Infrared-to-visible upconversion flurescence of Er3+/Yb3+-codoped bismuthate glasses

Er3+/Yb3+-codoped bismuthate glasses for developing potential upconversion lasers have been fabricated and characterized. The optimal Yb3+ doping content was investigated in the glasses with different Yb3+-Er3+ concentration ratios and the optimal Yb3+-Er3+ concentration ratio is 5:1. Under 975 nm excitation, intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions H-2(11/2) -> I-4(15/2), S-4(3/2) -> I-4(15/2) and F-4(9/2) -> I-4(15/2), respectively, were observed at room temperature. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation. (c) 2004 Elsevier Ltd. All rights reserved.

Influences of initial nitridation process on the optical and structural characterization of GaN layer grown on sapphire (0001) by metalorganic chemical vapor deposition

In this paper. we investigate the influences of the initial nitridation of sapphire substrates on the optical and structural characterizations in GaN films. Two GaN samples with and without 3 min nitridation process were investigated by photoluminescence (PL) spectroscopy in the temperature range of 12-300 K and double-crystal X-ray diffraction (XRD). In the 12 K PL spectra of the GaN sample without nitridation, four dominant peaks at 3.476, 3.409 3.362 and 3.308 eV were observed, which were assigned to donor bound exciton, excitons bound to stacking faults and extended structural defects. In the sample with nitridation, three peaks at 3.453, 3.365. and 3.308 eV were observed at 12 K, no peak related to stacking faults. XRD results at different reflections showed that there are more stacking faults in the samples without nitridation.

带非吸收窗口的大功率657 nm半导体激光器

在激光器腔面处制作非吸收窗口(NAW)可以有效地减少光吸收,防止激光器过早出现光学灾变损伤(COD),是提高大功率半导体激光器的功率特性的重要手段之一.采用金属有机化学气相沉积(MOCVD)技术二次外延生长了大功率657 nm红光半导体激光器结构,通过闭管扩散Zn的方法在腔面附近制作了非吸收窗口.实验发现扩散温度550 ℃,扩散时间20 min时,得到的非吸收窗口最为有效,激光器连续工作的无扭折输出功率大于100 mW,超过常规的无窗口结构激光器的最大输出功率的两倍,激光器的斜率效率提高了23%.测量该类器件的温度特性发现,环境温度为20～70 ℃时,其输出功率均可大于50 mW,计算得到激光器的特征温度约为89 K,波长增加率约为0.24 nm/℃.

Influences of initial nitridation process on the optical and structural characterization of GaN layer grown on sapphire (0001) by metalorganic chemical vapor deposition

In this paper. we investigate the influences of the initial nitridation of sapphire substrates on the optical and structural characterizations in GaN films. Two GaN samples with and without 3 min nitridation process were investigated by photoluminescence (PL) spectroscopy in the temperature range of 12-300 K and double-crystal X-ray diffraction (XRD). In the 12 K PL spectra of the GaN sample without nitridation, four dominant peaks at 3.476, 3.409 3.362 and 3.308 eV were observed, which were assigned to donor bound exciton, excitons bound to stacking faults and extended structural defects. In the sample with nitridation, three peaks at 3.453, 3.365. and 3.308 eV were observed at 12 K, no peak related to stacking faults. XRD results at different reflections showed that there are more stacking faults in the samples without nitridation.

岩石介质细观非均匀性对宏观破裂过程的影响

运用岩石破裂过程分析系统，研究了岩石介质非均匀性对宏观力学行为的影响。通过对均质度系数ｍ＝１.５的１０个样本破裂过程的模拟，发现样本的宏观力学行为在变形初期只有微弱的统计涨落，但在失稳阶段表现出显著的样本个性行为。对８种不同均质度系数的８０个样本破裂过程的模拟结果表明，岩石介质的非均匀性对岩样宏观强度和变形非线性行为有显著影响，而且也显著影响试样破裂模式。随着均质度系数的提高，宏观强度呈现指数规律

海洋工程中的流固耦合问题

本文主要讨论海洋工程中的流固耦台问题，着重研究小尺度结构、大尺度结构、地基等不同结构与波浪和海流的相互作用，分析了这一领域中流固耦合的主要特征和前沿科学问题。

In Situ Observations On The Fracture Process Of Metallic Glassy Ribbons

Crack propagation and strain field evolution in two metallic glassy ribbons are studied using in situ scanning electron microscopy and the white digital speckle correlation method. Strain state at the crack tip, which depends heavily on the fracture toughness, plays a key role in fracture. A high degree of shear strain concentration in tough glassy ribbon can satisfy the critical shear strain, resulting in shear fracture, whereas a high degree of linear strain concentration in brittle glassy ribbon can initiate normal tensile fracture. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

平面混合流拟序结构的直接数值模拟

采用高阶精度差分格式,求解二维可压缩N-S方程,直接数值模拟了可压缩平面混合流的二维拟序结构.给出了流动失稳,Kelvin-Helmholtz不稳定波的发展.展向大涡的卷起和相邻两涡卷对并,包括3次对并的发展过程.研究了平面混合流时-空的发展和可压缩效应对其发展的影响.

微重力科学进展

微重力科学是研究重力极大地减小的一种极端环境中的流体科学、材料科学、生物科 学和技术,以及物理学若干基础问题的新兴学科.近年来,由于地面研究的加强,空间实验 的增加以及实验定量化水平的提高,微重力研究取得了很好的进展.

湍流壁函数的一般形式

本文给出了一般形式的湍流壁函数。该壁函数适用于具有壁面引射的湍流计算。广为应用的无引射的湍流壁函数是该壁函数的特例。

晶体生长中的流体力学问题

流体力学与晶体生长有密切的关系.在晶体生长中,流体中的输运过程和流动的不稳定性对生成晶体的结构和质量有重要影响.本文概述了晶体生长中与流体力学有关的若干问题.首先对晶体生长的一些基本方法作了简介,然后对晶体生长中的流动现象及其与晶体缺陷的关系进行了讨论,最后提出了几种控制流动以改进晶体生长技术的可能途径.

弱扭转非轴对称磁场的静力学平衡

<正> 我们曾证明非轴对称磁流体静力学位形的存在性,并讨论了其基本特征。从观测上看,宇宙磁场往往有三维位形。比如太阳黑子有明、暗的纤维。从理论上分析,外加重力场引进以后改变了宇宙磁场必须有某种对称性的结论。高分辨率的白光观测还表明,太阳黑子磁场及其暗条经常扭转呈螺旋形。本文将文献[1]的结论推广到具有扭转位形的非轴对称磁流体静力学平衡,仍然采用摄动展开的方法求解。

A note on the heat-transfer to nonspherical particles from plasma

Heat transfer from plasma to a nonspherical partical in the free-molecular regime is studied in the present paper under thin plasma sheath condition. Analytical expressions for the floating potential charge and heat fluxes of an ellipsoid particle of revolution are derived and curves are given for key parameters for arbitrary plasma flow direction. On the basis of these results, an equivalent sphere with the same surface area as the nonspherical particle is suggested to be used for calculating the total heat flux of nonspherical particle in engineering application with acceptable accuracy. Furthermore, the effects of particle rotation, which occurs in most aerosol systems, on the heat transfer are also discussed.

Design and application of three-zone annular filters

We design three-zone annular filters to be applied to optical storage system. The designed filters extend the depth of focus and realize transverse superresolution simultaneously, which will improve the performance of optical storage system greatly. And we propose two feasible schemes to improve imaging resolution of three-dimensional imaging system. One scheme depends on a complex filter formed by cascading of a three-zone phase filter and a three-zone amplitude filter. The complex filter converge the optimized transverse superresolution and the optimized axial superresolution of two different filters onto a single filter. It can improve the three-dimensional imaging performances greatly. Another scheme depends on a single three-zone complex filter. We propose a three-zone complex filter with phase shift 0.8 pi, which presents bigger design margin, better imaging quality and stronger three-dimensional superresolution capability. (c) 2006 Elsevier GmbH. All rights reserved.