1 Phase seeding of a terahertz quantum cascade laser

The amplification of spontaneous emission is used to initiate laser action. Since the phase of spontaneous emission is random, the phase of the coherent laser emission (the carrier phase) will also be random each time laser action begins. This prevents phase resolved detection of the laser field. Here, we demonstrate how the carrier-phase can be fixed in a semiconductor laser: a quantum cascade laser. This is performed by injection seeding a quantum cascade laser with coherent terahertz pulses, which forces laser action to start on a fixed phase. This permits the emitted laser field to be synchronously sampled with a femtosecond laser beam, and measured in the time-domain. We observe the phase-resolved build-up of the laser field, which can give insights into the laser dynamics. In addition, as the electric field oscillations are directly measured in the time-domain, quantum cascade lasers can now be used as sources for time-domain spectroscopy. ha l-