Chip-Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching

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, an integrated architecture is conceived for the generation of high power (5.5–8.0 mW) THz FCs comprising an ultrafast THz polaritonic reflector, exploiting intersubband (ISB) cavity polaritons, and a broad bandwidth (2.3–3.8 THz) heterogenous THz QCL. By tuning the group-delay-dispersion in an integrated geometry, through the exploitation of light-induced bleaching of the ISB-based THz polaritons, spectral reshaping of the QCL emission and stable FC operation over an operational range up to 38%, characterized by a single and narrow (down to 700 Hz) intermode beatnote are demonstrated. This 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 toward the generation of mode-locked THz microlasers that can strongly impact a broad range of applications in ultrafast sciences, data storage, high-speed communication, and spectroscopy.