分数阶双涡旋光束的实验研究

具有分数阶拓扑荷数的涡旋光束的产生及其传输是近几年来人们感兴趣的研究课题． 本文提出了一种新型的分数阶双涡旋光束, 该光束是由两束带有不同分数阶拓扑荷数的涡旋光束共轴叠加产生, 其光强分布为双环结构．我们对该光束分别进行了理论模拟和实验研究．研究表明, 分数阶双涡旋光束的双环携带不同的轨道角动量, 且相互独立地传输．这种新型的涡旋光束相对于整数阶或单个分数阶拓扑荷数的涡旋光束更具有控制多样性, 有望在光学镊子、光学扳手等微粒子操控领域开发新的应用．     

Measuring the topological charge of integer and fraction vortices using multipoint plates

The topological charge of integral vortex beams with a single circle of multipoints has previously been measured. In this work, we theoretically and experimental study the diffraction patterns of vortex beams of integral and fractional topological charges using a single circle and two circles of multipoint plates. It is found that the diffraction patterns are dependent not only on the multipoint plates, but also on the topological charge of the vortex beams. On the basis of this property, we can measure the topological charges of integral and fractional vortex beams.     

High-harmonic-generation-inspired preparation of optical vortex arrays with arbitrary-order topological charges

In the process of high-harmonic generation with a Laguerre-Gaussian(LG) mode, it was well established that the topological charge could be of an N-fold increase due to angular momentum conservation. Here, by mimicking the effect of high-harmonic generation, we devise a simple algorithm to generate optical vortex arrays carrying arbitrary topological charges with a single phase-only spatial light modulator. By initially preparing a coaxial superposition of suitable low-order LG modes, we demonstrate experimentally that the topological charges of the embedded vortices can be multiplied and transformed into arbitrarily high orders on demand, while the array structure remains unchanged. Our algorithm offers a concise way to efficiently manipulate the structured light beams and holds promise in optical micromanipulation and remote sensing.     

Superposition of orbital angular momentum of photons by a combined computer-generated hologram fabricated in silica glass with femtosecond laser pulses

This paper introduces a novel method to realize the superposition of orbital angular momentum of photons by combined computer-generated hologram （CCGH） fabricated in silica glass with femtosecond laser pulses. Firstly, the two computer-generated holograms （CGH） of optical vortex were obtained and combined as a CCGH according to the design. Then the CCGH was directly written inside glass by femtosecond laser pulses induced microexplosion without any preor post-treatment of the material. The vortex beams with different vortex topological charges （including new topological charges） have been restructured using a collimated He-Ne laser beam incidence to the CCGH normally. A theoretical and experimental explanation has been presented for the generations of the new topological charges.     

The curious arithmetic of optical vortices

The superposition of noncoaxial light beams containing screw wave-front dislocations is shown to create light patterns with a richer vortex content than that given by the arithmetic of the topological charges of the individual beams. We report the experimental observation of this phenomenon.     

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。     

Shared aperture metasurface antenna for electromagnetic vortices generation with different topological charges

Vortex beams carrying orbital angular momentum (OAM) have aroused great interest of both scientific and engineering communities. Encouragingly, generating OAM with different topological charges in a shared aperture is regarded as a potential route to expanding the communication capacity, which yet is an academic challenging task. In this work, a paradigm of designing metasurface-based shared aperture antenna for generating polarization-dependent vortex beams with distinct topological charges is proposed. Anisotropic unit cells that can tailor different resonance phase profiles in two orthogonal orientations are used to assemble a metasurface reflector. As a proof-of-concept, a planar reflector antenna is designed with two Vivaldi sources, which can generate x- and y-polarized vortex beams with topological charges of l=-1 and l=-2, respectively. Both the simulation results and the measurement results are in good agreement, which demonstrates the feasibility of our design. Significantly, this work provides a new route to achieving vortex beams carrying different topological charges in the same frequency band, which may have potential applications in communication systems.     

单环多段光强分布检测光学涡旋拓扑荷值

针对涡旋光束检测范围局限这一问题, 提出了一种新的光学涡旋拓扑荷值检测方法-单环多段光强分布检测法, 它以分段数和环半径为两大检测常数, 将检测涡旋光束拓扑荷值范围扩大到了128种, 与以往利用旁瓣调控光学涡旋检测拓扑荷值方法相比, 检测范围扩大了1个数量级. 单环多段光强分布是基于计算机全息图实现在远场衍射焦平面上环半径相等的两束携带不同拓扑荷数的涡旋光束叠加后形成的光强分布. 计算机模拟和光学实验验证了所提出方法的可行性, 该方法在自由空间光通信领域具有一定的研究价值和应用潜力.     

相位片角向衍射产生拉盖尔-高斯光束的实验研究

设计了一种角向分布的相位片,利用离子束刻蚀技术加工成0和π二级的相位片.利用角向衍射理论对相位片的衍射场分析表明,衍射场为拓扑指数相反的两束拉盖尔-高斯光束的叠加场.用直径为4 mm的近平行光照射相位片,获得径向指数为零,拓扑荷相反的叠加拉盖尔-高斯光场.采用较大孔径的光束入射时,仍为拓扑荷相反的两束光的叠加,但径向指数会发生变化. 关键词： 信息光学 拉盖尔-高斯光束 相位片 离子束刻蚀     

The anomalous radiation force of light on a left-handed material

This paper derives the force of the electromagnetic radiation on left-handed materials (LHMs) by a direct applica-tion of the Lorentz law of classical electrodynamics.The expressions of radiation force are given for TE-polarised and TM-polarised fields.The numerical results demonstrate that electromagnetic waves exert an inverse lateral radiation force on each edge of the beams,that is,the lateral pressure is expansive for TE-polarised beams and compressive for TM-polarised beams.The investigation of the radiation force will provide insights into the fundamental properties of LHMs and will provide to better understanding of the interaction of light with LHMs.     

Experimental verification on tightly focused radially polarized vortex beams

The theoretical and experimental results of tightly focused radially polarized vortex beams are demonstrated. An auto-focus technology is introduced into the measurement system in order to enhance the measurement precision, and the radially polarized vortex beams are generated by a liquid-crystal polarization converter and a vortex phase plate. The focused fields of radially polarized vortex beams with different topological charges at numerical apertures （NAs） of 0.65 and 0.85 are measured respectively, and the results indicate that the total intensity distribution at focus is dependent not only on the NA of the focusing objective lens and polarization pattern of the beam but also on the topological charge l of the beam. Some unique focusing properties of radially polarized vortex beams with fractional topological charges are presented based on numerical calculations. The experimental verification paves the way for some practical applications of radially polarized vortex beams, such as in optical trapping, near-field microscopy, and material processing.     

随机电磁涡旋光束的深聚焦特性

基于德拜矢量积分理论,研究了随机电磁涡旋光束经过大数值孔径透镜之后的聚焦特性及透镜的数值孔径、入射光束的偏振度、拓扑荷以及横向相干长度对焦平面附近聚焦光束的光强分布和相干度的影响.结果表明:适当地选择相关参量,可在焦平面上得到椭圆形光斑的光强分布以及平顶光强分布.随机电磁涡旋光束在焦平面上同一点处两个相互垂直分量之间的相干度,不同两点处两个相同分量之间的相干度以及不同两点处两个相互垂直分量之间的相干度研究表明,入射光束的拓扑荷和横向相干长度对聚焦光束的相干性有着十分明显的影响.     

Free-space propagation of second harmonic beams carrying optical vortices

A vorticity of free propagating SH beam produced in nonlinear crystal by combined beam composed of two coaxial Laguerre–Gaussian vortex beams is analyzed. It is revealed that the vortical structures of SH beam in the near and far fields are different when two vortex beams are carrying vortices with topological charges of different signs. The diffraction of SH beam under free propagation is investigated. A qualitative agreement of the theoretical predictions with experimental results is obtained.     

Generation of rotating intensity blades by superposing optical vortex beams

A kind of optical beam with controllable rotating intensity blades is generated by coaxially superposing two optical vortex beams with frequency difference (Δω), different topological charges (m₁ and m₂) and equal amplitude. It is shown theoretically that the number of the blades is determined by the subtraction of topological charges (m₁ − m₂) and the angular velocity of the rotating pattern is equal to Δω / (m₁ − m₂). In our experiment, the rotating beams were generated by two optical vortex beams with opposite topological charges, where the frequency difference is acquired by a rotating plate glass. The results are quite in accordance with the simulations.     

Conversion of topological charge of optical vortices in a parametric frequency converter

It is shown that sum-frequency generation of light beams with vortices allows to construct vortices of different topological charges. An optical parametric amplifier enables to produce a vortex with opposite topological charge (to the reverse vortex). In this way, a frequency converter can be seen as a prototype of an optical processor which allows to produce simple arithmetical operations with topological charges of the interacting vortices.     

Interactions of optical vortex solitons superimposed on different background beams

The interaction between two optical vortex solitons (OVS), formed on different background beams is analyzed numerically. Analogous to the one-dimensional case, vector OVS seem obtainable [12]. The relative topological charges of the interacting (off-axis) vortices are found to rule their propagation characteristics. Attraction is found in the case of equal charges, in contrast to the opposite case, where repulsion is present.     

部分相干双曲正弦-Gauss涡旋光束叠加形成的合成相干涡旋在非 Kolmogorov 大气湍流中的动态演化

一些实验表明, 实际大气会偏离理想Kolmogorov模型. 本文基于广义Huygens-Fresnel原理和Toselli等提出的非Kolmogorov湍流模型, 推导出部分相干双曲正弦-Gauss (HSG)涡旋光束通过非Kolmogorov大气湍流的解析传输公式, 并用以对两束部分相干HSG涡旋光束相干叠加和非相干叠加形成的合成相干涡旋在非Kolmogorov大气湍流中的动态演化进行了研究. 结果表明, 合成光束平均光强的演化过程与非Kolmogorov湍流的广义指数α, 源平面上叠加涡旋光束拓扑电荷的符号, 以及叠加方式有关. 合成相干涡旋在非Kolmogorov大气湍流中传输时会出现移动、产生和湮灭. 广义指数α, 拓扑电荷符号, 以及叠加方式都会影响其演化行为. 最后, 将本文所得结果与相关文献做了比较.     

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.     

Composite optical vortices in noncollinear Laguerre--Gaussian beams and their propagation in free space

Taking two Laguerre-Gaussian beams with topological charge 1 = ±1 as an example, this paper studies the composite optical vortices formed by two noncollinear Laguerre-Gaussian beams with different phases, amplitudes, waist widths, off-axis distances, and their propagation in free space. It is shown by detailed numerical illustrative examples that the number and location of composite vortices at the waist plane are variable by varying the relative phase β, amplitude ratio η, waist width ratio ξ, or off-axis distance ratio μ. The net topological charge lnet is not always equal to the sum lsum of charges of the two component beams. The motion, creation and annihilation of composite vortices take place in the free-space propagation, and the net charge during the propagation remains unchanged and equals to the net charge at the waist plane.     

Propagation of partially coherent Bessel-Gaussian beams in turbulent atmosphere

The characteristics of partially coherent Bessel-Gaussian beams propagating in turbulent atmosphere are investigated. Based on the extended Huygens–Fresnel principle, the influence of topological charges and coherence of the source on the intensity and the degree of coherence in the received plane are considered. The influence of atmospheric turbulence on beam profile and coherence in the received plane is also analyzed. It is found that a Bessel-Gaussian shaped intensity distribution will eventually transform into a Gaussian distribution after propagating in turbulent atmosphere. Meanwhile, topological charges, coherence of the source and atmospheric turbulence will also influence the propagation characterizations of the beams.