Design of cylindrical conformal transmitted metasurface for orbital angular momentum vortex wave generation

We propose a cylindrical conformal transmitted metasurface for orbital angular momentum vortex wave generation. Formulas for calculating the phase distributions of cylindrical conformal transmitted metasurface is presented. A prototype of the proposed conformal transmitted metasurface is designed, fabricated and measured. Measured results shows that the proposed conformal transmitted metasurface can effectively generate vortex waves, which verifies the effectiveness of our method. The proposed method may pave the way of vortex wave generation with cylindrical conformal devices.     

Evaluation of gold nanowire pairs as a potential negative index material

A metamaterial consisting of pairs of metallic nanowires, separated by a dielectric spacer, has been fabricated and spectrally characterized in the visible and near-infrared spectral domain. It is shown, that the structure exhibits both a plasmonic and a magnetic resonance depending on its geometry and orientation with respect to the illuminating wave field. In particular, we investigate the influence of the thickness of the spacer layer on the spectral position of the resonances and show that, for an appropriate adjustment, both resonances coincide. Measurements of the amplitude and the phase of the transmitted wave are presented. It is also shown that the material is highly anisotropic with respect to the angle of incidence, as the plasmonic resonance wavelength depends strongly on it whereas the magnetic resonance does not show this sensitivity. All experimental results are supported by numerical simulations. PACS 81.07.-b; 78.67.-n; 78.20.-Ci     

Compound plasmonic vortex generation based on spiral nanoslits

In view of wide applications of structured light fields and plasmonic vortices, we propose the concept of compound plasmonic vortex and design several structured plasmonic vortex generators. This kind of structured plasmonic vortex generators consists of multiple spiral nanoslits and they can generate two or more concentric plasmonic vortices. Different from Laguerre–Gaussian beam, the topological charge of the plasmonic vortex in different region is different. Theoretical analysis lays the basis for the design of radially structured plasmonic vortex generators and numerical simulations for several examples confirm the effectiveness of the design principle. The discussions about the interference of vortex fields definite the generation condition for the structured vortex. This work provides a design methodology for generating new vortices using spiral nanoslits and the advanced radially structured plasmonic vortices is helpful for broadening the applications of vortex fields.     

Plasmonic waveplate: incident polarization modulation

Abstract We demonstrate that the rectangular nanohole arrays perforated in a 100 nm gold film can be used to tune the polarization direction of the transmitted light with maximum rotation angle of about 30 degrees. Theoretical analysis with the three-dimensional finite-difference time-domain simulations indicates that this phenomenon is attributed to the excitation of the surface plasmon wave on the gold film surface and the resonance of localized surface plasmon in the hole. With multiple plasmon resonances, the plasmonic waveplate can realize multi-wavelength polarization modulation. Our results may be useful to understanding the physical mechanism of enhanced plasmon mediated transmission and potential applications in plasmonic optical components.     

Shock tube exit flow fields through thin membranes

The flow field characteristics that form for a shock wave propagating through a membrane-like termination at the exit of a shock tube are studied. The strength of the shock wave reflected back into the tube, as well as the strength of the shock wave transmitted, is examined. Six different materials are used, ranging from copper and aluminium foils to a variety of elastic and plastic sheets, in a few cases with different initial pressure differentials. High-speed shearing interferometry imaging is done of the external flow. Three principal characteristics are present in the transmitted flow: a diffracted shock wave, an expansion wave and a re-compression shock wave. It is found that the prominence of these features varies depending on the material type. For the later flow development and material rupture, there are a number of principal characteristics: small vortices, secondary shock waves, a vortex ring, oblique waves and a Mach disc.     

Generation of a vortex ultrasonic beam with a phase plate with an angular dependence of the thickness

Vortex-wave beams are beams that carry angular momentum. Their specific feature is a ring-like transverse distribution of wave intensity with zero intensity at the axis. A method for generating an ultrasonic vortex beam by combining a single-element transducer and a phase plate with a nonuniform thickness is proposed. The method is examined theoretically and tested experimentally. In the theoretical analysis, the acoustic field was calculated using the Rayleigh integral. Experiments were performed in water with a focusing piezoceramic source with a frequency of the order of 1 MHz; the radiation from it was transmitted through a 12-sector organic-glass phase plate. The beam vorticity was established by setting the correct thickness of sectors. The results of scanning the field with a miniature hydrophone confirmed that the amplitude and phase distributions of the generated wave field were in fact consistent with a vortex beam. The capacity of the obtained beam to induce the rotation of scatterers positioned in the focal region was demonstrated.     

Light funneling mechanism explained by magnetoelectric interference

We investigate the mechanisms involved in the funneling of optical energy into subwavelength grooves etched on a metallic surface. The key phenomenon is unveiled thanks to the decomposition of the electromagnetic field into its propagative and evanescent parts. We unambiguously show that the funneling is not due to plasmonic waves flowing toward the grooves, but rather to the magnetoelectric interference of the incident wave with the evanescent field, this field being mainly due to the resonant wave escaping from the groove.     

Plasmonic wave plate based on subwavelength nanoslits

We propose a quarter-wave plate based on nanoslits and analyze it using a semianalytical theory and simulations. The device comprises two nanoslits arranged perpendicular to one another where the phases of the fields transmitted by the nanoslits differ by λ/4. In this way, the polarization state of the incident light can be changed from linear to circular or vice versa. The plasmonic nanoslit wave plate is thin and has a subwavelength lateral extent. We show that the predictions for the phase shift obtained from a semianalytical model are in very good agreement with simulations by the finite difference time domain method.     

Critical coupling in plasmonic resonator arrays

We report critical coupling of electromagnetic waves to plasmonic cavity arrays fabricated on Moiré surfaces. Dark field plasmon microscopy imaging and polarization dependent spectroscopic reflection measurements reveal the critical coupling conditions of the cavities. The critical coupling conditions depend on the superperiod of the Moiré surface, which also defines the coupling between the cavities. Complete transfer of the incident power can be achieved for traveling wave plasmonic resonators, which have a relatively short superperiod. When the superperiod of the resonators increases, the coupled resonators become isolated standing wave resonators in which complete transfer of the incident power is not possible. Analytical and finite difference time domain calculations support the experimental observations.     

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.     

Adaptive correction of vortex laser beam in a closed-loop system with phase only liquid crystal spatial light modulator

We propose and demonstrate the wave front correction of a vortex laser beam by using dual phase only liquid crystal spatial light modulators (LC-SLMs) and a stochastic parallel gradient descent (SPGD) algorithm. One phase only LC-SLM is used to generate vortex laser beam by loading spiral phase screen onto the wave front of input quasi-Gaussian beam. The other phase only LC-SLM under SPGD controller based on the subzone control method adaptively compensates the wave front of vortex laser beam. Numerical simulation and experimental results show that after correction, vortex doughnut like beam is focused into a beam with airy disk pattern distribution in the far field. The adaptive corrections of vortex laser beam with different optical topological charges are studied. The results show that the optical topological charge has little influence on adaptive correction. The powers in the main lobe of far field intensity distributions of vortex laser beams with different optical topological charges are all greatly improved by adaptive correction. The technique proposed in this paper can be used in optical communication, relay mirror and atmospheric turbulence correction.     

Comparison of three kinds of polarized Bessel vortex beams propagating through uniaxial anisotropic media

A comparison of differently polarized Bessel vortex beams propagating through a uniaxial anisotropic slab is discussed in terms of the vector wave function expansions.The magnitude profiles of electric field components, the transformation of polarization modes, and the distributions of orbital angular momentum(OAM) states of the reflected and transmitted beams for different incident angles are numerically simulated.The results indicate that the magnitude profiles of electric field components for different polarization modes are distinct from each other and have a great dependence on the incident angle,thus the transformation of polarization modes which reflects the change of energy can be affected largely.As compared to the x and circular polarization incidences, the reflected and transmitted beams for the radial polarization incidence suffer the fewest transformation of polarization modes, showing a better energy invariance.The distributions of OAM states of the reflected and transmitted beams for different polarization modes are diverse as well, and the derived OAM states of the transmitted beam for radial polarization present a focusing effect, concentrating on the state between two predominant OAM states.     

Anomalous behavior of the transverse acoustoelectric effect in superconductors with a moving vortex structure and the formation of acoustic vortex domains

It is shown that a longitudinal ultrasonic wave in superconductors with a moving vortex structure creates an additional transverse acoustoelectric field proportional to the power of this wave. This field changes its sign when the velocity of the vortex structure is lower than the velocity of the ultrasonic wave. This behavior qualitatively distinguishes this effect from a similar effect on the conduction electrons in semiconductors. Such a behavior of the acoustoelectric field, together with the effect of the amplification of the ultrasonic wave, can give rise to the negative differential conductivity and, therefore, to the formation of acoustic vortex domains in the superconductor.     

Optical vortex solitons in parametric wave mixing

We analyze two-component spatial optical vortex solitons supported by parametric wave mixing processes in a nonlinear bulk medium. We study two distinct cases of such localized waves, namely, parametric vortex solitons due to phase-matched second-harmonic generation in an optical medium with competing quadratic and cubic nonlinear response, and vortex solitons in the presence of third-harmonic generation in a cubic medium. We find, analytically and numerically, the structure of two-component vortex solitons, and also investigate modulational instability of their plane-wave background. In particular, we predict and analyze in detail novel types of vortex solitons, a "halo-vortex," consisting of a two-component vortex core surrounded by a bright ring of its harmonic field, and a "ring-vortex" soliton which is a vortex in a harmonic field that guides a ring-like localized mode of the fundamental-frequency field.     

Multi-function terahertz wave manipulation utilizing Fourier convolution operation metasurface

We propose a novel metasurface based on a combined pattern of outer C-shaped ring and inner rectangular ring. By Fourier convolution operation to generating different predesigned sequences of metasurfaces, we realize various functionalities to flexible manipulate terahertz waves including vortex terahertz beam splitting, anomalous vortex terahertz wave deflection, vortex terahertz wave splitting and deflection simultaneously. The incident terahertz wave can be flexibly controlled in a single metasurface. The designed metasurface has an extensive application prospect in the field of future terahertz communication and sensing.     

Broadband nulling of a vortex phase mask

A pulse transmitted through a helical vortex phase mask undergoes a temporal Hilbert transform. The fluence transmitted into the unfavorable plane wave mode is found to increase as the square of the bandwidth and, to first order, is independent of the topological charge.     

对称槽道涡波流场流动特征的POD分析

为深入分析层流状态下对称槽道内涡波流场的流动特性及其变化规律,对流场进行了二维粒子图像测速(2DPIV)测量获取瞬态速度矢量数据,利用本征正交分解(POD)技术进行模态分解以及涡波流场的重构,然后根据重构的流场对对称槽道内涡波流场进行了平均速度剖面、流场脉动强度以及特征点的速度和频谱分布等方面的分析。结果表明:POD的前15阶模态能够表征涡波流场的主导结构,第1,3阶模态主要表现为一对旋向相反的涡对特征,第2阶模态具有涡旋和波状主流的特征;提取了5个涡旋涡核的位置作为流场流动特性的特征点;根据POD重构流场分析发现流向平均速度呈抛物线形状分布,法向平均速度呈对称分布特征;流向脉动强度受壁面的影响较大,法向脉动强度呈现抛物线形状分布;距离中心主流较近的1#,4#,5#特征点的速度脉动程度受主流的脉动强度影响较大,速度的脉动主频0.15 Hz与次频、流场的自然频率0.35 Hz共同影响特征点的速度分布;2#,3#特征点的流向速度呈衰减趋势,法向速度在初期幅度变化较大。     

Surface modes in metal–insulator composites with strong interaction of metal particles

We theoretically examine plasmonic resonance excited between two close metallic grains embedded into a dielectric matrix. The grains sizes are assumed to be much less than the wavelength of the electromagnetic wave in the dielectric medium and the grain’s separation is assumed to be much smaller than the grains sizes. A qualitative scheme is developed that enables one to estimate frequency of the plasmonic resonance and value of the field enhancement inside the gap. Our general arguments are confirmed by rigorous analytic solution of the problem for simplest geometry—two identical spherical grains.     

Semiclassical dynamics of electron wave packet states with phase vortices

We consider semiclassical higher-order wave packet solutions of the Schr?dinger equation with phase vortices. The vortex line is aligned with the propagation direction, and the wave packet carries a well-defined orbital angular momentum (OAM) variant Planck's over 2pil (l is the vortex strength) along its main linear momentum. The probability current coils around the momentum in such OAM states of electrons. In an electric field, these states evolve like massless particles with spin l. The magnetic-monopole Berry curvature appears in momentum space, which results in a spin-orbit-type interaction and a Berry/Magnus transverse force acting on the wave packet. This brings about the OAM Hall effect. In a magnetic field, there is a Zeeman interaction, which, can lead to more complicated dynamics.     

Positively and negatively large Goos–Hänchen lateral displacements from a symmetric gyrotropic slab

A detailed study on the lateral displacements of a transverse magnetic (TM) wave transmitted and reflected from a symmetric gyrotropic slab is presented. We give the analytic formulas for the transmission coefficient and the reflection coefficient, as well as the corresponding lateral displacements. It is found that due to the external magnetic field the displacement of a transmitted beam is different from that of reflected one, even for a lossless symmetric configuration. Furthermore, within the chosen frequency band, when the incident angle is near the Brewster angle, the shift of a reflected wave can be large with nonzero reflectance, and can be positive or negative depending on the direction of the applied magnetic field and the incident wave.