Comparisons of the vector method and tensor method for simulating liquid crystal devices

In calculating the director configuration in a liquid crystal device, two methods are commonly employed: a vector model and a tensor model. In this paper, we compare and contrast these methods for liquid crystal devices consisting of a layer of liquid crystal sandwiched between two plates. We compare the reliability and accuracy of the results, the speed of computation and the complexity of implementations of each method.     

A study of the transition of liquid-crystal alignment from homeotropic to planar on a polyimide layer

The alignment of nematic liquid crystals by rubbed polyimide surfaces has been well-studied and developed. A novel polyimide film which induced a homeotropic alignment of the nematic liquid crystal without rubbing or with weak rubbing strength was presented. However, there was a transition from homeotropic to planar alignment of the nematic liquid crystal after strong rubbing. In order to study the transition, the polyimide surface was investigated by atomic force microscopy, surface free energy measurement and angle-resolved analysis X-ray photo-electron spectroscopy before and after rubbing with a velvet fabric. It was found that both the change of surface polarity and surface morphology were not the reasons for the transition. The droop of the side chain on the polyimide surface after the rubbing treatment was detected by angle-resolved analysis X-ray photo-electron spectroscopy. Owing to the special structure of the novel polyimide, the X-ray photo-electron spectroscopy was successfully used for the first time to analyse the conformational change of the side chain of a polymer. In conclusion, the transition of nematic liquid crystal alignment from homeotropic to planar after rubbing was influenced by the side chain conformation of the polyimide.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^?2 on reversal of an applied electric field.     

Self‐Assembled Binary Mixtures of Partially Etched Nanowires

Arrays of anisotropic particles are sought after for applications in optics, electronics, and energy. Structures assembled from multiple micro‐ or nanoparticles could incorporate the distinct properties of each component to achieve functions not possible from single‐population assemblies. In mixed‐particle populations, the assembly forces may differ between the particle types, which will in turn influence the final assembled structures. Here, binary particle mixtures are studied and compared to assemblies formed from each of the component particles alone. The particles are partially etched nanowires (PENs, ≈300 nm diameter, and 3–8 μm overall length), which are formed by the silica coating and subsequent etching of striped metal nanowires, such that what remains are silica nanotubes containing segments of metal core (Au, Pt, Rh, or Pt/Au) with controllable location and number, spaced by “empty” regions that fill with water. Binary mixtures of PENs with different core metals and segment patterns are examined here to explore how the different core segment material, length, position, and number affects overall self‐assembly behavior.     

Bent-core liquid crystal-functionalised flexible polymer substrates for liquid crystal alignment

ABSTRACT

A transparent flexible polymer film is chemically functionalised with a bent-core liquid crystal (BCLC) compound for effective alignment of the bulk BCLC sample at the substrate–LC interface. The surface attachment was achieved via a simple procedure which involved pre-treatment of the polymer film (commercial name: over head projector film) using piranha solution followed by chemically attaching the BCLC compound through silane condensation reaction. Surface characterisation of the unmodified and BC-modified flexible films was carried out through X-ray photoelectron spectroscopy (XPS), attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy, contact angle (CA) and atomic force microscopy (AFM) techniques. The BC-modified flexible substrates are analysed for their efficiency to orient the bulk LC sample. Remarkably, the chemically modified polymer substrates are highly efficient in vertically aligning both the BC and rod-like LC samples at the substrate–LC interface, in comparison to their unmodified counterparts. The described method is simple, reproducible, surface modified substrates are highly stable and more importantly reusable. The demonstrated method for the alignment of BCLCs advances a step forward towards the realisation of applications proposed for these fascinating compounds.     

圆形阵列光电探测系统双目标识别方法

针对双弹丸同时着靶情况下的立靶坐标测量问题,提出一种圆形阵列光电探测系统的双目标识别方法.采用光电探测器件组成1个圆形的探测阵列,并将3个发光角度均为60°的扇形一字线激光器均匀设置于圆形探测阵列上组成探测光幕.当2发弹丸同时穿过探测光幕时,会在圆形探测阵列上产生6个弹丸投影,通过信号处理电路识别6个弹丸投影的中心位置...     

遥感技术在上海立体绿化调查与规划中的优点与局限分析

由于立体绿化对于改善城市环境具有明显效应,上海近些年来正在大力推广立体绿化。然而,立体绿化在调查与规划上面临着难以掌握现状和潜在资源的难题。在上海的应用中,使用高分辨率航空遥感影像有效解决了上述难题。实践结果表明,遥感技术具有较多优点,同时也存在一些局限性。对此进行了分析,并针对局限性提出了相关对策。     

光栅对压缩器在刻线不平行时的色散研究

针对光栅对压缩器刻线平行性失调会对压缩器色散特性造成严重影响这一问题,对刻线不平行的光栅对压缩器的衍射特性进行了分析,推导出了脉冲光束通过光栅对的光程。数值计算了2阶、3阶和4阶色散量,以及与光栅对严格平行相比的色散差。计算了输出脉冲的垂直谱位移,并将其作为刻线夹角的函数进行了分析。结果表明,存在刻线失调时,低刻线密度的光栅具有更好的优越性。     

高反射率p-GaN欧姆接触电极

根据光学薄膜原理计算了GaN/Ti/Ag、GaN/Al和GaN/Ni/Au/Ti/Ag、GaN/Ni/Au/Al多层电极结构的反射率,得出Ag基和Al基反射电极均能在全角范围内提供较高的反射率。实验测量结果表明,反射率能高于80%的Ag基反射电极,具有低欧姆接触的电学特性。并将GaN/Ni/Au/Ti/Ag多层反射电极应用在上下电极结构的GaN基LED中。实验上采用两步合金法获得了低接触电阻、高反射率的电极结构,并引入Ni/Au覆盖层克服了Ag高温时的团聚和氧化现象。解决了Ag电极的稳定性问题,显著地提高了LED的出光效率,成功制备了具有上下电极结构的GaN基LED管芯。     

电离层垂直探测中扰动观测模式研究

电离层高频反射回波的多普勒频移观测是研究电离层扰动的一个重要方法.利用电离层垂直探测中的组合脉冲控制和回波相位测量分析方法,开发并实现了一种电离层扰动观测模式.实验观测表明,新开发的观测模式在电离层高频反射回波为窄带信号的情况下,可实时获得高精度多普勒频高图,并给出电离层扰动速度的时空变化曲线,能够满足电离层扰动观测研究的需要.