Increase Of Topological Charge Research Articles

Perfect Off-Axis Optical Vortex Lattice

Optical vortex lattices (OVLs) with diverse modes show potential for a wide range of applications, such as high-capacity optical communications, optical tweezers, and optical measurements. However, vortices in typical regulated OVLs often exhibit irregular shapes, such as being narrow and elongated. The resulting increase in asymmetry negatively impacts the efficiency of particle trapping. Additionally, the vortex radii expand with an increase in topological charge (TC), limiting the TC value of the vortices and hindering their ability to fully utilize orbital angular momentum (OAM). Herein, we propose an alternative approach to custom OVLs using off-axis techniques combined with amplitude modulation. Amplitude modulation enables the precise generation of an OVL with perfect vortex properties, known as a perfect off-axis OVL. Further, the number of vortices in the perfect off-axis OVL, the off-axis distances, and the TC can be freely modulated while maintaining a circular mode. This unique OVL will promote new applications, such as the complex manipulation of multi-particle systems and optical communication based on OAM.

Paraxial propagation characteristics of a controllable anomalous hollow vortex beam in free space

Propagation of a recently proposed controllable anomalous hollow vortex (CAHV) beam is investigated. Based on the integral formula of generalized Huygens–Fresnel diffraction, analytical expression for the CAHV beam through a paraxial ABCD optical system is derived. The factors that affect the intensity pattern are determined by the beam’s controllable parameters a, c x, c y, and the topological charge m. Results show that the Gaussian distribution features are controlled by parameter a, and the horizontal and vertical stretching deformations of the beam are adjusted by parameters c x and c y, respectively. For a controllable anomalous hollow (CAH) beam, when propagating in free space, it could initially maintain anomalous hollow property and the size of the spot increases with the increase of the propagation distance. Due to the CAHV beam carries the optical vortex, a dark hollow channel appears in the center of the beam during propagation, and the channel structure changes with the increase of topological charge. Additionally, the Poynting vector of CAHV beam proves the direction of energy flow corresponding to the intensity distribution. Results obtained in this paper could have potential applications in particle trapping and optical control.

Study of the acoustic scattering characteristics of a rigid sphere in a vortex acoustic field

This paper investigates the scattering sound field of a rigid sphere positioned in a vortex beam generated by a phase-modulated circular transducer array. The effects of the dimensionless parameter ka, the number of array elements, the topological charge, the radius of the transducer array, the distance between the array and the sphere, and its offset position on the scattering sound field distribution of a sphere are studied. Simulation results show that as the ka increases, the incident wave intensity and the scattering sound field of a rigid sphere increase, and the angle between the two main lobes decreases. Once ka is determined, the number of the array elements has no effect on the scattering pattern. However, the topological charge of the vortex beam has greater effect on the scattering sound field. As the topological charge increases, the incident wave intensity decreases, but the angle between two main lobes of the scattering field increases. The radius of the array has an effect on the intensity of the scattering field and the larger the radius, the larger the area of low intensity radiation between the two main lobes. In addition, the distance between the sphere and the array has a weak effect on the scattering pattern when it exceeds a certain value. For an off-axis sphere, the scattering pattern becomes more complex as the topological charge changes. These results can be used to adjust the constituent factors of the transducer array and to investigate the advantages of the vortex beam for underwater target detection.

Spin splitting of vortex beams on the surface of natural biaxial hyperbolic materials

We investigated the spin splitting of vortex beam on the surface of biaxial natural hyperbolic materials (NHMs) rotated by an angle with respect to the incident plane. An obvious asymmetry of spatial shifts produced by the left-handed circularly (LCP) component and right-handed circularly polarized (RCP) component is exhibited. We derived the analytical expression for in- and out-of-plane spatial shifts for each spin component of the vortex beam. The orientation angle of the optical axis plays a key role in the spin splitting between the two spin components, which can be reflected in the simple expressions for spatial shifts without the rotation angle. Based on an α-MoO3 biaxial NHM, the spatial shifts of the two spin components with the topological charge were investigated. As the topological charge increases, the spatial shifts also increase; in addition, a tiny spatial shift close to zero can be obtained if we control the incident frequency or the polarization of the reflected beams. It can also be concluded that the maximum of the spin splitting results from the LCP component at p-incidence and the RCP component at s-incidence in the RB-II hyperbolic frequency band. The effect of the incident angle and the thickness of the α-MoO3 film on spin splitting is also considered. These results can be used for manipulating infrared radiation and optical detection.

Magneto- and thermo-optic manipulation of imbert-fedorov shift for Laguerre-Gaussian beam in a graphene-VO2 multilayer structure

Imbert-Fedorov (IF) shift of Gaussian beams has been widely studied, while the IF shift of singular beams carrying Orbital Angular Momentum (OAM) also shows unique properties when reflected or refracted at optical interfaces. Here, through theoretical deduction and analysis, the IF shift of Laguerre-Gaussian beams with varying OAM in a graphene-VO2 multilayers structure is explored. The results demonstrate that the IF shift increases with the increase of topological charge l. Under the configuration of transverse magneto-optical Kerr effect (TMOKE), the IF shift increases by a factor of 10 compared to the Gaussian beam when l = 3. Additionally, under the polar magneto-optical Kerr effect (PMOKE) conditions, graphene demonstrates the magneto-optical (MO) effect and cross-polarization effect in the terahertz frequency range, and can effectively control the IF shift through the magnetic field. Both left- and right-handed circular polarization (LHCP and PHCP) exhibit asymmetry and non-reciprocity. Moreover, temperature control is achieved through the unique phase transition characteristics of VO2. These results offer novel insights into precision measurements, sensing techniques and the development of terahertz devices.

Experimental Realization of Orbital Angular Momentum Multiplexed Four-Beam Quadrature Squeezing

We experimentally generate 9 sets of orbital angular momentum (OAM) multiplexed four-beam quadrature squeezing by utilizing four-wave mixing processes in a rubidium vapor. The degree of four-beam quadrature squeezing decreases with the increase of topological charge |l|. In addition, we investigate the four-beam quadrature squeezing with OAM superposition modes. Our results provide an efficient way to generate OAM multiplexed multi-beam quadrature squeezing and may find potential applications in multi-parameter estimation.

Quantitative characterization of autofocusing and trapping of multi-Airy vortex beams

Abruptly autofocusing vortex beams were proposed for a variety of applications such as optical manipulation, but their quantitative characterization of their trapping capability remains largely unexplored. In this paper, we propose a type of autofocusing vortex beams named multi-Airy vortex beams (MAVBs) and investigate their autofocusing and trapping characteristics theoretically and experimentally. It is found that the autofocusing performance and trapping ability of MAVBs can be enhanced by adjusting the relevant parameters and increasing the number of superposed Airy within a certain range. Although the trapping force and trapping stiffness of MAVBs decrease as the topological charge increases, we found that MAVBs have advantages in capturing and manipulating large-sized particles in our experiments. In the process of manipulating multiple particles for rotation, the larger the topological charge, the more particles can be rotated simultaneously by MAVBs. Moreover, the speed and even the direction of rotation can be easily adjusted by controlling the degree of dispersion between particles. Our work provides a guide to quantitatively exploring the optical trapping capabilities of autofocusing vortex beams, which may lead to different photonic tools for optical trapping and manipulation.

Scattering analysis of a Laguerre Gaussian beam for target detection.

The propagation and scattering characteristics of a Laguerre Gaussian (LG) beam and Gaussian beam are theoretically and experimentally comparatively studied. The phase of the LG beam is almost free of scattering when the scattering is weak, resulting in a much weaker loss of transmission compared with the Gaussian beam. However, when scattering is strong, the phase of the LG beam is completely disturbed by it, and the transmission loss of the LG beam is stronger than that of the Gaussian beam. Moreover, the phase of the LG beam becomes stabler as the topological charge increases, and the beam radius increases as well. Therefore, the LG beam is suitable for short-range target detection in a weak scattering medium, not for long-range in a strong scattering medium. This work will contribute to the development of target detection, optical communication, and other applications of orbital angular momentum beams.

Radiation force of a special correlated beam with off-axis multiple vortices on Rayleigh particles

In order to explore the potential application of a superimposed Hermite-Gaussian correlated Schell-model beams with off-axis multi-vortices in optical trapping, based on the generalized Collins formula, the focused beam intensity and radiation force on particles are derived in detail. Numerical results show that the gradient force and capture stability range increase with the increase of topological charge. By changing the vortex off-axis distance, the precise position control of the particles can be made. Adjusting the number of vortices can control the existence of vortex singularity on the side lobe of the beam, and generate or close the optical potential well or optical cage in real time. By selecting the appropriate beam order, two kinds of particles with different refractive indexes can be captured. The beams are expected to be applied to optical trapping of multiple particles at different positions.

Scattering characteristics of a terahertz Bessel vortex beam by 3D dielectric-coated targets.

In order to explore the application of terahertz (THz) vortex beams in 3D dielectric-coated target detection and imaging, scattering characteristics of a THz Bessel vortex beam by 3D dielectric-coated conducting targets are investigated in terms of the combination of planewave angular spectrum expansion and a physics optics method in this paper. The accuracy of the proposed method is verified by comparing with the results of FEKO software. The scattering characteristics of a THz Bessel vortex beam by several typical 3D dielectric-coated targets are analyzed in detail. The effects of the beam parameters (topological charge, half-cone angle, incident angle and frequency) are discussed. The results show that with an increase of topological charge, the magnitude of the radar cross section (RCS) decreases, and the maximum value moves away from the incident direction gradually; the distribution of the RCS does not keep symmetry as the incident angle increases, and the corresponding orbital angular momentum state distribution of the far-scattered field is distorted remarkably.

Incoherent Scattering of a Bessel Vortex Electromagnetic Beam From a Nonmagnetized Plasma

Starting from the description of plane wave angular spectrum superposition of a Bessel vortex beam, with the aid of Thomson scattering theory of a Bessel vortex electromagnetic beam (BVEB) by a low-velocity election, the incoherent scattering power spectra of a BVEB by a non-magnetized plasma is investigated in this paper. The incoherent scattering power spectrum for a BVEB incidence is defined for the backscattered case, and the expression of the scattered power spectral density function is derived. The proposed method is verified in the physical sense by comparing the profiles of distribution parameter of scattered power and the magnitude of average Poynting vector of the incident beam. The effects of the beam parameters (topological charge and half-cone angle) on the scattered power spectral density function are analyzed for the plasmas with Maxwellian velocity distribution. Furtherly, the expression of the scattered power spectral density function is extended to the case of macroscopic moving plasmas. Taking plasma vortices as an example, the results indicate that the direction of the frequency shift of the scattered power spectral density function due to the rotational Doppler effect of the vortex beam is related to angular velocity of the plasma vortex. Additionally, as the topological charge increases, the magnitude of the frequency shifts also increases. Our proposed method provides a theoretical basis for exploring the feasibility of using vortex beams with single or multi orbital angular momentum (OAM) modes in the identifying and inverting parameters of plasma irregularities in ionospheric plasma or laboratory plasma environments.

Propagation factor of partially coherent vector vortex beam in inhomogeneous turbulent atmosphere

We undertake the propagation properties of a partially coherent vector vortex beam (PCVVB) through the inhomogeneous atmospheric turbulence. Analytical expressions for M2-factor of partially coherent radially polarized vortex beam (PCRPVB) in turbulence with the extended Huygens–Fresnel principle and the second-order moments of Wigner distribution function (WDF) are derived. Numerical results indicate that the M2-factor and root-mean-square (rms) angular width mainly depend on the light source parameters and zenith angle. We find that M2-factor and rms angular width of PCRPVB in turbulence will become saturated with increasing propagation distance. It can be also found that the relative M2-factor and relative rms angular width will decrease with the increase of topological charge and wavelength as well as the decrease of initial coherent length, which indicates that the PCRPVB with larger topological charge and wavelength, smaller initial coherent length has a stronger anti-turbulence ability. Our studies may be useful for the lidar distance measurement and free-space optical communications.

Propagation properties of partially coherent radially and azimuthally polarized vortex beams in turbulent atmosphere

Analytical formulas for the average intensity of the partially coherent radially and azimuthally polarized vortex (PCRPV and PCAPV) beams propagating through the turbulent atmosphere are derived based on the extended Huygens-Fresnel principle. The effect of the different topological charge number m of PCRPV beam and PCAPV beam in turbulence on average intensity and degree of hollowness (DOH) has been studied in detail. It can be found that PCRPV beam and PCAPV beam in turbulence with the larger topological charge m can greatly keep dark hollow constructure on propagation. And we have studied the orientation angles of the polarization ellipse of the PCRPV and PCAPV beams with different m in atmospheric turbulence. The results show that the capacity to keep radial polarization decline with the increase of topological charge m. In addition, we combined the complex screen method and multi-phase screens method to simulate the propagation of PCRPV beam and PCAPV beam in atmospheric turbulence for verifying our theoretical results. It is shown that the simulated results are consistent with theoretical results. Our study could be important for free-space optical communications.

The tight-focusing properties of radially polarized symmetrical power-exponent-phase vortex beam

In this paper, the radially polarized (RP) new kind of power-exponent-phase vortex (NPEPV) beam, with rotationally symmetrical phase structure, was introduced and the tightly focused properties of the RP NPEPV beam passing through a high numerical aperture objective lens were studied numerically. The results show that with the increase of topological charge l, there are multiple intensity points in the focal region, and the number is consistent with the topological charge. In addition, as the power order n increases, the light intensity gradually concentrates on the central optical axis and the surrounding intensity points gradually disappear, which finally presents a Gaussian intensity distribution with the dark cores gradually move away from the optical axis and disappear. These unique properties will have potential applications in particle trapping and laser fabrication, especially for simultaneous trapping of multiple particles and fabrication of chiral microstructures.

A phase compensational elliptical Fresnel zone plate for generating elliptical optical vortices

A phase compensational elliptical Fresnel zone plate (PCEFZP) is proposed to control the light field in this paper. It compensates the optical path difference from PCEFZP to the major focal point to control the beam focus on the specified position. Besides, the PCEFZP and the spiral phase plate are combined to produce vortex beams. By changing different phase plates, three types of elliptical vortex beams (EVB) are generated. Simulation and experimental results show that there are obvious differences among the three beam fields: EVB of type B has the longest focal depth among them, in addition, when the topological charge increases, beam fields of type A and B can maintain a perfect elliptical shape, while the central null of the type C EVB field intensity splits into a different number of nulls and the number of nulls is equal to topological charge. These parameter-tunable vortex beams can find applications in different fields, such as particle manipulation and optical communication.

Focusing properties of power order space-variant phase modulate Bessel-Gaussian vortex beam

The purpose of this paper is to study the focusing properties of power order space-variant phase modulate the Bessel-Gaussian (BG) vortex beam, which is based on vector diffraction theory. Results show that under different power order of space-variant phase, the modulation of topological charge number and phase parameter have different evolution rules. Specifically, when power order n = 1, the position of the focal pattern will be arbitrarily adjusted on the X-axis by modulating the phase parameter C. Under the condition of n = 2, the focal spot will be expanded along y = -x with the increase of topological charge l. Meanwhile, by modulating C, two opposite phenomena of focal spot separation will be obtained. When n = 3, the increase of l will make the focal spot expand along the positive and negative directions of the Y-axis. Besides, increasing the value of C, the original focal spot intensity will converge to the X-axis, and a new intensity peak will be formed on the X-axis. A series of focal properties have been obtained as the power order space-variant phase introduced. These novel properties have potential applications in many fields such as particle trapping, particle movement and optical modulation related fields.

Generation of diverse acoustic vortices by superimposed multipole emissions

Acoustic vortices (AVs) based on artificial metamaterials have attracted intense research efforts for their potential use in subdiffraction imaging, particle manipulation, and acoustic communication. However, to date, major strategies to generate AVs have an intrinsic limitation of fundamentally depending on complex phase modulations, which require a higher number of multiple elements as the desired topological charge increases, let alone the elaborate dephasing between the individual microstructured elements in the switching process of topological charges. In this paper, we propose an annulus anisotropic structure coupled with a pair of eccentric asynchronous monopole sources that can realize multiple-order AV modes at different frequencies. The generated AVs originate from the interactions of two superimposed multipoles characterized with orthogonal emission patterns and $\ensuremath{\pi}/2$-phase differences. The AVs with topological charges from 1 to 5 are achieved simultaneously at dipole, quadrupole, hexapole, octupole, and decapole resonances, respectively. Numerical simulations and experimental results clearly confirm the feasibility and practical applicability of the proposed design strategy in realizing topologically diverse AVs. Our design, exempted from complex phase modulations, possesses flexible topological charge modes at the expense of frequency, which may foresee potential capabilities in multiplexing and demultiplexing of AVs.

Bessel-Gauss beams in the generalized Lorenz-Mie theory using three remodeling techniques

In the analysis of light scattering by small particles, the Generalized Lorenz-Mie Theory (GLMT) describes the incident beam with a set of Beam Shape Coefficients (BSCs) that can be calculated with three different approaches, viz., quadratures, finite series and localized approximations. Choosing between them may not be self-evident. A Bessel-Gauss beam (BGB) is a finite energy, physically realizable wave field resulting from the apodization of a Bessel beam by a Gaussian function. This paper provides a comparison between the aforementioned techniques for the evaluation of the BSCs of scalar BGBs with distinct axicon angles and confinement parameters, including field reconstructions. All three methods agree quite well in the paraxial regime, although as the axicon angle or the topological charge increases, differences in the BSCs for each method become more and more evident.

Propagation characteristics of non-diffracting Lommel beams in a gradient-index medium

We investigate the propagation of the Lommel beams in a gradient-index medium. Based on the ABCD transfer matrix and Collins diffraction integral formula, the analytical expressions of the complex amplitude of the Lommel beams through the gradient-index medium are derived. The intensity distribution and the Poynting vector of the Lommel beams are studied analytically and numerically. The results show that the Lommel beam is focused at the singularities and the corresponding periodic beam profile is induced by the symmetrical refractive index pattern during the beam propagating in the gradient-index medium. The effects of the asymmetry factor and the topological charge on the propagation characteristics of the Lommel beams in the gradient-index medium are discussed in detail. Especially, the propagation trajectories of the Lommel beams become visible as the topological charge increases and the symmetry of the Lommel beam’s transverse intensity pattern can be controlled by varying the asymmetry factor. It is expected that the results obtained in this paper may be useful for potential applications of optical control and trapping.

Focus properties of cosh-Gaussian beams with the power-exponent-phase vortex.

Vector diffraction theory is used to investigate the focusing properties of cosh-Gaussian beams with the power-exponent-phase vortex. The effects of the decentered parameter, the power order, and the topological charge on the normalized intensity distribution are examined. Results show that intensity distribution in the focal region can be altered significantly by the topological charge, the power order, and the decentered parameter. The pattern of the optical intensity slowly enlarges with the increase of topological charge. The strongest intensity part of the annulus rotates by the changing topological charge. As the power order increases, the intensity distribution is more concentrated. As the decentered parameter increases, there occur multiple relatively strong intensity peaks, and the entire focus pattern extends outward. Some optical gradient force distributions are investigated to illuminate the applications of these alterable foci patterns.