1.Institute of Micro-Nano Optoelectronic Technology, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 2.College of Big Data and Internet, Shenzhen Technology University, Shenzhen 518118, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61275167) and the Basic Research Project of Shenzhen, China (Grant Nos. JCYJ20180305125430954, JCYJ20170817102315892, JCYJ2017081701827765)
Received Date:02 July 2020
Accepted Date:05 December 2020
Available Online:01 April 2021
Published Online:20 April 2021
Abstract:In this paper, we propose a new method to realize both polarization-multiplexing and wavelength-multiplexing using a simple structure, which can realize hologram by the multiplexing of double wavelengths and double polarization in the visible band. Our design can reduce color cross-talk and have a higher diffraction efficiency. We design a transmission metasurface composed of simple rectangular cells. Firstly, we establish the relationship of structural parameters with the transmission phase under various incident conditions of light beams. Then we propose a fitness function that can optimize the structural parameters of the unit cell at each pixel point, which can display different images by 532 nm x-polarization and 633 nm y-polarization incident light beams respectively. Finally, finite difference time domain method is used to analyze the structure, and the holographic result fits the theoretical design very well. This work proposes using single metasurface structure to solve the problems of wavelength cross-talk appearing when using simple structures, and will have great importance in coding and anti-counterfeiting. Keywords:metasurface/ holographic/ double wavelengths/ double polarization
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2.超表面微元设计本文运用单一结构超表面微元对波长和正交线偏振光具有不同相位调控能力的特性, 提出了一种基于超表面的波长和偏振态同时复用全息显示的新方法. 首先运用时域有限差分法(finite difference time domain, FDTD)建立矩形微元尺寸与透过相位的映射关系; 然后根据两个不同波长和偏振态相应的期望输出的目标字符计算出两幅相应的相位全息图; 再根据所计算出来的两幅相位全息图, 设定评价函数, 优化出单一结构超表面微元来表示两幅全息图上各点的相位; 最后构建了实现波长和线偏振同时复用的透射式全息显示超表面. 所设计的双波长、线偏振复用全息显示的超表面微元结构示意图如图1(a)所示, 图1(b)为超表面在532 nm波长和633 nm波长、正交线偏振态入射下, 全息显示示意图. 其中, 超表面由周期性排布的矩形柱微元组成, 设计的目标是: 波长为532 nm的x线偏振光、波长为633 nm的y线偏振光同时垂直入射到超表面后, 重建出绿色“CET”图像和红色“SZU”图像. 图 1 (a) 超表面微元结构示意图; (b) 超表面在532 nm波长和633 nm波长、正交线偏振态下, 全息显示示意图 Figure1. (a) Schematic of unit cell structure consisting of Si nanobrick on the SiO2 substrate; (b) schematic of hologram metasurface at wavelength of 532 nm and 633 nm with orthogonal linear polarizations.
其中φx(x, y), φy(x, y)分别表示微元长宽为L(x, y)和W(x, y)时532 nm波长、x偏振光和633 nm波长、y偏振光对应的透过相位值, tx(x, y), ty(x, y)分别表示微元长宽为L(x, y)和W(x, y)时532 nm波长x偏振光和633 nm波长y偏振光对应的透过效率. (2)式第一、二项考虑了全息图中(x, y)处的矩形微元, 能同时表示两幅全息图对应位置上的相位, 第三、四项考虑了矩形微元的透过效率. 评价函数Δ(x, y)值越小, 则表示φx(x, y), φy(x, y)和φ1(x, y), φ2(x, y)偏差越小, 并且保证透过效率最大, 说明此时的微元长宽更接近理想值. 字符CET和SZU对应的全息图八阶量化后各像素点的相位有8种值, φ1(n), φ2(m)(n = 1, 2,···, 8; m = 1, 2,···, 8), 由于要应用一个微元表示不同入射条件下的两种相位, 并且同一位置的像素点上对应的两种相位组合最多有64个(φ1(n), φ2(m)), 那么根据(2)式和图2所示的透过相位与微元尺寸之间的关系, 搜索出这64个组合所对应64个最优的硅矩形柱几何参数L(n, m), W(n, m). 硅矩形柱几何参数L(n, m), W(n, m)对应的在532 nm 波长x偏振光入射下的透过相位为φx(n, m), 透过效率为tx(n, m), 而在633 nm波长、y偏振光入射下的透过相位为φy(n, m), 透过效率为ty(n, m). 图4(a),(b)为筛选得出的64种硅矩形柱对应的透过相位与64种理想组合相位的差值. 图4(c),(d)为筛选得出的64种硅矩形柱对应的透过效率. 由图4(a),(b)可知, 相位差值基本都小于π/8, 绝大部分都接近于0. 由图4(c),(d)可知, 硅矩形柱几何参数对应的透过效率变化较大, 根据图2, 为了满足相位差值尽量小, 所筛选的硅矩形柱几何参数无法避开透过效率过低的区间. 图 4 64种硅矩形柱对应的透过相位与理想组合相位的差值 (a) 532 nm波长、x线偏振态, (b) 633 nm波长、y线偏振态; 64种硅矩形柱对应的透过效率 (c) 532 nm波长、x线偏振态, (d) 633 nm波长、y线偏振态 Figure4. The deviation plot between the designed and ideal phase (a) at 532 nm for x-polarization light and (b) at 633 nm for y-polarization light. The transmission of the designed metasuface nanoblock (c) at 532 nm for x-polarization light and (d) at 633 nm for y-polarization light.
根据字符全息图的相位分布, 获得全息图上所有像素点对应的矩形柱尺寸L(x, y)和W(x, y), 组成双波长、线偏振复用全息显示的超表面. 其示意图如图5(a)所示. 图5(b)为根据超表面3 × 3 像素点内硅矩形柱几何参数的尺寸L(n, m), W(n, m), 计算得到的532 nm波长、x偏振光和633 nm波长、y偏振光对应的透过相位值和透过效率. 图 5 (a) 超表面结构示意图; (b) 超表面3 × 3 像素点内硅矩形柱几何参数的尺寸L(n, m), W(n, m), 分别在532 nm波长、x偏振光和633 nm波长、y偏振光入射下对应的透过相位值和透过效率 Figure5. (a) Schematic of metasurface; (b) phase matrix, transmission matrix, length of rectangular unit cell matrix and width of rectangular unit cell matrix. This is shown for 3 × 3 pixel subsection of the metasurface.