Analysis of sound propagation in a fluid through a screen of scatterers

A multiple scattering analysis in a nonviscous fluid is developed in detail in order to predict the coherent sound motion in the presence of disordered heterogeneities, such as particles, fibers, bubbles, or contrast agents. Scatterers can be homogeneous, layered, shell-like with encapsulated liquids or gas, nonabsorbing, or absorbing, and can take a wide variety of shapes. A priori imposed limitations or physical assumptions are absent in the derivation, whether they concern the expected response of the fluid-scatterer mixture, the scatterer size relative to wavelength, or the scatterer concentration or the screen thickness. However, as in any multiple scattering formulation, a closure assumption is invoked. Closed-form results for the backscattered and forward-scattered wave motions on either side of the screen of scatterers are obtained. The fluid-scatterer mixture is shown to behave as an effective dissipative medium from the standpoint of the coherent motion. It is found that the effective medium is fully described once two parameters are determined: the effective wave number and the reflection coefficient for the associated half-space screen. Remarkably, both parameters depend only on the far-field scattering properties of a single scatterer.     

Origin of dynamic heterogeneities in miscible polymer blends: A quasielastic neutron scattering study

In order to investigate the origin of the often invoked nanoheterogeneities in miscible polymer blends, we have performed quasielastic neutron scattering experiments on the component dynamics within the miscible polymer blend polyisoprene/polyvinyl ether including the pure components as a reference. We find that the apparent local heterogeneities observed by spectroscopic techniques originate from the chain specific crossover properties between entropy driven and local chain dynamics and are, thus, a purely dynamical phenomenon.     

Simulation study on the avalanche process of the fiber bundles with strong heterogeneities

The fiber bundle with strong heterogeneities is an extension of a fiber bundle model based on the classical fiber bundle model to describe the failure process of strongly heterogeneous materials. In order to explore the breaking dynamic properties of strongly heterogeneous materials in short-range correlation, the fiber bundle model with strong heterogeneities in local load redistribution is numerically studied in detail. The impacts of the proportion of two kinds of fibers and the distribution of the failure thresholds on the macroscopic constitutive behavior, the avalanche size distribution and increasing step number of the external load are investigated, respectively. The numerical results show that there is a local plastic plateau in the constitutive curve at a critical proportion of two kinds of fibers. Strong intensity fibers in material can nontrivially increase the intensity and stability of the system by altering the microscopic properties of the failure process.     

Wave scattering effects in elastic percolation models

The study of the propagation of waves in randomly diluted models is presented. Porosity (crack-like) models are simulated by constructing typical elastic percolation networks with random microscopic heterogeneities in order to resemble rock media. Central and bond-bending forces (Born Hamiltonian) models are considered. For each experimental case, the elastic energy of the system is relaxed in equilibrium and then the model is excited by a pulse source in order to produce wave propagation. First, a review is presented of the well established velocity-porosity relationship from rock physics, which shows a linear trend from small porosities up to the critical porosity (percolation threshold) where the rocks fall apart. From the wave propagation analysis a general trend is observed for the attenuation of waves, from the small to the large porosity models, suggesting multiple scattering effects similar to those reported from effective-medium approximations of wave scattering due to random heterogeneities. Finally, the results are compared with those obtained from laboratory experiments on dry rocks with different porosities and different applied stress regimes.     

Sound scattering by the seafloor: Results of recent theoretical and experimental research

The state of theoretical and experimental research in sound scattering by the seafloor is considered. Current geoacoustic models are discussed. In general, these models make it possible to consider bottom sediments as an arbitrary stratified medium having statistically irregular interfaces and random volume heterogeneities, both continuous and/or discrete. The outlook for further development and practical use of the model are analyzed with emphasis on advancing techniques for acoustic sensing of marine sediments. Results of comparisons with data of the latest large-scale experiments on sound scattering by a shallow-water seafloor are given.     

Detection of submicron-sized raft-like domains in membranes by small-angle neutron scattering

Using coarse grained models of heterogeneous vesicles we demonstrate the potential for small-angle neutron scattering (SANS) to detect and distinguish between two different categories of lateral segregation: 1) unilamellar vesicles (ULV) containing a single domain and 2) the formation of several small domains or “clusters” (~10 nm in radius) on a ULV. Exploiting the unique sensitivity of neutron scattering to differences between hydrogen and deuterium, we show that the liquid ordered (lo) DPPC-rich phase can be selectively labeled using chain deuterated dipalymitoyl phosphatidylcholine (dDPPC), which greatly facilitates the use of SANS to detect membrane domains. SANS experiments are then performed in order to detect and characterize, on nanometer length scales, lateral heterogeneities, or so-called “rafts”, in ~30 nm radius low polydispersity ULV made up of ternary mixtures of phospholipids and cholesterol. For 1:1:1 DOPC:DPPC:cholesterol (DDC) ULV we find evidence for the formation of lateral heterogeneities on cooling below 30 °C. These heterogeneities do not appear when DOPC is replaced by SOPC. Fits to the experimental data using coarse grained models show that, at room temperature, DDC ULV each exhibit approximately 30 domains with average radii of ~10 nm.     

Endoscopically compatible near-infrared photon migration probe

We have developed a 2.3-mm-diameter fiber-optic probe for near-infrared photon migration spectroscopy that can be inserted into the body through an endoscope or biopsy needle. This probe is specifically designed to be inserted into a core biopsy needle to facilitate optical sampling of lesions during breast needle biopsy. This probe was tested on tissue phantoms containing heterogeneities (to stimulate breast lesions) of various sizes and optical properties. Under the conditions tested, the probe can measure the absorption coefficient to within 30% for heterogeneities with radii as small as 10 mm.     

Microscopic simulation of semiconductor lasers at telecommunication wavelengths

We consider two GaAs-based laser materials emitting at telecommunication wavelengths, namely the dilute nitride (GaIn)(NAs) as well as Ga(AsSb), and model their optical properties by including scattering and dephasing on a microscopic basis. The theory shows an excellent agreement with experiment without the inclusion of fit parameters such as phenomenological scattering times. By careful comparison of measurements and computations, one can extract controversial bandstructure parameters such as the band offset.     

Effect of the network on the director fluctuations in a nematic side-group elastomer analysed by static and dynamic light scattering

Static and dynamic light scattering experiments were performed on monodomains of a crosslinked nematic side-group polysiloxane. The depolarized scattering caused by fluctuations of the nematic director was analysed. Due to a superposed strong static scattering produced by heterogeneities the fluctuating part of the scattering intensity was found to be rather small. In contrast to low molar mass nematics, the relaxation rates and strength did not show a significant dependence on the scattering vector. The results can be explained by restoring torques on the director originating from the coupling to the network elasticity which dominate those originating from the Frank elasticity. Quantities describing the coupling as well as effective viscosities for the director reorientation were obtained.     

Cascaded four-wave mixing generation in photonic crystal fibers

We investigate the efficient generation of broadband cascaded four-wave mixing products seeded at different wavelengths in photonic crystal fibers, with a high-peak power picosecond pulse pumping in the anomalous dispersion region as well as the normal dispersion regime. Multiple four-wave mixing products with different frequency intervals have been experimentally achieved when pumping in the anomalous dispersion region. Measured results present bandwidths of over 500 nm. We have also experimentally demonstrated that only seeding at the Stokes or anti-Stokes peaks can we get the efficient generation of cascaded four-wave mixing or cascaded stimulated Raman scattering when pumping in the normal dispersion regime.     

Transient transport equations for high-frequency power flow in heterogeneous cylindrical shells

We use some recent mathematical results obtained for the high-frequency asymptotics of hyperbolic partial differential equations to derive exact transient power flow equations for vibrations of randomly heterogeneous cylindrical shells. The theory shows that the angularly resolved energy densities of an heterogeneous, elastic medium satisfy transient transport equations at higher frequencies. The behaviour of solutions of such equations short of their diffusion limit—if any—is fundamentally different from that of the solution of a diffusion equation, although the latter one is often invoked in the analyses of high-frequency vibrations of elastic structures. A condition by which diffusion equations can be obtained from transport equations is the presence of reflectors or heterogeneities such that scattering mean free paths are short with respect to the characteristic dimensions of the structure. The diffusion limit is reached in this study taking account of scattering by random heterogeneities of the background medium at the scale of the wavelength. This approach fills the gap between transport theory and the diffusion approach in structural dynamics, and clarifies the range of validity of the latter. Our results can be extended to fully coupled dynamic equations for compression, shear and bending of Timoshenko beams or Mindlin plates.     

High-power supercontinuum generation in highly nonlinear, dispersion-shifted fibers by use of a continuous-wave Raman fiber laser

High-power supercontinua are demonstrated in highly nonlinear, dispersion-shifted fibers with a continuous-wave Raman fiber laser. Supercontinuum growth is experimentally studied under different combinations of fiber length and launch power to show output powers as high as 3.2 W and bandwidths greater than 544 nm. Modulation instability (MI) is observed to seed spectral broadening at low launch powers, and the interplay between MI and stimulated Raman scattering plays an important role in the growth of the continuum at high launch powers. The effect on continuum generation of parametric four-wave mixing coupled with the higher-order dispersion properties of the fiber is investigated.     

SS-HORSE Method for Analysis of Resonances: Charged-Particle Scattering

The method for analyzing resonance states based on the harmoni©oscillator representation of scattering equations (HORSE) and on analytic properties of partial-wave scattering amplitudes which has been proposed earlier, is generalized to the case of charged-particle scattering. The method in question is tested by applying it to the model problem of pα scattering and can be used to study resonance states on the basis of microscopic calculations within various versions of the shell model.     

Broadband attosecond pulse shaping

We use semiconductor (Si) and metallic (Al, Zr) transmission filters to shape, in amplitude and phase, high-order harmonics generated from the interaction of an intense titanium sapphire laser field with a pulsed neon gas target. Depending on the properties of the filter, the emitted attosecond pulses can be optimized in bandwidth and/or pulse length. We demonstrate the generation of attosecond pulses centered at energies from 50 to 80 eV, with bandwidths as large as 45 eV and with pulse durations compressed to 130 as.     

SS-HORSE method for studying resonances

A new method for analyzing resonance states based on the Harmonic-Oscillator Representation of Scattering Equations (HORSE) formalism and analytic properties of partial-wave scattering amplitudes is proposed. The method is tested by applying it to the model problem of neutral-particle scattering and can be used to study resonance states on the basis of microscopic calculations performed within various versions of the shell model.     

纳米颗粒近场空间光谱散射建模与显微成像分析

用非直观偏振参数显微成像,即采用在传统显微光路中插入模式化装置,通过拟合过滤后对所得数据反演成像,通过对金属纳米颗粒在近场空间散射光谱分析,来解决空间散射现象。用非直观偏振参数显微成像与传统直观成像做对比,并通过时域有限差分法建模仿真,来描述近场直观与非直观散射光谱的差异,对比结果发现,非直观偏振参数显微反演成像的分辨率比直观成像更高,不但能够清晰的探测到纳米颗粒的形状和电场分布,而且比直观成像获得更广泛的空间散射光谱。     

中子散射技术及其应用

热中子的波长和凝聚态物质的原子／分子间距具有相同的量级，而其能量又和原子／分子的热运动能量相近．因此，利用热中子的弹性和非弹性散射效应，可以从微观层次上获取物质的结构和动力学知识。目前，中子散射技术在物理、化学、化工、生物和材料科学等研究领域的应用已经获得了许多用其他方法无法得到的知识，文章介绍了中子散射的基本原理和特点，并列举了中子散射技术在相关研究领域中的典型应用     

Light scattering in a finite multi-particle system

We use the discrete dipole approximation (DDA) to perform electromagnetic scattering calculations of particles in a 3D volume. We adjust the spacing between the particles to change the volume densities of the scattering systems from approximately 10% to 100%. For very large volume densities, e.g. >50%, it is difficult to assign unambiguously whether the system is composed of a single heterogeneous particle or of multiple particles. Our calculations demonstrate optical effects attributable to multiple scattering in systems having volume densities as high as ～90%. This suggests that heterogeneities within naturally occurring particle systems can produce multiple-scattering effects. We also see evidence of very deep negative polarization branches (NPBs) (～?6%) that may have implications in interpreting polarization phase curves of cometary circumnuclear halos.     

短脉冲激光与介质相互作用的扩散模型

推导了扩散近似方程,通过半无限大均匀介质计算,用扩散理论分析解验证了数值方法的有效性.模拟了光在非均匀介质内的传输过程,给出了介质内光通量随时间变化的空间分布.结果表明,该基于扩散模型的数值方法能够模拟短脉冲光在强散射介质中的传播过程以及漫散射光的时间变化特性,并且借助于光通量空间分布能够准确模拟非均匀介质内内含物的位置.     

Determine mechanical properties of particulate composite using ultrasound spectroscopy

It is known that microscopic spherulite growth plays an important role in macroscopical properties such as elastic moduli of some semicrystalline polymers. Ultrasonic spectroscopy can be used to quantitatively determine the role of spherulites. As a first approximation, spherulitic polymers are modeled as a material with spherical inclusions in an amorphous matrix. This two-phase composite model is then physically realized by embedding glass micro-spheres in an epoxy. The dynamic mechanical properties of these composites are experimentally determined by measuring their acoustic properties such as phase velocity and attenuation. Acoustic scattering theories are then applied to this model to test their predictive capabilities for the real composite's mechanical properties.