Laser beam manipulation by composite material electro-optic devices

Liquid crystals are anisotropic materials whose optical properties can be easily controlled applying an electric field. For this reason, they are widely used in electro-optical devices. Besides other applications, liquid crystals electro-optical devices are suitable for laser beam manipulation. In this paper, we present some devices developed or studied in our laboratory: a twisted nematic liquid crystal (TN-LC) polarization rotator, a TN-LC protected light sensor, a polymer dispersed liquid crystal (PDLC) light modulator, a PDLC optoelectronic polarizer and a polymer liquid crystal (PoLiCryst) light modulator.     

Optical and electro-optical properties of photonic crystals based on polymer-dispersed liquid crystals

Two-dimensional (2-D) square lattice (SL) photonic crystals (PCs) are fabricated, and their optical/electro-optical properties are studied. The PCs are based on polymer-dispersed liquid crystals (PDLC) that are formed using two-beam interference with double exposures. The PC structure that is observed using a scanning electron microscope matches with the calculated interference pattern. The results of optical/electro-optical studies demonstrate that superprism and negative refraction effects occur at certain incident angles over a range of frequencies and are consistent with the simulated ones. Moreover, the negative refraction efficiency is electrically controllable.     

Electro-optical phase shift in polymer dispersed liquid crystals

An anisotropic version of the Maxwell Garnett approximation is applied for studying the electro-optical phase modulation by polymer dispersed liquid crystals (PDLC). The PDLC contain bipolar liquid crystal droplets that can be reoriented by an external field causing a change in the optical birefringence. This approach provides an explicit link between the droplet orientation distribution and the electro-optical phase shift. For aligned droplets we find that the sharpness of the change in the birefringence may be controlled by selecting the initial orientation. For a planar distribution we find sharp transitions with a hysteresis loop whose width depends on the droplet concentration. For a random distribution, the droplet orientation and the optical phase shift change more gradually with the applied field. These results demonstrate that PDLC may be suitable for a wide range of electro-optic applications based on their field-induced phase modulation properties. In addition, it is apparent that the optical phase shift is quite sensitive to changes in droplet orientation. It should therefore be useful for studying reorientation phenomena in PDLC, overcoming the problems due to light scattering in these materials. Received 25 November 1999 and Received in final form 20 January 2000     

Fast electro-optical switching and high contrast ratio in epoxy-based polymer dispersed liquid crystals

We report the observation of an electro-optical switching effect from an opaque to a transparent state occurring at a threshold value of the applied field in a polymer dispersed liquid crystal (PDLC). Optical responses of the composite film under the conditions of an externally applied ac electric field (2–5 V_p−p/μ) and a film thickness (50 μm), were determined using an Argon laser (wavelength 514 nm). The experimental results showed promising switching times with a rise time of 190 μs and a decay time of 2 ms and an exceptionally high contrast ratio up to 410. These results demonstrate the validity of employing this new PDLC in electro-optical devices.     

Conducting polymers as driving electrodes for Polymer-Dispersed Liquid-Crystals display devices: On the electro-optical efficiency

Intrinsically conducting polymer (ICP) thin films are used as driving electrodes for Polymer-Dispersed Liquid-Crystals (PDLC) display devices. In order to investigate the electro-optical efficiency of these organic electrodes, three different kinds of conducting polymers, i.e. polyaniline doped with 10-camphorsulfonic acid (PANI(HCSA)), polypyrrole doped with dodecylbenzenesulfonic acid (PPY(DBSA)), and polyethylenedioxythiophene doped with polystyrenesulfonate (PEDOT(PSS)), were prepared or purchased, and coated either on glass or plastic substrates. Optical absorption studies in the UV-Vis range of the conducting polymer-coated substrates were first performed showing the presence of conducting species for the three types of polymers. The electrical characteristics of the resulting films were measured with the four-probes technique. PANI(HCSA) exhibits a higher conductivity ( ) compared to PPY(DBSA) ( ), and PEDOT(PSS) ( ). It is also shown that for a given conducting polymer, its electrical conductivity decreases when a plastic substrate is used. These observations have been related to significant morphological changes observed by scanning electron microscopy (SEM). A mixture of Norland Optical Adhesive 65 and nematic liquid-crystal E7 in the weight ratio (35:65) was used as precursor of the PDLC material. Better electro-optical responses (transmission properties, drive voltages and switching times) of PDLC films were obtained for devices prepared with (PPY(DBSA))-based electrodes. The electro-optical performances of the PDLC display devices also depend on the nature of the ICP substrate used.Received: 25 September 2002, Published online: 29 July 2003PACS: 73.61.Ph Polymers; organic compounds - 42.79.Kr Display devices, liquid-crystal devices - 82.35.Cd Conducting polymers - 42.70.Df Liquid crystals F. Roussel: rfred@purple.univ-littoral.fr     

Fabrication of PDLC films based on acrylic monomers and study of the electric field effect on the alignment structure of liquid crystal droplets in PDLC films

Polymer Dispersed Liquid Crystal (PDLC) films are prepared by the polymerization induced method of phase separation. Initial configurations of the director of liquid crystal droplets inside the films are found using a polarized microscope. The effect of an electric field on the alignment structure of droplets is studied. Two mechanisms of droplet orientation by the electric field are determined as smooth and spontaneous. The processes observed are compared with the results of electro-optical studies.     

纳米银掺杂的高效率全息聚合物分散液晶光栅制备

报道了一种基于掺杂纳米银聚合物分散液晶(PDLC)材料的高衍射率全息电控光栅的制备及特性。通过在原有聚合物分散液晶材料体系中添加适量的纳米银颗粒以制备体全息光栅,实验研究了掺杂不同质量比的纳米银颗粒对全息聚合物分散液晶(H-PDLC)体光栅的衍射效率、驱动阈值电压、响应时间的影响。实验结果表明,通过掺杂纳米银材料,能够优化聚合物和液晶两相分离结构,使聚合物与液晶分离更加彻底,显著提高H-PDLC体光栅的一级衍射效率,同时能改善体光栅的电光特性,缩短响应时间。初步分析表明,由于纳米银颗粒的表面等离子体效应和体系折射率匹配的优化改善了H-PDLC光栅的特性。     

PDLC-based VOA with a small polarization dependent loss

A polymer-dispersed liquid crystal (PDLC)-based variable optical attenuator (VOA) was demonstrated to exhibit a relatively small polarization dependent loss (PDL) in the optical communication C band. The influence of the PDLC characteristics, the temperature, as well as the wavelength of the laser irradiation, on the PDLC VOA performance was analyzed. The PDL, the insertion loss at saturated voltage, and the operating voltage of the PDLC-based VOA were reduced by optimizing the fabrication conditions and applying a bias voltage to the PDLC cells during polymerization-induced phase separation.     

Polysulfone as polymer matrix for a novel polymer-dispersed liquid crystals system

We report the preparation and thermotropic characterization of a novel polymer-dispersed liquid crystal (PDLC) system based on the polysulfone (PSU) UDEL-1700 as a polymer matrix and on 4-[4-(n-butyloxy)phenylene-imino-methylidene]benzonitrile as a liquid crystal. PDLC films have been prepared using the thermal-induced phase separation (TIPS) technique, with different compositions in the two components. A detailed optical polarized microscopy and differential scanning calorimetry study has been carried out to evaluate a correlation between the composite composition and the PDLC formation. We found that PSU could be a good matrix for the formation of PDLC systems.     

Photo-differential Scanning Calorimetry Study on Photopolymerization of Polyacrylate/E7 Liquid Crystal Blends

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the cure kinetics of a photo-initiated polymerization of tripropylene glycol diacrylate (TPGDA) monomers mixed with a eutectic liquid crystal mixture, E7, chosen due to its significant optical properties. The thermal photo-polymerization reactions were performed in isothermal mode in order to identify and evaluate the different thermal effects occurring during the photo-polymerization reaction under UV light. The results obtained help to better understand the curing mechanisms as well as the kinetics of the curing reactions. The isothermal photo-DSC provided complex thermograms showing the presence of two thermal effects: an exothermic heat (positive activation energy) generated during the conversion of monomers and an endothermic heat produced by the UV lamp. The latter phenomenon was observed in our previous work and was associated to the heating of the polymer dispersed liquid crystals (PDLC) material by the UV irradiation lamp. The dependence of photopolymerization on the time and intensities of the curing UV light were investigated. The results showed that the high intensity UV light heats the material and slows the process of phase separation which, in turn, affects the electro-optical properties of the material under study. The photo DSC results also revealed that the heating of the system was insignificant and can, therefore, be neglected, for materials exposed to UV radiation with an irradiation time of 5?seconds and a low intensity of 13.80?mW/cm².     

Polarization of light transmitted through a polymer film with nanosized droplets of liquid crystal

A method is developed for analyzing the state of polarization of a plane wave transmitted through a polymer-dispersed liquid-crystal (PDLC) film with nanosized liquid-crystal (LC) droplets. This method is based on the anisotropic-dipole approximation for describing scattering by a separate droplet and on the Foldy-Twersky approximation for describing propagation of light in a film. Equations are obtained that relate the ellipsometric parameters of coherent (direct) light transmitted through a PDLC film to the order parameters that characterize the morphological and structural properties of the film. Elliptic and circular polarizations and the rotation of the plane of polarization of a wave transmitted through a film are investigated under the normal illumination of the PDLC film by a linearly polarized plane wave. The order parameters of the PDLC film are determined as a function of a control field under the transition from a partially ordered structure of optical axes of LC droplets to a homeotropic structure.     

Evidence for droplet reorientation and interfacial charges in a polymer-dispersed liquid-crystal cell

We observe a new feature ( plateau), sandwiched between two exponentially decaying components, in the switching response of a polymer-dispersed liquid-crystal (PDLC) cell. Upon application of square-wave electrical pulses, the cell response exhibits a complex decay whose characteristics are significantly different within and outside the applied field. We interpret these results in terms of reorientation dynamics of nonspherical droplets in the PDLC. Irradiation of the PDLC with gamma rays induces significant changes in the cell response in a manner that is consistent with redistribution of charges at the liquid-crystal-polymer interface in the PDLC.     

聚合物弥散液晶材料在1550nm的电光特性研究

实验研究了不同配方及工艺条件下,聚合物弥散液晶（PDLC）材料在1550nm处的电控光透射特性。由此筛选合适的配方和工艺,研制了聚合物弥散液晶电控可调光衰减器样品。实验说明,在1550nm工作点,50％左右合适的液晶含量的聚合物弥散液晶样品能够获得更好的电控光学性能。采用配方质量比ω（DHPA）：ω（E7）：ω（RB）：ω（NVP）：ω（NPG）=100：100：3：2：1．5的配方,制作在1550nm工作的聚合物弥散液晶光衰减器样品。经检测,输入光源为1550mm,223μW时,器件插入损耗为2．3dB,衰减调节范围为13dB,线性工作区域为20～70V。     

Effect of clay surface modification on the structure and electro-optical properties of liquid crystal/clay nanocomposites

FTIR spectroscopy, atomic force microscopy (AFM) and electro-optical measurements were used to study nanocomposites consisting of nematic liquid crystal (LC) ZLI-2293 and organophilic montmorillonite (MMT) nanoparticles. The initial MMT was modified by two different surfactants: octadecylbenzyldimethylammonium chloride (OBDM) and dioctadecylldimethylammonium chloride (DODM). According to spectroscopic results, Van der Waals interactions between the components of the LC/clay composites are stronger in OBDM-MMT case. AFM images of two types of the LC/clay nanocomposites showed different morphology consisting of large-scale structures. Correlation between the spectroscopic results and electro-optical behaviour of the LC/clay nanocomposites was observed. It was inferred that the memory effect is more pronounced for composites with OBDM-MMT due to colloid network formation, whereas the electro-optical contrast is higher for DODM-MMT, where the clay particles are not organized in any ordered structure.     

Combined dielectrophoretic field cages and laser tweezers for electrorotation

Symmetric rotating octode field cages do not allow simultaneous particle trapping and electrorotation. Laser-trapped particles can stably be held in inhomogeneous rotating electric fields. Therefore, in a combination of laser tweezers and field-cage (electro-optical cage) dielectric single-particle spectroscopy (DSPS) of particles and cells can be carried out at low electric field strength. The electro-optical cage overcomes the limitation of low enough effective particle conductivity ensuring repelling dielectrophoretic forces in conventional field cages. At low power, forces in field cages and laser tweezers hardly interfere and can be used to calibrate each other. Received: 3 May 1999 / Revised version: 16 August 1999 / Published online: 27 October 1999     

应变液晶的负压光效应和反式压光效应

介绍聚合物分散液晶和应变液晶概念,给出聚合物分散液晶调光玻璃的"正压光效应"、"负压光效应"和"反式压光效应"三种效应定义.实验制备出负压光效应和反式压光效应新型应变调光玻璃样品,测试样品散射态雾度90%以上,半透明态透光率接近30%.用偏光显微术研究压光效应原理,表明对样品施加垂面按压或拉伸的应力诱导作用会引起液晶微滴中液晶分子具有某些特殊排列方式,导致样品光学性质发生显著变化.建立垂面拉伸液晶微滴模型,计算模拟所绘出的图形与偏光显微镜照片独特花样十分相似,进而合理解释了实验现象.应变液晶压光效应研究具有聚合物分散液晶基础研究意义和开发非电控调光玻璃的实际应用价值.     

Color dispersion in two-dimensional phase array based on polymer-dispersed liquid crystal film

This work proposes a feasible method for fabricating a two-dimensional periodic structure with a sub-micrometer periodicity using a single laser beam, based on polymer-dispersed liquid crystal (PDLC) films. The resulting nano-PDLC morphology is highly symmetrical, and similar to that written using multi-beam interference. The increase in the electric-tunability of the optical behavior, including spatial diffraction and color dispersion, is examined. The color dispersion provides optical evidence of the periodic structure of the PDLC film.     

Small-angle light scattering from polymer-dispersed liquid-crystal films

A method is developed for modeling and computing the angular distribution of light scattered forward from a single-layer polymer-dispersed liquid-crystal (PDLC) film. The method is based on effective-medium approximation, anomalous diffraction approximation, and far-field single-scattering approximation. The angular distribution of forward-scattered light is analyzed for PDLC films with droplet size larger than the optical wavelength. The method can be used to study field-and temperature-induced phase transitions in LC droplets with cylindrical symmetry by measuring polarized scattered light intensity.     

Low threshold and high contrast polymer dispersed liquid crystal grating based on twisted nematic polarization modulator

An electrically tunable mode is proposed to overcome the bottleneck problem of a conventional polymer dispersed liquid crystal (PDLC) grating and realize its low threshold and high contrast through an alignment-controlled PDLC. The results indicate that the threshold voltage V_PP of the grating is as low as 0.75–0.8 V, the saturation voltage V_pp is 7 V and the contrast ratio of the grating reaches 245, which is about three times larger than the conventional PDLC grating. Stability testing indicates a stable state of contrast during a long time. PACS 42.40.Eq; 42.70.Df; 42.79.Kr     

聚合物分散液晶的制备及拉曼光谱分析

用溶致相分离的方法制备了以PMMA为基体的四种不同配比的聚合物分散液晶(PDLC),四氢呋喃为引发剂,并且通过傅立叶拉曼光谱仪对其进行检测,分析其光谱,观察各单体混合前后性质的变化,鉴别聚合物分散液晶的物质构成。此外,在指定的组分前提下,根据液晶最强峰与PMMA特征峰的相对强度拟合曲线得出PDLC中液晶与PMMA的质量百分比,为下一步研究聚合物分散液晶系统的光折变特性提供指导。