利用非传统螺旋相位调控高阶涡旋光束的拓扑结构

本文提出一种利用非传统螺旋相位调控高阶涡旋光束拓扑结构的方法.数值模拟并实验研究了具有不均匀旋转梯度的非传统螺旋相位对高阶涡旋光束的调控行为.结果表明, 携带有非传统螺旋相位的高阶涡旋光束在传输过程中, 将退化为沿一条直线排列的多个一阶相位奇点, 并且, 这种非传统螺旋相位对高阶涡旋光束的调控特性, 可抑制相位噪声等扰动所引起的拓扑结构随机退化现象.本文的结论为涡旋光束拓扑结构的调控提供了一种可行的新途径, 在基于涡旋光束的光学通信、光学操控等方面具有潜在应用.     

基于涡旋光束的超快速角向集束匀滑方案

针对惯性约束聚变装置对激光集束辐照均匀性的需求,提出了一种基于涡旋光束的超快速角向匀滑方案,即利用螺旋相位板使2×2集束中的两子束由超高斯光束变换为涡旋光束,而其余两子束不变,进而通过对子束偏振态和中心波长的调控,使集束中的涡旋光束和超高斯光束在靶面两两相干叠加.相干叠加后的焦斑以皮秒量级为周期超快速旋转,从而在极短时间内快速抹平焦斑强度调制,改善靶面辐照均匀性.通过建立基于螺旋相位板的激光超快速角向集束匀滑方案的物理模型,分析了其角向匀滑特性,并与光谱角色散技术和径向匀滑技术进行了比较分析.结果表明,这一新型激光集束匀滑方案能实现对焦斑的超快速角向匀滑,且能在数皮秒时间内达到最佳辐照均匀性.     

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.     

利用周期型正交二元相位板产生正方阵列涡旋和螺旋光场

设计了一种周期型正交二元相位板,其具有周期型矩形调制单元,各单元的相位调制量为0或π。该相位板的空间频谱具有中心直流分量为0的特点,分别对该频谱中心区域的4个一级频谱点和8个二级频谱点进行相位调制,可得到正方阵列光斑和正方阵列涡旋。阵列涡旋中各涡旋光束的拓扑荷数l=±1,在空间交错分布。由于两个正方阵列光场在垂直传播方向的横向周期恰好重合,且两个光场沿光轴方向的波数不同,因此叠加之后可以形成强度分布随传输距离旋转的正方阵列螺旋,阵列中存在两种具有相反螺旋方向的螺旋光束,同样在空间呈交错分布。此外,讨论了产生最佳螺旋光束的相位板设计条件,并给出理想情况下系统的能量利用率,所得实验结果验证了该方法的可行性。     

基于Metasurface的轨道角动量光束的产生与调控

提出了一种基于metasurface产生与调控轨道角动量光束的新方法。光场在偏振态的演变过程中可以获得附加的Pancharatnam-Berry(PB)几何相位。因此,可以通过调控光场的偏振态获得所需相位。所构造的metasurface具有空间变化的光轴分布,能够精确地操控光束的偏振态。当metasurface的光轴方向在方位角方向连续变化时,就能产生与方位角坐标相关的PB相,也就是涡旋相。该相位可以用来产生轨道角动量光束,也可以用来操控涡旋光束的轨道角动量。实验结果验证了这种方案的可行性。所得到的结果为轨道角动量光束的产生及光束的轨道角动量调控提供了一种新方法,对基于轨道角动量光束的量子通信和光学微操控等也有应用价值。     

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.     

部分相干涡旋光束在大气湍流中的远场传输特性

运用广义惠更斯-菲涅耳原理,详细研究了部分相干涡旋光束在湍流介质中的远场传输规律.研究表明,部分相干涡旋光束的光谱相干度及光强分布与光束的拓扑电荷数、空间相对相干长度及湍流介质的折射率结构常数等因素有关.在湍流介质中,光谱相干度存在相位奇点,并且随着空间相对相干长度的增大,相干涡旋逐渐演化为强度涡旋,而湍流介质的强弱对部分相干涡旋光束的影响则相反,随着湍流介质的折射率常数的增大,强度涡旋逐渐演化为相干涡旋.     

涡旋光束与相位全息光栅不对准时的衍射特性研究

研究了涡旋光束与相位全息光栅不对准时衍射光束的解析特性.利用理论推导的方法得出涡旋光束经相位全息光栅接收后一阶衍射光束的解析表达式.然后通过仿真分析分别得出在发生正常对准、横向偏移、角向倾斜及横向偏移和角向倾斜两者同时出现时衍射光束的质心偏移特性和中心强度变化特性.研究表明: 拉盖尔-高斯光束经相位全息光栅衍射后得到的光场表达式为合流超几何函数形式.光束与相位全息光栅间的不对准会引起衍射光束质心的偏移,而且光束质心的偏移量随入射光束偏移距离和偏离角的增加而增加,与入射光束的偏移方向和方位角无关.角向倾斜时 关键词： 拉盖尔-高斯光束 相位全息光栅 横向偏移 角向倾斜     

光波场中多边位错向螺旋位错的转化

实验研究及理论分析了光波场中边位错向螺旋位错的转化行为. 设计了围绕中心沿角向周期排列的多边位错相位结构,实验上通过计算全息图法研究了其对高斯光束的调制作用以及调制光束的传播特性. 结果表明,随着调制光束逐渐演化为类涡旋光束,多边位错结构可以转化为螺旋位错,且位错结构的总相位变化守恒. 结合光场的线动量密度图详细分析了该演化过程,并进一步讨论了多边位错结构中的相位突变值对螺旋位错转化的影响,给出了决定螺旋位错拓扑荷值的定量关系式. 关键词： 边位错 螺旋位错 涡旋光束 拓扑荷     

强非局域介质中基于古依相位的旋转涡旋光孤子

利用强非局域非线性介质中傍轴光束传输的修正Snyder-Mitchell模型讨论了两束共线(即光束中心和传输方向都相同)拉盖尔-高斯型光孤子(CLGS)构成的涡旋光孤子传输过程。在一定条件下,涡旋光束在传输过程中,光束截面光斑发生旋转现象,但光束的束宽保持不变,称之为旋转涡旋光孤子。涡旋光孤子旋转的现象可以通过叠加光场中的古依相位来解释。结果展现了几个旋转涡旋光孤子在传输过程中的旋转现象和强非局域介质中多环形旋转涡旋光孤子的传输。     

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.

     

Optimal phase steps of multi-level spiral phase plates

We analyze vortex properties of the optical beams generated by a multi-level spiral phase plate (MLSPP) and find that this kind of beams consists of vortex components with topological charges of L − kM, where k is zero or any integer, M is the level number and L is the intrinsic topological charge of the MLSPP. We proved that the orbital angular momentum of the beams generated by a MLSPP reaches its maximum only if the phase steps (or the ratio of M to L) satisfies some special conditions.     

Optical vortex beam shaping by use of highly efficient irregular spiral phase plates for optical micromanipulation

Optical dark traps such as Laguerre-Gaussian beams, modulated optical vortices, and high-order Bessel beams have been used in the micromanipulation of microparticles. Such optical traps are highly versatile, as they are able to trap both high- and low-index microparticles as well as to set them into rotation by use of the orbital angular momentum of light. Holography has been widely used to modulate the shape of an optical vortex for new optical traps. We show that, by designing the shape of a spiral phase plate and using electron-beam lithography for fabrication, one can modulate the amplitude and the phase of an optical vortex with respect to the specific shape of the spiral phase plate as required. Furthermore, to the best of our knowledge this is the first report of transferring orbital angular momentum from a spiral phase plate to an absorptive microparticle in an experiment. Hence, with this technique, optical dark traps can easily be designed and fabricated.     

多环涡旋光束的实验研究

涡旋光束的产生、传输与应用是当前光学领域热门的研究课题之一.本文提出的新型多环涡旋光束,包括双环涡旋及三环涡旋光束,它是由多束携带不同拓扑电荷数且束腰半径不同的拉盖尔-高斯涡旋光束共轴叠加而成,其光强分布为多环结构.从理论上研究了多环涡旋光束的形成与分布特征,基于共轭对称延拓Fourier计算全息方法生成了多环涡旋光束的计算全息图,并利用一个空间光调制器实验产生了与理论一致的高质量的多环涡旋光束.研究表明多环涡旋光束的各环携带不同的轨道角动量,空间分布保持相互独立.这种新型的多环涡旋光束相对于携带单一拓扑电荷数的涡旋光束,提供了更多的控制参数和更加多样化的结构分布,因此在光学镊子、光学捕获等微操控以及光通信领域具有潜在的应用潜力.     

Tight focusing of spirally polarized vortex beams

The tight focusing of spirally polarized focused vortex beams is analyzed numerically based on the vectorial Debye theory. The expressions for the electric field and the orbital angular momentum of focused beams are derived. It is shown that the intensity distribution in the focal plane is dependent on the specific spirally polarized state and the coefficient of the spiral polarization function. By presenting the phase contours of the component polarized in the radial direction, it is found that the radii of dislocation lines will increase with the increase of the power of the spirally polarization function. It is reveled that the same orbital angular momentum can be obtained for different spirally polarized state at certain distance along the propagation direction in the focal region. Besides, the orbital angular momentum distributions for different polarized states have fewer crossover points with each other for higher topological charge. The influence of the spirally polarized state on the orbital angular momentum in the focal plane is also studied.     

Experimental observation of tangent-azimuthal non-spiral optical vortex

Optical vortices have attracted much attention recently due to their novel properties and widening applications. And lots of optical vortices can be obtained though most of them turn on spiral pattern on increasing azimuthal angle. In this paper, one kind of non-spiral optical vortex was proposed whose front phase distribution is tangent function of azimuthal angle. And this kind of optical vortices were also observed experimentally by computer-generated hologram method. It was found that when topological charge is smaller than unit one, vortex beam shape changes considerably on increasing topological charge, from hollow pattern to curve shape. When topological charge is bigger than unit and is times of 0.5, vortex beam turns symmetrical polygonal pattern though there is crack between adjacent sides, and the side number is twice of topological charge.     

Rotational motions of optically trapped microscopic particles by a vortex femtosecond laser

The rotational motions of the optically trapped microscopic particles by the vortex femtosecond laser beam are investigated in this paper.Black particles can be trapped and rotated by a vortex femtosecond laser beam very effectively because the vortex beam carries orbital angular momentum due to the helical wave-front structure in assoication with the central phase singularity.Trapped black particles rotate in the vortex beam due to the absorption of the angular momentum transferred from the vortex beam.The rotating directions of the trapped particles can be modulated by reversing the topological charge of the optical vortex in the vortex femtosecond beam.And the rotating speeds of the trapped microscopic particles greatly depend on the topological charges of the vortex tweezer and the used pulse energies.     

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.     

Orbital angular momentum density and spiral spectra of Lorentz–Gauss vortex beams passing through a single slit

Based on the Hermite–Gaussian expansion of the Lorentz distribution and the complex Gaussian expansion of the aperture function, an analytical expression of the Lorentz–Gauss vortex beam with one topological charge passing through a single slit is derived. By using the obtained analytical expressions, the properties of the Lorentz–Gauss vortex beam passing through a single slit are numerically demonstrated. According to the intensity distribution or the phase distribution of the Lorentz–Gauss vortex beam, one can judge whether the topological charge is positive or negative. The effects of the topological charge and three beam parameters on the orbital angular momentum density as well as the spiral spectra are systematically investigated respectively. The optimal choice for measuring the topological charge of the diffracted Lorentz–Gauss vortex beam is to make the single slit width wider than the waist of the Gaussian part.