We report an experimental study of terahertz (THz) quantum cascade lasers (QCLs) exploiting an intra-cavity self-detection process, where we demonstrate the generation of trains of self-mode-locked pulses. The detection technique is based on a well-known property of these devices, namely the fact that their gain dynamics unfolds on an ultra-fast time scale, in the order of tens of ps (for THz QCLs), determined by intersubband relaxation . The resulting modulation of the population inversion can therefore be observed as a function of time over very large bandwidths, in the ~10-100GHz range, simply by monitoring the QCL current . To fully exploit this possibility, in this work we processed an exceptionally long single-plasmon ridge waveguide of 15mm, based on an ultra-low threshold, 4.2THz active-region design [3,4].
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