Fig. 1: Topological edge state cavity (TESC) and its comparison with conventional whispering-gallery mode (WGM) and photonic crystal (PC) cavities.
Schematics of a WGM cavity with a microring resonator coupled with a silicon (Si) strip waveguide, b PC defect cavity coupled with a line-defect waveguide, c TESC coupled with a topological waveguide. Inset of c shows the unit cell of valley photonic crystal (VPC) consisting of two equilateral triangular air holes with side lengths L1 = (a + ΔL)/2 and L2 = (a ‒ ΔL)/2. a is the lattice constant. Band diagrams of d Si strip, e PC, and f edge states in VPCs. Waves are confined and guided through total internal reflection (TIR) in WGM cavity, photonic bandgap in PC, and topological edge states in TESC, respectively. When the waves interfere constructively within the cavity, resonant modes (red dots) are formed with discrete momentum (k, wavevector) and energy (ω, frequency). g Evolution of free spectral range (FSR) and quality factor (Q) with the round-trip length R of WGM cavities. h Evolution of FSR and Q with R and ΔL of TESCs. By shifting the edge state from above to below the light line through reducing ΔL, as shown in (f), the Q of TESCs can be enhanced by more than three orders of magnitude, along with a simultaneous improvement in FSR
