Gain engineering for terahertz quantum cascade lasers

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The objective of this research project is the investigation and demonstration of novel active-region-design strategies for the fabrication of terahertz quantum cascade lasers (THz QCLs) with high optical gain. High optical gain will translate into better laser performance in terms of a high energy conversion efficiency, high light output power and high operation temperature of devices. All three aspects are crucial for the development of future technology and applications using terahertz light sources, which lie at the boundary between optics and electronics.THz QCLs represent by far the best combination of power and compactness in lasers emitting within the THz spectral range. First demonstrated in 2002, THz QCLs with emission frequencies between 1-10 THz have been reported since then with best performance in the 2-5 THz range. The state of the art performance of THz QCLs is however limited to low optical powers (< 1 W) and operation temperatures below 200 K.Within the scope of this project laser physics issues driving the laser gain including the design of laser levels, electron injection, electron extraction, and population inversion will be addressed. First, we will formulate a realistic model for electron transport in THz QCLs, including interface-roughness scattering and hot electron effects. Interface-roughness scattering arises from the perturbation potential induced by rough crystal interfaces in semiconductor heterostructures. Hot electron effects arise from the slow heat exchange between the electron system and the crystal lattice. Both mechanisms reduce laser gain and deteriorate laser performance. Second, we will investigate different approaches to engineer interface-roughness scattering in THz QCLs in order to enhance optical gain. We will also investigate approaches to reduce carrier leakage in THz QCLs by engineering the transport of hot electrons. Finally, we will propose and demonstrate novel active region designs for high-gain THz QCLs based on the well-established GaAs/AlGaAs heterosystem and on novel semiconductor heterosystems as InGaAs/InAlAs and InGaAs/InGaAs.