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Optically controlled coherent dynamics of charge ͑excitonic͒ degrees of freedom in a semiconductor quantum dot under the influence of lattice dynamics ͑phonons͒ is discussed theoretically. We show that the dynamics of the lattice response in the strongly nonlinear regime is governed by a semiclassical resonance between the phonon modes and the optically driven dynamics. We stress on the importance of the stability of intermediate states for the truly coherent control.

The role of phonons for adiabatic rapid passage in semiconductor quantum dots is studied theoretically. While in an ideal system adiabatic rapid passage results in a full inversion of the quantum dot occupation, phonons hamper this behavior drastically. We show that the transitions between the adiabatic states lead to a temperature-dependent decrease of the final exciton occupation. In contrast to the ideal evolution, the phonon-related perturbation induces dependencies on the pulse power and on the sign of the chirp.Recent experiments 1,2 have confirmed the theoretical predictions 3 that preparation of the state of an exciton confined in a quantum dot (QD) by a chirped (frequency-swept) laser pulse, referred to as adiabatic rapid passage (ARP), leads to high-fidelity control of the QD occupation. Unlike the resonant Rabi rotation method 4 or its generalizations based on voltage control, 5,6 where the final state critically depends on the excitation or control conditions, the ARP technique guarantees that the dot will be occupied by an exciton as soon as the pulse intensity exceeds the threshold for the adiabatic passage. In the ideal case, the system state during the pulsed excitation follows the adiabatic eigenstates across the anticrossing, which leads to occupation swapping between the empty dot and exciton states.On the other hand, theoretical analysis of charge dynamics in QDs has led to the conclusion that the fidelity of quantum control is limited by phonon-induced dephasing. 7-13 The essential role of phonons in the dephasing of optically driven confined excitons has indeed been confirmed experimentally. 14,15 One can, therefore, expect that the carrier-phonon coupling will also limit the fidelity of control achievable in the ARP control scheme. One decoherence channel that may play a role is related to transitions between the adiabatic branches, 16 which can be expected to introduce the temperature-dependent difference between the two directions of the frequency sweeping, as they correspond to the evolution along the energetically higher or lower branch of the adiabatic spectrum. Recently, effects induced by coupling to longitudinal optical phonons have been analyzed in the range of very short and strongly chirped pulses. 17 However, in the parameter range relevant to the experiments, 1,2 where the spectral characteristics of the pulses exclude excitation of high-energy optical modes, the coupling to acoustic phonons can be expected to be the main source of decoherence.In this Rapid Communication, we study the evolution of the exciton system in a QD driven by a chirped pulse in the regime of ARP in the presence of the coupling to acoustic phonons. We show that at low temperatures the phonon-induced decoherence leads to a strong asymmetry in the final exciton occupation depending on the sign of the chirp and to a dependence on the pulse power, resulting in an optimum for pulse areas in the range between about π and 2π , as indeed observed in the experiment. 1 We model the QD in the strong confinement lim...

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We present a quantum-kinetic theory of the excitation transfer in a quantum dot molecule. We derive the consistent Markovian limit for the system kinetics, which leads to a description in terms of a single transfer rate for weak coupling. We show that the transfer rate is a strongly varying, nonmonotonic function of the spatial separation and energy mismatch between the dots.

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We study experimentally and theoretically polarization-dependent luminescence from an ensemble of quantum-dot-like nanostructures with a very large in-plane shape anisotropy (quantum dashes). We show that the measured degree of linear polarization of the emitted light increases with the excitation power and changes with temperature in a non-trivial way, depending on the excitation conditions. Using an approximate model based on the k.p theory, we are able to relate this degree of polarization to the amount of light hole admixture in the exciton states which, in turn, depends on the symmetry of the envelope wave function. Agreement between the measured properties and theory is reached under assumption that the ground exciton state in a quantum dash is trapped in a confinement fluctuation within the structure and thus localized in a much smaller volume of much lower asymmetry than the entire nanostructure.Comment: 13 pages, 9 figures; considerably extended, additional discussion and new figures include