CN110646992B - A dual-period composite liquid crystal polarization grating - Google Patents
A dual-period composite liquid crystal polarization gratingDownload PDFInfo
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Abstract
Description
Translated fromChinese技术领域technical field
本发明属于主动光电系统中的非机械式光束偏转技术领域，具体是指一种双周期复合液晶偏振光栅。The invention belongs to the technical field of non-mechanical beam deflection in an active optoelectronic system, in particular to a dual-period composite liquid crystal polarization grating.
本发明涉及偏振光栅，并且更特别地涉及液晶偏振光栅的设计和相关器件的制作。The present invention relates to polarization gratings, and more particularly to the design of liquid crystal polarization gratings and the fabrication of related devices.
背景技术Background technique
液晶偏振光栅是一种能够实现大角度光束偏转的新型元件，基于光的偏振态调制和几何相位调制原理，它能够将圆偏振光以近乎100％的衍射效率偏向+1级或者-1级，偏转的方向取决于入射光的偏振态。按照工作方式分类，液晶偏振光栅可以分为有源液晶偏振光栅和无源液晶偏振光栅。有源液晶偏振光栅是指受电压控制的液晶偏振光栅，其一般与半波片组合使用，可以在电压和半波片控制下使光束以近乎100％的衍射效率偏向三个级次(0级和±1级)。无源液晶偏振光栅结构简单，但是需要精确控制液晶层的厚度，自身不能加电控制，光束无法直接透过(没有零级)。要想实像无源液晶偏振光栅的动态光束调控，必须将其与液晶波片结合使用，通过电控液晶波片用以实现对入射光的偏振态进行调制，从而控制入射到无源液晶偏振光栅上面光束偏向+1级或者-1级，实现动态电控光束偏转。Liquid crystal polarization grating is a new type of component that can realize large-angle beam deflection. Based on the principle of polarization state modulation and geometric phase modulation of light, it can deflect circularly polarized light to +1 or -1 order with nearly 100% diffraction efficiency. The direction of deflection depends on the polarization state of the incident light. According to the classification of working methods, liquid crystal polarization gratings can be divided into active liquid crystal polarization gratings and passive liquid crystal polarization gratings. Active liquid crystal polarization grating refers to a liquid crystal polarization grating controlled by a voltage, which is generally used in combination with a half-wave plate. and ±1 level). The passive liquid crystal polarization grating has a simple structure, but requires precise control of the thickness of the liquid crystal layer, and cannot be controlled by power supply itself, and the light beam cannot be directly transmitted (there is no zero order). In order to realize the dynamic beam control of passive liquid crystal polarization grating, it must be used in combination with liquid crystal wave plate, and the polarization state of incident light can be modulated by electronically controlled liquid crystal wave plate, so as to control the incident light to passive liquid crystal polarization grating. The upper beam is deflected to +1 level or -1 level to realize dynamic electronically controlled beam deflection.
传统的激光都是良好的线偏光，为了实现光束偏转，需要将线偏光转换成圆偏光，这必然带来能量损失；另一方面，如果入射光不是良好的圆偏振光，而是椭圆偏振光，就无法使光束完全偏转到+1级或者-1级，而是存在两个级次。本发明提出的一种双周期复合液晶偏振光栅对于线偏振入射光具有良好的响应，只需要控制线偏振光的偏振方向，就可以使光束以近乎100％的衍射效率偏向+1级和-1级。相较于之前已经实现的液晶偏振光栅在系统控制和装调方面更加简洁、能量利用率等方面有显著提升。Traditional lasers are well linearly polarized light. In order to achieve beam deflection, it is necessary to convert linearly polarized light into circularly polarized light, which will inevitably lead to energy loss; on the other hand, if the incident light is not well circularly polarized light, but elliptically polarized light , the beam cannot be completely deflected to the +1 or -1 order, but there are two orders. The dual-period composite liquid crystal polarization grating proposed by the present invention has a good response to linearly polarized incident light, and only needs to control the polarization direction of the linearly polarized light, the beam can be deflected to +1 order and -1 order with nearly 100% diffraction efficiency class. Compared with the liquid crystal polarization grating that has been realized before, it is simpler in system control and adjustment, and the energy utilization rate is significantly improved.
发明内容SUMMARY OF THE INVENTION
本发明的目的在于设计和制作一种双周期复合液晶偏振光栅，解决目前液晶偏振光栅因需要将入射光转换成圆偏振光而带来的能量损失严重、结构复杂等问题。The purpose of the present invention is to design and manufacture a dual-period composite liquid crystal polarization grating to solve the problems of serious energy loss and complex structure caused by the need to convert incident light into circularly polarized light in the current liquid crystal polarization grating.
一种双周期复合液晶偏振光栅，其特征在于：该双周期复合液晶偏振光栅包括两层周期满足二倍关系的液晶层，第一层液晶周期为Λ，厚度为d，第二层液晶周期Λ′＝Λ/2，厚度为d′＝d/2，两层光栅配合调制偏振态，最终实现动态光束偏转的控制。A dual-period composite liquid crystal polarization grating is characterized in that: the dual-period composite liquid crystal polarization grating comprises two layers of liquid crystal layers whose periods satisfy a double relationship, the first layer of liquid crystal has a period of Λ and a thickness of d, and the second layer of liquid crystal has a period of Λ '=Λ/2, the thickness is d'=d/2, the two-layer grating cooperates to modulate the polarization state, and finally realizes the control of dynamic beam deflection.
一种双周期复合液晶偏振光栅，其结构包括如下部分：A dual-period composite liquid crystal polarization grating, the structure of which includes the following parts:
光控取向基板：用于旋涂光控取向膜和对液晶分子施加电场，电场方向垂直于基板平面；Light control alignment substrate: used for spin coating light control alignment film and applying electric field to liquid crystal molecules, the electric field direction is perpendicular to the plane of the substrate;
光控取向膜：经激光照射后发生光交联反应，诱导液晶分子取向；Photo-controlled alignment film: photocrosslinking reaction occurs after laser irradiation, inducing the alignment of liquid crystal molecules;
液晶：包括向列相液晶和聚合物液晶。Liquid crystal: including nematic liquid crystal and polymer liquid crystal.
本发明中光控取向基板可以选择导电或者不导电，这取决于要制作的光栅是有源或者无源。In the present invention, the light-controlled orientation substrate can be electrically conductive or non-conductive, depending on whether the grating to be fabricated is active or passive.
本发明中液晶层被分为两层，第一层周期为Λ，厚度为d；第二层周期Λ′＝Λ/2，厚度为d′＝d/2。In the present invention, the liquid crystal layer is divided into two layers, the period of the first layer is Λ and the thickness is d; the period of the second layer is Λ'=Λ/2, and the thickness is d'=d/2.
本发明中双周期复合液晶偏振光栅薄膜第一层液晶分子沿基板平面周期性排布，液晶分子的光轴在一个周期内连续变化，并满足如下关系式：In the present invention, the liquid crystal molecules of the first layer of the dual-period composite liquid crystal polarization grating film are periodically arranged along the plane of the substrate, and the optical axis of the liquid crystal molecules changes continuously within a period, and the following relational formula is satisfied:
式中
本发明中双周期复合液晶偏振光栅薄膜第二层液晶分子沿基板平面周期性排布，液晶分子的光轴在一个周期内连续变化，并满足如下关系式：In the present invention, the liquid crystal molecules of the second layer of the dual-period composite liquid crystal polarization grating film are periodically arranged along the plane of the substrate, and the optical axis of the liquid crystal molecules changes continuously within one period, and the following relational formula is satisfied:
式中
本发明中液晶偏振光栅薄膜第一层的厚度满足半波条件，即Δnd＝λ/2，其中Δn为液晶分子的双折射率，d为液晶偏振光栅薄膜厚度，λ为入射光波长。In the present invention, the thickness of the first layer of the liquid crystal polarization grating film satisfies the half-wave condition, that is, Δnd=λ/2, where Δn is the birefringence of liquid crystal molecules, d is the thickness of the liquid crystal polarization grating film, and λ is the wavelength of incident light.
本发明中液晶偏振光栅薄膜第二层的厚度满足四分之一波条件，即Δnd′＝λ/4，其中Δn为液晶分子的双折射率，d′为液晶偏振光栅薄膜厚度，λ为入射光波长。In the present invention, the thickness of the second layer of the liquid crystal polarization grating film satisfies the quarter-wave condition, that is, Δnd′=λ/4, where Δn is the birefringence of the liquid crystal molecule, d′ is the thickness of the liquid crystal polarization grating film, and λ is the incident light wavelength.
本发明中的双周期复合液晶偏振光栅打破了传统以半波条件为主的液晶偏振光栅，将其中一个液晶偏振光栅作为一种针对周期性非均匀偏振态分布矢量光场的波片。The dual-period composite liquid crystal polarization grating in the present invention breaks the traditional liquid crystal polarization grating mainly based on half-wave conditions, and uses one of the liquid crystal polarization gratings as a wave plate for the periodic non-uniform polarization state distribution vector light field.
一种双周期复合液晶偏振光栅，其特征在于：其中一种无源型双周期复合液晶偏振光栅的制备步骤如下，A dual-period composite liquid crystal polarization grating is characterized in that: the preparation steps of one of the passive dual-period composite liquid crystal polarization gratings are as follows:
步骤一，在玻璃基板上面旋涂光控取向膜，高温下使光控取向膜固化；
步骤二，调整光路使周期刚好为Λ，照射上述玻璃基板，使光控取向膜发生光交联反应，记录曝光图案；
步骤三，在步骤二的玻璃基板上多次旋涂可聚合液晶，在氮气保护环境下利用紫外光对其进行照射、固化，按照半波条件确定旋涂次数和转速等，制备形成第一层液晶偏振光栅薄膜；Step 3: Spin-coating the polymerizable liquid crystal on the glass substrate in
步骤四，调整光路使周期刚好为Λ/2，照射步骤一所述玻璃基板，使光控取向膜发生光交联反应，记录曝光图案；
步骤五，在步骤四的玻璃基板上多次旋涂可聚合液晶，在氮气保护环境下利用紫外光对其进行照射、固化，按照四分之一波条件确定旋涂次数和转速等，制备形成第二层液晶偏振光栅薄膜；
步骤六，将步骤三和步骤六所述液晶偏振光栅薄膜按照正确的相对位置将薄膜一侧紧贴在一起，形成双周期复合液晶偏振光栅。In
本发明公布的一种双周期复合液晶偏振光栅可以实现高效率、大偏转角度、快速便捷的非机械式光束偏转与扫描，这将使其在激光通信、激光对抗、激光雷达及存储显示等领域具有广阔的应用前景。The dual-period composite liquid crystal polarization grating disclosed in the present invention can realize high-efficiency, large deflection angle, fast and convenient non-mechanical beam deflection and scanning, which will make it useful in the fields of laser communication, laser countermeasures, laser radar, storage display and the like with broadly application foreground.
附图说明Description of drawings
图1为本发明公布的一种双周期复合液晶偏振光栅的结构示意图，其中包括基板1和2，光控取向膜3和4，液晶层5和6。FIG. 1 is a schematic structural diagram of a dual-period composite liquid crystal polarization grating disclosed in the present invention, which includes
图2为液晶偏振光栅在右旋圆偏振(图2(a))和线偏振入射(图2(b))情况下，通过厚度满足半波延迟的液晶层后近场偏振态的分布情况，其中1为均匀右旋圆偏振输入，2为均匀线偏振输入，3、4为入射偏振态分布，5、6为近场出射偏振态分布，7为液晶层，其厚度d满足半波条件Δnd＝λ/2，8、9、10为等相位面，11、12为出射光。Figure 2 shows the distribution of the near-field polarization state of the liquid crystal polarization grating after passing through a liquid crystal layer whose thickness satisfies half-wave retardation under the conditions of right-handed circular polarization (Fig. 2(a)) and linearly polarized incidence (Fig. 2(b)). Among them, 1 is the uniform right-handed circularly polarized input, 2 is the uniform linearly polarized input, 3 and 4 are the incident polarization state distribution, 5 and 6 are the near-field outgoing polarization state distribution, and 7 is the liquid crystal layer whose thickness d satisfies the half-wave condition Δnd =λ/2, 8, 9, and 10 are iso-phase planes, and 11 and 12 are outgoing lights.
图3为双周期复合液晶偏振光栅的衍射效率特性曲线，其中横轴为入射线偏振光的偏振方向，纵轴为0、+1和-1级次的衍射效率。FIG. 3 is the diffraction efficiency characteristic curve of the dual-period composite liquid crystal polarization grating, wherein the horizontal axis is the polarization direction of the incident line polarized light, and the vertical axis is the diffraction efficiency of 0, +1 and -1 orders.
图4为本发明公布的一种双周期复合液晶偏振光栅的实现实例，其中1为旋涂了光控取向膜的玻璃基板甲，2为旋涂了光控取向膜的玻璃基板乙，3为聚合物液晶层A，4为聚合物液晶层B，5为入射光方向。4 is an implementation example of a dual-period composite liquid crystal polarization grating disclosed in the present invention, wherein 1 is a glass substrate A spin-coated with a light-controlled alignment film, 2 is a glass substrate B spin-coated with a light-controlled alignment film, and 3 is a The polymer liquid crystal layer A, 4 is the polymer liquid crystal layer B, and 5 is the incident light direction.
图5为液晶偏振光栅的双光束干涉曝光光路，它由325nm激光器1、显微物镜2、小孔3、准直透镜4、反射镜5和6、分光棱镜7、四分之一波片8和9、样品架10组成，其中四分之一波片8和9的光轴方向相互垂直，与入射光偏振方向分别成45°和135°，Θ是两束光的夹角。Figure 5 shows the double-beam interference exposure light path of the liquid crystal polarization grating, which consists of a
图6为本发明公布的一种双周期复合液晶偏振光栅的测试衍射效果图。FIG. 6 is a test diffraction effect diagram of a dual-period composite liquid crystal polarization grating disclosed in the present invention.
具体实施方式Detailed ways
1、对液晶偏振光栅近场出射偏振态的分析1. Analysis of the near-field exit polarization state of the liquid crystal polarization grating
图2给出了满足半波延迟Δnd＝λ/2的液晶偏振光栅分别在右旋圆偏振输入和线偏振输入情况下近场出射光偏振态分布情况。当右旋圆偏振输入时，近场出射光为具有不同相位的等强左旋圆偏振光，进一步发现出射偏振态分布的周期刚好是光栅周期的一半，图上绘出的等相位面能够解释液晶偏振光栅正入射时偏转角满足的公式tanθm＝mλ/Λ,m＝±1。对于线偏振输入时，近场出射光为具有不同偏振方向的线偏振光，进一步发现出射偏振态分布的周期刚好是光栅周期的一半。线偏振光可以分解为等强的正交圆偏振，图上绘出的等相位面能够解释线偏振入射液晶偏振光栅时出射光有±1级两个级次。Figure 2 shows the polarization state distribution of the near-field outgoing light under the conditions of right-handed circularly polarized input and linearly polarized input of the liquid crystal polarization grating satisfying the half-wave retardation Δnd=λ/2, respectively. When the right-handed circularly polarized input is input, the near-field outgoing light is equal-intensity left-handed circularly polarized light with different phases. It is further found that the period of the outgoing polarization state distribution is exactly half of the grating period. The iso-phase plane drawn on the figure can explain the liquid crystal When the polarization grating is incident normally, the deflection angle satisfies the formula tanθm =mλ/Λ, m=±1. For linearly polarized input, the near-field outgoing light is linearly polarized light with different polarization directions, and it is further found that the period of the outgoing polarization state distribution is exactly half of the grating period. Linearly polarized light can be decomposed into equal-intensity orthogonal circular polarization, and the iso-phase plane drawn on the figure can explain that when linearly polarized incident on the liquid crystal polarization grating, the outgoing light has two orders of ±1 order.
2、双周期复合液晶偏振光栅的设计2. Design of double-period composite liquid crystal polarization grating
基于上述液晶偏振光栅近场出射光的偏振态分布周期刚好是光栅周期的一半，提出了双周期复合液晶偏振光栅。它将一个周期是原光栅周期一半的光栅置于其后，用来调节通过第一个周期为Λ的光栅的出射偏振态，从而实现新的光学特性。Based on the above-mentioned polarization state distribution period of the near-field outgoing light of the liquid crystal polarization grating is exactly half of the grating period, a dual-period composite liquid crystal polarization grating is proposed. It places a grating whose period is half the period of the original grating behind it, and is used to adjust the outgoing polarization state through the first grating with a period of Λ, thereby realizing new optical properties.
该双周期复合液晶偏振光栅主要由两个液晶偏振光栅复合而成(图1)。其中第一个液晶偏振光栅周期为Λ，厚度为d/2，满足半波条件；第二个液晶偏振光栅周期为Λ′＝Λ/2，厚度为d′＝d/2，满足四分之一波条件。The dual-period composite liquid crystal polarization grating is mainly composed of two liquid crystal polarization gratings (Fig. 1). The first liquid crystal polarization grating has a period of Λ and a thickness of d/2, which satisfies the half-wave condition; the second liquid crystal polarization grating has a period of Λ'=Λ/2 and a thickness of d'=d/2, which satisfies the quarter-wave condition. A wave of conditions.
3、双周期复合液晶偏振光栅的特性3. Characteristics of dual-period composite liquid crystal polarization gratings
图3给出了双周期复合液晶偏振光栅对于不同入射偏振态的特性示意图。对于不同偏振方向的线偏振光入射，能够以固定角度θ偏向±1级两个级次，通过控制线偏振的偏振方向，可以分配±1级的能量占比，当45°线偏振入射时，出射光能量全部集中在+1级；当135°线偏振入射时，出射光能量全部集中在-1级。Figure 3 shows a schematic diagram of the characteristics of the dual-period composite liquid crystal polarization grating for different incident polarization states. For the incidence of linearly polarized light with different polarization directions, it can be deviated to two orders of ±1 order at a fixed angle θ. By controlling the polarization direction of the linear polarization, the energy ratio of ±1 order can be allocated. When the linear polarization of 45° is incident, the The outgoing light energy is all concentrated in the +1 level; when the 135° linear polarization is incident, the outgoing light energy is all concentrated in the -1 level.
4、双周期复合液晶偏振光栅的一种实现方案4. An implementation scheme of dual-period composite liquid crystal polarization grating
1)图4给出了无源式复合液晶偏振光栅的一种实现方案，其包括光控取向玻璃基板、可聚合液晶、聚合物液晶层A、聚合物液晶层B和光控取向剂。1) FIG. 4 shows an implementation scheme of a passive composite liquid crystal polarization grating, which includes a light-controlled alignment glass substrate, a polymerizable liquid crystal, a polymer liquid crystal layer A, a polymer liquid crystal layer B, and a light-controlled alignment agent.
2)玻璃基板的准备。首先对基板进行清洗，待烘干后在基板一侧上面旋涂光控取向剂，其厚度约为几十纳米，120℃下使光控取向剂固化。2) Preparation of glass substrate. First, the substrate is cleaned, and after drying, a light-controlled alignment agent is spin-coated on one side of the substrate with a thickness of about tens of nanometers, and the light-controlled alignment agent is cured at 120°C.
3)光控取向基板的曝光。将上述旋涂过光控取向剂的基板分为甲和乙两类，分别放置于图5所示的全息干涉光路中进行曝光，使光控取向剂发生光交联反应，记录曝光图案。注意在曝光前需要通过调节角度Θ使得甲类基板周期满足Λ，乙类基板周期满足Λ′＝Λ/2。3) Exposure of the photo-controlled alignment substrate. The above substrates spin-coated with the light-controlled alignment agent are divided into two categories, A and B, and placed in the holographic interference optical path shown in FIG. Note that before exposure, it is necessary to adjust the angle Θ so that the period of type A substrate satisfies Λ, and the period of type B substrate satisfies Λ'=Λ/2.
4)在照射之后的光控取向玻璃基板上面旋涂可聚合液晶，在氮气保护环境下利用全波段紫外光对其进行照射、固化5min，按照532nm激光半波条件反复旋涂四次，使得甲类基板上的液晶层A厚度满足半波条件Δnd＝λ/2，使得乙类基板上的液晶层B厚度满足四分之一波条件，最终制备形成液晶偏振光栅薄膜。4) Spin-coat the polymerizable liquid crystal on the photo-controlled orientation glass substrate after irradiation, irradiate and cure it with full-band ultraviolet light under nitrogen protection environment for 5 minutes, and repeat the spin-coating four times according to the half-wave laser condition of 532 nm, so that the The thickness of the liquid crystal layer A on the type B substrate satisfies the half-wave condition Δnd=λ/2, so that the thickness of the liquid crystal layer B on the type B substrate satisfies the quarter-wave condition, and finally a liquid crystal polarization grating film is prepared.
5)将带有光栅薄膜的基板甲和乙按照图4(b)所示方式正确叠放，确保液晶层A和B的位置关系，具体是以光栅周期方向严格一致为标准，最后将两个基板粘连在一起，形成无源式双周期复合液晶偏振光栅。5) Stack the substrates A and B with the grating film correctly according to the method shown in Figure 4(b) to ensure the positional relationship between the liquid crystal layers A and B. Specifically, the grating period direction is strictly consistent as the standard, and finally the two The substrates are glued together to form a passive dual-period composite liquid crystal polarization grating.
5、无源式双周期复合液晶偏振光栅的特性检测5. Characteristic detection of passive dual-period composite liquid crystal polarization grating
利用一束532nm的线偏振光激光器垂直照射本发明制备的偏振光栅，旋转偏振光栅，观察衍射效果，可以使﹢1级或者-1级能量实现最大(图6)。可以看到0级非常弱(<0.02％)，这说明液晶层A的厚度基本满足半波消光的条件。如图6(a),此时认为是45°线偏振入射，出射光全部集中到+1级，消光比达到96.7％(ηm＝Im/(I+1+I-1),m＝±1)，说明液晶层B的厚度基本满足四分之一波条件；如图6(b)，此时是135°线偏振入射，出射光全部集中到-1级，消光比达到97.1％。A beam of 532nm linearly polarized laser is used to vertically irradiate the polarization grating prepared by the present invention, and the polarization grating is rotated to observe the diffraction effect. It can be seen that the 0-level is very weak (<0.02%), which indicates that the thickness of the liquid crystal layer A basically satisfies the condition of half-wave extinction. As shown in Figure 6(a), at this time, it is considered to be 45° linearly polarized incident, the outgoing light is all concentrated to +1 level, and the extinction ratio reaches 96.7% (ηm =Im /(I+1 +I-1 ), m = ±1), indicating that the thickness of the liquid crystal layer B basically meets the quarter-wave condition; as shown in Figure 6(b), at this time, the incident is 135° linearly polarized, the outgoing light is all concentrated to the -1 level, and the extinction ratio reaches 97.1%.
该无源式双周期复合液晶偏振光栅的检测结果验证了该类光栅的特性。The detection results of the passive dual-period composite liquid crystal polarization grating verify the characteristics of this type of grating.
显然，上述实施例仅仅是为清楚地说明所作的举例，而并非对实施方式的限定。对于所属领域的普通技术人员来说，在上述说明的基础上还可以做出其它不同形式的变化或变动。这里无需也无法对所有的实施方式予以穷举。而由此所引伸出的显而易见的变化或变动仍处于本发明创造的保护范围之中。Obviously, the above-mentioned embodiments are only examples for clear description, and are not intended to limit the implementation manner. For those of ordinary skill in the art, changes or modifications in other different forms can also be made on the basis of the above description. There is no need and cannot be exhaustive of all implementations here. And the obvious changes or changes derived from this are still within the protection scope of the present invention.
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