A system of two coupled CdTe quantum dots, one of them containing a single Mn ion, was studied in continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information in the spin state of the Mn ion has been demonstrated, using orientation of the Mn spin by spin-polarized carriers transferred from the neighbor quantum dot. Mn spin orientation time values from 20 ns to 100 ns were measured, depending on the excitation power. Storage time of the information in the Mn spin was found to be enhanced by application of a static magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple rate equation models were found to describe correctly static and dynamical properties of the system.One of important research directions that may influence the future of information processing, especially of spintronics [1], is focused on physical phenomena occurring in nanoscale-size quantum objects. One of such objects, close to the ultimate limit of information storage miniaturization, is a single Mn atom in a semiconductor quantum dot (QD) [2,3]. After intensive studies of semimagnetic QD containing many magnetic ions [4,5,6,7], single Mn atoms in CdTe [8] and InAs [9] QDs have been observed in photoluminescence (PL) experiments. Many experiments supplied substantial knowledge on physical properties of single Mn atoms, especially in CdTe QDs. In particular, they revealed a strong influence of the position of the Mn atom in the QD, reflecting the symmetry of the system [10]. They demonstrated an efficient optical read-out of the Mn spin state [8]. Furthermore, the dynamics of this state has been studied in photon correlation experiments [11], revealing an important influence of photo-created carriers on Mn spin relaxation. The writing and storing of the information in the Mn spin state has received less attention so far. These issues represent the focus of the present work.In particular, we demonstrate optical writing of information in the spin state of a single Mn ion and we test the stability of this state in the time range up to 0.2 ms.CdTe QDs containing single Mn ions were grown by molecular beam epitaxy. A single layer of self-assembled QDs was deposited in a ZnTe matrix. Manganese was added by briefly opening the Mn effusion cell during the growth of the CdTe layer [12]. The opening time and the Mn flux were adjusted to achieve a large probability of growth of QDs with a single Mn ion in each dot. The selection of single QDs was done by spatial limitation of PL excitation and detection to an area smaller than 0.5 micrometer in diameter, with microscope objective immersed in pumped liquid helium. Continuous wave excitation was used either above the ZnTe barrier gap (at 457 nm) or by a tunable dye laser in the range 570 -600 nm. Well separated photoluminescence lines from individual QDs were observed in the low energy part of the PL spectrum. We were able to select numerous lines showing a PL pattern characteristic for the presence of a...
Asymmetrical horizontal interdot coupling was demonstrated to exist in a system of a single plane of self-assembled CdTe/ZnTe quantum dots. Photoluminescence excitation ͑PLE͒, second-order photon correlation, and optical orientation were used as main experimental tools. Each individual absorbing dot was identified by a sharp PLE resonance assigned to the neutral exciton transition, while the corresponding emission contained several excitonic transitions of different charge states in another single quantum dot different from the absorbing one. Energy and spin transfer dynamics were studied. A high efficiency of spin transfer was found from the optical orientation in a vertical magnetic field ͑70%͒ as well as without the magnetic field ͑40%͒, in spite of a significant anisotropic exchange splitting of the absorbing dot. Coherent mechanism of linear-to-circular polarization conversion was identified, with an efficiency ͑43%͒ close to the theoretical limit of 50%.
We study the formation dynamics of a spontaneous ferromagnetic order in single self-assembled Cd1−xMnxTe quantum dots. By measuring time-resolved photoluminescence, we determine the formation times for QDs with Mn ion contents x varying from 0.01 to 0.2. At low x these times are orders of magnitude longer than exciton spin relaxation times evaluated from the decay of photoluminescence circular polarization. This allows us to conclude that the direction of the spontaneous magnetization is determined by a momentary Mn spin fluctuation rather than resulting from an optical orientation. At higher x, the formation times are of the same order of magnitude as found in previous studies on higher dimensional systems. We also find that the exciton spin relaxation accelerates with increasing Mn concentration. Doping semiconductor quantum dots (QDs) with magnetic ions offers a possibility of controlling magnetic properties of matter at nanoscale. Notably, several theoretical reports have proposed tailoring of QD magnetization by tuning the number of carriers in a dot.1-3 However, in order to achieve the control over magnetization a detailed knowledge of its dynamics is required. In compound II -VI QDs the Mn doping is performed routinely enabling studies of very dilute systems including QDs with single Mn ions 4 and of highly doped ones with molar contents up to 7%.5 Magnetic properties are comfortably monitored through optical experiments, since exchange interaction between the localized magnetic ions and the band carriers leads to pronounced magnetooptical effects.6 In particular, energy minimization of a complex consisting of a photocreated electron-hole pair (an exciton) interacting with Mn ions, results in a spontaneous formation of a local ferromagnetic order -a magnetic polaron (MP).Static and dynamic properties of MPs have been subject to intensive experimental and theoretical studies 5,[7][8][9][10][11][12][13][14][15][16][17][18][19] Experimental fingerprint of the MP formation is a redshift of the exciton photoluminescence (PL) by polaron energy E P -the energy gained by formation of the ferromagnetic order. The development of the magnetization can therefore be monitored in a time-resolved (TR) PL experiment, in which a transient shift of the exciton energy is observed allowing to evaluate the MP formation time, τ f .10,11 However, in bulk and 2D systems a prerequisite for the MP formation is an initial localization of the exciton.12 A precise experimental identification of E P and τ f is then hindered by processes related to trapping of the exciton. On the other hand, excitons in QDs are inherently localized by the QD potential, and thus the studies of MP formation dynamics in these nanostructures are free of the obscuring localization effects.14,18 The studies reported so far were performed on QD ensembles, in which the obtained τ f may be inaccurate due to inhomogeneities in dot morphology leading to variations in exciton lifetimes, 20 τ X , affecting in turn the TRPL transients. Previous reports have also lef...
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