Enhanced Fluorescence from Periodic Arrays of Silver Nanoparticles

Electron beam lithography was used to fabricate silver nanoparticle arrays and study the effects of geometrical properties of particles on metal-enhanced fluorescence. Nanoparticle size, shape, interparticle spacing, and nominal thickness were varied in a combinatorial pattern for investigation of the particle plasmon resonance effect on enhancement of fluorescence from three different fluorophores; Fluorescein, Cy3, and Cy5. A specific geometric property for optimal enhancement from each fluorophore was determined. For interparticle spacings greater or equal to 270 nm, the enhancement matched what is expected for a single-particle model. For those particles smaller than 210 nm, the enhancement was lower than for the larger spacing in the range studied. Triangular-shaped particles gave similar enhancement to those of square-shaped particles. This combinatorial pattern by e-beam lithography was found to be useful for studying how individual parameters enhance the fluorescence that are important for rational design of enhanced fluorescence sensors.     

Giant magnetic susceptibility enhancement in field-structured nanocomposites

We demonstrate through experiment and simulation that when mono-domain Fe nanoparticles are formed into chains by the application of a magnetic field, the susceptibility of the resulting structure is greatly enhanced (11.4-fold) parallel to the particle chains and is much larger than transverse to the chains. Simulations show that this significant enhancement is expected when the susceptibility of the individual particles approaches 5 in MKS units, and is due to the spontaneous magnetization of individual particle chains, which occurs because of the strong dipolar interactions. This large enhancement is only possible with nanoparticles, because demagnetization fields limit the susceptibility of a spherical multi-domain particle to 3 (MKS). Experimental confirmation of the large susceptibility enhancement is presented, and both the enhancement and the susceptibility anisotropy are found to agree with simulation. The specific susceptibility of the nanocomposite is 54 (MKS), which exceeds the highest value we have obtained for field-structured composites of multi-domain particles by a factor of four.     

Strong electromagnetic field localization near the surface of hemicylindrical particles

We theoretically study the forward and backward plane wave illuminations of hemicylindrical dielectric particles with different radii and refractive indices. Near field effects providing strong field enhancement and narrow beam width are obtained from hemicylindrical particles, which can be associated with whispering gallery modes (WGMs) and photonic nano-jets (PNJs). It should be noted that existence of PNJ and WGM effects depending on forward/backward light illuminations is studied for the first time in all-dielectric hemicylindrical particles. Compared to dielectric cylinders, hemicylindrical aided PNJs show relatively small influences of WGM regime, when exists, to PNJ behavior. Under the backward excitation, on the other hand, typical nano-jet is produced. The designed hemicylindrical structure could be implemented in high-resolution optical imaging and sensing applications.     

Simultaneous forward- and backward-hemisphere elastic-light-scattering patterns of respirable-size aerosols

Two-dimensional angular optical scattering (TAOS) patterns of aerosols are measured simultaneously from the forward hemisphere 15 degrees < theta < 90 degrees as well as the backward hemisphere 90 degrees < theta < 165 degrees (detecting 63% of the 4pi sr of scattered light) by using an ellipsoidal reflector and an intensified CCD detector. TAOS patterns were obtained from polystyrene-latex spheres (individuals and aggregates) and from single Bacillus subtilis spores. These information-rich patterns, measured with a single laser pulse for individual particles on the fly, suggest that forward-TAOS and backward-TAOS measurements may be used for rapid classification of single aerosol particles.     

Exciton condensation in coupled quantum wells

Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly.     

Microscopic theory of intrinsic shear and bulk viscosities

A microscopic theory of intrinsic shear and bulk viscosities of solutions is given for a model of particles that interact with hard-sphere cores and weak longrange attraction. The approximation considered (the velocity chaos assumption of the Enskog theory) can be expected to yield quantitatively useful values for viscosities of the model solute-solvent system when the solute particles are not much larger than the solvent particles. Under solute-solvent mixing conditions of constant pressure and temperature we find that the intrinsic viscosities of a hard-sphere solute in a hard-sphere solvent can be positive or negative, depending upon size and mass ratios; for solute and solvent particles whose mass ratio equals their volume ratio, the intrinsic shear and bulk viscosities are always positive for solute particles larger than solvent particles: in the opposite case, the intrinsic shear viscosity is always negative while the intrinsic bulk viscosity is for the most part negative, becoming positive again when the solute particle is sufficiently small. For solute particles smaller than solvent particles, this result is sensitive to change in mass ratio. The addition of solvent-solvent attraction is found to lower the intrinsic viscosities substantially; the addition of solute-solvent attraction raises it. Detailed quantitative analysis of these effects is given.     

Large angle elastic alpha scattering on a N=Z nucleus above A=40

Scattering of alpha particles from 44Ti, the lightest unstable alpha-particle nucleus above A=40, has been measured at backward angles. The "anomalous" order-of-magnitude enhancement that is characteristic of 40Ca and other light alpha-particle nuclei is not observed. Instead, the backward yield is similar to that observed for other nuclei heavier than 40Ca, and is well described with average optical model parameters.     

A Modeling Study on Particle Dispersion in Wall-Bounded Turbulent Flows

Three physical mechanisms which may affect dispersion of particle's motion in wall-bounded turbulent flows, including the effects of turbulence, wall roughness in particle-wall collisions, and inter-particle collisions, are numerically investigated in this study. Parametric studies with different wall roughness extents and with different mass loading ratios of particles are performed in fully developed channel flows with the Eulerian-Lagrangian approach. A low-Reynolds-number $k-\epsilon$ turbulence model is applied for the solution of the carrier-flow field, while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. It is shown that the mechanism of inter-particle collisions should be taken into account in the modeling except for the flows laden with sufficiently low mass loading ratios of particles. Influences of wall roughness on particle dispersion due to particle-wall collisions are found to be considerable in the bounded particle-laden flow. Since the investigated particles are associated with large Stokes numbers, i.e., larger than $\mathcal{O}(1)$, in the test problem, the effects of turbulence on particle dispersion are much less considerable, as expected, in comparison with another two physical mechanisms investigated in the study.     

Spin mass of an electron liquid

We show that in order to calculate correctly the spin current carried by a quasiparticle in an electron liquid one must use an effective "spin mass" m(s) that is larger than both the band mass m(b), which determines the charge current, and the quasiparticle effective mass m(*), which determines the heat capacity. We present two independent estimates of the spin mass enhancement, m(s)/m(b), in two- and three-dimensional electron liquids, based on (i) previously calculated values of the Landau parameters and (ii) a recent theory of the dynamical local field factor in the spin channel. Both methods yield a significant spin mass enhancement, which is larger in two dimensions than in three.     

Rigorous bounds on the radiative interaction between real gases and scattering particles

The radiative interaction due to the simultaneous presence of a real (non-gray) gas and isotropically scattering particles is examined rigorously for an isothermal plane layer, allowing completely general, frequency-dependent gas and particle radiative properties. A correction factor is defined to characterize the effects of interaction on the total hemispherical emittance of the layer, and rigorous bounds on the correction factor are determined, using the exact normal mode expansion technique of Case. It is shown that, under certain conditions for high albedo particles, interaction effects may lead to a slight enhancement of the total hemispherical emittance of the gas/particle mixture, compared to the sum of the emittances for the gas and particles considered separately, with the maximum enhancement possible being 3.2%. Under most conditions, however, the presence of the particles would be expected to shield the gas emittance. These rigorous bounds on the extent of the interaction effects are in contrast to the results of a previous numerical study, which predicted strong enhancement effects (nearly 100% enhancement for some cases) due to gas/particle radiative interaction.     

Backward enhanced emission from multiphoton processes in aerosols

We have investigated, both theoretically and experimentally, multiphoton-induced processes in aerosol particles using femtosecond laser pulses. More specifically, we have demonstrated that both multiphoton (1, 2 and 3 photon)-induced fluorescence (MPEF) and laser-induced breakdown (LIB) emissions are strongly enhanced in the backward direction. The backward enhancement increases from 1.8 to 35 (emission ratio between the backward direction and 90°) with increasing non-linear process order n. Application to non-linear lidar of biological aerosols is discussed. Received: 24 April 2002 / Revised version: 3 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-472/431507, E-mail: wolf@lasim.univ-lyon1.fr     

Constraints on PSC particle microphysics derived from lidar observations

Based on extensive T-matrix computations of light scattering by polydispersions of randomly oriented, rotationally symmetric nonspherical particles, we analyze existing lidar observations of polar stratospheric clouds (PSCs) and derive several constraints on PSC particle microphysical properties. We show that sharp-edged nonspherical particles (finite circular cylinders) exhibit less variability of lidar backscattering characteristics with particle size and aspect ratio than particles with smooth surfaces (spheroids). For PSC particles significantly smaller than the wavelength, the backscatter color index and the depolarization color index β are essentially shape independent. Observations for type Ia PSCs can be reproduced by spheroids with aspect ratios larger than 1.2, oblate cylinders with diameter-to-length ratios greater than 1.6, and prolate cylinders with length-to-diameter ratios greater than 1.4. The effective equal-volume-sphere radius for type Ia PSCs is about 0.8 μm or larger. Type Ib PSCs are likely to be composed of spheres or nearly spherical particles with effective radii smaller than 0.8 μm. Observations for type II PSCs are consistent with large ice crystals (effective radius greater than 1 μm) modeled as cylinders or prolate spheroids.     

Two distinct photoluminescence responses of CdTe quantum dots to Ag (I)

Four sizes of water-soluble thiol-capped CdTe quantum dots (QDs) have been synthesized and used to investigate the photoluminescence (PL) responses to Ag⁺ ions. For small particles, the CdTe QDs exhibit PL enhancement in the presence of lower concentration of Ag⁺ but show obvious quenching with the further increase of Ag⁺; for larger particles, however, PL of CdTe QDs is quenched all the time with the Ag⁺ addition, no PL enhancement is observed. Mechanism study shows that small QDs with more traps on the particle surface are effectively passivated by initial adsorbed Ag⁺, which accounts for the PL enhancement observed; after the initial traps are saturated, the excess Ag⁺ facilitates nonradiative recombination, resulting in PL quenching. For larger particles, the nonradiative recombination dominates the whole process even for the lower concentration of Ag⁺, due to the fewer traps on the QD surface. Compared with larger particles, the small CdTe QDs are more suitable for sensing Ag⁺ because of the more sensitive and selective PL response. To our best knowledge, this is the first systematical study on the interaction of Ag⁺ with different-sized CdTe QDs.     

沙尘气溶胶粒子群的散射和偏振特性

根据Mie散射理论,以对数正态分布函数描述沙尘气溶胶粒子群的粒径尺度分布,计算了沙尘气溶胶粒子群在0.2～40μm波段间对太阳短波辐射和地球大气长波辐射的单次散射反照率、散射相矩阵函数,揭示了不同相对湿度时,沙尘粒子群对入射辐射的散射和偏振的特征。结果表明,沙尘粒子群的单次散射反照率随着入射波长的增加有较大起伏,不同相对湿度条件下,变化趋势基本一致;在可见光、近红外波段单次散射反照率随湿度增加而变大,湿度95%时非常接近于1;大于10μm的热红外波段单次散射反照率随相对湿度增加而减小,具有较强的吸收辐射能力。散射辐射强度受湿度影响较小,随散射角的增加呈现先减小后增大的趋势,且增大的趋势随着波长的增加而减弱;不同波段上,线偏振和圆偏振随散射角和相对湿度变化存在差异;在前向和后向仅对入射辐射为圆偏振辐射产生圆偏振散射;散射光的偏振特性及其湿度差异主要表现在后向散射区,多以拱形形式体现。拱顶峰值散射角位置存在差异,且峰值散射角随相对湿度的降低向后向漂移。     

Multiplicities of forward-backward particles in16O-emulsion interactions at 4.5 AGeV/c

An exclusive study of the characteristics of interactions accompanied by backward emission (θlab≥ 90°) of shower and grey particles in collisions of a 4.5 AGeV/c 16O beam with emulsion nuclei is carried out. The experimental multiplicity distributions of different particles emitted in the forward (θlab<90°) and backward hemispheres due to the interactions with the two emulsion components (CNO, AgBr) are presented and analyzed. The correlations between the different emitted particles are also investigated. The results indicate that there are signatures for a collective mechanism, which plays a role in the production of particles in the backward hemisphere. Hence, the backward multiplicity distribution of the emitted shower and grey particles at 4.5 AGeV/c incident momentum can be represented by a decay exponential law formula independent of the projectile size. The exponent of the power was found to increase with decreasing target size. The experimental data favor the idea that the backward particles were emitted due to the decay of the system in the latter stages of the reactions.     

Lattice Boltzmann Simulations of Particle-Particle Interaction in Steady Poiseuille Flow

The moving behaviour of two- and three-particles in a pressure-driven flow is studied by the lattice Boltzmann simulation in two dimensions. The time-dependent values, including particles＇ radial positions, translational velocities, angular velocities, and the x-directional distance between the particles are analysed extensively. The effect of flow Reynolds number on particle motion is also investigated numerically. The simulation results show that the leading particle equilibrium position is closer to the channel centre while the trailing particle equilibrium position is closer to the channel wall. If Reynolds number Re is less than 85.30, the larger flow Reynolds number results in the smaller x-directional equilibrium distance, otherwise the x-directional distance increases almost linearly with the increase of time and the particles separate finally. The simulation results are helpful to understand the particle-particle interaction in suspensions with swarms of particles.     

Scattering and Transmission by a Monolayer of Spheres: A Study on the Monolayer Structure

The angular distribution of scattered light and the transmission of radiation through a monolayer of monodisperse spherical particles at variable particle concentration are studied. The scattering of light by a single particle is calculated with the classical Lorentz‐Mie theory. For a monolayer of mono‐dispersed spherical particles, if the monolayer density is less than 0.5 and the particle size parameter is larger than 5, effects from multiple scattering and dependent scattering can be excluded so that only steric interactions are considered. It is found that the scattering pattern, especially in the forward and backward directions, and the transmission are strongly dependent on the monolayer density.     

The effect of particle motion character upon the rate of the resonant transfer of electronic excitation

Equations are derived for describing a classical system interacting arbitrarily with a quantum two-level system. The solutions for the case of two converging electron terms are analysed and applied to the problem of electronic excitation resonant transfer or resonant charge exchange. It is supposed that the classical motion of the particles (molecules) possesses a stochastic character and may be represented by the Fokker-Plank equation, when the interaction with the quantum degrees of freedom are absent. The model allows us to trace theoretically the change of excitation transfer kinetics when passing from gas to liquid and from liquid to the solid state. The general formula for one-, two- and three-dimensional relative motions are found and the three-dimensional case is discussed in detail.     

振幅指数衰减正弦干涉信号线性预测模型的求解

在光谱成像的光谱分辨率增强技术中,对振幅衰减干涉信号的线性预测是其中的关键之一.文中在已知振幅指数衰减正弦干涉信号的个数、波数和衰减率的前提下,给出了线性预测系数直接计算公式及其前向线性预测系数与后向线性预测系数之间的关系;定义了该类正弦迭加信号的前向和后向线性可预测性条件.研究结果表明,振幅指数衰减的正弦迭加信号的前向和后向线性预测系数不再是简单的共轭对称关系了,但是二者依然是有联系的.该研究为一般的衰减干涉信号的光谱分辨率增强算法提供了理论基础. 关键词： 光谱分辨率增强 线性预测 可预测条件 预测系数     

火灾烟颗粒的分形结构形状模拟与光散射计算

针对火灾烟颗粒的形状特点，提出并建立火灾烟颗粒分形结构凝团的形状模型，并对烟颗粒扫描电镜(SEM)图像进行分析，获取分形结构模型中的单个凝团中基本颗粒个数、凝团分形维数、基本颗粒半径等参数.利用该模型对火灾烟颗粒的形状进行模拟的结果表明，该模型能够较好反映出烟颗粒的形貌特征.利用形状模型对火灾烟颗粒散射进行初步计算表明，在其他参数相同的情况下，相对于同体积的球形颗粒，分形凝团具有前向散射较弱，后向散射较强的特征.