A review of liquid crystal spatial light modulators:devices and applications

Spatial light modulators,as dynamic flat-panel optical devices,have witnessed rapid development over the past two decades,concomitant with the advancements in micro-and opto-electronic integration technology.In particular,liquid-crystal spatial light modulator(LC-SLM)technologies have been regarded as versatile tools for generating arbitrary optical fields and tailoring all degrees of freedom beyond just phase and amplitude.These devices have gained significant interest in the nascent field of structured light in space and time,facilitated by their ease of use and real-time light manipulation,fueling both fundamental research and practical applications.Here we provide an overview of the key working principles of LC-SLMs and review the significant progress made to date in their deployment for various applications,covering topics as diverse as beam shaping and steering,holography,optical trapping and tweezers,measurement,wavefront coding,optical vortex,and quantum optics.Finally,we conclude with an outlook on the potential opportunities and technical challenges in this rapidly developing field.

A high-contrast imaging coronagraph for segmented-mirror large aperture telescopes using a spatial light modulator

The primary mirrors of current and future large telescopes always employ a segmented mirror configuration.The small but non-negligible gaps between neighboring segments cause additional diffraction,which restricts the performance of high-contrast coronagraph.To solve this problem,we propose a coronagraph system based on a single liquid crystal spatial light modulator(SLM).This spatial light modulator is used for amplitude apodization,and its feasibility and potential performance are demonstrated using a laboratory setup using the stochastic parallel gradient descent(SPGD)algorithm to control the spatial light modulator,which is based on point spread function(PSF)sensing and evaluation and optimized for maximum contrast in the discovery working area as a merit function.The system delivers a contrast in the order of 10−6,and shows excellent potential to be used in current and future large aperture telescopes,both on the ground and in space.

Diffractive characteristics of the liquid crystal spatial light modulator

The liquid crystal spatial light modulator （LC SLM） is very suitable for wavefront correction and optical testing and can produce a wavefront with large phase change and high accuracy. The LC SLM is composed of thousands of pixels and the pixel size and shape have effects on the diffractive characteristics of the LC SLM. This paper investigates the pixel effect on the phase of the wavefront with the scalar diffractive theory. The results show that the maximum optical path difference modulation is 41μm to produce the paraboloid wavefront with the peak to valley accuracy better than λ/10. Effects of the mismatch between the pixel and the period, and black matrix on the diffraction efficiency of the LC SLM are also analysed with the Fresnel phase lens model. The ability of the LC SLM is discussed for optical testing and wavefront correction based on the calculated results. It shows that the LC SLM can be used as a wavefront corrector and a compensator.

Fast generation of controllable equal-intensity four beams based on isosceles triangle multilevel phase grating realized by liquid crystal spatial light modulator

Liquid crystal spatial light modulator （LCSLM） realizing equal-intensity multiple beams often has some features, i.e., phase valley between two adjacent pixels, flybaek region when phase decreases immediately from 2~r to 0, and inevitable backplane curvature, which are different from those of most conventional diffractive optical elements （DOEs）, such as static DOEs. For optimal intensity uniformity, equal-intensity multi-beam generation must be considered for these artifacts. We present a tunable-grating method in which the intensity uniformity can be improved by considering the LCSLM artifacts. For instance, tuning phase modulation depth of the grating, called isosceles triangle multilevel phase grating （ITMPG）, can be used not only to improve the intensity uniformity, but also to fast steer four beams with narrow beamwidths, determined by the same effective aperture of ITMPG. Improved intensity uniformity and high relative diffraction efficiency are demonstrated through experiments with phase-only LCSLM.

COMPENSATION EFFECT OF POLYIMIDE THIN FILMS ON NORMALLY WHITE TWISTED NEMATIC LIQUID CRYSTAL DISPLAYS

The disadvantages of Normally White Twisted Nematic Liquid Crystal Display (NW-TN-LCD) were discussed. The reason that the negative birefringent polyimide thin films were used to compensate NW-TN-LCD to decrease off-axis leakage, improve contrast ratios and enlarge viewing angles was explained in this paper. A certain polyimide thin film was taken as an example to show compensation effect on NW-TN-LCD.

Two-Dimensional Spatial Solitons in Nematic Liquid Crystals

We study the propagation of spatial solitons in nematic liquid crystals, using the self-similar method. Analytical solutions in the form of self-similar solitons are obtained exactly. We confirm the stability of these solutions by direct numerical simulation, and find that the stable spatial solitons can exist in various forms, such as Oaussian solitons, radially symmetric solitons, multipole solitons, and soliton vortices.

Liquid Crystal Television Spatial Light Modulator

The liquid crystal television spatial light modulator (LCTVSLM) characterized is usable in optical processing applications,e.g.,optical pattern recognition,associative memory, optical computing,correlation detection and optical data processing systems.The array performance and real-time optical correlation applications are reviewed.

Simulation and optimization of spatial light modulation of twisted-nematic liquid crystal display

The approaches to obtaining desired intensity or phase modulation by twisted-nematic liquid crystal display （TN-LCD） have been extensively studied based on the knowledge of the LCD＇s internal structure parameters. Generally, the TN-LCD placed between two linear polarizers （P） produces coupled intensity and phase modulation. To obtain the commonly used phase-only modulation, quarter wave plates （QWPs） are often used in front of and/or behind the LCD. Here we present a method to optimize the optical modulation properties of the TN-LCD to obtain phase-only modulation in the configuration of P-QWP-LCD-QWP-P each with proper orientation. Our method is based on the macroscopical Jones matrix descriptions for the LCD, the QWPs, and the linear polarizers. Through Jones matrix calculations, the orientations of the polarizers and QWPs can be optimized to satisfy differently desired modulation demands. In contrast to the traditional method, which requires knowledge of the LCD＇s internal structure parameters, our method simplified the complicated theory analysis and can work in the absence of information on the LCD＇s internal structure parameters, which are usually not available for the commercial products.

基于扭矩平衡的分时锚定能测量方法

光控取向技术一直是液晶光学器件制备中的关键环节,精确测量光控取向技术中的锚定强度与曝光条件的关系对于提高器件性能至关重要。然而,目前锚定强度的测量方法还存在一定的局限性,比如无法实现对同一取向膜的单点前后多状态的测量等,这导致实际应用中的诸多不便。鉴于此,本文提出了一种基于相位检测原理的新型方位锚定能测量方法,通过采用微分归一化拟合的方法同时结合全方位透射特性校正有效消除了起始放置误差,不仅极大地优化了测量过程,使测量误差控制在8%以内,而且大幅提高了测量时效性。理论与实验结果表明,该方法能够快速有效地测量光控取向膜锚定能随曝光参数的变化,具有较高的准确性和稳定性,能够为液晶光学元件的精密制造过程提供实时在线监测手段,对掌握光控取向物理过程和提高制造质量至关重要。

计算全息综合性光电教学实验设计

基于傅里叶光学原理实现对目标光场进行精确重建,设计了从显示目标绘制,相应计算全息图生成到光学重建的综合性教学实验。具体的实验流程包含基于Visio的显示目标设计,基于MATLAB的相位型计算全息图生成算法编写和基于相位型液晶空间光调制器的目标重建光路搭建。实验设计结合了光学、数字电路和计算机软件技术,具有流程清晰、易于教学、学生参与度高和获得感强的优势,可有效增强学生对计算全息技术与光电信息技术的理论理解与实操能力。

液晶调控下的亚波长金光栅反射相位

空间光调制器(Spatial Light Modulator,SLM)是实现光场调控的关键器件。本文研究了一种将亚波长金光栅嵌入硅基液晶(Liquid Crystal on Silicon,LCoS)构成的SLM(G-LCoS)器件模型的反射相位特性,该器件模型以ITO透明导电层和液晶构成G-LCoS中亚波长金光栅的非对称边界。首先运用散射矩阵原理给出了亚波长金光栅反射相位的计算表达式,其次给出了下表面液晶的有效折射率计算公式。在此基础上,利用TechWiz 3D软件模拟加电情况下金光栅下表面液晶指向矢的变化,给出金光栅下表面液晶有效折射率的变化值。将得到的有效折射率数据导入到FDTD Solutions软件,对该器件结构开展了相关的仿真实验和结果分析。综合考虑了上表面的ITO色散情况、金光栅高度变化以及不同电压下液晶有效折射率值,实现了可见光范围内多波长的近2π相位调制。本文研究的器件也为AR/VR显示系统以及全息视频显示的优化提供了一种新思路。

竞争向列相液晶中(1+2)维空间光孤子

基于JUNG等人提出的竞争非局域模型,研究了竞争型向列相液晶中(1+2)维空间光孤子的传输特性。利用变分法给出了孤子临界功率解析表达式,发现当热非局域程度固定时,临界功率随分子取向效应非局域程度的变化规律与随热非线性系数的变化规律一致:随着分子取向非局域程度或热非线性系数的增加,临界功率由功率不等的两个分支逐渐重合为功率相等的一支;当分子取向效应非局域程度和热非线性系数固定时,随着热非局域程度的增加,临界功率由功率相等的一支分为功率不等的两个分支。利用光束传输方法发现,只有功率不等分支上的点对应的(1+2)维空间光孤子才能稳定传输。该研究结果可为竞争非局域介质中(1+2)维空间光孤子的实验研究以及全光互联应用提供理论基础。

基于液晶空间光调制器的附面层效应模拟

为了给航空动平台激光通信的附面层效应校正系统提供验证条件,设计了一种基于液晶空间光调制器来模拟附面层效应的模拟器。首先,从几何光学的角度分析附面层效应,将其等效为负透镜。然后,利用计算机对不同飞行条件下附面层效应与等效负透镜焦距之间的关系进行数值分析仿真。之后在液晶空间光调制器上导入不同焦距透镜的相位灰度图以实现变焦透镜的功能,通过改变变焦透镜的焦距来模拟航空平台不同飞行条件下的附面层效应。最后通过实验验证模拟的准确性,在环境温度下,模拟附面层效应的液晶空间光调制器得到的光斑图像由红外相机拍摄,之后进行图像处理分析实际光斑大小,并与理论计算光斑大小比较,得出误差曲线图。研究结果显示,基于液晶空间光调制器模拟附面层效应引起散斑效应的理论光斑大小与实际光斑大小的均方根误差为0.04375,验证了所提出的方法的可行性和有效性。

贝塞尔双光子光片显微镜在活体生物成像中的应用

双光子光片显微镜具有分辨率高、光漂白低等优势,在生物学领域有着广泛的应用。但由于照明光片厚度与长度之间的取舍矛盾问题,导致轴向分辨率和成像视场无法兼顾,因此在应用中受到限制。为了解决这一问题,本文采用液晶空间光调制器将普通高斯照明光束调制为贝塞尔光束,利用贝塞尔光束的“无衍射”特性实现了大且薄的光片照明,光片长度与厚度之比达到了163。在此基础上,设计并搭建了一套贝塞尔双光子光片显微镜,在310μm的视场范围内实现了横向440 nm、轴向1.9μm的三维高分辨率成像。本文成功实现了斑马鱼胚胎的活体成像,可以做到血液细胞流速的测量和脊柱骨细胞的分割计数,验证了贝塞尔双光子光片显微镜在活体生物成像中的应用潜力。

向列相液晶器件光调制性能研究

液晶的折射率易受外加电压和温度的影响,据此可获得电场和温度可调制器件。为改善液晶器件对光的调制性能,制备了向列相液晶器件,并对其调制性能进行理论模拟和实验研究。首先研究液晶器件的透过光强和相位延迟随电压变化关系,结果显示,相同电压下,增大器件厚度,光强调制曲线的峰谷数增多,相位调制量增大,其中电压在0.8~3 V范围内,相位延迟曲线变化较明显,厚度为4、6和10μm的器件可分别获得约1.21π、1.87π和3.19πrad的相位调制量。其次对厚度为4μm的器件进行温度可调制性能的研究,结果显示,温度从20℃升至45℃时,获得的温度可调制量约为0.29πrad。改变电场或温度均能引起液晶的折射率变化,从而实现对光的调控,适当增加液晶器件厚度可获得较大的相位调制量。研究结果对液晶光调制器件的性能改善及应用具有重要参考意义。

Smart beam via electrically addressing the dye-doped and polymer-stabilized cholesteric liquid crystals

Smart beams play a vital role in modern intelligent vehicles and have recently attracted significant attention.A spatial light modulator with high optical efficiency,low cost,and compact size is crucial for designing smart beams.Here,we mix cholesteric liquid crystals with dichroic black dye and a monomer.After UV polymerization,the sample exhibits a low driving voltage of 26 V,a high transmittance of over 70%,and an On-off ratio over 280,thanks to the joint contribution of both the absorption and the scattering effect.A smart beam device is demonstrated by electrically addressing the dye-doped and polymer-stabilized cholesteric liquid crystal with pixelated electrodes.Light patterns with arbitrary designs are projected dynamically.The switching time reaches several tens of milliseconds.This strategy brings new designs to intelligent vehicles and may also inspire applications in public information displays,advertising,and even AR/VR displays.

基于编码成像的激光能量调控方法研究

为了有效提升光电成像系统焦平面探测器在外界存在激光干扰条件下的环境适应性,同时不显著降低原成像系统成像性能,提出了一种基于编码成像的激光能量调控方法,构建了一套以液晶空间光调制器为核心器件的编码成像实验系统并对该系统的成像性能进行了测试,设计了一套基于编码成像的激光能量调控实验验证系统并开展了多参数条件下编码成像抗激光干扰实验研究。实验结果表明,基于编码成像的激光能量调控方法可在不显著降低成像质量前提下有效提升焦平面探测器的抗激光干扰性能。

Laser light scattering technique for non-invasive in situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals

The laser light scattering technique for non-invasivein situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals is introduced. By measuring the autocorrelation function of the scattered light from nematic liquid crystals at different scattering angles, the splay and twist elastic constantsK 11 andK 22 are obtained from the amplitudes of the autocorrelation function, and the viscosity coefficients ofη Splay andη Twist are determined using the viscoelastic ratiosK 11/η Splay andK 22 η Twist from the telaxation parameters of the two modes.

Characterizing a liquid crystal spatial light modulator at oblique incidence angles using the self-interference method

The phase modulation characteristics of a reflective liquid crystal （LC） spatial light modulator （SLM） under oblique incidence are studied by using our proposed self-interference method. The experimental setup of the method is very simple and has good robustness to mechanical vibrations. By changing the gray value of the combined grayscale loaded on the LC-SLM, different sheared fringe patterns, generated by the interference between the constant phase-modulated beam and the ＋1-order diffracted beam of the blazed grating, can be obtained. The amount of phase modulation of the LC-SLM is obtained by subtracting the phase of the two side lobes in the frequency domain. By turning the turntable where the SLM is mounted, the phase modulation characteristics at different incident angles can be measured. The experimental results show that the phase modulation curves do not change significantly with the small angle. When the angle is large （i.e. larger than 10°）, the phase modulation curves become different, especially for the high gray levels. With the increase of the incident angle, the phase modulation depth is reduced. The results indicate that the incident angle plays an important role in the performance of the phase modulation of an LC-SLM.

基于深度学习的计算全息显示进展

计算全息作为一种三维显示手段,能够基于衍射计算实现对目标光场的精确重建,在元宇宙通讯、AR/VR头戴显示、车载抬头显示等方向均有着重要的应用。如何实现高速且高质量的相位全息图生成是计算全息领域发展的关键问题,也是当前该方向的重要研究课题。近年来,深度学习技术的飞跃式发展为上述问题的解决提供了一条新的技术路径。本文介绍了计算全息技术的基本原理及算法分类,综述了近年来所提出的基于深度学习的计算全息解决方案,比较了各类方案的优势与不足,展望了深度学习技术在计算全息领域的发展与挑战。