Customizable dual-resonance sensing empowered by coupled quasi-bound states in the continuum

The quasi-bound states in the continuum (QBIC) have drawn increasing attention in optical metasurfaces derived from their ultrahigh quality factors, and show the utility to enhance the sensitivity of optical sensing. However, conventional single-resonance sensing may be inaccurate and unreliable, and then the dual-resonance sensing governed by the coupled QBIC is desired but remains elusive. Here, we show that the coupled QBIC modes can be leveraged to unfold dual-resonance refractive index sensing in the hybrid all-dielectric metasurface. Specifically, it is revealed that the toroidal dipole mode can be realized with strong electric field enhancement, enabling the implementation of anapole mode in the telecom short-wavelength band (1460-1530 nm). Under different linearly polarized illuminations, the dual symmetry-protected QBIC modes dominated by the electric quadrupole resonance can be fulfilled in the telecom extended-wavelength band (1360-1460 nm). Within this framework, the polarization-dependent dual symmetry-protected QBIC modes selectively coupled with the toroidal dipole mode or the anapole mode can not only uncover the transformation from Fano resonance to analog of electromagnetically induced transparency, but also manifest two types of high-sensitivity dual-resonance refractive index sensing in the telecom extended-wavelength and short-wavelength bands. The dual-resonance refractive index sensing can also be extended to telecom long- wavelength band (1565-1625 nm) and ultra-long-wavelength band (1625-1675 nm) with enhanced sensitivity. These results offer exploration potential for multi-channel sensing, optical modulators, and slow-light devices.