  Dissipative Kerr solitons are self-organized optical waves arising from the
interplay between Kerr effect and dispersion. They can form spontaneously in
nonlinear microresonators pumped with an external continuous-wave laser, which
provides the parametric gain for the proliferation of an ultrastable frequency
comb. These miniaturized and battery driven microcombs have become a disruptive
technology for precision metrology, broadband telecommunication and ultrafast
optical ranging. In this work, we report on the first experimental observation
of dissipative Kerr solitons generated in a ring cavity with a fast
semiconductor gain medium. The moderate quality factor of the ring cavity is
compensated by the giant resonant Kerr nonlinearity of a quantum cascade laser,
which is more than a million times larger than in Si3N4. By engineering the
dispersion of the cavity, we observe the formation of bright dissipative Kerr
solitons in the mid-infrared range. Soliton formation appears after an abrupt
symmetry breaking between the two lasing directions of the ring cavity. The
pump field of the soliton is generated by direct electrical driving and closely
resembles the soliton Cherenkov radiation observed in passive microcombs. Two
independent techniques shed light on the waveform and coherence of the solitons
and confirm a pulse width of approximately 3 ps. Our results extend the
spectral range of soliton microcombs to mid-infrared wavelengths and will lead
to integrated, battery driven and turnkey spectrometers in the molecular
fingerprint region.
