On-chip topological edge state cavities.

Confining light in an on-chip photonic cavity with strong light-matter interactions is pivotal for numerous applications in optical and quantum sciences. Recently, topological valley photonics has introduced new schemes for light confinement with topological protection, enabling robust on-chip light manipulation. Here, we present a topological edge state cavity that confines light within a topological bandgap while robustly guiding it to circulate around the cavity via topological edge states. We demonstrate a giant enhancement in the intrinsic quality factor by three orders of magnitude, while simultaneously increasing the free spectral range from 5.1 to 7.1 GHz through tailoring the radiation leakage and group index of topological valley edge state. Our work provides a novel and robust on-chip cavity platform for a wide range of applications, including high-capacity communications, nonlinear optics, atomic clocks, and quantum photonics.