Daniel C. Cole, Erin S. Lamb, Pascal P. Del'Haye, Scott A. Diddams, Scott B. Papp
Solitons are pulses which propagate without spreading due to a balance between nonlinearity and dispersion or diffraction, and are universal features of systems exhibiting these effects. They are important in plasma physics, fluid dynamics, atomic physics, and biology. In the context of photonics, bright dissipative cavity solitons in chip-integrable Kerr microresonators have been identified as candidates for inclusion in next-generation communication, computation, and measurement systems. Kerr microresonators convert a continuous-wave pump laser into a soliton waveform circulating inside the resonator. The soliton is periodically out-coupled to yield a repetitive pulse train, which may be stabilized to give a frequency comb. Here we report on the generation of soliton crystals in Kerr microresonators collectively-ordered ensembles of co-propagating, spatially separated solitons in which the separations between pulses take on discrete allowed values. Spectral interference between solitons in these crystals yields distinctive optical spectra. Soliton crystallization in Kerr microresonators is driven by a perturbation to the frequency distribution of the optical resonator modes, which leads to soliton interactions through the incorporation of an extended wave into the localized pulse. Simulated generation of soliton ensembles in the presence of this perturbation reveals the sharply peaked correlation function expected of a crystal. This contrasts with liquid-like ensembles which are generated without the perturbation. We observe a rich space of possible crystal configurations, including crystals exhibiting vacancies, Frenkel defects, and superstructure. To confirm our inference of crystal structure from spectral data, we measure the cross-correlation between a crystal and a reference pulse. Our work adds to the rich physics of microresonator frequency combs, and will be important for the preparation of soliton ensembles for chip-based photonic communication systems.
Frequency comb, microcomb, microresonator frequency comb, photonics, soliton