Spin decoherence and relaxation processes in zero-dimensional semiconductor nanostructures

International audienceIn recent years, interest in spin physics has been renewed due to its potential application in spintronics and quantum information. In these frameworks, the main required property is the presence of long spin memory. We present a short review of recent results concerning spin decoherence and relaxation processes in zero-dimensional (0D) nanostructures, especially in quantum dots

Nuclear field effect on the spin dynamics of electron localized on a donor in a single quantum well

International audienceWe use photoinduced Faraday rotation (PFR) in presence of an applied magnetic field to study the spin dynamics of localized electrons. The sample is a CdTe/CdMgTe quantum well (QW) of width 80 angstrom containing a layer of iodine donors at its center, with, concentration 10(11) cm(-2). The spin polarization of donor bound electrons is built via the optical polarization of donor-bound excitons, their hole spin relaxation, and their recombination. In a transverse (in-plane) magnetic field, PFR shows damped Larmor oscillations from which we deduce a 18 ns electron-spin decoherence time, and a transverse Lande factor of 1.29. In addition, for oblique optical incidence the electron-nuclei hyperfine interaction builds a nuclear spin polarization in presence of polarized electrons. This leads to the construction of an effective magnetic field, the Overhauser field, acting on the electronic spins. The Larmor frequency is then different for sigma+ or sigma- polarizations of the exciting light. The dependence of the phenomenon on the optical incidence allows the determination of the maximal Overhauser field, which is about 10mT, at least two orders of magnitude weaker than for III-V semiconductors. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Anisotropic spin splitting of the electron ground state in InAs quantum dots

International audiencePhotoinduced circular dichroism experiments in an oblique magnetic field allow measurements of Larmor precession frequencies, and so give a precise determination of the electron Lande g factor and its anisotropy in self-assembled InAs/GaAs quantum dots emitting at 1.32 eV. In good agreement with recent theoretical results, the authors measure parallel to g(perpendicular to)parallel to=0.397 +/- 0.003 and parallel to g(parallel to)parallel to=0.18 +/- 0.02. (c) 2007 American Institute of Physics

The so-called ``Trion line'': A singular many-body effect between one composite exciton and many electrons

International audienceAlthough commonly called ``trion line'', the low-energy line observed in the absorption spectrum of doped quantum wells, is not due to a bound 3-body object, the trion. In fact, it comes from a many-body object, spectrally wide by nature, made of one composite exciton and a large number of electrons, interacting through both Coulomb and Pauli scatterings, i.e., Coulomb interaction without carriers exchange, and carriers exchange without Coulomb process. This understanding is experimentally supported by the highly asymmetrical shape of the circular dichroism spectrum measured with a spin-polarized electron gas: this spectrum shows a sharp edge at low energy and a long tail a high energy, which are well explained by a singular many-body effect, while they completely rule out an explanation in terms of trion, either free or bound

Energy dependence of the linear and dynamical photo-induced dichroisms of InAs/GaAs quantum dots

International audienceWe present pump-probe measurements of the photo-induced dynamics of the lowest electron-hole (e-h) transitions in ensembles of InAs/GaAs quantum dots, as a function of interband emission energy 1.30-1.40 eV. We used two different beam-polarization configurations to obtain information on the electronic structure of both e-h states, and on their relaxation and decoherence. In linear dichroism experiments, we address the population evolution of the e-h states: we measured two different e-h radiative times, with almost constant mean value 1 ns and ratio 0.8-0.9. In dynamical dichroism experiments, we observe the evolution of the coherence between the e-h states: we deduced an inhomogeneous decoherence time 10-25 ps, and an energy splitting monotonically decreasing with emission energy, from 80 to 20 mu eV

Two time scales of the electron-hole spin relaxation in InAs/GaAS quantum dots

4th International Conference on Semiconductor Quantum Dots (QD2006), Chamonix Mont Blanc, FRANCE, MAY 01-05, 2006International audienceWe have performed time-resolved differential transmission on an ensemble of InAs/GaAs self-assembled quantum dots. We have calculated the polarization rate decay, on which two time scales in the electron-hole spin relaxation dynamics are observed. A study versus the energy of the interband transition reveals that the fast spin relaxation becomes important when excited and mixed states are involved. (C) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Energy dependence of the electron-hole in-plane anisotropy in InAs/GaAs quantum dots

International audienceIn pump-probe experiments, we measured the anisotropic exchange energy splitting of the low-lying electron-hole pair states in an ensemble of as-grown InAs/GaAs self-assembled quantum dots. This splitting shows a monotonic decrease with the energy of interband emission. We discuss this result considering different parameters: shape anisotropy, wavefunction confinement, size of the quantum dots, and piezoelectric effect. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Energy dependence of the electron-hole in-plane anisotropy in InAs/GaAs quantum dots (vol 3, pg 3900, 2006)

International audienceIn pump-probe experiments, we measured the anisotropic exchange energy splitting of the low-lying electron-hole pair states in an ensemble of as-gown InAs/GaAs self-assembled quantum dots. This splitting shows a monotonic decrease with the energy of interband emission. We discuss this result considering different parameters: shape anisotropy, wavefunction confinement, size of the quantum dots, and piezoelectric effect. (c) 2007 WILEY-VCH Vertag GmbH & Co. KGaA, Weinheim

Linear and dynamical photoinduced dichroisms of InAs/GaAs self-assembled quantum dots: Population relaxation and decoherence measurements

International audienceWe present and model our pump-probe experiments measuring the photoinduced dynamics of an ensemble of self-assembled InAs/GaAs quantum dots. A pulsed pump beam, linearly polarized along an in-plane symmetry axis of the quantum dots, photoinduces a linear dichroism. We show that the dynamics of this linear dichroism is consistent with a long spin relaxation time and allows us to measure different radiative lifetimes for both nondegenerate, low-lying, electron-hole pair states. In another experimental configuration, when the pump beam creates a coherent superposition of these electron-hole pair states, it photoinduces a dynamical dichroism, which gives information about quantum decoherence and provides the electron-hole anisotropic exchange interaction energy splitting, 41 +/- 8 mu eV at 1.343 eV. A model is developed, which accounts for the dynamical dichroism and predicts the experimental observations with good accuracy