Ulf Quentin, Karl-Heinz Leitz, Lutz Deichmann, Ilya Alexeev, and Michael Schmidt
Particle based near-field nanostructuring is an excellent possibility to overcome the optical diffraction limit in laser based material processing. In the near-field of microspheres which are irradiated with pulsed laser radiation, it is possible to generate nanoholes with diameters below 100 nm using a laser wavelength of 800 nm. To improve this approach, it is possible to position the microparticles with an optical trap to generate arbitrary structure geometries. In this paper, the authors describe the basic principle of optical trap assisted nanostructuring and present simulational and experimental results demonstrating the potential of this innovative nanoscale optical material processing technology.
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Kang Soo Lee, Sang Youl Yoon, Kyung Heon Lee, Sang Bok Kim, Hyung Jin Sung, and Sang Soo Kim
This paper describes a method for particle manipulation in a liquid-core/liquid-cladding optical waveguide system. Step-index and graded-index waveguides were modeled with consideration for, respectively, miscible and immiscible core and cladding fluids. The characteristic motions of four different particles with refractive indices of 1.59, 1.48, 1.37, and 1.22 were examined. The guided beam was assumed to be Gaussian in shape. Our results showed that high-refractive-index particles converged at the center of the core fluid due to a positive gradient force, whereas low-refractive-index particles converged at the flow periphery. The nonlinearity of the particle motion increased as the flow velocity and the guided beam waist decreased and the laser power and the particle size increased. The initial beam waist of the guided beam in the graded-index waveguide did not significantly affect the characteristics of the particle motion due to the effects of diffusion.
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Ling Lin, Guo Hong-Lian, Huang Lu, Qu E, Li Zhao-Lin and Li Zhi-Yuan
We set up a system of multiple optical tweezers based on a spatial light modulator, and measured the displacement and optical force of the trapped particles simultaneously. All of the trapped particles can be clearly imaged in three dimensions by several CCDs. The displacement is obtained by calculating the gray weighted centroid in the trapped particle's image. The stiffness of the trapped particles in the optical traps is measured by oscillating the sample stage in a triangular wave based on Stokes fluid dynamics. The optical force of each trapped particle can be calculated by the measured displacement and stiffness.
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Hong Cai and Andrew W. Poon
We demonstrate planar optical tweezers using the evanescent field of a silicon nitride tapered-waveguide junction between a singlemode waveguide and a multimode waveguide. Our experiments show that the junction embedded in a fluidic channel holds up to one and two polystyrene particles of sizes of 2.2 μm and 1 μm, respectively. The trapped particles are successively substituted by the incoming particles. Our experiments and numerical modeling reveal that the junction particle trapping depends on particle size and number.
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E. Messina, L. D’Urso, E. Fazio, C. Satriano, M.G. Donato, C. D’Andrea, O.M. Maragò, P.G. Gucciardi, G. Compagnini, F. Neri
The plasmon resonance of metallic Au/Ag nano-alloys in the colloidal state was tuned from 400 nm to 500 nm using a laser irradiated technique performed directly in the liquid state. Interesting optical nonlinearities, trapping effects and spectroscopic enhancements were detected by modifying the gold molar fraction in the nano-alloys. In particular a reduction of the limiting threshold was observed by increasing the gold amount. The SERS activity of the Au/Ag alloys was tested in liquid and in solid state in presence of linear carbon chains as probe molecules. The increased Raman signals for nanoparticles with different Au/Ag atomic ratio are presented. Finally, studies and prospects for optical and Raman tweezers experiments are discussed.
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Damien Baigl
Using light to control liquid motion is a new paradigm for the actuation of microfluidic systems. We review here the different principles and strategies to induce or control liquid motion using light, which include the use of radiation pressure, optical tweezers, light-induced wettability gradient, thermocapillary effect, photosensitive surfactants, chromocapillary effect, optoelectrowetting, photocontrolled electroosmotic flows and optical dielectrophoresis. We analyze the performance of these approaches to control using light many kinds of microfluidic operations involving discrete pL- to µL-sized droplets (generation, driving, mixing, reaction, sorting) or fluid flows in microchannels (valve operation, injection, pumping, flow rate control). We show that a complete toolbox is now available to control microfluidic systems by light. We finally discuss the perspectives of digital optofluidics as well as microfluidics based on all optical fluidic chips and optically reconfigurable devices.
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Zhen Ma, Qiuying Liu, Honghai Liu, Huaxiao Yang, Julie X. Yun, Meifeng Xu, Carol A. Eisenberg, Thomas K. Borg, Roger Markwald and Bruce Z. Gao
Normal cardiomyocytes are highly dependent on the functional expression of ion channels to form action potentials and electrical coupling with other cells. To fully determine the scientific and therapeutic potential of stem cells for cardiovascular-disease treatment, it is necessary to assess comprehensively the regulation of stem-cell electrical properties during stem cell-cardiomyocyte interaction. It has been reported in the literature that contact with native cardiomyocytes induced and regulated stem-cell cardiogenic differentiation. However, in conventional cell-culture models, the importance of cell–cell contact for stem-cell functional coupling with cardiomyocytes has not been elucidated due to insufficient control of the cell-contact mode of individual cells. Using microfabrication and laser-guided cell micropatterning techniques, we created two biochips with contact-promotive and -preventive microenvironments to systematically study the effect of contact on cardiogenic regulation of stem-cell electrical properties. In contact-promotive biochips, connexin 43 expression was upregulated and relocated to the junction area between one stem cell and one cardiomyocyte. Only stem cells in contact with cardiomyocytes were induced by adjacent cardiomyocytes to acquire electrophysiological properties for action-potential formation similar to that of a cardiomyocyte.
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Huijuan You, Ryota Iino, Rikiya Watanabe and Hiroyuki Noji
A molecular system of a nanometer-sized reel was developed from F1–ATPase, a rotary motor protein. By combination with magnetic tweezers and optical tweezers, single-molecule double-stranded DNA (dsDNA) was wound around the molecular reel. The bending stiffness of dsDNA was determined from the winding tension (0.9–6.0 pN) and the diameter of the wound loop (21.4–8.5 nm). Our results were in good agreement with the conventional worm-like chain model and a persistence length of 54 ± 9 nm was estimated. This molecular reel system offers a new platform for single-molecule study of micromechanics of sharply bent DNA molecules and is expected to be applicable to the elucidation of the molecular mechanism of DNA-associating proteins on sharply bent DNA strands.
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O. A. Schmidt, M. K. Garbos, T. G. Euser, and P. St. J. Russell
We report a novel optothermal trapping mechanism that occurs in air-filled hollow-core photonic crystal fiber. In the confined environment of the core, the motion of a laser-guided particle is strongly influenced by the thermal-gradient-driven flow of air along the core surface. Known as “thermal creep flow,” this can be induced either statically by local heating, or dynamically by the absorption (at a black mark placed on the fiber surface) of light scattered by the moving particle. The optothermal force on the particle, which can be accurately measured in hollow-core fiber by balancing it against the radiation forces, turns out to exceed the conventional thermophoretic force by 2 orders of magnitude. The system makes it possible to measure pN-scale forces accurately and to explore thermally driven flow in micron-scale structures.
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Graham Gibson, Louise Barron, Fiona Beck, Graeme Whyte and Miles Padgett
We report the development of a joystick controlled gripper for the real-time manipulation of micron-sized objects, driven using holographic optical tweezers (HOTs). The gripper consists of an arrangement of four silica beads, located in optical traps, which can be positioned and scaled in order to trap an object indirectly. The joystick can be used to grasp, move (lateral or axial), and change the orientation of the target object. The ability to trap objects indirectly allows us to demonstrate the manipulation of a strongly scattering micron-sized metallic particle.
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