分子束外延ZnSe/ZnSxSe1-x超晶格光学特性

用分子束外延方法在GaAs(100)衬底上生长了高质量的ZnSe/ZnSxSe1-x(x=0.12)超晶格结构,通过X射线衍射谱和光致发光谱,对其结构特性和光学特性进行了研究.结果表明:在4.4K温度下,超晶格样品显示较强的蓝光发射,主发光峰对应于阱层ZnSe的基态电子到重空穴基态的自由激子跃迁,而且其峰位相对于ZnSe薄膜的自由激子峰有明显蓝移.从理论上分析计算了由应变和量子限制效应引起的自由激子峰位移动,理论和实验结果相吻合.     

用调制光谱研究半导体体材料及微结构的非线性极化率

提出了用调制光谱信号强度表征半导体体材料及微结构的非线性极化率,研究了测量的原理和方法．对玻璃中量子点电反射调制光谱信号强度随团簇颗粒尺寸的不同而产生几个数量级变化的原因作出了解释     

激光诱导生成锗纳米晶体量子点

采用氧化和析出的方法在氧化硅中凝聚生成锗纳米晶体量子点结构.其形成的锗晶体团簇没有突出的棱角和支晶结构,锗晶体团簇的轮廓较圆混,故可以用球形量子点模型来模拟实际的锗晶体团簇.对比了在高温(800℃～1000℃)条件下和在低温(200℃～500℃)用激光照射条件下所生成的锗纳米晶体结构的PL光谱和对应的锗纳米晶体团簇的尺寸分布.低温用激光照射条件下所生成的锗纳米晶体较小,其PL光谱出现蓝移.用量子点受限模型计算了锗纳米晶体团簇的能隙结构,用Monte Carlo方法模拟了PL光谱和对应的锗纳米晶体团簇的尺寸分布,分别与实验结果吻合较好.     

用调制光谱研究半导体体材料及微结构的非线性极化率

提出了用调制光谱信号强度表征半导体体材料及微结构的非线性极化率,研究了测量的原理和方法。对玻璃中量子点电反射调制光谱信号强度随团簇颗粒尺寸的不同而产生几个数量级变化的原因作出了解释。     

半导体量子点的尺寸分布对其荧光的影响

基于量子局域模型，用类似Ｋａｎｅ平均场方法和Ｌｉｆｓｈｉｔｚ几率观点获得半导体量子点体系的光荧光谱．对直径为ｄ的半导体量子点尺寸用对数高斯或高斯分布描述，研究表明：光荧光谱的线型在短波边不对称，与实验观测一致；尺寸服从一定的分布导致光荧光峰红移，可用于获得表观激子束缚能；尺寸分布对光荧光峰的宽度起重要作用     

飞秒激光流动烧蚀制备ZnSe有机复合量子点

采用飞秒激光流动烧蚀法制备了聚乙烯亚胺(PEI)包覆的ZnSe量子点水相分散液。水相分散液外观呈现亮黄色透明液体状。多个ZnSe有机复合量子点在水中聚集形成球形胶束,通过扫描电子显微镜(SEM)检测发现,胶束粒径为40~100 nm。利用高分辨透射电子显微镜(HRTEM)和X射线衍射(XRD)研究ZnSe晶体学特性变化,结果表明制得的ZnSe量子点保持了块体ZnSe的立方闪锌矿晶型。该ZnSe有机复合量子点的水分散液在365 nm紫外光照射下显示出明亮的绿色荧光,荧光中心波长约在500 nm处。采用光致发光光谱和紫外可见吸收光谱研究了该有机复合量子点的pH值响应特性,结果表明随着分散液中pH值由9降低到4,光致发光光谱显示出蓝移特性,波长最大蓝移量为25 nm。讨论了绿色荧光的来源与荧光波长调谐的可能机理。     

量子点超辐射发光管研究进展

简要回顾了超辐射发光管(SLD)的发展历史、量子点SLD的提出及优点,介绍了SLD的工作原理、器件结构及表征参数,详细分析了近年来国内外各研究小组在提高量子点SLD性能方面的研究进展。基于自组织量子点的尺寸非均匀分布特征、优化的有源区结构设计以及高的光学质量,量子点SLD目前的研究水平已远远超过量子阱SLD。例如,量子点SLD的输出光谱宽度可达到150nm以上,输出功率可达到百mW量级。简要介绍了SLD在光纤陀螺仪、光学相干断层成像术、光纤通信、宽带外腔可调谐激光器等方面的应用,讨论了量子点SLD研制中存在的问题、解决方法和发展趋势。量子点SLD在展宽光谱和提高输出功率上展示了巨大的潜力,它的成功有力地推动了其他宽增益谱器件的研制。     

扫描切变力/近场光学显微镜研制及应用

在发展光纤探针制备和探针与样品近场间距非光学控制等关键技术基础上，我们研制成能与倒置光学显微镜联合使用的扫描切变力／近场光学显微镜，并具有反射和透射等工作模式以及能在溶液环境中工作。利用这套系统，获得了多种样品的表面形貌和近场光学图像以及细胞内的荧光光谱。本文将对该系统和有关实验结果作简要介绍。     

ZnSe／ZnS超晶格激子能级的计算及实验研究

从理论和实验上研究了ZnSe/ZnS超晶格的激子能级,采用LCAO理论和Kronig-Penney模型计算了超晶格中电子、空穴和激子能带随垒宽的变化,测量了垒宽为5.8nm超晶格的低温吸收光谱和发光光谱,实验结果与理论一致。     

溶液环境中工作的扫描近场光学/荧光光谱显微术

我们采用双压电陶瓷片发展了一种切变力非光学检测探针一样品间距控制方法。基于这种方法，研制成能与倒置光学显微镜联合使用的扫描近场光学，荧光光谱显微系统，实现了在溶液环境中探针一样品间距的可靠控制和成像，获得了人体乳腺癌细胞内的近场荧光光谱。本文简要介绍该系统的工作原理以及利用该系统得到的实验结果。     

Exciton photoluminescence and energy in a percolation cluster of ZnSe quantum dots as a fractal object

The results of studies of samples containing ZnSe quantum dots with a density corresponding to or considerably higher than the exciton percolation threshold, at which quantum dots form conglomerates, are reported. Excitonic emission from a percolation cluster of bound quantum dots as a fractal object is observed for the first time. Analysis of the structure of the photoluminescence spectra shows that the spectra are determined by the contribution of exciton states that belong to different structural elements of the percolation cluster, specifically, to the skeleton (backbone), dangling (dead) ends, and internal hollow spaces. A qualitative model is proposed to interpret the dependence of the exciton energy in these structural elements on the concentration of quantum dots in the material.     

硒化锌量子点的热解组装及光学性质研究

报道了在无水无氧环境下利用热解反应将硒化锌量子点组装入介孔二氧化硅有序孔道内的方法,采用X射线衍射、透射电子显微成像及能量散射谱等测试方法分析了体系的微观结构,并采用紫外-可见光吸收光谱仪和荧光光谱仪研究了其光学性质.结果表明硒化锌量子点被组装在介孔二氧化硅的孔道内;与硒化锌体材料本征吸收谱相比,组装在介孔内的量子点的吸收光谱表现出显著的蓝移,这归结于量子尺寸效应.     

Growth and photoluminescence study of ZnSe quantum dots

We report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.     

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.     

Evolution of exciton states near the percolation threshold in two-phase systems with II–VI semiconductor quantum dots

From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, “a dielectric trap” is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.     

Spectroscopy of exciton states of InAs quantum molecules

Tunnel-coupled pairs of InAs quantum dots (quantum molecules) were formed by molecular beam epitaxy in a GaAs matrix. Optical and structural properties of the obtained quantum molecules were studied. Four molecular exciton states forming a photoluminescence spectrum were revealed. The photoluminescence decay times indicate the possibility of interlevel radiative recombination from the second excited state, which is of particular importance for designing mid-infrared devices.     

Photoluminescence and Confinement of Excitons in Disordered Porous Films

The exciton confinement effect in quantum dots at the surface of SiO₂ spheres and the percolation phase transition in films based on a mixture of pure SiO₂ spheres and spheres covered by CdS quantum dots (SiO₂/CdS nanoparticles) are studied. It is found that, due to the high surface energy of spheres, the quantum dots deposited onto their surface are distorted, which modifies the exciton confinement effect: the effect is retained only in one direction, the direction normal to the surface of the spheres. As a result, the energy of the exciton ground state exhibits a complex dependence on both the quantum-dot radius and sphere size. In the optical spectra of films based on this mixture, the clustering of small-sized nanoparticles and then, at a critical concentration of nanoparticles of ~60%, the formation of a percolation cluster are detected for the first time. The critical concentration is twice higher than the corresponding quantity given by the model of geometrical “colored percolation”, which is a consequence of interaction between submicrometer nanoparticles. The relation between the basic parameters of the percolation transition, such as the film porosity, coordination number, and the quantity defining the number of particles in the percolation cluster, is obtained and analyzed.     

Quantum-Dot Sources for Single Photons and Entangled Photon Pairs

Quantum dots show excellent promise as triggered sources of both single and polarization entangled photons for quantum information applications. Our recent progress developing nonclassical light sources with single quantum dots is presented in this paper. Following radiative emission of an exciton confined in a quantum dot, there is a finite delay before re-excitation can occur; this results in an anti-bunching of the photons emitted providing a source of single photons. Excitation of a quantum dot with two electrons and two holes leads to the emission of a pair of photons; we show here that, provided the spin splitting of the intermediate exciton state in the decay is erased, the photon pair is emitted in an entangled polarization state. The fidelity of this entangled state is shown to exceed 70%. Using quantum dots to generate quantum light allows contacts for electrical injection to be integrated into a compact and robust device. A cavity may also be integrated into the semiconductor structure to enhance the photon collection efficiency and control the recombination dynamics. We detail a process to form a submicrometer current aperture within an electrical device, allowing individual quantum dots to be addressed electrically in devices.     

MO-PPV/ZnSe量子点复合材料的制备及光致发光特性研究

采用水相法制备了颗粒尺寸为3.75nm的硒化锌(ZnSe)量子点,采用表面活性剂将ZnSe量子点转移到有机相聚(2-甲氧基-5-辛氧基)对苯乙炔(MO-PPV)中,获得了MO-PPV/ZnSe复合材料。通过对MO-PPV和ZnSe量子点的吸收光谱(ABS)和光致发光(PL)光谱的研究发现,随着ZnSe量子点掺杂浓度的提高,复合材料的发光强度明显增强,发光峰位置出现了蓝移。当ZnSe∶MO-PPV的质量比为1∶0.181时,发光峰位置蓝移10nm。结果表明,MO-PPV与ZnSe量子点之间存在着能量传递,这是导致MO-PPV/ZnSe量子点复合材料具有PL增强的重要原因。