Absorption and Scattering of Light by Small Particles

In many fields of science and engineering, accurate information about light scattering properties of small nonspherical particles are necessary. In the so-called resonance region of particle size parameters, numerical methods for computing nonspherical scattering must be based on directly solving Maxwell's equations, because Rayleigh and geometric optics approximations are inapplicable. In this paper, we review the status of Waterman's T-matrix approach and we present some results obtained by computing orientationally-averaged light scattering characteristics for ensembles of nonspherical particles.

Abstract The aim of this work is to evaluate the tribological behavior of decorative thin film nanocomposites consisting of gold nanoparticles dispersed in a dielectric matrix of TiO 2 . The main focus is given to the changes in the friction coefficient and wear rate, as well as on the overall tribological features resulting from the morphological changes induced by annealing experiments in vacuum, at increasing temperatures up to 500 °C. In particular, the gold clustering, increase of grain size and the crystallization of the TiO 2 dielectric matrix are correlated with the changes in the tribological parameters. Linearly reciprocating ball-on-flat sliding tests were performed under 0.1 N applied load and constant frequency of 1 Hz. AISI 52100 ball bearing steel was used as the opponent surface and the sliding occurred at room temperature, in the absence of external lubrication. It was found that the annealing process at different temperatures had no significant effect on the stoichiometric condition of the TiO 2 matrix, deepness homogeneity of the Au nanoclusters and film thickness, but important changes occurred in terms of clusters size and crystallization of the film. Those structural changes resulted in distinct friction regimes and wear responses. Annealing at 300 °C resulted in the best frictional response ( µ ~0.15 during an initial friction plateau) and good wear resistance ( K =2×10 −6  mm 3  N −1  m −1 ). Thermal annealing at higher temperature had a detrimental effect on the tribological properties of the films due to important structural changes as increasing of crystallinity and nanoparticle gain size. Annealing at 500 °C presented the worst tribological properties of the films ( µ ~0.22 and K =4×10 −5  mm 3  N −1  m −1 ), with extensive early delamination. A direct correlation was found between the tribological properties of the TiO 2 /Au decorative thin films and the diffusion mechanisms induced by the thermal annealing.

The mechanisms at the origin of the ultrafast third-order optical nonlinearity of metallic and plasmonic materials are described focusing on the dominant resonant processes associated to energy absorption. Optical nonlinearity is discussed in terms of ultrafast modifications of the dielectric function of the constituting metal as a consequence of electron and lattice heating, using a bulk-like model. Based on this description, the time- and spectral-dependent nonlinear changes of the optical response of plasmonic materials due to interaction with a femtosecond light pulse are modeled. The results are illustrated in the case of an individual gold nanoparticle for different shapes (sphere, ellipsoid, and rod), and for an ensemble of gold nanoparticles dispersed in a dielectric matrix. The same approach can be extended to more complex plasmonic materials or meta-materials.

The aim of this paper is to provide some conceptual basis of a measurement method for non-invasive in situ diameter characterization of a thin, glass fibre. The method involves small-angle scattering of incoherent light. A few selected results achieved with the aid of mathematical models of light scattering indicate that a practical implementation of the method is possible. Streszczenie. W niniejszej pracy przedyskutowano koncepcje metody nieinwazyjnego pomiaru in situ średnicy wlokna szklanego, wykorzystującej promieniowanie niespojne jako narzedzie poznawcze. W drodze modelowania matematycznego i symulacji numerycznych zbadano wlaściwości pola rozproszonego pod malym kątem oraz zaproponowano metode jednoznacznego rozwiązania problemu odwrotnego w pomiarze średnicy.(Wykorzystanie dyfrakcji promieniowania niespojnego w pomiarze średnicy wlokna szklanego).

This chapter discusses the dispersed metal clusters from metal vapor chemistry. The realization of the potential of metal clusters in catalysis can be achieved by harnessing the unique properties that are anticipated for groupings of metal atoms having low nuclearity. Large anionic metal-carbonyl clusters serve as models for small metallic crystallites covered (or poisoned) by ligands. The chapter focuses on the metal atoms in polymers, and outlines the specific features of their interactions that are significant in promoting the growth and stabilization of metal clusters prepared directly from the atomic vapor. Adapting the techniques of nanogram-microgram matrix isolation synthesis and spectroscopy, a microscale metal vapor technique was introduced for assessing the outcome of controlled depositions of 1 to 3 different metal atoms into thin polymer or oligomer films or organic solvent/solute systems.

Since the first inductively coupled plasma (ICP) torch was developed in the 60's, RF thermal plasmas have found a variety of applications in material processing such as crystal growth, thermal spray coatings, spheroidization and vaporization of refractory materials. In recent decades, inductively coupled thermal plasmas have been used for the synthesis of high purity nanoparticles, thank to their remarkable advantages such as high energy density, variable operating pressures and low product contamination. The formation of nano-sized particles in thermal plasmas particularly using solid refractory precursor is a complex process. Hereby the injected precursor powders are heated by the high temperature of the thermal plasma. The heated powders are vaporized and decomposed into vapor species. During the following cooling, the vapor species are condensed to nano-sized particles. In this synthesis process, characterized by evaporation-condensation, the properties of the synthesized particles such as particle size, size distribution, phases and morphology are affected by various process parameters. Gas pressure, flow rates of the different torch gases, size of the precursor powders and powder loading in the plasma are considered as the influencing process parameters. In order to optimize and control the process for tailor-made nanoparticle synthesis, it is necessary to understand the effects of the process parameters on plasma properties and on the particle-plasma interactions resulting in final properties of the synthesized powders. In the present work, firstly, the enthalpy probe technique, a well-established plasma diagnostic tool for high pressure thermal plasma, has been applied to characterize the inductively coupled Ar-H2 thermal RF plasma used for alumina nanoparticle synthesis at different operating conditions. The plasma enthalpy has been determined, and the plasma temperature under a given gas pressure could subsequently be calculated assuming local thermodynamic equilibrium (LTE), once the gas composition has been determined by mass spectrometry. The gas velocity of the plasma jet could also be obtained by using the Bernoulli equation and stagnation pressure. Secondly, the influence of flow rates of central gas and precursor feed rates on the Al2O3 precursor evaporation has been investigated by process monitoring. Optical emission spectroscopy (OES) and laser light extinction (LE) measurements have been carried out to monitor in-situ the injection of the alumina precursors and to obtain information on the ongoing precursor evaporation. The emission line intensities of the aluminum vapor were used to determine the dependence of process parameters on precursor evaporation. Furthermore, the laser light extinction was used to calculate the number density of non plasma-treated powders, from which the number fraction of evaporated powders could be deduced. The size and morphology of the synthesized particles have been characterized ex-situ. Finally, the influence of quenching conditions on the condensation and formation of the nano-sized alumina particle has been studied by process analysis of the gas phase reactions of alumina. Optical emission spectroscopic was performed at different axial positions to monitor the gas phase reactions of the vaporized particles. Axial profiles of the emission line intensities of the main vapor species of alumina, Al(g) and AlO(g), were compared with the results of the enthalpy probe measurements and the thermal decomposition reactions of alumina found in literature. The results allow explanation of the alumina reactions in the thermal plasma as well as gives indications for the optimal axial position of the quenching gas injection. In addition to the determination of the optimal quenching position, the influence of the flow rates of quenching gas on the nanoparticle formation has been investigated. The synthesis of the submicron alumina using vapor-condensation principle predominantly leads to various thermodynamically metastable crystal structures called transition alumina. Therefore, the occurrence of different transition aluminas is discussed with regards to the flow rates of the quenching gas. The results presented in this thesis show that the synthesis process of the nanoparticle can be described on the basis of experimental results and basic information on the powder materials. The explanation of the influence of process parameters on powder evaporation and nanoparticle formation shows the usefulness of the in-situ plasma process monitoring, and the large potential for optimizing and controlling the nanopowder synthesis process in an inductively coupled RF thermal plasma.

In this paper, we present the forward (optical gating) and inverse (reconstruction) approaches to obtain high-resolution imaging in tissue-like media under single-photon (1-p) and two-photon (2-p) excitation. Effective point spread functions (EPSF) for fluorescence microscopic imaging are introduced for fast image modeling and reconstruction. It is demonstrated that a deeper penetration depth can be achieved under 2-p excitation due to the use of a longer illumination wavelength and the non-linear dependence of the fluorescence on excitation intensity. The fundamental difference between 1-p and 2-p fluorescence imaging is that the penetration depth is limited by the degradation in image resolution in 1-p fluorescence, while the penetration depth is limited by the loss in signal strength in 2-p fluorescence imaging. Based on the EPSF that derived in the simulation, image reconstruction using deconvolution methods can partially recover the resolution loss.

This chapter provides an overview of wave scattering and the effective medium. In the classical scalar wave case, the single scatterer is taken to be a sphere of radius R with dielectric constant. When there are infinitely many scatterers, the T -matrix and the exact Green's function are impossible to obtain accurately. However, it is determined that in the wave vector representation, the averaged Green's function is given by a particular function. For the three-dimensional (3D) coherent potential approximation (CPA), the evaluation of the Green's function is perhaps the most time-consuming part of the numerical calculation. A useful and efficient approach to the calculation of the 3D Green's function is to compile a numerical table of the density of states for the ordered lattice and store it in computer memory. The diagonal Green's function can then be obtained from an equation, which involves only one integration.

Silver and gold nanoparticles were prepared by grinding method using stabilizers such as de-ionized water, ethylene glycol (EG) and 5 wt% polyvinyl alcohol (PVA). From the observation, it was found that the solution’s color of silver particle changed from clear color to light yellow or brown and that of gold particle changed from yellow to light blue. This indicated that the size of silver and gold particle was reduced. In addition, the optical properties of silver and gold nanoparticles were studied by UV-vis spectroscopy. It was found that the absorbance and transmission spectra of silver and gold nanoparticles depended on the type of stabilizing agent and grinding time. In addition, the observed color of nanoparticles could be explained using Maxwell triangle and in agreement with the color observed by naked eyes.

A rigourous theory for the determination of the van der Waals interactions in colloidal systems is presented. The method is based on fluctuational electrodynamics and a multiple-scattering method which provides the electromagnetic Green's tensor. In particular, expressions for the Green's tensor are presented for arbitrary, finite, collections of colloidal particles, for infinitely periodic or defected crystals as well as for finite slabs of crystals. The presented formalism allows for {\it ab initio} calculations of the vdW interactions is colloidal systems since it takes fully into account retardation, many-body, multipolar and near-fields effects.

Various optical methods for assessing the retinal nerve fiber layer (RNFL) depend on reflected light, but little is known about the characteristics of the RNFL as a reflecting structure. The authors investigated the angular dependence of light reflected by the unmyelinated nerve fibers of the toad eyecup using a small 500-nm light source that could illuminate the retina from various directions and a movable low-power microscope that imaged the retina onto a cooled charge-coupled device in a digital camera system. Measured areas had nerve fiber bundles separated by gaps. Therefore, the reflectance of a bundle alone could be determined from the difference in intensity between the bundle and an adjacent gap. The RNFL reflectance showed striking directional dependence; nerve fiber bundles seen when illuminated from one direction disappeared completely when illuminated from another. Light reflected by a bundle was confined to a conical sheet concentric with the axis of the bundle. The apex angle of the cone was twice the angle between the incident light and the bundle axis, and the orientation of the cone changed with the orientation of the RNFL. This behavior was consistent with the theory of light scattering by cylinders. Therefore, it was concluded that the RNFL reflectance arises from cylindric structures. These results have clinical significance for imaging the RNFL in the human eye because the apparent intensity of the RNFL will depend, not just on its thickness, but also on its orientation relative to the imaging system.