Terahertz Quantum Cascade lasers are very versatile sources of terahertz radiation. Frequency comb operation, surface emitting arrays, external cavity tuning have been demonstrated. For all these implementations broadband gain is strongly demanded. The intersubband gain mechanism allows to the design of different wavelength active region and their integration in the same waveguide. We have developed active regions consisting of up to four different intersubband designs. To enable a common operation not only the gain curve needs to be aligned over all sections but also the alignment electric field and subsequently the operating current. Fabry-Perot devices fabricated from the four-section active region show lasing over more than one octave. Ring resonators show also broadband laser operation and comb formation. Broadband operation is a large advantage of random lasers which we turn into useful devices by an optical machine learning approach. This allows the control of the emission wavelength beyond discrete cavity modes.
Ring resonators are interesting alternative cavity solutions to the commonly used ridge type waveguide for THz Quantum Cascade lasers. They either support a standing wave pattern showing spatial hole burning if there are defects implemented or a traveling mode in a defect-free cavity. We have fabricated two devices structures. The first one is episide-up with bonding pads. The measurements show a complex behavior of comb-formation most probably influenced by spatial hole burning. The second structure is a pure ring mounted episode down on Si-substrate. This structure shows a totally different comb formation as well as much reduced threshold currents.
The study of high Al containing barriers in Terahertz Quantum Cascade lasers has led to the improvement of operation temperature and of the quantum efficiency. This is mainly caused by the reduction of transport channels through higher states. In consequence, the electron transport in these new devices is dominated by photon assisted tunneling. The originating non-linearity provides a huge potential for different operation modes. We try to further study this by coupling distributed QCL devices on a chip which has led to the observation of bi-stable operation and THz switching. We use the non-linear behavior for the control of the emission spectra of surface emitting random laser structures. Furthermore, ring structures can be realized which can be tuned from single mode to frequency comb operation.
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