光电反馈抑制掺铒光纤激光器的低频强度噪声

通过光电反馈电路对掺铒光纤激光器的中低频噪声进行了抑制。根据速率方程理论,分析了影响掺铒光纤激光器强度噪声的因素,通过电路仿真分析优化反馈电路参数,重点讨论反馈信号的相位对噪声抑制的影响。实验表明：激光器的低频(小于20KHz)强度噪声平均降低了10dB,中频弛豫振荡峰处（30KHz附近）抑制达35dB,并且克服了光电反馈抑制强度噪声使弛豫振荡峰向高频移动,导致高频噪声增大的问题。优异的噪声特性使光纤激光器在光传感领域具有很高的实用价值。     

Determination of the exciton energy from electron beam excited luminescence in direct gap semiconductors

The exciton radiative recombination spectra of CuCl, ZnSe, CuGaS₂ and AgGaSe₂ have been investigated at 300 K. The sample was excited by electron beams of different energies. The reabsorption is determined by the ratio η of the spectra obtained for different electron beam energies such as 5 and 40 kV. This ratio is independant of hv when the absorption coefficient α is small (< 10³cm⁻¹). It increases when α becomes larger than the inverse of the penetration depth ( 3 μ) of the 40 kV electron beam. In this case, η is approximately proportional to the absorption coefficient. In particular, its maxima give the excitonic energies E_ex. We find the following values, in agreement with previous reflectivity and absorption measurements: for CuCl, E_ex(Z₃) = 3.26 eV and E_ex(Z₁₂) = 3.33 eV: for ZnSe, E_ex = 2.70 eV; for CuGaS₂, E_ex(A) = 2.46 eV and E_ex(B) = 2.59 eV. In the case of AgGaSe₂, there is no peak in the η spectrum and we have fitted our data to a broadened exciton model. We obtain E_ex(A) = 1.80 eV.     

Application of valence electron energy-loss spectroscopy and plasmon energy mapping for determining material properties at the nanoscale

Measuring material properties at the nanoscale is critical to understanding the behavior of nanostructured materials. In this paper, we demonstrate a novel technique that allows direct determination and imaging of physical properties of individual nanoprecipitates and nanoparticles using energyfiltering transmission electron microscopy combined with valence electron energy-loss spectroscopy (VEELS). We show that strong scaling correlations exist between the plasmon energy and elastic properties, hardness, valence electron density and cohesive energy. We apply these scaling relationships to characterize the elastic properties of metastable nanoprecipitates in a Ti-based structural alloy and the hardness of diesel-engine soot particles. We also discuss additional factors that need to be considered when using plasmons as a quantitative tool for nanoscale property measurement. The results show that VEELS has the potential to determine multiple solid-state properties of materials at the nanoscale, establishing a new capability for analytical electron microscopy.     

Influence of RIE-induced damage on luminescence and electron transport properties of AlGaAs-GaAs heterostructures

Reactive ion etching induced damage was systematically studied by photoluminescence (PL), cathodoluminescence (CL) and electronic microwave absorption in GaAs/AlGaAs multiple quantum well (MQW) and two-dimensional electron gas (2DEG) heterostructures. Using QW’s of differing widths at various depths, PL and CL characterization of the individual quantum wells allowed a depth sensitive detection of RIE induced damage. Etching was done with CC1₂F₂ at constant pressure and exposure time, while the bias voltage was successively increased from 55 to 320 V. A remarkable degradation in PL-intensity was observed for the topmost 1 nm QW located 30 nm beneath the surface, even at the lowest etch bias voltage. In 2DEG heterostructure samples investigated electrically, both mobility and carrier concentration of the 2DEG were seen to be strongly reduced. After illumination however, the initial values were almost completely restored, indicating that RIE damage predominantly reduces the electron supply efficiency of the AlGaAs barrier, whereas the 2DEG channel itself is not severely degraded even at the highest etch bias voltage.     

钴硅化合物的理论电子能量损失谱

本文采用扩展平面波加局域轨道方法和广义梯度近似对钴硅系中Co2Si，CoSi和CoSi：三种不同硅化物的电子结构以及电子能量损失近边结构（ELNEs）进行了理论计算。结果表明计算得到的硅化物中Co的ELNES很好反映了Co在费米能级以上d的未占据态密度分布，其中Co2Si和CoSi2具有金属性质；而CoSi呈现出半金属性质。计算还表明电子能量损失谱仪应具有足够高的能量分辨率（0．2ev），电子能量损失近边结构才能正确反映出钴硅化合物的电子结构特征。     

Determination of the LO-phonon and Γ→L intervalley scattering time in GaAs from hot electron luminescence spectroscopy

Both the LO-phonon scattering time and the Γ→L intervalley scattering time for electrons in the conduction band of GaAs are of fundamental importance, and they are needed for the modelling of devices. We measure the steady state distribution of hot electrons in lightly p-doped bulk GaAs under carrier densities of 10¹³–10¹⁴cm⁻³, which is orders of magnitude lower than in pulsed laser experiments. Using a 16×16 k.p Hamiltonian and taking into account the transition matrix elements in a dipole model, we determine the hot electron lifetime from comparison with the experimentally found lifetime broadening. For electrons with a kinetic energy of 100meV to 300meV we obtain τ_LO=(132±10)fs. We also determine the Γ→L intervalley separation as E_ΓL=(300±10)meV. We find Γ→L scattering times around 150fs to 200fs, corresponding to a value for the associated deformation potential of D_ΓL=(9.4±1.5)×10⁸eV/cm.     

新型功能化合物的晶体结构和电子能量损失谱研究

本文深入研究了六方层状结构钴氧化物中的层间阳离子排列,电荷/轨道序和金属超导体NbB2中的电子能量损失谱,阐述了现代透射电子显微镜(TEM)技术和电子能量损失谱在功能化合物研究领域的应用.基于MxCoO2(M=Na,Sr或Ca)化合物的TEM结构分析,总结给出了反映这种层状结构化合物中阳离子含量和结构特性关联的相图.以Na0.5CoO2材料作为研究主体,系统分析了材料结构随温度的变化,并对Na离子有序和结构相变进行了深入探讨.在100 K到20 K温度区间观测到了两个超结构相,其基本特性可以用电荷/轨道有序模型很好地解释.NbB2是典型的层状超导体,电子能量损失谱表现出很强的各向异性.结合第一性原理计算对其电子结构和电子能量损失谱的特性进行了仔细分析,获得了费米能级附近B的2p轨道未占据态信息.在这些实验和理论结果的基础上,系统分析了六方层状钴氧化物中结构相变和物理性能的关联,并对NbB2和MgB2超导材料电子结构区别进行了讨论.     

利用电子能量损失谱对二氧化铀电子结构的研究

锕系元素因具有复杂的5f电子结构,其显示出神秘而又独特的物理、化学特性.为了加深对锕系金属、合金和化合物的特异行为的理解,对其电子结构精确表征的重要性日益凸显.电子显微技术与电子能量损失谱技术的结合在锕系元素原子结构和电子结构研究中发挥了至关重要的作用.本文采用电子能量损失谱研究二氧化铀的5f电子结构,并讨论了铀5f电子的相互作用特性.     

X射线能谱分析方法及其在电镜酶细胞化学中的应用

近年来,X射线能谱分析和电镜酶细胞化学在许多领域中得到了广泛的应用.两者的结合把超微分析和超微观察密切结合起来,为超微结构的研究提供了可靠的分析手段.本文对X射线能谱分析的原理作简单地介绍,同时对X射线能谱分析在电镜酶细胞化学中的应用和实验方法进行了分析和讨论.     

Investigation of up-conversion luminescence of Er3+ and Er3+/Yb3+ ions doped in PLZT for active electro-optical applications

The up-conversion luminescence of Er3+ and Er3+/Yb3+ ions doped in lanthanum-modified lead zirconate titanate (PLZT) under 980 nm excitation at room temperature has been investigated. The green upconversion luminescence at 540 and 566 nm was observed in Er3+:PLZT; the greater the concentration of Er3+ ions, the stronger the intensity. In Er3+-Yb3+:PLZT, other than the green up-conversion luminescence at 540 and 566 nm, a relatively weak red up-conversion luminescence at 668 nm was also observed. Both green and red up-conversion luminescence intensities presented an approximately quadratic dependence on excitation power, which indicated that two excitation photons are involved in the up-conversion process of Er3+:PLZT and Er3+-Yb3+:PLZT. The characteristics of PLZT ceramic material were also studied by Raman spectroscopy. This work shows a promising future for developing multifunctional up-conversion electrooptical devices using Er3+ and Er3+/Yb3+ ions doped PLZT.     

Quantitative electron energy loss spectroscopy of Si nanoclusters embedded in SiOx

We have used a methodology, based on electron energy loss spectroscopy combined with energy filtered images, which allows us to quantify the clustered silicon concentration in annealed sub-stoichiometric silicon oxide layers (SiO_x). This information was coupled to the chemical silicon and oxygen concentrations determined from Rutherford backscattering analyses. The silicon agglomeration kinetics was investigated as a function of the gas flows used during the plasma enhanced chemical vapor deposition process. Although the clustered Si concentration and the average cluster radius do not follow a well defined dependence on Si, O, or N concentration, separately, the ratio between the former parameter and the silicon excess concentration decreases monotonically as a function of the nitrogen concentration approaching a saturation value equal to 0.2 for nitrogen concentrations above 10 at.%.     

并行电子能量损失谱仪中多重四极透镜系统物方视场的模拟

通过对三重四极透镜成像系统的渐晕模拟,研究了并行电子能量损失谱仪中物方视场、漂移管径以及与电荷耦合探测器（CCD）幅面尺寸的对应关系,并由它们间的相互关系确定了与CCD幅面二维探测器尺寸相匹配的漂移管径。