Résumé
We show that it is possible to fix the carrier phase of a quantum cascade laser (QCL) by using injection seeding. Terahertz (THz) pulses with a fixed phase are injected into the QCL cavity and coincide with the gain of the QCL being turned on rapidly to avoid gain clamping (gain switching). The externally injected THz pulses are greatly amplified through multiple passes and can initiate laser action (instead of the spontaneous emission) and set the carrier-phase. Consequently, as well as the generation of large THz fields, this enables the electric field of the laser emission to be measured as a function of time, from initiation of lasing to the steady-state lasing regime using coherent sampling techniques. The phased-resolved field of the QCL is thus directly measured in the time-domain. This work enables the laser emission to be measured in the time domain and the QCL to be used as a powerful source for time-domain spectroscopy. We also use this scheme to imprint a fixed phase relationship between the multiple longitudinal modes of a THz QCL, resulting in the emission of ultrashort THz laser pulses.