Phase-locking a THz quantum cascade laser to a THz comb through an all-optical beating

We demonstrate the phase-locking of a 2.5 THz Quantum Cascade Laser (QCL) to an air-propagating, 6 octaves-spanning, THz Optical Frequency Comb Synthesizer (OFCS). We directly observe the beating signal using a commercial detector, therefore proving that this approach provides a mature metrological tool in the THz region. We show that only a small fraction (<;100 nW) of the radiation emitted from the QCL is sufficient to generate a beat-note suitable for the phase-lock to the comb. These results pave the way to metrological-grade THz applications, especially in view of QCLs operating at higher temperatures.