1 Nanophotonics Theory and Signal Processing, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark3 Nanophotonic Devices, Department of Photonics Engineering, Technical University of Denmark4 Nanophotonics, Department of Photonics Engineering, Technical University of Denmark
This thesis concerns the propagation of optical pulses in semiconductor waveguide structures with particular focus on methods for achieving slow light or signal delays. Experimental pulse propagation measurements of pulses with a duration of 180 fs, transmitted through quantum well based waveguide structures, are presented. Simultaneous measurements of the pulse transmission and delay are measured as a function of input pulse energy for various applied electrical potentials. Electrically controlled pulse delay and advancement are demonstrated and compared with a theoretical model. The limits of the model as well as the underlying physical mechanisms are analysed and discussed. A method to achieve slow light by electromagnetically induced transparency (EIT) in an inhomogeneously broadened quantum dot medium is proposed. The basic principles of EIT are assessed and the main dissimilarities between an atomic and a quantum dot medium are discussed. Three generic schemes are compared, showing that only one of the schemes are viable for slow light in an inhomogeneously broadened medium. The principal differences between the schemes are analysed and discussed. Propagation calculations of the three schemes are presented and compared together with estimates of the achievable delay and transmission. Finally, measurements of the ultra fast gain dynamics of a quantum dot semiconductor optical amplifier are presented. The experiment is based on degenerate pump-probe transmission spectroscopy using 180 fs pulses. Both the wavelength dependence as well as the applied current density dependence are investigated. Two characteristic relaxation rates of 0.2 ps and 1 ps are extracted based on a theoretical model. The choice of model and the underlying physical processes of the measurements are discussed.