Realization of a Chiral Topological Whispering-Gallery-Mode Cavity in Gyromagnetic Photonic Crystals

Abstract

Photonic whispering-gallery-mode (WGM) cavities formed by total internal reflection of light around closed paths have played a central role in modern optics. Recently, extensive efforts have been devoted to achieving topologically photonic WGM cavities that are robust against defects or obstacles. However, previous experimental studies on topological photonic WGM cavities are limited to systems with time-reversal symmetry (TRS) and weak topological protection due to the existence of backscattering channels. Here, the study reports the experimental realization of a chiral topological WGM cavity in a gyromagnetic photonic crystal with broken TRS. Using microwave near-field measurements, it is directly observed that the chiral topological WGM cavities are inherently nonreciprocal and exhibit superior robustness against various obstacles with strong topological protection owing to the absence of backscattering channels. More interestingly, it is demonstrated that the resonance frequencies and free spectral range (FSR) of the chiral topological WGM cavities can be tailored by adjusting the insertion length of a metallic scatterer. The work extends the research realm of topological photonic WGM cavities and may find important applications in designing arbitrary-shaped and nonreciprocal topological photonic cavities, filters, lasers, and frequency combs.