Modeling of quantum cascade laser frequency combs in the mid-infrared and terahertz spectral region

• Applicant: Professor Dr.-Ing. Christian Jirauschek
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: from 2016 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 323277022

In this research project, we focus on the modeling of quantum cascade laser (QCL) frequency combs in the mid-infrared (MIR) and terahertz (THz) regimes. The QCL is an extremely versatile light source, employing artificially engineered optical transitions in the conduction band of a semiconductor nanostructure rather than electron-hole recombination. In this way, frequency ranges are opened up which are otherwise inaccessible to compact semiconductor lasers. In addition, giant optical nonlinearities can be integrated into the QCL nanostructure, which has recently been exploited for the realization of compact MIR and THz frequency comb sources. These provide an equidistant line spectrum which serves as a ruler in the frequency domain, enabling many innovative applications in metrology and spectroscopy, e.g., in chemical sensing.To describe the underlying coherent nonlinear optical phenomena, we use the Maxwell-Bloch equations. In contrast to previously employed perturbative solutions, we envisage a fully time dependent numerical treatment. By coupling the resulting numerical scheme to ensemble Monte Carlo (EMC) carrier transport simulations, we obtain a self-consistent approach which does not require empirical input parameters. In this way, a quantitative numerical model including all relevant effects, such as four-wave mixing, dispersion and spatial hole burning, is obtained. Specifically, such a fully numerical approach not only provides an accurate description of frequency comb operation, but also allows us to identify the allowed parameter regime for the formation of stable combs and to investigate imperfect combs extending only over a fraction of the laser modes.In detail, the proposal involves the development of a stable and efficient numerical scheme for solving the Maxwell-Bloch equations, suitable for long-time simulations as required to eliminate transients. Building on this, a self-consistent modeling tool will be set up by coupling the scheme to EMC simulations. Furthermore, we will include resonant tunneling into our model, which plays an important role primarily in THz QCL structures and can significantly affect the spectral comb characteristics. In close collaboration with experimental groups, we will use the modeling tool for the analysis of QCL-based MIR and THz combs and the development of improved comb sources with, e.g., extended spectral bandwidth. A further goal is to theoretically assess the feasibility of broadband THz combs, e.g., obtained by down-conversion of MIR combs. This involves the development of a stable and efficient numerical approach for solving the Maxwell-Bloch equations without the commonly used rotating-wave approximation.

• DFG Programme: Research Grants