Tunable comb operation in terahertz quantum cascade ring lasers

We demonstrate a versatile and compact approach to generate and control terahertz frequency combs by applying radio frequency modulation to a ring-shaped quantum cascade laser. By harmonically driving the device well above threshold, we induce the formation of phase-coherent sidebands with tunable spacing across a broad radio frequency range (1.5-14.5 GHz), independent of the laser's cavity modes. The RF generated comb exhibits narrow beat notes with linewidths down to 3.5 Hz and a linear phase profile, as verified by optical beat note analysis, indicating the important role of the phase modulation. Unlike classical phase modulation, we observe a simultaneous increase in total terahertz output power and driving current, reflecting an enhanced laser efficiency and photon-assisted transport. The presented approach enables flexible manipulation of frequency grids and spectral power distribution in both single-mode and multimode regimes, including subharmonic injection locking for spectral broadening. Our results highlight the potential of radio frequency-modulated quantum cascade lasers as customizable, high-efficiency terahertz comb sources for spectroscopy, sensing, and communication applications.