激光功率对光镊光阱刚度标定的影响

光阱刚度是描述光镊对粒子进行操控的重要力学指标,实际使用过程中会受到激光功率的影响。采用均方位移法及玻尔兹曼统计法对搭建的光镊系统进行光阱刚度的标定,利用图像采集方法进行微粒位移的测量,并对两种方法的测量结果进行了比较。为了提高光阱刚度的标定结果的准确性,分析了光路放大倍数、温度变化对最后标定结果的精度影响。结果表明,两种方法进行标定的结果基本相同;光阱刚度在低激光功率（1 mW ~20 mW）范围时随功率近似线性增加,在高功率情况下（25 mW~60 mW）随功率增加不再线性增加,而是趋于一个饱和值。此外,光路放大倍数标定的精确性对标定的精度影响较大,10%的相对误差时,标定结果产生23%的变化,温度对标定的精度影响较小,0.1 ℃的温度变化导致标定结果0.034%的变化。     

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.     

Videomicroscopy calibration of optical tweezers by position autocorrelation function analysis

We present a simple method to calibrate optical tweezers by using only videomicroscopy to measure the position autocorrelation function of the trapped bead in the potential well of the tweezers. To accomplish this task, we use a high-speed camera with ??500 fps (frames per second), which provides a precise measurement of the relaxation time of the bead Brownian fluctuations. We also study the variation of the trap stiffness as a function of some parameters of interest such as the laser power and the distance from the bead center to the microscope coverslip, showing that the presented method returns precise results.     

小型化可调光镊设计

光镊利用强会聚激光对微粒产生的梯度力来捕获微粒,可以进行无损、远程操控,同时具有皮牛精度的测力特性,已经成为物理学、生命科学和胶体化学等研究领域中不可缺少的研究工具。光镊效应可以表现微小的光子动量和角动量,是物理学的重要教学工具。本文根据高斯光束传播和变换规律,设计具有稳定捕获性能的最小化光镊,并给出了典型参数。光镊系统由捕获激光、光束耦合系统、倒置生物显微镜和大数值孔径物镜组成,成像系统由物镜、摄影目镜和CCD相机组成。本光镊系统具有紧凑特性,同时通过保持物镜后瞳充满度来实现稳定捕获。在该最小光镊系统上,可以根据用户需求增加光镊阱位操控系统、刚度调节系统和其他辅助设备以满足不同操控要求,可以很好地满足科研和教学需求。     

利用光镊计算工具箱实现光镊系统的参量评价

光镊所捕获的微球尺度常常落在中间尺度上,导致相关参量难于计算.OTT1光镊工具箱是一种基于广义Lorenz-Mie理论的T-matrix方法,它的发展使得对光镊系统的详细计算和评价成为可能.本文对光镊的轴向捕获特性曲线、线性性和刚度,以及杜克系列微球的互换性做了计算和评价.结果表明:光镊所用物镜的数值孔径越接近水的折射率捕获效果越好;0.8~1.2 μm的聚苯乙烯微球组成的光镊刚度较大;直径在2 μm以下的聚苯乙烯微球组成的光镊线性度较好;0.8~2 μm的Duke系列聚苯乙烯微球的互换性较好,便于纳米光镊的修正与实验;要避免米共振微球的直径要在2.5 μm以下.     

Computer simulation of the collision frequency of two particles in optical tweezers

Optical tweezers have been successfully used in the study of colloid science. In most applications people are concerned with the behaviour of a single particle held in the optical tweezers. Recently, the ability of the optical tweezers to simultaneously hold two particles has been used to determine the stability ratio of colloidal dispersion. This new development stimulates the efforts to explore the characteristics of a two-particle system in the optical tweezers.An infinite spherical potential well has been used to estimate the collision frequency for two particles in the optical trap based on a Monte Carlo simulation. In this article, a more reasonable harmonic potential, commonly accepted for the optical tweezers, is adopted in a Monte Carlo simulation of the collision frequency. The effect of hydrodynamic interaction of particles in the trap is also considered. The simulation results based on this improved model show quantitatively that the collision frequency drops down sharply at first and then decreases slowly as the distance between the two particles increases. The simulation also shows how the collision frequency is related to the stiffness of the optical tweezers.     

Optical tweezers for confocal microscopy

In confocal laser scanning microscopes (CLSMs), lasers can be used for image formation as well as tools for the manipulation of microscopic objects. In the latter case, in addition to the imaging lasers, the light of an extra laser has to be focused into the object plane of the CLSM, for example as optical tweezers. Imaging as well as trapping by optical tweezers can be done using the same objective lens. In this case, z-sectioning for 3D imaging shifts the optical tweezers with the focal plane of the objective along the optical axis, so that a trapped object remains positioned in the focal plane. Consequently, 3D imaging of trapped objects is impossible without further measures. We present an experimental set-up keeping the axial trapping position of the optical tweezers at its intended position whilst the focal plane can be axially shifted over a distance of about 15 μm. It is based on fast-moving correctional optics synchronized with the objective movement. First examples of application are the 3D imaging of chloroplasts of Elodea densa (Canadian waterweed) in a vigorous cytoplasmic streaming and the displacement of zymogen granules in pancreatic cancer cells (AR42 J). Received: 24 March 2000 / Revised version: 23 June 2000 / Published online: 11 October 2000     

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.     

有限远共轭显微镜光镊设计和误差分析

光镊是研究单分子生物物理特性的独特工具, 因而光镊设备的研发是一个极为重要的课题. 本文根据矩阵光学, 对基于有限远共轭显微镜的光镊操控光路进行计算, 得出了阱位径向操控和轴向操控方程, 并分析了光束调控系统、 共焦系统后置透镜和耦合透镜安装位置误差及物镜轴向位置调整对光镊阱位径向及轴向操控精度的影响. 计算结果显示, 当物镜初级像面和耦合透镜像方焦面完全重合, 光束调控系统和耦合透镜的距离误差对阱位径向和轴向操控精度没有影响. 光镊系统元器件定位不准时, 基于无限远共轭显微镜光镊的阱位径向操控误差和轴向操控误差都小于基于有限远共轭显微镜光镊的阱位径向操控误差和轴向操控误差. 当光镊耦合透镜定位误差控制在小于10 mm时, 基于有限远共轭显微镜光镊的径向和轴向操控误差分别小于5.9%和11.4%, 有限远共轭显微镜仍然存在改造为光镊的价值.本文理论为基于有限远共轭显微镜的光镊设计、改造和操控提供理论和实验指导. 关键词： 光镊 光学设计 矩阵 误差     

泽尼克多项式校正全息阵列光镊像差的实验研究

像差会影响光镊对粒子的捕获效果. 全息阵列光镊中, 像差不仅来自光学元件, 由特定算法设计的光阱相位片也会在光路中引入像差. 本文通过液晶空间光调制器加载泽尼克多项式相位图, 对全息阵列光镊中由光栅透镜组型算法引起的像差进行校正. 结果显示: 利用三阶泽尼克多项式可有效消除光路中由光栅透镜组型算法引 起的慧差, 使得捕获2 μm聚苯乙烯小球的阵列光阱刚度提高了约40%; 对比不同项的像差校正结果发现, 全息阵列光镊中由算法引起的慧差 与光学元件引起的像差一样, 也会对阵列光阱的捕获效果产生较大影响; 同时根据一阶像差校正结果可得光栅透镜 组型算法对于一阶泽尼克像差具有鲁棒性. 实验结果表明, 对全息阵列光镊中由 算法引起的像差进行校正, 对于提高光阱的捕获效果和深化对算法特性的认识都具有重要意义.     

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.     

Trap position control in the vicinity of reflecting surfaces in optical tweezers

Shift of the trap position from the laser beam waist of optical tweezers is studied experimentally in the presence of a reflecting surface in the vicinity of the focal plane. A standing wave is formed owing to the interference of waves forming the waist and reflected from the surface. The standing wave is shown to affect significantly the resulting trap position. The distance between the surface and the stable optical trap as a function of the trapped particle size is studied numerically. A new method to stabilize the position of the microparticle relative to the surface is proposed. The localization accuracy is determined by the Brownian fluctuations in optical tweezers and is about 10 nm for effective trap stiffness of 4 × 10^?5 N/m.     

Measurement of the effective focal shift in an optical trap

The focus of an oil-immersion microscope objective is shifted because of the refractive-index mismatch between the cover glass and the aqueous sample. We present a procedure with which to determine the focal shift by use of an inverted microscope equipped with optical tweezers. As the position of the sample chamber is scanned vertically, we measure the axial displacement of an optically trapped bead; the relative motion of the bead with respect to the surface supplies the effective focal shift. Measurements of this quantity deviate from electromagnetic calculations of the focal shift, a discrepancy attributable to the depth-dependent decrease in axial trap stiffness that arises from spherical aberration.     

Surface plasmon optical tweezers: tunable optical manipulation in the femtonewton range

We present a quantitative analysis of 2D surface plasmon based optical tweezers able to trap microcolloids at a patterned metal surface under low laser intensity. Photonic force microscopy is used to assess the properties of surface plasmon traps, such as confinement and stiffness, revealing stable trapping with forces in the range of a few tens of femtonewtons. We also investigate the specificities of surface plasmon tweezers with respect to conventional 3D tweezers responsible for their selectivity to the trapped specimen's size. The accurate engineering of the trapping properties through the adjustment of the illumination parameters opens new perspectives in the realization of future optically driven on-a-chip devices.     

采用超连续谱激光的双光束光纤光阱实验

以超连续谱激光器作为捕获光源,首次提出并搭建了超连续谱双光束光纤光阱实验系统,实现了聚苯乙烯微球的捕获和操控。通过改变光纤端面间隔和调整捕获光功率的方式精确控制微球的位置,采用CCD图像分析方法实现了微球位置的精确测量。对微球受限布朗运动下的位置变化进行傅里叶变换,计算得到功率谱,与理论功率谱函数拟合后求出了其光阱刚度。结果表明,捕获光束的功率为28 mW时,光阱刚度达到1.3×10-6N/m,高于相同实验条件下单波长光纤光阱的刚度。与传统采用单色光作为捕获光源的光镊系统不同,超连续谱双光束光阱系统利用其宽谱优势,通过研究被捕获微粒的散射光谱信息可获取其尺寸、折射率等物理特征参数。     

Manipulation on human red blood cells with femtosecond optical tweezers

Different types of femtosecond optical tweezers have become a powerful tool in the modern biological field. However, how to control the irregular targets, including biological cells, using femtosecond optical tweezers remains to be explored. In this study, human red blood cells (hRBCs) are manipulated with femtosecond optical tweezers, and their states under different laser powers are investigated. The results indicate that optical potential traps only can capture the edge of hRBCs under the laser power from 1.4 to 2.8 mW, while it can make hRBCs turn over with the laser power more than 2.8 roW. It is suggested that femtosecond optical tweezers could not only manipulate biological cells, but also subtly control its states by adjusting the laser power.     

Monte-Carlo simulation of effective stiffness of time-sharing optical tweezers

<正>The Brownian motion of a polystyrene bead trapped in a time-sharing optical tweezers(TSOT) is numerically simulated by adopting Monte-Carlo technique.By analyzing the Brownian motion signal,the effective stiffness of a TSOT is acquired at different switching frequencies.Simulation results confirm that for a specific laser power and duty ratio,the effective stiffness varies with the frequency at low frequency range,while at high frequency range it keeps constant.Our results reveal that the switching frequency can be used to control the stability of time-sharing optical tweezers in a range.     

Manipulation of Nanoparticles Using Dark-Field-Illumination Optical Tweezers with Compensating Spherical Aberration

Based on our previous investigation of optical tweezers with dark field illumination [Chin. Phys. Left. 25（2008）329] nanoparticles at large trap depth are better viewed in wide field and real time for a long time, but with poor forces. Here we present the mismatched tube length to compensate for spherical aberration of an oil-immersion objective in a glass-water interface in an optical tweezers system for manipulating nanoparticles. In this way, the critical power of stable trapping particles is measured at different trap depths. It is found that trap depth is enlarged for trapping nanoparticles and trapping forces are enhanced at large trap depth. According to the measurement, 70-nm particles are manipulated in three dimensions and observed clearly at large appropriate depth. This will expand applications of optical tweezers in a nanometre-scale colloidal system.     

Numerical modeling and theoretical analysis of femtosecond laser tweezers

Optical trapping (also called optical tweezers) is a widespread technique, with a large number of applications in biology and other fields. Taking femtosecond laser pulses as a sampling of CW light, we theoretically demonstrate the feasibility of femtosecond laser tweezers and present the formulae of the induced forces which femtosecond laser pulses exert on micrometer-sized spheres. We also demonstrate the stability condition for femtosecond laser tweezers. As an example, we present the numerical results for a sphere with a radius of 10 mm.     

Optical tweezers technique and its applications

Since their advent in the 1980s,optical tweezers have attracted more and more attention due to their unique non-contact and non-invasion characteristics and their wide applications in physics,biology,chemistry,medical science and nanoscience.In this paper,we introduce the basic principle,the history and typical applications of optical tweezers and review our recent experimental works on the development and application of optical tweezers technique.We will discuss in detail several technological issues,including high precision displacement and force measurement in single-trap and dual-trap optical tweezers,multi-trap optical tweezers with each trap independently and freely controlled by means of space light modulator,and incorporation of cylindrical vector optical beams to build diversified optical tweezers beyond the conventional Gaussian-beam optical tweezers.We will address the application of these optical tweezers techniques to study biophysical problems such as mechanical deformation of cell membrane and binding energy between plant microtubule and microtubule associated proteins.Finally we present application of the optical tweezers technique for trapping,transporting,and patterning of metallic nanoparticles,which can be harnessed to manipulate surface plasmon resonance properties of these nanoparticles.