Photon-Driven Broadband Emission and Frequency Comb RF Injection Locking in THz Quantum Cascade Lasers

We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime, they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultrabroadband operation exceeding 1.83 THz (Delta f/f = 50%) with high phase noise, exhibiting selfsustained, periodic voltage oscillations. The low CW threshold (115 A cm(-2)) and broadband comb operation (Delta f/f = 25%) make these sources extremely appealing for on-chip frequency comb applications.