Polarization-independent high-Q monolayer Ge-assisted near-perfect absorber through quasibound states in the continuum

Narrow-band metasurface perfect absorbers with a high-quality $(Q)$ factor play a vital role in governing the performance of optoelectronic devices. However, it is quite challenging to realize polarization-independent high-$Q$ perfect absorption at a near-infrared wavelength by only a single-layered structure without using the back reflectors. Here, we theoretically propose a monolayer high-$Q$ near-perfect metasurface absorber with unit cells comprised of four germanium (Ge) split rings. Based on quasibound states in the continuum, the Mie modes with low radiative decay rates are induced by the orientation of each split ring to match the low nonradiative decay rates and achieve high-$Q$ absorption performance. The high reflection background mode excited by four split rings can transform the symmetrically coupled Mie modes into asymmetrically coupled ones and decrease the radiative decay rate of transmission, enabling the monolayer structure to break through the absorption limit of 50% to near 100%. We have theoretically demonstrated that the maximum absorption of the presented structure is as high as 93% at 1530 nm of the $Q$ factor up to 470. The structure is polarization independent due to its ${C}_{4}$ symmetry. Our proposed monolayer all-dielectric metasurface without any back reflectors achieves near-perfect absorption at a very narrow bandwidth. This work may provide a paradigm for designing polarization-independent high-$Q$ monolayer perfect absorbers, which is potentially applicable in ultrafast optoelectronic devices and integrated photon systems.