Dual-band high-Q near-perfect absorption by tilted Si-assisted metasurfaces

All-dielectric metasurface perfect absorbers (MPAs) based on quasibound states in the continuum ( Q BICs) play a crucial role in optical and photonic devices as they can excite high- Q resonances. These structures require adding back reflectors or placing at least two asymmetric elements in each unit to break the absorption limit of 50%, which will increase the design complexity. In this work, we propose a high- Q monolayer MPA (MMPA) composed of a tilted Si nanocube array. By tuning the tilted angle of the nanocube, dual- Q BIC modes at two different wavelengths are excited, which corresponds to magnetic quadrupole (MQ) and toroidal dipole (TD) modes, respectively. The high-reflection but low- Q magnetic dipole (MD) background mode excited by such a dual-band structure can decrease the radiative loss of transmission of MQ and TD modes, enabling the structure to break the absorption limit of 50%. The maximum absorption achieves 94% simultaneously at the wavelength of 933 and 961 nm, with the Q factors of 759 and 986, respectively. Our work provides a simple paradigm for designing dual-band high- Q MMPAs, which would greatly expand their range of applications, such as multiplexed optical nanodevices.