Chip-scale terahertz frequency combs through integrated intersubband polariton bleaching

Abstract: 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.