Dephasing of Coherent Excitations

The mean-field description of coherent interactions fails under low-excitation conditions and/or in the ultra-fast time regime. Here not enough scattering events between the quasi-particles occur to warrant averaged (mean-field) conditions. This is the regime of non-Markovian dynamics where the Boltzmann equation and Fermi’s golden rule are not valid, memory effects occur and ongoing scattering processes can even be reversed. We will present some illustrative experimental results and give some short introduction into quantum kinetics. The coherent Markovian dynamics can also be deduced from the homogeneous linewidth of a resonance in the frequency domain. We discuss the role of homogeneous and inhomogeneous broadening in semiconductor structures and how to address these contributions in various experiments. Then dephasing by different processes like exciton-phonon scattering or related to the excited particle density (excitation-induced dephasing) is considered. We describe the homogeneous linewidth of localized states like single quantum dots and how macroscopic decoherence evolves in homogeneously and inhomogeneously broadened systems. The generation of photon echoes is described in detail using the Bloch-vector representation. This chapter further introduces variations of the experimental technique four-wave mixing (FWM): coherent control using three-pulse FWM, non-degenerate FWM and detection photon echos with FWM.