Theoretical Demonstration of a Newly Designed Micro-Rotator Driven by Optical Pressure on a Light Incident Surface

A newly designed cylindrical optical micro-rotator which has slopes for trapping and rotation on its upper surface is proposed. The cylindrical shape is effective in decreasing viscous drag force (damping factor) in the medium. A ray-tracing method considering the beam waist is employed to analyze the radiation pressure exerted on the upper surface of the rotator. We have demonstrated optical trapping and high speed rotation for various optical beam parameters such as the lens numerical aperture and the Gaussian mode profile as well as rotator shape parameters including oblique angle, height and diameter.     

Rotation rate of a three-wing rotor illuminated by upward-directed focused beam in optical tweezers

The optical torque and the trapping position (focal point) in optical tweezers are analyzed for upward-directed focused laser illumination using a ray optics model, considering that laser light is incident at not only the lower surface but also the side surface of a 3-wing rotor. The viscous drag force due to the pressure and the shearing stress on all surfaces of the rotor is evaluated using computational fluid dynamics. The rotation rate is simulated in water by balancing the optical torque with the drag force, resulting in 500 rpm for an SU-8 rotor with 20 μm diameter at a laser power of 200 mW. The trapping position is estimated to be 7.6 μm in the rotor with an upward-directed laser at 200 mW via an objective lens having a numerical aperture of 1.4. Both the rotation rate and the trapping position agree well with the values obtained in the experiment.     

Improvement of the axial trapping effect using azimuthally polarised trapping beam

A dual optical tweezers system,which consists of a doughnut mode optical tweezer (DMOT) with the azimuthally polarised trapping beam and a solid mode optical tweezer (SMOT) with the Gauss trapping beam was constructed to compare the axial trapping effect of DMOT and SMOT.The long-distance axial trapping of ST68 microbubbles (MBs) achieved by DMOT was more stable than that of SMOT.Moreover the axial trapping force measured using the viscous drag method,was depended on the diameter of the particle,the laser power,and the numerical aperture (NA) of the objective lens.The measurement of the axial trapping force and the acquisition of CCD images of trapping effect confirmed that the DMOT showed excellent axial trapping ability than SMOT.A simple and effective method is developed to improve axial trapping effect using the azimuthally polarized beam as trapping beam.This is helpful for the long-distance manipulating of particles especially polarised biological objects in axial direction.     

聚焦光场俘获微球的FDTD分析

基于动量守恒原理,采用2维时域有限差分方法(2D FDTD)建立了激光场对微米量级微球的作用力模型,讨论了入射高斯光场的波长、束腰半径、微球的折射率和半径等对聚焦光场俘获力的影响.结果表明：位于聚焦光场中特定位置的微球可被俘获,当离轴距离增加,俘获力减小.微球所受到的俘获力与微球的折射率有关,当小于环境折射率时(如汽泡),不能形成俘获,而被推离光场.模拟结果与其他文献中报道的实验结果一致. 关键词： 光镊 俘获力 时域差分有限方法(FDTD) 动量守恒     

Optical Trapping with Focused Surface Waves

Near-field optical trapping can be realized with focused evanescent waves that are excited at the water–glass interface due to the total internal reflection, or with focused plasmonic waves excited on the water–gold interface. Herein, the performance of these two kinds of near-field optical trapping techniques is compared using the same optical microscope configuration. Experimental results show that only a single-micron polystyrene bead can be trapped by the focused evanescent waves, whereas many beads are simultaneously attracted to the center of the excited region by focused plasmonic waves. This difference in trapping behavior is analyzed from the electric field intensity distributions of these two kinds of focused surface waves and the difference in trapping behavior is attributed to photothermal effects due to the light absorption by the gold film.     

New calibration method for position detector for simultaneous measurements of force constants and local viscosity in optical tweezers

We present a new method to calibrate a quadrant photodiode used as position detector to monitor latex beads trapped in optical tweezers. The method combines the dragging Stoke’s force with the thermal noise analysis (power spectral density (PSD)) of the Brownian motion of the trapped bead. Position detector calibrations used in other similar methods normally utilise a bead attached to the coverslip: the voltage-position calibration factor is found by translating the bead across the waist of a laser beam. The so determined calibration factor is then assumed to be the same when beads are investigated in the optical trap. This procedure presents some drawbacks since attached beads can be affected by proximity effects due to the coverslip glass surface which alter the position sensor response itself. On the contrary, our method is able to provide, simultaneously, the calibration factor, the trap stiffness, and the local viscosity of the medium making use of a single trapped bead.     

Monte-Carlo simulation of optical trap stiffness measurement

The high precision calibration of optical trap stiffness is the foundation of the weak force measurement in an optical tweezers system. And the accuracy of the trap stiffness measurement is limited by the bandwidth of the acquisition system. In this article, such an influence is analyzed and discussed. The stiffness measuring process using an acquisition system with a finite acquisition time is numerically simulated by using Monte-Carlo method. Then the simulated results are analyzed by thermal motion analysis method to deduce the trap stiffness for different trapping system and for measuring systems with different acquisition time. As a comparison the power spectrum analysis method is used to study the thermal motion of the bead and to compute the trap stiffness for the same acquisition system, from which it is concluded that the bandwidth of the acquisition system is determined by its acquisition time, not the sampling frequency. The influence of the finite acquisition time or the limited bandwidth on the trap stiffness measurement is discussed. The numerical simulation shows that the measured position, which is here the average position within the acquisition time, shifts to the trap center due to the trapping force, which gives an alternative interpretation for the deviation of the measured stiffness from the true trap stiffness.     

阻塞态颗粒介质的慢速阻力

报导了不同尺寸的大球缓慢压入颗粒床过程中所受阻力随深度变化的实验测量结果,发现阻力曲线在不同的深度区域有不同的增长规律,存在凹—凸转变.阻力在较浅的区域满足1.3次方的幂率增长,而在较深的区域趋向0.3次方的幂率增长.通过研究凹凸转变中拐点的性质,发现这种阻力增长速度趋缓的转变不是来源于前人认为的器壁支撑导致的介质压强饱和,而是来源于侵入物自身的体积效应.此外,适用于颗粒介质快速阻力的静水压力描述并不适用于阻塞态颗粒介质的慢速阻力行为,实验表明慢速阻力对深度的依赖关系不是线性,且测得的静水压力系数也远大于理论估算值. 关键词： 颗粒介质 阻力 力链重组     

时域有限差分法数值仿真单光镊中微球受到的光阱力

本文采用三维时域有限差分法(FDTD)和Maxwell应力张量法建立了单光镊在焦点附近俘获球形微粒的光阱力模型,采用基于球矢量波函数(VSWF)的五阶高斯光源作为仿真光源,得到了准确的光场传播.讨论了光源的波长、束腰、偏振态和微球的半径、折射率对光阱力的影响,分析了在单光镊俘获微球时,邻近微球对光阱力的影响.特别研究了光源的偏振态对微球所受光阱力的作用效果,仿真结果表明圆偏振光比线偏振光对微球的俘获力更大;被光镊稳定俘获的微球,会受到邻近微球干扰,失去平衡状态,改变光源的偏振态可以改变微球的受力状态. 关键词： 光镊 光阱力 介质微球 时域有限差分法(FDTD)     

Trapping multiple particles in single optical tweezers

Optical trapping of multiple particles in three dimensions in a single inverted optical tweezers is investigated. The effect of trapping pairs of beads on the voltage signal recorded by a quadrant photo diode detector is examined, with particular emphasise on its power spectral density. It is found that trapped pairs of beads could be mistaken for single beads in a trap of half the strength. Stokes drag measurements reveal that both beads of the pair are confined less than a single particle in the same trap and that the quadrant signal provides some information of their mean displacements.     

光镊力影响因素的计算分析

运用基于T矩阵算法的开源光镊计算工具包对可能影响光镊力的微粒尺寸、相对折射率以及光束模式进行了研究,计算结果表明,这三方面因素都会对光镊力产生显著影响,微粒直径与波长相等、相对折射率尽可能大时选择恰当的光束模式能够产生最佳的光镊捕获效果.     

Radiation torque and force on optically trapped linear nanostructures

We study the optical trapping of highly elongated linear nanostructures in the focal region of a high-numerical aperture lens (optical tweezers). The radiation torque and trapping force on these nanostructures that are modeled as chains of identical spherical scatterers are calculated by means of multipole field expansions in the framework of the transition matrix approach. We investigate both orientational and trapping stability and calculate force constants and trap parameters in order to clarify the role of the linear geometry in the optical trapping mechanism. Furthermore, we calculate optical trapping of nanowires of different materials and compare our theoretical findings with available experimental results.     

非线性晶体产生的空心光束中大尺寸粒子囚禁与导引

提出了一种基于非线性ZnSe晶体产生的空心光束与光泳力的大尺寸粒子二维囚禁与一维导引、三维囚禁方案.理论上分析并计算了单个非线性ZnSe晶体产生的空心光束内粒子受到的横向与纵向光泳力,纵向光泳力的大小同粒子尺寸与光束尺寸比例的四次方成正比,与空心光束功率成正比,方向与光束传播方向一致.粒子尺寸与空心光束尺寸越接近时,横向光泳力的大小越大.结果表明该光泳力可以实现对大尺寸粒子的二维囚禁,同时可对粒子进行长距离(米量级)一维定向导引;理论上分析并计算了基于双非线性ZnSe晶体产生的局域空心光束内粒子所受横向与纵向光泳力情况,光泳力与系统参数的依赖关系与单个非线性晶体产生的空心光束中的粒子受力情况类似,不同的是该条件下纵向光泳力指向光束中心.结果表明该局域空心光束可以实现大尺寸粒子的三维有效囚禁.基于非线性ZnSe晶体产生的空心光束或者局域空心光束可以作为大尺寸粒子非接触式有效操控的工具,在现代光学以及生物医学中有潜在的应用.     

A study of multi-trapping of tapered-tip single fiber optical tweezers

We develop a pair of tapered-tip single fiber optical tweezers, and study its multi-trapping characteristic. The finite difference time domain method is employed to simulate the trapping force characteristic of this pair of single fiber optical tweezers, and the results show that the number of trapped particles depends on the refractive index and the size of the particles. The trapping force of this pair of tapered-tip single fiber optical tweezers is calibrated by the experimental method, and the experimental results are consistent with the theoretical calculation results. The multi-trapping capability realized by the tapered-tip single fiber optical tweezers will be practical and useful for applications in biomedical research fields.     

Polarization-induced stiffness asymmetry of optical tweezers

A tightly focused, linearly polarized laser beam, so-called optical tweezers, is proven to be a useful micromanipulation tool. It is known that there is a stiffness asymmetry in the direction perpendicular to the optical axis inherited from the polarization state of the laser. In this Letter, we report our experimental results of stiffness asymmetry for different bead sizes measured at the optimal trapping condition. We also provide the results of our generalized Lorenz-Mie based calculations, which are in good agreement with our experimental results. We also compare our results with previous reports.     

Atomistic simulations of nonequilibrium crystal-growth kinetics from alloy melts

Nonequilibrium kinetic properties of alloy crystal-melt interfaces are calculated by molecular-dynamics simulations. The relationships between the interface velocity, thermodynamic driving force, and solute partition coefficient are computed and analyzed within the framework of kinetic theories accounting for solute trapping and solute drag. The results show a transition to complete solute trapping at high growth velocities, establish appreciable solute drag at low growth velocities, and provide insights into the nature of crystalline anisotropies and solute effects on interface mobilities.     

Multifocal optical trapping using counter-propagating radially-polarized beams

A model of optical tweezers which can trap a chain of Rayleigh particles is proposed by using two counter-propagating equal highly focused radial polarized beams. Calculations show that a multifocal distribution along the optical axis is formed and the scattering force is equal to zero in the total focal filed, consequently a chain of metallic Rayleigh particles can be stably trapped. The trap force and the trap stiffness using two counter-propagating Radially-polarized beams are larger than those using two counter-propagating linearly-polarized beams. The trapping stability is calculated and analyzed in detail. The trapping number of particles in a trapping chain can be controlled by adjusting the aperture angle of the objective and the parameters of the filter used in the proposed trap system.     

Optimal beam diameter for lateral optical forces on microspheres at a water-air interface

Optical tweezers with a low numerical aperture microscope objective is used to manipulate the microspheres at the water-air interface. In this letter, we determine the optimal optical trap for the lateral manipulation of microspheres at a water-air interface. The experimental results show that the trapping force is influenced by the expansion of the trapping beam at the back aperture of the objective. The optimal filling ratio of 0.65 is suggested for lateral optical manipulation at the water-air interface. The lateral trapping forces at the water-air interface are theoretically investigated with the ray-optics model. The numerical results show that the lateral trapping forces can be changed by shrinking the diameter of the trapping laser beam. The numerical results are in accordance with the experimental results.     

Enhancement of Axial Trapping Force in a Single-beam Gradient Optical Trap by a Pair of Axicons

1　Ｉｎｔｒｏｄｕｃｔｉｏｎ　　ＴｈｅｆｉｒｓｔｗｏｒｋｗｉｔｈｏｐｔｉｃａｌｔｒａｐｐｉｎｇｏｎｓｍａｌｌｐａｒｔｉｃｌｅｓｗｅｒｅｄｏｎｅｂｙＡｓｈｋｉｎｅｔａｌ.ｉｎ1 985ａｎｄ 1 986 [1,2 ] ,Ｗｈｏｓｕｃｃｅｓｓｆｕｌｌｙｄｅｍｏｎｓｔｒａｔｅｄｔｈａｔｍｉｃｒｏｓｃｏｐｉｃｐａｒｔｉｃｌｅｓｃｏｕｌｄｂｅｔｒａｐｐｅｄｂｙａｓｉｎｇｌｅｆｏｃｕｓｓｅｄｌａｓｅｒｂｅａｍ .Ａｎｕｍｂｅｒｏｆｄｅｍｏｎｓｔｒａｔｉｏｎｓｏｆｔｒａｐｐｉｎｇａｎｄｍａｎｉｐｕｌａｔｉｏｎｏｆｂｉｏｌｏｇｉｃａｌ…     

激光捕获技术及其发展

激光捕获技术是利用光辐射力来捕捉、移动和操纵微粒的先进技术。光镊即单光束梯度力光阱是通过在高度会聚的激光束束腰附近所产生的极高的场强梯度来形成皮牛顿量级的力,可以三维地捕获和操纵微小粒子。阐述了激光捕获技术的模型和原理以及系统的基本结构;追踪了激光捕获技术的最新研究进展;介绍了非高斯型光阱、光纤光阱和全息光镊等几种特殊形式,并分析了每种形式的特点。展望了激光捕获技术的发展前景。