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.