Bound state in the continuum (BIC) has been widely employed in designing metamaterials with high quality factor (Q-factor) resonances. BIC is a state that can still maintain localization in the continuum and can be explained by phase-canceling interference. When the system parameters are continuously adjusted, the coupling of the BIC resonance mode to all radiated waves disappears, which leads to an infinitely long lifetime and an infinitely high Q-factor. If the vanishing of the coupling constants is due to symmetry, the BIC is also called symmetry-protected BIC. Ideal BICs exist only in lossless and infinite structures, exhibiting infinite Q-factors and vanishing resonance linewidths, or transmission spectra with zero linewidths. In practice, the BIC can be changed to quasi-BIC by breaking the symmetry and generating a leakage resonance. Although the Q-factor and resonance linewidth will be limited at this point, the metamaterial can still have an ultra-high Q-factor with promising applications in sensors.

By setting up two pairs of split ring resonators (SRRs) with high refractive index in a periodic cell, we design a terahertz all-dielectric metamaterial (Fig. 1). Based on the symmetry-preserving principle of superlattice modes, we obtain observable quasi-BIC (QBIC) modes by varying the distance between the two SRRs. Meanwhile, the variation rule of the Q-factor is obtained by calculating the energy distribution of the multipole to determine its resonance mode as shown in Fig. 4 and by changing the different structural parameters as shown in Fig. 6. Additionally, the transmission spectra with different background refractive indices are simulated to evaluate the sensing performance of the proposed metamaterial.

We simulate the transmission spectra of this all-dielectric SRR structure with different asymmetry parameters, and the Q-factor of QBIC decreases significantly under the increasing asymmetry parameter. The relationship between the Q-factor and the asymmetry parameter follows an inverse quadratic ratio. Fig. 4(a) shows the multipole scattering energy distribution at a=0 μm (BIC) and Fig. 4(b) shows the multipole scattering energy distribution at a=12 μm (QBIC). Near the resonant frequency of 0.6467 THz, the electric quadrupole increases significantly and dominates the far-field scattering energy distribution.

We design a terahertz sensor based on an all-dielectric metamaterial structure, with the Q-factor of the sensor as high as 2420. By simulating and analyzing the sensing performance of the designed metamaterial, the sensor achieves a sensitivity and an FOM of 254.8 GHz/RIU and 509.6 when the refractive index of the material to be measured varies from 1.00 to 1.04 respectively, and the sensor performance can be further improved by optimizing the structure. The sensor features a simple structure, low fabrication cost, and high sensitivity and FOM, and can be adopted as one with a high-sensitivity refractive index.