5-ps-long terahertz pulses from an active-mode-locked quantum cascade laser

A significant research effort toward the generation of short terahertz (THz) pulses using quantum cascade lasers (QCLs) has been undertaken over the past few years. This is motivated by the desire to realize compact, electrically driven THz pulsed sources to study the dynamics of a variety of solid-state systems using ultrafast spectroscopy techniques, or to replace standard THz time domain spectroscopy systems for real-life applications. In this work we demonstrate the generation of 5-ps-long, transform-limited pulses by actively mode locking a 2.5 THz QCL. The pulse duration is a factor of 2 shorter than what has been reported to date with this technique, and has been achieved thanks to the use of a metal–metal waveguide, which favors lasing over a broader spectral bandwidth compared to previously employed single-plasmon waveguides. Active mode locking is obtained using a low-power radio-frequency (RF) signal to modulate the QCL current at the cavity round-trip frequency. The modulation does not affect the QCL emission spectrum. This is in striking contrast with previous reports of active mode locking where multimode emission was the direct consequence of the RF modulation.