Excitation and generation mechanism of high Q factors resonance in terahertz all-dielectric metasurfaces

In optical applications, optical resonators with high quality (Q) factors are crucial for enhancing light–matter interactions. However, achieving high Q factors in terahertz (THz) devices is often challenging. The unique properties of bound states in the continuum (BICs) enable the excitation of optical resonances with high Q factors in low loss dielectric structures. Here, we present a cross-shaped all-dielectric metasurface (MS) that can generate symmetry-protected BIC in the transverse electric mode. It is demonstrated that the BICs are protected by topological vortices of polarization vector. When the symmetry of the MS is disrupted, the BIC transforms into quasi-BIC (QBIC) with finite Q factors. Furthermore, as the degree of asymmetry increases, the Q factors obey inverse-square law with the asymmetric parameter. Finally, we experimentally demonstrated the existence of QBIC with ultra-high Q factor resonances in the THz range by fabricating a series of symmetry-broken MS samples and measuring their transmission spectra, with the maximum Q factors reaching 102. Our results not only provide valuable insights for achieving optical resonance modes with high Q factors but also promote the development of efficient THz optoelectronic devices and components across a wide range of applications.