Low RF-Power Injection-Locking and Beatnote Control of Terahertz Quantum Cascade Laser Frequency Combs

Quantum Cascade lasers (QCLs), relying on intersubband transitions in semiconductor quantum well structures, show very short carrier lifetimes of the order of picoseconds [1]. As a consequence, relaxation oscillations remain over-damped up to modulation frequencies of several tens of GHz [2], enabling efficient amplitude modulation of the gain medium in this frequency range. These properties make QCLs ideally suited for RF-injection-locking. We demonstrate that the round-trip frequency of THz QCLs, as observed from the beatnote, can be injection-locked by RF-modulating the bias current. Within a certain locking range we observe mutual phase-locking of approximately 20 longitudinal modes for significantly lower RF-powers than in previous studies [3]. Apart from injection-locking, we demonstrate beatnote control by means of an external cavity.