| In contrast to atomic systems, demonstrations of coherent optical phenomena associated with radiative and non-radiative coherences have remained elusive in semiconductor systems, due to inherent rapid decoherence and many-body effects. Recent studies, however, have raised the hopes for observations and applications of coherent optics in semiconductors, as ultralong dipole dephasing times in quantum dots (QDs), as well as a remarkable robustness of the electron spin coherence have been revealed.; In the first part of this dissertation, we present experimental studies of dipole dephasing in CdSe-QDs using a novel technique based on spectral hole burning. In contrast to previous studies, we are able to suppress effects of spectral diffusion, and therefore, to provide first time measurements of the intrinsic dipole dephasing rate. The dipole dephasing times measured exceed those of previous studies by over one order of magnitude. We thus find that despite significantly enhanced electron-phonon interactions, dipole dephasing times in II-VI based QDs can be as long as in III-V based QD-systems.; Based on the remarkable robustness of the electron spin coherence, we reveal in the second part of this dissertation nonlinear optical processes unique to extended optical excitations in semiconductors. Employing frequency- and time-domain pump-probe techniques, we find that signatures of electron spin coherence are absent in the third-order nonlinear optical response from QW-excitons subject to in-plane magnetic fields. This observation is unexpected based on considerations involving atom-like systems. Insight into our findings is gained by formally including many-body correlations in a phenomenological description of elementary optical excitations in QWs. We show that, although the electron spin coherence is created, effects thereof cancel in their respective contributions to one-exciton and two-exciton coherences. Furthermore, we reveal the emergence of quantum beats associated with electron spin coherence in an unusual fifth-order nonlinear optical process, an observation which can again be explained invoking higher-order correlations. In addition to providing fundamental insight into the role of many-body correlations on excitonic optical nonlinearities, our studies provide novel means for coherent optical control of electron spin coherence. The coherent Raman scattering study in particular presents a precursor for future observations of electromagnetically-induced-transparency (EIT) and slow-light applications based on electron spin coherence in semiconductors. |
