T. Gurung

Optically-induced magnetization of CdMnTe self-assembled quantum dots

We demonstrate that resonant excitation of CdMnTe self-assembled quantum dots creates an ensemble of spin-polarized magnetic polarons at B=0 T. The strong spatial confinement characteristic of quantum dots significantly increases the stability of magnetic polarons so that the optically-induced spin alignment is observed for temperatures >120 K.

Exciton-controlled magnetization in single magnetic quantum dots

We report on low temperature polarization-resolved imaging of single magnetic self-assembled CdMnTe quantum dots (QDs) in the absence of magnetic field. Using longitudinal optical phonon-assisted absorption to photoexcite spin-polarized excitons into a QD ground state, we find that the magnetic impurities within CdMnTe QDs can be aligned ferromagnetically with a single emission lines exhibiting a circular polarization as large as 65%. These results demonstrate that the magnetization of a single magnetic QD can be optically controlled with a suitably polarized laser.

Sensitivity of exciton spin relaxation in quantum dots to confining potential

We observe a strong dependence of the exciton spin relaxation in CdTe quantum dots on the average dot size and the depth of the confining potential. After rapid thermal annealing, which increases the average dot size and leads to weaker confinement, we measure the spin relaxation time of the quantum dot excitons to be 1.5ns, as compared to 4.8ns found previously for the as-grown CdTe quantum dots. The annealed CdTe quantum dots exhibit also smaller values of the absolute polarization of the quantum dot emission. This dramatic enhancement of the spin scattering efficiency upon annealing is attributed to increased mixing between different spin states in larger CdTe quantum dots.We observe a strong dependence of the exciton spin relaxation in CdTe quantum dots on the average dot size and the depth of the confining potential. After rapid thermal annealing, which increases the average dot size and leads to weaker confinement, we measure the spin relaxation time of the quantum dot excitons to be 1.5ns, as compared to 4.8ns found previously for the as-grown CdTe quantum dots. The annealed CdTe quantum dots exhibit also smaller values of the absolute polarization of the quantum dot emission. This dramatic enhancement of the spin scattering efficiency upon annealing is attributed to increased mixing between different spin states in larger CdTe quantum dots.

Optical studies of zero-field magnetization of CdMnTe quantum dots: Influence of average size and composition of quantum dots

We show that through the resonant optical excitation of spin-polarized excitons into CdMnTe magnetic quantum dots (QD), we can induce a macroscopic magnetization of the Mn impurities. We observe very broad (4-meV linewidth) emission lines of single dots, which are consistent with the formation of strongly confined exciton magnetic polarons. Therefore, we attribute the optically induced magnetization of the magnetic dots to the formation of spin-polarized exciton magnetic polarons. We find that the photoinduced magnetization of magnetic polarons is weaker for larger dots which emit at lower energies within the QD distribution. We also show that the photoinduced magnetization is stronger for quantum dots with lower Mn concentration, which we ascribe to weaker Mn–Mn interaction between the nearest neighbors within the dots. Due to particular stability of the exciton magnetic polarons in QDs, where the localization of the electrons and holes is comparable to the magnetic exchange interaction, this optically i...

Ultralong spin memory of optically excited single magnetic quantum dots

We study the magnetization dynamics in CdMnTe quantum dots using subwavelength optical microscopy imaging at B=0 T. For continuous laser illumination each dot exhibits strong and unique circular polarization despite completely unpolarized ensemble emission. This implies that after an exciton recombines, the spontaneous ferromagnetic alignment of magnetic impurities persists for over 100 μs, which is a million times longer than in CdMnTe quantum wells. The spin memory effect points toward a qualitatively different picture of magnetization dynamics in the zero-dimensional limit.

Exciton spin relaxation in quasiresonantly excited Cd Te ∕ Zn Te self-assembled quantum dots

We study exciton spin-relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. Experiments on single CdTe QDs and on large QD ensembles show that by combining LO phonon-assisted absorption with circularly polarized excitation, spin-polarized excitons are photoexcited directly into the ground states of QDs. We find that for single symmetric QDs at

Optical studies of spin relaxation in CdTe self-assembled quantum dots

B=0\phantom{\rule{0.3em}{0ex}}\mathrm{T}

Magnetization imaging of single CdMnTe/ZnTe quantum dots

, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by only a fraction of a Kelvin through the application of small magnetic fields, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. Similar behaviors are found in a large ensemble of CdTe QDs. Finally, due to strong electronic confinement in CdTe QDs, the large spin polarization of excitons shows no dependence on the number of LO phonons emitted between the virtual state and the ground state of the exciton.

Optically Induced Zero‐Field Magnetization Of CdMnTe Quantum Dots

We study spin dynamics of excitons confined in self-assembled CdSe quantum dots by means of optical orientation in magnetic field. At zero field the exciton emission from QDs populated via LO phonon-assisted absorption shows a circular polarization of 14%. The polarization degree of the excitonic emission increases dramatically when a magnetic field is applied. Using a simple model, we extract the exciton spin relaxation times of 100 ps and 2.2 ns in the absence and presence of magnetic field, respectively. With increasing temperature the polarization of the QD emission gradually decreases. Remarkably, the activation energy which describes this decay is independent of the external magnetic field, and, therefore, of the degeneracy of the exciton levels in QDs. This observation implies that the temperature-induced enhancement of the exciton spin relaxation is insensitive to the energy level degeneracy and can be attributed to the same excited state distribution.

Exciton Spin Relaxation In Symmetric Self-Assembled Quantum Dots

Exciton magnetic polarons in CdMnTe QDs form through the spontaneous ferromagnetic alignment of the Mn. We show evidence that after EMP recombination, the Mn magnetization persists, directly impacting the next occupation.