Axial Trapping Research Articles

Heat Transfer on MHD Peristaltic Rotating Flow of a Jeffrey Fluid in an Asymmetric Channel

This paper addresses heat transfer and rotation effects on MHD peristaltic flow of a Jeffrey fluid in an asymmetric channel with partial slip. Mathematical modeling is carried out by utilizing long wavelength and small Reynolds number assumptions. The analytical solution has been computed for the stream function, velocity and temperature distribution. The expression for pressure rise is carried out using numerical integration. The effects of various emerging parameters on the flow characteristics are shown and discussed with the help of graphs. The pumping characteristics, axial pressure gradient and trapping phenomenon have been studied. The axial velocity depresses with increasing of Hartmann number, Jeffrey fluid parameter and slip parameter. The size of trapping bolus enhances when an increase in Hartmann number, Jeffrey fluid parameter and rotation parameter. The temperature diminishes as the larger values of slip parameter. A comparison of a special case of our results with the one previously reported in the literature shows a very good agreement.

Theory of optical-tweezers forces near a plane interface

Optical-tweezers experiments in molecular and cell biology often take place near the surface of the microscope slide that defines the bottom of the sample chamber. There, as elsewhere, force measurements require force-calibrated tweezers. In bulk, one can calculate the tweezers force from first principles, as recently demonstrated. Near the surface of the microscope slide, this absolute calibration method fails because it does not account for reverberations from the slide of the laser beam scattered by the trapped microsphere. Nor does it account for evanescent waves arising from total internal reflection of wide-angle components of the strongly focused beam. In the present work we account for both of these phenomena. We employ Weyl's angular spectrum representation of spherical waves in terms of real and complex rays and derive a fast-converging recursive series of multiple reflections that describes the reverberations, including also evanescent waves. Numerical simulations for typical setup parameters evaluate these effects on the optical force and trap stiffness, with emphasis on axial trapping. Results are in good agreement with available experimental data. Thus, absolute calibration now applies to all situations encountered in practice.

Acoustic equations for a gas stream in rigid-body rotation

The classical topic of wave propagation in a rotating gas is revisited by deducing scalar wave equations for propagation of acoustic and rotational waves through a plug flow of gas in rigid-body rotation with arbitrary intensities of the radial stratification. In the light of these novel equations, wave propagation is analyzed in two different base gas states: isothermal and homentropic. In both cases, previous findings are recovered that assess the validity of the equations and new results are established. In the non-homentropic but isothermal case, the set of governing equations is reduced to two coupled scalar wave equations with space dependent coefficients for the disturbances of density and pressure. Travelling wave solutions with variable amplitude have been obtained in the limit of weak stratification both for inertial waves as for acoustic waves which, in general, propagate on different frequency bands that overlap in the small wavenumber region. Furthermore, the entropy stratification in the base state is stable and compels the propagation of internal waves, leading to hybrid acoustic-inertial-vortical modes. In the homentropic case, the adiabatic relation between pressure and density disturbances allows to reduce further the governing equations to a single fourth-order scalar wave equation. In this case, the sound propagation velocity depends on the distance to the rotation axis and solutions are found by multiple-scale analyses in the form of waves with slowly varying amplitude and wavenumber. The corresponding eikonal equation shows that acoustic rays are refracted towards the rotation axis, propagating and spinning along and around it. In that way, the swirling gas behaves as an axial waveguide trapping inside any acoustic ray propagating in the vortex with large enough azimuthal and/or vertical wavenumber component.

Theoretical investigation on nonlinear optical effects in laser trapping of dielectric nanoparticles with ultrafast pulsed excitation.

The use of low-power high-repetition-rate ultrafast pulsed excitation in stable optical trapping of dielectric nanoparticles has been demonstrated in the recent past; the high peak power of each pulse leads to instantaneous trapping of a nanoparticle with fast inertial response and the high repetition-rate ensures repetitive trapping by successive pulses However, with such high peak power pulsed excitation under a tight focusing condition, nonlinear optical effects on trapping efficiency also become significant and cannot be ignored. Thus, in addition to the above mentioned repetitive instantaneous trapping, trapping efficiency under pulsed excitation is also influenced by the optical Kerr effect, which we theoretically investigate here. Using dipole approximation we show that with an increase in laser power the radial component of the trapping potential becomes progressively more stable but the axial component is dramatically modulated due to increased Kerr nonlinearity. We justify that the relevant parameter to quantify the trapping efficiency is not the absolute depth of the highly asymmetric axial trapping potential but the height of the potential barrier along the beam propagation direction. We also discuss the optimal excitation parameters leading to the most stable dipole trap. Our results show excellent agreement with previous experiments.

Single fiber optical trapping of a liquid droplet and its application in microresonator

We propose and demonstrate an optical trapping of a liquid droplet and its application based on an annular core microstructured optical fiber. We grind and polish the annular core fiber tip to be a special frustum cone shape to make sure the optical force large enough to trap the liquid droplet non-intrusively. The axial and transverse optical trapping forces are simulated. In addition, we investigate the whispering gallery modes resonance characteristic of the trapped liquid droplet as the example of applications. The whispering gallery modes spectrum is sensitive to the size of the micro liquid droplet. Due to the simple construction and flexible manipulation, the fiber-based optical trapping technology for micro liquid droplets trapping, manipulating, and controlling has great application penitential in many fields, such as physics, biology, and interdisciplinary studies.

Parametric cooling of a degenerate Fermi gas in an optical trap

We demonstrate a novel technique for cooling a degenerate Fermi gas in a crossed-beam optical dipole trap, where high-energy atoms can be selectively removed from the trap by modulating the stiffness of the trapping potential with anharmonic trapping frequencies. We measure the dependence of the cooling effect on the frequency and amplitude of the parametric modulations. It is found that the large anharmonicity along the axial trapping potential allows to generate a degenerate Fermi gas with anisotropic energy distribution, in which the cloud energy in the axial direction can be reduced to the ground state value.

Improved Axial Optical Trapping

Conventional optical tweezers suffer from several complications when applying axial forces to surface-tethered molecules. Aberrations from the refractive-index mismatch between an oil-immersion objective's medium and the aqueous trapping environment degrade the trapping strength with focal depth. Furthermore, interference effects from back-scattered light make it difficult to use back-focal-plane interferometry for high-bandwidth position detection. Holographic optical tweezers were employed to correct for aberrations to achieve a constant axial stiffness and modulate artifacts from backscattered light. By discarding light from the centre of the trapping beam, interference from back-reflections could be reduced, but not eliminated without significant degradation to the trapping strength. However, careful calibration of the detector sensitivity allows for the application and measurement of axial forces on small surface-tethered molecules. The use of a donut beam to reduce the enhanced photobleaching of fluorophores by the trapping light was also investigated with the goal of combining our holographic axial tweezers with single-molecule fluorescence.

Optical trapping of a dielectric-covered metallic microsphere

In this paper we propose a ray-optics model of single-beam gradient trap for a dielectric-covered metallic microsphere and calculate the optical forces exerted by a single incident optical ray striking the microsphere. The axial trapping forces experienced by the copper and silver microspheres covered with polystyrene are analyzed, respectively. The results show that, unlike the case for a purely dielectric Mie particle, the trapping positions for a metallic microsphere covered with dielectric depend on the thickness of the dielectric shell. In the experiments copper particles covered with polymer are trapped in the bright region rather than in the dark region of a focused vortex beam and rotate owing to the orbital angular momentum of the beam. The experimental results are in good agreement with the theoretical analysis.

Intrinsic heating in optically trapped Au nanoparticles measured by dark-field spectroscopy.

Assessing the degree of heating present when a metal nanoparticle is trapped in an optical tweezers is critical for its appropriate use in biological applications as a nanoscale force sensor. Heating is necessarily present for trapped plasmonic particles because of the non-negligible extinction which contributes to an enhanced polarisability. We present a robust method for characterising the degree of heating of trapped metallic nanoparticles, using the intrinsic temperature dependence of the localised surface plasmon resonance (LSPR) to infer the temperature of the surrounding fluid at different incident laser powers. These particle specific measurements can be used to infer the rate of heating and local temperature of trapped nanoparticles. Our measurements suggest a considerable amount of a variability in the degree of heating, on the range of 414-673 K/W, for different 100 nm diameter Au nanoparticles, and we associated this with variations in the axial trapping position.

Radiation forces of highly focused radially polarized hollow sinh-Gaussian beams on a Rayleigh metallic particle

Based on the vector diffraction theory, the tight focusing properties of radially polarized hollow sinh-Gaussian (HsG) beams are theoretically studied. It is found that the radially polarized HsG beams can form a longitudinally polarized sub-wavelength focal spot. Moreover, the radiation forces acting on a Rayleigh metallic particle are calculated for the case where the radially polarized HsG beams are applied. Compared with the use of conventional Gaussian beams, the high-order radially polarized HsG beams can largely enhance the radial trap stiffness and broaden the axial trap distance. The influence of the beam order m on the focusing properties and trap stiffness is investigated in detail.

Study on particle size dependence of axial trapping efficiency

An optical tweezers system using laser beams with a Gaussian intensity profile and doughnut intensity profiles made by hollow core optical fiber and axicon lenses, respectively, was constructed. The axial trapping efficiencies for the three intensity profiles were measured and compared with each other. The particle size dependence of axial trapping efficiencies in the range of the particle diameter from 1 to 20 μm were analyzed by using the modified ray optics model [Appl. Opt.33, 1735 (1994)].

Flow Characteristics of Distinctly Viscous Multilayered Intestinal Fluid Motion.

The goal of this investigation is to study the three layered (core layer, intermediate layer, and peripheral layer) tubular flow of power law fluids with variable viscosity by peristalsis in order to investigate the strength of the role played by an artificially generated intermediate layer to ease constipation. The solution is carried out under the long wavelength and low Reynolds number approximations in the wave frame of reference as the flow is creeping one. The stream functions for each layer such as core layer, intermediate layer, and peripheral layer are determined. The expressions for axial pressure gradient, interfaces, trapping, and reflux limits are obtained. The effects of power law index and viscosities on pressure across one wavelength, mechanical efficiency, and trapping are discussed numerically. It is found that the pressure required to restrain flow rates and the mechanical efficiency increase with the viscosities of the intermediate and peripheral layers as well as with the flow behaviour index. It is observed that the axisymmetric flow in intestines is less prone to constipation than two-dimensional flow and may be more easily overcome with introducing a viscous intermediate layer.

Radiation force exerted on a sphere by focused Laguerre-Gaussian beams.

The generalized Lorenz-Mie theory is employed to calculate the force exerted on a sphere by focused Laguerre-Gaussian beams. The key parameters of the theory, namely, the multipole coefficients of the beams, are exactly derived from the beams' angular spectra in terms of some auxiliary coefficients. Several recurrence formulas, which can improve the calculation of the auxiliary coefficients and accordingly the force, are also derived. According to the calculated force, the trapping performances of the beams are investigated in the Mie regime. It is found that low(high)-azimuthal-order beams usually have advantages in the radial trapping of the high(low)-refractive-index sphere and the axial trapping of the low(high)-refractive-index sphere. The influences of the parameters of the beams, lens, and sphere on the trapping performance are also investigated.

On-chip optical trapping and Raman spectroscopy using a TripleX dual-waveguide trap.

We present a new approach to the dual-beam geometry for on-chip optical trapping and Raman spectroscopy, using waveguides microfabricated in TripleX technology. Such waveguides are box shaped and consist of SiO2 and Si3N4, so as to provide a low index contrast with respect to the SiO2 claddings and low loss, while retaining the advantages of Si3N4. The waveguides enable both the trapping and Raman functionality with the same dual beams. Polystyrene beads of 1 μm diameter can be easily trapped with the device. In the axial direction discrete trapping positions occur, owing to the intensity pattern of the interfering beams. Trapping events are interpreted on the basis of simulated optical fields and calculated optical forces. The average transverse trap stiffness is 0.8 pN/nm/W, indicating that a strong trap is formed by the beams emitted by the waveguides. Finally, we measure Raman spectra of trapped beads for short integration times (down to 0.25 s), which is very promising for Raman spectroscopy of microbiological cells.

Optical trapping of core-shell magnetic microparticles by cylindrical vector beams

Optical trapping of core-shell magnetic microparticles is experimentally demonstrated by using cylindrical vector beams. Second, we investigate the optical trapping efficiencies. The results show that radially and azimuthally polarized beams exhibit higher axial trapping efficiencies than the Gaussian beam. Finally, a trapped particle is manipulated to kill a cancer cell. The results make possible utilizing magnetic particles for optical manipulation, which is an important advantage for magnetic particles as labeling agent in targeted medicine and biological analysis.

First and second sound of a unitary Fermi gas in highly elongated harmonic traps

Using a variational approach, we present the full solutions of the simplified one-dimensional two-fluid hydrodynamic equations for a unitary Fermi gas trapped in a highly elongated harmonic potential, which was recently derived by G. Bertaina and coworkers [Phys. Rev. Lett. 105, 150402 (2010)]. We calculate the discretized mode frequencies of first and second sound along the weak axial trapping potential, as a function of the temperature and the form of superfluid density. We show that the density fluctuations in second-sound modes, due to their coupling to first-sound modes, are large enough to be measured in current experimental setups such as that exploited by M. K. Tey et al. at the University of Innsbruck [Phys. Rev. Lett. 110, 055303 (2013)]. Owing to the sensitivity of second sounds on the form of superfluid density, the high precision of the measured second-sound frequencies may provide us a promising way to accurately determine the superfluid density of a unitary Fermi gas, which so far has remained elusive.

Influence of Lateral Walls on Peristaltic Flow of a Couple Stress Fluid in a Non-Uniform Rectangular Duct

In the present investigation we have studied the peristaltic flow of a couple str ess fluid in a non-uniform rectangular duct. The flow is investigated in a wave frame of reference moving with the velocity c away from the fixed frame. The peristaltic waves propagating on the horizontal side walls of a non-uniform rectangular duct is studied under lubrication approach. The exact solutions of velocity and pressure gradient have been found under lubrication approach. The pumping characteristics, axial pressure gradient, velocity field and trapping phenomena have been discussed to highlight the physical features of emerging parameters of couple stress fluid.

Adiabatic quantum simulation with a segmented ion trap: Application to long-distance entanglement in quantum spin systems

We investigate theoretically systems of ions in segmented linear Paul traps for the quantum simulation of quantum spin models with tunable interactions. The scheme is entirely general and can be applied to the realization of arbitrary spin-spin interactions. As a specific application we discuss in detail the quantum simulation of models that exhibit long-distance entanglement in the ground state. We show how tailoring of the axial trapping potential allows for generating spin-spin coupling patterns that are suitable to create long-distance entanglement. We discuss how suitable sequences of microwave pulses can implement Trotter expansions and realize various kinds of effective spin-spin interactions. The corresponding Hamiltonians can be varied on adjustable time scales, thereby allowing the controlled adiabatic preparation of their ground states.

Sensitivity analysis of light force accelerometer based on optical trapping Mie microsphere

For achieving the greatest sensitivity, numerical simulation is proposed to optimize the parameters of light force accelerometer (LFA). Firstly, the basic principle of the LFA is analyzed, and the mathematical model is established to describe the axial and radial optical trapping forces on microspheres in the fundamental mode Gaussian laser beam. Secondly, the axial and radial optical trapping forces applied on Mie spheres are simulated to obtain the optimal parameters of open-loop LFA. Results show that the sensitivity can reach 103 μm/g. Finally, the LFA system is proposed based on the closed-loop scheme with dual beams. This can provide reference for the design and fabrication of LFA in future.

Comparative Analysis of Trapping Efficiencies between Gaussian and Laguerre-Gaussian beams for Optical Tweezers

Using numerical calculations based on a mathematical model, this paper shows how the axial trapping efficiency (Qz) and the radial trapping efficiency (Qr) of optical tweezers depend on specific parameters such as size of the dielectric particle (r), numerical aperture of the microscope objective (NA), refractive index of the dielectric particle (np) to be trapped and the refractive index of the surrounding medium (nm) in which the particle is being trapped. The model is used to calculate the trapping efficiencies and the forces acting on a particle. The calculations are done for two types of laser beams: Gaussian and LaguerreGausssian. Results for both types are then compared in terms of effective trapping. For all of the simulations, the particle being trapped is considered to be a micrometer-sized spherical polystyrene bead.