High-resolution terahertz semiconductor spectroscopy using quantum-cascade lasers: Develop-ment of appropriate laser sources for 2.7-3.3 and 5.0-5.7 THz

Paul-Drude-Institut （PDI）和Technische Universität Berlin （TUB）的研究小组的共同目标是演示基于多模式、窄线宽量子级联激光器（qcl）和光栅光谱仪的高分辨率太赫兹频率半导体激光光谱的光谱仪，以及对杂质态（特别是同位素纯Ge和Si）的线宽、线形和弛豫时间的研究。PDI小组的目标是开发一种基于适当建模水平的改进设计工具，包括选择必要的理论组件，探索有效设计过程的原则和自动化设计过程的呈现，将太赫兹qcl的发射范围扩展到5太赫兹以上，了解在更高频率下操作的限制物理过程，以及开发宽带。可调谐qcl工作在2.7和3.3太赫兹（Ge）和5.0和5.7太赫兹（Si）之间。最后，我们计划共同演示整个系统的运行，并确定基于qcl的光谱仪的最佳分辨率。为了开发一个适合设计目的的模型，我们打算改进子带间散射率的近似，以纵向光学声子散射为例。将探讨横向电位波动、界面分布和相干效应对太赫兹量子激光器工作特性的影响。该模型将用于有效设计过程的研究和自动化设计过程的描述。这些知识将用于优化2.7和3.3太赫兹之间合适的qcl。将太赫兹量子激光器的发射范围扩展到5太赫兹以上，了解在更高频率下运行的限制物理过程，以及几种可能结构的有效设计和各自激光器的制造，将允许开发工作在5.0和5.7太赫兹之间的可调谐量子激光器。整个系统的演示将在与TUB小组密切互动的情况下进行，特别是在必要操作参数的定义方面。与TUB小组合作，我们还将利用高分辨率傅立叶变换红外光谱对同位素纯Ge和Si及其各自的杂质状态进行比较光谱研究。

The common goal of the groups at the Paul-Drude-Institut (PDI) and the Technische Universität Berlin (TUB) consists in the demonstration of a spectrometer for high-resolution laser spectroscopy of semiconductors at THz frequencies based on multi-mode, narrow-line-width quantum-cascade lasers (QCLs) combined with a grating spectrometer as well as the investigation of the line width, line shape, and the relaxation time of impurity states in particular in isotopepure Ge and Si. The objectives of the PDI group are the development of an improved design tool based on an appropriate level of modeling including the selection of necessary theoretical components, the exploration of principles for efficient design processes and the presentation of an automated design procedure, the extension of the emission range of THz QCLs beyond 5 THz and the understanding of the limiting physical processes for operation at higher frequencies, as well as the development of broad-band, tunable QCLs operating between 2.7 and 3.3 THz (for Ge) and 5.0 and 5.7 THz (for Si). Finally, we plan to jointly demonstrate the operation of the complete system and the determination of the best possible resolution of the QCL-based spectrometer. With regard to the development of a suitable model for design purposes, we intend to improve our approximation for the intersubband scattering rates, starting exemplarily with longitudinal optical phonon scattering. The influence of lateral potential fluctuations, interface profiles, and coherent effects on the operation properties of THz QCLs will be explored. The model will be used for the investigation of efficient design processes and the description of an automated design procedure. This knowledge will be used to optimize appropriate QCLs between 2.7 and 3.3 THz. The extension of the emission range of THz QCLs beyond 5 THz and the understanding of the limiting physical processes for operation at higher frequencies as well as the efficient design of several possible structures and the fabrication of the respective lasers will allow for the development of tunable QCLs operating between 5.0 and 5.7 THz. The demonstration of the complete system will be carried out in close interaction with the group at TUB, in particular with respect to the definition of the required operating parameters. In cooperation with the TUB group, we will also carry out comparative spectroscopic investigations using high-resolution Fourier transform infrared spectroscopy on isotopepure Ge and Si as well as on the respective impurity states.