Quantum Kinetics and Non-Equlibrium Many-Body Theory in Semiconductors
Optically excited semiconductors provide a non-eqilibrium many-body system with a variable electron-hole density. The interactions between electronic excitations and with other excitations in the crystal determine the linear and nonlinear optical optical properties.
On femtosecond time scales the relaxation and dephasing kinetics can no longer be described by semiclassical Boltzmann kinetics, but has to be described by a new quantum kinetics which takes the partly coherent wave nature of the excited electron and holes into account. The quantum coherence causes memory effects in the quantum kinetic scattering integrals.
The study of the quantum kinetics - derived with non-equilibrium Green functions - for phonon and Coulomb scattering in semiconductors and in semiconductor microstructures is the central topic of our research group. We calculate with quantum kinetics for time-resolved four-wave-mixing and pump-and-probe spectra which can be measured with femtosecond laser pulses. In microstructures with quantum confinement we also study low-dimensional many-body systems. Collective quantum coherent phenomena in these low-dimensional systems such as the formation of exciton and Wigner crystals, the relaxation of excitons toward a Bose-Einstein condensed state and Bloch oscillations in superlattice structures belong also to our research projects.
Kinetics of the Bose-Einstein Condensation of Exciton Polaritons in Microcavities
Polaritons in semiconductor microcavities are mixed bosons – partly photons and partly excitons. At sufficiently high pumping they undergo a non-equilibrium Bose-Einstein condensation (BEC). The condensation kinetics and the spontaneous build-up of coherence determined by the first- and second- order coherence functions, as well as limiting decoherence processes are in the focus of our recent research.
Our recent effort was a quantum kinetical derivation of the nonequilibrium Gross-Pitaevskii equation fort he condensate wave function, which allows to study a rich scenario of spatio-temporal structures, such as solitons, quantized vortices and spontaneous structure formation.