单个球面散射的电磁场的空间分布研究

近年来,关于发生在直径远大于入射光波长的圆柱或圆球散射体外表面的光学喷射翼的研究受到了很多关注.本文采用严格的Mie散射理论,借助计算机仿真软件Matlab 7.1,模拟计算出关于单个球体散射体电磁场在圆球内部以及圆球外部近场的空间分布.实验中,我们对电磁场边界条件的数值误差进行了检查,讨论了平面波入射下,在圆球外表面产生的光学喷射翼的一些基本性质,如喷射翼衰减长度,光强峰值及其空间位置.同时,我们详细分析了出现在圆球内部的两处显著光强峰的基本性质.另外,散射球体半径以及折射率对空间电磁场的影响在文中得以分析,相关的解释在文中得以提出.     

Investigation of laser-induced damage by nanoabsorbers at the surface of fused silica

A model for describing laser-induced damage in optical materials by nanosecond laser pulses is investigated. The laser-damage critical fluence is obtained based on calculating the light absorption of nanoabsorbers by using Mie theory and solving the heat equation. Considering a power law distribution of nano-absorbers, we calculated the damage probability at the surface of fused silica including Pt particles. The theoretical results calculated with appropriate parameters are applied to fit the experimental data in order to identify the properties of nanodefects.     

Theoretical Modeling of the Radiative Properties and Effective Thermal Conductivity of the Opacified Silica Aerogel

In this paper, we investigate the radiative properties and the effective thermal conductivity (ETC) of the opacified silica aerogel by theoretical method. The radiative properties of the opacified silica aerogel are obtained by the modified Mie Scattering Theory that is used for particle scattering in absorbing medium. The modified gamma distribution is used to take account of the non-uniformity of the particle size. The solid thermal conductivity of the composite material is obtained by considering the scale effect of the particles. Based on these calculated thermophysical properties the coupled heat conduction and radiation through the evacuated opacified aerogel are solved by the finite volume method. And the radiation flux is computed by the P-1 approximation combined with the gray-band model. Results show that, the calculated thermophysical properties of the TiO₂-doped silica aerogel are close to the experimental data. The optimal mean radius for the largest radiation extinction of the SiC particles is about 1μm. The presented data of optimal doping amount of the SiC particles at different temperature conditions for the evacuated silica aerogel is very useful for thermal insulation material design.     

Fabrication and technological properties of nanoporous spinel/forsterite/zirconia ceramic composites

In this work, nanoporous spinel/forsterite/zirconia ceramic composites were fabricated at 1600 °C for 2 h. The influence of zirconia content (up to 10 mass%) on the technological properties, nanopores formation, phase compositions, microstructure and thermal diffusivity of nanoporous ceramic composites was investigated. Nanospinel and nanoforsterite powders were synthesized via a modified co-precipitation and sol–gel techniques, respectively. Results indicated that apparent porosity of the fired nanoporous ceramic composites is mostly in the range 14.26–56.14% with the average pores diameter 35.8 nm. Using of nanopowders (spinel and forsterite) as the staring materials were achieved high mechanical (cold crushing strength ∼ 235–164 MPa) and elastic (Young’s modulus ∼ 123.6–4.5 GPa) properties of the prepared nanoporous ceramic composites. Microstructure analysis exhibited all of the crystalline phases and pores of the nanoporous ceramic composites are in the nanosize (35–40 nm). These nanoporous ceramic composites are promising porous ceramic materials for using in advanced applications due to their excellent combination properties.     

Visible and near-infrared backscattering spectroscopy for sizing spherical microparticles

Scattering is a useful tool for the determination of particle size in solution. In particular, spectroscopic analysis of backscattering renders the possibility of a simplified experimental setup and direct data processing using Mie theory. We show that a simple technique based on near-infrared (NIR) backscattering spectroscopy together with the development of the corresponding algorithm based on Fourier transform (FT) and Mie theory are a powerful tool for sizing microparticles in the range from 8 to 60 microm diameter. There are three wavelength intervals in the NIR, within which different diameter ranges were analyzed. In each one, the FT yields a coarse diameter value with an uncertainty dependent on the wavelength range. A more accurate value is obtained by further applying cross correlation between experimental and theoretical spectra. This latter step reduces the uncertainty in diameter determination between 30% and 40%, depending on wavelength interval and particle diameter. These results extend previous information on visible backscattering spectroscopy applied to sizing microparticles in the range between 1 and 24 mum diameter. This technique could be the basis for the construction of a portable and practical instrument.     

Images of finite sized spherical particles in confocal and conventional microscopes when illuminated with arbitrary polarization

We investigate the form of the image of a finite sized spherical particle in confocal and conventional microscopes when the illuminating light has an arbitrary polarization. In particular, we take the cases of radial and azimuthal polarizations and use the Mie theory to find the scattered field from differently sized particles for these cases. We present numerical results for the changes in the detected intensity when subresolution and resolvable spherical particles are illuminated with particular wavelengths and polarizations. Further, we find the limiting size of a particle for which it can be considered a point scatterer for a particular wavelength.     

Light scattering and absorption by fractal-like carbonaceous chain aggregates: comparison of theories and experiment

This study compares the optical coefficients of size-selected soot particles measured at a wavelength of 870 nm with those predicted by three theories, namely, Rayleigh-Debye-Gans (RDG) approximation, volume-equivalent Mie theory, and integral equation formulation for scattering (IEFS). Soot particles, produced by a premixed ethene flame, were size-selected using two differential mobility analyzers in series, and their scattering and absorption coefficients were measured with nephelometry and photoacoustic spectroscopy. Scanning electron microscopy and image processing techniques were used for the parameterization of the structural properties of the fractal-like soot aggregates. The aggregate structural parameters were used to evaluate the predictions of the optical coefficients based on the three light-scattering and absorption theories. Our results show that the RDG approximation agrees within 10% with the experimental results and the exact electromagnetic calculations of the IEFS theory. Volume-equivalent Mie theory overpredicts the experimental scattering coefficient by a factor of approximately 3.2. The optical coefficients predicted by the RDG approximation showed pronounced sensitivity to changes in monomer mean diameter, the count median diameter of the aggregates, and the geometric standard deviation of the aggregate number size distribution.     

Validation of the Infrared Emittance Characterization of Materials Through Intercomparison of Direct and Indirect Methods

A comparison of the spectral directional emittance of samples as a function of wavelength was performed at the Fourier Transform Infrared Spectrophotometry (FTIS) and the Advanced Infrared Radiometry and Imaging (AIRI) facilities at NIST. At the FTIS, the emittance is obtained indirectly through the measurement of near-normal directional-hemispherical reflectance (DHR) using an infrared integrating sphere. At the AIRI, the normal directional emittance is obtained directly through the measurement of the sample spectral radiance referenced to that from blackbody sources, while the sample is located behind a black plate of known temperature and emittance. On the same setup at the AIRI, the normal emittance at near ambient temperatures is also measured indirectly by a “two-temperature” method in which the sample spectral radiance is measured while the background temperature is controlled and varied. The sample emittance measurements on the comparison samples are presented over a wavelength range of 3.4 μm to 13.5 μm at several near-ambient temperatures and for near-normal incidence. The results obtained validate the two independent capabilities and demonstrate the potential of the controlled background methods for measurements of the radiative properties of IR materials.     

Antireflection coating with enhanced anti-scratch property from nanoporous block copolymer template

We prepared a sponge-like nanoporous silica film with a dense skin layer by the infiltration of silica precursor into nanoporous polystyrene-block-poly(methyl methacrylate) copolymer template followed by calcinations at high temperature. This film showed not only excellent antireflection at visible light wavelength range but also very good resistance to scratching compared with antireflection materials made of polymeric film. We expect that this film could be used for antireflection film with anti-scratching property for flat panel displays or touch panels.     

Predictions and measurements of scattering and absorption over broad wavelength ranges in tissue phantoms

Predictions from Mie theory regarding the wavelength dependence of scattering in tissue from the near UV to the near IR are discussed and compared with experiments on tissue phantoms. For large fiber separations it is shown that rapid, simultaneous measurements of the elastic scatter signal for several fiber separations can yield the absorption coefficient and reduced scattering coefficient. With this information, the size of the scattering particles can be estimated, and this is done for Intralipid. Measurements made at smaller source detector separations support Mie theory calculations, demonstrating that the sensitivity of elastic scatter measurements to morphological features, such as scatterer size, is enhanced when the distance between the source and detector fibers is small.     

Modulus-density scaling behaviour and framework architecture of nanoporous self-assembled silicas

Natural porous materials such as bone, wood and pith evolved to maximize modulus for a given density. For these three-dimensional cellular solids, modulus scales quadratically with relative density. But can nanostructuring improve on Nature's designs? Here, we report modulus-density scaling relationships for cubic (C), hexagonal (H) and worm-like disordered (D) nanoporous silicas prepared by surfactant-directed self-assembly. Over the relative density range, 0.5 to 0.65, Young's modulus scales as (density)n where n(C)相似文献   

Nanoporous carbon synthesized from sol–gel template for adsorbing gibberellic acid in aqueous solution

A novel method, based on dynamic carbonization and silica template formed by sol–gel, was developed to prepare nanoporous carbon materials with tailored pore structures. The effects of the sol–gel reaction and carbonization process on the final nanoporous carbon product were investigated by pore features such as specific surface area, the total pore volume, and pore size distribution, which were systemically characterized by iodine index, transmission electron microscopy, and nitrogen adsorption. The experimental results indicate that the pore structures of the prepared nanoporous carbon are tunable on the nano-scale by controlling the preparation process in the proposed method. The nanoporous carbon prepared under the optimal conditions has a high total pore volume of 1.26 cm³/g, a large specific surface area of 1744 m²/g, and a maximal adsorption capacity of 9.2 mg/g to gibberellic acid in aqueous solution, which is nearly 6 times that of commercial activated carbon.     

Synthesis of SiC nanorods from bleached wood pulp

Unbleached and bleached soft wood pulps have been used as templates and carbon precursors to produce SiC nanorods. Hydrolyzed tetraethylorthosilicate (TEOS), silicic acid was infiltrated into the pulps followed by a carbothermal reduction to form SiC nanorods at 1400 °C in Ar. Residual carbon formed along with SiC was removed by gasification at 700 °C in air. The SiC materials prepared from unbleached pulp were non-uniform SiC with a thick SiO₂ coating, while the SiC nanorods prepared from the bleached pulp were uniform and straight with dimensions of 250 nm in diameter and 5.0 mm long. The formation of uniform camelback structure of SiC in the reaction between silica and bleached pulp is attributed to more silica deposited in the amorphous region of cellulose.     

Surface effects on the mechanical properties of nanoporous materials

Using the theory of surface elasticity, we investigate the mechanical properties of nanoporous materials. The classical theory of porous materials is modified to account for surface effects, which become increasingly important as the characteristic sizes of microstructures shrink to nanometers. First, a refined Timoshenko beam model is presented to predict the effective elastic modulus of nanoporous materials. Then the surface effects on the elastic microstructural buckling behavior of nanoporous materials are examined. In particular, nanoporous gold is taken as an example to illustrate the application of the proposed model. The results reveal that both the elastic modulus and the critical buckling behavior of nanoporous materials exhibit a distinct dependence on the characteristic sizes of microstructures, e.g. the average ligament width.     

Multi-scale Modeling of Radiation Heat Transfer through Nanoporous Superinsulating Materials

In this contribution, the extraordinarily high level of thermal insulation produced by nanoporous materials, which can achieve thermal conductivities down to a few mW·m⁻¹·K⁻¹ when they are evacuated to a primary vacuum, is highlighted. The objective here is to quantify the level of radiation heat transfer traveling through a nanoporous material in relation with its composition. The model used here is based on the “non-gray anisotropically scattering Rosseland approximation,” which allows the definition of a “radiation thermal conductivity” expressed as a function of the optical properties (complex optical index spectra), mean sizes and volume fractions of the different populations of particles constituting the material. With the help of this simple model, one can draw interesting conclusions concerning the impacts of different parameters related to the microstructure of the nanoporous material on the amplitude of the radiation heat transfer. In the future, this model should help to orient the formulation of new nanoporous materials with optimized radiative properties. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Experimental and Theoretical Characterization of Emission from Ceramics at High Temperature: Investigation on Yttria-Stabilized Zirconia and Alumina

This study is a contribution to characterizations of non-isothermal ceramic samples, for mean temperatures higher than 1000 °C. The objective is to determine the infrared radiative emission, of materials often used as thermal barrier coatings, under realistic thermal boundary conditions. The problem is treated by both experimental and numerical approaches that reveal additional ways of investigation. Results from sintered zirconia and plasma-sprayed alumina are presented. The experimental bench, designed and built to perform emission measurements on semi-transparent ceramics at high temperature, is described. The numerical code used to solve coupled conduction–radiation heat transfers in semi-transparent media is introduced. Special attention is paid to calculation of radiative properties of plasma-sprayed samples characterized by their complex internal microstructure. Experimental and theoretical emission factors obtained from these kinds of ceramics samples are compared and analyzed. The influence of the inside temperature gradient on emission is discussed.     

Three-dimensional imaging analysis of confocal and conventional polarization microscopes by use of Mie scattering theory

We present a three-dimensional imaging analysis of confocal and conventional polarization microscopes by using the extended Mie scattering theory. In the analysis, we calculate the images of a Mie particle whose diameter is comparable with the wavelength of confocal and conventional microscopes. It was found that, when we observe a Mie particle, polarization confocal microscopy is not affected by the polarization distortion that is due to focusing with high-numerical-aperture lenses and does not produce pseudopeaks in the images in comparison with conventional polarization microscopy. The three-dimensional resolution of the polarization microscope and the verification of the proposed analysis method are also discussed.     

Patterning of periodic nano-cavities on PEDOT-PSS using nanosphere-assisted near-field optical enhancement and laser interference lithography

A simple approach for creating periodic nano-cavities and periodic stripes of nano-cavity arrays on poly (3,4-ethylene dioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) thin films using a combination of optical near-field enhancement through self-assembled silica nanospheres and laser interference lithography is presented. Monolayers of close-packed silica nanospheres (800, 600, and 430 nm in diameter) are self-assembled on 2 μm thick PEDOT-PSS electropolymerized films and are subsequently irradiated with 10 ns pulses of 355 nm wavelength laser light. Over areas spanning 2 cm(2), circular nano-cavities with central holes of size 50-200 nm and surrounding craters of size 100-400 nm are formed in the PEDOT-PSS films directly underneath the nanospheres due to strong enhancement (11-18 fold) of the incident light in the near-field, which is confirmed through Mie scattering theory. Predictions from theoretical simulations examining the combined effects of near-field enhancement and interference are in good agreement with the experimental results. The results illustrate the versatility of the described technique for creating nano-cavity arrays or nano-pores in PEDOT-PSS over large areas with designed periodicity and hole size.     

Reconfigurable Grating Diffraction Structural Color in Self-Assembled Colloidal Crystals

Self-assembled colloidal crystals display structural colors due to light diffracted from their microscale, ordered structure. This color arises due to Bragg reflection (BR) or grating diffraction (GD); the latter mechanism is much less explored than the former. Here the design space for generating GD structural color is identified and its relative advantages are demonstrated. Electrophoretic deposition is used to self-assemble crystals with fine crystal grains from colloids of diameter 1.0 µm. The structural color in transmission is tunable across the full visible spectrum. The optimum optical response—represented by both color intensity and saturation—is observed at low layer number (≤5 layers). The spectral response is well predicted by Mie scattering of the crystals. Taken together, the experimental and theoretical results demonstrate that vivid grating colors with high color saturation can be produced from thin layers of micron-sized colloids. These colloidal crystals extend the potential of artificial structural color materials.     

Radiation forces in the discrete-dipole approximation

The theory of the discrete-dipole approximation (DDA) for light scattering is extended to allow for the calculation of radiation forces on each dipole in the DDA model. Starting with the theory of Draine and Weingartner [Astrophys. J. 470, 551 (1996)] we derive an expression for the radiation force on each dipole. These expressions are reformulated into discrete convolutions, allowing for an efficient, O(N logN) evaluation of the forces. The total radiation pressure on the particle is obtained by summation of the individual forces. The theory is tested on spherical particles. The resulting accumulated radiation forces are compared with Mie theory. The accuracy is within the order of a few percent, i.e., comparable with that obtained for extinction cross sections calculated with the DDA.