  Quantum Cascade Lasers (QCL) constitute an intriguing opportunity for the
production of on-chip optical Dissipative Kerr Solitons (DKS): self-organized
optical waves which can travel while preserving their shape thanks to the
interplay between Kerr effect and dispersion. Originally demonstrated in
passive microresonators, DKS were recently observed in mid-IR ring QCL paving
the way for their achievement even at longer wavelengths. To this end we
realized defect-less THz ring QCLs featuring anomalous dispersion leveraging on
a technological platform based on waveguide planarization. A concentric
coupled-waveguide approach is implemented for dispersion compensation whilst a
passive broadband bullseye antenna improves the device power extraction and far
field. In these devices, comb spectra featuring sech$^2$ envelopes are
presented for free-running operation. This first hint of the presence of
solitons is further supported by the observation of highly hysteretic behaviour
and by phase-sensitive measurements which show the presence of self-starting 12
ps-long pulses in the reconstructed time profile of the emission intensity.
These observations are in very good agreement with our numeric simulations
based on a Complex Ginzburg-Landau equation time-domain solver. Such devices
constitute a new experimental platform for the study of soliton phenomena in
the THz range, allowing as well on-chip, passive ultrashort THz pulse
generation appealing for a variety of applications.
