Topological soliton frequency comb in nanophotonic lithium niobate.

Frequency combs have revolutionized metrology, ranging and optical clocks, motivating substantial efforts on the development of chip-scale comb sources. Some on-chip comb sources exist and have been implemented through electro-optic modulation, mode-locked lasers, quantum cascade lasers or soliton formation by Kerr nonlinearity. However, widespread deployment of on-chip comb sources has remained elusive, as they still require radiofrequency sources, high-Q (high-quality factor) resonators or complex stabilization schemes while facing efficiency challenges. Here, we demonstrate an on-chip frequency comb source based on the integration of a lithium niobate nanophotonic circuit with a semiconductor laser that can alleviate these challenges. We show the formation of temporal topological solitons in an on-chip nanophotonic parametric oscillator with quadratic nonlinearity and low finesse. These solitons, independent of the dispersion regime, consist of phase defects separating two π-out-of-phase continuous wave solutions at the signal frequency, which is half the input pump frequency. We use on-chip cross-correlation for temporal measurements and confirm formation of topological solitons as short as 60 fs around 2 μm, in agreement with a generalized parametrically forced Ginzburg-Landau theory. Moreover, we demonstrate a proof-of-concept turn-key operation of a hybrid-integrated source of topological frequency comb. Topological solitons are potential candidates for the development of integrated comb sources, which are dispersion-sign agnostic and do not require high-Q resonators or high-speed modulators, and can provide access to hard-to-reach spectral regions, including mid-infrared regions.