Near-unity light absorption at normal incidence via a single critically coupled unidirectional guided resonance in all-dielectric zero-contrast gratings.

Perfect light absorption under one-sided illumination in a conventional two-port planar photonic system typically requires either a reflecting element or a pair of degenerate resonances. In this study, we show that near-perfect light absorption can be achieved by exploiting a single critically coupled Γ-point unidirectional guided resonance (UGR) in an all-dielectric zero-contrast grating (ZCG), without requiring either a reflecting element or a pair of degenerate resonances. Because downward radiation is suppressed, the ZCG effectively behaves as a one-port system, enabling near-perfect absorption (≥ 99%) of normally incident light from the top, whereas light incident from the bottom is predominantly reflected. We present an analytical expression, Amax = FηFr, which separately quantifies the effects of radiation asymmetry Fη and critical coupling Fr on the maximum absorptance at resonance. This relation shows that near-perfect absorption can be maintained despite deviations from the ideal UGR condition, the exact critical-coupling condition, or both, provided that FηFr ≥ 0.99. This analytical prediction is confirmed by finite-element simulations. Our findings provide valuable insight for developing high-performance all-dielectric light absorbers based on UGRs for diverse optoelectronic applications.