  Quantum cascade lasers (QCLs) represent a fascinating accomplishment of
quantum engineering and enable the direct generation of terahertz (THz)
frequency radiation from an electrically-biased semiconductor heterostructure.
Their large spectral bandwidth, high output powers and quantum-limited
linewidths have facilitated the realization of THz pulses by active
mode-locking and passive generation of optical frequency combs (FCs) through
intracavity four-wave-mixing, albeit over a restricted operational regime.
Here, we conceive an integrated architecture for the generation of high power
(10 mW) THz FCs comprising an ultrafast THz polaritonic reflector, exploiting
intersubband cavity polaritons, and a broad bandwidth (2.3-3.8 THz)
heterogeneous THz QCL. Quantum cascade lasers (QCLs) represent a fascinating
accomplishment of quantum engineering and enable the direct generation of
terahertz (THz) frequency radiation from an electrically-biased semiconductor
heterostructure. By tuning the group delay dispersion in an integrated
geometry, through the exploitation of light induced bleaching of the
intersubband-based THz polaritons, we demonstrate spectral reshaping of the QCL
emission and stable FC operation over an operational dynamic range of up to
38%, characterized by a single and narrow (down to 700 Hz) intermode beatnote.
Our concept provides design guidelines for a new generation of compact,
cost-effective, electrically driven chip-scale FC sources based on ultrafast
polariton dynamics, paving the way towards the generation of mode locked THz
micro-lasers that will strongly impact a broad range of applications in
ultrafast sciences, data storage, high-speed communication and spectroscopy.
