Intermittent Emission of Particles from a Bose-Einstein Condensate in a 1D Lattice

Particle emission from a Bose-Einstein condensate with periodically modulated interactions is investigated in a 1D lattice. Within perturbative analysis, which leads to instabilities for discrete modes, the main regimes are obtained where the system can emit a large particle jet, and the distinctly intermittent rather than continuous emission is found. The time evolution of the trapped particles exhibits a stair-like decay, and a larger drive induces a more significant intermittency. This study further sheds light on the dynamics of the stimulating process, and demonstrates that instead of a real suspension, the intermittency represents a build-up stage of the system. The theoretical framework might be generalized to the explorations on multiple-site systems with analogous configurations and couplings, and offer new insights into other fundamental nonequilibrium problems.     

Calculation and optical measurement of laser trapping forces on non-spherical particles

Optical trapping, where microscopic particles are trapped and manipulated by light is a powerful and widespread technique, with the single-beam gradient trap (also known as optical tweezers) in use for a large number of biological and other applications. The forces and torques acting on a trapped particle result from the transfer of momentum and angular momentum from the trapping beam to the particle. Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult. Calculations can be performed using approximate methods, but are only applicable within their ranges of validity, such as for particles much larger than, or much smaller than, the trapping wavelength, and for spherical isotropic particles. This leaves unfortunate gaps, since wavelength-scale particles are of great practical interest because they are readily and strongly trapped and are used to probe interesting microscopic and macroscopic phenomena, and non-spherical or anisotropic particles, biological, crystalline, or other, due to their frequent occurance in nature, and the possibility of rotating such objects or controlling or sensing their orientation. The systematic application of electromagnetic scattering theory can provide a general theory of laser trapping, and render results missing from existing theory. We present here calculations of force and torque on a trapped particle obtained from this theory and discuss the possible applications, including the optical measurement of the force and torque.     

Optical torque controlled by elliptical polarization

We show theoretically and demonstrate experimentally that highly absorbing particles can be trapped and manipulated in a single highly focused Gaussian beam. Our studies of the effects of polarized light on such particles show that they can be set into rotation by elliptically polarized light and that both the sense and the speed of their rotation can be smoothly controlled.     

Entanglement generation and multiparticle interferometry with neutral atoms

We study the preparation and manipulation of states involving a small number of interacting particles. By controlling the splitting and fusing of potential wells, we show how to interconvert Mott-insulator-like and trapped BEC-like states. We also discuss the generation of "Schr?dinger cat" states by splitting a microtrap and taking into practical consideration the asymmetry between the resulting wells. These schemes can be used to perform multiparticle interferometry with neutral atoms, where interference effects can be observed only when all the participating particles are measured.     

Incoherent light transport in anisotropic media: Form factor influence for oriented prolate ellipsoids

In turbid media, the partial or global orientation of anisotropic particles induces anisotropic light transport. In this study, we discuss the anisotropic incoherent transport of light in media where prolate ellipsoids are oriented in the same direction. In these anisotropic media, incoherent light transport is investigated using Monte Carlo simulations where the influence of particle anisotropy, size and optical properties are explored in a systematic way, from the local scattering event up to the diffusion limit. The database allows inverting the anisotropy of the backscattered image to yield the form factor of the particles. We then illustrate the relevance of such an analysis to assess the deformability of human erythrocytes in blood samples under normal physiological conditions.     

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.     

光散射的逆向思考

长期以来人们已习惯于光被物质散射的思考而形成的“光谱物理学”,而对它的逆向思维、即物质(如原子)被‘光’散射的诸多专题问津的较少;其中的某些问题,如物质(如原子)被光(散射)作用的经典物理图像;物质(如原子)被光加(减)速;物质(如原子)被光俘获、陷落、致冷和速度的聚束。此外,密度效应、光场与原子的互作用,(量子化)光(电磁)场对物质(如原子)散射等是近期取得的成果,都将在该文中给出。     

Characterisation of nanoparticle size and state prior to nanotoxicological studies

Before commencing any nanotoxicological study, it is imperative to know the state of the nanoparticles to be used and in particular their size and size distribution in the appropriate test media is particularly important. Particles satisfying standards can be commercially purchased; however, these invariably cannot be used directly and need to be dispersed into the relevant biological media. Often such changes in the environment or ionic strength, or a change in the particle concentration, results in some aggregation or a shift in the particle size distribution. Such unexpected aggregation, dissolution or plating out, if unaccounted for, can have a significant effect on the available nanoparticle dose and on interpretation of any results obtained thereafter. Here, we demonstrate the application of characterisation instrumentation that sizes nanoparticles based on their Brownian motion in suspension. Unlike classical light-scattering techniques, the nanoparticle tracking and analysis (NTA) technique allows nanoparticles to be sized in suspension on a particle-by-particle basis allowing higher resolution and therefore better understanding of aggregation than ensemble methods (such as dynamic light scattering (DLS) and differential centrifugation sedimentation (DCS)). Results will be presented from gold (standard) nanoparticles in biologically relevant media that emphasise the importance of characterisation of the nanoparticle dispersion. It will be shown how the NTA technique can be extended to multi-parameter analysis, allowing for characterization of particle size and light scattering intensity on an individual basis. This multi-parameter measurement capability allows sub-populations of nanoparticles with varying characteristics to be resolved in a complex mixture. Changes in one or more of such properties can be followed both in real time and in situ.     

空间散斑场捕获大量吸光性颗粒及其红外显微观测

光镊技术被广泛应用于捕获和操纵微纳米尺寸颗粒,主要包括捕获水中透明性颗粒和空气中吸光性颗粒两种类型.本文用激光束照射毛玻璃散射片,透射光经透镜会聚后在透镜的像平面附近产生了主观散斑场.该散斑场为空间分布,包含大量的亮斑和暗斑.大量由亮斑包围的暗斑如同一个个空间能量陷阱,被用来捕获大量的吸光性墨粉颗粒,被捕获颗粒的尺寸约2—8μm,密度约1—2 g/cm3.采用红外显微镜拍摄到空间散斑场捕获颗粒的红外像,红外图像显示被捕获颗粒吸光后温度升高,证实了空间散斑场捕获吸光性颗粒的机理为光泳力原理.     

On the observation of negative magnetization under zero-field-cooled process

We have addressed problems associated with the measurement of zero-field-cooled (ZFC) magnetization and its interpretation in the light of negative magnetization reported in certain ferrimagnetic materials such as CoCr₂O₄. We demonstrate that a small negative trapped field in the sample space as well as large coercive fields are responsible for the observed negative magnetization. The problem is commonly encountered while working with magnetometers and a superconducting magnet where the sign of the trapped field can be positive or negative depending on the way the field is reduced to zero.     

The effect of multiple anisotropies in fine particles

To investigate the effects of multiple anisotropies, morphology and size on magnetic properties of fine particles, cobalt-modified materials with different shapes were tested at temperatures from liquid nitrogen to 400 K. Some interesting and original conclusions were drawn: (a) When multiple easy axes are available, thermal fluctuations can induce the magnetization to switch from one axis to the other; the overall effect will be an increase of the fraction of particles with superparamagnetic behaviour. (b) The phenomenon will be greater for materials where the conflicting anisotropy constants are similar (isotropic particles); thus, for a given composition, the lower the shape anisotropy and the larger the superparamagnetic fraction. (c) Porosity and particle defects will contribute to increase the super-paramagnetic fraction. (d) In practical media (tapes) the effect of the superparamagnetic fraction is much lower than expected: a “constricted magnetization” phenomenon could account for such behaviour. (e) The lack of interactions predicted for truly isotropic media is experimentally verified only at extremely low temperatures. (f) Partial orientation in the plane of the strongest anisotropy axis must be taken into account for explaining the behaviour of SFD; under such assumption, “quasi-spherical” particles will behave quite differently from elongated ones. (g) Rotational hysteresis, CF and (1 ? S¹) for isotropic particles seems to indicate that the rotational mechanism might not be accounted for by known models.     

Polydispersed Powders (Nd3+:YVO4) for Ultra Efficient Random Lasers

Random lasers hold the potential for cheap, coherent light sources that can be miniaturized and molded into any shape with several other added benefits such as speckle‐free imaging; however, they require improvements specifically in terms of efficiency. This paper details for the first time a strategy for increasing the efficiency of a random laser that consists in using smaller particles, trapped between large particles to serve as absorption and gain centers whereas the large particles control mainly the light diffusion into the sample. Measurements of backscattering cone, sample absorption, reflection, and laser emission are used to determine the samples' transport mean free path, fill fractions, laser efficiency, and the average photon path lengths inside the scattering medium for backscattered pump photons. A record slope efficiency of 50% is reached by optimizing pump photon diffusion and absorption in a powder pellet composed by a polydispersed particle size distribution (smaller particles between bigger ones) from a grinded and sieved 1.33 mol% yttrium vanadate doped with neodymium crystal with mean particle size of 54 µ m.     

Scattering of a Light Particle by a System of Harmonic Oscillators

We discuss the asymptotic wave function of a quantum system in ?³ composed by heavy and light particles, in the case where the light particles are in scattering states and no interaction is assumed among particles of the same kind. We first review a recent result concerning the case of K heavy and N light particles, where the one-particle potential acting on each heavy particle decays at infinity. Then we consider the case of one light particle interacting with a system of harmonic oscillators and prove the same kind of result following, with some modification, the proof of the previous case. A possible application to the analysis of the scattering of a light particle from condensed matter is also outlined.     

Intrinsic and extrinsic nature of the orbital angular momentum of a light beam

We explain that, unlike the spin angular momentum of a light beam which is always intrinsic, the orbital angular momentum may be either extrinsic or intrinsic. Numerical calculations of both spin and orbital angular momentum are confirmed by means of experiments with particles trapped off axis in optical tweezers, where the size of the particle means it interacts with only a fraction of the beam profile. Orbital angular momentum is intrinsic only when the interaction with matter is about an axis where there is no net transverse momentum.     

Experimental investigation of magnetically induced unusual emission of light from a ferrodispersion

An unusual emission of light is observed when a coherent light beam is passed through a mixture of a magnetorheological suspension and a ferrofluid that is subjected to a critical magnetic field. When first the incident light is removed and then the field is switched off, a flash of light is observed. In this Letter certain characteristics of this unusual emission are reported. Our findings suggest that a part of the incident light energy is magnetically trapped within the medium. Upon removal of the field, the same is released. Several physical phenomena that may give rise to such emission are discussed. The magnetically tunable emission will be useful to develop photonic devices.     

Features of the Mechanism of Formation of the Radiation Belt of High-Energy Electrons and Positrons

New results of the PAMELA experiment revealed significant difference in the electron and positrons flux and fraction between trapped particles of the radiation belt and quasitrapped particles. A decrease in the ratio of the electron-to-positron fluxes both with increasing altitude and with decreasing energy was observed for the inner radiation belt, but this does not fit in currently used models. The residual atmosphere density in the trapping region for L ~ 1.15?1.2 is estimated on the basis of calculations of trapped-particle trajectories in the Earth’s magnetosphere. It is shown that processes leading to the energy loss for trapped electrons and positrons in the interactions with residual-atmosphere atoms play an important role in the formation of fluxes of these particles, but these processes cannot cause the reduction of the positron fraction in the total flux at energies below some 100 MeV. The role of the process of δ-electron production in the formation of the belt of trapped electrons and positrons is considered. Allowance for this process makes it possible to explain the above reduction at least partly.     

Particle levitation and laboratory scattering

Measurements of light scattering from aerosol particles can provide a non-intrusive in situ method for characterising particle size distributions, composition, refractive index, phase and morphology. When coupled with techniques for isolating single particles, considerable information on the evolution of the properties of a single particle can be gained during changes in environmental conditions or chemical processing. Electrostatic, acoustic and optical techniques have been developed over many decades for capturing and levitating single particles. In this review, we will focus on studies of particles in the Mie size regime and consider the complimentarity of electrostatic and optical techniques for levitating particles and elastic and inelastic light scattering methods for characterising particles. In particular, we will review the specific advantages of establishing a single-beam gradient force optical trap (optical tweezers) for manipulating single particles or arrays of particles. Recent developments in characterising the nature of the optical trap, in applying elastic and inelastic light scattering measurements for characterising trapped particles, and in manipulating particles will be considered.     

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.     

Hydrodynamic Stress Tensor in Inhomogeneous Colloidal Suspensions: an Irving-Kirkwood Extension *

Based on statistical mechanics for classical fluids, general expressions for hydrodynamic stress in inhomogeneous colloidal suspension are derived on a molecular level. The result is exactly an extension of the Iving-Kirkwood stress for atom fluids to colloidal suspensions where dynamic correlation emerges. It is found that besides the inter-particle distance, the obtained hydrodynamic stress depends closely on the velocity of the colloidal particles in the suspension, which is responsible for the appearance of the solvent-mediated hydrodynamic force. Compared to Brady's stresslets for the bulk stress, our results are applicable to inhomogeneous suspension, where the inhomogeneity and anisotropy of the dynamic correlation should be taken into account. In the near-field regime where the packing fraction of colloidal particles is high, our results can reduce to those of Brady. Therefore, our results are applicable to the suspensions with low, moderate, or even high packing fraction of colloidal particles.     

Particle Manipulation by Ultrasonic Standing Wave Fields to complement dynamic light scattering experiments

Dynamic light scattering is a widely used technique for the sizing of colloidal suspensions. It is capable of measuring particles across the size range from approximately 1 nm to several microns. However the larger particle sizes tend to pose problems for the interpretation of the scattered light signal either by virtue of their light scattering efficiency relative to the smaller species present or the departure of the scattered light signal from Gaussian statistics. Rapid removal of such particles in-situ could extend the use of dynamic light scattering particularly in on-line analysis or laboratory automated measurement. In this paper a method is demonstrated for the in-situ removal of larger particles in suspension by means of ultrasonic standing waves and concurrent dynamic light scattering measurement. The theory behind ultrasonic particle manipulation and its effect on the motion of the particles is discussed. Data from a scattering cell designed to incorporate the ultrasonic technology is presented showing that dynamic light scattering measurements may be carried out under such conditions. Varying the energy density of the ultrasonic field allows particles greater than a defined cut-off diameter to be removed from the measurement region. Theory shows that the minimum cut-off size may be as small as 100 nm. Results presented here demonstrate complete removal at a lower diameter threshold of approximately 2000 nm.