利用衍射光栅探测涡旋光束轨道角动量态的研究进展

涡旋光束是一种携带有轨道角动量的光束,在光学扳手、光通信、旋转探测等领域具有重要的应用价值.由于轨道角动量态是涡旋光束的特征值,因此如何探测光束的轨道角动量态分布至关重要.国内外学者已经提出了多种探测涡旋光束的技术,如干涉法、衍射光栅法、多普勒分析法、超材料表面法等.这些技术中,衍射光栅测量法较为简单易行,应用较广.本综述主要介绍了几种当前利用衍射光栅测量涡旋光束轨道角动量态的主流方法,同时也介绍了如何利用衍射光栅来测量光束的轨道角动量谱.     

矢量涡旋光束的生成与模式识别方法

与宏观物体类似，光子等微观粒子也可携带角动量。光子的角动量包括自旋角动量和轨道角动量，两种角动量的共同作用产生了一种新型结构光束，即矢量涡旋光束。矢量涡旋光束具有各向异性的波面和偏振分布，提供了多种光场自由度，在量子技术、光通信、激光探测、激光加工、高分辨成像、光镊等前沿领域展现了巨大的应用潜力，吸引了国内外学者的广泛关注。高效地生成矢量涡旋光束，以及高精度地识别矢量涡旋光束的模式分布，是其应用的关键。本文简要回顾了国内外学者在矢量涡旋光束的生成与模式识别方面的研究工作，同时系统梳理了本文作者过去十年在该方面的研究进展，重点介绍了其相关代表性成果。     

具有空间结构的强激光场与原子相互作用研究的新进展(特邀)

随着超快超强激光技术的发展,强激光场光与物质相互作用的研究得到了广泛的关注。与此同时,光场调控技术在经典光学领域中的快速发展为光学操控提供了一个新的自由度。近年来,这两个不同领域之间的结合——具有空间结构的强激光场与物质相互作用,成为了强场物理前沿研究的热点之一。光电离和高次谐波是传统强场科学中两个十分基本又非常重要的物理过程。本文总结和综述了涡旋强激光场对光电离过程的影响和控制方面的研究进展,以及利用涡旋光束或者柱矢量光束驱动气体高次谐波产生等方面的最新工作,最后展望了具有空间结构强激光场与物质相互作用物理和光场调控研究的发展方向。     

部分相干涡旋光束研究进展

涡旋光束具有螺旋波前、携带相位奇点和轨道角动量等物理特性,在粒子操控、量子信息、超分辨成像、光通信等领域具有重要的应用,并已成为学术界的研究热点。得益于相干光学理论的快速发展,将相干性作为新的自由度引入涡旋光束中,提出新型涡旋光束即部分相干涡旋光束。相较于完全相干涡旋光束,部分相干涡旋光束具有独特的物理内涵和光学特性,尤其是对其相干性和拓扑荷的联合调控会引发一系列奇特的新物理效应(如相干奇点、光束整形、偏振态转换、自修复等)。回顾了部分相干涡旋光束的基本理论及发展历程,着重对部分相干涡旋光束的理论模型、传输特性、实验产生、实验测量和应用基础研究进行了阐述。     

光子高阶轨道角动量制备、调控及传感应用研究进展

光子既是经典信息也是量子信息的理想载体. 单个光子不仅可以携带自旋角动量(与光波的圆偏振相关), 还可以携带轨道角动量(与光波的螺旋相位相关). 而轨道角动量的重要意义在于可利用单个光子的量子态构建一个高维的Hilbert空间, 从而实现高维量子信息的编码. 自Allen等于1992年确认光子轨道角动量的物理存在以来, 轨道角动量在经典光学和量子光学领域展现了诸多诱人的应用前景, 目前已成为国际光学领域的研究热点之一. 本综述将着重介绍高阶轨道角动量光束的制备与调控技术, 特别是高阶轨道角动量的量子纠缠态操控、旋转Doppler 效应测量及其在远程传感和精密测量技术中的应用.     

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。     

石墨烯表面涡旋光束的局域动力学特性分析

结合角谱展开方法和矢量势方法,推导了拉盖尔高斯涡旋光束在石墨烯表面反射后的电磁场分量显式表达式。分析了光束的入射角和拓扑荷数,以及石墨烯-衬底系统的费米能量和磁场对涡旋光束在石墨烯表面反射后局域动力学特性的影响。数值结果表明,石墨烯表面反射涡旋光束的能量、动量、自旋/轨道角动量密度分布随着光束入射角和拓扑荷数的变化发生显著变化。同时,改变石墨烯-衬底系统的费米能量和磁场可以有效调节反射涡旋光束的局域动力学特性。为基于涡旋光束局域动力学特性对石墨烯的表征以及基于石墨烯对涡旋光束的调控奠定了理论基础。     

轨道角动量指向可控的紧聚焦时空波包(特邀)EI北大核心CSCD

利用Debye积分,研究了三个相互正交的轨道角动量(包括两个正交的横向轨道角动量以及一个纵向轨道角动量)光场在紧聚焦条件下的复杂耦合现象,并演示了焦场中相位奇点在三维时空间中的演化。此外,还研究了具有不同拓扑荷数的纵向轨道角动量对聚焦波包整体轨道角动量指向的影响。数值结果表明,聚焦波包的整体轨道角动量指向可由纵向轨道角动量的拓扑荷数进行调控,进而实现紧聚焦时空波包的轨道角动量指向可控。这种角动量指向可控的时空波包在光学微操作、微纳加工、自旋-轨道耦合以及量子通信等领域具有潜在的应用价值。     

暗态多极赝局域等离子模式的太赫兹涡旋光激发

理论分析和实验验证了太赫兹涡旋光与带有周期性亚波长金属褶皱圆盘中暗态多极赝局域等离子模式的相互作用.研究结果表明,携带不同轨道角动量和自旋角动量的太赫兹涡旋光可以决定最终激发出的太赫兹暗态多极等离子模式,此结果和光频段的理论分析一致.利用太赫兹近场扫描方法对涡旋光的自旋和轨道角动量与暗态多极等离子模式的对应关系进行了实验论证,实验结果与理论仿真符合较好.研究成果将对太赫兹物理、等离子体以及成像领域研究起到一定的促进作用.     

扭转对称光束的产生及其变换过程中的轨道角动量传递

研究了采用扭转柱面镜光学系统将厄米-高斯光束变换成为具有轨道角动量的拉盖尔-高斯扭转对称光束. 采用本征模式分解的方法分析了扭转柱面镜光学变换系统实现光束变换的原理.利用光束传输矩阵和二阶矩理论分析计算了光束经过扭转柱面透镜变换过程中的轨道角动量传递过程，证明光束与透镜系统的轨道角动量交换发生在第一个柱面透镜处，光束经过第一个柱面透镜后，具有的轨道角动量保持不变. 关键词： 轨道角动量 光束变换 扭转对称光束 轨道角动量传递     

Proposal for generating brilliant x-ray beams carrying orbital angular momentum

We consider use of a variable polarizing undulator for generating brilliant x-ray beams carrying orbital angular momentum. We find that higher harmonics of the radiation correspond to Laguerre-Gaussian modes with azimuthal mode indices l equal to one less than the harmonic number when the undulator is operated to produce circularly polarized light. Beams with nonzero l carry orbital angular momentum quantized in units of lvariant Planck's over 2pi per photon. When operated to produce linear polarization, the harmonics correspond to Hermite-Gaussian modes. Selection of these modes with conventional monochromator optics opens the door for new research with x-ray synchrotron and free-electron laser sources.     

Mechanical equivalence of spin and orbital angular momentum of light: an optical spanner

We use a Laguerre-Gaussian laser mode within an optical tweezers arrangement to demonstrate the transfer of the orbital angular momentum of a laser mode to a trapped particle. The particle is optically confined in three dimensions and can be made to rotate; thus the apparatus is an optical spanner. We show that the spin angular momentum of +/-?per photon associated with circularly polarized light can add to, or subtract from, the orbital angular momentum to give a total angular momentum. The observed cancellation of the spin and orbital angular momentum shows that, as predicted, a Laguerre-Gaussian mode with an azimuthal mode index l=1 has a well-defined orbital angular momentum corresponding to ? per photon.     

Light with a twist in its tail

Polarized light is a phenomenon familiar to anyone with a pair of polaroid sunglasses. Optical components that change the nature of the polarization from linear to circular are common in any undergraduate laboratory. Probably only physicists know that circularly polarized light carries with it an angular momentum that results from the spin of individual photons. Few physicists realize, however, that a light beam can also carry orbital angular momentum associated not with photon spin but with helical wavefronts. Beams of this type have been studied only over the last decade. In many instances orbital angular momentum behaves in a similar way to spin. But this is not always so: orbital angular momentum has its own distinctive properties and its own distinctive optical components. This article outlines the general behaviour of such beams; how they can be used to rotate microscopic particles; how they interact with nonlinear materials; the role they play in atom-light interactions and how the rotation of such beams results in a measurable frequency shift.     

A simple analytical model of the angular momentum transformation in strongly focused light beams

A ray-optics model is proposed to describe the vector beam transformation in a strongly focusing optical system. In contrast to usual approaches based on the focused field distribution near the focal plane, we use the beam pattern formed immediately after the exit aperture. In this cross section, details of the output field distribution are of minor physical interest but proper allowance is made for transformation of the beam polarization state. This enables the spin and orbital angular momentum representations to be obtained, which are valid for any cross section of the transformed beam. Simple analytical results are available for a transversely homogeneous, circularly polarized incident beam confined by a circular aperture. Variations of the spin and orbital angular momenta of the output beam with change of the focusing strength are analyzed. The analytical results are in good qualitative and reasonable quantitative agreement with the results of numerical calculations performed for the Gaussian and Laguerre-Gaussian beams. The model supplies an efficient and physically transparent means for qualitative analysis of the spin-to-orbital angular momentum conversion. It can be generalized to incident beams with complex spatial and polarization structure.     

Properties of circularly polarized vortex beams

The properties of circularly polarized vortex beams in cylindrical polarization bases are studied. A circularly polarized vortex beam is decomposed into radial and azimuthal polarization. With the proper combination of vortex charge and the handedness of the circular polarization, a focal field with an extremely strong longitudinal component as well as a flat-topped profile can be obtained. The cylindrical decomposition also sheds light on the connections between orbital angular momentum and the spin of the light beams.     

Linear and angular momenta in tightly focused vortex segmented beams of light(Invited Paper)

We investigate the linear momentum density of light,which can be decomposed into spin and orbital parts,in the complex three-dimensional field distributions of tightly focused vortex segmented beams.The chosen angular spectrum exhibits two spatially separated vortices of opposite charge and orthogonal circular polarization to generate phase vortices in a meridional plane of observation.In the vicinity of those vortices,regions of negative orbital linear momentum occur.Besides these phase vortices,the occurrence of transverse orbital angular momentum manifests in a vortex charge-dependent relative shift of the energy density and linear momentum density.     

Investigation of the center intensity of first- and second-order Laguerre-Gaussian beams with linear and circular polarization

The vectorial Debye integral shows that tightly focused Laguerre-Gaussian (LG) beams have a residual intensity at the focal point for linear polarization, for a topological charge of m=1 and 2. We measured the shapes of linearly and circularly polarized LG beams and found that a central intensity appeared at m=1 and 2 for linear and right-handed circular polarization, however, it is completely canceled for left-handed circular polarization. In general, when the orbital angular momentum of the LG beam is parallel to the spin angular momentum of the photons, zero intensity is always achieved at the focus.     

Quantized rotation of atoms from photons with orbital angular momentum

We demonstrate the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of variant Planck's over 2pi, using a 2-photon stimulated Raman process with Laguerre-Gaussian beams to generate an atomic vortex state in a Bose-Einstein condensate of sodium atoms. We show that the process is coherent by creating superpositions of different vortex states, where the relative phase between the states is determined by the relative phases of the optical fields. Furthermore, we create vortices of charge 2 by transferring to each atom the orbital angular momentum of two photons.     

碳酸钙微粒光致旋转的实验和理论研究

理论分析了由于光束轨道角动量和自旋角动量传递以及微粒的特殊形状导致微粒旋转的机理.实验建立了单光束激光光镊装置,不仅可以捕获并移动直径为微米量级的微小粒子,而且利用圆偏振光与微粒之间角动量的传递,实现了对具有双折射特性的碳酸钙微粒的光致旋转.实验中发现微粒的旋转不仅取决于光束的偏振态,还与微粒本身的形状有关,解释了实验中观察到的几种旋转现象.碳酸钙微粒旋转的最高转速达到12转/秒,转速与激光功率成正比.