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.

Broadband frequency translation by space-time interface with weak permittivity temporal change.

Breaking spatial and temporal homogeneities simultaneously incurs the combination of wavenumber and frequency translations. In this work, broadband frequency translations with both redshifts and blueshifts triggered by a single photonic space-time interface (PSTI) with weak temporal change of permittivity across which a homogeneous medium suddenly becomes a one-dimensional photonic crystal is proposed. Mode conversions induced by the PSTI are analyzed, according to which the frequency translation amplitudes are independent of the change of permittivity and the initial frequency but are given by the product of the phase speed of the homogeneous medium and the spatial modulation frequency of the photonic crystal. Hence, a static field can be partially converted into dynamic fields by imposing the PSTI. Our findings pave the way for the study of PSTIs and provide a new scheme to realize broadband frequency translations.

Kerker-type positional disorder immune metasurfaces.

Metasurfaces that can operate without a strictly periodic arrangement of meta-atoms are highly desirable for practical optical micro-nano devices. In this paper, we propose two kinds of Kerker-type metasurfaces that exhibit immunity to positional disorder. These metasurfaces consist of two distinct core-shell cylinders that satisfy the first and second Kerker conditions, respectively. Despite significant positional disorder perturbations of the meta-atoms, the metasurfaces can maintain excellent performance comparable to periodic ones, including total transmission and magnetic mirror responses. This positional disorder immunity arises from the unidirectional forward or backward scattering of a single core-shell cylinder, which results in minimal lateral scattering coupling between neighboring cylinders, thereby having little impact on multiple scattering in either the forward or backward direction. In contrast, the response of positional disorder non-Kerker-type metasurfaces decreases significantly. Our findings present a new approach for designing robust metasurfaces and expanding the applications of metasurfaces in sensing and communications within complex practical scenarios.