Role of unit-cell defects in terahertz topological ring resonators

We demonstrate a terahertz topological ring resonator based on all-dielectric valley photonic crystals. A triangular ring-shaped topological edge is coupled to a straight topological waveguide by suitably merging two valley photonic crystals. The resonant characteristics are investigated for different asymmetry parameters as well as for different domain interfaces (zigzag and bridge type) constituting the ring resonator. In the topological ring resonator, ultra-narrowband high quality (Q)-factor resonances are excited. Then, multiple defect states are intentionally introduced into the domain interfaces, and the influences of defects on the terahertz propagation and the Q-factors are examined for both types of domain interfaces. Numerical simulation studies on terahertz transmission reveal a resonance redshift of ∼ 10 G H z due to the introduction of multiple defect states. Such resonance redshift is attributed to the alteration in the effective refractive indices of the structure. Very high Q-factors within the range of 1240–840 are achieved even in the presence of several unit-cell defects owing to strong field confinements in the domain interface. This research can open up new routes for the development of nonlinear technologies and other advanced on-chip topological devices at THz frequencies.