Optical gain and laser properties of semiconductor quantum-dot systems

Abstract

For practical applications of quantum dots in light emitters as well as for fundamental studies of their emission properties, the understanding of many-body processes plays a central role. We employ a microscopic theory to study the optical properties of semiconductor quantum dots. The excitation-induced polarization dephasing due to carrier-phonon and carrier-carrier Coulomb interaction as well as the corresponding lineshifts of the optical interband transitions are determined onthe basis of a quantum-kinetic treatment of correlation processes.Our theoretical model includes non-Markovian effects as well as renormalized single-particle states.Thus we achieve an accurate description of the partial compensation between different dephasing contributions and are able to systematically study their temperature and density dependencies.Applications of this theoretical model include optical gain spectra for quantum-dot systems that reveal a novel effect, not present in other gain materials. For large carrier densities, the maximum gain can decrease with increasing carrier density. This behavior arises from a delicate balancing of state filling and dephasing, and implies the necessity of an accurate treatment of the carrier-density dependence of correlations. Measurements of the coherence properties of the light emitted from semiconductor quantum-dot lasers have raised considerable attention in recent years. We study the correlations between individual emission events on the basis of a microscopic semiconductor laser theory. This allows for a study of effects like Pauli blocking, modifications to the source term of spontaneous emission,and the absence of complete inversion, that strongly influence the emission characteristics of quantum dot based devices. A new and challenging material system for applications in the visible spectral range are nitride semiconductors. As crystal symmetry and bandmixing effects strongly influence the opticalselection rules, the single particle properties of quantum dot and wetting layer states are determinedon an atomistic level from tight-binding calculations. The resulting tight-binding wave functionsare used to calculate dipole transition matrix elements and Coulomb interaction matrix elements.As an example for the combination of microscopic single-particle calculations and many-body theory, optical spectra of quantum-dot wetting-layer systems including multiple subbands and the influence of theatomic structure and strong bandmixing effects are presented.