Observation of topological frequency combs

On-chip generation of optical frequency combs using nonlinear ring
resonators has enabled numerous applications of combs that were otherwise
limited to mode-locked lasers. Nevertheless, on-chip frequency combs have
relied predominantly on single-ring resonators. Here, we experimentally
demonstrate the generation of a novel class of frequency combs, the
topological frequency combs, in a two-dimensional lattice of hundreds of
ring resonators that hosts fabrication-robust topological edge states with
linear dispersion. By pumping these edge states, we demonstrate the
generation of a nested frequency comb that shows oscillation of multiple
edge state resonances across $\approx$40 longitudinal modes and is
spatially confined at the lattice edge. Our results provide an opportunity
to explore the interplay between topological physics and nonlinear
frequency comb generation in a commercially available nanophotonic
platform.