High-Q slow light and its localization in a photonic crystal microring

We introduce a photonic crystal ring cavity that resembles an internal gear and unites photonic crystal (PhC) and whispering gallery mode (WGM) concepts. This ‘microgear’ photonic crystal ring (MPhCR) is created by applying a periodic modulation to the inside boundary of a microring resonator to open a large bandgap, as in a PhC cavity, while maintaining the ring’s circularly symmetric outside boundary and high optical quality factor (Q), as in a WGM cavity. The MPhCR targets a specific WGM to open a large PhC bandgap up to tens of free spectral ranges, compressing the mode spectrum while maintaining the high-Q, angular momenta and waveguide coupling properties of the WGM modes. In particular, near the dielectric band edge, we observe modes whose group velocity is slowed down by 10 times relative to conventional microring modes while supporting Q = (1.1 ± 0.1) × 106. This Q is around 50 times that of the previous record in slow-light devices. Using the slow-light design as a starting point, we further demonstrate the ability to localize WGMs into photonic crystal defect modes, enabling a more than 10 times reduction of mode volume compared with conventional WGMs while maintaining a high Q value of up to (5.6 ± 0.1) × 105. Importantly, this additional photonic crystal defect localization is achievable without requiring detailed electromagnetic design. Moreover, controlling their resonance frequencies and waveguide coupling is straightforward in the MPhCR, owing to its WGM heritage. In using a PhC to strongly modify the fundamental properties of WGMs, such as group velocity and localization, the MPhCR provides an exciting platform for a broad range of photonics applications, including sensing/metrology, nonlinear optics and cavity quantum electrodynamics. Researchers demonstrate a microring cavity with a photonic crystal on its inside edge, which enables a simultaneous high quality factor (1,000,000) and slow light (10 times slower than for conventional whispering gallery modes). Defect modes with a high quality factor (600,000) and high localizations (20 times smaller) are also enabled.