High-<i>Q</i> perfect light absorption enabled by degenerate merging BICs

Achieving high-<i>Q</i> light absorption within a thin dielectric film is highly desired for optoelectronic applications. However, controlling the absorption linewidth remains challenging, as it requires simultaneous control over both radiative and non-radiative loss to satisfy the critical coupling condition. In this paper, we report the design of a metasurface-based, polarization-independent perfect light absorber with a tunable absorption <i>Q</i> factor. The radiative decay rate can be tuned by adjusting the lattice constant of the metasurface, leveraging the mechanism of degenerate merging bound states in the continuum (BICs). Meanwhile, the non-radiative loss rate is controlled by introducing a waveguiding layer that modulates the overlap between the resonant field and the absorbing medium. This dual-control mechanism enables the metasurface absorber to achieve arbitrarily high absorption <i>Q</i> factors at a desired wavelength under critical coupling. Our approach offers a versatile platform for narrowband light absorption with tunable spectral linewidths, opening new opportunities for photodetection and spectral sensing applications.