Experimental Analysis of Optical Trapping System Using Tapered Hemispherically Lensed Optical Fiber

We describe the operation and performance of an optical fiber trap realized using a tapered hemispherically lensed optical fiber. Axial and transverse trapping forces exerted on a microsphere are experimentally analyzed to corroborate the optical trapping using an optical fiber. Experimental results are as follows. (i) Transverse force F_tr acting on a sphere is a restoring force that acts to pull the microsphere back to the center of trap. (ii) Axial force F_ax always acts to push a sphere in the direction of the beam away from the trapping fiber end. (iii) Vector sum of F_tr and F_ax acting on a sphere gives a restoring force directed back to the stable point. (iv) Transverse force F_tr plays a significant role in trapping a micro-sized object by means of an optical fiber.This paper was originally presented at the 5th International Conference on NEAR FIELD OPTICS and RELATED TECHNOLOGIES (NFO-5), which was held on December 6–10, 1998 at Coganoi Bay Hotel, Shirahama, Japan, in cooperation with the Japan Society of Applied Physics and Mombusho Grant-in Aid for Scientific Research on Priority Areas “Near-field Nano-optics” Project, sponsored by Japan Society for the Promotion of Science.     

Single laser beam fiber optic trap

Optical forces acting on a sphere were experimentally analyzed to investigate the single-beam fiber optic trap using a cleaved optical fiber or a lensed optical fiber. A stable optical trap could be created at the point where the x-directed (horizontal) optical forces were precisely balanced, and the vector sum of axial and transverse forces acting on a sphere gave a restoring force directed back to the stable point. As compared with other embodiments, such as a single-beam gradient trap (optical tweezers) and dual-beam fiber optical traps, this single-beam fiber optic trapping was most economical, much simpler to operate, and required relatively low optical power to capture an object. Furthermore, a lensed optical fiber could easily trap and manipulate a micro object in comparison with a cleaved optical fiber because of the strong transverse optical confinement.     

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.     

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.     

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.     

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     

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.     

Quantitative analysis of charge-carrier trapping in organic thin-film transistors from transfer characteristics

A dynamic method for quantifying the amount and mechanism of trapping in organic field effect transistors (OFETs) is proposed. It exploits transfer characteristics acquired upon application of a triangular waveform gate sweep V _G. The analysis of the transfer characteristics at the turning point V _G=−V _max between forward and backward gate sweeps, viz. around the maximum gate voltage V _max applied, provides a differential slope Δm which depends exclusively on trapping. Upon a systematic change of V _max it is possible to extract the initial threshold voltage, equivalent to one of the observables of conventional stress measurements, and assess the mechanism of trapping via the functional dependence on the current. The analysis of the differential logarithmic derivative at the turning point yields the parameters of trapping, as the exponent β and the time scale of trapping τ. In the case of an ultra-thin pentacene OFET we extract β=1 and τ=10²–10³ s, in agreement with an exponential distribution of traps. The analysis of the hysteresis parameter Δm is completely general and explores time scales much shorter than those involved in bias stress measurements, thus avoiding irreversible damage to the device.     

Integration of fluorescence collection optics with?a?microfabricated surface electrode ion trap

We have successfully demonstrated an integrated optical system for collecting the fluorescence from a trapped ion. The system, consisting of an array of transmissive, dielectric micro-optics and an optical fiber array, has been intimately incorporated into the ion-trapping chip without negatively impacting trapping performance. Epoxies, vacuum feedthrough, and optical component materials were carefully chosen so that they did not degrade the vacuum environment, and we have demonstrated light detection as well as ion trapping and shuttling behavior comparable to trapping chips without integrated optics, with no modification to the control voltages of the trapping chip.     

Improvement of optical trapping effect by using the focused high-order Laguerre-Gaussian beams

We investigate the optical trapping effect of high-order Laguerre-Gaussian (LG) beams acting on a dielectric sphere in Rayleigh regime. For LG beams with the azimuthal mode index l=0, it is found that under the same input power, the transverse trapping effect can be enhanced several times with increasing the radial mode index p, compared with that of the Gaussian beam; while its axial trapping effect is exactly the same as that of Gaussian beam, although the central trapping region reduces as p increases. For LG beams with l≥1, we find that the maximal transverse gradient forces increase with the increasing of p and the axial radiation forces reduces slightly, therefore an optimal choose on p and l is necessary for obtaining an optimal optical guiding. Our result is useful for analyzing the trapping efficiency of LG beams applied in micromanipulation technologies.     

Systematical study of the trapping forces of optical tweezers formed by different types of optical ring beams

The technique of optical tweezers has been improved a lot since its invention, which extends the application fields of optical tweezers. Besides the conventionally used Gaussian beams, different types of ring beams have also been used to form optical tweezers for different purposes. The two typical kinds of ring beams used in optical tweezers are the hollow Gaussian beam and Laguerre--Gaussian (LG) beam. Both theoretical computation and experiments have shown that the axial trapping force is improved for the ring beams compared with the Gaussian beam, and hence the trapping stability is improved, although the transverse trapping forces of ring beams are smaller than that of Gaussian beam. However, no systematic study on the trapping forces of ring beam has ever been discussed. In this article, we will investigate the axial and transverse trapping forces of different types of ring beams with different parameters systematically, by numerical computation in which the ray optics model is adopted. The spherical aberration caused by the refractive index mismatch between oil and water is also considered in the article. The trapping forces for different objectives that obey the sine condition and tangent condition are also compared with each other. The result of systematical calculation will be useful for the applications of optical tweezers formed by different types of ring beams.     

Improvement of Transverse Trapping Efficiency of Optical Tweezers

Transverse trapping efficiency of optical tweezers is important in many force measurement applications. For improving the transverse trapping efficiency, we propose a simple scheme in which the Gaussian beam does not fully cover the aperture of the objective. Both experiment and theoretical simulation qualitatively demonstrate the scheme. It is expected that the results will be useful for the design of optical tweezers.     

高斯光束中吸收双层球形微粒的横向光俘获

为了研究吸收双层球形微粒的横向光俘获,基于几何光学模型提出了双层带吸收球形微粒的光俘获模型,对TEM₀₀模式高斯光束照射下外层有光吸收的双层电介质球形微粒受到的横向光俘获力进行了数值模拟,取得了光俘获力特性的一系列结果.结果显示,双层球形微粒的外层吸收系数对包括稳态俘获位置,峰值强度,稳态俘获的刚度等光俘获特性有很大影响.此外,内外径的比率对吸收双层球形微粒的光俘获特性也有调制性的影响.在一定条件下,带吸收的双层球形微粒可以被俘获在光轴上,也可能被俘获在中心在光轴上的圆环上. 关键词： 光俘获 几何光学模型 高斯光束 吸收双层球     

Optimized free-form optical trapping systems

We report a comprehensive process for designing and prototyping new and optimized optical trapping systems. A combination of traditional lens design strategies, simulation of optical forces, and high-end ultraprecision machining of optical free-form surfaces is applied to the realization of a highly specialized optical trapping system. The resulting compact and lightweight optical modules potentially open new classes of applications for optical manipulation. As an example we present a customized 3D trapping module made of a single piece of polymethylmethacrylate, with a large working distance of 650?μm.     

A neutral atom and a wire: towards mesoscopic atom optics

A large variety of trapping and guiding potentials can be designed by bringing cold atoms close to charged or current-carrying material objects. Using a current-carrying wire we demonstrate how to build guides and traps for neutral atoms and using a charged wire we study a 1/r ² singularity. The simplicity and versatility of the principles demonstrated in our experiments will allow for miniaturization and integration of atom optical elements into matter-wave quantum circuits. Received: 13 December 1998 / Revised version: 8 July 1999 / Published online: 8 September 1999     

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.     

Multiple-spot optical tweezers created with microlens arrays fabricated by proton beam writing

We report two different applications for using arrays of microlenses on glass substrate to facilitate multiple-spot optical trapping of colloidal microbeads. The array of microlenses was made of SU8 or PMMA resist and created by a combination of Proton-Beam writing followed by thermal reflow processes. Firstly, similar to previous reports [8, 9, 10 ], the lenses were utilized as an optical element in generating multiple laser spot arrays that were subsequently focused down to impose a microbead array. In addition, we demonstrated the feasibility of a novel approach of integrating the microlens array into a sample chamber to provide localized optical trapping. PACS 07.60.Pb; 41.75.Ak; 42.15.Eq; 42.65.Jx; 42.79.Bh     

Feasibility of narrow-line cooling in optical dipole traps

We have investigated the influence of narrow-line laser cooling on the loading of Ca atoms into optical dipole traps. To describe the narrow-line cooling of alkaline-earth atoms in combination with optical dipole trapping, we have developed a model that takes into account the light shifts of the cooling transition in three dimensions. The model is compared with two experimental realizations of optical dipole traps for calcium at the wavelengths 514 nm and 10.6 μm.     

Optomagnetically Controlled Microparticles Manufactured with Glancing Angle Deposition

Optical trapping and magnetic trapping are common micromanipulation techniques for controlling colloids including micro‐ and nanoparticles. Combining these two manipulation strategies allows a larger range of applied forces and decoupled control of rotation and translation; each of which are beneficial properties for many applications including force spectroscopy and advanced manufacturing. However, optical trapping and magnetic trapping have conflicting material requirements inhibiting the combination of these methodologies. In this paper, anisotropic microscaled particles capable of being simultaneously controlled by optical and magnetic trapping are synthesized using a glancing angle deposition (GLAD) technique. The anisotropic alignment of dielectric and ferromagnetic materials limits the optical scattering from the metallic components which typically prevents stable optical trapping in three dimensions. Compared to the current state of the art, the benefits of this approach are twofold. First, the composite structure allows larger volumes of ferromagnetic material so that larger magnetic moments may be applied without inhibiting the stability of optical trapping. Second, the robustness of the synthesis process is greatly improved. The dual optical and magnetic functionality of the synthesized colloids is demonstrated by simultaneously optically translating and magnetically rotating a magnetic GLAD particle using a custom designed optomagnetic trapping system.     

Optical Fiber Trapping in Space

A novel dual-beam trapping system was proposed and discussed for a three-dimensional optical trapping of an object with large relative refractive index. From theoretical studies, we confirmed that our system was useful for manipulating objects in cosmic space.