Optical micro-manipulation has seen a resurgence of interest in recent years which has been due in part to new application areas and the use of tailored forms of light beams particularly using holographic optical tweezers technology. Optical landscapes using evanescent waves can hold large arrays of particles (~1000) and may be enhanced using surface plasmon interactions. Dynamics in a static 2D landscape of a Bessel beam can lead to optically induced separation.
Experimental results on laser tweezers technology application for material science researche performed at the Institute for Lasers, Photonics and Biophotonics, University at Buffalo, SUNY are presented. Computer controlled dual-beam laser tweezers for highly efficient trapping and manipulation of micron and sub-micron size objects was designed and built. A novel technique for the calibration of laser tweezers that utilize two-photon excited fluorescence of commercial dye stained microspheres has been demonstrated. Laser tweezers technology has been used to monitor the bulk solution viscosity during the sol-gel gelation process at different depths from an interface. The gelation rate is the same in depth ranges 2 - 20 microns from the bounding surface. Optical trapping and manipulation of transparent microparticles suspended in a thermotropic nematic liquid crystals with small and large birefringence was also demonstrated. We employ the particle manipulation to measure line tension of a topologically stable disclination line and to determine colloidal interaction of particles with perpendicular surface anchoring of the director. Fast scanning beam multiple trap option of laser tweezers to construct and dynamically control micro-array structures was developed and characterized. Main parameters of scanning multiple trap setups were studied and optimized. Combination of optical trapping with the hot fluorescence phenomena has been used for local temperature monitoring in liquid samples,with under micron size resolution.
Several methods to rotate and align microscopic particles controllably have been developed. Control of the orientation of a trapped particle allows full three dimensional manipulation, whereas rotating particles are tools for the development of optically-driven micromachines. It has been shown that the orientation of an object in the laser trap depends on its birefringence as well as on its shape. The effect of shape is often referred to as form-birefringence. We report on the trapping, rotation, and in-situ growth of birefringent tetragonal lysozyme crystals in optical tweezers operating at a wavelength of 1064 nm. Variation of the temperature, pH and lysozyme concentration of the solution during growth was used to alter the size, as well as the length to width ratio of the crystals, and hence their orientation in the tweezers. Thus this system serves as a model to study the relative importance of birefringence versus form-birefringence for particle orientation. Crystals with the optical axis skewed or perpendicular to the trapping-beam axis could be rotated by changing the orientation of linearly polarized light. We observed spontaneous spinning of some asymmetric crystals in the presence of linearly polarized light, due to radiation pressure effects. Addition of protein to the solution in the tweezers permitted real-time observation of crystal growth.
A new technique, the near-field correlation spectroscopy (NFCS), is introduced which will be useful for monitoring individual binding events between the biomolecular complex precursors and a functionalized nanoparticle core in a physiological fluid. From jumps in the measured correlation times of single particles trapped in nanoapertures in a metal film, one will be able to determine the rate of binding/dissociation events and, most importantly, the nature of the binding entity, for e.g. discriminate the arrival and binding of a pentamer from a hexamer of protein subunits. This report deals only with the initial steps that are required to implement the new technique.
We are engaged in applying the Generalized Phase Contrast (GPC) method that enables lossless light conversion of spatial phase patterns to highly photon efficient light distributions. The GPC-method can be used in a number of applications requiring parallel light-beam encoding such as in advanced user-controlled optical micro-manipulation, wavefront sensing and generation for common-path interferometry and adaptive optics, optical phase-only encryption and integrated micro-optical implementations. In this work, we will outline the concept for a complete GPC-platform for advanced and user-interactive manipulation of fluid-borne colloidal structures with state-of-the-art controllability and versatility in 3D space and time, hence true 4D. Real-time reconfigurable light patterns with sub-micron accuracy are obtained from a direct map of phase patterns addressed on programmable phase-only spatial light modulator devices. A graphical user interface enables real-time, interactive and arbitrary control over the dynamics and geometry of synthesized light patterns. Experimental demonstrations have shown that GPC-driven micro-manipulation can be used for guided assembly of particles in a plane, control of particle stacking along the beam axis, manipulation of multiple hollow beads and real-time sorting of inhomogeneous mixtures of micro-particles. These experiments illustrate that GPC-driven micro-manipulation can be utilized not only for the improved synthesis of functional microstructures but also for their non-contact and parallel actuation crucial for sophisticated micro-fluidic based lab-on-a-chip demonstrations in the future.
We have developed an interactive user-interface that can be used to generate phase holograms for use with spatial light modulators. The program utilises different hologram design techniques allowing the user to select an appropriate algorithm. The program can be used to generate multiple beams, interference patterns and can be used for beam steering. We therefore see a major application of the program to be within optical tweezers to control the position, number and type of optical traps.
Laser tweezer trapping technology has been applied to monitor the bulk local solution viscosity during the sol-gel gelation process. The gelation rate is the same in depth ranges 2 - 20 microns from the bounding surface. Simultaneously with the laser tweezer study, a micro-viscosity kinetic measurement of the sol-gel process was performed using fluorescent anisotropy and quantum yield measurements. The differences between the bulk- and micro-viscosities obtained in the experiment reflect the intrinsic differences in solution environment sensed by the laser tweezer on the macro level and by other optical techniques within the probe microscopic environment.
We present a technique to measure the viscosity of microscopic
volumes of liquid using rotating optical tweezers. The technique
can be used when only microlitre (or less) sample volumes are
available, for example biological or medical samples, or to make
local measurements in complicated micro-structures such as cells.
The rotation of the optical tweezers is achieved using the
polarisation of the trapping light to rotate a trapped
birefringent spherical crystal, called vaterite. Transfer of
angular momentum from a circularly polarised beam to the particle
causes the rotation. The transmitted light can then be analysed to
determine the applied torque to the particle and its rotation
rate. The applied torque is determined from the change in the
circular polarisation of the beam caused by the vaterite and the
rotation rate is used to find the viscous drag on the rotating
spherical particle. The viscosity of the surrounding liquid can
then be determined. Using this technique we measured the viscosity
of liquids at room temperature, which agree well with tabulated
values. We also study the local heating effects due to absorption
of the trapping laser beam. We report heating of 50-70 K/W in the
region of liquid surrounding the particle.
SNARF-1 fluorochrome was used to functionalize 3μm diameter latex spheres making them sensitive to the pH of their environment, manifested as a change in their fluorescence. The fluorescence emission at 580nm was excited using a filtered xenon arc lamp at 515nm. A solution of functionalized latex spheres was placed between gold microelectrodes in a microfluidic channel. Optical tweezers were used to trap and manipulate the spheres in the vicinity of the microelectrodes, to map out the pH profile in the electrolyte solution, induced by passing 20 microsecond transient current pulses through the microelectrodes.
We investigate the dynamics of microscopic flow vortices. We create flow vortices by rotation of birefringent particles in optical tweezers. We then use either highly sensitive drag force measurements or video tracking to map the fluid velocity around that particle. The results obtained from these different methods are compared. Velocity profiles obtained for water agree very well with theoretical predictions. In contrast, we find a strong deviation of velocity profiles in a complex fluid from those predicted by simple theory.
We demonstrate the generation of second-harmonic radiation in transmission through periodic and disordered arrays of sub-wavelength metallic apertures. For circular apertures in a square lattice, the second-harmonic signal peaks at incidence angles corresponding to enhanced transmission of the fundamental beam of 800~nm wavelength except at small incidence angles where the local symmetry minimizes the effective second-order nonlinear susceptibility of the apertures. Even though the linear transmission of the fundamental beam can be more than five times greater through the periodic array as compared to a disordered array, the strength of the second harmonic from the disordered array is greater at large incidence angles. By breaking the local symmetry through the use of apertures of non-centrosymmetric shape, the second-harmonic output occurs at fundamental transmission resonances at small incidence angles.
We report a study on the optical forces between a pair of dielectric particles, based on quantum electrodynamics. At a fundamental level these forces result from a stimulated scattering process which entails a virtual photon relay between the two particles. Results for a variety of systems are secured from a completely general analysis that accommodates a system with arbitrary dielectric properties (with regard to shape, frequency response etc.) in an optical field of arbitrary complexity. Specific results are obtained and exhibited for: (a) optical forces between nanoparticles, and specifically between carbon nanotubes; (b) the effects of optical ordering, clustering and trapping associated with twisted (Laguerre-Gaussian) laser beams.
Fully-metal-coated near-field optical probes, based on a cantilever design, have been studied theoretically and experimentally. Numerical simulations prove that these structures allow non-zero modal emission of the electromagnetic field trough a 60 nm thick metallic layer, that is opaque when deposited on flat substrates. The far-field intensity patterns recorded experimentally correspond to the ones calculated for the fundamental and first excited LP modes. Moreover, this study demonstrates that a high confinement of the electromagnetic energy can be reached in the near-field, when illuminated with radially polarized light. Finally, it was verified that the confinement of the field depends on the volume of the probe apex. The coupling and transmission of transverse and longitudinal fields into the probes has been also investigated. Two kinds of probes with different metal coating roughness are considered. Transverse and longitudinal field distributions are obtained by focusing azimuthally and radially polarized beams produced by means of a liquid crystal plate. The focal plane is scanned using microfabricated probes in a collection mode configuration. It is found that the roughness of the metal coating plays an important role in the coupling strength of transverse fields into the probes: the relative coupling efficiency for transverse fields diminishes with a rough metal coating, while that of longitudinal fields does not.
There has been a tremendous interest about the trapping of a single biomolecule using nearfield optical trapping. The optical trapping of a biomolecule can be accomplished by controlling both scattering force on the molecule and field gradient force. In order to achieve nearfield optical trapping of the biomolecule, it seems that the radiant trapping force should be greater than the Brownian motion of the molecule in the liquid and the gravity. The radiation force is proportional to the nearfield intensity of the aperture. Though, the throughput of the conventional fiber probe is known to have weak light intensity due to the long, narrow waveguide. In order to better confine the molecule around the aperture, the greater throughput of the light intensity through the aperture is desirable due to wider tapered angle of waveguide. In this report, the nanosize circular metal shape around the subwavelength-size oxide aperture was designed and fabricated using physical metallic deposition of Au or bimetallic Al and Ti. The circular metallic shape (metallic nanoflower) around the subwavelength-size metallic aperture is supposed to focus the horizontal evanescent electromagnetic field toward the propagating direction. This can provide an enhanced evanescent field and an increased gradient force toward the axis of propagating direction. Therefore the nanoflower around the nano-aperture would be expected to better confine a bio-molecule in a nanoscale region.
We report experiments aimed at measuring the orbital angular momentum of light by means of a torsion pendulum, in the spirit of the classical spin angular momentum experiment by Beth (1936) but using present-day technology. Although our set-up has adequate sensitivity and resolution to measure orbital angular momentum of light, the systematic errors that are caused by the inherent asymmetry in the conversion of orbital angular moment remain a problem.
We describe basis sets of general astigmatic modes that are solutions of the paraxial wave equation. The fundamental modes are Gaussians with elliptical shapes of the spot size and elliptical or hyperbolic wave fronts. For a given fundamental mode, higher-order modes can be generated by the repeated application of two raising operators. The nature of the set of higher-order modes, corresponding to the Hermite-Gaussian, Laguerre-Gaussian, or intermediary modes, can be characterized by a point on a sphere, in direct analogy to the representation of polarization on the Poincare sphere. For general astigmatism, even the fundamental mode can carry high values of orbital angular momentum per photon. The additional angular momentum of higher modes as well as the vortex structure of the modes depends both on the degree of astigmatism and on the point on the sphere.
We elucidate the paraxial orbital angular momentum of entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) in different non-collinear geometries. To date, most investigations addressed SPDC in nearly collinear phase-matching geometries, where the pump, the signal and idler photons propagate coaxially almost along the same direction. However, non-collinear geometries introduce a variety of new features. The OAM of the entangled photons strongly depend on the propagation direction of the photons. Here we show that locally paraxial measurements of the OAM conducted with entangled photons generated in non collinear geometries, they do not comply with the known selection rules for the spiral index of the pump, signal and idler mode functions (Mair et al., Nature 412, 313 (2001)). In particular, we find the orbital angular momentum of entangled pairs generated in purely transverse-emitting configurations, where the entangled photons counter-propagate perpendicularly to the direction of propagation of the pump beam. In transverse emitting configurations, the spatial shape of the down converted in one transverse dimensions strongly depends on the corresponding spatial shape of the input pump beam, while in the other transverse dimension, the shape is tailored by the longitudinal phase matching. The spatial walk-off of all interacting waves in the parametric process also determines the OAM content of the down-converted photons, and here its influence is also revealed.
We examine novel features that might emerge from the interaction of Laguerre-Gaussian beams with liquid crystals. We study the response of nematic liquid crystal media to the throughput of twisted laser light. Specific attention is focused on the spatial evolution of the director orientation angle.
Propagation of light beams with apparent nondiffracting properties have intrigued the scientific community since they were introduced. In this talk we will introduce the fundamentals of nondiffracting beams and discuss the dynamics of optical vortices embedded in the new two families of nondiffracting beams we have recently discovered, Mathieu and parabolic beams.
We study the geometric phase that is acquired when the second-order mode of an optical beam undergoes a cyclic transformation. We find that a geometric phase appears when the initial and intermediate modes have different quantities of orbital angular momentum. The phase is similar to the one measured previously for transformations of first-order modes. However, we find no accumulated geometric phase when the initial and intermediate modes have zero orbital angular momentum.
The two-photon state generated by spontaneous parametric down-conversion (SPDC) exhibit spatial entanglement embedded in the corresponding mode function. The control of the spatial characteristics of the generated two-photon state is an issue of paramount importance. For example, the spatial entanglement of the two down converted photons forms the basis of quantum imaging, and entanglement in orbital angular momentum has opened a new scenario for implementing multidimensional Hilbert spaces. We put forward several techniques to engineer the spatial structure of entangled two-photon states generated in SPDC. The first strategy we consider for spatial control of the quantum state makes use of the direct manipulation of the pump beam. This technique makes feasible to prepare arbitrary engineered entangled states in any d-dimensional Hilbert space. The second strategy is based on the proper preparation of the down-converting crystal itself, namely quantum state manipulation by quasi-phase-matching (QPM) engineering. We use properly designed transversely varying QPM gratings in nonlinear crystals.
We describe the production of BECs on a new type of atom chip based on silver foil. Our atom chip is fabricated with thick wires capable of carrying currents of several amperes without overheating. The silver surface is highly reflective to light resonant with optical transitions used for Rb. The pattern on the chip consists of two parallel Z-trap wires, capable of producing two-wire guide, and two additional endcap wires for varying the axial confinement. Condensates are produced in magnetic microtraps formed within 1 mm of surface of the chip. We have observed the fragmentation of cold atom clouds when brought close to the chip surface. This results from a perturbed trapping potential caused by nanometer deviations of the current path through the wires on the chip. We present results of fragmentation of cold clouds at distances below 100 μm from the wires and investigate the origin of the deviating current. The fragmentation has different characteristics to those seen with copper conductors. The dynamics of atoms in these microtraps is also investigated.
Optical micro-manipulation has seen a resurgence of interest in recent years which has been due in part to new application areas and the use of tailored forms of light beam. In this paper, experimental observations of fluctuation-driven transport of silica microspheres within a two-dimensional optical potential of circular symmetry are observed. The potential is created by a Bessel light beam. The optical field is tailored to break the symmetry and create a static tilted periodic (washboard) potential. Transitions between locked and running modes may be observed. The running mode manifests itself by rapid accumulation of particles at the beam centre. We discuss what happens with mixtures of particles in such an optical potential.