14 Externe wissenschaftliche Einrichtungen
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15
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Item Open Access Spatio-temporal and polarisation dynamics of semiconductor microcavity lasers(2004) Hamm, Joachim; Hess, Ortwin (Prof. Dr.)Microcavity semiconductor lasers are known for their inherent tight coupling between active material and light-field. The dynamic interaction between the carrier and the photon subsystems is influenced equally strong by both, the dynamics of carriers within the quantum-well and the intra-cavity light-field dynamics. In this work, we develop theoretical models and investigate the nonlinear spatio-temporal behaviour of two prominent types of microcavity lasers, the vertical-cavity surface-emitting laser (VCSEL) and the vertical extended cavity surface-emitting laser (VECSEL). Today's aim to build faster and more powerful semiconductor laser devices goes hand in hand with a miniaturisation of the semiconductor laser structures down to the nanometerscale. Difficult even for simple bulk semiconductor devices, the even tighter coupling of the carrier and light-field sub-systems with respect to time- and length-scales disallow a separate dynamical treatment of the physical processes which take place within such novel microcavity semiconductor lasers. Due to their flexibility and their physical nature, time-domain simulations constitute an appropriate tool for targeting the entangled dynamics within the cavity, the structure and the active quantum-wells. We predict that along with the technological progress of microcavity semiconductor lasers and the availability of inexpensive computing power time-domain methods will gain more importance and constitute a valuable tool to analyse the optical and electronic properties of these devices.Item Open Access Three dimensional finite difference time domain simulations of photonic crystals(2004) Hermann, Christian; Hess, Ortwin (Prof. Dr.)In this work fundamental optical properties of various photonic crystal structures are analysed numerically within the framework of three dimensional finite-difference time-domain (FDTD) simulations. After a discussion of the underlying physical and mathematical principles from electrodynamics and solid state physics leading to the formation of a photonic bandgaps, two important example systems are discussed in detail. First, we study two-dimensionally patterned layer-by-layer systems. These system are promising with respect to applications in integrated optics, but suffer severely from out-of-plane radiation losses. With fully three dimensional simulations we analyse this loss mechanism for several vertical layer-setups, obtaining new and important results regarding the understanding of the interaction of in-plane structure and surrounding. E.g. we describe for the first time the existence of cladding modes in complicated layer-by-layer structures. Second, we analyse the strong space-, frequency- and polarisation dependance of spontaneous emission within the weak coupling limit in an three-dimensionally structured inverted opal crystallite of finite size. The inverted opal, exhibiting a complete bandgap, shows (beside the supression of emission for bandgap frequencies) strong enhancement for emitters placed on dielectric interfaces. These results are important for interpretating luminescence experiments of inhomogeneously infiltrated dye molecules. In the last part of the work we discuss the numerical method itself with special focus on the necessary adaptations for the treatment of photonic crystal structures.Item Open Access Microscopic spatio-temporal dynamics of semiconductor quantum well lasers and amplifiers(2007) Böhringer, Klaus; Hess, Ortwin (Prof. Dr.)This work discusses light-matter interaction and optical nonlinearities in semiconductor nanostructures and presents a detailed numerical analysis of the spatio-temporal dynamics in novel high-power diode lasers. We derive a microscopic, spatially resolved model that combines a density matrix approach to the carrier and gain dynamics in semiconductor quantum well gain media with the macroscopic Maxwell equations for the electromagnetic field dynamics. We present Maxwell semiconductor Bloch equations in full time-domain that cover many-body interactions, a diversity of time scales and gain saturation mechanisms, and comprise the fast-oscillating carrier wave and a sub-wavelength spatial resolution. Our work focuses on ultrafast carrier effects, a quantitative understanding of optical nonlinearities, the engineering of the mode structure in microcavities, and their impact on the laser emission characteristics. Optical dephasing and carrier relaxation due to the screened Coulomb interaction and scattering with phonons are explored in detail. This work aims to improve the quantitative understanding of lasing systems of technological or fundamental relevance by performing numerical experiments: Within the framework of the paraxial wave approximation, we study the excitation of multiple transverse modes, multi-mode dynamics and the occurrence of unstable optical filaments in broad area edge-emitting lasers. We analyse vertical cavity surface-emitting laser devices with a periodically structured defect as an example of a photonic band edge band gap laser. In particular, we explore the utilisation of photonic crystal structures: gain enhancement for band edge modes and the reduction of optical losses. The complex interplay between the intracavity optical field and quantum well gain dynamics is investigated for realistic optically pumped external cavity surface-emitting laser structures. We also consider the interaction of high-intensity femtosecond and picosecond pulses with semiconductor optical amplifiers and absorbers. We identify the microscopic origin of the fast nonlinearities, obtain nonlinear gain coefficients and recovery rates, and analyse the nonlinear pulse reshaping, i.e. changes and asymmetries in the amplified pulse shape and spectrum. Built upon efficient numerical algorithms and the increased availability of inexpensive high-performance computing resources, our microscopic time-domain approach is well suitable for the engineering and design optimisation of modern nanostructured high-power diode lasers.Item Open Access Spatio-temporal non-linear dynamics of lasing in micro-cavities : full vectorial Maxwell-Bloch FDTD simulations(2004) Klaedtke, Andreas; Hess, Ortwin (Prof. Dr.)This work explores the ultra fast and non-linear effects in dielectric micro-cavities and micro-cavity lasers on the basis of full vectorial Maxwell-Bloch finite difference in time domain simulations. Micro-cavity lasers promise low lasing pump thresholds and ultra fast amplitude modulations. Micro-cavities with low pump thresholds may function as single photon sources which are required for quantum optic data processing. For the first time the full three dimensional complete polarisation dynamics during the initial transient of a lasing process in a micro-cavity are calculated and discussed. So called microdisks and microgears were chosen as the cavity geometries. In order to simulate the non-linear, ultra short effects in micro cavities, a three dimensional finite element in time domain (FDTD) code was set up, adapted to different types of computer architectures and verified. The FDTD algorithm describes the temporal evolution of the electromagnetic fields on a Yee type numerical grid by using the Faraday and the generalised Ampere law to calculate the fields at a new time step from the former time step in a leap frog like, second order accurate scheme. The Maxwell material equations introduce the polarisation into the FDTD algorithm. It is described for the first time in a semi-classical, self consistent three dimensional formalism by the discretised optical Bloch equations as separate difference equations. Resonators with distributed feedback (DFB resonators), including one or two dimensional corrugations, pose the first type of micro-resonator for which the numerical method is applied. This type of resonator is used in so called "plastic lasers" which are entirely made from organics in order to form extremely flexible and shapeable, large area light sources. The simulations in this work lead to qualitative results which give insight in the type of DFB mechanism, and they give quantitative results for DFB structures which are used in experiments. The last two chapters of this work deal with disk type lasers with length scales in the range of a few wavelengths. Starting with smooth shaped dielectric cylinders, cold cavity modes are computed and measured. The results are then compared to experiments and different theoretical computations from literature. Subsequently, the influence of a periodic modulation of the cylinder radius - which characterises the microgear laser - with variable depth on the properties of cold cavity modes in smooth shaped disks is studied. Quantitative investigations are performed on selected modes. These dielectric disks with a periodic modulation of the radius are called microgears. The simulations of the initial transient relaxation oscillation of the active material in microdisk lasers visualise the quasi stationary spatial hole burning process. But this initially static behaviour is immediately replaced by a rotational movement of the electromagnetic mode. We ascribe this effect to the continuous azimuthal degeneracy of the eigenmodes of the cold cavity and the non-linearity of the combined optical Maxwell-Bloch equations. The continuous azimuthal degeneracy is removed by the transition to the microgear geometry. The simulations of lasing in this kind of resonator shows a spatially static hole burning of the inversion from the first relaxation oscillations to the steady state lasing process. It is thereby demonstrated that the mode decomposition of a cold dielectric cavity is an indication of the properties which are to be expected of a lasing resonator, but proves to be insufficient to predict non-linear effects in micro and macro lasers.Item Open Access Theorie der quantenoptischen und nichtlinear-dynamischen Eigenschaften von Halbleiterlasern(2001) Preißer, Dietmar; Hess, Ortwin (PD Dr.)Die Frage nach Eigenschaften wie der Laserlinienbreite, die in gewissem Umfang eine voll quantenmechanische Beschreibung des Lasers erfordert, ist ein erster Schwerpunkt dieser Arbeit. Im Verlauf eines detaillierten Modells bzgl. quantenmechanischer Eigenschaften in einer Kavität werden die Bedingungen herausgearbeitet, unter denen eine Langevin-Beschreibung möglich ist, als auch eine Korrektur zur Formel von Schawlow-Townes gegeben. Auf Basis der Maxwell-Bloch-Gleichungen und weiterer vereinfachender Modelle werden eine Reihe von Simulationen zur Veranschaulichung des Phasenübergangs bei transversal ausgedehnten Halbleiterlasern gezeigt. Der Vergleich der Simulationen ermöglicht die Abschätzung der Güte der jeweils zugrunde liegenden Approximation. Darüber hinaus wird auch eine analytische Untersuchung des Phasenübergangs gegeben. Ein weiterer Schwerpunkt liegt in den speziellen dynmischen Eigenschaften kurzer optischen Pulsen. Bereits bestehende Modellansätze zur Beschreibung von Gaslasern, insbesondere von Verstärkern, werden wiederum auf die Güte der Beschreibung unter den Bedingungen von Halbleitersystemen untersucht. Dabei zeigt sich, daß eine aus der Theorie zu Gaslasern wichtige Erkenntnis von forminvarianten Pulsprofilen (Solitonen) sich unter gewissen Bedingungen auch auf Halbleitersysteme übertragen lassen.