Observation of an exceptional point in a non-Hermitian metasurface

Abstract: Exceptional points, also known as non-Hermitian degeneracies, have been observed in parity-time symmetric metasurfaces as the parity-time symmetry breaking point. However, the parity-time symmetry condition puts constraints on the metasurface parameter space, excluding full examination of the unique properties that stem from an exceptional point. Here, we thus design a general non-Hermitian metasurface with a unit cell containing two orthogonally oriented split-ring-resonators with overlapping resonance but different scattering rates and radiation efficiencies. Such a design grants us full access to the parameter space around the exceptional point. The parameter space around the exceptional point is first examined by varying the incident radiation frequency and coupling between split-ring resonators. We further demonstrate that the exceptional point is also observable by varying the incident radiation frequency along with the incident angle. Through both methods we validate the existence of an exceptional point by observing unique level crossing behavior, eigenstate swapping under encirclement and asymmetric transmission of circularly polarized light.