Parametrically driven pure-Kerr temporal solitons in a chip-integrated microcavity

The discovery that externally driven nonlinear optical resonators can sustain ultrashort pulses (solitons) corresponding to coherent optical frequency combs has enabled landmark advances in applications from telecommunications to sensing. Most previous research has focused on resonators with purely cubic (Kerr-type) nonlinearity that are externally driven with a monochromatic continuous-wave laser—in such systems, the solitons manifest themselves as unique attractors whose carrier frequency coincides with that of the external driving field. Recent experiments have, however, shown that a qualitatively different type of temporal soliton can arise via parametric downconversion in resonators with simultaneous quadratic and cubic nonlinearity. In contrast to conventional solitons in pure-Kerr resonators, these parametrically driven solitons come in two different flavours with opposite phases, and they are spectrally centred at half of the frequency of the driving field. Here we theoretically predict and experimentally demonstrate that parametrically driven solitons can also arise in resonators with pure-Kerr nonlinearity under conditions of bichromatic driving. In this case, the solitons arise through four-wave-mixing-mediated phase-sensitive amplification, come with two distinct phases and have a carrier frequency in between the two external driving fields. Our experiments are performed in an integrated silicon nitride microcavity, and we observe frequency comb spectra in remarkable agreement with theoretical predictions. In addition to representing the discovery of a new type of temporal dissipative soliton, our results constitute an unequivocal realization of parametrically driven soliton frequency combs in a microcavity platform that is compatible with foundry-ready mass fabrication.