Twins of Exceptional Points with Opposite Chirality for Non-Hermitian Metasurfaces

Exceptional points (EPs) in non-Hermitian systems have recently emerged as an intriguing way to manipulate certain physical processes interacting with the external environment, giving rise to numerous fascinating phenomena. For the non-Hermitian metasurface, most works thus far have concentrated on a single isolated EP due to only one EP being easily accessible in such resonance-induced loss systems (scattering and absorption). In this work, we explore how the twins of chiral EPs can be realized in a non-Hermitian metasurface consisting of three submicrometer-sized silver (Ag) rectangular bars arranged in a chiral "tau" shape in each unit cell. As anticipated, the topologically guaranteed complete 2 pi phase accumulation can be achieved by encircling a path around each EP in the twins. Thus, we can introduce two sets of optical phases with different chiral engineering by exploiting topological features of the non-Hermitian metasurface operating near the twins of EPs. As an example in applications, the phase-only and polarization multiplexing hologram of left circularly polarized (LCP) and right circularly polarized (RCP) light can be easily achieved by utilizing the separate phase engineering on each EP.