As applications such as heterodyne spectroscopy require only single mode operation, the selection, suppression and tuning of individual lasing modes in THz QCLs has received considerable attention over the last decade. By periodically patterning the QCL in one- or two dimensions (e.g. distributed feedback (DFB) or photonic crystal lasers), single mode emission can be enforced. An alternative approach which requires no modification of the QCL waveguide is based on injection seeding technique with tunable narrowband THz seeds. Using this technique, we will show how the same QCL can be operated in both multi-mode and single mode regimes.
On the other hand, short pulses allow for time-resolved measurements and the generation of frequency combs. As the duration of a pulse is limited by its spectral bandwidth, a multimode operation of the QCL is highly desirable. By addition of a microwave modulation at the round-trip frequency, where the spacing and phase of the QCL modes is consequently fixed, results in active modelocking. This leads to laser emission of a train of THz pulses separated by the round-trip frequency. Coupled to coherent detection and a novel application of dispersion compensation, we demonstrate the generation of a stable 4 ps train pulse train. This opens up the possibility to reach sub-picosecond pulses and potentially the single cycle regime.
To conclude, we will show two methods to control the THz QCL emission from single mode regimes to the generation of short THz pulses.