Juan José Sáenz
Scientists have theoretically proposed that it is possible to pull objects from a far distance towards a light source in the absence of axial optical gradient forces.
DOI
DOI
Tuesday, August 30, 2011
Optical pulling force
Jun Chen, Jack Ng, Zhifang Lin & C. T. Chan
A photon carries 
k of momentum, so it may be anticipated that light will ‘push’ on any object standing in its path by means of the scattering force. In the absence of an intensity gradient, using a light beam to pull a particle backwards is counter-intuitive. Here, we show that it is possible to realize a backward scattering force that pulls a particle all the way towards the source without an equilibrium point. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simultaneous excitation of multipoles in the particle, and if the projection of the total photon momentum along the propagation direction is small, an attractive optical force is possible. This possibility adds ‘pulling’ as an additional degree of freedom to optical micromanipulation.
DOI
k of momentum, so it may be anticipated that light will ‘push’ on any object standing in its path by means of the scattering force. In the absence of an intensity gradient, using a light beam to pull a particle backwards is counter-intuitive. Here, we show that it is possible to realize a backward scattering force that pulls a particle all the way towards the source without an equilibrium point. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simultaneous excitation of multipoles in the particle, and if the projection of the total photon momentum along the propagation direction is small, an attractive optical force is possible. This possibility adds ‘pulling’ as an additional degree of freedom to optical micromanipulation.DOI
Note: Direct force and ionic-current measurements on DNA in a nanocapillary
O. Otto, L. J. Steinbock, D. W. Wong, J. L. Gornall, and U. F. Keyser
We have developed optical tweezers, with force measurements based on fast video tracking, for analysis and control of DNA translocation through nanocapillaries. Nanocapillaries are single-molecule biosensors with very similar characteristics to solid-state nanopores. Our novel experimental setup allows for ionic-current measurements in which the nanocapillary is oriented perpendicular to the trapping laser. Using video-based particle tracking, we are able to measure the position of DNA coated colloids at sub-millisecond resolution and in real-time. We present the first electrophoretic force and simultaneous ionic-current measurements of a single DNA molecule inside the orifice of a nanocapillary.
DOI
We have developed optical tweezers, with force measurements based on fast video tracking, for analysis and control of DNA translocation through nanocapillaries. Nanocapillaries are single-molecule biosensors with very similar characteristics to solid-state nanopores. Our novel experimental setup allows for ionic-current measurements in which the nanocapillary is oriented perpendicular to the trapping laser. Using video-based particle tracking, we are able to measure the position of DNA coated colloids at sub-millisecond resolution and in real-time. We present the first electrophoretic force and simultaneous ionic-current measurements of a single DNA molecule inside the orifice of a nanocapillary.
DOI
Monday, August 29, 2011
BioPhotonics workstation: A versatile setup for simultaneous optical manipulation, heat stress, and intracellular pH measurements of a live yeast cell
Thomas Aabo, Andrew Raphael Banás, Jesper Glückstad, Henrik Siegumfeldt, and Nils Arneborg
In this study we have modified the BioPhotonics workstation (BWS), which allows for using long working distance objective for optical trapping, to include traditional epi-fluorescence microscopy, using the trapping objectives. We have also added temperature regulation of sample stage, allowing for fast temperature variations while trapping. Using this modified BWS setup, we investigated the internal pH (pHi) response and membrane integrity of an optically trapped Saccharomyces cerevisiae cell at 5 mW subject to increasing temperatures. The pHi of the cell is obtained from the emission of 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester, at 435 and 485 nm wavelengths, while the permeability is indicated by the fluorescence of propidium iodide. We present images mapping the pHi and permeability of the cell at different temperatures and with enough spatial resolution to localize these attributes within the cell. The combined capability of optical trapping, fluorescence microscopy and temperature regulation offers a versatile tool for biological research.
DOI
In this study we have modified the BioPhotonics workstation (BWS), which allows for using long working distance objective for optical trapping, to include traditional epi-fluorescence microscopy, using the trapping objectives. We have also added temperature regulation of sample stage, allowing for fast temperature variations while trapping. Using this modified BWS setup, we investigated the internal pH (pHi) response and membrane integrity of an optically trapped Saccharomyces cerevisiae cell at 5 mW subject to increasing temperatures. The pHi of the cell is obtained from the emission of 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester, at 435 and 485 nm wavelengths, while the permeability is indicated by the fluorescence of propidium iodide. We present images mapping the pHi and permeability of the cell at different temperatures and with enough spatial resolution to localize these attributes within the cell. The combined capability of optical trapping, fluorescence microscopy and temperature regulation offers a versatile tool for biological research.
DOI
Waveguide trapping of hollow glass spheres
Balpreet Singh Ahluwalia, Pål Løvhaugen, and Olav Gaute Hellesø
Microparticles can be trapped and propelled by the evanescent field of optical waveguides. As the evanescent field only stretches 100–200 nm from the surface of the waveguide, only the lower caps of the microparticles interact directly with the field. This is taken advantage of by trapping hollow glass spheres on waveguides in the same way as solid glass spheres. For the chosen waveguide, numerical simulations show that hollow microspheres with a shell thickness above 60 nm can be stably trapped, while spheres with thinner shells are repelled. The average refractive index of the sphere–field intersection volume is used to explain the result in a qualitative way.
DOI
Microparticles can be trapped and propelled by the evanescent field of optical waveguides. As the evanescent field only stretches 100–200 nm from the surface of the waveguide, only the lower caps of the microparticles interact directly with the field. This is taken advantage of by trapping hollow glass spheres on waveguides in the same way as solid glass spheres. For the chosen waveguide, numerical simulations show that hollow microspheres with a shell thickness above 60 nm can be stably trapped, while spheres with thinner shells are repelled. The average refractive index of the sphere–field intersection volume is used to explain the result in a qualitative way.
DOI
On-a-chip surface plasmon tweezers
H. M. K. Wong, M. Righini, J. C. Gates, P. G. R. Smith, V. Pruneri, and R. Quidant
We report on an integrated optical trapping platform operated by simple fiber coupling. The system consists of a dielectric channel optical waveguide decorated with an array of gold micro-pads. Through a suitable engineering of the waveguide mode, we achieve light coupling to the surface plasmon resonance of the gold pads that act as individual plasmonic traps. We demonstrate parallel trapping of both micrometer size polystyrene beads and yeast cells at predetermined locations on the chip with only 20 mW total incident laser power.
DOI
We report on an integrated optical trapping platform operated by simple fiber coupling. The system consists of a dielectric channel optical waveguide decorated with an array of gold micro-pads. Through a suitable engineering of the waveguide mode, we achieve light coupling to the surface plasmon resonance of the gold pads that act as individual plasmonic traps. We demonstrate parallel trapping of both micrometer size polystyrene beads and yeast cells at predetermined locations on the chip with only 20 mW total incident laser power.
DOI
Robust trapping and manipulation of airborne particles with a bottle beam
Vladlen G. Shvedov, Cyril Hnatovsky, Andrei V. Rode, and Wieslaw Krolikowski
We demonstrate that micron-sized light-absorbing particles can be trapped and transported photophoretically in air using an optical bottle formed inside the focal volume of a lens with a controlled amount of spherical aberration. This optical fiber-based single beam trap can be used in numerous applications where true 3D manipulation and delivery of airborne micro-objects is required.
DOI
We demonstrate that micron-sized light-absorbing particles can be trapped and transported photophoretically in air using an optical bottle formed inside the focal volume of a lens with a controlled amount of spherical aberration. This optical fiber-based single beam trap can be used in numerous applications where true 3D manipulation and delivery of airborne micro-objects is required.
DOI
Calculation of radiation forces exerted on a uniaxial anisotropic sphere by an off-axis incident Gaussian beam
Zheng-Jun Li, Zhen-Sen Wu, and Qing-Chao Shang
Using the theory of electromagnetic scattering of a uniaxial anisotropic sphere, we derive the analytical expressions of the radiation forces exerted on a uniaxial anisotropic sphere by an off-axis incident Gaussian beam. The beam’s propagation direction is parallel to the primary optical axis of the anisotropic sphere. The effects of the permittivity tensor elements εt and εz on the axial radiation forces are numerically analyzed in detail. The two transverse components of radiation forces exerted on a uniaxial anisotropic sphere, which is distinct from that exerted on an isotropic sphere due to the two eigen waves in the uniaxial anisotropic sphere, are numerically studied as well. The characteristics of the axial and transverse radiation forces are discussed for different radii of the sphere, beam waist width, and distances from the sphere center to the beam center of an off-axis Gaussian beam. The theoretical predictions of radiation forces exerted on a uniaxial anisotropic sphere are hoped to provide effective ways to achieve the improvement of optical tweezers as well as the capture, suspension, and high-precision delivery of anisotropic particles.
DOI
Using the theory of electromagnetic scattering of a uniaxial anisotropic sphere, we derive the analytical expressions of the radiation forces exerted on a uniaxial anisotropic sphere by an off-axis incident Gaussian beam. The beam’s propagation direction is parallel to the primary optical axis of the anisotropic sphere. The effects of the permittivity tensor elements εt and εz on the axial radiation forces are numerically analyzed in detail. The two transverse components of radiation forces exerted on a uniaxial anisotropic sphere, which is distinct from that exerted on an isotropic sphere due to the two eigen waves in the uniaxial anisotropic sphere, are numerically studied as well. The characteristics of the axial and transverse radiation forces are discussed for different radii of the sphere, beam waist width, and distances from the sphere center to the beam center of an off-axis Gaussian beam. The theoretical predictions of radiation forces exerted on a uniaxial anisotropic sphere are hoped to provide effective ways to achieve the improvement of optical tweezers as well as the capture, suspension, and high-precision delivery of anisotropic particles.
DOI
Potential mapping of optical tweezers
Tahmineh Godazgar, Rouzbeh Shokri, and S. Nader S. Reihani
Optical tweezers are very often used for measurement of piconewton range forces. Depending on the displacement of the trapped bead, the trap may become stiffer which causes considerable underestimation of the measured force. We have shown, both by theory and experiment, that such a stiffening occurs for beads larger than 0.5 μm in radius. For the first time, we have shown that the displacement at which the stiffening starts is size dependent and that the stiffening starts at higher forces for larger beads. We have shown that for the applications, which simultaneous force measurement and position sensing are on demand (such as biopolymer stretching), mid-range sized (∼1.5 μm in radius) beads could be the best choice.
DOI
Optical tweezers are very often used for measurement of piconewton range forces. Depending on the displacement of the trapped bead, the trap may become stiffer which causes considerable underestimation of the measured force. We have shown, both by theory and experiment, that such a stiffening occurs for beads larger than 0.5 μm in radius. For the first time, we have shown that the displacement at which the stiffening starts is size dependent and that the stiffening starts at higher forces for larger beads. We have shown that for the applications, which simultaneous force measurement and position sensing are on demand (such as biopolymer stretching), mid-range sized (∼1.5 μm in radius) beads could be the best choice.
DOI
Acceleration control of Airy beams with optically induced refractive-index gradient
Zhuoyi Ye, Sheng Liu, Cibo Lou, Peng Zhang, Yi Hu, Daohong Song, Jianlin Zhao, and Zhigang Chen
We demonstrate both experimentally and theoretically controlled acceleration of one- and two-dimensional Airy beams in optically induced refractive-index potentials. Enhancement as well as reduction of beam acceleration are realized by changing the index gradient, while the beam shape is maintained during propagation through the linear optical potential. Our results of active acceleration manipulation in graded media are pertinent to Airy-type beam propagation in various environments.
DOI
We demonstrate both experimentally and theoretically controlled acceleration of one- and two-dimensional Airy beams in optically induced refractive-index potentials. Enhancement as well as reduction of beam acceleration are realized by changing the index gradient, while the beam shape is maintained during propagation through the linear optical potential. Our results of active acceleration manipulation in graded media are pertinent to Airy-type beam propagation in various environments.
DOI
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