CdMnTe Quantum Dots Research Articles

Temperature Dependence of Faraday Rotation of Glass Optical Fibers Doped with Quantum Dots of CdSe and CdMnTe

Specialty optical fibers incorporated with CdSe and CdMnTe quantum dots are fabricated using the modified chemical vapor deposition and high temperature fiber drawing process. Temperature dependence of Faraday rotation angle (FRA) is investigated under magnetic field of 0.126 Tesla (T) upon temperature change from 25 to 120 °C. The FRAs of the CdSe and CdMnTe doped fibers are found to increase linearly with magnetic field and no appreciable change in the FRA of the CdSe doped fiber with the increase of temperature is found regardless of the magnetic field as compared to the CdMnTe doped fiber. Average temperature dependence of the FRA are 16.40 × 10−4 and −104.90 × 10−4 degree °C−1 at 0.126 T for the CdSe and CdMnTe doped fibers and the resultant Verdet constants are estimated to be 3.75 ± 0.04 and 3.50 ± 0.12 rad T−1 m−1, respectively.

Synthesis and characterization of high-quality water-soluble CdMnTe quantum dots capped by N-acetyl-L-cysteine through hydrothermal method

High-quality water-soluble Mn2+ doped CdTe quantum dots (QDs) with N-acetyl-L-cysteine (NAC) as capping reagent have been synthesized through hydrothermal route, allowing a rapid preparation time (<1h), tunable emitting peaks (from 530 to 646nm) and excellent quantum yields (approximately 50%). The influences of various experimental variables, including Mn-to-Cd ratio, Te-to-Cd ratio, pH value, and reaction time on the growth rate and luminescent properties of the obtained QDs have been systematically investigated. And the optimum reaction conditions (Cd:Mn:NAC:Te=1.0:1.0:2.4:0.2, pH=9.5, 35min, 200°C) are found out. The optical features and structure of the obtained CdMnTe QDs have been characterized through fluorescence spectroscopy, UV absorption spectroscopy and TEM. In particular, we realized qualitative, semi-quantitative and quantitative studies on the doping of Mn to CdTe QDs through XPS, EDS, and AAS. The actual molar ratio of Mn to Cd in CdMnTe QDs (551nm) is 1.166:1.00, very close to the feed ratios (1:1).

Identification of Optical Transitions from CdTe and CdMnTe Quantum Dots Embedded in ZnTe Nanowires

We study photoluminescence properties of CdTe and CdMnTe quantum dots embedded in ZnTe nanowires. The nanowires were grown by molecular beam epitaxy, applying the vapor-liquid-solid growth mechanism. Linear polarization anisotropy measurements allow us to assess that the excitonic transitions originate from a single nanowire. We identify the optical transitions by comparing observed spectroscopic shifts with the universal emission pattern from the epitaxial CdTe dots. We support this identi cation by analyzing the photoluminescence intensity dependence on excitation power.

Influence of exciton spin relaxation on the photoluminescence spectra of semimagnetic quantum dots

We present a comprehensive experimental and theoretical studies of photoluminescence of single CdMnTe quantum dots with Mn content x ranging from 0.01 to 0.2. We distinguish three stages of the equilibration of the exciton-Mn ion spin system and show that the intermediate stage, in which the exciton spin is relaxed, while the total equilibrium is not attained, gives rise to a specific asymmetric shape of the photoluminescence spectrum. From an excellent agreement between the measured and calculated spectra we are able to evaluate the exciton localization volume, number of paramagnetic Mn ions, and their temperature for each particular dot. We discuss the values of these parameters and compare them with results of other experiments. Furthermore, we analyze the dependence of average Zeeman shifts and transition linewidths on the Mn content and point out specific processes, which control these values at particular Mn concentrations.

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 CdMnTe 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.

Optically Controlled Nanomagnets

AbstractWe report on single dot photoluminescence imaging and spectroscopy at B=0T on magnetically doped CdMnTe self-assembled quantum dots with average Mn concentration of several percent. Quasi-resonant excitation with circularly polarized laser leads to formation of magnetic polarons with magnetization induced by the laser light. In this case all quantum dots are polarized in the same direction. In contrast, when the dots are populated using above the barrier excitation, with randomly polarized excitons, the resultant magnetization is random and varies from dot to dot. These experiments demonstrate a way to control the magnetization of magnetically doped quantum dots by means of light excitation. In addition, they point towards extremely long spin memory times in these structures, reaching hundreds of microseconds, making CdMnTe quantum dots promising candidates for local magnetic field sources on the nanoscale.

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-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.

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 induced spin alignment persists to temperatures as high as 160K.

Optically controlled magnetization of zero‐dimensional magnetic polarons in CdMnTe self‐assembled quantum dots

AbstractWe find strong optically induced magnetization of zero‐dimensional magnetic polarons confined to CdMnTe quantum dots. The spin alignment of magnetic ions at B = 0 T is created through circularly polarized resonant excitation and it is probed by polarization‐resolved photoluminescence. This observation clearly demonstrates that in the CdMnTe quantum dots the exciton spin relaxation time is significantly longer than the formation time of magnetic polaron. Importantly, the spatial confinement characteristic of quantum dots increases the stability of magnetic polarons so that significant polarizaton of the emission is observed for temperatures higher than 120 K. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optical Properties of Semimagnetic Quantum Dots

ABSTRACTMeasurements 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.

Exciton Spectroscopy of Single CdTe and CdMnTe Quantum Dots

Single dot optical spectroscopy results obtained for CdTe and CdMnTe self-assembled quantum dots in ZnTe matrix are reported. In order to resolve the emission from individual dots apertures with diameter below 200 nm were opened in a metal masks. In the case of CdTe dots, sharp emission lines with a linewidth below 200 μeV were observed. From a magnetic field dependence of the emission energy the exciton effective g-factor is deduced to be equal to —3 and it is found to be independent of the energy of the quantum dot emission at B = 0 T. On the other hand, for CdMnTe quantum dots, broad (∼3–5 meV) emission lines related to a single dot emission were observed. Moreover, the Zeeman splitting of these lines is found to be comparable with their linewidth. The observed behavior is attributed to either high manganese concentration in the dots or to confinement induced decrease of exchange constants in magnetic quantum structures.

Micro photoluminescence spectra of CdTe and CdMnTe self-organized quantum dots

We report on micro photoluminescence (μ-PL) experiments in CdTe and CdMnTe quantum dots (QDs). PL spectra of the CdTe QDs indicate an ensemble of discrete narrow lines, each of which arises from a single dot emission. High emission efficiency of the present QDs allows the time resolving of the μ-PL signal. Dependence of the PL decay time on the QD size shows the evidence for the quantum size effect of the radiative recombination process. For CdMnTe QDs, it is found that the discrete spectral feature disappears in the μ-PL spectrum, reflecting the fast relaxation kinetics of excitons in the manganese doped QDs.