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Physics

Crystal Vibrations Invert Quantum Dot Exciton

Stephen Hughes, H. J. Carmichael

Published Mar 30, 2015• 6 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 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.

DOI
10.1103/physics.8.29

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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 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 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.
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.
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.
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.
Introduction Quantum dots are often referred to as artificial atoms, since they trap carriers in discrete energy-levels due to the nanoscale three-dimensional finite potential energy well they provide. As such, dots exhibit a coherent light–matter interaction that is similar to an atom. This is evidenced by observations of atom–optics phenomena such as Rabi oscillations [43, 26], power broadening [27], Autler–Townes doublet [14, 42], Mollow triplet [42, 8], and coherent population trapping [6]. In this chapter, Rabi rotation measurements are used to examine how an exciton transition deviates from an ideal two-level atom due to its interaction with a reservoir of phonons. The neutral exciton transition may be regarded as a two-level system, or qubit, composed of the crystal ground-state ∣0〉 and a single electron-hole pair ∣ X 〉. The state-vector of a qubit can be described as a pseudo spin-half. When an oscillating electro-magnetic field resonantly excites the two-level transition it drives an oscillation in the population inversion known as a Rabi oscillation. This results from the oscillations of the driving field and the dipole of the two-level system being synchronous, such that in its rotating frame, the driving field acts as a static magnetic field that causes the pseudo-spin to rotate. Coherent control of the pseudo-spin can be achieved by applying well-defined driving fields, enabling the preparation, and manipulation of superposition states. Such coherent control concepts have found widespread use in electron spin, and nuclear magnetic resonance spectroscopy.