Pushing Q-factor limit of guided resonances by harnessing topologically protected terahertz bound states in the continuum

Controlling and enhancing light–matter coupling at subwavelength scales is an essential requirement in the realm of meta-photonics. Recently, all-dielectric metasurfaces (MSs) governed by the physics of bound states in the continuum (BICs) have emerged as a standout platform for delivering high-quality (Q) factor resonances and near-field electromagnetic hotspots. However, in the terahertz (THz) domain, experimental validation of high-Q BICs resonances with strong robustness and advanced maneuverability in such all-dielectric photonic systems remains a long-standing challenge. Here, we demonstrate a simple and feasible fabrication approach to unlock the full potential of BICs-inspired resonances within the array of silicon cross elliptical resonators. Our results suggest that the designed THz-MS can support two symmetry-protected BICs with a topological charge of ±1 and several accidental BICs with a topological charge of +1 simultaneously. By introducing small perturbations to the individual resonator, the original two symmetry-protected BICs transform into quasi-BICs that bow to the inverse-square law. Astoundingly, for larger symmetry breaking, two additional BICs can be observed in the asymmetric THz-MSs surpass typical inverse-square rule, hence presenting a supererogatory degree of freedom for tailoring BICs resonances on demand. We bear out theoretical findings by transmission experiments implemented on the fabricated samples. We observe experimentally ultrasharp dual quasi-BICs resonances with a highest measured Q factor of up to 371, a level of performance that was previously unattainable with all-dielectric THz-MS on a substrate. The results mark an important step toward enriching the family of BICs and promise exciting opportunities in the field of THz optoelectronic devices and metadevices.