Magneto-optical metasurfaces for high-Q perfect absorption with quasi-bound states in the continuum.

We present a novel, to the best of our knowledge, magneto-optical (MO) metasurface composed of a bismuth iron garnet (BIG) nanocube array, designed to achieve near-perfect absorption through quasi-bound states in the continuum (QBICs). This metasurface supports a stable QBIC mode induced by MO-induced permittivity terms that break the symmetry of the permittivity tensors, corresponding to a longitudinal electric dipole (ED) mode. By integrating graphene to introduce material loss, the absorption reaches 99.6% at a wavelength of 1512.3 nm with a Q factor of 9440, despite monolayer graphene's inherent absorption being only 2.3%. The inherent transverse ED background mode, with high reflection and low Q, helps decrease the radiative loss of the QBIC mode, allowing the structure to surpass the 50% absorption limit. This approach offers a simplified pathway for designing high-Q metasurface perfect absorbers, with potential applications in optical switches and modulators.

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 (QBICs) 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-QBIC 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.

Achieving bi-anisotropic coupling through uniform temporal modulations without inversion symmetry disruption.

Temporal modulations provide a new approach for realizing metamaterials. In this study, through the imposition of uniform temporal modulations, we achieve two types of reciprocal bi-anisotropic metamaterials. Notably, these achievements do not rely on any spatial modulation, preserving inversion symmetry at any instantaneous time. This stands in sharp contrast to the scenario of traditional bi-anisotropic metamaterials, where the disruption of inversion symmetry by spatial arrangements is necessary. Conditions for realizing nonzero bi-anisotropic coupling are discussed and verified through full-wave simulations. Our work will stimulate research in the field of temporal bi-anisotropic metamaterials, as well as the application of temporal modulations in manipulating photonic spin angular momentum.