Polarization-independent perfect absorber enabled by quasibound states in the continuum

Light absorption plays a vital role in governing the performance of optoelectronic devices, such as solar cells and photodetectors. However, it is quite challenging to realize perfect absorption for materials with low intrinsic dissipative losses and requires the careful design of nanoscale structures. This study theoretically proposes a polarization-independent germanium (Ge) metasurface absorber at the telecommunication C band by exploring the quasibound state in the continuum (QBIC). Such a QBIC is excited by breaking the in-plane symmetry of the Ge metasurface in which each unit cell is made of four subwavelength Ge nanodisks. The Q factor of QBIC is mainly governed by the asymmetry parameter, providing a straightforward way of optimizing the light absorption by the critical coupling. We demonstrate two designs that boost the absorption up to 50% in the transmission and up to 100% in the reflection modes.