Theory and modelling of infrared optical devices

 

S. Rotter

 

Project P14 will be dedicated to the numerical modelling of infrared and THz optical devices with a focus on the lasers fabricated in the experimental projects of the SFB (P03, P04, P11). To describe these devices both the material aspects of the active medium as well as the nonlinear optics induced by the light-matter interaction will have to be addressed.

 

Each one of the two Vienna-based investigators (S. Rotter, H. Kosina) will address one of these two complementary tasks: S. Rotter will be responsible for the optical aspects of the project for which the ideal tool will be the Steady-state Ab-initio Laser Theory (SALT) which he helped to develop. The main focus here will be to study the behaviour of devices which combine an amplifying and a dissipating optical component. Such gain-loss structures which have recently attracted much attention can be ideally realized with quantum-cascade structures where gain and loss are electrically tunable. We will propose new concepts which can be realized with variants of already available structures to demonstrate interesting new laser device properties like bistability and mode switching. To deliver device and material-specific predictions our SALT formalism will require a set of input parameters like the voltage and frequency dependent gain curves. These inputs will be provided by electronic transport calculations which constitute the second major task of this project.

 

Planar QCLs will be addressed by H. Kosina utilizing the semi-classical simulator developed in the previous phase. To further improve the agreement between simulation and measurement which we have demonstrated for both MIR and THz QCL designs, we shall refine the physical model set. Based on the Pauli master equation solved in the simulator, which relies on the diagonal approximation of the density matrix, we devised a method to include selected off-diagonal elements. The goal is to incorporate coherent tunneling in an extended master equation approach which still allows for an efficient numerical solution. We will extensively apply our transport simulator to the devices fabricated in P03 and P11 with the aim to optimize QCL designs. Additionally, we plan to include electron-photon scattering in our solver to realize a self-consistent coupling of electronic transport with the optical cavity field. Optical solvers existing in the groups of Rotter and Kosina will be extended to three dimensions by means of a common numerical library.

 

As our external collaborator, the group of P. Vogl will bring in its expertise on multiple levels of our project, in particular, on the transport calculations where its sophisticated NEGF code will serve as an important benchmark. Also the interesting predictions for room temperature operation of nanowire THz QCLs put forward in the last funding period will be pursued further in collaboration with projects P03 and P11.