  Dissipative Kerr soliton generation in chip-scale nonlinear resonators has
recently observed remarkable advances, spanning from massively-parallel
communications, self-referenced oscillators, to dual-comb spectroscopy. Often
working in the anomalous dispersion regime, unique driving protocols and
dispersion in these nonlinear resonators have been examined to achieve the
soliton and soliton-like temporal pulse shapes and coherent frequency comb
generation. The normal dispersion regime provides a complementary approach to
bridge the nonlinear dynamical studies, including the possibility of square
pulse formation with flat-top plateaus, or platicons. Here we report
observations of square pulse formation in chip-scale frequency combs, through
stimulated pumping at one free-spectral-range and in silicon nitride rings with
+55 fs2/mm normal group velocity dispersion. Tuning of the platicon frequency
comb via a varied sideband modulation frequency is examined in both spectral
and temporal measurements. Determined by second-harmonic auto-correlation and
cross-correlation, we observe bright square platicon pulse of 17 ps pulsewidth
on a 19 GHz flat frequency comb. With auxiliary-laser-assisted thermal
stabilization, we surpass the thermal bistable dragging and extend the
mode-locking access to narrower 2 ps platicon pulse states, supported by
nonlinear dynamical modeling and boundary limit discussions.
