Damping of a unitary Fermi gas

We measure the temperature dependence of the radial breathing mode in an optically trapped, unitary Fermi gas of 6Li, just above the center of a broad Feshbach resonance. The damping rate reveals a clear change in behavior which we interpret as arising from a superfluid transition. We suggest pair breaking as a mechanism for an increase in the damping rate which occurs at temperatures well above the transition. In contrast to the damping, the frequency varies smoothly and remains close to the unitary hydrodynamic value. At low temperature T, the damping depends on the atom number only through the reduced temperature, and extrapolates to 0 at T = 0.     

Transient enhancement and detuning of laser-driven parametric instabilities by particle trapping

Kinetic simulations of backward stimulated Raman scattering (BSRS), where the Langmuir wave coherence time is greater than the bounce time for trapped electrons, yield transient reflectivity levels far above those predicted by fluidlike models. Electron trapping reduces the Langmuir wave damping and lowers the Langmuir wave frequency, and leads to a secular phase shift between the Langmuir wave and the BSRS beat ponderomotive force. This phase shift detunes and saturates BSRS and a similar effect, due to ion trapping, is the saturation mechanism for backward stimulated Brillouin scattering. Competition with forward SRS is discussed.     

Marginally trapped surfaces in spaces of oriented geodesics

We investigate the geometric properties of marginally trapped surfaces (surfaces which have null mean curvature vector) in the spaces of oriented geodesics of Euclidean 3-space and hyperbolic 3-space, endowed with their canonical neutral Kaehler structures. We prove that every rank one surface in these four manifolds is marginally trapped. In the Euclidean case we show that Lagrangian rotationally symmetric sections are marginally trapped and construct an explicit family of marginally trapped Lagrangian tori. In the hyperbolic case we explore the relationship between marginally trapped and Weingarten surfaces, and construct examples of marginally trapped surfaces with various properties.     

Simulating the Klein tunneling of pseudospin-one Maxwell particles with trapped ions

We propose an experimental scheme to simulate and observe the Klein tunneling of relativistic Maxwell particles with trapped ions. We explore the scattering dynamics of the pseudospin-one Maxwell particles and demonstrate that the scattered state should be a superposition of a reflection state, a localization state, and a transmission state. The probabilities of these states can be analytically obtained by the approach of Landau-Zener transition. We further show that the Maxwell Hamiltonian and the associated scattering dynamics can be mimicked with two trapped ions. The Maxwell spinors are encoded by three internal states of the first ion, the position and momentum are described by those of the motional modes, and the desired linear potential barrier is built by the second ion.     

Measurement of elastic light scattering from two optically trapped microspheres and red blood cells in a transparent medium

Optical tweezers can be used to manipulate small objects and cells. A trap can be used to fix the position of a particle during light scattering measurements. The places of two separately trapped particles can also be changed. In this Letter we present elastic light scattering measurements as a function of scattering angle when two trapped spheres are illuminated with a He-Ne laser. This setup is suitable for trapping noncharged homogeneous spheres. We also demonstrate measurement of light scattering patterns from two separately trapped red blood cells. Two different illumination schemes are used for both samples.     

Sound absorption coefficient measurements by phase-conjugate ultrasonic waves

Measurements of the sound absorption coefficient in test objects containing solid microparticles randomly distributed over the object volumes are carried out. Two methods are used for this purpose: the standard echo-pulse insert-substitution method and a modified method using phase conjugation of ultrasound. The test objects are made from gelatin, and the size of the particles introduced in it is chosen to allow measurements in both the long- and medium-wavelength scattering modes of the probing beam. It is shown that, in the first scattering mode, in which the presence of particles causes additional viscous and temperature losses, the two aforementioned methods give identical results. In the second scattering mode, in which the dominant mechanism of additional loss is elastic scattering, the use of phase conjugation allows an almost complete reconstruction of the scattered field and, hence, a more reliable upper estimate for the coefficient of ultrasonic absorption in the test objects.     

Three-dimensional optical trapping of partially silvered silica microparticles

We demonstrate three-dimensional trapping of micrometer-diameter silica particles, partially coated with silver, within conventional optical tweezers. Although metallic particles are usually repelled from the beam focus by the scattering force, we show that transparent spheres partially coated with silver can be trapped with efficiencies comparable with dielectric particles. The trapping characteristics of these particles are examined as a function of metallic coverage, and the application of these particles to surface-enhanced resonance Raman scattering is investigated.     

Manipulation of Particles with Counter-Propagating Evanescent Waves

Two counter-propagating evanescent beams are used to align and manipulate polystyrene particles on a prism surface. Since the radiation pressure transferred laterally from the evanescent wave is negated on both sides, particles can be stably aligned. By projecting a circular and a linear beam spot onto the interface, both multiple and single arrays of particles are achieved. Arrays of particles trapped on the interface can be easily moved adjusting the intensity of incident beams on either side. We also simulate electromagnetic distribution of scattering light that is converted from the evanescent wave using the FDTD method. The results show that scattering light converts from an evanescent wave propagating through a particle array and has a distance longer than that propagating from a normal evanescent wave.     

Collisionless Trapped Ion Temperature Gradient Instabilities

Under the fluid limit, the collisionless electrostatic trapped ion temperature gradient instabilities are investigated by retaining the trapped ion parallel compressibility in the long wavelength limit. The two-scale analysis should be employed in view of the fact that the eigenfunctions vary over the connection width scale with an envelope varying over a secular scale. The results show that the slab branch of trapped ion modes, corresponding to the aperiodic trapped ion instability discovered by Kadomtzev, propagates along ion diamagnetic direction. The toroidd branch is also obtained in low-frequency and long wavelength limit. In this case, besides the unstable one propagating in ion diamagnetic direction, there exists a marginally stable toroidal odd mode propagating in electron diamagnetic direction. The eigenfrequencies and perturbative structures of these unstable modes are given analytically and numerically. The numerical results are in good agreement with the analytical ones.     

Chaotic scattering,transport, and fractals in a simple hydrodynamic flow

Advection of passive tracers in an unsteady hydrodynamic flow consisting of a background stream and a vortex is analyzed as an example of chaotic particle scattering and transport. A numerical analysis reveals a nonattracting chaotic invariant set Λ that determines the scattering and trapping of particles from the incoming flow. The set has a hyperbolic component consisting of unstable periodic and aperiodic orbits and a nonhyperbolic component represented by marginally unstable orbits in the particle-trapping regions in the neighborhoods of the boundaries of outer invariant tori. The geometry and topology of chaotic scattering are examined. It is shown that both the trapping time for particles in the mixing region and the number of times their trajectories wind around the vortex have hierarchical fractal structure as functions of the initial particle coordinates. The hierarchy is found to have certain properties due to an infinite number of intersections of the stable manifold in Λ with a material line consisting of particles from the incoming flow. Scattering functions are singular on a Cantor set of initial conditions, and this property must manifest itself by strong fluctuations of quantities measured in experiments.     

Dipole oscillations of a Bose-Einstein condensate in the presence of defects and disorder

We consider dipole oscillations of a trapped dilute Bose-Einstein condensate in the presence of a scattering potential consisting either in a localized defect or in an extended disordered potential. In both cases the breaking of superfluidity and the damping of the oscillations are shown to be related to the appearance of a nonlinear dissipative flow. At supersonic velocities the flow becomes asymptotically dissipationless.     

Resonant waveguide field enhancement in dimers

It is shown that a waveguide mode, which exists between metal dimer particles and does not cut off, produces an enhanced electric field and hence a mechanism for surface-enhanced Raman scattering. Structures can thus be designed that provide large average field enhancement over significant length scales.     

Light scattering studies of Brownian motion in confined geometries

Summary Dynamic light scattering can be a useful tool to determine the confinement of Brownian particles whose motion is restricted to dimensions comparable to the wavelength of the light. The theoretical form of the correlation function of the electric field scattered from such trapped particles has been derived and compared with the signal obtained both in a simulated experiment and in a real experiment where the particles are trapped in a glass wedge. This new result can be of relevance for particles trapped in various media such as a porous (transparent) media, a gel, a suspension of lamellar phases or even a concentrated colloidal suspension where a particle is ?trapped? by its neighbours. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Saturation of backward stimulated scattering of a laser beam in the kinetic regime

Stimulated Raman (SRS) and Brillouin scattering (SBS) are examined in the kinetic regime using particle-in-cell simulations. Wave front bowing of electron-plasma waves (ion-acoustic waves) from trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time. Self-focusing from trapped particle modulational instability (TPMI) is shown to occur in 2D and 3D SRS simulations. The key physics of SRS saturation is identified as a combination of wave front bowing, TPMI, and self-focusing: Bowing marks the beginning of SRS saturation and self-focusing terminates SRS. Ion-acoustic wave bowing also contributes to SBS saturation. Velocity diffusion by transverse modes and rapid loss of hot electrons in regions of small transverse extent formed from self-focusing dissipate wave energy and increase Landau damping, despite trapping that reduces Landau damping initially.     

Image enhancement in near-field scanning optical microscopy with laser-trapped metallic particles

A trapped-particle near-field scanning optical microscope is constructed by use of submicrometer- or micrometer-sized metallic particles (gold and silver) to increase scattering efficiency. The image contrast of the evanescent-wave interference pattern on the surface of a prism upon total internal reflection, obtained with trapped gold particles of diameter 0.1 and 2microm , is improved by a factor of approximately 2 and 1.5, respectively, compared with that obtained with trapped polystyrene particles of similar size. The use of a 2-microm gold particle leads to image contrast that is approximately three times as great as that obtained with a 0.1-microm gold particle, and interference patterns of a subwavelength period are obtained in both cases.     

Trapped-particle instability leading to bursting in stimulated Raman scattering simulations

Nonlinear, kinetic simulations of stimulated Raman scattering (SRS) under laser-fusion conditions present a bursting behavior. Different explanations for this regime have been given in previous studies: saturation of SRS by increased nonlinear Landau damping [K. Estabrook et al., Phys. Fluids B 1, 1282 (1989)]], and detuning due to the nonlinear frequency shift of the plasma wave [H. X. Vu et al., Phys. Rev. Lett. 86, 4306 (2001)]]. Another mechanism, also assigning a key role to the trapped electrons is proposed here: the breakup of the plasma wave through the trapped-particle instability.     

Plasma wave frequency shift in a weak transverse magnetic field due to trapped particle acceleration

A simple model of nonlinear electrostatic wave–particle interaction in a weak magnetic field perpendicular to the direction of wave propagation is developed. The damping of the wave loaded with the phase bunched groups of trapped particles is considered with the aid of the model equations. To determine the nonlinear frequency shift of the wave in the process of the trapped particle acceleration, the nonlinear dispersion equation is derived. It is shown that the corresponding variation of the phase velocity may affect the interaction process and hence must be taken into account in the self-consistent treatment of the time evolution of the wave.     

Enhanced damping of electrostatic waves due to combination scattering from density oscillation in plasmas

Experiments demonstrating the combination scattering of the electrostatic wave from the electron density oscillation in plasmas are described. The spatially periodic energy transfer among the satellite modes and the primary mode is observed, which results in an enhanced damping of the primary mode.     

Trapped waves in a cranked waveguide with hard walls

The presence of a trapped mode in an asymmetric waveguide with slightly deformed hard walls—in particular, in a cranked waveguide—is revealed. The perturbation profile is subject to certain integral conditions. The result is obtained by analyzing the augmented scattering matrix.     

Metrology of laser-guided particles in air-filled hollow-core photonic crystal fiber

Micrometer-sized particles are trapped in front of an air-filled hollow-core photonic crystal fiber using a novel dual-beam trap. A backward guided mode produces a divergent beam that diffracts out of the core, and simultaneously a focused laser beam launches a forward-propagating mode into the core. By changing the backward/forward power balance, a trapped particle can be selectively launched into the hollow core. Once inside, particles can be optically propelled along several meters of fiber with mobilities as high as 19 cm·s(-1) W(-1) (precisely measured using in-fiber Doppler velocimetry). The results are in excellent agreement with theory. The system allows determination of fiber loss as well as the mass density and refractive index of single particles.