Ref HAL: hal-00465011_v1
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We discuss the decoherence dynamics in a single semiconductor quantum dot and analyze two dephasing mech- anisms. In the ¯rst part of the review, we examine the intrinsic source of dephasing provided by the coupling to acoustic phonons. We show that the non-perturbative reaction of the lattice to the interband optical transition results in a composite optical spectrum with a central zero-phonon line and lateral side-bands. In fact, these acoustic phonon side-bands completely dominate the quantum dot optical response at room temperature. In the second part of the paper, we focus on the extrinsic dephasing mechanism of spectral di®usion that determines the quantum dot decoherence at low temperatures. We interpret the variations of both width and shape of the zero- phonon line as due to the °uctuating electrostatic environment. In particular, we demonstrate the existence of a motional narrowing regime in the limit of low incident power or low temperature, thus revealing an unconventional phenomenology compared to nuclear magnetic resonance.

Ref HAL: hal-00284909_v1
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We report systematic measurements on the broadening of the emission spectrum of single quantum dots as a function of temperature and incident power. Spectral diffusion effects in the motional narrowing regime provide a quantitative interpretation of our experimental results. We show that, at low incident power, the thermal activation of spectral diffusion results in a Lorentzian zero-phonon line with a width that increases linearly with temperature. Our study provides a unified interpretation to the widely debated issue of the dispersion of the data on the temperature dependence of this zero-phonon linewidth. Our explanation is based on an original model where acoustic phonons interact with carriers outside the quantum dot.

Ref HAL: hal-00105812_v1
Ref Arxiv: cond-mat/0610346
DOI: 10.1038/nphys433
WoS: 000242477600014
Ref. & Cit.: NASA ADS
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180 Citations
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Motional narrowing refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower when increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance since the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance.

Ref HAL: tel-00011932_v1
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Le modèle de l'atome artificiel est l'image physique intuitive qui découle de la discrétisation du spectre énergétique des électrons, qui sont confinés dans les trois directions de l'espace dans une boîte quantique de semiconducteurs. Cette analogie avec les systèmes atomiques s'est révélée commode et fructueuse pour étudier les propriétés électroniques et optiques des boîtes quantiques de semiconducteurs. Elle a conduit à des expériences élégantes qui utilisent les concepts de base de la physique quantique de systèmes élémentaires et qui montrent l'intérêt des boîtes quantiques pour l'information quantique. Ces expériences ont cependant toutes en commun d'utiliser des boîtes quantiques à basse température et les mesures de spectroscopie optique sont faites sur l'état excitonique fondamental de la boîte quantique. Cette constatation lève d'emblée le problème des limites de validité du modèle de l'atome artificiel dont l'utilisation, certes fertile, semble pourtant se resteindre à des conditions expérimentales très précises. Dans ce document, nous allons aborder plus généralement l'étude des propriétés électroniques et optiques de boîtes quantiques dans le système modèle de nanostructures auto-organisées InAs/GaAs afin de cerner les limites de validité du modèle de l'atome artificiel.

Ref HAL: hal-00546632_v1
DOI: 10.1063/1.1854733
WoS: 000226761400021
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34 Citations
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We present experimental evidence of giant optical anisotropy in single InAs quantum dots. Polarization-resolved photoluminescence spectroscopy reveals a linear polarization ratio with huge fluctuations, from one quantum dot to another, in sign and in magnitude with absolute values up to 82%. Systematic measurements on hundreds of quantum dots coming from two different laboratories demonstrate that the giant optical anisotropy is an intrinsic feature of dilute quantum-dot arrays. (C) 2005 American Institute of Physics.

Ref HAL: hal-00546623_v1
DOI: 10.1103/PhysRevB.71.233304
WoS: 000230276800016
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43 Citations
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Exciton spin relaxation is investigated in single epitaxially grown semiconductor quantum dots in order to test the expected spin relaxation quenching in this system. We study the polarization anisotropy of the photoluminescence signal emitted by isolated quantum dots under steady-state or pulsed nonresonant excitation. We find that the longitudinal exciton spin relaxation time is strikingly short (<= 100 ps) even at low temperature. This result breaks down the picture of a frozen exciton spin in quantum dots.

Ref HAL: hal-00018428_v1
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Carrier dynamics is investigated in both luminescent and non luminescent samples of single wall carbon nanotubes -obtained by laser ablation- by means of two-color pump-probe experiments. The recombination dynamics is monitored by probing the transient photobleaching observed on the interband transitions of semi-conducting nanotubes. Interband and inter-subband relaxation times are about one order of magnitude slower in isolated nanotubes than in ropes of nanotubes bringing evidence of the environment influence on the carrier dynamics. The relaxation dynamics is non-exponential and is interpreted as a consequence of the inhomogeneity of the sample. Slow components up to 250 ps are measured which is significantly greater than values previoulsy reported in HiPCo nanotubes. These observations show the great dependence of the electronic properties of carbon nanotubes on the synthesis method and on their environment.