H. E. Jackson

Tuning the properties of magnetic CdMnTe quantum dots

We show that CdMnTe self-assembled quantum dots (QDs) can be formed by depositing a submonolayer of Mn ions over a ZnTe surface prior to deposition of the CdTe dot layer. Single-dot emission lines and strongly polarized QD photoluminescence (PL) in an applied magnetic field confirm the presence of Mn in individual QDs. The width of PL lines of the single CdMnTe dots is 3 meV due to magnetic moment fluctuations (MMFs) of the Mn ions. After rapid thermal annealing, the emission lines of individual magnetic QDs narrow significantly to 0.25 meV, showing that effect of MMFs is strongly reduced, most probably due to an increase in the average QD size. These results suggest a way to tune the spin properties of magnetic QDs.

Exciton spin relaxation time in quantum dots measured by continuous-wave photoluminescence spectroscopy

We demonstrate a method of measuring the exciton spin relaxation time in semiconductor nanostructures by continuous-wave photoluminescence. We find that for self-assembled CdTe quantum dots (QDs) the degree of circular polarization of emission is larger when exciting polarized excitons into the lower energy spin state (σ−-polarized) than in the case when the excitons are excited into the higher energy spin state (σ+-polarized). A simple rate equation model gives the exciton spin relaxation time in CdTe QDs equal to τS=4.8±0.3u2002ns, significantly longer than the QD exciton recombination time τR=300u2002ps.

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...

Optical studies of spin relaxation in CdTe self-assembled 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

We show that by using circularly polarized quasi‐resonant spectroscopy, one can probe the exciton spin processes in symmetric self‐assembled quantum dots, where at B=0T the excitonic levels are degenerate. While in the case of CdTe dots we observe no net polarization of the emission, a strongly circularly polarized luminescence is detected for CdSe quantum dots. We ascribe this difference to longer spin relaxation time in CdSe quantum dots.

Photoluminescence Imaging Of CdTe/ZnTe Self‐Assembled Quantum Dots

We demonstrate that by combining solid immersion lens (SIL) imaging with slit‐confocal microscopy, photoluminescence emission of single quantum dots (QDs) can be imaged with a spatial resolution of 400 nm. As an example we use this method to study the internal structure of an exciton confined to a single asymmetric CdTe QD.

Optical Properties of Semimagnetic Quantum Dots

Measurements of optical properties of epitaxially grown CdMnTe quantum dots are presented. Magnetic ions are incorporated into the structure by passivating the substrate surface with Mn prior the quantum dot deposition. As expected, the intensity of intra-Mn transition increases with the time of manganese passivation. The formation of magnetic quantum dots is evidenced by presence of single emission lines in the micro-photoluminescence spectrum. The width of single dot emission lines is much broader than observed for non-magnetic quantum dots. The broadening is caused by thermal fluctuations of magnetization in quantum dots. Resonant spectroscopy results indicate that the exciton-optical phonon scattering is the main energy relaxation mechanism in these structures. The measurements suggest that post-growth thermal processing offers the possibility of tuning the magnetic properties of these structures.