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International Conference on Solid State Devices and Materials (SSDM)

Exciton Spin Stability in InP Quantum Dots at the Elevated Temperatures

Yasuaki Masumoto, Takashi Suzuki, K. Kawana, Michio Ikezawa

Published Sep 25, 2008• 0 citations
Publisher

Abstract

Exciton spin stability in InP quantum dots at the elevated temperatures is studied by measuring resonant spin orientation of excitons. With the increase of temperature, spin relaxation time becomes shorter and resonant spin orientation of excitons is reduced. It is 12 times longer than the lifetime of 260 ps at 5 K but is comparable with the lifetime around 50 K. The amplitude of the resonant spin orientation structure changes with the change of the excitation photon energy, reflecting the competition between the spin relaxation through the energy relaxation and the energy relaxation. Spin relaxation time through the energy relaxation is comparable with phonon-mediated relaxation time and is much shorter than the spin relaxation time within the bright and dark exciton states.

DOI
10.7567/ssdm.2008.p-12-3

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Exciton spin stability in InP quantum dots at the elevated temperatures is studied by measuring resonant spin orientation of excitons. With the increase of temperature, spin relaxation time becomes shorter and resonant spin orientation of excitons is reduced. It is 12 times longer than the lifetime of 260 ps at 5 K but is comparable with the lifetime around 50 K. The amplitude of the resonant spin orientation structure changes with the change of the excitation photon energy, reflecting the competition between the spin relaxation through the energy relaxation and the energy relaxation. Spin relaxation time through the energy relaxation is comparable with phonon-mediated relaxation time and is much shorter than the spin relaxation time within the bright and dark exciton states.
The dynamics of single photon emitters based on InP/(Ga,In)P quantum dots (QDs) has been studied by time‐resolved photo luminescence (TRPL) and photon correlation spectroscopy (PCS) up to 50 K. The increase of temperature produces marked effects on the exciton photoluminescence (PL) decay time (τX) and on the anti‐bunching time (τR) in the second order photon correlation function. Both times are found to depend on the QD size. A competition of the thermally activated Dark‐to‐Bright (D→B) exciton transition and the thermal excitation of the carriers to‐ and from the QD give a good qualitative understanding of the observed results. Resonant excitation at the QD p‐states produces a marked decrease of τR together with the appearance of time bunching in a longer time scale.
Exciton spin and related optical polarization in self-assembled InAs/In0.53Ga0.23Al0.24As/InP(001) quantum dashes emitting at 1.55 μm are investigated by means of polarization- and time-resolved photoluminescence, as well as photoluminescence excitation spectroscopy, at cryogenic temperature. We investigate the influence of highly non-resonant and quasi-resonant optical spin pumping conditions on spin polarization and spin memory of the quantum dash ground state. We show that a spin pumping scheme, utilizing the longitudinal-optical-phonon-mediated coherent scattering process, can lead to the polarization degree above 50%. We discuss the role of intrinsic asymmetries in the quantum dash that influence values of the degree of polarization and its time evolution.
The dark exciton state strongly affects the optical and quantum optical properties of flat InP/GaInP quantum dots. The exciton intensity drops sharply compared to the biexciton with rising pulsed laser excitation power while the opposite is true with temperature. Also, the decay rate is faster for the exciton than the biexciton and the dark-to-bright state spin flip is enhanced with temperature. Furthermore, long-lived dark state related memory effects are observed in second-order cross-correlation measurements between the exciton and biexciton and have been simulated using a rate-equation model.
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.
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.
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.