Ultrafast gain switching of THz quantum cascade lasers: THz pulse amplification and injection seeding

Laser action normally is initiated by the amplification of spontaneous emission. As spontaneous emission is a random process, the carrier phase of a laser is different each time a laser is turned on. As a consequence, it is not possible to measure the time-resolved field of a free-running laser using coherent detection techniques. The authors 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. The externally injected THz pulses are greatly amplified through multiple passes and 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.