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

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The common goal of the groups at the Paul-Drude-Institut (PDI) and the Humboldt-Universität zu Berlin (HUB) is the demonstration of lasing based on gain due to quantum coherence as well as the comprehensive understanding of the 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 PDI group are a suitable description of the interplay of coherent and dissipative as well as dephasing processes in TQL structures, the implementation of a model, and the demonstration of improved TQL structures with increased gain and enhanced internal coupling of the pump light to the respective transitions. Finally, the coherent and dissipative processes in TQLs will be analyzed. In cooperation with the group at HUB, we will develop appropriate methods for external coupling of the pump beam into the TQL and plan to demonstrate the lasing of the complete system. For a suitable description of the interplay of coherent and dissipative as well as dephasing processes in TQL structures, we will combine the Maxwell-Bloch equations with a generalized Fermi's golden rule. The initial objective consists in the numerical implementation of this model, which is expected to significantly improve the design of TQL structures. In particular, the design strategy has to include the optimization of the gain due to quantum coherence. We will develop, realize, and investigate several advanced structures with different numbers of relevant states, injector properties, and operating field strengths. The advanced designs focus on a further improvement of the internal coupling efficiency of the MIR pump light, on larger total optical gain, and on the complete suppression of transport instabilities in the operating field strength regime. In cooperation with the group at HUB, we will adjust the designs for the MIR pump QCLs and the TQL structures so that a better matching of the emission wavelength of the QCL to the excitation wavelength of the TQL can be achieved. For the analysis of coherent and dissipative processes in TQLs, the comparison of the simulations with experimental investigations is expected to allow for the distinction of lasing due to conventional gain from lasing due to gain originating from quantum coherence. In addition to the improvement of the internal coupling efficiency, we intend to improve the external coupling of the pump beam into the TQL by developing lateral-coupling higher-order gratings.