Terahertz Frequency Combs Exploiting an On-Chip, Solution-Processed, Graphene-Quantum Cascade Laser Coupled-Cavity

The ability to engineer quantum-cascade-lasers(QCLs) with ultrabroad gain spectra, and with a full compensationof the group velocity dispersion, at terahertz (THz) frequencies, iskey for devising monolithic and miniaturized optical frequency-comb-synthesizers (FCSs) in the far-infrared. In THz QCLs four-wave mixing, driven by intrinsic third-order susceptibility of theintersubband gain medium, self-locks the optical modes in phase,allowing stable comb operation, albeit over a restricted dynamicrange (∼20% of the laser operational range). Here, we engineerminiaturized 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 mixingno longer provides dispersion compensation. This enables a high-power (8 mW) FCS with over 90 optical modes, through 55% of the laser operational range. We also achieve stable injection-locking, paving the way to a number of key applications, including high-precision tunable broadband-spectroscopy and quantum-metrology.