Near-perfect light absorption via degenerate quasi-bound states in the continuum in double-sided gratings with broken in-plane <i>C</i> ₂ symmetry

We demonstrate that near-perfect light absorption can be achieved by exploiting degenerate quasi-bound states in the continuum (quasi-BICs) supported by one-dimensional double-sided gratings (DSGs) with broken in-plane C 2 symmetry. Using a two-step iterative optimization procedure, the grating parameters are identified such that two quasi-BICs become spectrally degenerate while their radiative and nonradiative quality ( Q ) factors are nearly balanced. Our results show that once exact frequency degeneracy is achieved, approximate satisfaction of the critical coupling condition is sufficient to yield near-unity absorption. We further demonstrate the universality of the proposed DSG-based absorber by designing structures operating in the telecommunication, terahertz, and microwave regimes using different materials, including germanium, doped silicon, and alumina. In particular, low-loss materials are shown to significantly enhance spectral selectivity through higher absorbance Q factors. These findings establish asymmetric DSGs as a flexible and effective platform for designing high-performance all-dielectric light absorbers for diverse optoelectronic applications.