Three-dimensional temperature measurement in turbulent thermal convection by extended range scanning liquid crystal thermometry

The usable temperature range of liquid crystal thermometry has been extended and used to measure three-dimensional temperature fields in turbulent thermal convection. The color of the liquid crystals is calibrated against temperature using the standard method in which hue is the single input variable and two new methods: hue/intensity as input variable, and hue, saturation and intensity as input variables to a neural network. Relative to the hue calibration, the new methods extend the range over which temperature can be measured by more than 100%. Three-dimensional temperature measurements of turbulent thermal convection over smooth surfaces were carried out at a flux Rayleigh number of 3 × 10⁹ by scanning a white light sheet normal to the visualized image plane and capturing a number of sequential images at various positions of the light sheet. Stacks of the planar data were composed into three-dimensional temperature distributions. The results indicate the presence of an irregular spoke pattern over the surface and the generation of plumes from the intersections of the patterns, consistent with other investigations.     

Optimization of thickness and uniformity of photonic structures fabricated by interference lithography

We study the influence of the absorption of materials used for holographic fabrication of photonic structures on their uniformity along the film thickness. We demonstrate theoretically and experimentally a strong dependence of structure thickness and uniformity on the exposure dose of the interference pattern. A novel technique is proposed to overcome the absorption effect and to fabricate thick two- and three-dimensional structures, which are uniform throughout the film thickness. It consists of exposing once again the sample by an additional and independent counterpropagating uniform beam, which allows to compensate the diminution of the light intensity of interference pattern. These results are very useful for the fabrication of high quality polymer-based photonic crystals.     

Charge conversion of uranium ions in CaF2

Single crystals of CaF₂ doped with uranium ions exhibiting a light red colour have been grown in an inert atmosphere. ESR spectra belonging to U³⁺ centers unstable at room temperature could be observed only after UV-irradiation at liquid nitrogen temperature. The formation and annealing of these centers as well as the optical absorption date indicate that a new charge conversion mechanism is effective in such crystals.     

Determination of the liquid crystals phase transition temperatures using optical rotation effect

Using optical rotation effect, a sensitive, simple optical analytical system is developed for determining the phase transition temperatures of liquid crystals (LCs). When a monochromatic polarized light passes through LCs sample and analyzer, the light intensity changes with temperature. Especially, during the phase transition process, the intensity varies greatly due to optical rotation effect. The variation of light intensity versus variation of temperature curve shows the phase transition temperatures of LCs clearly. The phase transition temperatures of three cholesteric liquid crystals (ChLCs) and a nematic liquid crystals (NLCs) were detected by this method, and compared with those of the differential scanning calorimetry (DSC) and polarized light microscope (PLM) methods.     

基于Ce:YAG单晶的白光发光二极管性能研究

以Ce:YAG单晶取代传统Ce:YAG荧光粉用于制备白光发光二极管(LED),研究了Ce:YAG单晶厚度及驱动电压的变化对其发射光谱、色坐标、亮度、光视效能和色温的影响。研究结果表明,在基于Ce:YAG单晶的白光LED中,发射光的色坐标以及蓝光与黄绿光之间的相对强度可通过对Ce:YAG单晶片厚度的改变进行调整。在恒定电压驱动下,白光LED样品的亮度、光视效能和色温均随单晶片厚度的减小而增加。当Ce:YAG单晶厚度为0.6mm时,可获得较纯的白发射光,并且其色坐标具有较高的可靠性和稳定性,基本不受驱动电压变化的影响。研究结果表明Ce:YAG单晶是一种可用于新型白光LED的理想荧光材料。     

向列相液晶中光致非线性效应的研究

分析了向列相液晶中的光致非线性效应.在此基础上,基于液晶显示器工作原理设计了光控光器件.提出简化模型并利用琼斯矩阵给出光在该模型中的传输理论;引入斜入射的控制光束代替外加电压来调控液晶盒中分子的排列,实现对出射信号光的位相和光强的光-光调控;利用5CB液晶的温度特性分析环境温度对光-光调控的影响.理论分析表明:出射光位...     

Orientation phenomena in nematic liquid crystals under a periodic compressional deformation

The orientation behavior of homogeneous planar layers of nematic liquid crystals with open and closed ends in the field of compressional deformations caused by an acoustic effect is studied. The mechanisms determining the connection of the optical response of a nematic liquid crystal (the variable component of an optical signal and its spectrum) with the acoustic parameters (the oscillation amplitude and frequency, and the amplitude of sound pressure) and the layer thickness are revealed. The factors responsible for the mechanism and modes of acoustooptic conversion are considered. It is demonstrated that, by varying the layer thickness, it is possible to implement different modes of signal conversion. The possibility of designing a new modification of a sound receiver based on a nematic liquid crystal and the specific features of this design are discussed. Its advantages over conventional sound pressure receivers based on nematic liquid crystals are indicated, in particular, the absence of limitation of the frequency of the received signal in the low-frequency range.     

Photoalignment of liquid crystals by cinnamate polyelectrolyte layer-by-layer ultrathin film

In this paper, a new layer-by-layer (LBL) ultrathin film with photosensitive cinnamoyl chromophores was prepared. It could induce uniform alignment of liquid crystals (LC) after irradiated by linearly polarized ultraviolet (LPUV) light. First, a photosensitive polycation containing cinnamoyl side groups was synthesized. Then a layer-by-layer ultrathin film was prepared in an aqueous solution of the polycations and poly (sodium 4-styrenesulphonate). The spectra of UV-vis absorption and ellipsometric measurement showed that the LBL film was uniform and suitable for the photoalignment of LC. When the film was irradiated by LPUV light, the cinnamoyl units in the film underwent [2 + 2] cycloaddition along the electric vector direction of the LPUV light. The polarized UV-vis spectra also proved that the film was anisotropic, and then the film could induce the homogeneous alignment of nematic LC. With the number of bilayers increasing, the alignment effect of multilayer film became better, and the contrast ratio became higher.     

脉冲激光沉积制备CsPbBr3厚膜及其蓝-绿光转换性能

金属卤化物钙钛矿CsPbBr3具有优异的光学性能,是作为波长转化层在液晶显示中实现全彩显示的理想材料。为了实现高效的蓝光到绿光的光转换,采用脉冲激光沉积技术(PLD)制备CsPbBr3微米级厚膜,通过设定激光脉冲数实现膜厚的有效调控,并借助扫描电子显微镜(SEM)、X射线粉末衍射(XRD)、紫外-可见吸收光谱等测试手段对其形貌、晶体结构和光学性质进行分析。然后,将CsPbBr3微米级厚膜沉积在发射波长为460 nm的蓝光发光二极管上,并测试光转换性能。实验结果表明,制备的CsPbBr3厚膜由(100)取向的柱状晶体组成,且膜厚与激光脉冲数呈线性关系,在膜厚为2.252μm时,在460 nm的蓝光激发下实现了完全、有效绿光的发射。此外,在空气氛围(温度20℃,湿度25%)下放置18 d,光致发光强度无明显衰退。     

Optical measurement of instantaneous liquid film thickness based on total internal reflection

An optical method of measuring the instantaneous transparent liquid film thickness, based on the effect of total internal reflection by the gas-liquid interface, is represented. A light circle whose diameter is proportional to the liquid film thickness and eccentricity is proportional to the slope gradient of the liquid film surface in the measurement region is formed around a small-size light source in the plane of vessel bottom. Using several light sources, the method allows measurement of the liquid film thickness field. The method is implemented in a multichannel meter of transparent liquid film thickness and curvature surface, with computer image processing.     

色偏振的理论及应用

本文就色偏振的理论作了详尽的分析。重点分析振幅Ａ１、Ａ２与角φ的关系，区别单色的干涉光与非干涉的白色背景光，得出在色偏振中光强、色纯度、互补色、变色位置和色调变化的规律。同时也对某些应用作了探讨。     

脉冲电场对新鲜干红葡萄酒酚类物质和色泽影响的研究

以新鲜干红葡萄酒为研究对象,采用高效液相色谱(HPLC)法测定分析了脉冲电场(PEF)处理前后葡萄酒中黄烷-3-醇(flavan-3-ols)和酚酸(phenolic acids)２种酚类物质的含量变化以及葡萄酒色度与色调值的变化。实验所采用的高压脉冲电场设备为清华大学自行研制,其输出波形为指数衰减波;PEF处理系统所采用的处理室为平板处理室,电极板间距为5 mm,并且在实验过程中进行了电场参数优化。经实验研究发现,脉冲电场(电场能量密度： 24.5,40.5,60.5 J·mL-1)处理后大部分酚类物质的含量都发生了显著变化,葡萄酒色度与色调值也有显著的提高,并且当注入电场能量密度为60.5 J·mL-1时处理效果最显著。     

积分光谱法测量塑料薄膜厚度研究

在谱带朗伯定律概念的基础上,通过分析光束在透过塑料薄膜时光强所发生的变化,提出了谱带积分透过率定义并建立基于谱带积分透过率的薄膜厚度测量模型。采用不同厚度的聚丙烯薄膜作为实验对象,使用光谱仪测量了其光谱透过率。按上述模型拟合了薄膜厚度与谱带积分透过率关系式。使用500K理想黑体,分析了基于此方法研制新型宽谱带塑料薄膜厚度在线检测装置的可行性。实验结果表明：使用积分光谱法可以较高精度的测量塑料薄膜厚度。基于此方法研制的塑料薄膜厚度在线检测装置,有望解决使用双单色光对比法测薄膜厚度时存在的准确性低、通用性差的问题。     

复合周期的介质-液晶光子晶体研究

本文研究了掺入各向同性周期性介质层缺陷的一维复合周期胆甾型液晶光子晶体的禁带特性.发现同一般的胆甾型液晶不同,复合介质-液晶周期结构对左旋偏振入射光和右旋入射光都会出现共同禁带.当各向同性介质层占一个周期的厚度比例越小,左旋偏振光的禁带效应逐渐消失,而右旋偏振光的禁带则逐渐合并变大;且各向同性介质的折射率越大,则两种偏振光的禁带效应越明显,更容易出现共同禁带;通过调整厚度比,可以在可见光范围内出现对应红、绿、蓝色波长的禁带,从而可以用于液晶显示的反射式彩色滤光片. 关键词： 胆甾型液晶 光子晶体 禁带 缺陷     

Coherent backscattering of light in nematic liquid crystals

Multiple light scattering by director fluctuations in nematic liquid crystals is considered. A uniform director orientation is assumed to be specified by an applied magnetic field. The coherent backscattering effect, which consists in the presence of a sharp light backscattering peak, is studied. The Bethe-Salpeter equation is used to calculate the multiple scattering intensity taking into account the contributions of ladder and cyclic diagrams. An analytical expression for the angular and polarization dependences of the coherent backscattering intensity is obtained in terms of the diffusion approximation. The calculation and experimental results are compared. The developed theory is shown to qualitatively describe the elliptical shape of the backscattering cone, to explain the absence of a coherent contribution for crossed polarizations, and to calculate the relative peak height.     

On the Hydrothermodynamics of the Icing of a Wing Profile in the Air-Crystalline Flow

An experimental and theoretical complex was created to study the physical processes accompanying the interaction of the air flow carrying ice crystals with the heated surface of the streamlined body. An effective coefficient of phase transformations (evaporation and transition—through melting and solidifying in a liquid film—into barrier ice) of the mass of crystals bombarding a dry or moistened heated surface taking into account their partial entrainment with flowing air has been found. The physical and mathematical model of hydrothermodynamics of a liquid film is developed, the numerical data for the thickness, velocity, and temperature, complementing the results of the experiment, are obtained.     

纳米级润滑膜分子排列取向的拉曼光谱表征技术

利用激光拉曼散射技术,对剪切作用下受限于钢球与石英盘之间的纳米级液晶5 CB的分子排列取向进行研究. 结果表明,在特定的实验条件下,可以得到高信噪比的纳米级润滑膜的拉曼散射信号(20∶1). 同时发现,当激光偏振方向与剪切运动方向平行(垂直)时,所得拉曼信号强度达到最大(小)值,表明纳米级液晶5 CB分子在剪切诱导作用下,沿剪切运动方向趋于定向排列. 另外,当钢球与石英盘之间的剪切速度逐渐增大时,受限的纳米级液晶5 CB的拉曼信号强度也逐渐增大. 最后,利用根据相对光强干涉原理研制的纳米膜厚测量仪对纳米级 关键词： 薄膜润滑 分子排列取向 拉曼散射     

聚合物网络稳定液晶的电光偏振片

制备了一种散射偏振片,它是由非晶态单体形成的聚合物网络、向列液晶和涂有聚酰亚胺的ＩＴＯ玻璃基板组成。聚酰亚胺被反向平行摩擦,液晶分子则沿摩擦方向取向,并被单体形成的网络所稳定。根据聚合物网络液晶的电光特性可知。入射偏振光束被散射或透射相应地取决于其偏振方向平行还是生趣于基板的摩擦方向。平行时,光的散射强度通过电场可被强烈地调节;垂直时,光的透过率很高,几乎不依赖施加的电场。这种偏振器件具有低的驱动     

钛酸锶钡薄膜的室温光学性能研究

采用溶胶-凝胶法制备了(Ba0.75Sr0.25)TiO3薄膜,研究了不同退火温度下样品的物相结构、薄膜的光致发光性能和光学透过率。结果表明:室温下非晶钛酸锶钡薄膜在蓝光激发下具有明显的发光现象,发光波长范围是500～650 nm,峰值在525 nm附近。延长非晶态薄膜的退火时间能够显著提高样品的发光强度,且发光强度随薄膜厚度增加而增大。晶态薄膜有微弱的发光现象。透射谱测试结果表明,钛酸锶钡薄膜在可见光范围内具有良好的光学透过率。     

Optical sensor protector using wide-bandgap semiconductors

Optical power limiting is the increase of optical absorption with light intensity. Optical absorption in dependences of the light intensity and thickness of single crystals of wide-bandgap semiconductor cadmium iodide was studied using laser light. The transmission of the crystals was found to increase with increasing light intensity. However, at higher light intensity, an increase of optical absorption with intensity was observed. This power limiting characteristics was also found to depend on the thickness of the crystals. The findings resulting from this investigation might have potential applications in optical sensor protector using wide-bandgap semiconductors.