Asymmetric Absorption of Non-Hermitian Metasurfaces Enhanced by Quasi-Bound States in the Continuum

Metasurfaces have emerged as a powerful platform for controlling light at subwavelength scales, enabling tailored reflection, transmission, and absorption properties. Among these, quasi-bound states in the continuum (qBIC) provide a mechanism for enhancing light-matter interactions by supporting high-Q resonances that can be selectively coupled to free-space radiation. Additionally, introducing loss into metasurfaces facilitates non-Hermitian physics, where the scattering matrix of the system can exhibit exceptional points, leading to unique optical phenomena such as reflection suppression and asymmetric absorption. In this work, we investigate an asymmetrically absorbing semitransparent metasurface composed of an inherently lossy layer coupled with a dielectric structure supporting qBIC modes. Due to the presence of inherent losses, the system can enter a non-Hermitian regime, giving rise to an exceptional point where reflectance is suppressed for the backward incidence direction while remaining significant for the forward incidence direction. This produces a pronounced forward-backward asymmetry in absorption, which is further enhanced by the presence of qBIC modes. Furthermore, we find that the system displays a phase singularity at the exceptional point. Our numerical simulations confirm the existence of this effect and its dependence on loss and symmetry-breaking parameters. These findings provide new insights into metasurface-based optical diodes and nonreciprocal light control applications, offering a passive and scalable approach to achieving asymmetric light absorption without requiring external biasing.