Exceptional point-mediated unidirectional reflectionlessness in non-Hermitian plasmonic metasurfaces

Abstract This study delves into the intriguing phenomenon of unidirectional reflectionlessness (UR) in the context of exceptional points (EPs) in a non-Hermitian plasmonic system. Unlike traditional Hermitian systems, non-Hermitian systems provide a distinct domain for the manifestation of UR. Here, we investigate the role of EPs in facilitating UR in terahertz plasmonic metasurfaces. These metasurfaces mimic a non-Hermitian system featuring near-field coupled split ring resonators (SRRs). At EP, we identify backward reflectionlessness occurring at 0.43 THz for one polarization of the incident terahertz probe. Further, we elucidate the intricate interplay between near-field coupling dynamics and the emergence of UR through systematic simulations and experimental validations. Our findings reveal that optimizing inter-SRR near-field coupling can enable precise control over UR, revealing an optimal inter-SRR separation of 2.85 µ m ( λ /240), where UR is achieved with an asymmetry factor Λ = 1. Conversely, while perfect UR is not observed for the orthogonal probe polarization, the reflection minima are observed for the backward direction as well. These insights underscore the significance of EPs in tailoring the reflection characteristics of planar metasurfaces by embracing the inherent richness of non-Hermitian physics, which can pave the way toward the development of unidirectional devices with enhanced functionality and performance, unlocking unprecedented opportunities for tailoring light–matter interactions.