Subharmonic RF locking in THz quantum walk comb lasers

We demonstrate advanced control capabilities in a fully integrated optical frequency comb platform featuring THz ring quantum cascade lasers by exploiting external RF modulation resonant to the fundamental cavity round-trip frequency and its subharmonics. We present a monolithic higly-tunable optical frequency comb source featuring a planarized double-ring THz quantum cascade laser with surface-emitting bullseye antenna. The device operates as single-mode in free-running around 3.0 THz and as a quantum walk comb source when applying external RF modulation resonant to the cavity roundtrip frequency (frt). We observe spectral broadening injecting at finj=frt~15.9 GHz, with the maximum bandwith growing with the injected power up to ~500 GHz. The coherence of the comb states is confirmed by SWIFT measurement. A similar broadening occurs when the injected RF frequency is swept around subharmonics of the fundamental round-trip frequency. The mode spacing follows the injected frequency and SWIFT analysis confirms the coherence of these new comb states, down to frt/10~1.59 GHz. Subharmonic injection enables the use of two-tones modulation techniques for spectral shaping of the optical frequency comb, overcoming the limitations imposed by RF components.