Polarization-controlled dual quasi-BICs in all-dielectric metasurfaces enabled by refractive-index asymmetry

Abstract

Bound states in the continuum (BICs) in all-dielectric metasurfaces have attracted considerable interest due to their ability to support ultra-high Q resonances. However, realizing multiple high-Q quasi-BICs (q-BICs) while preserving geometric symmetry remains a significant challenge. In this work, we demonstrate an all-dielectric metasurface supporting dual q-BICs with high-Q resonances in the near-infrared region, uniquely enabled by refractive-index asymmetry introduced into an otherwise symmetric nanocross architecture. Unlike conventional approaches relying on structural perturbations, our strategy leverages material-index contrast to activate two spatially distinct q-BICs localized predominantly along orthogonal arms of the nanocross and enables mode-selective spectral tuning. Multipole decomposition and electromagnetic-field analysis elucidate the physical mechanism, confirming that the dual q-BICs are primarily governed by toroidal dipole modes. Moreover, the metasurface exhibits efficient polarization-controlled switching with an amplitude modulation depth exceeding 90%. By exploiting the extreme near-field intensity contrast between BICs and q-BICs, we further design an intensity-encoded metasurface capable of subwavelength near-field imaging. This work provides a versatile strategy for engineering multi-band q-BICs and suggests promising applications in high-resolution imaging and optical information encoding.