Time-reversal-symmetry-protected topological photonic cavity via adiabatic interface engineering.

Topological optical microcavities utilize protected boundary states to achieve robust optical field localization, which makes them ideal candidates for high-stability on-chip light sources and quantum interfaces. However, existing schemes that rely on gyromagnetic effects require external magnetic fields, a requirement that compromises their stability and integrability. In this work, we present a ring resonator constructed at the adiabatic interface between a lattice in a higher-order topological insulator (HOTI) phase and a trivial insulator. Our design operates without breaking time-reversal symmetry or requiring external fields, yet it achieves a level of topological protection comparable to that of gyromagnetic systems. It demonstrates robustness against defects and effectively suppresses whispering-gallery mode (WGM) splitting. Furthermore, we demonstrate directional "waveguide-microcavity-waveguide" transmission in a coupled system, which is enabled by the chiral excitation of topological interface states. This work provides a new strategy, to the best of our knowledge, for developing magnetic-field-free, robust topological microcavities, highlighting their significant potential for applications in quantum information processing and integrated photonic chips.