Picosecond optical excitation was used to coherently control the excitation in a single quantum dot on a time scale that is short compared with the time scale for loss of quantum coherence. The excitonic wave function was manipulated by controlling the optical phase of the two-pulse sequence through timing and polarization. Wave function engineering techniques, developed in atomic and molecular systems, were used to monitor and control a nonstationary quantum mechanical state composed of a superposition of eigenstates. The results extend the concept of coherent control in semiconductors to the limit of a single quantum system in a zero-dimensional quantum dot.
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In transient laser-induced grating experiments, the diffusion coefficients and lifetimes of free excitons are determined in CdSe at temperatures between 2 and 40 K and for different densities. We find that the diffusion coefficient D decreases and that the recombination lifetime T, increases with increasing temperature. The increase of T& with temperature is due to the release of excitons bound to impurities and to an increase of the radiative lifetime. From the measured D values for motion parallel and perpendicular to the crystal c axis, we extract momentum relaxation rates which are discussed in terms of excitonacoustic-phonon scattering. In pure samples, and for lattice temperatures T (10 K, the interaction via the deformation potential is dominating, while for higher temperatures, the interaction via the piezoelectric potential becomes significant.
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