Quantum Cascade concepts for optical and THz lasers have been successfully developed and used for  various applications. However, there are still significant improvements possible, such as room-temperature generation of THz signals. Therefore we introduced new structural concepts and further materials such as wide bandgap III-V and II-VI-based structures.The optimized conditions for room-temperature operation of TeraHertz QCLs are variable barrier heights in ZnSe/Zn1-xMgxSe material systems. The performance is improved also due to the suppression of thermally activated carrier leakage via higher-energy parasitic levels.
We also proposed the design of blue InGaN/GaN light-emitting diodes with Mg-Si pin-doped GaN barrier. The simulation results demonstrate that the Mg-Si pin-doping in the GaN barrier effectively reduces the polarization-induced electric field between the InGaN well and the GaN barrier in the multiple quantum well, and enhances the hole injection and electron confinement in the active region. For this LED device structure, we found that the turn-on voltage is 2.8 V, peak of light emission is at 415.3 nm and internal quantum efficiency (IQE) is 85.9% at 100 A/cm2. It is established that the LED device with Mg-Si pin-doping in the GaN barrier has significantly improved efficiency and optical output power performance, compared with LED device structures with undoped or Si(Mg)-doped GaN barriers.
Lasing means coherent emission of signals, so that interesting beam forming opportunities exist. Some of such possibilities are reviewed.

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