Dokument

Summary:
In laser physics, for the construction of new devices as well as the command and optimization of established configurations, it is crucial to properly understand the interplay between the different physical effects underlying the laser operation. In many cases, it may be rather difficult to experimentally access relevant key parameters. Furthermore, a decent understanding of the operation principles may be required already in the design stadium before the fabrication of the first prototype. Hence, an appropriate laser model is highly desirable since it allows for the simulation and analysis of important aspects of the laser performance. Modeling and simulation may help to represent or characterize, understand or analyze, asses or solve research problems encountered in experiments and verify assumptions made in theoretical investigations.The operation of modern microcavity lasers is governed by a complicated interplay of a variety of interaction processes. The systematic modeling of such devices therefore poses a significant challenge to the formulation of a physics driven laser theory based on the description of microscopic processes rather than on phenomenological approaches that mimic empirical observations. Especially, this applies to the modeling of high power VECSEL applications in extreme parameter regions well above the threshold. Since the emission characteristics of the laser systems is strongly dependent on the actual geometry of the setup, on the material parameters of both the dielectric structure and the gain region, and on the actual excitation state of the optically active material, the simulation of the experimentally observed features poses a highly non-trivial problem. While in many situations, it is sufficient to assume that the high carrier densities and the related fast carrier-carrier scattering effects in lasers lead to a sufficiently fast thermalization of the carrier system, under high power VECSEL operation conditions, such an assumption is usually not satisfied. In these systems, the carrier distribution in the valence and conduction bands may deviate significantly from Fermi-Dirac functions provided by the quasi-equilibrium conditions of a thermalized carrier system as the stimulated emission tends to burn kinetic holes into the carrier distributions leading to gain modifications which affect the emission characteristics of the device. A theory appropriate for the simulation of non-equilibrium laser performance, therefore, must be able to closely track the quantum kinetic carrier dynamics in the optically active material. This implies that for the simulation of the VECSEL performance a model has to be used that allows for the monitoring of the carrier dynamics on a microscopic time scale (i.e. a few femtoseconds) for macroscopic time periods (i.e. several nanoseconds) corresponding to the build up of stable laser oscillations - all the while being computationally thus feasible that even parametric studies do not become impractical.In our approach, the system consisting of semiconductor gain medium and laser field is described within the context of semi-classical laser theory by the Maxwell-semiconductor-Bloch equations (MSBE). Many-body Coulomb effects are included at the level of the screened Hartree–Fock approximation, and the effective relaxation rate approximation is used to account for the effects of carrier–carrier and carrier–phonon collisions. Here, we follow this approach to analyze the dominant non-equilibrium effects in the multimode operation of VECSEL devices under high excitation conditions. To test our model and to study the relevance of non-equilibrium effects, we pick the examples (i) of a traditional device configuration that exhibits dual-mode emission and (ii) of ultrashort pulse generation via modelocking in a VECSEL For the two-color operation, a model study is presented demonstrating that the non-linear laser theory on the basis of the MSBE is fully adequate for the description of high power applications where non-equilibrium effects gain increased importance. Altogether, our microscopic simulations give the first numerical verification that dynamically, stable two-wavelength oscillation of a semiconductor laser can occur when the mode coupling between two wavelengths is weak. Using the same model approach, we further carry out the first microscopic simulations of modelocking in a simple VECSEL configuration. Thus, we present a preliminary analysis of ultrafast mode-locking in order to expose the role of hot carrier distributions in establishing this feature.

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