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 wide-angle incidence and the scattering losses caused by manufacturing defects. Here, we propose a novel THz resonator based on photonic crystal (PhC) slab with C_2v symmetry and flat TE mode bands. Theoretical results indicate, symmetry-protected bound states in continuum (S–P BIC) can occur at the Γ point and four accidental BICs in the ΓX and ΓY 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 Γ point, forming flat bands with Q values as high as 10¹¹ in the ΓX and ΓY 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 10⁹. Furthermore, as the incidence angle of the THz waves increases from 0° to 20°, the relative shift and bandwidth variation of resonant transmission peak are close to 0%, which demonstrates that the resonator has extremely strong robustness.