  Solitons are shape preserving waveforms that are ubiquitous across nonlinear
dynamical systems and fall into two separate classes, that of bright solitons,
formed in the anomalous group velocity dispersion regime, and `dark solitons'
in the normal dispersion regime. Both types of soliton have been observed in
BEC, hydrodynamics, polaritons, and mode locked lasers, but have been
particularly relevant to the generation of chipscale microresonator-based
frequency combs (microcombs), used in numerous system level applications in
timing, spectroscopy, and communications. For microcombs, both bright solitons,
and alternatively dark pulses based on interlocking switching waves, have been
studied. Yet, the existence of localized dissipative structures that fit
between this dichotomy has been theoretically predicted, but proven
experimentally elusive. Here we report the discovery of dissipative structures
that embody a hybrid between switching waves and dissipative solitons, existing
in the regime of (nearly) vanishing group velocity dispersion where third-order
dispersion is dominant, hence termed as `zero-dispersion solitons'. These
dissipative structures are formed via collapsing switching wave fronts, forming
clusters of quantized solitonic sub-structures. The switching waves are formed
directly via synchronous pulse-driving of a photonic chip-based Si3N4
microresonator. The resulting frequency comb spectrum is extremely broad in
both the switching wave and zero-dispersion soliton regime, reaching 136 THz or
97% of an octave. Fourth-order dispersion engineering results in
dual-dispersive wave formation, and a novel quasi-phase matched wave related to
Faraday instability. This exotic unanticipated dissipative structure expands
the domain of Kerr cavity physics to the regime near zero-dispersion and could
present a superior alternative to conventional solitons for broadband comb
generation.
