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

• Applicant: Professor Dr. Holger Grahn
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 269855421

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.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Lutz Schrottke