渐逝波耦合半导体量子点光纤放大器

基于半导体量子点的特性,结合光纤渐逝波耦合器,提出了一种新型的光纤放大器件,它将以溶液形式的硫化铅(PbS)半导体量子点材料沉积于耦合器熔锥区,信号光和抽运光通过渐逝波共同与半导体量子点材料相互作用,实现光的放大作用。PbS量子点材料是采用工艺容易控制的反胶束法制备的,通过透射电镜(TEM)测量得到其粒子尺寸小于10 nm。利用工作波长为980 nm,功率为30 mW的半导体激光器抽运光源对该光纤放大器抽运,在1310 nm波段得到了大于4 dB的增益,这是半导体量子点尺寸效应引起的光谱蓝移现象的体现。因此,这种有源区短、器件结构紧凑的光纤放大器在高速、宽带光纤接入等领域具有重要的实际意义和应用价值。     

Photoluminescence properties of cadmium-selenide quantum dots embedded in a liquid-crystal polymer matrix

The photoluminescence properties of cadmium-selenide (CdSe) quantum dots with an average size of ～3 nm, embedded in a liquid-crystal polymer matrix are studied. It was found that an increase in the quantum-dot concentration results in modification of the intrinsic (exciton) photoluminescence spectrum in the range 500–600 nm and a nonmonotonic change in its intensity. Time-resolved measurements show the biexponential decay of the photoluminescence intensity with various ratios of fast and slow components depending on the quantum-dot concentration. In this case, the characteristic lifetimes of exciton photoluminescence are 5–10 and 35–50 ns for the fast and slow components, respectively, which is much shorter than the times for colloidal CdSe quantum dots of the same size. The observed features of the photoluminescence spectra and kinetics are explained by the effects of light reabsorption, energy transfer from quantum dots to the liquid-crystal polymer matrix, and the effect of the electronic states at the CdSe/(liquid crystal) interface.     

Photoluminescence of ZnS:Cu in a Polymethyl Methacrylate Matrix

The method of agents arising directly in the (poly)methyl methacrylate medium is used to synthesize ZnS:Cu quantum dots fixed in an optically transparent polymer matrix. The optical transmittance of the polymer composites in the visible spectral region reaches 92% (at the thickness 5 mm). The photoluminescence of the (poly)methyl methacrylate/ZnS:Cu composite is defined by defects in the crystal structure of ZnS and by the system of energy levels in the band gap of ZnS. The photoluminescence signal depends on the Cu-ion concentration, the reabsorption of radiation in ZnS and (poly)methyl methacrylate, and other factors.     

Green synthesis of yellow emitting PMMA–CdSe/ZnS quantum dots nanophosphors

We herein report the fabrication of highly fluorescent yellow emitting nanophosphors using CdSe/ZnS quantum dots (QDs) dispersed in polymethyl methacrylate (PMMA). The QDs were synthesised via a simple, non-phosphine and one pot synthetic method in the absence of an inert atmosphere. The as-prepared nanocrystallites were characterised by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) and photoluminescence spectroscopy, energy-dispersive spectroscopy (EDS), Raman spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) microscopy. Optical analysis confirmed that the as-synthesised CdSe/ZnS QDs were of high quality with sharp absorption peaks, bright luminescence, narrow emission width and high PL quantum yield (up to 74%). The electron microscope images showed that the QDs are small and spherical in shape with narrow size distributions while the HRTEM micrograph confirmed the high crystallinity of the material. The Raman analysis of the QDs revealed the formation of core–shell structure and the energy dispersive spectroscopy confirmed the presence of the corresponding elements (i.e., Cd, Se, Zn and S). The dispersion of the core–shell QDs in PMMA matrix led to the red-shifting of the emission position from 393 nm in the neat PMMA to 592 nm in the nanocomposite. The fabricated highly fluorescent yellow emitting PMMA–CdSe/ZnS core–shell QDs polymer nanocomposite film display excellent optical properties without loss of luminescence. Furthermore, the as-synthesised organic soluble CdSe/ZnS QDs were successfully converted into highly water soluble QDs after ligand exchange with mercaptoundecanoic acid (MUA) without the loss of their emission properties. The simplicity of the method and the quality of the as-synthesised nanocomposite make it a promising material for the large scale fabrication of diverse optical devices.     

Nonradiative recombination in GaN quantum dots formed in the AlN matrix

The mechanisms of temperature quenching of steady-state photoluminescence are studied for structures with hexagonal GaN quantum dots embedded in the AlN matrix. The structures are grown by molecular beam epitaxy. The study is conducted for structures with differently sized quantum dots, for which the peak of the photoluminescence band is in the range from 2.5 to 4.0 eV. It is found that the activation energy of thermal quenching of photoluminescence varies from 27 to 110 meV, as the quantum-dot height is decreased from 5 to 2 nm. A model is suggested to interpret the results. According to the model, the photo-luminescence signal is quenched because of the transfer of charge carriers from energy levels in the quantum dots to defect levels in the matrix.     

Residual strain in Si-Si1−xGex quantum dots

Quantum dots of 50–60 nm diameter fabricated from both Si-Si_1−xGe_x (x = 0.1−0.3) strained layer superlattices and a strain symmetrized Si₉-Ge₆ superlattice were investigated by photoluminescence and Raman scattering. It was found that the residual strain in 50 nm etched quantum dots can be as large as 50% of a corresponding pseudomorphic structure. In addition, both the photoluminescence intensity and quenching temperature of these dots increase as compared to the corresponding as-grown superlattices. These improved optical properties may be due to a combined effect of lateral quantum confinement and a possible indirect-direct bandgap transition in the dots induced by the change of strain field during the nanofabrication process.     

Hole transfer from PbS nanocrystal quantum dots to polymers and efficient hybrid solar cells utilizing infrared photons

Polymer/inorganic-nanocrystals bulk heterojunction solar cells, where inorganic semiconductor nanocrystals such as CdSe, CdS, CdTe, ZnO, TiO₂, and silicon, replace the fullerene molecules as the electron acceptors, typically exhibit a power conversion efficiency (PCE) below 3% even after tremendous engineering efforts to optimize the nanocrystal size, shape, and nanoscale morphology. One promising feature of polymer hybrid solar cells is the ability to sensitize conjugated polymers, which on their own absorb only in the visible part of solar spectrum, into the infrared spectral range using infrared-active lead salt nanocrystal quantum dots (NQDs). Here we observed for the first time hole transfer from PbS NQDs to polymers as evidenced by the quenching of the PbS photoluminescence (PL), a sign of the presence of charge separating type II heterojunction. The type II band-offset at the NQD/polymer heterojunction enables efficient hole extraction from NQDs and leads to a record PCE of 3.80%, realized in a planar junction configuration under simulated air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm². The photocurrent has an extended spectral range spanning from the ultraviolet (UV) to the infrared (IR). Contributions from the polymer and PbS to the photocurrent were identified. Infrared photons (>700 nm) contribute about 30% of the photocurrent and yield a high external quantum efficiency (EQE) of 20% at 1050 nm.     

RGO-CdSe纳米复合材料的制备及其光学非线性吸收特性

通过两相法制备了氧化石墨烯 -硒化镉(GO-CdSe)纳米复合材料,并进一步通过水合肼还原, 制备了还原氧化石墨烯-硒化镉(RGO-CdSe)纳米复合材料。通过透射电镜(TEM)、 X射线衍射仪(XRD)、紫外可见光吸收(UV-Vis)光谱和荧光(PL)光谱对复合材料的 形貌、结 构和光学特性进行了表征。CdSe量子点为闪锌矿结构,粒径在6nm左右,基本均匀的分布在 RGO片上。在波长为532nm、脉冲宽度为30ps 的激光作用下,采用单光束Z扫描 技术,对复合材料的三阶光学非线性吸收性质进行了研究。研究发现,RGO-CdSe纳米复合 材料展现出反饱和吸收的非线性光学特性;相对于未附着CdSe量子点的 RGO,复合材料的光学非线性吸收特性有所增强。RGO-CdSe纳米复合材料的非线性吸收系数 为18.3cm/GW,高于纯RGO的11.2cm/GW。实 验 结果表明,RGO-CdSe纳米复合材料在光限幅器、光开关等光学器件方面有着潜在的应用前 景。     

Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.     

Reprint of: Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.     

Study of the evanescent wave coupled semiconductor quantum dot amplifying fiber

Optical amplifiers are the key parts in many fiber-optic transmission systems such as fiber CATV network, subscribe access network and some military fields. Amplifying fiber i the core part in an optical amplifier, and till now most ampli fying fibers in …     

Trapping of charge carriers into InAs/AlAs quantum dots at liquid-helium temperature

The problem of how the probability of trapping of charge carriers into quantum dots via the wetting layer influences the steady-state and time-dependent luminescence of the wetting layer and quantum dots excited via the matrix is analyzed in the context of some simple models. It is shown that the increase in the integrated steady-state luminescence intensity of quantum dots with increasing area fraction occupied by the quantum dots in the structure is indicative of the suppression of trapping of charge carriers from the wetting layer into the quantum dots. The same conclusion follows from the independent decays of the time-dependent luminescence signals from the wetting layer and quantum dots. The processes of trapping of charge carriers into the InAs quantum dots in the AlAs matrix at 5 K are studied experimentally by exploring the steady-state and time-dependent photoluminescence. A series of structures with different densities of quantum dots has been grown by molecular-beam epitaxy on a semi-insulating GaAs (001) substrate. It is found that the integrated photoluminescence intensity of quantum dots almost linearly increases with increasing area occupied with the quantum dots in the structure. It is also found that, after pulsed excitation, the photoluminescence intensity of the wetting layer decays more slowly than the photoluminescence intensity of the quantum dots. According to the analysis, these experimental observations suggest that trapping of excitons from the wetting layer into the InAs/AlAs quantum dots at 5 K is suppressed.     

InP quantum dots in (1 0 0) GaP: Growth and luminescence

We describe the growth and optical emission from strained InP quantum wells and quantum dots grown on GaP substrates using gas-source molecular beam epitaxy. Self-organized quantum dot formation takes place for InP coverage greater than 1.8 monolayers on the (1 0 0) GaP surface. Atomic force and scanning-electron microscopy studies indicate that unburied dots have a lateral size of 60–100 nm and are about 20 nm high, with dot densities in the range of 2–6×10⁸ cm⁻² for InP coverage between 1.9 and 5.8 MLs. Intense photoluminescence is emitted from both the quantum wells and the quantum dots at energies of about 2.2 and 2.0 eV, respectively. Time-resolved measurements indicate rather long carrier lifetimes of about 19 ns in the quantum wells and about 3 ns in the quantum dots. The data indicate that the InP/GaP quantum wells form a type-II band system, with electrons in the X valleys of the GaP recombine with holes in the InP. Furthermore, in the InP/GaP quantum dot system, the conduction band edge in the X valley of the GaP is nearly aligned with that in the Γ valley of the InP. Rapid thermal annealing of the quantum dots results in at least a six-fold enhancement of integrated emission intensity as well as some Ga-In interdiffusion. The low interdiffusion activation energy indicates that the material near the interface between the GaP matrix and the InP dots is not free of defects.     

Exciton characteristics and exciton luminescence of silicon quantum dot structures

The exciton binding energy, the energies of the basic radiative exciton transition, and the zerophonon radiative lifetime of excitons in silicon quantum dots embedded in the SiO_x matrix are calculated in effective mass approximation with quadratic dispersion relation. In addition, the spectra of steady-state photoluminescence and of time-resolved photoluminescence of excitons in the silicon quantum dots are calculated, and the kinetics of the photoluminescence relaxation is considered. The theory is compared with the experiment. It is shown that, for nanostructures involving silicon quantum dots with diameters smaller than 4 nm, the governing factor in the broadening of the spectral photoluminescence bands is the effect of mesoscopic quantum fluctuations. In this case, either an even one dangling bond at the interface, or one intrinsic point defect, or one foreign atom located inside the small-sized nanocrystallite or in its close surroundings produces a pronounced effect on the energy of the exciton transition.     

Relaxation and recombination in ultrasmall InAs quantum dots

We report a photoluminescence study of self-organized nanometer-size InAs quantum dots grown by molecular beam epitaxy on a GaAs substrate. High optical excitation has been used in order to observe emission from higher states of the quantum dots. The energy difference between adjacent states turns out to be of the order of 40–50 meV for dot diameters around 20 nm. The photoluminescence decay time at the fundamental transition is found to be of the order of 700 ps, decreasing down to 100 ps for the highest confined states. Finally, a cascade-like mechanism for the carrier relaxation in these structures is strongly suggested by the time resolved data.     

Specific features of photoluminescence properties of copper-doped cadmium selenide quantum dots

The effect of doping with copper on the photoluminescence properties of cadmium selenide quantum dots 4 nm in dimension is studied. The quenching of the excitonic photoluminescence band related to the quantum dots and the appearance of an impurity photoluminescence band in the near-infrared region are observed after doping of the quantum dots with copper. It is established that, on doping of the quantum dots, the photoluminescence kinetics undergoes substantial changes. The photoluminescence kinetics of the undoped quantum dots is adequately described by a sum of exponential relaxation relations, whereas the photoluminescence kinetics experimentally observed in the region of the impurity band of the copper-doped samples follows stretched exponential decay, with the average lifetimes 0.3–0.6 μs at the photon energies in the range of 1.47–1.82 eV. The experimentally observed changes in the photoluminescence properties are attributed to transformation of radiative centers in the quantum dots when doped with copper atoms.     

Effect of anisotropy of band structure on optical gain in spherical quantum dots based on PbS and PbSe

Results of an experimental and theoretical study of the optical absorption spectra of spherical quantum dots based on PbS and PbSe are presented. A rigorous theoretical analysis of the energy spectra and optical transitions is performed within the framework of the four-band k·p approximation with full account of the effect of anisotropy of the band structure. It is shown that strong anisotropy of the band structure of PbS and PbSe leads to the appearance of optical transitions that are forbidden in the isotropic approximation. These transitions were detected in the optical absorption spectra of the investigated quantum dots. Fiz. Tekh. Poluprovodn. 33, 1450–1455 (December 1999)     

Stabilization of Black Phosphorous Quantum Dots in PMMA Nanofiber Film and Broadband Nonlinear Optics and Ultrafast Photonics Application

The stabilization of black phosphorous quantum dots (BPQDs) for optical application under ambient conditions is highly challenging. Here, a facile approach is presented to substantially stabilize BPQDs by making a uniform BPQDs/polymethyl methacrylate (PMMA) composite nanofiber film via an electrospinning technique. As verified by femtosecond laser Z‐scan measurement, the BPQDs/PMMA composite nanofiber film that has been stored for three months exhibits almost the same nonlinear optical properties as the fresh BPQDs. Additionally, the BPQDs/PMMA composite nanofiber film demonstrates broadband nonlinear optical response ranging from the visible bandwidth (400 nm) to the mid‐IR bandwidth (at least 1930 nm). By employing the BPQDs/PMMA composite nanofiber film as an optical saturable absorber, an ultrashort pulse with the pulse duration of ≈1.07 ps centered at the wavelength of 1567.6 nm is generated in a mode‐locked fiber laser. These results suggest that the BPQDs/PMMA composite nanofiber film can combine the advantage of convenient integration and mitigation of the drawback of the easy oxidation of black phosphorous and pave the way for BP‐based practical optoelectronic devices.     

Molecular beam epitaxy of AlGaAs/Zn(Mn)Se hybrid nanostructures with InAs/AlGaAs quantum dots near the heterovalent interface

Features of the growth of InAs quantum dots in an Al_0.35Ga_0.65As matrix by molecular beam epitaxy at different substrate temperatures, deposition rates, and amounts of deposited InAs are studied. The optimum conditions for growing an array of low-density (≤2 × 10¹⁰ cm^?2) small (height of no more than 4 nm) self-organized quantum dots are determined. The possibility of the formation of optically active InAs quantum dots emitting in the energy range 1.3–1.4 eV at a distance of no more than 10 nm from the coherent heterovalent GaAs/ZnSe interface is demonstrated. It is established that inserting an optically inactive 5-nm GaAs quantum well resonantly coupled with InAs quantum dots into the upper AlGaAs barrier layer enhances the photoluminescence efficiency of the quantum-dot array in hybrid heterostructures.     

Photoluminescence kinetics slowdown in an ensemble of GaN/AlN quantum dots upon tunneling interaction with defects

The carrier recombination dynamics in an ensemble of GaN/AlN quantum dots is studied. The model proposed for describing this dynamics takes into account the transition of carriers between quantum dots and defects in a matrix. Comparison of the experimental and calculated photoluminescence decay curves shows that the interaction between quantum dots and defects slows down photoluminescence decay in the ensemble of GaN/AlN quantum dots.