Wavelength Selective Charge Storage in self-assembled InGaAs-GaAs Quantum Dots
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Wavelength Selective Charge Storage in self-assembled InGaAs-GaAs Quantum Dots M. Kroutvar, Y. Ducommun, J. J. Finley, M. Bichler and G. Abstreiter Walter Schottky Institut, Technische Universität München, D-85748, Garching, Germany A. Zrenner University Paderborn, FB6, Warburger Str. 100, 33098 Paderborn, Germany ABSTRACT We present the first demonstration of spectrally selective charge storage in self-assembled quantum dots that utilizes optical charge generation and readout of the stored charge distribution. Resonant optical excitation charges only a sub-ensemble of the inhomogeneous broadened dots. The optical readout process directly recovers the spectral distribution of stored charges. Wavelength selectivity is demonstrated in a two-color experiment exhibiting charge storage in two independent QD sub-ensembles. Resonant charge storage is observed at least up to 85K depending on excitation energy. Lack of time evolution of the spectral emission indicates charge storage times ∆t much longer than 25µs. Introduction The fully quantized electronic structure of self-assembled quantum dots (QDs) results in unique optical properties such as a discrete emission [1-3] and absorption spectra [4-6] for individual dots and enhanced few-particle interactions between localized charge carriers [1-3]. These properties have recently led to proposals for novel applications such as the realization of quantum operations based on isolated excitons [7] or electron spins [8,9] in isolated dots, inter-band lasers with improved performance [15] and novel optical memory devices [10-14] that potentially combine ultrahigh data storage densities with optical addressing and readout [10]. The latter application stems from the inhomogeneously broadened absorption spectrum of large ensembles of dots, arising from dot-dot size, shape and compositional fluctuations, and provides the mechanism for frequency selective excitation of a sub-ensemble of QDs. In the present paper, we present such a QD-based charge storage device with optically stimulated charge (electron or hole) storage and the first demonstration of direct optical readout of the stored charge distribution in the wavelength domain. Analysis of the spectral distribution of stored charges provides new information regarding the absorption mechanisms as well as the evolution of the stored charge distribution with lattice temperature and excitation intensity. Experimental Details and Device Structure The devices investigated consist of a single layer of In0.5Ga0.5As QDs embedded within the intrinsic region of either an n- or p-type Schottky photodiode for hole or electron storage respectively. A schematic band diagram of the hole storage device (n-type) is presented in Fig. 1a. Following resonant optical excitation, the built-in axial electric field results in efficient ionization of the photo-generated excitons and selective storage of one charge species (electron or hole) due to the presence of a 50nm wide Al0.33Ga0.67As barrier that inhibits tunneling escape of the other charge.
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