Optical anisotropy and photoluminescence polarization in single InAlAs quantum dots

Abstract

We have investigated the optical anisotropy in individual self-assembled quantum dots. The linear polarization analysis of the positive trion photoluminescence reveals the effect of the strain-induced valence band mixing since the positive trion has the spin-paired holes and therefore exchange interaction has no influence. Meanwhile, the neutral exciton indicates the complex polarization states due to both the in-plain asymmetries of the dot shape and the strain distributions. The experimental and theoretical polarization analysis has been performed for tens of InAlAs quantum dots and the correlation between the important parameters was investigated. © 2011 American Institute of Physics. ͓doi:10.1063/1.3583453͔Studies on localized spins in semiconductor quantum dots ͑QDs͒ have been attracting considerable interest. This is because the discrete electronic levels involved in the optical transitions serve the fascinating applications in which QDs are used as emitters of single, indistinguishable, and entangled photons.1 For these applications, it is crucial to study the polarization of the emitted photons associated with exciton annihilation. For ideal QDs as artificial atoms, the relevant eigenstates are bright excitons with the angular momentum of Ϯ1, and the circularly polarized photons ͑ + or − ͒ are to be absorbed ͑emitted͒ to ͑from͒ the eigenstates. However, actual QDs have the anisotropic distributions of shape and strain, and as a result, the confinement potential symmetry is reduced from D 2d to C 2v or lower. It is well known that the shape anisotropy induces the change in the emission polarization as well as the level splitting as an exciton fine structure.2-5 Also, QDs formed by self-assembly in the Stransky-Krastanov ͑SK͒ growth mode is believed to have a large strain originating from the QD formation process, and the strain with the anisotropic distribution more or less remains inside a QD even after QD formation is complete. Consequently, the emission polarization is affected by the anisotropic exchange interaction ͑AEI͒ and the straininduced valence-band mixing ͑SI-VBM͒. The former originates from the QD shape asymmetry and the latter comes mainly from the in-plane anisotropic relaxation of strains. Accordingly, the polarization of the QD emissions is one of valuable probes for the origin of the QD symmetry lowering. Since the strain anisotropy may be different largely from QD to QD, the investigation of the polarization is necessary for the individual QDs.In this work, we investigated the polarization of the photoluminescence ͑PL͒ in single InAlAs QDs in order to probe the interactions that reduce the QD potential symmetry. By comparing the positive trion PL with the neutral exciton PL in the same single QD, the intrinsic parameters for AEI and SI-VBM were deduced. Since III-V semiconductor QDs generally have a smaller strain anisotropy than that in II-VI QDs, 6,7 the precise estimation of the composite of AEI and SI-VBM in III-V semiconductor QDs have not been reported so far. The simple method pr...

Valence-band-ordering of a strain-free bulk ZnO single crystal identified by four-wave-mixing spectroscopy technique

“…45 This phenomenon has been found in GaN epilayers grown on foreign substrates suffering from in-plane uniaxial compressive stress [39][40][41][42] and also in self-assembled GaN quantum dots, which suffer from anisotropic stresses. 46,47 In order to verify if the present ZnO sample is strain-free (even locally), the strain anisotropy analysis is carried out with the aid of the high sensitivity of FWM to exciton polarizations: the FWM response is proportional to the fourth power of the transition oscillator strength jlj 2 . Indeed, this advantage plays an important role in detecting strain anisotropy even for the epilayers grown on isotropic substrates and bulk samples, in which only negligible strain anisotropy is expected.…”

Polarization properties of excitonic qubits in single self-assembled quantum dots

“…Indeed, in II-VI CdTe/ZnTe 12,13 or III-V InAlAs/AlGaAs 14 QDs, VBM has been related to the strain anisotropy induced during growth process. In strain-free GaAs QDs grown by droplet epitaxy it has been shown that VBM exists due to the anisotropy of the confinement potential, 15 while in strained self-assembled InAs/GaAs QDs, VBM effects have been investigated in order to estimate the exciton spin relaxation time, 16 its influence on the exciton FSS, 17,18 or the hyperfine coupling.…”

Polarization anisotropic luminescence of tunable single lateral quantum dot molecules

“…More importantly, even in the case where there exists an observable polarization anisotropy, no distinct and reproducible directionality is observed, and the angle between the polarization components can differ from 90 , which is in good agreement with recent theoretical considerations and experimental reports. 24,31,32 AEI and VBM could well explain the observed small polarization of the single QD PL under the assumption of a much weaker electron-light hole transition compared to the electron-heavy hole transition. However, these contributions alone cannot explain the generally higher polarization of the lateral QDM PL.…”

“…[3][4][5] Strain-induced anisotropic exchange interaction (AEI) and valence-band mixing (VBM) might have an impact on the exciton FS, even for highly symmetric single QDs and QDMs, and are recently also discussed as possible sources for polarization anisotropy. 23,24 Especially the symmetry of QD structures and their confinement potentials, as well as their impact on the polarization of the related excitonic recombination, are important topics in current theoretical and experimental studies. 25,26 In this report, we investigate the polarization properties of single photons emitted from lateral InGaAs/GaAs QDMs manipulated by an electric field.…”