Bound States in the Continuum in Metasurface Absorbers: A Comparison with Metasurfaces

Bound states in the continuum (BICs) in metasurfaces enable excellent local field enhancement and ultrahigh quality ( Q ) factors, significantly boosting light–matter interactions. While BICs have been extensively explored in transmission‐mode configurations, their radiation characteristics in reflection‐mode configurations are still vague. In this study, the radiation properties of symmetry‐protected BICs in metasurfaces and their reflective absorber counterparts are comparatively analyzed through multipolar decomposition and experimental validation. It is demonstrated that while the mode properties of symmetry‐protected BICs are preserved in absorber configurations, their Q factors are reduced due to the emergence of an additional radiative magnetic dipole component. This behavior is further validated by analyzing two additional band‐folded BICs, indicating the generality of the observation across different types of BICs. A comprehensive understanding of the radiation properties of BICs in absorber configurations is crucial for designing high‐performance sensors, detectors, and modulators.