1.International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology, and Engineering Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province, Shenzhen University, Shenzhen 518060, China 2.College of New Materials and New Energies, Shenzhen Technology University, Shenzhen 518118, China 3.Synergetic Innovation Center for Quantum Effects and Applications, School of Physics and Electronics, Hunan Normal University, Changsha 410081, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61805149, 61575127), the Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030310065), the Educational Commission of Guangdong Province, China (Grant No. 2016KCXTD006), the Program of Fundamental Research of Shenzhen Science and Technology Plan (Grant No. JCYJ20180507182035270), the Science and Technology Project of Shenzhen, China (Grant No. ZDSYS201707271014468), and the Fund of the International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Shenzhen University, China (Grant No. 2DMOST2018003)
Received Date:04 August 2019
Accepted Date:16 October 2019
Available Online:13 December 2019
Published Online:05 January 2020
Abstract:With the rapid development of metasurface and metamaterials, the image edge detection based on the optical spatial differential calculation becomes an interesting topic in recent years. There have been a certain number of studies in this region, but most of them are applicable only to one-dimensional optical spatial differential calculation. In this work, a two-dimensional optical differentiator using Pancharatnam-Berry (P-B) phase metasurface is proposed and implemented in optical image two-dimensional edge detection. Based on the principle of the spin-dependent splitting from P-B phase devices, this metasurface is capable of separating the left-handed circularly polarized light from the right-handed circularly polarized light at a certain spatial distance. After filtering out the overlapped linear polarization, the left optical information is the result of the two-dimensional optical spatial differential. Meanwhile, the resolution of the image edge information is adjustable by changing the optic axis distribution of this two-dimensional optical differentiator. These results indicate that our P-B phase metasurface can be applied to the extraction of the optical image two-dimensional edge information, and the extracted edge information is more complete than the previous one-dimensional grating metasurface. For these advantages, this two-dimensional optical differentiator shows great potential applications in ultrafast optical calculation and image processing. Keywords:metasurface/ Pancharatnam-Berry phase/ photonic spin Hall effect
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2.1.P-B相位超表面设计原理
不同于传输相位通过控制光程来改变相位[17-20], P-B相位是一种通过改变光场偏振态而产生的几何相位[21,22], 其相位变化与偏振变化相关. 例如, 当LHCP光通过一块半波片后, 其偏振会被转换为RHCP态, 同时携带上一个附加相位, 这个相位即为P-B相位. 其遵守$\left| L \right\rangle \!\to \!{{\rm{e}}^{{\rm{2 i}}\phi }}\left| R \right\rangle $与$\left| R \right\rangle \!\to \!{{\rm{e}}^{ - 2{\rm{i}}\phi }}\left| L \right\rangle $原则, 其中$\phi $为半波片的光轴旋转角. 由于P-B相位型超表面对LHCP和RHCP光束具有不同的相位响应, 若设计恰当的相位分布, 便可使入射的光子发生自旋分离[23-26], 这种自旋分离是实现光学边缘检测的关键. 根据光子自旋分离原理, 为了使LHCP与RHCP光束通过超表面后产生的附加相位相互共轭, 超表面的每个单元结构都需满足半波片条件, 即${\delta _x} - {\delta _y} = {\text{π}}$, 其中${\delta _x}, {\delta _y}$分别为单元结构对x与y方向偏振的相位响应. 由于非晶态TiO2的透明窗口达360 nm, 其带间跃迁刚好处于可见光谱之外, 在整个可见光波段具有很高的传输效率且可达到0—2π的相位变化[27,28]. 因此, 选择TiO2作为P-B相位超表面中介质柱的材料. 图1(a)为设计的微结构单元, 其中基底材料为SiO2, 介质柱材料为TiO2. 介质柱高度h = 600 nm, 晶格大小为325 nm, 即Px = Py = 325 nm. 介质柱长为l, 宽为w, 其长轴与x轴的夹角为$\phi $. 首先, 为使得每个单元结构都满足半波条件, 分别以波长为532 nm的x与y方向的线偏振光作为入射光, 对单元结构中介质柱的长宽$(l, w)$进行参数扫描, 得到${\delta _x}$、${\delta _y}$与$(l, w)$的关系如图1(b)和(c)所示. 图1(d)为${\delta _x}$、${\delta _y}$之间的的相位差值与$(l, w)$的关系, 为满足${\delta _x} - $${\delta _y} = {\text{π}} $以达到半波条件, 选择l = 300 nm, w = 105 nm. 在确定介质柱长与宽之后, 将入射光源设置为圆偏振光, 对单元结构中介质柱旋转角度进行参数扫描, 所得圆偏振光通过单元结构得到的附加相位与介质柱旋转角度的关系曲线如图1(e)所示. 从图中可看出, 圆偏振光入射后得到的附加相位可以覆盖整个0—2π区间. 因此, 根据二维边缘检测所需P-B相位分布可以设计超表面上介质柱的排布方式. 图 1 (a)单元结构示意图; (b)与(c) x与y方向线偏振入射光相位响应与介质柱长(l)、宽(w)之间的关系; (d) x和y方向上的相位差随l和w变化关系; (e)介质柱的旋转角与附加相位关系图. Figure1. (a) Schematic for basic unit structure; (b) and (c) phase response of different length (l) and width (w) of the dielectric column under x- and y- LP incident beams; (d) phase difference between the x- and y-polarized light for different length (l) and width (w) of the dielectric column; (e) relationship between the rotation angle of the dielectric column and the additional phase.
22.2.基于P-B相位超表面实现二维边缘检测的原理 -->
2.2.基于P-B相位超表面实现二维边缘检测的原理
图2(a)为二维光学边缘检测原理示意图. 当一束线偏振(linearly polarized, LP)平面波入射至设计好的P-B相位超表面, 经过傅里叶变换后在像平面中LHCP分量沿着径向向外扩大, RHCP分量沿着径向向内缩小, 中间重叠部分仍为LP. 通过检偏器将LP消光, 仅留下边缘位置光强, 便可达到边缘检测的效果. 由于超表面光轴方向为局部变化[29], 根据琼斯理论, 超表面的光学传输矩阵可以表示为[21] 图 2 (a)光学二维边缘检测原理图; (b) LHCP与RHCP通过PB相位超表面后获得的相位梯度变化; (c) P-B相位超表面示意图; (d)和(e) RHCP与LHCP平面波通过超表面后波前变化图 Figure2. (a) Schematic diagram of the 2D optical edge detection; (b) phase gradient of the LHCP and RHCP component after the P-B phase matesurface; (c) diagram of the metasurface; (d) and (e) wavefront changes of RHCP and LHCP plane waves through the metasurface.