Using merging BICs in photonic crystal slabs to enhance incident angle robustness of Ultrahigh-Q terahertz resonance

Ultrahigh-quality (Q) terahertz (THz) resonances hold tremendous potential for applications in nonlinear optics, sensing, and other fields. However, their performance is often affected by the dispersion effects caused by wideangle incidence and the scattering losses caused by manufacturing defects. Here, we propose a novel THz resonator based on photonic crystal (PhC) slab with C2v symmetry and flat TE mode bands. Theoretical results indicate, symmetry-protected bound states in continuum (S-P BIC) can occur at the Gamma point and four accidental BICs in the Gamma X and Gamma Y directions. Leveraging the topological properties of BICs, by judiciously tuning the structural parameter of resonator, the S-P BIC and accidental BICs are merged at the Gamma point, forming flat bands with Q values as high as 10(11) in the Gamma X and Gamma Y directions. Consequently, based on the flat TE mode bands and high-Q flat bands, when X or Y-polarized THz waves are incident on this PhC, it exhibits a Q value as high as 10(9). Furthermore, as the incidence angle of the THz waves increases from 0 degrees to 20 degrees, the relative shift and bandwidth variation of resonant transmission peak are close to 0%, which demonstrates that the resonator has extremely strong robustness.