A Terahertz Metamaterial Refractive Index Sensor Based on Symmetry-Protected BIC

Abstract Bound states in the continuum (BICs) feature an infinite quality factor and zero bandwidth. When incorporated into metamaterials, quasi-BIC excited via BIC symmetry breaking effect gives rise to resonance with an ultrahigh quality factor (Q-factor). This study designs a novel terahertz metamaterial sensor rooted in symmetry-protected BIC. By inducing structural symmetry breaking of the sensor’s unit cell, the intrinsically ideal BIC state will be converted into a quasi-BIC state. Through analyzing the spatial distribution of the resonant electric field intensity of BIC, a relationship is derived in which the Q-factor exhibits an inverse proportionality to the square of the asymmetry parameter, thereby verifying that the two newly generated resonant dips are both originated from quasi-BIC. Meanwhile, the contribution rates of different multipole components of the induced current density on the device surface to the total field are calculated. In addition, the influence of the target analyte’s thickness on the sensor’s performance is investigated. Simulation results demonstrate that this designed sensor achieves excellent sensing performance, with a Q-factor of 70 and a sensitivity (S) of 191GHz/RIU. The refractive index sensor proposed here holds broad application potential in high-sensitivity sensing, chirality enhancement, fingerprint spectrum detection and other fields.