Fig. 5: Simulated frequency response of a THz QCL using the Maxwell-Bloch formalism.

a Schematic of a self-generated microwave signal within a QCL cavity that modulates the gain, resulting in interlaced loss and gain regions in the cavity. b Simulated response with constant gain (without loss regions), resulting in a mode spacing at the fundamental frequency. c Simulated response with loss regions, resulting in a mode spacing at the second harmonic frequency and frequency bands separated by 100GHz