Yuqiang Jiang, Tetsuya Narushima & Hiromi Okamoto
The optical trapping technique has been widely used in various areas to manipulate particles, cells, and so forth. The principle of trapping is based on the interaction between optical electric fields and induced linear polarizations. Here we show a novel phenomenon of trapping arising from nonlinear polarization when we trap gold nanoparticles by ultrashort near-infrared laser pulses. That is, the stable trap site is split into two equivalent positions (we call this ‘trap split’). The trap positions are aligned along the direction of the incident laser polarization. The dependencies of trap split on the trapping-laser power and wavelength were investigated. The results were successfully interpreted in terms of the nonlinear polarization caused by the femtosecond pulses. This method may give novel applications to micromachining, nanofabrication, and biological samples as well as atomic and molecular trapping at low temperatures.
DOI
Lin Ling, Fei Zhou, Lu Huang, and Zhi-Yuan Li
Discrete dipole approximation (DDA) method is an efficient method for computing electromagnetic (EM) field of nanometer/micrometer-sized dielectric particles with arbitrary geometric shape and topology. In this work we employ the DDA method to calculate the optical force of dielectric shaped particles embedded in optical tweezers made from focused Gaussian laser beams. The EM force is calculated based on the self-consistent solution of EM field distribution and discrete dipole moment distribution within the particles. The DDA method agrees well with the Mie theory for spherical dielectric particles and this supports the effectiveness of the DDA method in handling optical forces in optical tweezers. The optical force for shaped particles such as cubes, rectangles, cylinders, and core-shell composite particles shows many interesting features. The force strongly depends on the orientation of the particle with respect to the laser beam propagation and polarization direction and the aspect ratio of the anisotropic particle. For a core-shell composite particle the zero-force balance point shifts from the particle center to its two sides. When an additional particle comes close a trapped particle, the perturbation effect strongly depends on the relative location of the center of the focused laser beam with respect to the two particles. Furthermore, the geometry of shaped particles not only affects the magnitude of the optical force but also influences the optical trap stiffness.
DOI
Carolin Wagner, Carsten Olbrich, Hergen Brutzer, Mathias Salomo, Ulrich Kleinekathöfer, Ulrich F. Keyser and Friedrich Kremer
The compaction of DNA by the HU protein from Thermotoga maritima (TmHU) is analysed on a single-molecule level by the usage of an optical tweezers-assisted force clamp. The condensation reaction is investigated at forces between 2 and 40 pN applied to the ends of the DNA as well as in dependence on the TmHU concentration. At 2 and 5 pN, the DNA compaction down to 30% of the initial end-to-end distance takes place in two regimes. Increasing the force changes the progression of the reaction until almost nothing is observed at 40 pN. Based on the results of steered molecular dynamics simulations, the first regime of the length reduction is assigned to a primary level of DNA compaction by TmHU. The second one is supposed to correspond to the formation of higher levels of structural organisation. These findings are supported by results obtained by atomic force microscopy.
DOI
Jinsong Li, Xiumin Gao, Shuqin Zhang and Songlin Zhuang
Vector diffraction theory is employed to investigate the focusing properties of the Gaussian beams with mixed screw and conical phase fronts. Numerical simulations show that the Gaussian beams with screw–conical phase fronts are different from both the ordinary Laguerre–Gaussian beams and the higher-order Bessel beams. Rather than forming the ring-shaped intensity distributions characteristic of optical vortices, focusing the Gaussian beams with screw–conical phase fronts produce non-symmetric spiral intensity distributions at the focal plane. The intensity distribution forms a counter-clockwise non-symmetric screw path around the focus. The rotation of intensity distributions was observed in the focal plane. The gradient force patterns of these beams focused with high NA are also investigated. The results show that the gradient force pattern shape depends principally on parameter topological charge n of the phase distribution. The gradient force pattern expands with increase in the parameter m of the phase distribution. Therefore, one can change the topological charge n or the parameter m of the phase mask to construct the tunable optical trap to meet different requirements. Its potential application might include rotational positioning of particles and accumulation of smaller non-symmetric particles towards the focus.
DOI
Yuan Dong, Xi-He Zhang, Feng-dong Zhang, Guo-Bin Ning, Guang-yong Jin, Wei Liang, Yan-Fei Lü and Kai Zhang
A new kind of hollow beams – hollow laser beams with three-dimension trap optical distribution – was put forward. With the help of the Collins formula in paraxial optical system, the analytical equation of propagation and transformation of the hollow laser beams was deduced. According to the analytical equation, the propagation properties of the kind of hollow beams that transform in free space were simulated. In the experiment, we obtained the hollow laser beams by means of the combinational optical system of reflecting positive-axis and negative-axis pyramids. The intensity of the vertical loop in different distances was tested, which shows that the analytical equation of propagation and transformation is in agreement with the result.
DOI
Xiumin Gao, Jinsong Li, Jian Wang and Songlin Zhuang
Focusing properties of the azimuthally polarized beam induced by a pure phase plate are investigated theoretically. The pure phase plate consists of two concentric portions, one center circle portion and one outer annular portion, through which the azimuthally polarized beam passed evolves into concentric piecewise azimuthally polarized beam. When the phase shift of the center portion is π, one ring focus may evolve into novel focal patterns with increasing radius of the center circle portion, such as cylindrical crust focus, two-ring focus, and three-ring focus. And if the geometrical parameters are unchanged, focal patterns also changes considerably with tunable phase of the center portion. Ring focus shifts along the optical axis on the increasing phase. Some optical gradient force distributions and dependence of focal shift on phase shift are also illustrated. This kind of concentric piecewise azimuthally polarized beam can be used in optical manipulation technology.
DOI
Takeshi Iwasa and Katsuyuki Nobusada
Optical forces induced by a near field are calculated for a 1-mm-sized metal particle mimicked by a jellium model and for C60 in the framework of real-time and real-space time-dependent density-functional theory combined with a nonuniform light-matter interaction formalism, fully taking account of multipole interaction. A highly localized near field nonuniformly polarizes these molecules. The locally induced polarization charges in the molecules are partly canceled by the screening charges. The polarization and screening charges generally contribute to the attractive and repulsive forces, respectively, and a sensible balance between these charges results in several peaks in the optical force as a function of the frequency of the near field. The resonance excitation does not necessarily maximally induce the net force, and the force exerted on the molecules strongly depends on the details of their electronic structures. The optical force is larger in the metal particle than in C60. We also found that the optical force depends linearly on the intensity of the near field.
DOI
Nobuhiko Kojima, Ken Miura, Tomoki Matsuo, Hidenari Nakayama, Kikuo Komori, Shoji Takeuchi, and Yasuyuki Sakai
Effectively organizing isolated cells to tissue elements having an appropriate microstructure is a fundamental issue in future tissue engineering, but biological cell-to-cell adhesion is too weak to assemble single cells directly. In order to overcome the difficulty, we applied an Avidin-Biotin Binding System (ABBS) to cell surfaces, and avidinylated and biotinylated cells could mutually bind in the short time they were mixed together. Unlike conventional intact cells, ABBS helped make larger spheroids. Interestingly, avidinylated and biotinylated cell adherence occurred within 1 sec using laser trapping, enabling single cell manipulation. We showed precise, direct single-cell-based tissue assembly using ABBS and optical tweezers, followed by damage-free tissue culture. The combination of ABBS and single cell manipulation has considerable potential for use in application such as tissue engineering, regenerative medicine, and drug screening system.
DOI
Bor-Wen Yang and Zhe Li
Agents that alter the dynamics of hemostasis form an important part in management of conditions such as atherosclerosis, cerebrovascular disease, and bleeding diatheses. In this study, we explored the effects of heparin and tranexamic acid on the efficiency of blood coagulation. Using optical tweezers, we evaluated the pN-range micro-interaction between coagulating red blood cells (RBCs) by measuring the minimum power required to trap them. By observing the mobility of RBCs and the intensity of cellular interactions, we found that the coagulation process can be separated into three phases. The effects of heparin and tranexamic acid were examined by observing variations in cellular interaction during the coagulation phases. Heparin attenuated the interaction between RBCs and prolonged the first phase whereas the samples containing tranexamic acid bypassed the first two phases and immediately proceeded to the final one.
DOI
1 μm particle length scale with 
10 ppm (weight percent), with the dynamics reflecting polymer solution concentration, flow rate, and polydispersity.