Friday, January 23, 2009

Nano-optical Trapping of Rayleigh Particles and Escherichia coli Bacteria with Resonant Optical Antennas

M. Righini, P. Ghenuche, S. Cherukulappurath, V. Myroshnychenko, F. J. García de Abajo and R. Quidant

Immobilizing individual living microorganisms at designated positions in space is important to study their metabolism and to initiate an in situ scrutiny of the complexity of life at the nanoscale. While optical tweezers enable the trapping of large cells at the focus of a laser beam, they face difficulties in maintaining them steady and can become invasive and produce substantial damage that prevents preserving the organisms intact for sufficient time to be studied. Here we demonstrate a novel optical trapping scheme that allows us to hold living Escherichia coli bacteria for several hours using moderate light intensities. We pattern metallic nanoantennas on a glass substrate to produce strong light intensity gradients responsible for the trapping mechanism. Several individual bacteria are trapped simultaneously with their orientation fixed by the asymmetry of the antennas. This unprecedented immobilization of bacteria opens an avenue toward observing nanoscopic processes associated with cell metabolism, as well as the response of individual live microorganisms to external stimuli, much in the same way as pluricellular organisms are studied in biology.

Tuesday, January 20, 2009

Microrheology of complex fluids using optical tweezers: a comparison with macrorheological measurements

G Pesce, A C De Luca, G Rusciano, P A Netti, S Fusco and A Sasso

The increasing interest in the mechanical properties of complex systems at mesoscopic scale has recently fueled the development of new experimental techniques, collectively indicated as microrheology. Unlike bulk-based approaches (macrorheology), these new techniques make use of micrometric probes (usually microspheres) which explore the mechanical properties of the surrounding medium.In this paper we discuss the basic idea of microrheology and we will focus on one specific technique based on optical tweezers (OT). The discussion starts from Newtonian fluids to tackle the more general case of complex fluids, also showing results of these techniques on solutions of a relevant biomolecule: hyaluronic acid (HA). In particular, we study the viscoelastic properties of low molecular weight HA (155 kDa) at low ionic strength over an extended frequency range (0.1–1000 Hz) and in a wide range of concentrations (0.01–20 mg ml−1), which include both the dilute and semidilute regime. In the concentration range here explored and within the test frequencies covered by our techniques, samples prevalently exhibit a viscous behavior, the elastic contribution becoming significant at the highest concentrations. By comparing OT outcomes to those obtained by a traditional rheometer, we found that they were in good agreement in the overlapping frequency range of the two techniques, thus confirming the reliability of the microrheological approach.

Longitudinal optical binding of several spherical particles studied by the coupled dipole method

V Karásek, O Brzobohatý and P Zemánek

We employed a coupled dipole method (CDM) to study theoretically the interaction among several spherical particles placed into two counter-propagating mutually incoherent Bessel beams. This interaction is mediated by the light scattering among the particles. It has already been demonstrated that, if the intensity of the incident beam is sufficiently high, the scattered light is strong enough to self-arrange the objects in the space. Namely, the counter-propagating and incoherent Bessel beams are extremely useful to be employed because the interaction among the particles via the scattered light is not superimposed by other optical forces coming from the radiation pressure of each beam and axial gradients of the beam intensities. Therefore so-called optical binding between the particles is enhanced and leads to several stable configurations of the particles. We studied these stable configurations using the CDM for various properties of the beams and particles and we also compared these theoretical results with the experimental observations.


Optical alignment of a cylindrical object

Chaolong Song, Nam-Trung Nguyen and Anand Krishna Asundi

This paper reports the use of theory of geometrical optics to analyze how an optical field interacts with a cylindrical object. Of great interest is the mechanism with which a laser beam with a special profile manipulates a particle which has a similar shape as the beam profile. The present paper investigates the interaction between a cylinder-shape fiber and a laser beam with a line-shape profile. Based on the Fresnel equation, a numerical model was formulated to describe the optical torque generated by a projected line-shape optical image. The drag force was also considered in the model to accurately describe the fiber's movement in a liquid. A differential equation is established to describe this damped movement of the cylinder. Parametric analysis was carried out to investigate the influence of the beam power and the liquid viscosity as well as the density, the length, and the diameter of the cylindrical object. The movement of a carbon fiber was measured with a CCD camera. The observed experimental results agree well with the theoretical results.


Holographic phase contrast for dynamic multiple-beam optical tweezers

Mike Woerdemann, Frank Holtmann and Cornelia Denz

We propose and demonstrate holographic phase contrast (HPC) as a new method to transfer a spatial phase distribution of arbitrary shape into a corresponding intensity pattern. A powerful application of HPC is the use in optical tweezers to dynamically control multiple traps like arrays or even more complex trapping geometries. Due to the image plane nature of HPC no hologram calculation is required and hence real-time control of complex tweezers configurations is possible. The inherent optical amplification by HPC can improve the fundamental limit in trapping power in optical tweezers that are based on common spatial light modulators.


Parallel particle identification and separation for active optical sorting

Ivan Perch-Nielsen, Darwin Palima, Jeppe S Dam and Jesper Glückstad

An instrument for rapidly and non-invasively sorting different cell specimens is a valuable tool in biological and medical research. Parallel identification of target specimens through image analysis can sort based on highly tuneable selection criteria and can enable high-speed optical sorting when matched with a rapidly reconfigurable optical sorting field. We demonstrate the potential of such a system using colloidal polystyrene microspheres. By combining machine vision with a parallel add-on optical manipulation scheme, we were able to move identified particles over a distance of several hundred micrometres at velocities that exceed 800 µm s−1 and are easily scalable to higher velocities.


Holographic twin traps

S Zwick, T Haist, Y Miyamoto, L He, M Warber, A Hermerschmidt and W Osten

We present a new method that enables the generation of arbitrary positioned dual-beam traps without additional hardware in a single-beam holographic optical tweezers setup. By this approach, stable trapping at medium numerical aperture and long working distance is realized on a standard Zeiss Axiovert 200 M research microscope. Simulations of focus separations and spherical aberrations were performed and first experimental results are presented.


Fabrication of microstructures for optically driven micromachines using two-photon photopolymerization of UV curing resins

Theodor Asavei, Timo A Nieminen, Norman R Heckenberg and Halina Rubinsztein-Dunlop

Two-photon photopolymerization of UV curing resins is an attractive method for the fabrication of microscopic transparent objects with size in the tens of micrometers range. We have been using this method to produce three-dimensional (3D) structures for optical micromanipulation, in an optical system based on a femtosecond laser. By carefully adjusting the laser power and the exposure time we were able to create micro-objects with well-defined 3D features and with resolution below the diffraction limit of light. We discuss the performance and capabilities of a microfabrication system, with some examples of its products.


Monday, January 19, 2009

Optical tweezers: not just for physicists anymore

Christine Piggee

This Product Review gives an overview of optical tweezers and some of their applications. Table 1 presents some basic specifications of commercial optical trapping systems. This article is not intended to be an exhaustive analysis of all optical trapping instruments; interested readers should contact the vendors for more information about their products and services.

Friday, January 9, 2009

Rotation of absorbing spheres in Laguerre-Gaussian beams

Stephen H. Simpson and Simon Hanna

It is well known that optical vortex beams carry orbital as well as spin angular momentum. This optical angular momentum can manifest itself mechanically, for example in tightly focused Laguerre-Gaussian beams, where trapped, weakly absorbing spheres rotate at a rate proportional to the total angular momentum carried by the beam. In the present paper we subject this system to a rigorous analysis involving expansions in vector spherical wave functions that culminates in a simple expression for the torque on the sphere. It is seen that, for large weakly absorbing spheres, the induced torque per unit power is independent of the detailed structure of the incident field, being a simple function of two indices that describe the helicity and polarization state of the beam, the relative refractive indices of the sphere and ambient medium, the absorption index of the sphere, and its radius. A number of relationships between the coefficients of these expansions are also developed.

Precision optical trapping via a programmable direct-digital-synthesis-based controller for acousto-optic deflectors

A. H. Mack, M. K. Trías, and S. G. J. Mochrie

We describe a simple-to-construct programmable direct-digital-synthesis-based controller for use with acousto-optic deflectors. Our controller corrects for nonlinear diffraction efficiency versus diffraction angle, provides superior stability, functionality, and configurability, and costs a fraction of commercially available systems. Using this instrument, we move a 1  µm diameter bead by 1-nm-sized steps and resolve these steps. 

Direct measurements of the frequency-dependent dielectrophoresis force

Ming-Tzo Wei, Joseph Junio, and H. Daniel Ou-Yang

Dielectrophoresis (DEP), the phenomenon of directed motion of electrically polarizable particles in a nonuniform electric field, is promising for applications in biochemical separation and filtration. For colloidal particles in suspension, the relaxation of the ionic species in the shear layer gives rise to a frequency-dependent, bidirectional DEP force in the radio frequency range. However, quantification methods of the DEP force on individual particles with the pico-Newton resolution required for the development of theories and design of device applications are lacking. We report the use of optical tweezers as a force sensor and a lock-in phase-sensitive technique for analysis of the particlemotion in an amplitude modulated DEP force. The coherent detection and sensing scheme yielded not only unprecedented sensitivity for DEP force measurements, but also provided a selectivity that clearly distinguishes the pure DEP force from all the other forces in the system, including electrophoresis, electro-osmosis, heat-induced convection, and Brownian forces, all of which can hamper accurate measurements through other existingmethods. Using optical tweezers-based force transducers already developed in our laboratory, we have results that quantify the frequency-dependent DEP forceand the crossover frequency of individual particles with this new experimental method.

Probing technique using circular motion of a microsphere controlled by optical pressure for a nanocoordinate measuring machine

Masaki Michihata, Yuto Nagasaka, Terutake Hayashi, and Yasuhiro Takaya

A new surface probing technique using the circular motion of an optically-trapped microsphere is proposed for a nanocoordinate measuring system. The probe sphere is oscillated circularly in the plane perpendicular to the probe axis and the circular orbit of the probe sphere is monitored for the detection of the position and normal vector direction of the surface. The principle of detection is based on changes in the circular orbit of the microsphere. When the probe approaches a work surface, the orbit of the probe sphere becomes elliptical. The minor-axis length and the minor-axis angle of the ellipse are then used as parameters to detect the position and normal vector direction of the surface, respectively. In this study, the circular motion probe is shown to have a resolution of position detection of 39 nm, and the accuracy of measuring a normal vector to the surface is on the order of 3 °.


Whispering gallery mode enhanced optical force with resonant tunneling excitation in the Kretschmann geometry

J. J. Xiao, Jack Ng, Z. F. Lin, and C. T. Chan

The boundary element method is applied to investigate the optical forces when whispering gallery modes (WGMs) are excited by a total internally reflected wave. Such evanescent wave is particularly effective in exciting the high-Q WGM, while the low angular or highradial order modes are suppressed relatively. This results in a large contrast between the forces on and off resonance, and thus allows for high size selectivity. We fully incorporate the prism-particle interaction and found that the optical force behaves differently at different separations. Optimal separation is found, which corresponds to a compromise between intensity and Q factor.

Saturday, January 3, 2009

Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides

Allen H. J. Yang, Sean D. Moore, Bradley S. Schmidt, Matthew Klug, Michal Lipson & David Erickson

The ability to manipulate nanoscopic matter precisely is critical for the development of active nanosystems. Optical tweezers are excellent tools for transporting particles ranging in size from several micrometres to a few hundred nanometres. Manipulation of dielectric objects with much smaller diameters, however, requires stronger optical confinement and higher intensities than can be provided by these diffraction-limited systems. Here we present an approach to optofluidic transport that overcomes these limitations, using sub-wavelength liquid-core slot waveguides. The technique simultaneously makes use of near-field optical forces to confine matter inside the waveguide and scattering/adsorption forces to transport it. The ability of the slot waveguide to condense the accessible electromagnetic energy to scales as small as 60 nm allows us also to overcome the fundamental diffraction problem. We apply the approach here to the trapping and transport of 75-nm dielectric nanoparticles and λ-DNA molecules. Because trapping occurs along a line, rather than at a point as with traditional point traps, the method provides the ability to handle extended biomolecules directly. We also carry out a detailed numerical analysis that relates the near-field optical forces to release kinetics. We believe that the architecture demonstrated here will help to bridge the gap between optical manipulation and nanofluidics.

Optically induced potential energy landscapes

Justo Rodriguez, Luciana C. Davila Romero, and David L. Andrews

Multi-dimensional potential energy surfaces are associated with optical binding. A detailed exploration of the available degrees of geometric freedom reveals unexpected turning points, producing intricate patterns of local force and torque. Although optical pair interactions outweigh Casimir-Polder coupling even over short distances, the forces are not always attractive. Numerous local potential minimum and maximum can be located, and mapped on contour diagrams. Islands of stability appear, and structures conducive to the formation of rings. The results, based on quantum electrodynamics, apply to optically trapped molecules, nanoparticles, microparticles and colloids.

Dual filtered backprojection for micro-rotation confocal microscopy

Danai Laksameethanasan, Sami S Brandt, Olivier Renaud and Spencer L Shorte

Micro-rotation confocal microscopy is a novel optical imaging technique which employs dielectric fields to trap and rotate individual cells to facilitate 3D fluorescence imaging using a confocal microscope. In contrast to computed tomography (CT) where an image can be modelled as parallel projection of an object, the ideal confocal image is recorded as a central slice of the object corresponding to the focal plane. In CT, the projection images and the 3D object are related by the Fourier slice theorem which states that the Fourier transform of a CT image is equal to the central slice of the Fourier transform of the 3D object. In the micro-rotation application, we have a dual form of this setting, i.e. the Fourier transform of the confocal image equals the parallel projection of the Fourier transform of the 3D object. Based on the observed duality, we present here the dual of the classical filtered back projection (FBP) algorithm and apply it in micro-rotation confocal imaging. Our experiments on real data demonstrate that the proposed method is a fast and reliable algorithm for the micro-rotation application, as FBP is for CT application.

Live cell lithography: Using optical tweezers to create synthetic tissue

Utkur Mirsaidov, Jan Scrimgeour, Winston Timp, Kaethe Beck, Mustafa Mir, Paul Matsudaira and Gregory Timp

We demonstrate a new method for creating synthetic tissue that has the potential to capture the three-dimensional (3D) complexity of a multi-cellular organism with submicron precision. Using multiple laminar fluid flows in a microfluidic network, we convey cells to an assembly area where multiple, time-shared optical tweezers are used to organize them into a complex array. The cells are then encapsulated in a 30 m × 30 m × 45 m volume of photopolymerizable hydrogel that mimicks an extra-cellular matrix. To extend the size, shape and constituency of the array without loss of viability, we then step to an adjacent location while maintaining registration with the reference array, and repeat the process. Using this step-and-repeat method, we formed a heterogeneous array of E. coli genetically engineered with a lac switch that is functionally linked to fluorescence reporters. We then induced the array using ligands through a microfluidic network and followed the space-time development of the fluorescence to evaluate viability and metabolic activity.


Electrokinetic patterning of colloidal particles with optical landscapes

Stuart J. Williams, Aloke Kumar and Steven T. Wereley

We demonstrate an opto-electrokinetic technique for non-invasive particle manipulation on the surface of a parallel-plate indium tin oxide (ITO) electrode that is biased with an alternating current (AC) signal and illuminated with near-infrared (1064 nm) optical landscapes. This technique can generate strong microfluidic vortices at higher AC frequencies (>100 kHz) and dynamically and rapidly aggregate and pattern particle groups at low frequencies (<100kHz).

Happy New Year!

Hello 2009! In this first entry of 2009, I focused on some of the recent published papers of optical tweezers. By the end on this month I will add some new features on this site.