Terahertz lasing without inversion based on quantum coherence between intersubband transitions: Pumping and lasing

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The common goal of the groups at the Humboldt-Universität zu Berlin (HUB) and the Paul-Drude- Institut (PDI) is the demonstration of lasing based on gain due to quantum coherence as well as the comprehensive understanding of underlying physical processes. The proposed system consists of a high-power mid-infrared quantum-cascade laser (MIR QCL) for pumping and an optically pumped, electrically driven terahertz quantum laser (TQL). The objectives of the HUB group to realize the common goal are the optimization of the MIR pump QCLs in terms of power, beam quality, and tunability within an external-cavity, the optimization of the coupling between the MIR QCL and the TQL structure using two new approaches, and, together with the PDI, the demonstration and investigation of a functional TQL. To achieve these objectives and the common goal, the active region of the MIR QCL must be optimized for injection efficiency, requiring a deeper understanding of the mechanisms that scatter electrons into higher-lying energy states. This understanding will allow for an increase of the emission power to the required 30 W. At the same time, the QCL active region also needs to be optimized for sufficient external-cavity tuning, maintaining a large transition efficiency. In addition, two approaches will be pursued to optimize the coupling of the MIR emission into the TQL structure: butt coupling of an external cavity QCL and lateral coupling using higher-order gratings on both the MIR QCL and the TQL structures. Finally, the response of the TQL structure to the incoupled MIR radiation will be investigated in the third objective in order to demonstrate the functionality of the TQL. In particular, THz emission as a function of MIR power, bias, coupling, and temperature will be measured. This proposal is specifically focused on developing the MIR QCLs for the optical pumping part of this system and coupling the 10-pm excitation beam into the TQL structure. This GaAs/(AI,Ga)Asbased cascade structure that defines the THz emission transitions will be developed and investigated within the parallel project submitted by the PDI group. The project goal, the demonstration of a complete THz laser based on quantum coherence, will be developed in collaboration with the PDI group.