Optical manipulation of particle ensembles in air

We demonstrate that airborne light-absorbing particles can be photophoretically trapped and moved inside an optical lattice formed by multiple-beam interference. This technique allows simultaneous three-dimensional manipulation of multiple micro-objects in gases.     

Microwave Induced Optical Nuclear Polarization at 75 GHz: A quantitative analysis

A quantitative analysis of Microwave Induced Optical Nuclear Polarization (MIONP) on crystals of fluorene-h₁₀ doped with phenanthrene-d₁₀ at 75 GHz and 1.4 K is presented. Two effects are studied in detail: the nuclear spin diffusion barrier and the phonon bottleneck. Experiments are presented that allow the identification of the proton spins on fluorene-h₁₀ molecules located inside the nuclear spin diffusion barrier surrounding phenanthrene-d₁₀ molecules excited in the photo-excited triplet state. Using this result the experimental values for the triplet spin-lattice relaxation (SLR) rates and the nuclear SLR rate can be related to each other without any fitting parameter. Microwave frequency and magnetic field modulation are used during MIONP to prove that the triplet SLR is phonon-bottlenecked. Subsequently a quantitative analysis of MIONP in the system fluorene-h₁₀ doped with phenanthrene-d₁₀ is obtained.     

Layering instability in a confined suspension flow

We show [J. Fluid Mech. 592, 447 (2007)] that swapping (reversing) trajectories in confined suspension flows prevent collisions between particles approaching each other in adjacent streamlines. Here we demonstrate that by inducing layering this hydrodynamic mechanism changes the microstructure of suspensions in a confined Couette flow. Layers occur either in the near-wall regions or span the whole channel width, depending on the strength of the swapping-trajectory effect. While our theory focuses on dilute suspensions, we postulate that this new hydrodynamic mechanism controls the formation of a layered microstructure in a wide range of densities.     

Free surface instability in a confined suspension jet

A suspension confined between two close parallel plates is studied in the Stokesian regime. The use of boundary integral equations and the lubrication approximation allows to compute the hydrodynamic forces acting on the particles. The forces are long ranged and depend on the orientation of the relative position and velocity of particles. This tensorial character predicts an “antidrag” that is observed in experiments. The effect of the computed hydrodynamic forces is studied in the dynamics of a jet of particles falling by a gravitational field, which shows a surface instability similar to the Kelvin–Helmholtz one. A theoretical model, based on hydrodynamic-like equations, is able to predict the instability that is produced by the interaction of the long-range forces and the free surface.     

Chemotaxis driven instability of a confined bacterial suspension

A suspension of bacteria in a thin channel or film subject to a gradient in the concentration of a chemoattractant, will develop, in the absence of an imposed fluid flow, a steady bacteria concentration field that depends exponentially on cross-stream position. Above a critical bacteria concentration, this quiescent base state is unstable to a steady convective motion driven by the active stresses induced by the bacteria's swimming. Unlike previously identified long-wavelength instabilities of active fluids, this instability results from coupling of the bacteria concentration field with the disturbance flow.     

Interface phenomena and optical properties of structurally confined InP quantum wire ensembles

Three-dimensional regular ensembles of InP quantum wires have been produced in channels of porous dielectric matrices by metal-organic chemical vapor deposition. These matrices differ both in the diameter of the channels (0.7, 3, and 8 nm) and in their spatial arrangement. The InP layer thickness does not exceed two-three monolayers. A comparative study of Raman, optical absorption, and photoluminescence spectra revealed the dependence of the optical properties of these quantum wires on interface effects, namely, atomic interaction in the wires, wire-matrix, and wire-wire interactions. It is shown that the wire-matrix interaction distorts the InP lattice, broadens the wire electronic density-of-states spectrum in the vicinity of the fundamental gap, and redistributes the relaxation of photoinduced excitations among states belonging to the wire itself and to defects in the matrix bound to the wire. Fiz. Tverd. Tela (St. Petersburg) 39, 727–734 (April 1997)     

Optical excitation of optically inactive photonic band modes

At the point Γ of the Brillouin zone of photonic crystals (PCs), there are many photonic band (PB) states that do not couple to the incoming plane wave beams. This paper investigates how to excite these optically inactive PB (OIPB) modes in slab PCs by using optical means. Three methods are proposed, using a prism, traveling charges and focused laser pulses, each making use of evanescent light to excite OIPBs. It is concluded that to accumulate photons in an OIPB, sequential passing of bunched charges or intermittent irradiation of focused laser pulses is effective. The effect of the lateral size of the practical PC slabs is also discussed and it is found that the conclusions for an ideal system of infinite size still work if the lateral size is order of magnitudes larger than the PC lattice constant.     

Energy absorption by a magnetic nanoparticle suspension in a rotating field

Heat generation by viscous dissipation in a dilute suspension of single-domain ferromagnetic particles in a rotating magnetic field is analyzed by assuming that the suspended particles have a high magnetic rigidity. The problem is solved by using a kinetic approach based on a rotational diffusion equation. Behavior of specific loss power (SLP) as a function of field strength H and frequency ω is examined at constant temperature. SLP increases as either of these parameters squared when the other is constant, eventually approaching a saturation value. The function SLP(H, ω) can be used to determine optimal and admissible ranges of magnetically induced heating.     

Optical probing of Eu ions confined in an RF trap

The Eu ions confined in an RF quadrupole trap, has been optically detected. Using a tunable dye laser which is pumped by a Nd-YAG pulsed laser system, the resonance ⁹S₄^9S_4–6p _3/2, J = 5 transition of the Eu ions have been excited and the resulting fluorescence to the metastable ⁹D_4-6^9D_{4-6} state has been detected. In preparation to determine the ground-state hyperfine splitting of the odd isotopes we found the optimum trapping operating point. We have also observed a number of instabilities inside the region of the stability for an ideal trap. These non-linear resonances arise from higher-order contributions to the ideal quadrupole potential.     

Probability diffusion in non-Markhovian,non-Gaussian molecular ensembles: A theoretical analysis and computer simulation

A theoretical generalisation of the Fokker/Planck equation for atomic and molecular diffusion is compared with the results of a molecular dynamics simulation of a triatomic molecule ofC _2v symmetry. The molecular dynamics results are non-Markhovian and non-Gaussian in nature, markedly so in the case of the centre of mass linear velocityV. This may be ascertained by simulating the long-time limit of the three dimensional kinetic energy autocorrelation function <V ²(t)V ²(0)>/<V ²(0)V ²(0)>, which falls well below the theoretical Gaussian value of 3/5. By expressing the Mori continued fraction as a multidimensional Markhovian chain of differential equations and expressing this in turn as a non-Gaussian probability-diffusion equation of the Kramers/Moyal type it is possible to account for the simulation results in a qualitative fashion.     

Classifier ensembles for fMRI data analysis: an experiment

Functional magnetic resonance imaging (fMRI) is becoming a forefront brain–computer interface tool. To decipher brain patterns, fast, accurate and reliable classifier methods are needed. The support vector machine (SVM) classifier has been traditionally used. Here we argue that state-of-the-art methods from pattern recognition and machine learning, such as classifier ensembles, offer more accurate classification. This study compares 18 classification methods on a publicly available real data set due to Haxby et al. [Science 293 (2001) 2425–2430]. The data comes from a single-subject experiment, organized in 10 runs where eight classes of stimuli were presented in each run. The comparisons were carried out on voxel subsets of different sizes, selected through seven popular voxel selection methods. We found that, while SVM was robust, accurate and scalable, some classifier ensemble methods demonstrated significantly better performance. The best classifiers were found to be the random subspace ensemble of SVM classifiers, rotation forest and ensembles with random linear and random spherical oracle.     

Acoustic wave in a suspension of magnetic nanoparticle with sodium oleate coating

The ultrasonic propagation in the water-based magnetic fluid with doubled layered surfactant shell was studied. The measurements were carried out both in the presence as well as in the absence of the external magnetic field. The thickness of the surfactant shell was evaluated by comparing the mean size of magnetic grain extracted from magnetization curve with the mean hydrodynamic diameter obtained from differential centrifugal sedimentation method. The thickness of surfactant shell was used to estimate volume fraction of the particle aggregates consisted of magnetite grain and surfactant layer. From the ultrasonic velocity measurements in the absence of the applied magnetic field, the adiabatic compressibility of the particle aggregates was determined. In the external magnetic field, the magnetic fluid studied in this article becomes acoustically anisotropic, i.e., velocity and attenuation of the ultrasonic wave depend on the angle between the wave vector and the direction of the magnetic field. The results of the ultrasonic measurements in the external magnetic field were compared with the hydrodynamic theory of Ovchinnikov and Sokolov (velocity) and with the internal chain dynamics model of Shliomis, Mond and Morozov (attenuation).     

Optical spectroscopy on Se clusters and chains confined in zeolites

The incorporation of selenium into the supercages of zeolite Y leads to the formation of Se ₈ rings and distorted Se chains in a ratio which is influenced by the nature of the zeolitic cations. Here we review Raman (including resonance Raman) and UV/vis absorption spectroscopy results on Se encapsulated into a number of cation-exchanged faujasite zeolites. Both rings and chains give rise to characteristic Raman bands. In particular, low-frequency Raman bands are attributed to localized vibrations in ordered segments of distorted chains. The UV/vis absorption spectra indicate an opening of the band gap of selenium upon confinement in these zeolites. This can be reversed through electronic interaction with zeolite cations.Received: 1 January 2003, Published online: 14 October 2003PACS: 78.30.-j Infrared and Raman spectra - 82.75.Mj Measurements and simulation of properties (optical, structural) of molecules in zeolites     

Director deformation in a nematic liquid crystal with a ferromagnetic nanoparticle suspension

In view of the recent interest regarding nematic liquid crystals with a nanoparticle suspension, we investigate the director dynamics of a nematic matrix in the perspective of a soliton framework. The nematic liquid crystal is dispersed with ferromagnetic nanoparticles in order to enhance the director activity associated with the improved magnetization of the ferronematic matrix. The free energy density of the nematic matrix is deduced to be the Ginzburg–Landau (GL) equation for the director reorientation. We obtain a series of localized solutions for the GL equation, and also the dynamics is expressed in terms of the soliton evolution for the director by using the standard Hirota bilinearization procedure.     

Optical nonlinearity enhanced by metal nanoparticle in CdTe quantum dots

The optical nonlinearity of a CdTe quantum dot enhanced by a gold nanoparticle has been theoretically studied by employing the multi-bands effective mass method. The energy levels have been computed using 6×6 k.p model for the valence band. The semiconductor quantum dot-size-dependent third-order susceptibility of third harmonic generation in a CdTe-Au nanocrystal complex has been analyzed. It is found that the metal nanoparticle enhances the optical nonlinearity of the semiconductor quantum dot due to the dipole/multipole interaction that will bring in the strong damping and the field enhancement. By choosing the radius of CdTe quantum dot, the third-order nonlinear susceptibility for third harmonic generation can be optimized for the one- and multi-photon resonance. Also, we can alter the optical nonlinearity by changing the ratio of semiconductor-metal nanoparticle distance to the metal nanoparticle radius.     

Time dependent magnetically induced variations in optical transmission of magnetite nanoparticle aqueous suspension

We observe time dependent variations in the light intensity transmitted through an aqueous suspension of Fe₃O₄ nanoparticles caused by applied DC magnetic field. Two types of variations can be distinguished. Fast response takes less than 1ms while slow variations occur at the time interval fromseconds to hundreds of minutes. Possible mechanisms of these variations are discussed. Formation of chain-like structures consisted from iron oxide nanoparticles is responsible for the slow variations. It is also accompanied by a diffraction pattern when the magnetic field is orthogonal to the light beam. Fast variations are due to particle rotation and reorientation of the magnetic moment inside a nanoparticle.     

Optical properties of a suspension of hard spheres

The optical properties of suspensions are studied in a wide range of concentrations. An expression for the polarization operator is obtained taking into account the contributions of two-and three-particle correlations. The extinction length l and the transport length l* are calculated in terms of a model of hard spheres. A detailed comparison of the results of calculations with experimental data is performed. In calculations, the structure factor is determined in the Percus-Yevick approximation, while the form factor is taken into account in the Rayleigh-Gans approximation and in terms of the Mie theory. It is shown that taking into account the contribution of three-particle correlations improves the agreement of the theory with experiment. It is found that, in the range of high suspension concentrations, the optical parameters are more sensitive to the choice of the model for the structure factor than for the form factor.     

Nature of the transition from two- to three-dimensional ordering in a confined colloidal suspension

We report the results of extensive molecular dynamics simulations of solid-to-solid transitions in two- to six-layer colloidal suspensions confined between two smooth parallel walls. The studies are designed to elucidate the ordered particle packings that interpolate between the structures of two- and three-dimensional crystals in a confined space. At a fixed density per layer, as the wall separation increases we find a sequence of stable phases, each characterized by uniform amplitude buckling along the normal to the layer planes. The buckling is coupled to an in-plane ordering transition. The buckled phases alternate with phases whose structures contain only parallel planes of particles. The relative densities of the positively and negatively displaced particles in a buckled layer, the in-plane structures, and the behavior with respect to increasing wall separation of the split density distribution that characterizes a buckled layer, clearly identify these layers as intermediates in the reconstructive transformations ntriangle up-->(n+1) square that occur when the character of the constrained space evolves from being two dimensional to being three dimensional (triangle up denotes layers with hexagonal packing symmetry, while square denotes layers with square packing symmetry). The two transitions, ntriangle up-->n-buckled-->(n+1) square, are found to be first order.