冰洲石晶体Wollaston棱镜分束角的温度效应

为了了解温度变化对冰洲石晶体W0llaston棱镜分束角的影响,首先需决定棱镜分束角的两个因素,即棱镜的结构角和晶体中o光、e光的主折射率;然后从温度对这两个因素的影响出发,从理论上对冰洲石晶体Wollaston棱镜中的o光、e光分束角的温度效应进行了研究。结果表明,随着温度的升高,两分束角减小,但o光减小的幅度要比e光大得多。建立了实验测试系统,实验测试结果与理论分析结果相吻合。     

飞秒激光倒锥微孔加工的反射式扫描装置设计

基于道威棱镜扫描原理,设计了由高速旋转电机驱动三片K字形排布反射镜组成的反射式扫描装置,并分析了扫描钻孔准确度.结果表明:该扫描装置可用于飞秒激光高准确度环切钻孔系统,钻孔准确度不受光束质量影响,钻孔扫描速度二倍于电机转速且钻孔效率高、兼容多光谱波段;与道威棱镜系统相比,该系统装调准确度要求低,能避免透射系统中色散引起的飞秒激光脉宽展宽现象.测试结果显示该装置可以实现直径0.05~0.2mm、加工准确度小于±2μm倒锥孔的高效率加工.     

Tc6三棱柱原子簇电子结构探讨

本文用SCC-DV-Xα方法计算了Tc_6Cl_(12)~(2-)、Tc_6Cl_(14)~(3-)、Tc_6Br_(12)等三棱柱原子簇.结果表明,Tc_6Cl_(12)~(2-)与Tc_6Cl_(14)~(3-)金属键轨道大体相似,只是Tc_6Cl_(14)~(3-)较前者多一个反键轨道.Tc_8Br_(12)有22个金属键轨道,分别由2个、4个或8个Tc原子组成,形式有双中心键,多中心键或离域键.过渡金属原子簇众多的d轨道与三棱柱构型的特点,使金属键变得多种多样。文中附有部分金属键轨道图。     

微棱镜阵列型定向反射器特性

介绍了微棱镜阵列型定向反射器的材料折射率对入射孔径角的限制,定向反射率随入射角的变化关系,特别是提出了单元有效反射面积的计算方法,并进行了计算机模拟和实验,为该元件的设计和制作提供了理论依据。     

“三位水平法”快速调节分光计

分光计构造精密,它的调节技术是光学实验中的基本技术之一。应用分光计测定棱镜折射率时,仪器的调节存在两个难点。在此介绍借助圆水准泡,应用"三位水平法"快速解决这两个难点,方法简单、易操作,可在物理实验教学中应用。     

DNS of heat transfer in turbulent and transitional channel flow obstructed by rectangular prisms

Direct numerical simulation (DNS) of heat transfer in a channel flow obstructed by rectangular prisms has been performed for Re_τ = 80–20, where Re_τ is based on the friction velocity, the channel half width and the kinematic viscosity. The molecular Prandtl number is set to be 0.71. The flow remains unsteady down to Re_τ = 40 owing to the disturbance induced by the prism. For Re_τ = 30 and 20, the flow results in a steady laminar flow. In the vicinity of the prism, the three-dimensional complex vortices are generated and heat transfer is enhanced. The Reynolds number effect on the time-averaged vortex structure and the local Nusselt number are investigated. The mechanism of the heat transfer enhancement is discussed. In addition, the mean flow parameters such as the friction factor and the Nusselt number are examined in comparison with existing DNS and experimental data.     

Regular graphs are not universal fixers

For any permutation π of the vertex set of a graph G, the graph πG is obtained from two copies G^′ and G^″ of G by joining u∈V(G^′) and v∈V(G^″) if and only if v=π(u). Denote the domination number of G by γ(G). For all permutations π of V(G), γ(G)≤γ(πG)≤2γ(G). If γ(πG)=γ(G) for all π, then G is called a universal fixer. We prove that regular graphs and graphs with γ=4 are not universal fixers.     

Periodic adjustment of the position of a laser beam spot on a semiconductor saturable absorber mirror in a passively mode-locked solid-state laser

A laser diode end-pumped passively mode-locked Nd:YVO₄ solid-state laser with a semiconductor saturable absorber mirror (SESAM), in which the intracavity laser beam spot on the SESAM can be adjusted periodically, is investigated. Inserting a rectangular prism (RP) into the laser cavity is a promising approach towards the goal of periodically moving the position of the focus spot of the intracavity pulse on the SESAM surface to avoid the long-time irradiation of the laser beam on the same position, thereby solving a series of problems caused by damage to the SESAM and greatly prolonging its usage life. The adjustment of the rectangular prism in the laser cavity does not break the stable continuous wave (CW) mode-locked condition. The laser generates a stable picosecond pulse sequence at 1064 nm with an output power of 3.6 W and a pulse width of 14 ps. The instabilities of the output power and the pulse width are 1.77% and 4.5%, respectively.     

Switchable photonic crystal for polymer dispersed liquid crystal

Single-prism systems are used to fabricate electrically switchable Photonic Crystal (PhC) structures in Polymer Dispersed Liquid Crystal (PDLC) films. The optical configuration is simple, stable and repeatable. A good agreement between theoretical and experimental results is obtained for the PhC structures. Hexagonal far-field diffraction patterns and electrical switching characteristics are also investigated. The smallest droplets of liquid crystals are 10 nm in diameter. The switching voltage can be decreased to 13.3 V/μm.     

折返式激光二极管侧抽运Nd:YAG激光器

提出一种明显改善激光二极管侧面直接抽运固体激光器输出光束质量的新型谐振腔——折返式谐振腔,它由三部分组成：直角棱镜、全反的腔镜以及耦合输出腔镜,采用振荡光顺次通过晶体内增益场强区的工作方式。它重叠利用不均匀增益场的对称强区,提高了晶体增益场和谐振腔基模的重叠度,达到改善器件输出光束质量和提高效率的目的。通过对受抽运晶体内的增益场分布及腔内振荡光的模拟分析以及实验测试,验证了这种谐振腔的优势。在此基础上,研制了采用准连续线阵列激光二极管抽运Nd：YAG的折返式激光器。研究比较了在不同类型谐振腔下,激光器的输出特性。例如：平-平直腔和平-平折返腔在耦合透过率为50％时的输出特性,其增益场和基模的重叠度由24％提高到53％,光-光斜效率由的20．2％提高到27．3％。