轴向多光阱微粒捕获与实时直接观测技术

传统的光镊技术使用单个物镜同时进行光学捕获与显微成像,使得捕获与成像区域被限制在物镜焦平面附近,无法同时观察到沿光轴方向(即Z向)捕获的多个微粒.本文提出一种轴平面(XZ平面)GerchbergSaxton迭代算法来产生沿轴向分布的多光阱阵列,将轴平面成像技术与光镊结合,实现了沿轴向对二氧化硅微球的多光阱同时捕获与实时观测.通过视频分析法测量了多个二氧化硅微球在轴向光镊阵列中的布朗运动,并标定了光阱刚度.本文提出的轴向多光阱微粒捕获与实时观测技术为光学微操纵提供了一个新的观测视角和操纵方法,为生物医学、物理学等相关领域研究提供了一种新的技术手段.     

Observing Nanometre Scale Particles with Light Scattering for Manipulation Using Optical Tweezers

Nanometre-scale particles can be manipulated using optical tweezers, but cannot be directly observed. We present a simple method that nanoparticles can be directly observed using optical tweezers combined with dark field microscopy. A laser beam perpendicular to a tightly focused laser beam for trap illuminates specimen and does not enter objective, nanoparticles in focal plane all can be directly observed in dark field because of light scattering. It is implemented that the polystyrene beads of diameter lOOnm can be directly observed and trapped.     

光学微操纵过程的轴平面显微成像技术

光学俘获技术利用光与物质相互作用产生的光势阱效应来实现对微粒的操控,已经成功应用于生物医学、材料科学等交叉领域.在对微粒进行三维俘获时,传统的宽场光学显微技术只能观测到某一平面内微粒的横向运动,对微粒沿轴向运动的观测受到很大限制.本文将轴平面显微成像技术引入光学微粒操控研究中,利用45?倾斜的反射镜把微粒的轴向运动信息转换到横向平面进行观测,与传统宽场显微成像技术相结合,实现了对二氧化硅小球俘获过程横向和轴向运动的同步观测.该成像方法无需扫描和数据重构,具有实时快速等优点,在新型光束光镊、厚样品三维观测和成像等领域具有潜在的应用价值.     

Manipulation and spectroscopy of a single particle by use of white-light optical tweezers

We demonstrate, for the first time to our knowledge, three-dimensional (3D) trapping and manipulation of microscopic objects by use of supercontinuum white light generated from photonic crystal fibers. Furthermore, we show that the supercontinuum white-light optical tweezers used have the unique capability to perform optical scattering spectroscopy of a single 3D trapped object over a broad wavelength range. These novel tweezers can potentially open a promising avenue toward simultaneous manipulation and characterization of microscopic objects.     

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。     

Trapping cavitation bubbles with a self-focused laser beam

We observed that laser-induced cavitation bubbles in water can be trapped in a self-focused laser beam. Both optical imaging and acoustic detection have been utilized to confirm bubble trapping. Transverse and longitudinal trapping forces were measured to be as large as 87 and 11 pN, respectively. This result is contrary to conventional wisdom, since the mechanism of trapping in conventional optical tweezers implies that a low-index particle (a bubble being the limiting case) should be antitrapped.     

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.     

Measurement of elastic light scattering from two optically trapped microspheres and red blood cells in a transparent medium

Optical tweezers can be used to manipulate small objects and cells. A trap can be used to fix the position of a particle during light scattering measurements. The places of two separately trapped particles can also be changed. In this Letter we present elastic light scattering measurements as a function of scattering angle when two trapped spheres are illuminated with a He-Ne laser. This setup is suitable for trapping noncharged homogeneous spheres. We also demonstrate measurement of light scattering patterns from two separately trapped red blood cells. Two different illumination schemes are used for both samples.     

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.     

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.     

Photocatalytic nano-optical trapping using TiO2 nanosphere pairs mediated with Mie-scattered near field

The localized enhanced near field on nanostructures has been attracting much attention for a template for size-selective optical trapping (tweezers) beyond the diffraction limit. The near-field optical trapping has mainly been studied using metallic substrates such as Au nanodot pairs, periodic Al nanoslits, nanoapertures on an Au film, etc. In this paper, we newly propose a Mie-scattered-near-field optical trapping scheme for size-selective photocatalytic application using pairs of poly-rutile TiO₂ nanospheres. The optical intensity distribution in a 3D-nanogap space between the nanospheres was simulated by a 3D FDTD method. The simulation system consists of the two TiO₂ nanospheres placed on a silica substrate in water. The 400-nm excitation laser is used for both the near-field trapping and the photocatalyst excitation. The optical trapping forces were calculated based on the near-field optical intensity distribution. The trapping stiffness for 20-nm polystyrene sphere at a gap distance of 20 nm was 6.4 pN/nm/W. The optical force vector shows that the object like virus can be trapped with sufficient forces into the nanogap space and then is driven into the direct surface of the TiO₂ sphere. This result suggests that this system works as a photocatalytic trapping for killing virus, protein, etc.     

Laser-induced hybrid trap for micro-bubbles

Micro-bubbles have been stably trapped in liquid ethanol by a focused argon laser beam. Two equilibrium points where bubbles can be trapped are observed to be at the center and the rim of the laser beam. The light force is also able to push a bubble into the liquid to a position well below the liquid surface. Both the light-pressure force and the fluid force induced by the convection of the liquid medium are calculated. The results clearly show that these two kinds of forces account for the formation of trapping potential. Received: 3 February 2000 / Revised version: 6 July 2000 / Published online: 8 November 2000     

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.     

一种用于细胞操作的单光纤光镊研究

采用一种抛物线形光纤针作为基本器件,设计制作了单光纤光镊系统.用时域有限差分（Finite Difference Time Domain,FDTD）的方法仿真了抛物线型光纤探针的出射场,并在稳态场下通过对麦克斯韦应力张量积分求出不同介质球半径和折射率情况下,介质球受到的横向和纵向光作用力.实验中此单光纤光镊系统实现了对水中酵母细胞的空间捕获,并且结合两个抛物线形光纤针实现了酵母细胞在两个光势阱中的转移交接.这种单光纤光镊系统结构简单、紧凑,操纵灵活,便于调整.可以适应更多的生物细胞和生物分子的光微操作需求.     

Calculation and optical measurement of laser trapping forces on non-spherical particles

Optical trapping, where microscopic particles are trapped and manipulated by light is a powerful and widespread technique, with the single-beam gradient trap (also known as optical tweezers) in use for a large number of biological and other applications. The forces and torques acting on a trapped particle result from the transfer of momentum and angular momentum from the trapping beam to the particle. Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult. Calculations can be performed using approximate methods, but are only applicable within their ranges of validity, such as for particles much larger than, or much smaller than, the trapping wavelength, and for spherical isotropic particles. This leaves unfortunate gaps, since wavelength-scale particles are of great practical interest because they are readily and strongly trapped and are used to probe interesting microscopic and macroscopic phenomena, and non-spherical or anisotropic particles, biological, crystalline, or other, due to their frequent occurance in nature, and the possibility of rotating such objects or controlling or sensing their orientation. The systematic application of electromagnetic scattering theory can provide a general theory of laser trapping, and render results missing from existing theory. We present here calculations of force and torque on a trapped particle obtained from this theory and discuss the possible applications, including the optical measurement of the force and torque.     

亚微米局域空心光束的产生及其在单原子囚禁与冷却中的应用理论研究

提出了一种采用单模光纤、环形二元相位板和微透镜组成的光束整形系统产生亚微米局域空心光束的方案. 根据瑞利-索莫菲衍射积分公式, 数值计算了微透镜焦平面附近的场分布, 详细研究了空心光束的暗斑尺寸与单模光纤模场半径和微透镜焦距的关系. 数值计算结果表明: 在微透镜焦平面附近光场分布近似对称, 在焦点处场强近似为零, 周围场强逐渐增大, 形成半径约为0.4 μm的三维封闭的球形空心光场区域, 即亚微米局域空心光束. 当局域空心光束为蓝失谐时, 光场中的原子将被囚禁在光场最弱处. 若加上抽运光, 原子将受到蓝失谐局域空心光束与抽运光共同激发的强度梯度Sisyphus冷却. 本文利用该方案产生的亚微米局域空心光束构建单原子的囚禁与冷却器件, 并以单个⁸⁷Rb原子为例, 利用Mont-Carlo方法研究亚微米局域空心光束中单原子囚禁与强度梯度冷却的动力学过程, 结果表明利用该器件可以获得温度在5.8 μK量级的超冷单原子.     

红外显微观测被俘获吸光性颗粒

光镊技术被广泛应用在俘获和操纵微纳米尺寸颗粒, 目前被研究学者普遍接受的俘获吸光性颗粒的机理为光泳力. 本文实现了对空气中被俘获的吸光性颗粒的红外显微观测. 当激光器功率为1.0 W时, 成功观测到被俘获墨粉颗粒(直径约7 μm)和甲苯胺蓝颗粒(直径约为1–20 μm)的温升约为14 K, 为光泳力理论提供了强有力的证据. 另外, 首次用可见光显微镜和红外显微镜同时观测到被俘获颗粒的周期振荡现象, 并分析了振荡现象的产生机理. 关键词： 光镊 光俘获 红外显微     

Laser manipulation of a smectic liquid-crystal droplet

The laser trapping of a smectic-A liquid-crystal micro-droplet was spatially traced during its transient into the trapped position. The lateral and angular orientation of the droplet were determined and followed in time during the axial descent of the micro-droplet into the stationary trapped position using the analysis of polarization changes of the light passed through the droplet with temporal resolution of a video refresh rate of 30 ms. The spatial resolution of 0.1-1μm has been achieved for typical laser trapping powers of 2-600 mW. The axial profile of a laser trapping force (an ellipticity of the focal spot) has been determined. The laser trapping mechanism of smectic micro-droplets is discussed in terms of minimization of a light-droplet interaction.     

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)