P. Wojnar

Optical manipulation of a single Mn spin in a CdTe-based quantum dot.

Two coupled CdTe quantum dots, selected from a self-assembled system, one of them containing a single Mn ion, were studied by continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information on the spin state of the Mn ion has been demonstrated, using the orientation of the Mn spin by spin-polarized carriers transferred from the neighboring quantum dot. Mn spin orientation time values from 20 to 100 ns were measured, depending on the excitation power. Storage time of the information on 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 the static and dynamical properties of the system.

Optically induced energy and spin transfer in nonresonantly coupled pairs of self-assembled CdTe/ZnTe quantum dots

Asymmetrical horizontal interdot coupling was demonstrated to exist in a system of a single plane of self-assembled CdTe/ZnTe quantum dots. Photoluminescence excitation (PLE), 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%.

Picosecond charge variation of quantum dots under pulsed excitation

We present a spectroscopic study of excitation dynamics in self-assembled CdTe/ZnTe quantum dots. Insight into details of kinetics is obtained from the time-resolved microphotoluminescence, single photon correlation, and subpicosecond excitation correlation measurements done on single quantum dots. It is shown that the pulsed excitation at an energy above the energy gap of the barrier material results in separate capture of electrons and holes. We found that the capture of electrons by the quantum dot is delayed with respect to the capture of holes: the electron capture takes place in 20\char21{}40 ps after the excitation pulse, while the capture of holes is much faster.

Giant Spin Splitting in Optically Active ZnMnTe/ZnMgTe Core/Shell Nanowires

An enhancement of the Zeeman splitting as a result of the incorporation of paramagnetic Mn ions in ZnMnTe/ZnMgTe core/shell nanowires is reported. The studied structures are grown by gold-catalyst assisted molecular beam epitaxy. The near band edge emission of these structures, conspicuously absent in the case of uncoated ZnMnTe nanowires, is activated by the presence of ZnMgTe coating. Giant Zeeman splitting of this emission is studied in ensembles of nanowires with various average Mn concentrations of the order of a few percent, as well as in individual nanowires. Thus, we show convincingly that a strong spin sp-d coupling is indeed present in these structures.

Brightening of dark excitons in a single CdTe quantum dot containing a single Mn2+ ion

A promising method to investigate dark exciton transitions in quantum dots is presented. The optical recombination of the dark exciton is allowed when the exciton state is coupled with an individual magnetic impurity (manganese ion). It is shown that the efficient radiative recombination is possible when the exchange interaction with the magnetic ion is accompanied by a mixing of the heavy-light hole states related to an in-plane anisotropy of the quantum dot. It is also shown that the dark exciton recombination is an efficient channel of manganese spin orientation.

Magnetic polaron formation and exciton spin relaxation in single Cd1-xMnxTe quantum dots

We study the formation dynamics of a spontaneous ferromagnetic order in single self-assembled Cd1-xMnxTe quantum dots (QDs). 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.

Magnetophotoluminescence study of intershell exchange interaction in CdTe/ZnTe quantum dots

T. Kazimierczuk, ∗ T. Smoleński, M. Goryca, 2 L. K lopotowski, P. Wojnar, K. Fronc, A. Golnik, M. Nawrocki, J.A. Gaj, † and P. Kossacki Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Hoża 69, 00-681 Warsaw, Poland Laboratoire National des Champs Magntiques Intenses, Grenoble High Magnetic Field Laboratory, CNRS, 38042 Grenoble, France Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/64, 02-688 Warsaw, Poland (Dated: June 28, 2011)

Coherent Precession of an Individual 5/2 Spin.

We present direct observation of a coherent spin precession of an individual Mn^{2+} ion, having both electronic and nuclear spins equal to 5/2, embedded in a CdTe quantum dot and placed in a magnetic field. The spin state evolution is probed in a time-resolved pump-probe measurement of absorption of the single dot. The experiment reveals subtle details of the large-spin coherent dynamics, such as nonsinusoidal evolution of states occupation, and beatings caused by the strain-induced differences in energy levels separation. Sensitivity of the large-spin impurity on the crystal strain opens the possibility of using it as a local strain probe.

In-plane radiative recombination channel of a dark exciton in self-assembled quantum dots

We demonstrate evidence for a radiative recombination channel of dark excitons in self-assembled quantum dots. This channel is due to a light hole admixture in the excitonic ground state. Its presence was experimentally confirmed by a direct observation of the dark exciton photoluminescence from a cleaved edge of the sample. The polarization resolved measurements revealed that a photon created from the dark exciton recombination is emitted only in the direction perpendicular to the growth axis. Strong correlation between the dark exciton lifetime and the in-plane hole g-factor enabled us to show that the radiative recombination is a dominant decay channel of the dark excitons in CdTe/ZnTe quantum dots.

Quantum interference in exciton-Mn spin interactions in a CdTe semiconductor quantum dot.

We show theoretically and experimentally the existence of a new quantum-interference effect between the electron-hole interactions and the scattering by a single Mn impurity. The theoretical model, including electron-valence-hole correlations, the short- and long-range exchange interaction of a Mn ion with the heavy hole and with electron and anisotropy of the quantum dot, is compared with photoluminescence spectroscopy of CdTe dots with single magnetic ions. We show how the design of the electronic levels of a quantum dot enables the design of an exciton, control of the quantum interference, and hence engineering of light-Mn interaction.