用四台阶相位板产生涡旋光束

涡旋光束的产生与应用是当前光学领域的研究热点. 利用傅里叶级数展开法分析了四台阶相位板的相位结构, 发现四台阶相位板可看作是由一系列不同拓扑荷数的螺旋相位板所组成, 用线偏振光直接照射相位板时, 将产生多级衍射光波, 各级衍射光均为不同拓扑荷数的涡旋光波, 由于多级衍射光波间的干涉导致光强分布偏离轴对称分布, 因而与涡旋光波有一定差距. 在此基础上, 提出了用四台阶相位板产生涡旋光束的新方案, 借助于Mach-Zehnder 干涉仪光路, 两块四台阶相位板产生的衍射光干涉叠加, 通过调节干涉仪光路的相位差, 使一部分衍射级干涉相消, 另一部分衍射级干涉相长, 相互加强, 从而把线偏振光转换为涡旋光束. 数值模拟计算了几种周期数不同的四台阶相位板衍射光强和角动量分布, 并与螺旋相位板进行比较, 证明用简单的四台阶相位板不仅能够获得与用螺旋相位板相同的涡旋光束, 而且可以用周期数较小的四台阶相位板产生具有大拓扑荷数的涡旋光束, 降低了制作相位板的难度.     

基于透射型几何相位编码超表面的太赫兹波束自由操控

为了实现太赫兹波束的自由操控,基于Pancharatnam-Berry几何相位理论,提出一种多层C型单元结构,实现在目标频率下0到2π的太赫兹波透射传输相位调控。基于广义斯涅耳定律,构建2-bit和3-bit编码超表面,在圆偏振入射条件下,实现透射型太赫兹波束的散射角度调控。采用数字信号处理理论中的傅里叶卷积运算,对不同周期序列的编码超表面进行四位制编码卷积运算,获得新的编码序列,实现对透射太赫兹波散射角度的自由调控。利用多层C型编码超表面单元结构对其进行旋转编码,实现1阶和2阶涡旋相位板,产生不同阶数太赫兹波涡旋光束。     

从平面波的线性干涉到涡旋光的角向干涉

依托中学生“英才计划”课题，以光场调控技术为选题，开展了从平面波线性干涉到涡旋光角向干涉演化过程的实验教学.从理论上模拟了涡旋光束与平面波的相干叠加，通过数值分析确认了拓扑荷数与干涉条纹的关系.实验上利用螺旋相位板制备涡旋光束，并采用马赫-曾德尔干涉法观测光强分布图.结果表明：涡旋光束与平面波的相干叠加，可以视为在线性干涉图样中移入与拓扑荷数相关的叉型条纹；而具有相反拓扑荷数的涡旋光束的角向干涉图样，则呈现与拓扑荷数相关的角向干涉条纹.     

线偏振相位涡旋光束的像散特性

提出采用像散系数表征涡旋光束的像散特性。利用螺旋相位板产生了线偏振相位涡旋光束,并对其光束质量及像散特性进行了实际测量。数值模拟了不同拓扑荷数的涡旋光束的传输特性及光束质量,分析了像散系数随拓扑荷数变化的规律,结果表明:当拓扑荷数为整数时,光束无像散,像散系数为零;当拓扑荷数为半奇数时,光束的像散特性明显,像散系数达到极大值;随着拓扑荷数整数部分的增加,像散系数的极大值减小。     

角向偏振涡旋光的紧聚焦特性研究以及超长超分辨光针的实现

系统研究了角向矢量涡旋光的紧聚焦焦斑特性,解释了焦平面自旋角动量局域化分布的形成原因.角向矢量涡旋光可分解为左、右旋圆偏振电场叠加,将分解所得的左、右旋分量分别经大数值孔径透镜聚焦,总聚焦电场可视为左、右旋分量聚焦电场的干涉叠加.经分析研究后发现,左、右旋分量各自聚焦电场的纵向分量大小相等、相位相反,完全干涉相消,使得总聚焦电场的纵向分量消失;而各自聚焦电场的横向分量则完全相反,几乎不发生干涉,总聚焦电场表现为非相干叠加.角向偏振光引入涡旋相位后,使得左、右旋电场分量的轨道角动量的拓扑荷数发生变化,拓扑荷数的绝对值不再相等,而是恒定差值为2.因此左、右旋电场的横向分量由于携带不同的拓扑荷数,分别聚在焦平面的不同位置,而横向分量发生非相干叠加,不相互影响,最终形成了总电场偏振态的局域化分布,即自旋角动量局域化分布的现象.随后,本文横向对比了1阶角向矢量涡旋光和径向偏振矢量光的超分辨焦斑特性,分析了各自的优、缺点以及影响焦斑尺寸的因素.最后,兼顾了超分辨光针的性能和实际实现难度,设计了6环带的二元相位板对1阶角向矢量涡旋光进行了波前调制,实现了横向半高全宽为0.391λ,纵向半高全宽为25...     

自旋-轨道相互作用下X型涡旋光束的传播特性

研究了 X型涡旋光束在左旋/右旋圆偏振状态下在紧聚焦系统中的传播特性,讨论了 自旋-轨道相互作用下焦平面光强的横向焦移和传播方向旋转现象的特点及其调控方法.研究发现:相位分布因子c是X型涡旋光束产生横向焦移和光强旋转的主要原因,而半孔径角α、偏振态和拓扑荷数对光强分布均有调控作用;由于自旋-轨道相互作用,入射光为左旋圆...     

超高斯涡旋光束在空间中的传输特性

研究了超高斯涡旋光束光强最大值、光斑半径以及环围能量半径等参数随传输距离和拓扑荷数的变化规律,并与高斯涡旋光束做了比较,结果表明:超高斯涡旋光束的光斑半径和环围能量半径随拓扑荷数及传输距离呈近似线性关系;对同一拓扑荷数和传输距离,高斯涡旋光束的能量较超高斯涡旋光束要发散;当拓扑荷数较大时,超高斯涡旋光束的光斑半径比高斯涡旋光束更大。针对光束质量研究了广义光束质量因子随传输距离和拓扑荷数的变化,结果表明传输距离足够远时,拓扑荷数较小的超高斯涡旋光束具有更好的光束质量。     

完美涡旋光束在大气湍流中的斜程传输特性

完美涡旋（POV）光束具有光束半径与拓扑荷数无关的特点,与其他涡旋光束相比具有更加稳定的空间强度分布特性。利用多相位屏法和傅里叶变换法,分析了POV光束在大气湍流中的斜程传输特性。采用光束漂移和孔径平均闪烁指数作为大气湍流影响光束质量的评价参数,对比了POV光束与高斯涡旋光束在相同传输条件下的光束质量。结果表明：相比于高斯涡旋光束,POV光束的光束稳定性更好。当拓扑荷数增大或天顶角减小时,POV光束抵抗大气湍流的能力增强。在不改变POV光束拓扑荷数的前提下增大其光束半径,也能提高POV光束对大气湍流的抵抗能力。     

螺旋微孔阵列对涡旋光束的调制机理研究

从理论和实验上研究涡旋光束通过具有费马螺线分布微孔结构的衍射板后拓扑荷数的变化情况,以及光束强度分布的衍射和聚焦特性。利用一个正透镜对衍射光束进行聚焦,观察和研究聚焦光束在焦平面后一定范围内的光强分布变化情况。衍射光束的最内环光强分布在束腰前后约有五个变化阶段。这一变化趋势适用于涡旋光束通过微孔沿不同费马螺线分布的衍射板的情况,无论是螺线旋转方向的改变,还是螺线结构数目的改变。通过衍射光束与球面波的干涉来验证拓扑荷数的变化规律。结果表明,这种衍射可以产生新的拓扑荷数,其变化与入射涡旋光束的相位波前和螺旋微孔阵列的相对旋向有关。     

利用全息法在偶氮聚合物薄膜中记录涡旋光场

利用全息技术在偶氮聚合物薄膜中记录了拓扑荷数q=–1,1,2,4的涡旋光场,并将记录的原始叉形光栅与计算全息光栅进行对比,对不同拓扑荷数涡旋光的记录速率和偶氮材料的可重复擦写性能进行了测试;记录完成后,将复现涡旋光与高斯光束干涉,并与原始涡旋光和原始叉形光栅对比,分析了记录质量.实验结果表明:高阶涡旋光场的全息叉形光栅会在记录过程中发生劈裂,轻微劈裂的涡旋光束仍维持一个稳定的环状结构;全息记录过程中不同拓扑荷数的涡旋光束记录速率较为统一,偶氮材料可经过上百次的擦写而不出现疲劳;再现涡旋光与原始涡旋光在光强分布结构上保持高度一致,再现涡旋光的干涉条纹与原始涡旋全息光栅保持高度一致,涡旋光及其携带的拓扑荷信息可被有效记录和读取.     

Broadband and high efficiency terahertz metasurfaces for anomalous refraction and vortex beam generation

The applications of metasurfaces are currently a highly active research field due to their extraordinary ability to manipulate electromagnetic waves. The ultra-thin characteristics of metasurfaces allow the miniaturization and integration of metasurface devices. However, these devices work typically under a low efficiency and narrow bandwidth condition. In this work, we design eight multilayered unit cells with similar amplitudes and a phase interval of π/4, which convert the polarization states of the terahertz (THz) waves between two orthogonal directions. The average cross-polarized transmission amplitudes of these cells are all around 0.9 in an ultra-broad frequency range from 0.5 THz to 1.4 THz. Furthermore, unit cells are used to construct both an ultra-thin anomalous refraction metalens and a vortex phase plate. Our simulation results show that the anomalous refraction for the transmitted linear polarization component is comparable to the theoretical prediction, and the maximum error is determined to be below 4.8%. The vortex phase plate can also generate an ideal terahertz vortex beam with a mode purity of 90% and more. The distributions of longitudinal electric field, intensity, and phase illustrate that the generated vortex beam has excellent propagation characteristics and a weak divergence. Simulations of the two types of metasurface devices, based on the eight unit cells, exhibit very high efficiencies in a wide bandwidth. Our research will assist in the improvement in the practical applications of metasurfaces. It also provides a reference for the design of high efficiency and broadband devices that are applied to other frequency ranges.     

Switchable vortex beam polarization state terahertz multi-layer metasurface

We propose a switchable vortex beam polarization state terahertz multi-layer metasurface, which consists of three-layer elliptical metal crosses, four-layer dielectrics, and two-layer hollow metal circles, which are alternately superimposed. Under the normal incidence of left-handed circularly polarized (LCP) wave and the right-handed circularly polarized (RCP) waves, the proposed structure realizes three independent control functions, i.e., focused and vortex beam, vortex beam with different topological charges, and polarization states switching, and azimuth switching of two vortex beams with different polarization states. The results show that the proposed metasurface provides a new idea for investigating the multifunctional terahertz wave modulation devices.     

High efficiency and broad bandwidth terahertz vortex beam generation based on ultra-thin transmission Pancharatnam–Berry metasurfaces

The terahertz(THz) vortex beam generators are designed and theoretically investigated based on single-layer ultra-thin transmission metasurfaces. Noncontinuous phase changes of metasurfaces are obtained by utilizing Pancharatnam–Berry phase elements, which possess different rotation angles and are arranged on two concentric rings centered on the origin.The circularly polarized incident THz beam could be turned into a cross-polarization transmission wave, and the orbital angular momentum(OAM) varies in value by lh. The l values change from ±1 to ±5, and the maximal cross-polarization conversion efficiency that could be achieved is 23%, which nearly reaches the theoretical limit of a single-layer structure.The frequency range of the designed vortex generator is from 1.2 THz to 1.9 THz, and the generated THz vortex beam could keep a high fidelity in the operating bandwidth. The propagation behavior of the emerged THz vortex beam is analyzed in detail. Our work offers a novel way of designing ultra-thin and single-layer vortex beam generators, which have low process complexity, high conversion efficiency and broad bandwidth.     

Multi-function terahertz wave manipulation utilizing Fourier convolution operation metasurface

We propose a novel metasurface based on a combined pattern of outer C-shaped ring and inner rectangular ring. By Fourier convolution operation to generating different predesigned sequences of metasurfaces, we realize various functionalities to flexible manipulate terahertz waves including vortex terahertz beam splitting, anomalous vortex terahertz wave deflection, vortex terahertz wave splitting and deflection simultaneously. The incident terahertz wave can be flexibly controlled in a single metasurface. The designed metasurface has an extensive application prospect in the field of future terahertz communication and sensing.     

Phase modulation of acoustic vortex beam with metasurfaces

We present a new acoustic metasurface structure to generate the stable acoustic vortex beams with controllable topological charge in the broadening of operating frequency. The proposed acoustic metasurface can dynamically achieve a full span of 360° phase modulation by three simple solutions. Acoustic plane beam and acoustic vortex beam can convert each other based on multi-metasurfaces combination. Potential applications of such phase modulation schemes can be anticipated.     

光在Metasurface中的自旋-轨道相互作用

探讨了光在Metasurface中的自旋-轨道相互作用, 理论分析了Metasurface 对圆偏振和线偏振光的转换. 结果表明: 光与具有空间非均匀性和各向异性性的Metasurface的相互作用导致了自旋-轨道角动量的耦合. 采用Metasurface与螺旋相位片组合在一起进行了验证实验, 所得实验结果与理论分析完全一致. 这些结论有助于我们更加深入理解Metasurface 对光的操控.     

一种光可调的多功能太赫兹几何相位超表面北大核心CSCD

基于金属-光敏硅组合圆环提出了一种光可调太赫兹超表面,可用于实现多功能的波前操控。该超表面单元由正反两面的结构层和中间介质层组成,其中结构层是开口方向相反的金属环,开口处由光敏硅进行填充。在光照强度较低时,超表面可以将入射的圆极化波转换为交叉极化的透射波;随着光照强度升高,透射波将逐渐被完全抑制。根据几何相位原理,通过旋转金属-光敏硅组合圆环,透射的交叉极化波会携带额外的相位因子,并可实现完全的2π范围相位覆盖。通过合理排列超表面单元结构,可以对透射波的波前实现任意操控。利用提出的光可调超表面,在较低光照条件下实现了高效的异常折射、透镜以及轨道角动量产生器;在较高光照条件下,抑制了其透射效率,可有条件地选择应用功能,表现出较好的灵活性。提出的光可调超表面在太赫兹成像、通信、雷达等方面具有较大的潜在应用价值。     

平板式螺旋相位板的设计与应用

传统的螺旋相位板是一种利用沿方位角方向介质材料高度递增实现对光束相位调控产生涡旋光束的光学器件,由于这种特殊的几何结构特征使其不能通过相位板的叠加而调控出射光束所携带的角量子数.本文基于坐标变换方法将介质材料沿方位角方向折射率不变而高度递增的传统螺旋相位板变换为一种介质材料沿方位角方向高度不变而折射率递增的平板式螺旋相位板.通过理论分析与数值模拟,发现本文所设计的平板式螺旋相位板不仅与传统螺旋相位板一样能够产生高质量的涡旋光束,而且平板式螺旋相位板的高度和涡旋光束携带的角量子数可以根据介质材料的折射率选取而任意调节.为了实际应用的需要,可以通过叠加多层平板式螺旋相位板以获得不同角量子数的涡旋光束.这种平板式螺旋相位板在光传输、光通信等领域具有广阔的潜在应用价值.     

Geometric metasurface fork gratings for vortex‐beam generation and manipulation

In recent years, optical vortex beams possessing orbital angular momentum have received much attention due to their potential for high‐capacity optical communications. This capability arises from the unbounded topological charges of orbital angular momentum (OAM) that provide infinite freedoms for encoding information. The two most common approaches for generating vortex beams are through fork diffraction gratings and spiral phase plates. While realization of conventional spiral phase plate requires complicated 3D fabrication, the emerging field of metasurfaces has provided a planar and facile solution for generating vortex beams of arbitrary orbit angular momentum. Among various types of metasurfaces, the geometric phase metasurface has shown great potential for robust control of light‐ and spin‐controlled wave propagation. Here, we realize a novel type of geometric metasurface fork grating that seamlessly combine the functionality of a metasurface phase plate for vortex‐beam generation, and that of a linear phase gradient metasurface for controlling the wave‐propagation direction. The metasurface fork grating is therefore capable of simultaneously controlling both the spin and the orbital angular momentum of light.