Hyperbolic Topological Frequency Combs

On-chip generation of optical frequency combs (OFCs) holds fundamental significance for a wide range of applications, such as precision metrology, optical communications, and so on. Recent investigations have shown that OFCs can be generated in a topologically coupled array of ring resonators, providing a novel avenue to engineer nested temporal solitons and robust comb spectra. However, limited by the bulk-edge correspondence, the implementation of topological frequency combs always requires a substantial number of auxiliary bulk ring resonators, diminishing the ratio of edge ring resonators for OFC generation. Here, we propose a new approach to generate OFCs and nested temporal solitons using hyperbolic photonic topological insulators. Our designed hyperbolic topological OFCs exhibit a significantly reduced frequency spacing compared to that of conventional topological OFCs with the same number of optical rings. Thus, for achieving the same frequency spacing, hyperbolic topological OFCs require much fewer ring resonators than conventional topological OFCs in Euclidean space, giving an extremely improved utilization efficiency of ring resonators. Furthermore, the robustness of hyperbolic topological OFCs is also demonstrated. Our results open up a promising avenue for establishing highly efficient nonlinear topological devices.