of two displacements of graphene strips along the horizontal and vertical directions, that is, RCP for the same sign and LCP for different signs. In addition, we show that the chirality of EPs is reversed when light is incident from the opposite direction of the metasurface. This study may have potential applications in polarization detection, manipulation, and ultrasensitive sensing by taking advantage of the non-Hermiticity present in the system.