Self-instability and bending behaviors of nano plates

This paper aims at investigating the size-dependent self-buckling and bending behaviors of nano plates through incorporating surface elasticity into the elasticity with residual stress fields. In the absence of external loading, positive surface tension induces a compressive residual stress field in the bulk of the nano plate and there may be self-equilibrium states corresponding to the plate self-buckling. The self-instability of nano plates is investigated and the critical self-instability size of simply supported rectangular nano plates is determined. In addition, the residual stress field in the bulk of the nano plate is usually neglected in the existing literatures, where the elastic response of the bulk is often described by the classical Hooke’s law. The present paper considered the effect of the residual stress in the bulk induced by surface tension and adopted the elasticity with residual stress fields to study the bending behaviors of nano plates without buckling. The present results show that the surface effects only modify the coefficients in corresponding equations of the classical Kirchhoff plate theory.

An Improved CE/SE Scheme for Numerical Simulation of Gaseous and Two-Phase Detonations

A new structure of solution elements and conservation elements based on rectangular mesh was pro- posed and an improved space-time conservation element and solution element (CE/SE) scheme with sec- ond-order accuracy was constructed. Furthermore, the application of improved CE/SE scheme was extended to detonation simulation. Three models were used for chemical reaction in gaseous detonation. And a two-fluid model was used for two-phase (gas–droplet) detonation. Shock reflections were simu- lated by the improved CE/SE scheme and the numerical results were compared with those obtained by other different numerical schemes. Gaseous and gas–droplet planar detonations were simulated and the numerical results were carefully compared with the experimental data and theoretical results based on C–J theory. Mach reflection of a cellular detonation was also simulated, and the numerical cellular pat- terns were compared with experimental ones. Comparisons show that the improved CE/SE scheme is clear in physical concept, easy to be implemented and high accurate for above-mentioned problems.

Two-Phase Flow In Anode Flow Field Of A Small Direct Methanol Fuel Cell In Different Gravities

An in-situ visualization of two-phase flow inside anode flow bed of a small liquid fed direct methanol fuel cells in normal and reduced gravity has been conducted in a drop tower. The anode flow bed consists of 11 parallel straight channels. The length, width and depth of single channel, which had rectangular cross section, are 48.0, 2.5 and 2.0 mm, respectively. The rib width was 2.0 mm. The experimental results indicated that when the fuel cell orientation is vertical, two-phase flow pattern in anode channels can evolve from bubbly flow in normal gravity into slug flow in microgravity. The size of bubbles in the reduced gravity is also bigger. In microgravity, the bubbles rising speed in vertical channels is obviously slower than that in normal gravity. When the fuel cell orientation is horizontal, the slug flow in the reduced gravity has almost the same characteristic with that in normal gravity. It implies that the effect of gravity on two-phase flow is small and the bubbles removal is governed by viscous drag. When the gas slugs or gas columns occupy channels, the performance of liquid fed direct methanol fuel cells is failing rapidly. It infers that in long-term microgravity, flow bed and operating condition should be optimized to avoid concentration polarization of fuel cells.

基于畸变相位波前分形特征产生矩形湍流相屏

在包含时间进程的光波大气传输及其自适应光学相位校正的数值模拟研究中,如长曝光成像和自适应光学系统的动态控制过程,矩形湍流相屏的产生和应用尤为重要.而现在通常使用的功率谱反演法产生的是正方形的湍流相屏,只采用其中的矩形部分显然造成计算机资源的浪费;并且谱反演法产生的湍流相屏需要进行低频补偿,从而明显地增加计算量.基于大气湍流所造成的畸变相位波前的分形特征,提出了一种产生矩形湍流相屏的新方法,并与解析理论结果进行对比,验证了这种矩形相屏产生方法的正确性.与已有的方法相比,此算法具有两个明显的优点:算法简单、计算效率高,节省计算机资源;与大气湍流介质统计特性无论在高频部分还是在低频部分均符合得较好.

高精度微结构聚合物光栅的复制技术

光栅复制是降低光栅制造成本,提高产量的一条有效途径。研究了利用紫外压印技术复制微结构光栅的方法。使用玻璃基底矩形浮雕结构的微结构光栅作为母光栅,给出了利用聚二甲基硅氧烷(PDMS)制作光栅模具和在光敏聚合物材料上复制微结构光栅的详细过程。经过优化工艺条件,成功地复制了一系列不同周期和开口比的微结构光栅,测试了复制光栅和母光栅的衍射图像和0级与±1级的衍射强度,结果表明,复制光栅和母光栅的衍射图像与光强分布基本一致。

Beam splitting of low-contrast binary gratings under second Bragg angle incidence

Beam splitting of low-contrast rectangular gratings under second Bragg angle incidence is studied. The grating period is between lambda and 2 lambda. The diffraction behaviors of the three transmitted propagating orders are illustrated by analyzing the first three propagating grating modes. From a simplified modal approach, the design conditions of gratings as a high-efficiency element with most of its energy concentrated in the -2nd transmitted order (similar to 90%) and of gratings as a 1 x 2 beam splitter with a total efficiency over 90% are derived. The grating parameters for achieving exactly the splitting pattern by use of rigorous coupled-wave analysis verified the design method. A 1 x 3 beam splitter is also demonstrated. Moreover, the polarization-dependent diffraction behaviors are investigated, which suggest the possibility of designing polarization-selective elements under such a configuration. The proposed concept of using the second Bragg angle should be helpful for developing new grating-based devices. (C) 2008 Optical Society of America.

A metal-mirror-based reflecting polarizing beam splitter

We theoretically investigated the design of a metal-mirror-based reflecting polarizing beam splitter (RPBS). The metal mirror is a silver slab, which is embedded in the substrate of a rectangular silica transmission grating. By using a modal analysis and rigorous coupled-wave analysis, an RPBS grating is designed for operation at 1550 nm. When it is illuminated in Littrow mounting, the transverse electric (TE) and transverse magnetic (TM) waves will be mainly reflected in the minus-first and zeroth orders, respectively. Moreover, a wideband RPBS grating is obtained by adopting the simulated annealing algorithm. The RPBS gratings exhibit high diffraction efficiencies (similar to 95%) and high extinction ratios over a certain angle and wavelength range, especially for the minus-first-order reflection. This kind of RPBS should be useful in practical optical applications.

Deep silicon grating as high-extinction-ratio polarizing beam splitter

A deep binary silicon grating as high-extinction-ratio reflective polarizing beam splitter (PBS) at the wavelength of 1550 nm is presented. The design is based on the phenomenon of total internal reflection (TIR) by using the rigorous coupled wave analysis (RCWA). The extinction ratio of the rectangular PBS grating can reach 2.5×105 with the optimum grating period of 397 nm and groove depth of 1.092 μm. The effciencies of TM-polarized wave in the 0th order and TE-polarized wave in the −1st order can both reach unity at the Littrow angle. Holographic recording technology and inductively coupled plasma (ICP) etching could be used to fabricate the silicon PBS grating.

宽光束进入展宽器提高输出脉冲对比度分析

根据分析啁啾脉冲对比度模型,通过光线追迹法详细分析了展宽器对输出脉冲对比度的影响.发现当扩大进入展宽器的入射光束口径,从2mm扩大到40mm时,输出脉冲的对比度提高了近两个数量级;同时发现在相同条件下,当输入脉冲为双曲正割函数时,其输出脉冲的对比度明显好于输入脉冲为高斯脉冲和矩形脉冲的情形。

侧面泵浦的短矩形掺钕磷酸盐玻璃光纤激光器

利用光纤－波导耦合器技术侧面泵浦一根4厘米长的短光纤，我们研制得到了一个红外波长的光纤激光器。该激光器的增益介质是横截面为矩形的掺钕磷酸盐玻璃光纤，其纤芯横截面尺寸为1.5×0.5 毫米，数值孔径为0.2。单横模激光可以通过该矩形光纤的增益-导引效应来获得。最大的激光输出功率为1.05瓦，斜率效率为10％。

Propagation properties of off-axis Hermite-cosh-Gaussian beam combinations through a first-order optical system

Based on the Collins integral formula, the analytic expressions of propagation of the coherent and the incoherent off-axis Hermite-cosh-Gaussian (HChG) beam combinations with rectangular symmetry passing through a paraxial first-order optical system are derived, and corresponding numerical examples are given and analysed. The resulting beam quality is discussed in terms of power in the bucket (PIB). The study suggests that the resulting beam cannot keep the initial intensity shape during the propagation and the beam quality for coherent mode is not always better than that for incoherent mode. Reviewing the numerical simulations of Gaussian, Hermite-Gaussian (HG) and cosh Gaussian (ChG) beam combinations indicates that the Hermite polynomial exerts a chief influence on the irradiance profile of composite beam and far field power concentration.

Structural and optical properties of a-plane ZnO thin films synthesized on γ-LiAlO2 (302) substrates by low pressure metal-organic chemical vapor deposition

Nonpolar a-plane (1120) ZnO thin films have been fabricated on gamma-LiAlO2 (302) substrates via the low-pressure metal-organic chemical vapor deposition. An obvious intensity variation of the E-2 mode in the Raman spectra indicates that there exhibits in-plane optical anisotropy in the a-plane ZnO thin films. Highly-oriented uniform grains of rectangular shape can be seen from the atomic force microscopy images, which mean that the lateral growth rate of the thin films is also anisotropic. It is demonstrated experimentally that a buffer layer deposited at a low temperature (200 degrees C) can improve the structural and optical properties of the epilayer to a large extent. (c) 2007 Elsevier B.V. All rights reserved.

Characterization of m-plane ZnO thin film on gamma-LiAlO2 (100) substrate by metal-organic chemical vapor deposition

Non-polar (1 (1) over bar 00)m-plane ZnO thin film has been prepared on gamma-LiAlO2 (100)substrate via the low pressure metal organic chemical vapor deposition. Obvious intensity variation of the E-2 mode in the polarized Raman spectra and the absorption edge shift in the polarized optical transmission spectra indicate that the m-plane film exhibits optical anisotropy, which have applications in certain optical devices, such as the UV modulator and polarization-dependent beam switch. From the atomic force microscopy images, highly-oriented uniform-sized grains of rectangular shape were observed. (c) 2008 Elsevier B.V. All rights reserved.