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光取向液晶技术是利用偏振光照射来实现液晶分子的定向排列, 该技术可以研制成偏振体全息光栅(polarization volume grating, PVG), 具有偏振和体全息选择特性, 且作为光耦合元件在光学波导及扩瞳输出方面有广泛应用前景. 本文报道了一种采用光控取向技术与偏振离轴全息光路相结合的方法, 制备了光斑直径为2 cm的液晶偏振体全息柱透镜(polarization variable line-space, PVLS). 实验过程中可通过控制曝光角度来获得所需光栅周期变化范围, 实现红绿蓝RGB三色光入射不同光栅周期下的衍射角角度相同. 实验结果表明: 在1721.2—5346.5 nm的光栅周期变化范围内, 当RGB三色光分别入射至光栅周期为3147 nm, 2649.1 nm, 2275.6 nm位置上时, 所测得的衍射角度均为11.59°, 实际衍射角度与理论衍射角度的误差在±0.5°以内; 在532 nm右旋圆偏光下, 正入射的衍射效率达90.6%, 满足布拉格条件的斜入射衍射效率为84.4%; 同时实现光斑在一维方向上扩束, 初步实现了PVLS在彩色波导领域应用的可行性验证.Photo-oriented liquid crystal technology utilizes polarized light illumination to achieve the directional alignment of liquid crystal molecules. This technology can be developed into polarization volume gratings (PVGs), which possess polarization and volume holographic selectivity characteristics, and also have a broad application prospect as an optical coupling element in optical waveguides and for pupil expansion output. This paper reports on the fabrication of a liquid crystal polarization volume holographic cylindrical lens (PVLS) with a beam diameter of 2 cm by using photo-oriented technology combined with a polarization off-axis holographic optical path. During the experiment, the exposure angle can be controlled to achieve the desired grating period variation range, enabling the diffraction angles of red, green, and blue light incident on different grating periods to be the same. The experimental results show that within the grating period variation range from 1721.2 to 5346.5 nm, when the red, the green, and the blue light are incident on grating with periods of 3147 nm, 2649.1 nm, and 2275.6 nm respectively, the measured diffraction angles are all 11.59°, with an error between the actual and theoretical diffraction angles within ±0.5°; under 532-nm right-handed circularly polarized light, the diffraction efficiency for 18 normal incidence reaches 90.6%, and the diffraction efficiency for oblique incidence satisfying the Bragg condition is 84.4%; simultaneously, beam expansion in one-dimensional direction is achieved, preliminarily verifying the feasibility of PVLS application in the field of color waveguides.
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Keywords:
- photocontrolled orientation /
- liquid crystal /
- variable spacing polarization grating /
- optical waveguide.
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图 1 (a)两束正交偏振平面波的干涉示意图; (b)正交偏振柱面波与平面波之间的干涉示意图; (c) PVG中偏振态的周期性分布; (d) PVLS光栅中偏振态的周期性分布
Fig. 1. (a) Schematic diagram of the interference between two orthogonally circularly polarized plane waves; (b) schematic diagram of the interference between an orthogonally circularly polarized cylindrical wave and a plane wave; (c) periodic distribution of polarization states in LCPG; (d) periodic distribution of polarization states in PVLS.
图 2 正交圆偏振全息示意图
Fig. 2. Schematic diagrams of orthogonal circularly polarization holography.
图 3 反射型PVLS光栅的偏振敏感及衍射特性①右旋圆偏振光入射样品表面1; ②左旋圆偏振光入射; ③右旋圆偏振光入射样品表面2(手性剂为右旋)
Fig. 3. Schematic diagram of the polarization sensitivity and diffraction characteristics of the reflective PVLS grating (with a right-handed chiral agent): ① Right-handed circularly polarized light incident on sample surface 1; ② Left-handed circularly polarized light incident; ③ Right-handed circularly polarized light incident on sample surface 2.
图 4 (a)入耦合光栅衍射; (b)出耦合光栅衍射.
Fig. 4. (a) In-coupling grating diffraction; (b) out-coupling grating diffraction.
图 7 变间距光栅制造实验装置示意图
Fig. 7. Schematic illustration of the experimental setup for fabricating PVLS grating.
图 8 (a)制备的反射式PVLS(反射绿光); (b)反射式 PVLS 光栅的衍射示意图; (c)入射光与衍射光的示意图; (d)一维方向x—光栅周期$\varLambda $关系图
Fig. 8. (a) Fabricated reflective PVLS (anti-green light); (b) schematic diagram of diffraction for the reflective PVLS grating; (c) schematic diagram of incident and diffracted light; (d) graph of the relationship between the one-dimensional direction x and grating period $\varLambda $.
图 9 532 nm右旋圆偏光斜入射PVLS光栅示意图
Fig. 9. Schematic diagram of right-handed circularly polarized 532 nm light obliquely incident on a PVLS grating.
图 10 (a), (b)制备的反射红光和反射蓝光PVLS光栅; (c)一维方向x—RGB衍射角θ关系图; (d)RGB衍射角θ—光栅周期$\varLambda $关系图
Fig. 10. (a), (b) Fabricated PVLS gratings for anti-red and anti-blue light; (c) graph of the relationship between the one-dimensional direction x and RGB diffraction angle θ as a function; (d) graph of the relationship between RGB diffraction angle θ and grating period $\varLambda $ as a function.
图 12 (a)光路示意图; (b), (c)532 nm激光和直径0.7 cm的532 nm右旋圆偏光照射至PVLS的传播示意图
Fig. 12. (a) Schematic diagram of the optical path; (b), (c) schematic diagram of the propagation of 532 nm laser and 532 nm right-handed circularly polarized light with a diameter of 0.7 cm incident on PVLS.
表 1 PVLS液晶层主要材料及配比
Table 1. Main materials and ratios of the PVLS liquid crystal layer.
5 CB R5011 反射红色 97.79% 2.21% 反射绿色 97.37% 2.63% 反射蓝色 96.94% 3.06% 
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表 2 反射红光(632 nm)PVLS测量结果
Table 2. Measurement results of PVLS for anti-red light (632 nm) reflectance.
x/cm 光栅周期/nm 衍射角/(°) 0.9 1739.4 21.31 0.5 2044.7 18 0 2655.9 13.77 –0.5 3567.5 10.2 –0.9 5304.5 6.84
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表 3 反射蓝光(457 nm)PVLS测量结果
Table 3. Measurement results of PVLS for blue light (457 nm) reflectance.
x/cm 光栅周期/nm 衍射角/(°) 0.9 1723.5 15.38 0.5 2081.9 12.68 0 2651.1 9.93 –0.5 3684.5 7.13 –0.9 5395.9 4.86
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