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Physical Review Letters

Phonon-Assisted Population Inversion of a Single InGaAs / GaAs Quantum Dot by Pulsed Laser Excitation

John H. Quilter, Alistair J. Brash, Feng Liu, Martin Glässl, A. M. Barth, V. M. Axt, A. J. Ramsay, Maurice S. Skolnick, A. M. Fox

Published Mar 30, 2015• 165 citations
Publisher

Abstract

We demonstrate a new method to realize the population inversion of a single InGaAs/GaAs quantum dot excited by a laser pulse tuned within the neutral exciton phonon sideband. In contrast to the conventional method of inverting a two-level system by performing coherent Rabi oscillation, the inversion is achieved by rapid thermalization of the optically dressed states via incoherent phonon-assisted relaxation. A maximum exciton population of 0.67±0.06 is measured for a laser tuned 0.83 meV to higher energy. Furthermore, the phonon sideband is mapped using a two-color pump-probe technique, with its spectral form and magnitude in very good agreement with the result of path-integral calculations.

DOI
10.1103/physrevlett.114.137401

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We demonstrate a new method to realize the population inversion of a single InGaAs/GaAs quantum dot excited by a laser pulse tuned within the neutral exciton phonon sideband. In contrast to the conventional method of inverting a two-level system by performing coherent Rabi oscillation, the inversion is achieved by rapid thermalization of the optically dressed states via incoherent phonon-assisted relaxation. A maximum exciton population of 0.67±0.06 is measured for a laser tuned 0.83 meV to higher energy. Furthermore, the phonon sideband is mapped using a two-color pump-probe technique, with its spectral form and magnitude in very good agreement with the result of path-integral calculations.
We demonstrate an approach to realize the population inversion of a single InGaAs/GaAs quantum dot, which is driven by a laser pulse tuned within the neutral exciton phonon sideband. The inversion is achieved by rapid thermalization of the optically dressed states via phonon-assisted relaxation. A maximum exciton population of 0.67 ±\pm± 0.06 is measured for a laser tuned 0.83 meV to higher energy and the phonon sideband is mapped using a two-color pump-probe technique. Our experiments reveal that, in accordance with theory, the phonon-bath provides additional functionality for an optically driven quantum dot qubit.
We demonstrate a new method to realize the population inversion of a single InGaAs=GaAs quantum dot excited by a laser pulse tuned within the neutral exciton phonon sideband.In contrast to the conventional method of inverting a two-level system by performing coherent Rabi oscillation, the inversion is achieved by rapid thermalization of the optically dressed states via incoherent phonon-assisted relaxation.A maximum exciton population of 0.67 Æ 0.06 is measured for a laser tuned 0.83 meV to higher energy.Furthermore, the phonon sideband is mapped using a two-color pump-probe technique, with its spectral form and magnitude in very good agreement with the result of path-integral calculations.
We demonstrate ultrafast incoherent depopulation of a quantum dot from above to below the transparency point using LA-phonon-assisted emission stimulated by a redshifted laser pulse. The QD is turned from a weakly vibronic system into a strongly vibronic one by laser driving which enables the phonon-assisted relaxation between the excitonic components of two dressed states. The depopulation is achieved within a laser pulse-width-limited time of 20 ps and exhibits a broad tuning range of a few meV. Our experimental results are well reproduced by path-integral calculations.
In the present study, we apply shortcut to adiabaticity pulses (time-dependent Rabi frequency and detuning) for the efficient population transfer from the ground to the exciton state in a GaAs/InGaAs quantum dot with phonon-induced dephasing. We use the time-evolving matrix product operator (TEMPO) method to propagate system in time and find that, for temperatures below $ 20 \ \text{K} $ and pulse duration up to $ 10 \ \text{ps} $, a very good transfer efficiency is obtained in general. We explain these results using a Bloch-like equation derived from a generalized Lindblad equation, which adequately describes system dynamics at lower temperatures. For higher temperatures, the transfer efficiency is significantly reduced except for subpicosecond pulses, where the shortcut Rabi frequency reduces to a delta pulse attaining a fast population inversion. The present work is expected to find application in quantum technologies which exploit quantum dots for single-photon generation on demand.
This thesis presents experiments carried out into the coherent properties of charge carriers confined within a single InGaAs/GaAs quantum dot. Specifically the ground state - single exciton qubit system is controlled with ultrafast optical pulses and then the population of the dot is measured by a high-sensitivity photocurrent detection technique. There are two principal results of these experiments, firstly by modulating the bias applied to the quantum dot diode, the detection efficiency of the photocurrent measurement technique is enhanced. The enhancement is possible as the modulated bias allows fast switching of the voltage-dependent carrier tunneling rates. Secondly, when the transition is addressed by an intense, positively detuned laser, the qubit system can be driven to a population inversion. This occurs due to phonon-assisted relaxation of the optical dressed states of the system. Here, the rapid thermalization due to the exciton-phonon interaction actually enables the population inversion, instead of simply being a source of decoherence.
Abstract A major challenge in generating single photons with a single emitter is to excite the emitter while avoiding laser leakage into the collection path. Ideally, any scheme to suppress this leakage should not result in a loss in the efficiency of the single-photon source. Here, we investigate a scheme in which a single emitter, a semiconductor quantum dot, is embedded in a microcavity. The scheme exploits the splitting of the cavity mode into two orthogonally-polarised modes: one mode is used for excitation, the other for collection. By linking the experiment to theory, we show that the best population inversion is achieved with a laser pulse detuned from the quantum emitter. The Rabi oscillations exhibit an unusual dependence on pulse power. Our theory describes them quantitatively, enabling us to determine the absolute population inversion. By comparing the experimental results with our theoretical model, we determine a population inversion of 98 % − 5 % + 1 % for optimal laser detuning. The Rabi oscillations depend on the sign of the laser-pulse detuning, a phenomenon arising from the non-trivial effect of phonons on the exciton dynamics. The exciton–phonon interaction is included in the theory and gives excellent agreement with all the experimental results.