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arXiv:0801.0571 · Physical Review B

Optical alignment and polarization conversion of the neutral-exciton spin in individual InAs∕GaAs quantum dots

K. Kowalik, O. Krebs, Aristide Lemaître, J. A. Gaj, Paul H. Voisin

Published Apr 28, 2008• 37 citations
arXivHTMLPDF

Abstract

We investigate exciton spin memory in individual $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ self-assembled quantum dots via optical alignment and conversion of exciton polarization in a magnetic field. Quasiresonant phonon-assisted excitation is successfully employed to define the initial spin polarization of neutral excitons. The conservation of the linear polarization generated along the bright exciton eigenaxes of up to 90% and the conversion from circular to linear polarization of up to 47% both demonstrate a very long spin relaxation time with respect to the radiative lifetime. Results are quantitatively compared to a model of pseudospin $1∕2$ including heavy-to-light hole mixing.

DOI
10.1103/physrevb.77.161305

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We investigate exciton spin memory in individual InAs∕GaAs\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}InAs∕GaAs self-assembled quantum dots via optical alignment and conversion of exciton polarization in a magnetic field. Quasiresonant phonon-assisted excitation is successfully employed to define the initial spin polarization of neutral excitons. The conservation of the linear polarization generated along the bright exciton eigenaxes of up to 90% and the conversion from circular to linear polarization of up to 47% both demonstrate a very long spin relaxation time with respect to the radiative lifetime. Results are quantitatively compared to a model of pseudospin 1∕21∕21∕2 including heavy-to-light hole mixing.
We report quantum beat phenomena between different polarization states in InAs quantum dots, observed in time-resolved photoluminescence and transient dichroism measurements. The polarization of the emitted light can be changed from linear to circular by applying a small magnetic field. Following quasiresonant excitation the electron and hole spin states remain stable during the exciton lifetime, independent of the applied magnetic field. We show that the exciton spin coherence is partially preserved in the energy relaxation process. These experiments enable us to measure the anisotropic exchange energy $\ensuremath{\Delta}{E}_{XY}$ and the exciton ggg factor. We find $\ensuremath{\Delta}{E}_{XY}\ensuremath{\simeq}30\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}$ and $g\ensuremath{\simeq}2.5$ for the quantum dot excitons.
Abstract We have studied the influence of an external electric field on the optical spin polarization of initially negatively charged InAs/GaAs quantum dots (QDs). The spin polarization of the carriers was controlled by measuring the photoluminescence (PL) circular polarization. We have found that the PL polarization of the QD is negative under an excitation above the GaAs barrier, and that it becomes positive under an excitation below the barrier. An applied negative electric field destroys both positive and negative polariza‐tion. The time decay of the negative polarization was found from time‐resolved measurements to exceed strongly the QD radiative lifetime. This fact indicates a long‐living spin memory in the dots under study.
We investigate the coherent exciton spin dynamics in a single InAlAs/AlGaAs quantum dot using time-resolved quantum state tomography. Under two-LO-phonon quasi-resonant excitation of neutral exciton, we observe pronounced quantum beats in the circular and diagonal polarization components, reflecting the fine-structure splitting (Δ≈19.6 μeVΔ\approx 19.6 \ μ\text{eV}Δ≈19.6 μeV). By employing a global fitting procedure across three orthogonal polarization bases, we demonstrate that the spin evolution is consistently described by a unified Hamiltonian dominated by the anisotropic exchange interaction. While the initial degree of circular polarization is limited to ≈0.28\approx 0.28≈0.28 due to fast relaxation processes during carrier cooling, the subsequent dynamics reveal a long-lived spin coherence (1.1±0.21.1 \pm 0.21.1±0.2 ns) that exceeds the exciton lifetime (∼767\sim 767∼767 ps). Our analysis reveals that the spin-formation time is significantly shorter than the instrument response function, and the absence of a discernible Overhauser shift confirms a negligible influence from the local nuclear environment under the present conditions. These results provide a quantitative benchmark for the three-dimensional reconstruction of spin trajectories using differential polarization signals, demonstrating the feasibility of using quasi-resonant excitation for stable spin initialization in semiconductor nanostructures.
Unambiguous experimental evidence for a significant difference in efficiency of excitonic vs. free carrier spin injection is provided in novel laterally arranged self-assembled InAs/GaAs quantum dot structures, from optical orientation and tunable laser spectroscopy. A lower efficiency of exciton spin injection as compared to free carrier spin injection from wetting layers into QDs results in a distinct feature in luminescence polarization of the QDs as a function of excitation photon energy. It is shown that this difference is not related to carrier density and state-filling effects arising from the difference in optical absorption efficiency between the excitons and free carriers. Rather, it is a genuine property for exciton spin injection that suffers stronger spin relaxation due to Coulomb exchange interaction.
We show storage of the circular polarization of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently read out through the electronically-triggered emission of a single photon. The emitted photon shares the same polarization as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 T, spin memory is preserved for at least 1000 times more than the exciton's radiative lifetime.