Polarization Spectroscopy of Charged Single Self-Assembled Quantum Dots
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Polarization Spectroscopy of Charged Single Self-Assembled Quantum Dots Morgan E. Ware1, Allan Bracker1, Daniel Gammon1, David Gershoni2 1 Naval Research Laboratory, Washington, DC 20375 2 Technion-Israel Institute of Technology, Haifa, 32000, Israel ABSTRACT We have demonstrated single dot spectroscopy of InAs/GaAs self-assembled quantum dots embedded in a bias controlled Schottky diode. The photoluminescence spectra exhibit discrete lines depending on bias, which we attribute to the recombination of positively charged, neutral, and negatively charged confined excitons. With excitation directly into the dot, large circular polarization memory is exhibited by the two charged exciton (trion) lines. This indicates long spin lifetimes for both the electron and the heavy hole in the quantum dots. INTRODUCTION Semiconductor quantum dots have been viewed in recent years as attractive components for implementing quantum computation schemes [1]. In particular, the unpaired spin of an electronically charged semiconductor quantum dot is especially exciting, because of its relatively long spin dephasing time, and because its spin can be controlled and measured optically [2]. Knowledge of the discrete energy spectrum of these dots and the ability to control the individual charges in the dots is vital for future progress in this field. Several groups have succeeded in identifying charging in single dot photoluminescence (PL) by high power excitation [3,4] or by controlling the bias across a field effect device containing quantum dots [5,6,7]. In order to control the charge on a dot at lower excitation power, in the single exciton regime, it is more useful to embed the dots in a gated structure. Regelman, et. al., demonstrated both hole and electron charging in high power excitation of InAs dots [3]. Ashmore, et. al., were able to make separate devices on n and p-type substrates which formed negatively and positively charged excitons respectively in In0.5Ga0.5As dots under optical excitation [6]. Both groups demonstrated that compared to the neutral exciton the positive exciton is blueshifted by ~2 meV and the negative exciton is redshifted by ~5 meV. In addition to being able to controllably charge the dot, a requirement for quantum information processing is the ability to control and measure the spin of the charges in the dot [1]. This is accomplished with circularly polarized light which through selection rules couples to certain spin states. In particular, if one defines the quantization axis as that of the incident photon, a spin up heavy hole and spin down electron pair is photogenerated by a right circularly polarized photon (σ+), and the opposite spins pair is photogenerated by a left circularly polarized photon, (σ–) [8]. The reverse process, recombination, involving these same spin polarized pairs will in turn result in the creation of a σ+ and σ– photon, respectively. In isolated quantum dots, recombination of single pairs can be readily identified spectrally [9]. This allows one to probe the spin
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