Passively Q-switched green laser operation using CdTe/CdS quantum dots

The direct generation of passively Q-switched lasers at a green wavelength has rarely been investigated in the past. In this Letter, we demonstrate a passively Q-switched praseodymium-doped yttrium lithium fluoride green laser at 522 nm using CdTe/CdS quantum dots as a saturable absorber. A maximum average output power of 33.6 m W is achieved with the shortest pulse width of 840 ns. The corresponding pulse energy and peak power reached 0.18 μJ and 0.21 W, respectively. To the best of our knowledge, this is the first demonstration in regard to a quantum dots saturable absorber operating in the green spectral region.     

Manifestation of CdTe quantum dots and interdiffusion in IR reflection spectra of CdTe/ZnTe structures with quantum dots

The IR reflection spectra of structures that consist of alternating layers of CdTe quantum dots and ZnTe barriers and are grown on ZnTe and CdTe/ZnTe buffer layers on GaAs(001) substrates are studied. The spectra are processed using dispersion analysis, and the parameters of the oscillators are determined. In the spectra, quantum dots manifest themselves in the form of a broad band at a frequency close to the frequency of the Fröhlich mode. It is revealed that the spectra contain features associated with the interdiffusion of the CdTe and ZnTe compounds, i.e., bands attributed to the local mode of Zn atoms in the CdTe compound and two gap modes of Cd atoms in the ZnTe compound.     

Interaction of water-soluble CdTe quantum dots with octacarboxy metallophthalocyanines: A photophysical and photochemical study

Water-soluble CdTe quantum dots capped with l-cysteine (QD-CYS) were found to improve the photophysical and photochemical properties of octacarboxy metallophthalocyanine (MOCPc, M=Zn(II), Al(III)(OH), Ge(IV)(OH)₂ and Si(IV)(OH)₂) complexes. Increased diffusional interactions between the molecules and the ground-state molecular oxygen were established resulting from the increase in the triplet-state quantum yield and lifetimes of the MOCPcs in the presence of QD-CYS. Energy transfer occurred from QD-CYS to the MOCPcs upon excitation of QD-CYS. It was found that an efficient energy transfer process, which is not directly related to the amount of spectral overlap between the donor (QD-CYS) and the acceptor (MOCPc) can occur. Singlet oxygen via fluorescence resonance energy transfer (FRET) mechanism was produced in the QD-MPc mixture.     

Imaging microscopic viscosity with confocal scanning optical tweezers

The techniques of confocal microscopy and optical tweezers have shown themselves to be powerful tools in biological and medical research. We combine these methods to develop a minimally invasive instrument that is capable of making hydrodynamic measurements more rapidly than is possible with other devices. This result leads to the possibility of making scanning images of the viscosity distribution of materials around biopolymer-producing cells. 100 x 100 images can be taken with 0.5-microm spatial resolution in 3 min. An image of the viscosity distribution around a pullulan-producing cell of Aureobasidium pullulans is shown as an example.     

低强度飞秒激光激发下CdTe和CdTe/CdS核壳量子点的荧光上转换研究

利用400 nm和800 nm不同波长的低强度飞秒激光,对CdTe和CdTe/CdS核壳量子点溶胶进行激发,研究其稳态和时间分辨荧光性质.800 nm飞秒激光激发下,CdTe和CdTe/CdS核壳量子点产生上转换发光现象,上转换荧光峰与400 nm激发下的荧光峰相比蓝移最多达15 nm,而且蓝移值与荧光量子产率有关.变功率激发确认激发光功率与上转换荧光强度间满足二次方关系,时间分辨荧光的研究表明荧光动力学曲线服从双e指数衰减.提出表面态辅助的双光子吸收模型是低激发强度上转换发光的主要机理.CdTe和CdT 关键词： CdTe量子点 CdTe/CdS核壳量子点 时间分辨荧光 上转换荧光     

CdTe量子点标记雌二醇衍生物的研究

以巯基乙酸为修饰剂合成高量子产率的CdTe量子点,并成功标记生物小分子17β-氨基雌二醇.紫外光谱、荧光光谱、红外光谱以及倒置显微镜照片分析表明CdTe量子点在活化剂N-羟基琥珀酰亚胺的作用下,通过表面的羧基与17β-氨基雌二醇的氨基以共价键结合,这为建立基于量子点探针的新型的药物筛选模型提供了实验基础.     

A one-dimensional model for the growth of CdTe quantum dots on Si substrates

Recent experiments involving CdTe films grown on Si(111) substrates by hot wall epitaxy revealed features not previously observed [S.O. Ferreira, et al., J. Appl. Phys. 93 (2003) 1195]. This system, which follows the Volmer–Weber growth mode with nucleation of isolated 3D islands for less than one monolayer of evaporated material, was described by a peculiar behavior of the quantum dot (QD) size distributions. In this work, we proposed a kinetic deposition model to reproduce these new features. The model, which includes thermally activated diffusion and evaporation of CdTe, qualitatively reproduced the experimental QD size distributions. Moreover, the model predicts a transition from Stranski–Krastanow growth mode at lower temperatures to Volmer–Weber growth mode at higher ones characterized through the QD width distributions.     

Raman scattering in multilayer structures with CdTe quantum dots in ZnTe

Raman spectra in superlattices composed of layers of self-assembled CdTe quantum dots separated by ZnTe barriers are investigated. As the barrier thickness increases, a high-frequency shift of all peaks is observed, which is explained by a decrease in the lattice constant averaged over the volume of the entire structure. Peaks are found at a CdTe TO mode frequency of 140 cm^?1 and also at 120 cm^?1. The first peak is assigned to the symmetric Coulomb (interface) mode of the quantum dot material, and the low-frequency peak is assigned to the symmetric mode of the phonons captured in the quantum dot. This combination of modes in structures with quantum dots has not been observed previously.     

Interaction of different thiol-capped CdTe quantum dots with bovine serum albumin

Due to their unique optical properties, quantum dots (QDs) are rapidly revolutionizing many areas of medicine and biology. Despite the remarkable speed of development of nanoscience, relatively little is known about the interaction of nanoscale objects with organism. In this work, interaction of CdTe QDs coated with mercaptopropanoic acid (MPA), L-cysteine (L-cys), and glutathione (GSH) with bovine serum albumin (BSA) was investigated. Fluorescence (FL), UV–vis absorption, and circular dichroism (CD) spectra methods were used. The Stern-Volmer quenching constant (K_sv) at different temperatures, corresponding thermodynamic parameters (ΔH, ΔG and ΔS), and information of the structural features of BSA were gained. We found that QDs can effectively quench the FL of BSA in a ligand-dependent manner, electrostatic interactions play a major role in the binding reaction, and the nature of quenching is static, resulting in forming QDs-BSA complexes. The CD spectra showed that the secondary and tertiary structure of BSA was changed. This study contributes to a better understanding of the ligand effects on QDs-proteins interactions, which is a critical issue for the applications in vivo.     

Long-wavelength IR spectroscopy of ZnTe/CdTe superlattices with quantum dots

This paper reports on the results of investigations of the lattice IR reflection spectra of ZnTe/CdTe multilayer superlattices with CdTe quantum dots grown by molecular-beam epitaxy on a GaAs substrate with a CdTe buffer layer. It is found that the lattice IR reflection spectra of the studied structures exhibit three intense bands associated with vibrational excitations in the GaAs substrate, ZnTe barriers separating the layers with CdTe quantum dots, and the CdTe buffer layer. An analysis of the reflection bands and shifts in the phonon frequencies has revealed internal elastic stresses both in the surface layer of the GaAs substrate and in the ZnTe barriers. It is established that elastic stresses undergo relaxation in the separating ZnTe layers with an increase in their thickness. An additional mode observed in the reflection spectra is explained by manifestations of ZnTe-like vibrations in the ZnCdTe alloy due to interdiffusion of Cd and Zn at the interfaces.     

The third-order optical nonlinearity and upconversion luminescence of CdTe quantum dots under femtosecond laser excitation

The aqueous CdTe quantum dots (QDs) were synthesized by the electrostatic reaction method. The optical properties of CdTe QDs were investigated by femtosecond Z-scan and time-resolved luminescence technique in nonresonant spectral region. The nonlinear absorption and refraction are ascribed to two-photon absorption, and time-resolved upconversion photoluminescence produces biexponential decay pattern at infrared femtosecond laser excitation. Upconversion luminescence is composed of a band-edge excitonic state and a photoinduced trapping state. The short-lived band-edge excitonic emission is independent of the detection wavelengths, and long-lived species becomes even longer with the increase of detection wavelengths, which indicates the size dependence of surface excitonic emission. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Determination of l-proline based on anodic electrochemiluminescence of CdTe quantum dots

A novel flow injection method for detection of l-proline was proposed in the presence of CdTe quantum dots (QDs). This method is based on the enhanced anodic electrochemiluminescence (ECL) emission of CdTe QDs l-proline in aqueous system. CdTe QDs were modified with thioglycolic acid to obtain stable water-soluble QDs and intensive anodic ECL emission in Na₂CO₃–NaHCO₃ buffer solution at an indium tin oxide (ITO) electrode, which was used for the sensitive detection of ECL enhancement using our homemade flow cell. Under the optimal conditions, the ECL intensity was correlated linearly with the concentration of l-proline over the range of 1.0×10^?8?1.0×10^?4 g mL^?1 (r=0.9996) and the detection limit was 5.0×10^?9 g mL^?1. The relative standard deviation was 1.12% for 6.0×10^?5 g mL^?1 l-proline (n=11). The possible mechanism was discussed. This method put forward a new efficient ECL methodology for enhancement-related determination of l-proline successfully.     

Quantum confinement effects on the optical phonons of CdTe quantum dots

We present Raman-scattering results for CdTe nanocrystals in doped glasses which clearly show the confinement effects on the phonon spectra as a function of the quantum-dot size. We observed optical phonon modes, surface phonons and some of their overtone combinations. We show that the surface-phonon scattering intensity increases as the quantum-dot size decreases. Our results also show a decrease in the electron–phonon coupling as the nanocrystal size is decreased. These confinement effects are observed by changing the laser excitation energy, and thus by tuning to resonance with the optical transitions for quantum dots of different sizes within their broad size distribution in semiconductor-doped glasses.     

A simple fluorescence quenching method for roxithromycin determination using CdTe quantum dots as probes

A new method for the determination of roxithromycin based on the fluorescence quenching of 3-mercaptopropionic acid-capped CdTe quantum dots (MPA-CdTe QDs) was developed. In ethanol medium, the fluorescence of CdTe quantum dots at 552 nm was quenched in the presence of roxithromycin. Based on this a simple, sensitive, and selective method for rapid determination of roxithromycin was described. Reaction time, interfering substances on the fluorescence quenching, and mechanism of the interaction of CdTe QDs with roxithromycin were investigated. After optimization, the proposed method allows the determination of roxithromycin over the range 25.0-350.0 μg ml⁻¹. The detection limit is 4.6 μg ml⁻¹. The proposed method was successfully applied to commercial capsules and tablets with satisfactory results. The recovery of the method was in the range of 96.8-102.5%.     

Shrinking of ultrathin polyelectrolyte multilayer capsules upon annealing: A confocal laser scanning microscopy and scanning force microscopy study

Heating-induced morphological changes of micrometer size capsules prepared by step-wise deposition of oppositely charged polyelectrolytes onto melamine formaldehyde (MF) latex particles and biological cells with subsequent dissolution of the core have been investigated by confocal laser scanning microscopy (CLSM) and scanning force microscopy (SFM). For poly(styrenesulfonate-Na salt)/poly(allylamine hydrochloride) polyelectrolyte capsules a remarkable heating-induced shrinking is observed. An increase of the wall thickness corresponding to the capsule diameter decrease is found. The morphology of these microcapsules after temperature treatment is characterized. The thickening of the polyelectrolyte multilayer is interpreted in terms of a configurational entropy increase via polyanion-polycation bond rearrangement. Received 20 January 2000     

Three-dimensional photon confinement in a spherical microcavity with CdTe quantum dots: Raman spectroscopy

We have studied the Raman spectra of a system consisting of a polystyrene latex microsphere coated by CdTe colloidal quantum dots. The cavity-induced enhancement of the Raman scattering allows the observation of Raman spectra from only one CdTe monolayer. Periodic structure with very narrow peaks in the Raman spectra of a single microsphere was detected both in Stokes and anti-Stokes spectral regions, arising from the coupling between the emission from quantum dots and spherical cavity modes.     

Preparation of water-soluble CdTe/CdS core/shell quantum dots with enhanced photostability

CdTe/CdS core/shell quantum dots (QDs) have been synthesized in an aqueous phase using thioacetamide as a sulfur source. The quantum yield was greatly enhanced by the epitaxial growth of a CdS shell, which was confirmed by X-ray photoelectron spectroscopy (XPS) results. The quantum yield of as-prepared CdTe/CdS core/shell QDs without any post-preparative processing reached 58%. The experimental results illustrate that the QDs with core/shell structure show better photostability than thioglycolic acid (TGA)-capped CdTe QDs. The cyclic voltammograms reveal higher oxidation potentials for CdTe/CdS core/shell QDs than for TGA-capped CdTe QDs, which explains the superior photostability of QDs with a core/shell structure. This enhanced photostability makes these QDs with core/shell structure more suitable for bio-labeling and imaging.     

Imaging biological specimens with the confocal scanning laser microscope/macroscope

A new confocal scanning laser microscope/macroscope (cslm/M) has recently been developed. It combines in one instrument the high resolution capability of a confocal scanning beam microscope for imaging small specimens, with good resolution confocal imaging of macroscopic specimens. Some of its main features include: (a) 0.25 μm lateral resolution in the microscope mode and 5 μm lateral resolution in the macroscope mode; (b) a field of view that can vary from 25 μm × 25 μm to 75,000 μm × 75,000 μm; (c) capability for acquiring large data sets from 512 × 512 pixels to 2048 × 2048 pixels; (d) 0.5 μm depth resolution in the microscope mode and 200 μm depth resolution in the macroscope mode.

In this work the cslm/M was used to image whole biological specimens (> 5 m diameter), including insects which are ideal specimens for the macroscope. Specimens require no preparation, unlike scanning electron microscope (SEM) specimens which require a conductive coating. The specimens described in this paper are too large to be imaged in their entirety by a scanning beam laser microscope, however they can be imaged by slower scanning stage microscopes. In the macroscope mode the cslm/M was used to acquire a large number (e.g. 20–40) of confocal image slices which were then used to reconstruct a three-dimensional image of the specimen. High resolution images were collected by the cslm/M by switching to the microscope mode where high numerical aperture (NA) objectives were used to image a small area of interest. Reflected-light and fluorescence images of plant and insect specimens are presented which demonstrate the morphological details obtained in various imaging modes. A process for three-dimensional visualization of the data is described and images are shown.     

Transmission confocal laser scanning microscopy with a virtual pinhole based on nonlinear detection

We present a transmission-mode confocal laser scanning microscope system based on the use of second-harmonic generation (SHG) for signal detection. Our method exploits the quadratic intensity dependence of SHG to preferentially reveal unscattered signal light and reject out-of-focus scattered background. The SHG crystal acts as a virtual pinhole that remains self-aligned without the need for descanning.     

Study on the interaction of CdTe quantum dots with ferulic acid and protocatechuic aldehyde by optical spectroscopy

Luminescent CdTe quantum dots (QDs) were synthesized using thioglycolic acid (TGA) as a stabilizing agent in aqueous medium and were characterized by Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and transmission electron microscopy (TEM). In weak basic media the fluorescence of TGA-CdTe QDs was quenched notably by ferulic acid (FA) and protocatechuic aldehyde (PA), and the quenching values were proportional to the concentration of the quenchers in a certain range. The addition of bovine serum albumin (BSA) to TGA-CdTe QDs-FA and TGA-CdTe QDs-PA systems rendered a large recovery of the fluorescence of TGA-CdTe QDs.