Gradient, scattering and other kinds of longitudinal optical forces exerted by off-axis Bessel beams in the Rayleigh regime in the framework of generalized Lorenz-Mie theory

Gérard Gouesbet

After Arthur Ashkin’s pioneering work in optical levitation and manipulation, the study of optical forces exerted by laser beams on particles has become an active field of research. The present paper is a contribution to this issue. The interest of Bessel beams is that their intensity does not have any longitudinal gradient along the direction of propagation leading to a trivial separation between gradient and scattering forces. Beside the classical gradient and scattering forces, we shall however exhibit a new kind of optical forces associated with the existence of non zero axicon angles.

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Modified plasmonic response of dimer nanoantennas with nonlocal effects: From near-field enhancement to optical force

Hancong Wang, Kaixi Chen, Jia Pan, Shihao Huang, Jinyang Lin, Wenming Xie, Xuhong Huang

The metallic nanoparticle dimer is a fundamental model system for enhancing and tuning localized surface plasmon resonances. In the past, it had been found that the far- and near-field optical properties of dimer antennas can be regulated by many parameters (e.g., gap, size, orientation, materials, and surrounding medium). In recent years, the quantum mechanical effects such as nonlocal screening and electron tunneling have been achieved when the gap distance in a dimer approaches 1 nm and subnanometer. In this communication, both the near-field enhancement and optical force in dimer are fully investigated and compared between classical and nonlocal models. Compared with classical theory, we found that both the resonant wavelength and peak intensity have smaller changes in nonlocal model when geometrical or material parameters changes. Besides, the extent of parameter-induced spectral changes is slightly different between near-field enhancement and optical force. These results make possible the quantitative analysis of nonlocal effects in surface-enhanced spectroscopy, nanoantennas, refractive-index sensing, surface-enhanced optical force, and quantum plasmonics.

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Optical radiation force on a dielectric sphere of arbitrary size illuminated by a linearly polarized Airy light-sheet

Ningning Song, Renxian Li, Han Sun, Jiaming Zhang, Bojian Wei, Shu Zhang, F.G. Mitri

Based on the generalized Lorenz–Mie theory (GLMT) and the angular spectrum decomposition method (ASDM), we calculate the optical radiation force exerted on a lossless dielectric sphere of arbitrary size illuminated by an Airy light-sheet. The beam shape coefficients (BSCs) of the Airy light-sheet are calculated using the vector angular spectrum decomposition and vector spherical wave functions methods. The optical radiation force acting on the spherical particle is obtained by the integral of Maxwell’s stress tensor. The transverse (Fy) and longitudinal (Fz) forces are numerically computed. Two kinds of polarization (TE and TM) are considered for the Airy light-sheet, and the negative longitudinal optical (pulling) force is particularly emphasized. The influence of the transverse scale parameter w0 and attenuation parameter γ of the Airy light-sheet on the force is discussed. The results of the present theory are verified using the dipole approximation method in which the gradient force has been also computed for a Rayleigh sphere. The numerical results show that when the transverse scale parameter w0 and attenuation parameter γ increase, the transverse and longitudinal forces decrease. Furthermore, the force caustic (i.e., maximum) shifts to the direction of y < 0 as the transverse scale parameter w0 increases. As the dimensionless size parameter of the sphere ka increases (where k is the wavenumber and a is the radius), the resonance peaks of the optical forces become larger. The results of this paper are of practical significance for the development of Airy light-sheet based optical manipulation technologies.

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Optical radiation force circular dichroism spectroscopy

F.G. Mitri

This work introduces the method of circular dichroism spectroscopy in the framework of the electromagnetic/optical radiation force theory. This analytical tool is defined here as the difference in radiation force of left-handed and right-handed circularly polarized electromagnetic waves illuminating an object exhibiting rotary polarization. The example of a lossless material, such as the perfect electromagnetic conductor (PEMC) cylinder having a circular geometric cross-section, is considered. The modal expansion method in cylindrical coordinates is used to obtain exact mathematical series expansions for the longitudinal radiation force per-length (i.e. acting along the direction of wave propagation) considering left-handed and right-handed circularly polarized cylindrically diverging waves emanating from a line source. The case of plane progressive waves is recovered when the source is located far from the cylinder. Numerical illustrative results for the dimensionless radiation force functions as well as the scattering, extinction and absorption energy efficiencies and their co-polarized and cross-polarized components are performed with particular emphasis on the size parameter of the cylinder, the dimensionless distance parameter from the line source, and the admittance parameter of the cylinder. The results reveal that the individual radiation force functions for left-handed and right-handed circularly polarized waves can be negative, zero, or positive depending on the cylinder distance from the source. Moreover, the optical radiation force circular dichroism (ORFCD) and the extinction energy efficiency circular dichroism (EEECD) are positive for a negative admittance of the cylinder, while they reverse sign for a positive admittance. While the EEECD shows some form of symmetry versus admittance sign change, the ORFCD does not. The possibility of achieving invisibility cloaking for a small PEMC cylinder is also investigated. The present ORFCD spectroscopy method is applicable to any cylinder material exhibiting rotary polarization such as chiral, topological insulator, plasma, liquid crystal etc.

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Dual optical trap created by tightly focused circularly polarized ring Airy beam

Zaili Chen, Yunfeng Jiang

The dual focus property of focused circularly polarized ring Airy beam (RAB) under the action of tightly focused lens is demonstrated in this paper. The radiation forces at two foci of tightly focused RAB are calculated, the numerical results show that the particle could be longitudinally and transversely trapped at the two foci. By varying corresponding parameters, we could control the property of two traps. The trapping force increases with NA and the scaling parameter w; and an appropriate initial radius r0 is necessary for the enhancement of either trap. The two traps could move closer as w increases or r0 decreases. To realize the dual optical trap, we should choose a smaller decaying parameter a and a larger NA, or the dual optical trap would degenerate into a single optical trap. Moreover, because of the influence of the Brownian motion and the scattering force, the size of the particle should be in a special range.

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Stability and dynamics of chiral nanoparticles in lateral optical binding induced by high-order Bessel beams

Jing Bai, Cheng-xian Ge, Zhen-sen Wu

The generalized multi-particle Mie equation (GMM) and electromagnetic momentum (EM) theory are applied to investigate the stability and dynamics of chiral nanoparticles in lateral optical binding induced by a high-order Bessel beam (HOBB). Such non-diffracting light suppressed the influence of the axial intensity profile of the illuminating beams on the self-organization process which then depended critically upon the inter-particles interactions. The illuminating HOBB is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz–Mie theories (GLMT). Utilizing the addition theorem of the vector spherical wave functions (VSWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the chiral nanoparticles is considered. The observed lateral binding force (BF) dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction when the chiral spheres degenerate into isotropic spheres. We discuss the influence of the different parameters of the incident Bessel beam and of the chiral body on lateral BF in detail. Linearly and circularly polarized incident Bessel beams are considered, and the corresponding lateral BFs are compared and analyzed. The polarizations of incident HOBB considerably influence the lateral BF of chiral nanoparticles. In binding chiral nanoparticles, the polarization of incident beams should be chosen in accordance with the chirality. This finding may provide a recipe to understand the light interaction with multiple chiral particles of arbitrary shapes with the aid of the analytical approach. It could be a promising avenue in controlling the optical micromanipulation on chiral structures self-arrangement.

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Internal and near-surface fields for a charged sphere irradiated by a vector Bessel beam

Yiming Yang, Zizhuo Nie, Yinan Feng, Renxian Li

The interaction of an axicon-generated vector Bessel beam (AGVBB) with a charged sphere is investigated in the framework of generalized Lorenz–Mie theory (GLMT). The incident, internal, and scattered fields are expanded using vector spherical wave functions (VSWFs), beam shape coefficients (BSCs), and internal and scattered coefficients. An analytical expressions of beam shape coefficients (BSCs), which are derived using angular spectrum decomposition method (ASDM), are given. The internal and scattered coefficients are derived by considering the boundary conditions. The internal and near-surface electric fields of a charged sphere illuminated by AVGBBs are numerical calculated, and the effects of polarization, order of beam, half-cone angle are mainly discussed. The results are compared with that for neutral particles. The effect of the surface charge are discussed by the comparison of the results for charged spheres with that for neutral particles. Numerical results show that the internal and near-surface fields are sensitive to the surface charge. The internal fields and the near-surface fields can be locally enhanced. Internal and near-surface fields, especially its local enhancement, are very sensitive to the beam parameters, including polarization, order, half-cone angle, etc.

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Optical trapping forces on Rayleigh particles by a focused Bessel-Gaussian correlated Schell-model beam

Zhang Hanghang, Han Yiping, Wang Jiajie, Guo Jirong

Optical trapping forces exerted by a focused Bessel-Gaussian correlated Schell-model (BGCSM) beam on Rayleigh dielectric spheres with different refractive indices are analyzed. The dependence of radiation forces on the transverse coherence width δ0, the coherence parameter β, refractive index of the particle, and particle radius are investigated. It is shown that the focused BGCSM beam can be used to trap particles with both high and low index of refraction near the focus, and as the transverse coherence width δ0 or the relative refractive indices of the particle with respect to the host medium increases, or the coherence parameter β decreases, the trapping stability increase. Furthermore, the limits of the radius for two types of particles stably captured are determined.

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Radiation force and torque on perfect electrically–conducting (PEC) corrugated circular and elliptical cylinders in TE or TM polarized plane progressive waves with arbitrary incidence

F.G.Mitri

In this work, theoretical modeling and numerical computations for the electromagnetic (EM) radiation force and torque (per-length) on perfect electrically conducting (PEC) cylinders of corrugated circular and elliptical geometrical cross-sections are developed and presented. A TE-polarized or TM-polarized plane progressive wave illumination with arbitrary incidence (in the polar plane) is assumed at normal incidence with respect to the axis of the cylindrical particle. The multipole expansion method in cylindrical coordinates is used and the scattering coefficients of the object of arbitrary shape (in 2D) are determined by imposing appropriate boundary conditions and solving numerically a linear system of equations by matrix inversion. Numerical computations are performed for the non-dimensional longitudinal and transverse radiation force functions as well as the axial radiation torque function. Suitable convergence plots confirm the validity of the multipole expansion approach to evaluate the radiation force and torque with no limitation to a particular frequency range (i.e. Rayleigh, Mie or geometrical optics regimes can be considered using the presented formalism). Particular emphases are given on the shape of the particle (i.e., circular or elliptical), its non-dimensional size, the corrugation/waviness characteristic of its surface, the polarization of the incident plane wave field, and the angle of incidence in the polar plane. The numerical predictions show that the longitudinal and transverse components of the radiation force vector are positive regardless of particle shape, size, corrugation properties, polarization and incidence angle [0 ≤ α ≤ 90°], while the axial torque component reverses sign at particular values of these parameters. The results are predominantly relevant in understanding the fundamentals of the optical/EM radiation force and torque theories and possible applications dealing with the interactions of EM waves with elongated tubular particles with circular or noncircular ribbed surfaces in particle manipulation and other areas. The acoustical analogue is also noted, which shows the universal characteristic of the radiation force and torque phenomena.

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Light-driven self-organization of gold clusters by linearly polarized Gaussian beam

Jiunn-Woei Liaw, Mao-Chang Huang, Cheng-Wei Huang, Yun-Cheng Ku, Mao-KuenKuo

The mechanism of optically organized and bound 2D clusters of multiple Au nanoparticles (NPs) is studied theoretically. Via the surface integration of Maxwell's stress tensor, the optical forces and torques upon every Au NPs are analyzed. The numerical results illustrate that 2D stable-equilibrium clusters (trimer, tetramer and pentamer) with wavelength-scale gaps can be produced at an off-focal plane of a linearly polarized (LP) Gaussian beam, besides 1D linear array patterns. For example, a trimer with an isosceles triangular pattern can be induced. These stably ordered cluster patterns are due to the plasmon-enhanced long-range interaction of light with these coupled Au NPs. Moreover, there is a range of the off-focal plane for inducing these ordered patterns. Our results are in agreement with the previous experiments [Yan et al. Nat. Commun. 2014; 5: 3751]. In addition, we found that these NPs are driven to transversely spin by the corresponding optical torque even though irradiated by a LP light. The study on optomechanics of the light-driven self-assembly of plasmonic colloids can be applied to 2D array patterning, which is worth further exploiting.

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Electromagnetic radiation force on a perfect electromagnetic conductor (PEMC) circular cylinder

F.G. Mitri

Unlike isotropic dielectric objects, the interaction of incident electromagnetic (EM) or optical waves with a material allowing rotary polarization produces coupled polarized internal and scattered fields. The aim of this investigation is to examine theoretically a novel physical effect, which concerns the contributions of the co-polarized and cross-polarized fields to the radiation force (per-length) experienced by an infinitely long perfect electromagnetic conductor (PEMC) cylinder having a circular cross-section and illuminated by TM-polarized plane progressive waves propagating perpendicularly to its axis. The multipole partial-wave series expansion method in cylindrical coordinates is used to derive exact series expansions for the co-polarized and cross-polarized components of the longitudinal radiation force per-length (i.e. acting along the direction of wave propagation). In contrast with the perfect electric or magnetic conductors (PECs or PMCs), or the dielectric cylinder case, numerical illustrative results for the radiation force function (which is the radiation force per unit energy density and cross-sectional surface) clearly demonstrate the contribution of the cross-polarized component of the radiation force for a PEMC cylinder allowing rotary polarization. The results show that the cross-polarized component of the radiation force function can be positive or negative as the dimensionless frequency parameter ka varies (where k is the wavenumber in the medium of wave propagation and a is the radius of the cylinder). Moreover, it vanishes for ka = 0.67946 regardless of the admittance of the PEMC cylinder. Notice that the total force (i.e. the sum of the co-polarized and cross-polarized components) is always repulsive (i.e., positive). It is also verified that the results are in complete agreement with the law of energy conservation applied to scattering. The present analysis generalizes the classical radiation force investigations by introducing extra new terms in the series expansion for the longitudinal radiation force function for cylindrical materials allowing rotary polarization.

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Generalized Lorenz--Mie theories and mechanical effects of laser light, on the occasion of Arthur Ashkin’s receipt of the 2018 Nobel prize in physics for his pioneering work in optical levitation and manipulation: A review

Gérard Gouesbet

Among the many works of Arthur Ashkin, many have been devoted to optical tweezers, optical levitation and optical manipulation of macroscopic particles (“macroscopic” being here to beunderstood as opposed to atoms or molecules). From a theoretical point of view, these experiments have been studied in the framework of two limiting regimes, namely Rayleigh regime for small size parameter and ray optics for large size parameter. The generalized Lorenz-Mie theory (GLMT, and more generally GLMTs) bridges the gap between these two regimes. The present paper therefore reviews GLMTs and mechanical effects of laser light, in Rouen where the GLMT had originally been built, but also worldwide. A story in the review concerns the first experimental validations of GLMT using optical levitation experiments.

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Generalized Lorenz-Mie theories and mechanical effects of laser light, on the occasion of Arthur Ashkin’s receipt of the 2018 Nobel prize in physics for his pioneering work in optical levitation and manipulation: A review

Gérard Gouesbet

Among the many works of Arthur Ashkin, many have been devoted to optical tweezers, optical levitation and optical manipulation of macroscopic particles (“macroscopic” being here to beunderstood as opposed to atoms or molecules). From a theoretical point of view, these experiments have been studied in the framework of two limiting regimes, namely Rayleigh regime for small size parameter and ray optics for large size parameter. The generalized Lorenz-Mie theory (GLMT, and more generally GLMTs) bridges the gap between these two regimes. The present paper therefore reviews GLMTs and mechanical effects of laser light, in Rouen where the GLMT had originally been built, but also worldwide. A story in the review concerns the first experimental validations of GLMT using optical levitation experiments.

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Determining the size and refractive index of single aerosol particles using angular light scattering and Mie resonances

Alison Bain, Aidan Rafferty, Thomas C. Preston

Optical trapping allows for high precision studies of many microphysical and chemical processes as it enables measurements on the single-particle level. This has been a tremendous benefit to fundamental aerosol research. In the vast majority of these experiments, trapped particles are characterized using light scattering – most often angular light scattering (phase functions) or Mie resonance spectroscopy. In this report, we compare the radii and refractive indices of best-fit found with these two light scattering methods by trapping single aerosol particles in a relative humidity-controlled cell where we can simultaneously measure both phase functions and Mie resonances, the latter of which are found using cavity-enhanced Raman scattering. Additionally, we compare best-fits found using both one- and two-dimensional phase functions. The application of Mie theory to these light scattering problems is thoroughly reviewed. Both the accuracy and uncertainty of the best-fits that these light scattering techniques produce are investigated using a model aqueous inorganic aerosol particle.

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Analysis of lateral binding force exerted on multilayered spheres induced by high-order Bessel beams with arbitrary polarization angles

J.Bai, Z.S.Wu, C.X.Ge, Q.C.Shang, Z.J.Li, L.Gong

Based on the generalized multi-particle Mie equation (GMM) and Electromagnetic Momentum (EM) theory, the lateral binding force (BF) exerted on multilayered spheres induced by an arbitrary polarized high-order Bessel beam (HOBB) is investigated with particular emphasis on the half-conical angle of the wave number components and the order (or topological charge) of the beam. The illuminating HOBB with arbitrary polarization angle is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz-Mie theories (GLMT). Utilizing the addition theorem of the spherical vector wave functions (SVWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the multilayered spheres is considered. Numerical results concerning the influences of different parameters of the incident Bessel beam and of the binding body on the lateral BF are displayed in detail. The observed dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction. Accurate investigation of BF induced by HOBB exerted on multilayered spheres could provide key support for further research on optical binding between more complex multilayered biological cells, which plays an important role in using optical manipulation on stratified particle self-assembly.

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Effect of nanoscale roughness on optical trapping properties of surface plasmon polaritons exerted on nanoparticle

Ge Cheng-Xian, Wu Zhen-Sen, Bai Jing, Gong Lei

Based on the three-dimensional dispersive finite difference time domain method and Maxwell stress tensor equation, the effect of nanoscale surface roughness on the optical trapping properties of nanoparticle in a vicinity of the composite gold film with periodic structure is investigated numerically. The periodic structure is observed as circular holes which can excite the surface plasmon polaritons on the metal-dielectric interface with particular emphasis on its crucial role in tailoring the optical force acting on a nearby nanoparticle. Utilizing the Monte-Carlo method, the surface roughness is added into the calculation model of the proposed method to accurately investigate the optical performance of the film-tuned nanoparticle system. Selected calculations on the effects of root mean square height and correlation length of rough surface are analyzed in detail to demonstrate that the negative effect of the surface roughness on the optical trapping force can be eliminated when the ratio of correlation length to root mean square height is equal to 10. Accurate investigation of optical trapping properties of nanoparticle in a vicinity of the composite gold film could provide guidelines for further research on the optical system design and manipulation of arbitrary composite nanoparticles.

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Optical Bessel tractor polarized beams on a charged sphere of arbitrary size

Renxian Li, Ping Li, Jiaming Zhang, Chunying Ding, Zhiwei Cui

Optical tractor beams can reel in an object towards the source, and are becoming a topic of significant worldwide research. Previous works considered the axial and transverse radiation pressure cross-sections (RPCSs) of optical tractor Bessel polarized beams on a dielectric sphere. However, most particles are charged, and it is important to investigate the tractor beam effect on charged particles. The aim of this work is therefore directed toward this goal, where the axial and transverse RPCSs for a charged sphere illuminated by a vector Bessel beam are computed in the framework of generalized Lorenz-Mie theory (GLMT). Numerical computations of the RPCSs are performed, with emphasis on the emergence of a negative pulling force and its dependence on the half-cone angle α0, the order l, and the polarization. A higher-order (l ≠ 0) Bessel beam possesses a hollow core and is of vortex nature, while the fundamental mode () is of non-vortex type and has a bright maximum intensity at the center of the beam. In our calculation, both and are considered. The axial PRCSs versus ka and α0 are first calculated, and the negative axial forces can arise. Moreover, the axial and transverse RPCSs in the plane perpendicular to the beam axis are computed. However, numerical results show that the RPCSs are same to that for a neutral particle. To explain this, the ratios of axial RPCSs for charged and neutral spheres are investigated taking ka as a parameter. The results show that charges only affect the RPCSs for small particles. Finally, the RPCSs for a charged sphere of relatively small are considered. The charge can affect the magnitude of the RPCSs, however, it does not affect the direction of axial optical forces. These results are of great importance in the development of novel optical tweezers and tractor beams.

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Optical tweezers and their applications

Paolo Polimeno, Alessandro Magazzù, Maria Antonia Iatì, Francesco Patti, Rosalba Saija, Cristian Degli Esposti Boschi, Maria Grazia Donato, Pietro G. Gucciardi, Philip H. Jones, Giovanni Volpe, Onofrio M. Maragò

Optical tweezers, tools based on strongly focused light, enable optical trapping, manipulation, and characterisation of a wide range of microscopic and nanoscopic materials. In the limiting cases of spherical particles either much smaller or much larger than the trapping wavelength, the force in optical tweezers separates into a conservative gradient force, which is proportional to the light intensity gradient and responsible for trapping, and a non-conservative scattering force, which is proportional to the light intensity and is generally detrimental for trapping, but fundamental for optical manipulation and laser cooling. For non-spherical particles or at intermediate (meso)scales, the situation is more complex and this traditional identification of gradient and scattering force is more elusive. Moreover, shape and composition can have dramatic consequences for optically trapped particle dynamics. Here, after an introduction to the theory and practice of optical forces with a focus on the role of shape and composition, we give an overview of some recent applications to biology, nanotechnology, spectroscopy, stochastic thermodynamics, critical Casimir forces, and active matter.

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Analytical description of lateral binding force exerted on bi-sphere induced by high-order Bessel beams

J.Bai, Z.S.Wu, C.X.Ge, Z.J.Li, T.Qu, Q.C.Shang

Based on the generalized multi-particle Mie equation (GMM) and Electromagnetic Momentum (EM) theory, the lateral binding force (BF) exerted on bi-sphere induced by an arbitrary polarized high-order Bessel beam (HOBB) is investigated with particular emphasis on the half-conical angle of the wave number components and the order (or topological charge) of the beam. The illuminating HOBB with arbitrary polarization angle is described in terms of beam shape coefficients (BSCs) within the framework of generalized Lorenz-Mie theories (GLMT). Utilizing the vector addition theorem of the spherical vector wave functions (SVWFs), the interactive scattering coefficients are derived through the continuous boundary conditions on which the interaction of the bi-sphere is considered. Numerical effects of various parameters such as beam polarization angles, incident wavelengths, particle sizes, material losses and the refractive index, including the cases of weak, moderate, and strong than the surrounding medium are numerically analyzed in detail. The observed dependence of the separation of optically bound particles on the incidence of HOBB is in agreement with earlier theoretical prediction. Accurate investigation of BF induced by HOBB could provide an effective test for further research on BF between more complex particles, which plays an important role in using optical manipulation on particle self-assembly.

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Optical trapping and manipulation of single particles in air: Principles, technical details, and applications

Zhiyong Gong, Yong-Le Pan, Gorden Videen, Chuji Wang

Trapping a single aerosol particle allows detailed investigation of its fundamental properties over extended time periods without external interferences. Optical trapping has developed into a powerful tool to perform such single-particle studies. However, trapping and manipulating a single particle in air, especially an irregularly shaped, absorbing particle, is much more challenging than that of a particle in a liquid solution. Even though the underlying mechanisms are not fully understood, recent experimental developments advanced the technique for trapping single particles in air, making it possible to manipulate and characterize a wide range of single particles. In this paper, we review recently demonstrated optical configurations for trapping and manipulating single airborne particles. Based on different trapping principles, we tentatively categorize them into radiation-pressure traps, photophoretic traps, and universal optical traps (UOTs). Radiation-pressure traps are based on the radiation pressure force resulting from photon momentum transfer; they include the early optical levitation configurations and the well-known optical tweezers. Photophoretic traps are based on the complex photophoretic forces that occur in absorbing particles; they are classified by the optical arrangements and include single-beam, dual-beam, and confocal-beam traps. UOTs can trap a variety of different types of particles, including transparent or absorbing, spherical or irregularly shaped, and liquid or solid particles. In order to evaluate each optical trapping scheme, four key aspects, i.e., simplicity, robustness, flexibility, and efficiency, of an optical trapping configuration are discussed. In addition to the stable optical trapping, optical manipulations from one dimension to three dimensions allow studying various single particles with great flexibility. With the single particle stably trapped and flexibly manipulated in air, other analytical techniques can be used to characterize these particles. Recent updates on optical methods for characterizing and monitoring single particles in air are discussed, such as light scattering, Raman spectroscopy, and cavity ringdown spectroscopy (CRDS).

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