Exceptional point-mediated UnidirectionalReflectionlessness in non-Hermitian PlasmonicMetasurfaces

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 μm (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.