Trapping colloidal dielectric microparticles with overlapping evanescent optical waves

We experimentally demonstrate the creation of a stable surface trap for colloidal microparticles in a high-intensity evanescent optical field that is produced by total internal reflection of two counter-propagating and mutually incoherent laser beams. While the particles confined in the trap undergo fast Brownian motion, they never “stick” to the surface – not even at high optical powers – but rather levitate above the surface. If many particles are stored in the trap, they tend to form a well ordered self-organized array. We apply a numerical model based on the general energy-momentum tensor formalism to evaluate the overall optical force acting on a trapped particle. The optical-field parameters are calculated using the finite element method. The simulations show that for small particles a sharp repulsive potential at the surface – required for the levitation – can have neither optical nor light-induced thermal origin. Among the possible non-optical forces, electrostatic double-layer repulsion is often considered to be the origin of the levitation. We find, however, that the experimentally observed levitation of small particles in a high-intensity evanescent-wave trap cannot be explained by this effect.     

Influence of Agglomerates of Conducting Spheres on the Response of a Phase-Doppler Anemometer

The influence of agglomerates of solid conducting spheres on the response of a phase-Doppler anemometer (PDA) is described for a two-sphere system by using a ray theory model. First- and second-order reflection and diffraction are considered for far-field calculations of the PDA phase difference. The numerical simulations are accompanied and supported by experimental results. Two-sphere systems of Sn63Pb37 alloy particles were captured inside an electrodynamic trap and investigated with a standard phase-Doppler system.     

Calibration and numerical simulation of Nanoparticle Surface Area Monitor (TSI Model 3550 NSAM)

TSI Nanoparticle Surface Area Monitor (NSAM) Model 3550 has been developed to measure the nanoparticle surface area deposited in different regions of the human lung. It makes use of an adjustable ion trap voltage to match the total surface area of particles, which are below 100 nm, deposited in tracheobronchial (TB) or alveolar (A) regions of the human lung. In this paper, calibration factors of NSAM were experimentally determined for particles of different materials. Tests were performed using monodisperse (Ag agglomerates and NaCl, 7–100 nm) and polydisperse particles (Ag agglomerates, number count mean diameter below 50 nm). Experimental data show that the currents in NSAM have a linear relation with a function of the total deposited nanoparticle surface area for the different compartments of the lung. No significant dependency of the calibration factors on particle materials and morphology was observed. Monodisperse nanoparticles in the size range where the response function is in the desirable range can be used for calibration. Calibration factors of monodisperse and polydisperse Ag particle agglomerates are in good agreement with each other, which indicates that polydisperse nanoparticles can be used to determine calibration factors. Using a CFD computer code (Fluent) numerical simulations of fluid flow and particle trajectories inside NSAM were performed to estimate response function of NSAM for different ion trap voltages. The numerical simulation results agreed well with experimental results.     

Single-file diffusion on a periodic substrate

An assembly of "nonpassing" particles diffusing on a one-dimensional periodic substrate is shown to undergo single-file diffusion for both noiseless (ballistic) and stochastic dynamics. The dependence of the corresponding diffusion coefficients on the density and temperature of the particles and on the substrate parameters is determined by means of numerical simulations and analytically interpreted within the formalism of standard Brownian motion.     

Non-monotonic size effects on the structure and thermodynamics of Coulomb clusters in three-dimensional harmonic traps

Finite-size effects on the static and thermodynamical properties of small three-dimensional clusters of identical charged particles confined by an harmonic trap are investigated using global optimization and numerical simulations. The relative stabilities of clusters containing up to 100 particles are estimated from the second energy derivatives, as well as from the energy gap between the two lowest-energy structures at a given size. We also provide a lower bound for the number of permutationally independent minima, as a function of size, up to n=75. Molecular dynamics and exchange Monte Carlo simulations are performed to get insight into the finite temperature behaviour of these clusters. By focusing on specific sizes, we illustrate the interplay between the stable structures, the possible competition between different isomers, and the melting point. In particular, we find that the orientational melting phenomenon known in two-dimensional clusters has an equivalent form in some three-dimensional clusters. The vibrational spectra, computed for all sizes up to 100, shows an increasing number of low-frequency modes, but comparing to hydrodynamical theory reveals strong correlation effects. Finally, we investigate the effects of the trap anisotropy on the general shape of Coulomb clusters, and on the melting point of a selected case.     

Optical trapping microfabrication with electrophoretically delivered particles inside glass capillaries

We have developed a new technique for rapid microfabrication that uses electrophoretically delivered particles and an optical trap. The material particles, micrometer- and nanometer-sized polystyrene beads in aqueous solution, are continuously delivered to an optical trap by means of the electrophoretic effect inside glass capillaries or similar microstructures. The optical trap is used to manipulate and deposit the polystyrene beads onto a substrate. The continuous, on-demand delivery of particles allows for microfabrication in two and three dimensions with high speed and high efficiency and without material waste. This new technique has many potential applications in microelectronics and biotechnology.     

Structure and Spectrum of Binary Classic Systems Confined in a Parabolic Trap

The static and dynamic properties of the two-dimensional classic system of two-species interacting charged particles in a parabolic trap are studied. The ground state energy and configuration for different kinds of binary systems are obtained by Monte Carlo simulation and Newton optimization. The spectrum and normal modes vectors can be gained by diagonalizing the dynamical matrix of the system. It is found that the total particle number, particle number and mass-to-charge ratio of each species are decisive factors for the system structure and spectrum. The three intrinsic normal modes of single species Coulomb clusters are inherent, concluded from our numerical simulations and analytical results.     

Role of quantum statistics in multi-particle decay dynamics

The role of quantum statistics in the decay dynamics of a multi-particle state, which is suddenly released from a confining potential, is investigated. For an initially confined double particle state, the exact dynamics is presented for both bosons and fermions. The time-evolution of the probability to measure two-particle is evaluated and some counterintuitive features are discussed. For instance, it is shown that although there is a higher chance of finding the two bosons (as oppose to fermions, and even distinguishable particles) at the initial trap region, there is a higher chance (higher than fermions) of finding them on two opposite sides of the trap as if the repulsion between bosons is higher than the repulsion between fermions. The results are demonstrated by numerical simulations and are calculated analytically in the short-time approximation. Furthermore, experimental validation is suggested.     

Trapping of neutral rubidium with a macroscopic three-phase electric trap

We trap neutral ground-state rubidium atoms in a macroscopic trap based on purely electric fields. For this, three electrostatic field configurations are alternated in a periodic manner. The rubidium is precooled in a magneto-optical trap, transferred into a magnetic trap, and then translated into the electric trap. The electric trap consists of six rod-shaped electrodes in cubic arrangement, giving ample optical access. Up to 10;{5} atoms have been trapped with an initial temperature of around 20 microkelvin in the three-phase electric trap. The observations are in good agreement with detailed numerical simulations.     

Dynamical density functional theory with hydrodynamic interactions and colloids in unstable traps

A density functional theory for colloidal dynamics is presented which includes hydrodynamic interactions between the colloidal particles. The theory is applied to the dynamics of colloidal particles in an optical trap which switches periodically in time from a stable to an unstable confining potential. In the absence of hydrodynamic interactions, the resulting density breathing mode exhibits huge single peaked oscillations in the trap center which become double peaked and damped by hydrodynamic interactions. The predicted dynamical density fields are in good agreement with Brownian dynamics computer simulations.     

一种可控的Ioffe型冷分子表面微电阱

囚禁于阱中的粒子(原子或分子)可获得更长的相互作用时间,因而在精密测量中可获得更高的分辨率.阱中的粒子与外界隔离,从而可以被冷却到更低的温度.因此原子(或分子)阱已广泛应用到许多研究领域.然而中心电场强度为零的势阱会导致粒子发生非绝热跃迁,这是原子或分子损失的主要来源.该损失曾是制备原子玻色-爱因斯坦凝聚的最后一道障碍.本文提出了一种可控的Ioffe型表面微电阱,其电场强度处处不为零,可有效避免分子的非绝热损失.另外,通过调节电压等参数,势阱中心电场强度以及势阱中心距芯片表面的高度可以在较大范围内调节,例如在本文参数下,势阱中心电场强度可在0.15—5.5 kV/cm变化,势阱中心高度可在6.0—17.0μm变化.本文通过有限元软件计算了芯片表面微电阱的电场分布,并用Monte Carlo模拟验证了该方案的可行性.该表面微电阱不仅可用于分子芯片的集成,而且可用于表面量子简并气体的制备.为精密测量、量子计算、表面冷碰撞和冷化学等领域提供了一个平台.     

The energy distribution of an ion in a radiofrequency trap interacting with a nonuniform neutral buffer gas

An ion in a radiofrequency (rf) trap sympathetically cooled by a simultaneously trapped neutral buffer gas exhibits deviations from thermal statistics caused by collision-induced coupling of the rf field to the ion motion. For a uniform density distribution of the buffer gas, the energy distribution of the ion can be described by Tsallis statistics. Moreover, runaway heating of the ion occurs if the buffer gas particles are sufficiently heavy relative to the ion. In typical experiments, however, ultracold buffer gases are confined in traps resulting in localised, non-uniform density distributions. Using a superstatistical approach, we develop an analytical model for an ion interacting with a localised buffer gas. We demonstrate theoretically that limiting collisions to the centre of the ion trap enables cooling at far greater mass ratios than achievable using a uniform buffer gas, but that an upper limit to the usable mass ratio exists even in this case. Furthermore, we analytically derive the functional form of the energy distribution for an ion interacting with a buffer gas held in a harmonic potential. The analytical distribution obtained is found to be in excellent agreement with the results of numerical simulations.     

Atom trap in an axicon mirror

We have realized a novel atom trap in an axicon (conical hollow) mirror, using a frequency-modulated, single-diode laser. Different spatial distributions of trapped atoms such as a ball and a ring are observed. We show that our numerical simulations are consistent with experimental results. In particular, the ring diameter is found to be approximately the separation between the mirror axis and the magnetic field axis. The axicon trap may be useful as a precooled atom source for cold atomic beams, atom funnels, and atom waveguides.     

Chaotic Solutions of a Typical Nonlinear Oscillator in a Double Potential Trap

We have obtained a general unstable chaotic solution of a typical nonlinear oscillator in a double potential trap with weak periodic perturbations by using the direct perturbation method. Theoretical analysis reveals that the stable periodic orbits are embedded in the Melnikov chaotic attractors. The corresponding chaotic region and orbits in parameter space are described by numerical simulations.     

Reaction kinetics of diffusing particles injected into a reactive substrate

We analyze the kinetics of trapping (A+B-->B) and annihilation (A+B-->0) processes on a one-dimensional substrate with homogeneous distribution of immobile B particles while the A particles are supplied by a localized source. For the imperfect reaction case, we analyze both problems by means of a stochastic model and compare the results with numerical simulations. In addition, we present the exact analytical results of the stochastic model for the case of perfect trapping.     

Structure of Multi-Species Charged-Particles with Competing Interactions in a Quadratic Trap

Multi-species charged-particles interacting with each other by a competing short-range attraction and long-range repulsion potential confined in a quadratic trap are studied with molecular dynamics simulations. It is found that particles with similar mass-to-charge ratio tend to populate a common shell, whose location depends on the particle mass-to-charge ratio, and that the greater the latter is, the closer the particles to the centre of the trap are. This rule for the ground-state configuration is independent of the total particle and species numbers in the system.     

Motional frequencies in a planar Penning trap

A Penning trap consisting of concentric ring electrodes on a substrate and a magnetic field perpendicular to the substrate plane has been loaded with electrons. In order to demonstrate the performance of the trap we have measured the motional frequencies of the trapped electrons. Frequencies, line shape and width agree well with simulations. Miniaturization of the device is at hand which opens novel possibilities for application in quantum computing. Contribution presented at the TCP06, Vancouver Island, 2006-09-21     

Numerical calculations of potential distribution in non-ideal quadrupole trap and simulations of anharmonic oscillations

A quadrupole ion trap consisting of electrode structures symmetric about z-axis is an important tool for conducting several precision experiments. In practice the field inside the trap does not remain purely quadrupolar, and can be calculated using numerical methods. We have used boundary element method to calculate the potential inside the truncated as well as symmetrically misaligned quadrupolar ion trap. The calculated potential values are fitted to multipole expansion and the weights of multipole moments have been evaluated by minimizing the least square deviation. The higher-order multipole contribution in the fabricated hyperbolic electrodes due to truncation and machining imperfections is discussed. Non-linear effects arising due to the superposition of octupole moment manifest as anharmonic oscillations of trapped ions in the non-ideal Paul trap. Theoretical simulations of non-linear effects have been carried out.      

具有局部畸变的常规势阱中玻色-爱因斯坦凝聚孤子的演化特性

运用数值模拟方法解Gross—Pitaevskii方程,研究了具有局部畸变（impurity）的常规势阱中玻色-爱因斯坦凝聚（BEC）孤子的演化特性,发现势阱的局部畸变对BEC孤子随时间演化的特性有重要影响,并且其影响与畸变的强度、势阱的幅度和势阱的类型有关。