Establishment of an optical trapping curve for prediction of trapping parameters

Ayush Owhal, Dipankar Boruah, Sachin U. Belgamwar

Optical trapping is widely used to manipulate a small-sized particle freely suspended in the isotropic fluidic domain. Trapping is done by means of optical forces developed by conversing light beam. The active gradient forces, depends upon parameters like light wavelength, particle size, and refractive index of medium and particle. The viscous drag forces, depends upon parameter like viscosity of fluid, relative velocity of particle with respect to medium. The necessary condition for particle trapping is to maintain > . In this paper, a graphical approach is applied to predetermine the value of specific parameter such as relative velocity of particle and light wavelength under the necessary condition for optical trapping, while keeping other parameters are as fixed. Software simulation is performed with set of relative velocities on polystyrene small-sized particle in water channel to validate the graphical approach.

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Calculating optical forces with skew line ray model for Gaussian beam

Shuhe Zhang, Meng Shao, Xiao Yang, Jinhua Zhou

Ray optics models in optical tweezers are confined to two specific cases: ray pencil model for a highly focused beam and spherical wave front model for a weakly focused beam. In this manuscript, the skew lines of one sheet hyperboloid are introduced as rays’ trajectories to trace the propagation of a Gaussian beam for calculating optical forces. The simulations of trapping efficiencies demonstrate the skew-line ray model is valid in comparison with traditional ray-optics models including spherical wave front model and ray pencil model. Our results of transverse and axial trapping efficiencies show that the skew-line ray model has good performances in both highly and weakly focused beams. Furthermore, the influence of the spherical aberration is discussed, and our results are accordance with that from traditional ray-optics models. The SLR method unifies ray pencil model and spherical wave front model into one way, and can be used to calculate optical forces in either paraxial or nonparaxial conditions. Thus, this model is more appropriate in extensive simulations in ray optics regime.

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Simulation of motion of charged particle within a fluid in an optical tube

Amin Mousavi, Fahimeh Hosseinibalam, Smaeyl Hassanzadeh, Zahra Taghaviyan

In this study, the behavior of charged particles within a fluid is investigated when exposed to the radiation of a Laguerre-Gaussian beam as an optical tube. Optical forces (e.g., Photophoretic, Radiation pressure and Lorentz forces) and non-optical forces (e.g., Drag and gravitational forces) determine the trajectories of ions movement within these beams. Trapping of charged particles in the direction of the beam axis in an optical tubes depends on the superiority of the radial component of the photophoretic force on the electromagnetic force. The factors affecting this, such as the amount and type of electrostatic charge and the radius of the particle, and the shape and power of the beam are studied. As a result, the charged particle can be guided to various paths based on their electrostatic charge and particle diameter. This could be done in the study of ions within a fluid by optical methods. One of the applications proposed in this regard is that we can counted charged and uncharged particles within a fluid (e.g., liquid ions or aerosols) by separating them in an optical tube.

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Optical trapping of sub-wavelength objects with point-like slot waveguides

Mufei Xiao

An optical tweezer capable of picking up dielectric as well as metallic objects of sizes much smaller than the light wavelength is proposed based on point-like two-dimensionally slotted waveguides. Slot waveguides transport light based on evanescent modes, therefore, with sub-wavelength resolution. A point-like slot waveguide confines light within a sub-wavelength cross-section, and a strong gradient electric field is formed. A radiation force is induced by the strong gradient field, and the force is strong enough for optically trapping and manipulating small objects of different sizes and refractive indice with sub-wavelength resolution. As an example, the proposed device is numerically simulated with a rectangular slot waveguide so as to confirm its feasibility. Since the assumed device profile and materials are common for chip formation, the proposed device has the potential to become a lab-on-a-chip opto-electric tool for biologic analysis.

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Optical trapping two types of particles using a focused vortex beam

Hanghang Zhang, Jinhong Li, Miaojun Guo, Meiling Duan, Zhifang Feng, Wen Yang

Propagations of Gaussian Schell-model (GSM) vortex beams through a focusing optical system are formulated. The radiation force acting on Rayleigh dielectric sphere with different refractive indices produced by focused GSM vortex beams is investigated theoretically. Numerical results demonstrate that the focused GSM non-vortex beam can not trap the low index of refraction particles, but can capture the high index of refraction particles. The focused GSM vortex beam can be used to trap high index of refraction particles to a bright ring of the focal plane, and simultaneously capture low index of refraction particles to z-axis. The larger the topological charge m is, the larger the value of the spatial correlation length σ0 is, the easier it is to trap two types of particles is. Trapping stability is also analyzed.

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Optical confined nanoparticles on a nanofiber microring with a microparticle decorated the junction

Ying Li, Yanjun Hu

This work demonstrates optical trapping and transportation of nanoparticles along a nanofiber microring. The optical fiber, 800 nm in diameter, was formed to be a microring with a bending radius of about 42 μm. A microparticle, stuck to the junction of the ring, plays a critical role for confining nanoparticles on the microring. The experimental results show that, when a 650 nm laser light was launched into the ring, nanoparticles dispersed in the solution were trapped to the ring surface and delivered along the direction of the light propagation. Because the stuck microparticle on the junction for perturbation, nanoparticles can be confined over entire microring circumference. This technology offers a new degree of control for particles and lead to various nanomanipulation applictions.

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Radiation force of highly focused modified hollow Gaussian beams on a Rayleigh particle

Bin Tang, Yanjie Li, Xin Zhou, Li Huang, Xianzhong Lang

The radiation force on a Rayleigh dielectric particle produced by highly focused modified hollow Gaussian (MHG) beams is investigated numerically and theoretically. The results show that the highly focused MHG beams can be used to trap and manipulate the particles with low or high index of refractive larger than that of ambient at the focus point and in the neighbourhood of the focal plane simultaneously in the different region. Also, the conditions for trapping stability are analyzed in this paper.

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Photon momentum and optically induced force in matter derived from the eikonal equation

V.P. Torchigin, A.V. Torchigin

We show thatthe photon trajectory calculated on the basis ofthe Newton laws of mechanics coincides with that calculated on the basis of laws of the geometrical optics on assumption that the momentum of the photon in matter increases by n times as compared with that in free space. The force applied to the photon is calculated from the third Newton law as the force that is opposite to the net force produced by the electrical field of the photon in the matter. It is shown that there is no Lorentz force and the Maxwell force produced by an electrical filed in a dielectric is responsible for a change of the photon momentum.

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Modifying vibrational properties of a fused silica cantilever with optical tweezers

Rijuparna Chakraborty

This research effort is dedicated to examine the effects of an optical tweezer on the vibration of a small cantilever fiber made up of fused silica and to manipulate certain aspects of its vibrational properties with this tool. Optical tweezers has been used here to change the resonance frequency of the cantilever fiber by a small amount and also to damp its vibration.

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The equation of motion of the photon in an optical medium

V.P. Torchigin, A.V. Torchigin

We show that the equation of the photon motion coincides with the eikonal equation on the assumption that the photon momentum in an optical medium increases by n times compared with that in free space and the density force applied to the photon is determined by the density force derived by the Maxwell for the force in the dielectric located in an electrical field.

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Photon and physical phenomena responsible for its momentum

V.P. Torchigin, A.V. Torchigin

We derive a magnitude of the momentum of light in matter by means of matching two irrefutable not contradictory thought experiments where no preliminary assumptions about kinds of optically induced forces responsible for a change of the momentum of light in matter are made. The total momentum increases in the matter by n times due to the Coulomb kind of force in a dielectric investigated by Maxwell. There are two different component of the total momentum. These are the mechanical component arising due to a motion of conventional material objects, mass of whose is non-zero and the electromagnetic component produced by a travelling electromagnetic wave, mass of which is equal to zero. The following types of optically induced forces provide a redistribution of the total momentum between these components. These are the kind of the Abraham-like force produced in matter by an electromagnetic wave, intensity of which is changed in time and the Helmholtz-like force arising in a field of an electromagnetic wave due to an inhomogeneity of the electrostriction pressure produced by the light wave. The mechanical component of the momentum of the light is negative and the electromagnetic component is greater than the total momentum.

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Nonlinear properties of gaseous optical mediums in a context of ball lightning explanation

V.P. Torchigin, , A.V. Torchigin
We consider a dependence of the refractive index of gases on the intensity of the conventional white light in a context of an analysis of properties of nonlinear optical mediums where a self-confined light radiation is possible. This is connected with the fact that a behavior of the self-confined light in a form of thin spherical layer of strongly compressed air where the intensive white light is circulating in all possible directions and a behavior of Ball Lightning in the terrestrial atmosphere are identical.

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Design of a simple, low-cost, computer-controlled, dual-beam optical tweezer system

C.J. Firby, K.N. Smith, S.R. Gilroy, A. Porisky, A.Y. Elezzabi

We present the design of a simple optical tweezer system. Our system modifies a simple compound microscope to provide one stationary and one steerable trap. Vertical integration of the optical components results in a device with a small footprint that is both compact and portable. Motorized mounting systems are constructed to achieve precise trap motion in three dimensions. Control and image acquisition are performed via an intuitive computer interface. Common and readily obtainable components were incorporated into the apparatus to reduce cost and complexity. The system was used to successfully trap and manipulate yeast cells, and by tuning the laser power, trapping can easily be extended to a wide range of biological and dielectric samples.

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Kinds of optically induced force derived from laws of conservation of the momentum and energy

V.P. Torchigin, A.V. Torchigin

Optically induced forces (OIF) applied to a transparent optical medium are analyzed. We deliberately do not use any assumptions about a nature of optically induced force and analyze thought experiments where only notions and laws of conservation are used. Two unambiguous but contradictory thought experiments are used as a ground of an analysis. It is shown that only two kinds of well-known OIF is sufficient to match contradictory results of the experiments. This is the kind of density force known in electrostatics that is responsible for the density force arising in an inhomogeneous dielectric located in an electrical field. This is the Abraham density force arising at propagation of a light pulse in an optical medium. The force is located in the regions of the optical medium where leading and trailing edges of a light pulse are propagating. OIF in a homogeneous optical medium located in an inhomogeneous electrical field is equal to zero at a steady-state. This result contradicts to that obtained by means of the widely used approach based on the Lorentz density force.

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Radiation pressure on plane dielectric surfaces

V.P. Torchigin, A.V. Torchigin

It is shown on the basis of unambiguous thought and real experiments regarding the pressure produced by light on a plane boundary of optical medium that the Abraham force takes part in the production of the pressure. As a result, all hitherto known information obtained by using another approach based on the Lorentz force should be corrected. Radiation pressures produced by a continuous light wave and a light pulse on the simplest plane dielectric surfaces are presented. The pressure produced by a continuous travelling light wave incident from ree space on a semi-infinite dielectric is negative and is equal -W0(n-1)(n + 1) where W0 is the energy density of light in free space, n is the refractive index. The pressure produced by the leading edge of a light pulse at the entrance to the dielectric is positive and equal W0(n-1)(n + 1). Pressures produced by a continuous light wave and light pulse on a semi-infinite dielectric with anti-reflection λ/4 coating are equal W0(1-n) and W0(1-1/n), respectively.

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Switchable optical cage by use of coated axicons for optical trapping

Qiang Song, Jing Zhu, Shiwang Tang, Baoxi Yang, Huijie Huang

We present a novel method to generate switchable optical trapping by together tight focusing of two noncoherent cylindrical vector beams, which were produced by two pairs of axicons with polarizing films. The simulation result shows that the optical trapping shape is controllable varying from three-dimensional cage to channel by smoothly tuning the distances between axicons. We verify the cylindrical vector characteristic of the output beams from proposed setup based on vector diffraction theory. Finally, gradient forces of the optical trapping were calculated to demonstrate the potential application of this system in the field of micro particle manipulation.

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Propagation dynamics and optical trapping of a radial Airy array beam

Ke Cheng, Xianqiong Zhong, Anping Xiang

Analytical propagation expression of a radial Airy array beam in coherent and incoherent combination passing through paraxial ABCD system is derived, and used to investigate the effect of combination scheme, array orientation and initial phase of Airy beamlet on propagation dynamics of the resulting beam in free space, where optical spot array and vortex array with different shapes are also found, respectively. And then taking four-beamlet Airy array beam in same array orientation as an example, square optical spot array obtained in focal field can be used for simultaneous trapping multiple Rayleigh particles with relative refractive index larger than 1. The transverse gradient forces serving as restore forces tend to push particles at different initial positions to their individual optical spot center. The analysis of trapping stability indicates that larger input peak intensity of Airy beamlet and smaller particle size are benefit to trapping particle owing to many deeper potential wells. Vortex array produced by coherent combined Airy array beam in this paper is expected to be useful for simultaneous trapping microparticles with relative refractive index smaller than 1.

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Focusing properties of Gaussian beam with superimposed left-handed and right-handed helical phase fronts

Jinsong Li, Na Meng

Vector diffraction theory is employed to investigate the focusing properties of the Gaussian beams with superimposed left-handed and right-handed helical phase fronts theoretically. Numerical simulations show that the intensity distribution in focal region can be altered considerably by adjusting topological charge m corresponding to right-handed helix and topological charge n corresponding to left-handed helix. Many novel focal pattern may occur. It was shown that the focal pattern evolves from one intensity peak to multiple intensity peaks with changing the topological charge m and n, and all the intensity peaks form in a circle. As the number of intensity peaks is the sum of m and n, the focal pattern can be controlled through adjusting the topological charge m and n. And in order to show the possible applications of these alterable foci pattern, some optical gradient force distributions were also calculated and illuminated.

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Compensation of the optically induced Lorentz force in a homogeneous optical medium

Vladimir Torchigin, Alexander Torchigin
It is shown on the basis of different approaches that the density of optically induced forces applied to a homogeneous optical medium embedded in a simplest 1D structure in a form of a plane optical resonator is equal to zero. In particular, the density forces calculated on the base of the energetic approach, where no assumptions about physical nature of optically induced forces are used, are also equal to zero. At the same time the same forces calculated by means of the approach based on the Lorentz force are different from zero. A conclusion is derived that there is an additional type of optically induced force which compensates the Lorentz density forces. Thus, the Lorentz force approach used for calculation of the density of optically induced force is inconsistent.
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Creation of a controllable three dimensional optical chain by TEM01 mode radially polarized Laguerre–Gaussian beam

Jianwei Cao, Qingkui Chen, Hanming Guo

Three dimensional (3D) multi sites optical trapping requires multi focal spots in the focal region, which is not easy to achieve. In this paper, we present a novel design to create a controllable 3D optical chain that can stably trap and deliver particles purposely. On the basis of the vector diffraction theory, the complex pupil filters (CPF) is successfully designed to modulate the phase of TEM01 mode radially polarized Laguerre–Gaussian beams. With the optimized parameters of CPF, the 3D optical chains with two and five focal spots are created, respectively. Also, the 3D optical chain with five focal spots is periodical and moves forward (i.e., the direction far away the aplanatic system) with the increasing of the phase ψ of the outer zone of CPF. Moreover, the movement of the 3D optical chain can be well controlled by changing ψ purposely, which makes particle manipulation more controllable and flexible. This work is important for micromanipulation, micromachines, and microscopy. 

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