  The ability to engineer quantum-cascade-lasers (QCLs) with ultrabroad gain
spectra and with a full compensation of the group velocity dispersion, at
Terahertz (THz) frequencies, is a fundamental need for devising monolithic and
miniaturized optical frequency-comb-synthesizers (FCS) in the far-infrared. In
a THz QCL four-wave mixing, driven by the intrinsic third-order susceptibility
of the intersubband gain medium, self-lock the optical modes in phase, allowing
stable comb operation, albeit over a restricted dynamic range (~ 20% of the
laser operational range). Here, we engineer miniaturized THz FCSs comprising a
heterogeneous THz QCL integrated with a tightly-coupled on-chip
solution-processed graphene saturable-absorber reflector that preserves
phase-coherence between lasing modes even when four-wave mixing no longer
provides dispersion compensation. This enables a high-power (8 mW) FCS with
over 90 optical modes to be demonstrated, over more than 55% of the laser
operational range. Furthermore, stable injection-locking is showed, paving the
way to impact a number of key applications, including high-precision tuneable
broadband-spectroscopy and quantum-metrology.
