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Volume 43 Issue 4
Jul.  2019
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Preparation and characteristics of large aperture liquid crystal q-wave-plates

  • Corresponding author: ZHANG Xinzheng, zxz@nankai.edu.cn
  • Received Date: 2018-10-19
    Accepted Date: 2018-12-04
  • In order to solve the problems of poor repeatability, cumbersome methods and aperture limitation in the preparation of liquid crystal q-wave-plates, a preparation method of liquid crystal q-wave-plates was adopted based on ultraviolet mask exposure and liquid crystal out-of-plane orientation technology. Theoretical analysis and experimental verification were carried out. The ultraviolet exposure system was built. Large aperture liquid crystal q-wave-plates were prepared with diameter of 2.54cm, topological charges q of 1 and initial angle of 0.The results show that the conversion efficiency of spin angular momentum to orbital angular momentum of the large aperture liquid crystal q-wave-plates constructed by ultraviolet mask method can reach 85%. By using the wave plate, the generation and conversion of vortices and vector vortices are realized. The method of constructing large aperture liquid crystal q-wave-plates based on ultraviolet mask has advantages of low cost, simple preparation process and fast speed. It can realize batch fabrication of liquid crystal q-wave-plates. It is conducive to the commercialization of liquid crystal q-wave-plates.
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Preparation and characteristics of large aperture liquid crystal q-wave-plates

    Corresponding author: ZHANG Xinzheng, zxz@nankai.edu.cn
  • TEDA Institute of Applied Physics and School of Physics, Nankai University, Tianjin 300457, China

Abstract: In order to solve the problems of poor repeatability, cumbersome methods and aperture limitation in the preparation of liquid crystal q-wave-plates, a preparation method of liquid crystal q-wave-plates was adopted based on ultraviolet mask exposure and liquid crystal out-of-plane orientation technology. Theoretical analysis and experimental verification were carried out. The ultraviolet exposure system was built. Large aperture liquid crystal q-wave-plates were prepared with diameter of 2.54cm, topological charges q of 1 and initial angle of 0.The results show that the conversion efficiency of spin angular momentum to orbital angular momentum of the large aperture liquid crystal q-wave-plates constructed by ultraviolet mask method can reach 85%. By using the wave plate, the generation and conversion of vortices and vector vortices are realized. The method of constructing large aperture liquid crystal q-wave-plates based on ultraviolet mask has advantages of low cost, simple preparation process and fast speed. It can realize batch fabrication of liquid crystal q-wave-plates. It is conducive to the commercialization of liquid crystal q-wave-plates.

引言
  • 随着社会的发展和科技的进步,光场调控技术作为光学领域的研究热点备受关注,特别是通过对其相位、振幅或偏振的调控产生各种特殊光场。具有螺旋相位的涡旋光可用于激光光镊[1-4]、粒子捕获[5-7]和量子信息存储等[8-10];偏振随空间变化的矢量光场可用于飞秒矢量光场微加工[11-12]、单分子探测与显微成像技术[13-15]、表面等离激元的激发[16-17]、光学微粒操纵[18-19];振幅随空间变化的艾里光场具有自愈、自加速等特性[20],使其在非线性光学[21-22]、表面等离子体光子学[23-25]、电子学[26]等领域有着广泛的应用。特殊光场具有众多新颖的特性和广泛的应用而备受研究者的关注,因此一种兼顾光束高质量、转换效率高、制作成本低、可灵活调节的光场调控方法一直是人们的研究热点。其中,基于液晶q波片产生特殊光场是一种简单、高效的方法。

    液晶q波片的传统制备采用摩擦法[27],在涂有液晶的玻璃基板表面涂覆一层聚酰亚胺取向层,用绒布在取向层表面旋转摩擦就可以制得q=1的波片。这种方法只能制备q=1的波片,但无法制备其它拓扑荷数的波片。2009年后,不同研究组通过液晶中自发产生或外加电场、光场诱导产生的拓扑缺陷获得了不同拓扑荷数的液晶指向矢分布[28-30]。2011年,CHIGRINOV实验组利用光控取向液晶结合同步旋转法制备了具有任意拓扑荷数的液晶q波片[31]。2016年, 南京大学利用自主研发的动态无掩模光刻系统制备出液晶q波片阵列[32]。这些方法的重复性差,且较为繁琐,产生的光束质量也有待提高。

    基于飞秒激光直写技术,作者所在研究组提出了一种新型液晶面外区域定向技术[33-34]。在此基础上,实现了任意图案的功能性液晶q波片的制备[35]。但是,由于飞秒系统操作复杂、加工时间长、成品率较低以及物镜孔径的限制,目前制备的液晶q波片多在平方毫米以下量级,无法实现大孔径液晶q波片的制备,从而无法实现对大面积光场的调控。因此,本文中提出了一种基于紫外掩模曝光法和液晶面外区域定向技术液晶q波片制备方法,搭建了紫外掩模曝光系统,制备了直径为2.54cm,q=1, α0=0的径向结构液晶q波片,利用该波片实现了涡旋光、矢量涡旋光的产生和转换。该方法操作简便、重复性好、成品率高、没有物镜孔径的限制,可以实现大孔径液晶q波片的批量化制备。

1.   紫外掩模曝光系统
  • 基于飞秒激光直写的面外区域定向技术是一种新型的液晶定向技术[33-35],利用飞秒多光子光聚合技术构建由聚合物条带构成的微结构,液晶在不同的区域内可以有不同的取向。其液晶定向的锚定能来自于由单束入射飞秒光与界面反射光之间干涉而在聚合物条带的侧壁形成的周期为256nm的光栅沟槽。为解决大孔径液晶q波片的制备问题,作者选用相干性好的紫外激光作为曝光光源,搭建紫外掩模曝光系统,从而利用预先设计好的掩模板可快速、简便地制备聚合物微结构。

    作者搭建了如图 1所示的紫外掩模曝光系统。该系统包括紫外激光器(长春新产业光电技术有限公司)、电控机械快门(大恒光电)、半波片(half wave-plate, HWP)、格兰-泰勒棱镜(Glan-Taylor prism, GT)、凹透镜、凸透镜、样品夹持器。紫外激光心波长为360nm,线宽为0.1nm,与常用的高压汞灯紫外光源相比,紫外激光具有更好的相干性。机械快门由精密电子定时器控制,可以根据需要设定开关的打开时间,定时精确度可达1ms。半波片和格兰-泰勒棱镜用来调整曝光光强的强弱,凹透镜和凸透镜构成扩束系统,孔径分别为2.54cm和5.08cm,该扩束系统可将直径1.5mm的平行光束扩束为直径50mm的平行光束。在凸透镜后5cm处为样品夹持器,将掩模板和待曝光样品固定在夹持器上。样品由玻璃基底和其上旋涂的SU-8光刻胶,紫外光通过掩模板对SU-8薄膜曝光。

    Figure 1.  Diagram of ultraviolet exposure system with photomask

2.   大孔径液晶q波片的制备
  • 液晶q波片的制备主要包括聚合物微结构的制作和液晶盒制作两大步骤。聚合物微结构制作流程包括:基底清洗、甩膜、前烘、曝光、后烘、显影、清洗干燥。利用紫外掩模曝光系统进行曝光制备图案化聚合物结构前需要先设计掩模板。作者设计了直径为2.54cm的径向结构掩模板,其图型结构如图 2a所示。黑色线条为透光部分,每条线线宽为2μm,相邻线条之间的最大间距小于30μm,中间空心圆直径为20μm。在距离径向结构两侧5mm的地方是两条长38mm,宽2mm的矩形条带,起到支撑和保护径向结构的作用。

    Figure 2.  a—layout of the mask b—the central part of the fabricated mask by optical microscopic image 10×

    紫外激光透过掩模板上的径向结构实现光刻胶的曝光,再经过后烘、显影、清洗干燥,可以得到径向结构的聚合物条带。当激光功率为40μW、曝光时间为510s时,可以得到如图 2b所示直立的、线条均匀的聚合物结构。将制备好的聚合物结构封装成液晶盒,并充入液晶,便可得到所需的大孔径径向结构的液晶q波片。在正交偏光显微镜(物镜放大倍数为10×)下观察,可以看到明显的十字消光现象,如图 3所示。当同时转动起偏器和检偏器时,消光位置随着仪器旋转,证明液晶分子沿径向取向。从图中也可以看到, 部分区域液晶定向不均匀,这是由于盖玻片和聚合物结构没有实现紧密接触而造成的。

    Figure 3.  Polarized-light optical microscopy images of a large-aperture liquid crystal q-wave-plates

3.   特殊光场的产生与转换
  • 液晶q波片内的液晶分子取向排布[36]如下式所示:

    式中,α(r, φ)是指向矢n(x, y)与x轴的夹角,α0为指向矢与x轴的初始夹角,q为整数或半整数,被称为液晶q波片的拓扑荷数。液晶q波片上的每一点都可以看作是一个相位延迟为δ的波片,每点波片光轴方向与x轴的夹角跟此点方位角φq倍相差一个固定的角度α0

    x-y面内任意一点的液晶分子指向矢用n(x, y)=n(r, φ)=(cosα, sinα)描述。其中r, φ为极坐标下的极径和极角,α为局部任一点液晶分子指向矢与x轴的夹角。如图 4所示,当取不同的qα0的值时,会得到不同的液晶分子指向矢排布。

    Figure 4.  Director distributions of liquid crystals with different topological charges q and different initial angl

    利用琼斯矩阵把入射光和q波片表示出来,则可以计算出出射光的琼斯矩阵[37]。当入射光为左旋圆偏光时,出射光的琼斯矩阵可以由下式表示:

    式中,δ表示液晶q波片引起的相位延迟。等号右侧第1项是保留项,偏振态与入射光保持一致,仍为左旋圆偏光;另一项是转换项,将左旋圆偏光变为右旋圆偏光,并携带有螺旋相位因子e-i2α。此外,可以通过外加电场对液晶q波片的相位延迟进行调控,实现对两项占比的调制。因此,可以分别通过改变液晶分子指向矢排布和液晶q波片的相位延迟来实现对入射光偏振和相位的调控。

  • 搭建了如图 5所示的光路进行特殊光场的产生和检测。使用He-Ne激光器产生的633nm激光,经过偏振片(polarizer, P)和λ/4波片(quarter wave-plate, QWP)形成左旋圆偏光,再利用空间滤波器(spatial filter, SF)滤掉激光光束中的高频部分。通过空间滤波器针孔的发散激光经透镜准直为大孔径平行光束,照射在液晶q波片上。在液晶q波片上施加频率为1kHz的方波电压,通过调节所施加的电压大小,实现对液晶q波片相位的调节,从而产生涡旋光和矢量涡旋光。为了检测所产生特殊光的种类,在液晶q波片后放置一个检偏器,通过检偏器的光场经透镜成像在CCD上。

    Figure 5.  Setup for generating and analyzing specific light fields

    当相位延迟分别为2π, 3π/2, π, π/2时,通过对(2)式化简,可以计算出不同相位延迟下出射光的琼斯矩阵。当电压为25V时,液晶q波片的相位延迟为2π,此时出射光的琼斯矩阵如下式所示:

    出射光与入射光保持一致,为左旋圆偏光。当转动检偏器时,光斑形状与光强几乎无变化。如图 6a所示。

    Figure 6.  States of polarization, phase fronts and intensity distributions at four different phase delays

    当电压为16.8V时,液晶q波片的相位延迟为3π/2,出射光的琼斯矩阵如下式所示:

    出射光为既具有轴对称偏振分布又具有螺旋相位分布的矢量涡旋光,用(p, φ0, l)来表示,其中p表示偏振阶数,φ0表示初始偏振角度,l表示拓扑荷数。由(4)式可知,当q波片相位延迟为3π/2时,出射(p=1, φ0=π/4, l=-1)的矢量涡旋光场。转动检偏器时,消光位置跟着一起旋转,如图 6b所示。

    当电压为10.4V时,液晶q波片的相位延迟为,由下式分析:

    出射光为拓扑荷数l=-2的涡旋光。转动检偏器时,光斑始终是亮环,如图 6c所示。

    当电压为6.8V时,液晶q波片的相位延迟为π/2,出射光的琼斯矩阵如下式:

    出射光与第2种情况类似,是矢量涡旋光场(p=1, φ0=-π/4, l=-1)。转动检偏器时,消光位置与理论分析的偏振分布一致。如图 6d所示。

  • 通过波片组可以实现不同偏振分布的矢量涡旋光之间的转换,实验光路如图 7所示。在液晶q波片后面加入一对λ/2波片(2HWP),通过调节两个波片快轴之间的夹角θ,可实现对矢量涡旋光场空间每一点偏振方向旋转2θ。以相位延迟为3π/2矢量涡旋光为例,调节两个λ/2波片的夹角分别为22.5°和67.5°,则光波横截面上任意位置的偏振方向都分别发生了45°和135°的旋转,最终分别产生了如图 8所示的角向矢量涡旋光场(p=1, φ0=π/2, l=-1)和径向矢量涡旋光场(p=1, φ0=0, l=-1)。

    Figure 7.  Setup for conversion of different vectorial light fields by using a pair of HWP

    Figure 8.  States of polarization, phase fronts and intensity distributions at different angles between the fast axes of the two HWP

  • 自旋角动量(spin angular momentum, SAM)到轨道角动量(orbital angular momentum,OAM)的转换(SAM-to-OAM conversion,STOC),其转换效率RSTOC可用来表征液晶q波片对入射光的能量利用率[38-39]。由前面分析可知,当以左旋圆偏光入射时,经过液晶q波片后的出射光可用(2)式表示。其中第1项为保留项,表示出射光仍为左旋圆偏光,第2项为转换项,表示出射光的自旋角动量发生反转变为右旋,并且携带有轨道角动量。利用如图 9a所示的光路,在液晶q波片后加入由λ/4波片(QWP)和偏振分光棱镜(polarizing beam splitter, PBS)构成的圆偏光检偏器将出射光场中的保留项和转换项分开,并用功率计分别测量它们的功率[40-43]:

    Figure 9.  a—setup for measuring STOC efficiency b—relationship between STOC efficiency and the applied voltage

    式中,P0=TPin是透过液晶q波片后的光总功率; T表示液晶q波片的透过率; Pin表示入射到液晶q波片的光功率; P1表示转换项的光功率; P2表示保留项的光功率。

    实验中调节液晶q波片两端施加的电压,测得P1, P2随电压实时变化的数据,并通过MATLAB对数据进行处理,可得如图 9b所示的P1, P2随电压的变化曲线以及STOC转换效率随电压变化曲线,该液晶q波片的STOC转换效率最高可达85%。

4.   结论
  • 本文中提出了一种基于紫外掩模曝光法和液晶面外区域定向技术的液晶q波片制备方法,该方法具有成本低廉、制备工艺简单、速度快等优点,可实现大孔径液晶q波片的批量化生产,有利于液晶q波片走向商业化。搭建了紫外掩模曝光系统,制备了直径为2.54cm的径向结构液晶q波片。本文中利用液晶q波片通过外加电场的调控和半波片对的引入,成功实现了涡旋光、矢量光和矢量涡旋光的产生和转换。此外,针对所需的特殊光场,推算出液晶分子的指向失分布,设计对应的掩模板,从而可快速制备出相应的液晶q波片,实现个性化定制。

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