Propagation dynamics of an optical vortex imposed on an Airy beam

In this Letter, we demonstrate the general propagation dynamics of an Airy beam (AiB) carrying unit phase singularity, i.e., optical vortices (OVs). For the OV with a unit charge, theoretical analysis indicates that the OV carried by the AiB will propagate along the parabolic trajectory with an acceleration velocity twice as fast as conventional AiBs before a critical position. Thereafter, the AiB main lobe destroyed by OV will be reconstructed and the phase singularity will reappear in the middle of the AiB profile.     

Extending the detection range of optical vortices by Dammann vortex gratings

We report a 2D static binary phase Dammann vortex grating that combines the features of a conventional vortex grating and a Dammann grating. This grating uniformly distributes energies among the diffraction orders, so the low-efficiency problem at higher diffraction orders of conventional vortex gratings is resolved and the detection range of the optical vortices (OVs) is greatly increased. Experimental results of OV detection using a fabricated 5×5 Dammann vortex grating are given, and the topological charge detection range from -12 to +12 is achieved. The potential applications of such gratings include transmitting, receiving, and multiplexing OV beams in optical communication systems.     

Optical vortex generation with a “fork” hologram under conditions of high-angle diffraction

Spatial characteristics of the optical-vortex (OV) beams created during the Gaussian beam diffraction by a grating with groove bifurcation are analyzed theoretically and numerically. In contrast to previous works, condition of small-angle diffraction is no longer required and the diffracted beam can be strongly deformed. This causes the intensity profile rotation and the high-order OV decomposition into a set of secondary single-charged OVs. These effects are studied quantitatively and confronted with similar properties of a Laguerre-Gaussian beam that undergoes astigmatic telescopic transformation. In contrast to the latter case, the secondary OVs do not lie on a single straight line within the beam cross section, and morphology parameters of the individual secondary OVs carried by the same beam are, in general, different. Conditions for maximum relative separation of the secondary OVs with respect to the beam transverse size are specified. The results can be used for practical generation of OV beams and OV arrays with prescribed properties.     

Generation of optical vortex light beams by volume holograms with embedded phase singularity

Special features of the optical-vortex (OV) beams generated by thick holographic elements (HE) with embedded phase singularity are considered theoretically. The volume HE structure is based on the 3D pattern of interference between an OV beam and a standard reference wave with regular wavefront. The incident beam diffraction is described within the framework of a linear single-scattering model in which the volume HE is represented by a set of parallel thin layers with the “fork” holographic structure. An explicit integral expression is derived for the complex amplitude distribution of the diffracted paraxial beam with OV. The numerical analysis demonstrates that the HE thickness may essentially influence not only selectivity and efficiency of the OV beam generation but also the amplitude and phase profile of the diffracted beam as well as regularities of its propagation. We have studied the generated OV morphology and laws of its evolution; in particular, the possibility of obtaining a circularly symmetric OV beam regardless of the diffraction angle is revealed.     

Overcoming the rayleigh criterion limit with optical vortices

We experimentally and numerically tested the separability of two independent equally luminous monochromatic and white light sources at the diffraction limit, using optical vortices (OV). The diffraction pattern of one of the two sources crosses a fork hologram on its center generating the Laguerre-Gaussian (LG) transform of an Airy disk. The second source, crossing the fork hologram in positions different from the optical center, generates nonsymmetric LG patterns. We formulated a criterion, based on the asymmetric intensity distribution of the superposed LG patterns so created, to resolve the two sources at angular distances much below the Rayleigh criterion. Analogous experiments in white light allow angular resolutions which are still one order of magnitude below the Rayleigh criterion. The use of OVs might offer new applications for stellar separation in future space experiments.     

Dynamic generation of plasmonic Moiré fringes using phase-engineered optical vortex beam

Dynamic generation of plasmonic Moiré fringes using a phase engineered optical vortex (OV) beam is experimentally demonstrated. Owing to the unique helical phase carried by an OV beam, the initial phase of surface plasmon polaritons (SPPs) emanating from a metallic grating can be adjusted dynamically by changing the phase hologram displayed on a spatial light modulator. Plasmonic Moiré fringes are readily achieved by overlapping two SPP standing waves with certain angular misalignment, excited by the positive and negative topological charge components, respectively, of a cogwheel-like OV beam. The near-field scanning optical microscopy measurement result of SPP distributions has shown a good agreement with the numerical predictions.     

Optical vortices in self-focusing Kerr nonlinear media

In this work we numerically compare the interaction of optical vortices (OVs) in self-defocusing and self-focusing Kerr nonlinear media. We find that the basic scenarios (attraction/repulsion, translation/rotation vs. background) in the interaction of two and three vortices with equal and alternative topological charges (TCs) are the same in both media. However, the vortex dynamics under self-focusing conditions is influenced by the reshaping of the surrounding part of the background. Square structure of OVs with alternating TCs is found to be stable with respect to the vortex positions in self-focusing media. This elementary cell is successfully generalized in a large square array of OVs with alternative TCs which brings ordering in the multiple filamentation of the background beam in self-focusing conditions.     

Composite optical vortices in two noncollinear cosh-Gaussian vortex beams and their propagation properties in free space

The composite optical vortices (OVs) formed by superimposing two parallel, noncollinear cosh-Gaussian (ChG) vortex beams at the waist plane and their propagation in free space are studied. The dependence of the number, position and net topological charge of composite OVs on the control parameter, such as the relative phase, amplitude ratio, waist-width ratio, relative off-axis distance and decentered parameter, and on the propagation distance is illustrated numerically. A comparison with the previous work is also made.     

Oxygen-vacancy-induced diffusive scattering in Fe/MgO/Fe magnetic tunnel junctions

By first principles analysis, we systematically investigate effects of oxygen vacancies (OV) in the MgO barrier of Fe/MgO/Fe magnetic tunnel junctions. The interchannel diffusive scattering by disordered OVs located at or near the Fe/MgO interface drastically reduces the tunnel magnetoresistance ratio (TMR) from the ideal theoretical limit to the presently observed much smaller experimental range. Interior OVs are far less important in influencing TMR, but they significantly increase the junction resistance. Filling OV with nitrogen atoms restores TMR to near the ideal theoretical limit.     

Pseudo angular momentum and topological charge conservation for nonlinear acoustical vortices

We study acoustical vortices (AVs) and their connection with optical vortices (OVs). We show that AVs and OVs have the same properties if the concept of pseudomomentum is used. Radiation stress and conservation of topological charge are obtained with the pseudomomentum rule. In a weak nonlinear regime, the conservation of pseudo angular momentum imposes a linear increase of the topological charge with the harmonic order. This last result is experimentally verified.     

Generation of a hollow Gaussian beam and its anomalous behavior

I attempted to produce a laser beam with no light in the central line of the beam, by Fresnel diffraction of the Gaussian beam by a spiral zone plate (SZP). The study imparts an arbitrary-order phase singularity on the light field. The experimental results show that the optical vortex radius depends on the singularity’s integer order n (also termed topological charge, or order of the dislocation). Also, the radius of maximum intensity is shown to depend on the singularity number. Anomalous behavior of the spectra at phase singularity produced by various zone plates near the focus of a converging Gaussian beam is also studied using a simple experimental technique. It is found that the spectrum of the beam on the SZP (spiral zone plate) consisting of a single spectral profile shows almost vanishing intensity at all frequencies with a tendency of splitting into two peaks at the on-axis point and shows redshift and blueshift around the phase singularity. A comparative study of the anomalous behavior of the spectra at phase singularity produced by various SZPs, with varying phase singularity order n, near the focus of a converging Gaussian laser beam is studied.     

Shearograms of an optical phase singularity

Shearograms are known to represent phase gradients but when vortices are present in the optical field, these do not represent true phase gradients. Phase gradients of an optical phase singularity are presented. A lateral shear interferometer is used for obtaining shearograms of optical fields with vortices. A diffractive phase element is used to generate vortices. It is shown that shearograms can be used in the detection of optical vortices. Shearogram of speckle field is also presented.     

离轴平顶高斯涡旋光束形成的合成光涡旋及其在自由空间中的传输

对两束拓扑电荷m_1,2=±1的平行、离轴平顶高斯涡旋光束在束腰面叠加形成的合成光涡旋及其在自由空间的传输做了研究.详细的数值计算和分析表明,合光涡旋的位置、数目和净拓扑电荷与光束的控制参数,包括相对位相,振幅比,束腰宽度比,相对离轴参数,光束阶数,以及传输距离有关,但拓扑电荷不总是守恒. 关键词： 奇点光学 合成光涡旋 平顶高斯涡旋光束 拓扑电荷     

Observation of an x-ray vortex

Phase singularities are a ubiquitous feature of waves of all forms and represent a fundamental aspect of wave topology. An optical vortex phase singularity occurs when there is a spiral phase ramp about a point phase singularity. We report an experimental observation of an optical vortex in a field consisting of 9-keV x-ray photons. The vortex is created with an x-ray optical structure that imparts a spiral phase distribution to the incident wave field and is observed by use of diffraction about a wire to create a division-of-wave-front interferometer.     

Unfolding of an unstable singularity point into a ring

It is well known that phase singularities, in general lines in space, are topologically stable features of a wave field. An exception is a pointlike singularity, which is unstable and deforms into a ring or disappears when a small perturbation is applied. Recently, Nye showed how such an event can be understood as an unfolding of a higher-order dislocation [J. Opt. Soc. Am. A (to be published)]. We present an optical implementation of this model and show experimentally that the focal region of a lens contains points of zero intensity on the optical axis that deform into rings when a small amount of spherical aberration is applied to the system.     

Wavelength-scalable micro-fabricated wedge for generation of optical vortex beam in optical manipulation

An optical wedge phase plate is fabricated for the study of vortex beam generation in optical manipulation. It is demonstrated that the wedge phase element provides scalability for generating the vortex beam at different wavelengths as long as the incident beam size is adjusted accordingly. The presence of singularity for different wavelengths and the associated orbital angular momentum are confirmed experimentally in optical manipulation. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. PACS 41.85.Ct; 42.82.Cr     

水下高斯界面背向散射超声散斑场的相位奇异

为了精确描述超声散斑场的特性,提出了采用计算机模拟产生超声散斑场的方法.利用模拟产生的高斯相关随机表面,获得了这类表面远场超声散斑场,同时得到了声强分布和相位分布.与实验产生的散斑场进行对比,建立了超声散斑场接收的实验系统,取与计算模拟相同的参数,获得了实验散斑场.通过对比发现:计算机模拟产生的超声散斑场相位存在奇异点,奇异点周围相位分布类似漩涡,计算机模拟产生的散斑场与实验得到的散斑场强度分布相似,强度值比实验产生的散斑场强度大,携带有用信息的高亮散斑较多,暗点较少,更利于研究和分析.     

Phase singularity of surface plasmon polaritons generated by optical vortices

We demonstrate an experimental result that shows the phase singularity of surface plasmon waves generated by the direct transform of optical vortices at normal incidence focused on a structureless metal surface. The near-field two-dimensional intensity distribution near the focal plane is experimentally examined by using near-field scanning optical microscopy and shows a good agreement with the finite-difference time-domain simulation result. The experimental realization demonstrates a potential of the proposed excitation scheme to be reconfigured locally with advantages over structures milled into optically thick metallic films for plasmonics applications involving plasmonic vortices.     

Measuring the orbital angular momentum of elliptical vortex beams by using a slit hexagon aperture

The far-field diffraction pattern of an elliptical vortex beam by a slit hexagon aperture is investigated theoretically and experimentally. It is found that the number of the dark spots or stripes in the Fraunhofer diffraction intensity distribution is just equal to the topological charge value of the measured optical vortex, and that the centre of each dark spot or stripe is just a phase singularity point. Based on this property, it provides us a simple way to detect the orbital angular momentum (OAM) of an optical vortex beam.