Dual Modulation Polarization-Independent Terahertz BIC Metasurface for Multi-Wavelength Sensing

The use of bound states in the continuum (BICs) has emerged as an effective tool to trap light at the nanoscale and has many potential applications in photonics. Breaking the structural symmetry is regarded as an effective way to excite quasi-BICs (QBICs) and generate high-Q resonances. However, this approach may impact the resonance polarization sensitivity, consequently limiting its practicality in multi-wavelength polarization-dependent applications. Furthermore, the introduction of different types of structural perturbations into the design to form BICs has yet to be explored in depth. In this study, we present an optical sensor consisting of an L-shaped metasurface that supports three quasi-BIC modes in the terahertz band, where specific displacements, collective perturbations, or both occur. Furthermore, we analyze the field distributions in detail and combine them with multipolar decomposition to reveal the underlying mechanisms of the different resonant modes. Multiple asymmetric perturbations are found to affect the sensitivity of the metasurface in refractive index sensing, thus allowing for a comparison of different resonant modes. The quasi-BIC mode can attain a Q-factor of 1067.6, a sensitivity (S) of 300 GHz/RIU, and a figure of merit (FOM) of 5367.8 RIU−1 for vertical light incidence. These three quasi-BIC modes are polarization-independent, and their properties are maintained even for circularly polarized light. The results reveal a novel design strategy for metasurface-based sensors with promising application potential in biosensing, filtering, and lasers.