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

Paul-Drude-Institut（PDI）和Humboldt-Universität zu柏林（HUB）的团队的共同目标是演示基于量子相干增益的激光以及对潜在物理过程的全面理解。该系统由高功率中红外量子级联激光器（MIR QCL）泵浦和光泵浦、电驱动的太赫兹量子激光器（TQL）组成。PDI组的目标是适当地描述TQL结构中相干和耗散以及退相过程的相互作用，模型的实现，以及具有增加的增益和增强的泵浦光到相应过渡的内部耦合的改进的TQL结构的演示。最后，分析了TQL中的相干过程和耗散过程。我们将与HUB的团队合作，开发将泵浦光束外部耦合到TQL的适当方法，并计划演示整个系统的激光发射。为了适当地描述TQL结构中相干和耗散以及退相过程的相互作用，我们将联合收割机麦克斯韦-布洛赫方程与广义费米黄金法则相结合。最初的目标包括在这个模型的数值实现，预计将显着提高TQL结构的设计。特别地，设计策略必须包括由于量子相干性而导致的增益的优化。我们将开发，实现和研究几种先进的结构，具有不同数量的相关状态，注入器特性和工作场强。先进的设计集中在进一步提高MIR泵浦光的内部耦合效率，更大的总光学增益，并在工作场强制度的传输不稳定性的完全抑制。与HUB的团队合作，我们将调整MIR泵浦QCL和TQL结构的设计，以便实现QCL的发射波长与TQL的激发波长的更好匹配。对于TQL中的相干和耗散过程的分析，模拟与实验研究的比较预计将允许区分由于常规增益的激光发射，由于来自量子相干性的增益。除了内部耦合效率的提高，我们打算通过开发横向耦合高阶光栅来提高泵浦光到TQL的外部耦合。

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.