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physica status solidi (b)

Optical spin polarization in negatively charged InAs self‐assembled quantum dots under applied electric field

Vladimir K. Kalevich, Michio Ikezawa, Tsuyoshi Okuno, A. Yu. Shiryaev, A. E. Zhukov, V. M. Ustinov, P. N. Brunkov, Yasuaki Masumoto

Published Jun 25, 2003• 10 citations

Abstract

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.

DOI: 10.1002/pssb.200303021

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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.

In photoluminescence kinetics of the InP quantum dots, we have found a long-lived component of the degree of circular polarization, showing practically no decay during the lifetime of excitation. It is shown that this effect is observed only in the charged quantum dots. The amplitude of the long-lived PL polarization can be controlled by an applied electric field and may be as high as 50% in the case of quasi resonance excitation. The existence of the long-lived component of the polarization is the clear evidence of large lifetime of the electron spin polarization in quantum dots.

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.

We demonstrate the all-optical ultrafast manipulation and read-out of optical transitions in a single negatively charged self-assembled InAs quantum dot, an important step towards ultrafast control of the resident spin. Experiments performed at zero magnetic field show the excitation and decay of the trion (negatively charged exciton) as well as Rabi oscillations between the electron and trion states. Application of a DC magnetic field perpendicular to the growth axis of the dot enables observation of a complex quantum beat structure produced by independent precession of the ground state electron and the excited state heavy hole spins.

By using nonlinear resonant optical control and polarized photoluminescence, the authors studied the intensity dependence of exciton spin relaxation and photoluminescence polarization in single self-assembled InGaAs quantum dots. Data from dots with different dipole moments reveal two distinctive channels for polarization memory loss: (i) an external pathway due to carrier escape and capture to and from the wetting layer that is responsible for memory loss increasing with intensity and (ii) an internal loss channel due to intrinsic spin relaxation. The values obtained rule out a universal freezing of exciton spin relaxation in single self-assembled quantum dots related to the wetting layer.