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.
We report on high performance Terahertz Quantum Cascade Lasers with InGaAs and GaAs active regions. Modified doping profiles derived from symmetric structures allowed achieving record output powers of double metal InGaAs/InAlAs THz Quantum Cascade Lasers. The increase of the Al concentration of the barriers in GaAs/AlGaAs devices helped to increase the operating temperature to above 191 K while keeping the threshold current low. This has enabled laser operation by thermoelectric cooling which is very important for application. We demonstrate laser wavelength switching by magnetic field and wavelength selection in Random THz Quantum Cascade Lasers by spatially controlled near-infrared excitation
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