  Development of ultrashort pulse sources has had an immense impact on
condensed-matter physics, biomedical imaging, high-field physics, frequency
metrology, telecommunications, nonlinear optics, and molecular spectroscopy.
Although numerous advancements of such sources have been made, it remains a
challenge to create a highly compact, robust platform capable of producing
femtosecond pulses over a wide range of wavelengths, durations, and repetition
rates. Recent observations of frequency comb generation via cascaded parametric
oscillation in microresonators11 suggest a path for achieving this goal. Here
we investigate the temporal and spectral properties of parametric combs
generated in silicon-nitride microresonators and observe a transition to
passive modelocking of the comb consistent with soliton-pulse formation,
resulting in the generation of 160-fs pulses at a 99-GHz repetition rate. This
platform offers the prospect of producing pulses from 10 fs to a few ps at
repetition rates from 10 GHz to > 1 THz and over a wavelength range of 0.8 - 6
\mu m.
