Excitation of nuclei by photon beams carrying orbital angular momentum

A drop in the efficiency of nuclear excitation through transitions of high multipolarity is related to the increase in the angular momentum difference between the nuclear states involved in the excitation transition. Such transitions need photons with a high angular momentum. It is well known that photon beams carrying a well-defined and arbitrarily high value of angular momentum about the beam axis can be produced. We discuss some features in the excitation of nuclei with the beams.     

Breakup of ring beams carrying orbital angular momentum in sodium vapor

We have observed filamentation due to azimuthal modulational instabilities in spinning ring solitons with orbital angular momentum m variant Planck's over 2pi in sodium vapor. We show experimentally that vortex beams with m values of 1, 2, and 3 tend to break into two, four, and six filaments, respectively. Treating the sodium vapor as a Doppler broadened two-level atomic system, we find that we can accurately model the propagation and breakup of these beams with numerical simulations.     

Fiber coupler for generating orbital angular momentum modes

We propose a fiber coupler consisting of a central ring and four external cores to generate up to ten orbital angular momentum (OAM) modes. Four coherent input lights are launched into the external cores and then coupled into the central ring waveguide to generate OAM modes. By changing the size of the external cores, one can selectively excite a high-order OAM mode. The quality of the generated OAM modes can be enhanced by adjusting the polarization state and the phase of input lights. We show the generation of OAM modes with odd charge numbers of -9 to +9 (i.e., 10 modes totally) with mode purity of >99% using <2?mm long fiber. This fiber coupler design can be extended to enable all-fiber spatial-mode (de)multiplexing.     

Up-conversion detection of mid-infrared light carrying orbital angular momentum

Frequency up-conversion is an effective method of mid-infrared (MIR) detection by converting long-wavelength photons to the visible domain, where efficient detectors are readily available. Here, we generate MIR light carrying orbital angular momentum (OAM) from a difference frequency generation process and perform up-conversion on it via sum frequency conversion in a bulk quasi-phase-matching crystal. The maximum quantum conversion efficiencies from MIR to visible are 34.0%, 10.4%, and 3.5% for light with topological charges of 0, 1, and 2, respectively, achieved by utilizing an optimized strong pump light. We also verify the OAM conservation with a specially designed interferometer, and the results agree well with the numerical simulations. Our study opens up the possibilities for generating, manipulating, and detecting MIR light that carries OAM, and will have great potential for optical communications and remote sensing in the MIR regime.     

Measuring orbital angular momentum of vortex beams in optomechanics

Measuring the orbital angular momentum (OAM) of vortex beams, including the magnitude and the sign, has great application prospects due to its theoretically unbounded and orthogonal modes. Here, the sign-distinguishable OAM measurement in optomechanics is proposed, which is achieved by monitoring the shift of the transmission spectrum of the probe field in a double Laguerre–Gaussian (LG) rotational-cavity system. Compared with the traditional single LG rotational cavity, an asymmetric optomechanically induced transparency window can occur in our system. Meanwhile, the position of the resonance valley has a strong correlation with the magnitude and sign of OAM. This originally comes from the fact that the effective detuning of the cavity mode from the driving field can vary with the magnitude and sign of OAM, which causes the spectral shift to be directional for different signs of OAM. Our scheme solves the shortcoming of the inability to distinguish the sign of OAM in optomechanics, and works well for high-order vortex beams with topological charge value±45, which is a significant improvement for measuring OAM based on the cavity optomechanical system.     

Microobject manipulations using laser beams with nonzero orbital angular momentum

Microobject manipulation with a laser is investigated. The functional capabilities of the laser manipulators can be significantly enhanced using beams with predetermined distributions of intensity and angular momentum. Beams with nonzero angular momentum are generated using phase and amplitude-phase masks. Such beams make it possible to rotate weakly absorbing particles and to move them along a given trajectory. Experimental results on the microobject motion along various trajectories (circle, triangle contour, self-intersecting curve, and Archimedes spiral) are presented.     

Delocalized correlations in twin light beams with orbital angular momentum

We generate intensity-difference-squeezed Laguerre-Gauss twin beams of light carrying orbital angular momentum by using four-wave mixing in a hot atomic vapor. The conservation of orbital angular momentum in the four-wave mixing process is studied as well as the spatial distribution of the quantum correlations obtained with different configurations of orbital angular momentum. Intensity-difference squeezing of up to -6.7 dB is demonstrated with beams carrying orbital angular momentum. Delocalized spatial correlations between the twin beams are observed.     

Nonparaxial TE and TM vector beams with well-defined orbital angular momentum

A new formalism is developed for diffraction-free vector beams in free space. The solutions of Maxwell's equations are separated into two polarization modes, TE and TM. We discuss the validity of the method by applying it to a particular solution, the vectorial Bessel beam of order m.     

Rotation of laser beams with zero of the orbital angular momentum

We show that among the multitude of rotating light beams whose complex amplitude can be represented as a linear superposition of the Laguerre-Gaussian (LG) modes with definite numbers there are light beams with zero orbital angular momentum (OAM) and vice versa, multi-mode LG beams that show no rotation and are lacking the radially symmetric intensity distribution can possess the non-zero OAM. Also, we give examples of the rotating light beams with zero OAM, represented as a superposition of the Bessel and new hypergeometric modes. Using an SLM, we generate a rotating Bessel beam with zero OAM for the first time.     

Vortex-bearing array of singular beams with very high orbital angular momentum

We consider theoretically and experimentally an array of singular beams whose axes lie on the surface of a hyperboloid of revolution. We show that such a singular array can carry a very high orbital angular momentum. Experimental results demonstrate a way of generating such singular arrays.     

Influence of atmospheric turbulence on states of light carrying orbital angular momentum

We have experimentally studied the degradation of mode purity for light beams carrying orbital angular momentum (OAM) propagating through simulated atmospheric turbulence. The turbulence is modeled as a randomly varying phase aberration, which obeys statistics postulated by Kolmogorov turbulence theory. We introduce this simulated turbulence through the use of a phase-only spatial light modulator. Once the turbulence is introduced, the degradation in mode quality results in crosstalk between OAM modes. We study this crosstalk in OAM for 11 modes, showing that turbulence uniformly degrades the purity of all the modes within this range, irrespective of mode number.     

复杂像散椭圆光束的轨道角动量的实验研究

提出一种直接测量光束的轨道角动量的方法，其原理是把具有轨道角动量的光束照射到一个金属靶上，使其在光束的角动量作用下发生转动. 通过测量靶转动的角度来计算光束的角动量值. 对几种不同参数的光束的轨道角动量进行了测量，获得了轨道角动量与光束参数之间的关系，实验结果与理论分析较好地符合. 关键词： 复杂像散椭圆光束 轨道角动量 测量     

螺旋相位光束轨道角动量态测量的实验研究

对螺旋光束的轨道角动量态的测量技术进行了实验研究.建立了一套利用Mach-Zehnder干涉仪测量光束轨道角动量态的系统,对利用空间光调制器生成的携带不同轨道角动量的光束进行了测量,通过Mach-Zehnder干涉仪分离出了具有不同角量子数的螺旋光束. 关键词： 轨道角动量 空间光调制器 Mach-Zehnder干涉仪     

Acoustic beams with angular momentum

A family of exact solutions of the Helmholtz equation is used to represent transversely bounded helicoidal sound beams. Simple results are obtained for the energy content per unit length, the momentum content per unit length, and the angular momentum content per unit length. The analysis is restricted to lossless media; scattering and viscous damping are neglected. The energy, momentum, and angular momentum are calculated to second order in the velocity potential. The angular momentum content is always equal to m/omega times the energy content, where m (an integer) is the topological charge and omega is the angular frequency.     

Reflection of the light beam carrying orbital angular momentum from a lossy medium

It is shown that after reflection from a lossy medium the s- or p-polarized paraxial light beam carrying the orbital angular momentum suffers the 2D shift of the beam's centre of gravity relative the geometric optic axis. The direction as well as the length of the 2D vector, which describes the shift, change smoothly with the change of the angle of incidence.     

光束偏移对轨道角动量信息传输系统的影响

研究了拉盖尔-高斯光束轨道角动量信息传输系统在未对准时轨道角动量谱的变化.采用数学推导和数值仿真的方法,得到在任意传播距离处,接收系统轴与光轴分别出现横向偏移、角向倾斜,以及两者同时存在时光束的表达式,并分析和比较了几种情况下螺旋谱的变化特征.研究表明,两种偏移量的增加会增加螺旋谱的弥散,而弥散现象与两者偏移方向无关.在传输起点,横向偏移和角向倾斜均会引起螺旋谱的对称弥散.然而,随着传输距离的增加,横向偏移时螺旋谱的弥散程度逐渐减小,而角向倾斜时螺旋谱弥散程度逐渐增加.角向倾斜时螺旋谱始终保持对称分布,而横向偏移引起的弥散螺旋谱呈非对称分布,且经过一定距离后得到稳定的螺旋谱分布.当两者同时存在时,光束螺旋谱的弥散程度随着距离的增加逐渐增加到一个最大值,之后平缓地下降.并进一步得出了横向偏移与角向倾斜对弥散度的影响是不独立的. 关键词： 拉盖尔-高斯光束 螺旋谱 横向偏移 角向倾斜     

Geometric phase associated with mode transformations of optical beams bearing orbital angular momentum

We present direct measurements of a new geometric phase acquired by optical beams carrying orbital angular momentum. This phase arises when the transverse mode of a beam is transformed following a closed path in the space of modes. The measurements were done via the interference of two copropagating optical beams that pass through the same interferometer parts but acquire different geometric phases. The method is insensitive to dynamical phases. The magnitude and sign of the measured phases are in excellent agreement with theoretical predictions.