Non-Hermitian interband coupling manipulation: from unidirectional reflectionless to unilateral perfect absorption.

Unilateral perfect absorption (UPA)-achievable through tailored system loss-holds broad application potential. It can be viewed as an extension of unidirectional reflectionlessness (UR), which is linked to non-Hermitian scattering exceptional points (EPs). However, the lack of a systematic design framework for UR-based UPA often results in confusion during the adjustment of coupling component parameters. Here, from a quasi-normal-mode perspective, we introduce a topology-supported design scheme that predicts and realizes UPA at an EP in continuum optical systems. By converting inter-resonator or inter-array coupling into interband coupling within energy bands, the asymmetry in mode radiation direction enables controlled asymmetric absorption at the EP. Maximum absorption asymmetry occurs under extreme radiation asymmetry, i.e., when unidirectional guided resonance (UGR) is supported. This framework provides what we believe to be new insight for asymmetric optical field manipulation and advanced photonic device design.