Rayleigh Particles Research Articles

Radiation force of a self-focused vortex beam on Rayleigh particles

The radiation force of a partially coherent self-focusing vortex beam on Rayleigh particles is studied in this paper. According to the generalized Huygens-Fresnel principle and Rayleigh scattering theory, the effects of two main parameters of the beam, namely relative coherence length and non-trivial phase factor, on the self-focusing characteristics and radiation force are respectively researched. We have also conducted a brief analysis of the stability of particle capture using this self-focusing vortex beam. It has been found that changing the values of such parameters can flexibly regulate the self-focusing effect of the beam on propagation so as to effectively adjust the magnitude of the radiation force and trapping range. The results show that such beams can be used to trap and manipulate particles without using a focusing lens. In addition, this beam is able to capture two different refractive index particles, that is, high refractive index particles are captured near the focus, and low refractive index particles are captured on the z-axis. The research results establish a theoretical basis for the application of this novel partially coherent self-focusing vortex beams in optical tweezers technology.

Dynamical characteristics of tightly focused flat-topped double vortex beam for enhanced optical manipulation

The Flat-topped Vortex (FTV) beam has shown remarkable utility in a diverse range of industrial procedures, primarily attributed to its central intensity plateau. Nevertheless, the substantial potential of the FTV beam in particle trapping and manipulation has been largely underappreciated. In this work, we conducted the first study on the focusing properties and dynamic characteristics of the Flat-topped double vortex (FTDV) beam under different polarization for the absorption of Rayleigh particles. Through superimposing linearly two FTV beams with equal but opposite topological charges, the FTDV beam emerged naturally. Moreover, a comprehensive numerical analysis was subsequently performed to decode the interactions between the focusing field of the FTDV beam and Rayleigh particles. The analysis demonstrated that the focusing field of FTDV beam can perform multi-particle capture and effectively manipulate particles by modulating the topological charge |m|, the flatness order n, and the polarization state of the FTDV. Furthermore, a comparison between the focusing field of FTDV beam with that of vector vortex (VV) beam revealed that the FTDV beam was capable to generate spin angular momentum (SAM) density and spin torque at least an order of magnitude higher. These findings highlight the superior capabilities of FTDV beam in optical manipulation, and provide essential theoretical support for its utilization in particle capture and management.

Controllable multi-autofocusing of chirped multi-Pearcey beams and their trapping performance on Rayleigh particles

The exploration of structured light created through autofocusing beams has attracted widespread attention in optical manipulation. Here, we investigate the propagation dynamics of chirped multi-Pearcey beams (CMPBs) in theory and experiment, especially exploring the impact of chirped factors on the autofocusing characteristics of CMPBs. It is found that the autofocusing characteristics of the beams can be significantly modulated by introducing first-order and second-order chirped factors. Specifically, the first-order chirped factor greatly improves the autofocusing coefficient of CMPBs, while the second-order chirped factor introduces multiple autofocusing propagations. By analyzing the transverse power flow of CMPBs both without and with chirped factors, we explain why the chirped factors can remarkably strengthen the autofocusing performance of CMPBs. Finally, our theoretical analysis on trapping force on Rayleigh particles indicates that the introduction of chirped factors optimizes the intensity gradient and trapping capability of CMPBs, potentially enabling multiple longitudinal trapping positions. These findings offer a comprehensive insight into the propagation dynamics of CMPBs, opening avenues for flexible optical manipulation of multiple particles.

Optical Force Effects of Rayleigh Particles by Cylindrical Vector Beams.

High-order cylindrical vector beams possess flexible spatial polarization and exhibit new effects and phenomena that can expand the functionality and enhance the capability of optical systems. However, building a general analytical model for highly focused beams with different polarization orders remains a challenge. Here, we elaborately develop the vector theory of high-order cylindrical vector beams in a high numerical aperture focusing system and achieve the vectorial diffraction integrals for describing the tight focusing field with the space-variant distribution of polarization orders within the framework of Richards-Wolf diffraction theory. The analytical formulae include the exact three Cartesian components of electric and magnetic distributions in the tightly focused region. Additionally, utilizing the analytical formulae, we can achieve the gradient force, scattering force, and curl-spin force exerted on Rayleigh particles trapped by high-order cylindrical vector beams. These results are crucial for improving the design and engineering of the tightly focused field by modulating the polarization orders of high-order cylindrical vector beams, particularly for applications such as optical tweezers and optical manipulation. This theoretical analysis also extends to the calculation of complicated optical vortex vector fields and the design of diffractive optical elements with high diffraction efficiency and resolution.

Multioptical bottles from the chirped circular Bessel Gaussian beams carrying annular spiral-zone power-exponent-phase

In this paper, we introduce a new structure of multioptical bottles formed by the chirped circular Bessel Gaussian beams carrying annular spiral-zone power-exponent-phase (ASP). The ASP is a general term for complex phases composed of the radial phase, equiphase, and power spiral phase. By combining the influence of autofocusing properties, chirp factor, and ASP mode, the propagation dynamics of the proposed multioptical bottles beam such as the sizes, lengths, number and locations can be flexibly manipulated. Intriguingly, the effect of the power order of ASP mode on the beam propagation dynamics is firstly introduced and examined. The proposed beams can enable diversification the optical tweezers system in multiparticle manipulation, which is verified by the radiation force analysis on a Rayleigh particle.

Optical trapping capability of tornado circular Pearcey beams

We systemically investigate optical trapping capability of a kind of tornado waves on Rayleigh particles. Such tornado waves are named as tornado circular Pearcey beams (TCPBs) and produced by combining two circular Pearcey beams with different radii. Our theoretical exploration delves into various aspects, including the propagation dynamics, energy flux, orbital angular momentum, trapping force, and torque characteristics of TCPBs. The results reveal that the orbital angular momentum, trapping force, and torque of these beams can be finely tuned through the judicious manipulation of their topological charges (l 1 and l 2). Notably, we observe a precise control mechanism wherein the force diminishes with |l 1 + l 2| and |l 1–l 2|, while the torque exhibits enhancement by decreasing solely with |l 1 + l 2| or increasing with |l 1–l 2|. These results not only provide quantitative insights into the optical trapping performance of TCPBs but also serve as a valuable reference for the ongoing development of innovative photonic tools.

On chip all-optical distinguishing of independently placed distinct types of single Rayleigh particle

In order to determine whether a particle is plasmonic, dielectric, or chiral, different complex processes and chemicals are applied in lab setups and pharmaceutical industries. Sorting or categorizing a particle based on distinct optical forces can be a novel technique. When a beam of light interacts with a particle, it usually pushes the particle in the direction of the light's propagation. Counterintuitively, it can also pull the particle toward the light beam or move it toward a lateral direction. As far as we know, to date, no comprehensive report exists regarding a single optical arrangement capable of inducing entirely distinct behaviors of force for three disparate types of independently placed single Rayleigh particle. This study introduces an all-optical technique aimed at effectively sorting nanoscale Rayleigh-sized objects employing a plasmonic substrate, when each distinct type of single particle is placed over the substrate independently. Unfortunately, this proposed technique does not work for the cluster or mixture of distinct particles. In our proposed configuration, a simple linearly polarized plane wave is incident onto the plasmonic substrate, thereby engendering completely different responses from three different types of nanoparticles: Gold (plasmonic), SiO2 (dielectric), and Chiral particles. We conducted individual tests for our setup using linearly polarized plane waves at angles of 30-degree, 45-degree, and 60-degree individually. Consistent results were obtained across all angles. In each of the three distinct setups involving the aforementioned particle, a dielectric Rayleigh particle experiences an optical pulling force, a plasmonic Rayleigh particle experiences an optical pushing force, and a chiral Rayleigh particle encounters an optical lateral force. These distinctive force behaviors manifest as a result of the intricate interplay between the material properties of the nanoparticles and the characteristics of the plane-polarized beam, encompassing aspects such as plasmonic response, chirality, and refractive index. Moreover, this technique presents an environmentally sustainable and economically viable alternative to the utilization of expensive and potentially hazardous chemicals in nanoparticle sorting processes within industrial domains.

Generation and propagation of circular Airy derivative beams carrying rotationally-symmetric power-exponent-phase vortices.

In this paper, the circular Airy derivative beams carrying rotationally-symmetric power-exponent-phase vortices are proposed for the first time, whose evolutionary properties are explored by theoretical analysis as well as experimental verification. The intensity and phase distributions of this kind of beam can be flexibly modulated by controlling its parameters such as derivative order, topological charge, and power order. Intriguingly, the evolution of such beams with different fractional topological charges is also investigated in detail by means of phase singularities distribution. In addition, from the perspective of transverse gradient force, the proposed beam is capable of efficiently protecting the trapped Rayleigh particles located at the beam center from the impact by surrounding particles. Furthermore, the beam width, orbital angular momentum (OAM) density and spiral spectrum are also compared and analyzed under different beam parameters. The proposed beams are expected to be useful for extending applications of optical vortices and autofocusing beam, especially for multi-regional particle gathering and central particle protecting.

Hyperbolic metamaterial induced optical pulling of distinct Rayleigh particles: Single particle or cluster of heterogeneous objects

In the field of optical manipulation, there has been a phenomenon where an illuminated object is propelled towards the direction of the light beam due to the momentum carried by the quanta of light known as photon ℏκ. Nonetheless, there is currently no work in literature that describes a method to draw all types of objects, including single and clustered nanoparticles, using a single optical setup. In this study, we propose a generic optical setup that utilizes the anomalous reflections of light generated by hyperbolic metamaterial to enable an Optical Tractor Beam (OTB) effect over a heterogeneous cluster of particles and single particle arrangements. We conducted numerous full-wave simulations to observe the optical force pattern under various conditions, such as varying wavelengths, interparticle distances, and particle distributions, as well as investigations in a variety of surrounding media, including air, half-submerged in water, and fully submerged in water. Our optical setup has been subjected to a near-field OTB effect from the clustered Rayleigh-sized particles, as well as single particles for all varied environments. This article demonstrates the mechanisms and outcomes that offer a fresh new avenue toward manipulating nanoscale homogeneous and heterogeneous objects, including dielectric, plasmonic, and chiral nanoparticles.

Optical force on a Rayleigh particle generated by photonic jet

Optical force on a Rayleigh particle generated by photonic jet

Controlling the counterintuitive optical repulsive thrust of nano dimers with counter propagating type waves and background medium.

This work focuses on the utilization of counter-propagating plane waves for optical manipulation, which provides a unique approach to control the behavior of Rayleigh and Dipolar nanoparticles immersed in a homogeneous or heterogeneous medium. Our study presents an interesting finding of a repulsive force between plasmonic-chiral heterodimers where the particles move away from each other in both near and far field regions. Interestingly, this repulsive thrust supports the wave like nature of light for the case of homogeneous background but particle type nature of light for heterogenous background. At first, we have investigated the theory underlying the optical trapping of the chiral particle and the impact of this phenomenon on the overall repulsive behavior of the heterodimers placed in air (homogeneous) background. After that, our proposed set-up has further been investigated putting in air-water interface (heterogenous background) and by varying light angle only a little bit. Our observation for this interface case is suggesting the transfer of Minkowski momentum of photon to each optically pulled Rayleigh or dipolar particle of the dimer set, which ultimately causes a broad-band giant repulsive thrust of the dimers. However, in absence of the other particle in the cluster, a single half-immersed particle does not experience the pulling force for the broad-band spectrum. The 'common' reason of the observed repulsive thrust of the dimers for both the aforementioned cases has been attributed to "modified" longitudinal Optical Binding Force (OBF). Technically, this work may open a new way to control the repulsion and attraction between the nanoparticles both in near and far field regions by utilizing the background and the counter-propagating waves. We also believe that this work manifests a possible simple set-up, which will support to observe a background dependent wave 'or' particle nature of light experimentally.

Self-induced back-action for aperture trapping: Bethe-Rayleigh theory.

A dielectric (nano)particle can influence the local electromagnetic field and thereby alter its interaction with that field through the process of self-induced back-action. While this phenomenon is usually considered theoretically as a change in a cavity resonance frequency, such theoretical approaches are not as appropriate when considering systems away from resonance, such as with a subwavelength aperture in a metal film. Here we consider the interaction between an aperture, modelled with Bethe theory as a magnetic dipole, and a Rayleigh particle, modelled as an electric dipole. Using this magnetic dipole - electric dipole interaction, we quantify the self-induced back-action of the particle on the aperture transmission and the optical trapping potential. The model shows quantitative agreement with finite-difference time-domain simulations. This shows that the physics of self-induced back-action for an aperture and a nanoparticle can be understood in terms of dipole-dipole coupling.

Influence of off-focus and off-axis distances on trapping of a Rayleigh particle

The influence of the off-focus and off-axis distances on trapping of a metallic particle is investigated theoretically using vector diffraction and dipole approximation. Numerical results show that when the off-focus and off-axis distances of a metallic Rayleigh particle are smaller, typically smaller than 0.5[Formula: see text], the particle can be three-dimensionally trapped to the focus by a highly focused real radially polarized beam. However, when the off-focus and off-axis distances are large, the particle can not be three-dimensionally trapped to the focus but it may be trapped to an annular region. The dependence of the trapping stability and trapping stiffness on the off-focus and off-axis distances are calculated. The effect of the beam parameter (the ratio of the radius of the lens to the beam waist) of the radially polarized light on optical trap is discussed.

Stable 3D particle manipulation using rotating elliptical Gauss–Schell model vortex partially coherent light sources

The Partially coherent light source can be applied to the noninvasive capture of particles, and increasing the freedom of capturing different types of particles simultaneously is of great significance. This paper studies the optical field distribution of a rotating elliptical Gaussian-Schell model vortex beam focused by a lens and the radiation force acting on Rayleigh particles. The beam’s rotation parameter, the vortex’s topological charge, and the spatial coherence length have different effects on the light intensity, which will determine the optical capture ability of Rayleigh particles with other refractive indices and generate equilibrium points at different positions. The longitudinal equilibrium points of high-refractive-index particles and the transverse equilibrium points of low-refractive-index particles are increased by adjusting the value of the rotation parameter and the topological charge. Finally, the appropriate size range for trapping Rayleigh particles is analyzed according to the comparison of Brownian force and radiant force. The results show that this partially coherent light source has a high degree of freedom in capturing particles with different refractive indices, which provides a new way for optical capture technology and three-dimensional (3D) particle manipulation.

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.

Analysis of magneto-natural-convection flow in a semi-annulus enclosure filled with a micropolar-nanofluid; a computational framework using CVFEM and FVM

Micropolar fluids are the media in which their behavior is affected by the microrotation of the substructure particles. In the present work, the natural-convection flow of a micropolar-nanofluid (Al2O3/water) in the presence of an inclined magnetic field in a semi-annulus enclosure is investigated. The influence of active parameters on magneto-natural convection is investigated. The governing equations (mass, momentum, microrotation, and energy) are numerically solved by CVFEM. Moreover, for some conservative comparisons, the equations are solved by Ansys Fluent CFD code which is conjugated by an innovative nondimensionlization scheme. The effects of important parameters are investigated on the streamlines, isotherms, microrotation contours, the maximum absolute value of stream function (Ψmax), the average Nusselt number (Nuave) and the maximum absolute value of microrotation Nmax∗. The results discovered that the values of Nuave descend with ascending the Hartmann number (Ha) and material parameter (K) while it ascends with ascending Rayleigh number (Ra) and particle volume fraction (ϕ). The value of Nuave descends 10.8% while Nmax∗ increases 19.8% when K increases from 1 to 2. The value of Nuave descends 44.83% when Ha increases from 25 to 75 at λ=45°. The minimum value of Nuave occurs at λ=45° for Ha=25 and at λ=30° for Ha=50 and Ha=75.

Holographic Optimization Method of Acoustic Radiation Force for Continuously Manipulating a Mie Particle through a Fluid

Acoustic holography is widely used in particle manipulation. However, previous methods focus on modulating the amplitude and phase of the target sound field. In general, the amplitude gradient and phase gradient of the sound field correspond to the gradient force and the scattering force, respectively. But the relationship between the force and the aforementioned gradient is nonlinear even for a Rayleigh particle (whose radius is much smaller than the wavelength), not to mention the Mie particle(whose radius is comparable with the wavelength). If we want to control the continuous motion of a Mie particle quantitatively, we need to design force based on the relationship among the force, acceleration, and velocity. Here we propose to design radiation force along the path of a particle directly in holography methods. Compared to the usual modulation of amplitude and phase, the method designing radiation force directly can control the acceleration and orbital limiting velocity of a two-dimensional orbit rotation of a Mie particle with radius $a=0.4\phantom{\rule{0.2em}{0ex}}\mathrm{mm}$ (approximately $2/3$ wavelength) in simulation. The experiment verified its feasibility in controlling the continuous motion of particle quantitatively. This work shows its superiority over the pure acoustic holography methods in quantitative particle manipulation.

Self-induced backaction in optical waveguides.

In this paper, we study the backaction effect on the force exerted upon Rayleigh particles in guided structures. We show that the backaction becomes stronger as the group velocity of the guided modes is decreased, which is not unexpected since the fall of group velocity increases the interaction time between the particle and the electromagnetic field. Interestingly, the sign of the group velocity affects the pushing and pulling nature of the exerted electromagnetic force. We specifically investigate the case of a single mode optical waveguide both in the propagating and evanescent regimes, and show that the backaction enables us to enhance the ratio of the potential depth to the trapping intensity, and thereby can be a beneficial tool for nondestructive trapping of small nanoparticles. We further show that backaction can induce some resonances in the optical force in the evanescent regime. These resonances can be employed for sorting of nanoparticles.

Linear and angular momentum properties induced by radial- and azimuthal-variant polarized beams in a strongly focused optical system.

Optical linear and angular momenta have attracted tremendous research interest in recent years. In this paper we theoretically investigate the electromagnetic fields and linear and angular momentum properties of tightly focused radial- and azimuthal-variant vector input beams. Calculations show that a uniform 3D optical cage can be achieved when the optical degree of freedom of polarization in the radial direction is introduced. Furthermore, the distributions of linear and angular momenta in the focal volume are revealed. Moreover, we numerically investigate the gradient, scattering, and total forces as well as spin and orbital torques on a Rayleigh particle generated by the optical cage. It is found that there are two equilibrium positions before and after the focal plane, both of which can achieve stable 3D particles capture. Most importantly, the longitudinal spin and orbital torques show the same patterns but in opposite directions in the two equilibrium positions, thus, the unwinding of the double helix can be expected to be achieved by virtue of this special optical torque.

Longitudinal optical trapping and manipulating Rayleigh particles by spatial nonuniform coherence engineering

Focused structured light fields with spatially nonuniform correlation can exhibit multiple focal points in the longitudinal direction, which suggests that they are suitable for longitudinal optical trapping. Due to the controllable nature of the longitudinal intensity distribution, this class of structured light fields not only can trap two high-index particles simultaneously, but also can manipulate them in the longitudinal direction.