Magneto-Optical Spectroscopy of Charge-Tunable Quantum Dots


Haruki Sanada, Tetsuomi Sogawa, Hideki Gotoh,
Yasuhiro Tokura, and Hidehiko Kamada
Optical Science Laboratory

 Spin states in semiconductor nanostructures are expected to act as quantum information carriers in solid-state systems. One approach that allows us to access single electron spin in semiconductor quantum dots (QDs) is to use negative trions, which serve as an intermediary for the initialization [1] and readout [2] of single electron spins. Most experimental studies have focused on their lowest radiative states, however, their excited states have a rich variety of configurations because Coulomb and/or exchange interactions between different shells are quite different from those in the lowest shells. In this study, we investigated the magneto-photoluminescence (PL) properties of excited trions in a charge-tunable QD [3].
 Our sample consists of monolayer-fluctuation GaAs QDs embedded in an n-i -Schottky diode grown by molecular beam epitaxy. For micro-PL measurements, the sample was cooled to 6 K in a cryostat placed in a magnetic field B, which was applied to the sample in the Faraday geometry. A Ti:sapphire laser excited QDs within an aperture in the metal mask/electrode, and the PL spectra were measured with a monochromator and CCD detector.
 Figure 1 shows typical PL spectra obtained at B=0. Two lines labeled X0 and X- are assigned to the lowest-lying neutral exciton and negative trion, respectively. Figure 2 shows a magnetic field dependence of polarization-resolved PL spectra. In addition to X0 and X- emissions, we found several PL lines (labeled A〜E), most of which exhibit much complex B dependencies. To understand the origin of these PL lines, we consider shell and spin configuration of excited trions. From a configuration-interaction calculation using an elliptic Fock-Darwin model, we obtained the B dependence of trion and electron energies, and found possible assignments of shell and spin configurations for the origin of lines A-E. We believe that the present work opens the possibility of developing a novel scheme for the optical control of single electron spins using the excited states of trions in QDs.
 This work was partly supported by KAKENHI (19310067).

[1] M. Atatüre et al., Science 312 (2006) 551.
[2] D. Press et al., Nature 456 (2008) 218.
[3] H. Sanada et al., Phys. Rev. B 79 (2009) 121303.

Fig. 1. PL spectra at zero magnetic field.
Fig. 2. Magnetic field evolution of the polarization resolved PL spectra at (a) 0, (b) 0.25 (c) 0.3 V.

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