基于聚合物分散液晶的二维六角晶格变间距光栅的研制 下载: 903次
1 引言
光栅作为重要的光学元件,因其独特的衍射特性,广泛应用于各种光学光子器件,如:输入输出耦合器、窄带滤波器、分布式反馈激光器及布拉格反射器[1-3]等。其中,变间距光栅是具有多种周期的光栅,其具有自聚焦、像差校正、高分辨率[4-5]等优点,近年来被广泛地研究。陈建文等[6]于1986年提出了制作变间距光栅的新方法;Wilbur等[7]使用弹性膜制造了变间距光栅;Liu等[8]用柱透镜代替三棱柱采用一步法制备了变周期布拉格反射光栅;Kim等[9]利用一片柱面镜代替平面镜的洛埃镜系统制备了变周期的二维(2D)全息光刻光栅。但目前对于2D六角晶格全息聚合物分散液晶(H-PDLC)变间距光栅的报道却很少。聚合物分散液晶(PDLC)是微米尺寸的液晶随机分散在聚合物基质中形成的一种可以电调控的材料,由于聚合物的光敏感性,PDLC可以作为全息记录材料[10-11]。PDLC被广泛地应用于变焦透镜[12]、光学开关[13]、全息一维、2D光栅[14-16]等。
本文报道了一种制作2D六角晶格H-PDLC变间距光栅的简便方法。在双路干涉光路中插入柱面镜使柱面波与平面波干涉,经过第一次曝光后逆时针旋转样品60°进行第二次曝光,将两次曝光的干涉条纹写入PDLC,形成2D六角晶格H-PDLC变间距光栅。聚合物在整个样品区域对角线方向的分布为正六边形,在非对角线区域的分布为不规则的六边形。此外,可以通过在氧化铟锡(ITO)导电膜上加载电压的方式改变光栅的一级衍射效率与中心透射率。当需要使用不同周期的光栅时,本文提出的2D变间距光栅可以取代不同周期光栅的组合,从而减小器件的尺寸。同时,作为一种具有变周期和电控特性的嵌入式光栅,它可用于可调谐多波长有机激光器的研究。
2 光栅的制备
在本实验中,制备PDLC材料的体系包括:液晶(TEB50,北京清华亚王液晶材料有限公司生产,质量分数为34.76%)、预聚合物单体(EB8301,比利时UCB公司生产,质量分数为44.71%)、交联剂(1-vinyl-2-pyrrolidinoe,
将上述材料在暗室中按照一定比例进行配比混合,用超声乳化仪混合、加热均匀后静置24~48 h,形成纳米银掺杂型PDLC材料,将材料均匀放置在两片镀有ITO导电膜的玻璃片间,用间隔子控制光栅厚度为20 μm。将此材料置于
实验光路如
实验中,为了使聚合物与液晶达到较好的分离效果,需要严格控制曝光光强以及两次曝光的时间。在实验过程中发现,当两束光的总光强为19 mW/cm2时,第一次曝光仅几秒后,聚合物与液晶相分离的结构已经固化,完全限制了第二次曝光。经多次测试,选取曝光光强为16 mW/cm2。在曝光光强确定的情况下,两次曝光时间也需严格控制,第一次曝光是为了使预聚物单体初始聚合,由于光引发自由基反应具有不可逆的特性,因此第一次曝光时间应远短于第二次曝光时间,此时生成的自由基少,固化速率小,单体生长时间充足,两者相互扩散非常充足。实验对比了曝光时间分别为2 s/60 s、4 s/60 s、5 s/60 s(首次曝光时间/二次曝光时间)时聚合物和液晶相的分离情况,如
图 1. (a)实验光路图;(b)曝光面干涉角度变化图;(c)样品旋转角度图;(d)理论模拟2D变间距光栅图
Fig. 1. (a) Experimental light path diagram; (b) interference angle variation diagram of exposure surface; (c) sample rotation angle diagram; (d) theoretical simulation diagram of 2D variable spacing grating
图 2. 不同曝光时间下聚合物与液晶分离的微观结构图。(a) 2 s/60 s;(b) 4 s/60 s;(c) 5 s/60 s
Fig. 2. Microstructural diagrams of polymer and liquid crystal separation at different exposure time. (a) 2 s/60 s; (b) 4 s/60 s; (c) 5 s/60 s
3 光栅周期变化的理论分析
根据
式中:
振幅为
该光束以与全息干涉记录面垂直方向呈
式中:
另外一束光以与全息干涉记录面垂直方向呈
因此,全息干涉记录面上的光强分布可以表示为
由(5)式可以知道全息干涉记录面上的条纹对于
式中:
同理,在
4 光栅特性分析
为了探测该光栅的衍射特性,用633 nm的氦氖激光器作为探测光垂直入射到光栅表面,在观察面处观察光栅的衍射光斑变化,并计算不同位置处的光栅周期,如
图 5. (a)对角线左上区域处、(b)对角线中心区域处、(c)对角线右下区域处、(d)非对角线区域处的衍射光斑;(e)对角线左上区域处、(f)对角线中心区域处、(g)对角线右下区域处、(h)非对角线区域处光栅的原子力显微镜微观结构图
Fig. 5. Diffraction spot maps at (a) diagonal upper left corner area, (b) diagonal center area, (c) diagonal lower right corner area, (d) off-diagonal area; atomic force microscope micro structure maps of grating at (e) diagonal upper left corner area, (f) diagonal center area, (g) diagonal lower right corner area, (h) off-diagonal area
图 6. (a)未加电压时和(b)加载电压时的衍射光斑
Fig. 6. Diffraction spots (a) without and (b) with applied voltage
对于该2D变周期H-PDLC光栅,除了具有周期变化的特性,还具备电调谐特性,其衍射效率随着外加电压的增加而逐渐减小。在该光栅两端加载频率为50 Hz的交流电,逐渐增加两端电压,中心透射光明显增强,一级衍射光强明显减弱,如
图 7. 一级衍射效率、零级衍射效率随加载电压的变化
Fig. 7. First-order diffraction efficiency and zero-order diffraction efficiency as functions of applied external voltage
5 影响光栅周期的参数分析
为了在实际应用中制备周期变化符合要求的2D六角晶格H-PDLC变间距光栅,需要对影响光栅周期的因素进行讨论。由(6)式可以知道周期的
变化与柱透镜焦距
图 8. (a)不同D下的周期变化曲线;(b)不同f下的周期变化曲线;(c)不同α下的周期变化曲线
Fig. 8. (a) Periodic variation curves at different D; (b) periodic variation curves at different f; (c) periodic variation curves at different α
的周期变化率增大,并且光栅面左上角的周期变化率小于右下角的周期变化率。
6 结论
通过柱面波与平面波干涉,二次曝光时将样品旋转60°的方法制备了2D六角晶格H-PDLC变间距光栅,不仅实现了光栅在两个方向上的周期变化,还使得光栅具备电控的特性。实验测得了在半径为6 mm的圆形光栅区域,周期的变化区间为[1.804 μm,2.281 μm],这与理论计算结果基本吻合;在对角线方向,衍射光斑呈现规则的正六边形,在非对角线区域,衍射光斑呈现不规则的六边形。此外,通过在ITO导电膜上加载电压,测得当电压加载到90 V时,零级衍射效率为73%,较未加电压时的零级衍射效率15.6%有了很大变化,说明该结构具备良好的电控特性。最后,模拟得出了柱透镜焦距、柱透镜与曝光面之间的距离以及干涉光夹角对光栅周期变化范围和变化率的影响。该光栅同时具备了两个维度周期变化的特点以及电控特性,在光谱分析、光电探测领域具有潜在的应用价值。
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Article Outline
缪涛, 郑继红, 王康妮, 刘悠嵘, 黄新荣, 朱天赟. 基于聚合物分散液晶的二维六角晶格变间距光栅的研制[J]. 中国激光, 2018, 45(8): 0809001. Miao Tao, Zheng Jihong, Wang Kangni, Liu Yourong, Huang Xinrong, Zhu Tianyun. Development of Two-Dimensional Hexagonal Lattice Variable Line-Space Grating Based on Polymer-Dispersed Liquid Crystal[J]. Chinese Journal of Lasers, 2018, 45(8): 0809001.