Optical equivalence of isotropic ensembles of ellipsoidal particles in the Rayleigh-Gans-Debye and anomalous diffraction approximations and its consequences

Light scattering by isotropic ensembles of ellipsoidal particles is considered in the Rayleigh-Gans-Debye approximation. It is proved that randomly oriented ellipsoidal particles are optically equivalent to polydisperse randomly oriented spheroidal particles and polydisperse spherical particles. Density functions of the shape and size distributions for equivalent ensembles of spheroidal and spherical particles are presented. In the anomalous diffraction approximation, equivalent ensembles of particles are shown to also have equal extinction, scattering, and absorption coefficients. Consequences of optical equivalence are considered. The results are illustrated by numerical calculations of the angular dependence of the scattering phase function using the T-matrix method and the Mie theory.     

Anomalous diffraction approximation for light scattering cross section: Case of random clusters of non-absorbent spheres

We previously [Jacquier S, Gruy F. Approximation of the light scattering cross-section for aggregated spherical non-absorbent particles. JQSRT 2008;109:789–810] reformulated the anomalous diffraction (AD) approximation to calculate the light scattering cross section of aggregates by introducing their chord length distribution (CLD). It was applied to several ordered aggregates. This new method is entitled ADr, with the r for rapid because this one is at least 100 times faster than the standard AD method. In this article, we are searching for an approximated expression for CLD suitable all at once for ordered and disordered aggregates. The corresponding scattering cross-section values are compared to the ones coming from the standard AD approximation.     

Light absorption and scattering by aggregates: Application to black carbon and snow grains

A geometric-optics surface-wave approach has been developed for the computation of light absorption and scattering by nonspherical particles for application to aggregates and snow grains with external and internal mixing structures. Aggregates with closed- (internal mixing) and open-cell configurations are constructed by means of stochastic procedures using homogeneous and core-shell spheres with smooth or rough surfaces as building blocks. The complex aggregate shape and composition can be accounted for by using the hit-and-miss Monte Carlo geometric photon tracing method. We develop an integral expression for diffraction by randomly oriented aggregates based on Babinet's principle and a photon-number weighted geometric cross section. With reference to surface-wave contributions originally developed for spheres, we introduce a nonspherical correction factor using a non-dimensional volume parameter such that it is 1 for spheres and 0 for elongated particles. The extinction efficiency, single-scattering albedo, and asymmetry factor results for randomly oriented columns and plates compare reasonably well with those determined from the finite-difference time domain (FDTD) and the discrete dipole approximation (DDA) computer codes for size parameters up to about 20. The present theoretical approach covers all size ranges and is particularly attractive from the perspective of efficient light absorption and scattering calculations for complex particle shape and inhomogeneous composition.We show that under the condition of equal volume and mass, the closed-cell configuration has larger absorption than its open-cell counterpart for both ballistic and diffusion-limited aggregates. Because of stronger absorption in the closed-cell case, most of the scattered energy is confined to forward directions, leading to a larger asymmetry factor than the open-cell case. Additionally, light absorption for randomly oriented snowflakes is similar to that of their spherical counterparts under the condition of equal geometrical cross section area for both external and internal mixing states; however, nonspherical snowflakes scatter less light in forward directions than spheres, resulting in a substantial reduction of the asymmetry factor. We further demonstrate that small soot particles on the order of 1 μm internally mixed with snow grains could effectively reduce snow albedo by as much as 5-10%. Indeed, the depositions of black carbon would substantially reduce mountain-snow albedo, which would lead to surface warming and snowmelt, critical to regional climatic surface temperature amplification and feedback.     

Calculation of the quantum efficiency for the absorption on confinement levels in quantum dots

The quantum efficiency of the absorption on quantum confinement levels is investigated. This is achieved by modeling the electron confinement in a spherical quantum dot (QD). The confinement levels are calculated using both infinite and finite rectangular quantum wells. The spectral internal quantum efficiency is evaluated within both the models, by computing Einstein’s coefficients for the transitions between confinement levels. The size of QDs (1–3 nm radius) leads to negligible many body effects. The nature of the QD material and of the matrix embedding is taken into account in the finite rectangular quantum well approximation and introduces only a small correction. The temperature dependence of the efficiency is also taken into account. A numerical application is performed for a silicon QD of 2.5 nm radius, embedded in amorphous silica. It is proved that the absorption threshold shifts toward the far infrared limit and that the spectral internal quantum efficiency reaches 4–5% at the threshold.     

Analytical characterization of gold nanoparticle primary particles,aggregates, agglomerates,and agglomerated aggregates

Gold nanoparticles have been studied for many biomedical applications. However, alterations in the gold nanoparticles’ environment frequently lead to the formation of aggregates and agglomerates, which have not been well characterized. These new structures could significantly change the biological impact of the nanoparticles, so the appropriate characterization of these structures prior to biological administration is vital for the correct interpretation of toxicology results. By varying the solvent or heating under pressure, four reproducible gold nanoparticles structures were created: 10 nm primary particles, aggregates of the primary particles that contain non-reversible bonds between the individual nanoparticles, agglomerates of primary particles that contain reversible interactions between the individual nanoparticles, and agglomerated aggregates that have reversible bonds linking individual aggregates. Ultraviolet–visible (UV–Vis) spectroscopy, thermal gravitational analysis, and neutron activation analysis were each found to accurately measure the concentration of the primary particles. The primary particles measured 10 nm by dynamic light scattering (DLS) and had a spherical morphology by transmission electron microscopy (TEM) while the aggregates measured 110 nm by DLS and had a distorted morphology by TEM. The agglomerate and aggregated agglomerate samples both measured >1,000 nm by DLS, but the individual particles had significantly different morphologies by TEM. Multiple other analytical techniques, including ultracentrifugation, gel electrophoresis, and X-ray diffraction, also showed unique traits for each structure. The structural differences did not change in the presence of cell culture media or rat serum. In addition, the primary particles, aggregates, and agglomerates each had a unique UV–Vis spectrum, allowing for an inexpensive, rapid method to differentiate between the structures.     

Thermophoretic transport of moderately large spherical and cylindrical particles in two-component gases

The problem of the thermophoretic motion of moderately large solid spherical and cylindrical particles in a two-component gas is solved for Re≪1. The formulas obtained permit direct estimation of the rate of thermophoretic motion of both single-layer and multilayer particles. Corrections which depend directly on the Knudsen number are taken into account in the derivation of these formulas. The thermal conductivity of the particles is assumed to be a function which depends on the radial coordinate. It is shown that thermal diffusion and the dependence of the thermal conductivity on the radial coordinate can have a significant influence on the rate of thermophoretic transport of particles. Zh. Tekh. Fiz. 69, 21–27 (August 1999)     

Angular structure of radiation scattered by monolayer of polydisperse droplets of nematic liquid crystal

We considered light scattering by a polydisperse ensemble of droplets of a nematic liquid crystal. To model light scattering by a monolayer of polymer-dispersed spherical droplets of a nematic liquid crystal with cylindrical symmetry of its internal structure, we proposed a semianalytical modeling method. The method is based on interference approximation of the theory of multiple wave scattering, anomalous diffraction approximation, and effective-medium approximation. The method takes into account cooperative optical effects in concentrated, partially ordered layers and can be used to analyze the small-angle structure of the intensity of scattered radiation in relation to the concentration, size, polydispersity of liquid crystal droplets, orientation of their optical axes, and refractive indices of the liquid crystal and polymer. The obtained relations can be applied to solving direct and inverse problems of light scattering in composite liquid crystal materials using data of polarization measurements. We present graphical results of solving the direct problem for components of the polarization vector of scattered wave. These results illustrate the formation of an angular structure for monolayers with a high concentration of polydisperse droplets of the liquid crystal in the range of small scattering angles (0 < θ _s ≤ 8°).     

Study of dispersion and absorption of an ensemble of spherical particles inside an optical cavity and new possibilities of predicting the optical characteristics of biological media by intracavity laser spectroscopy

Optical characteristics, namely, dispersion and absorption spectra of an ensemble of spherical particles randomly oriented inside an optical cavity are investigated. The study is based on the self-consistent matching of new data from the inhomogeneous optical cavity with data from the scattering of an ensemble of spherical particles of different size, randomly oriented in free space. As a result, a new model, which self-consistently accounts for multiple scattering in the optical cavity, has been developed to predict absorption and dispersion of ensembles of spherical particles. This model is supposed to enhance potentiality of the intracavity method for plotting wavelength dependences of optical characteristics of media. A specific calculation of dispersion and absorption dependences on the wavelength shows that this method can be used for investigation of biological media consisting of spherical particles, in particular, erythrocyte suspension.     

Anomalous diffraction approximation for light scattering cross section: Case of ordered clusters of non-absorbent spheres

The anomalous diffraction (AD) method has proved to be an adequate approximation of the exact method (GMM code computed by Xu) to obtain the scattering cross section for aggregates whose primary particle size parameter is greater than 2. Indeed, the error of this approximated method is less than approximately 10% when the primary particles are in SiO₂.However, this method is strongly related to the aggregate morphology since it includes in its formulation the concept of chord length. We initially studied the chord distribution for various aggregates, and then we reformulated the anomalous diffraction method, to approximate the scattering cross section, now completely analytically. This new expression is entitled ADr with the r for rapid because this one is at least a hundred times faster than the standard AD method.     

Light-scattering by optically soft randomly oriented spheroids

In the framework of the Rayleigh–Gans–Debye approximation and anomalous diffraction approaches, the light scattering characteristics of randomly oriented spheroids have been investigated. It has been proved that the system of randomly oriented spheroids is equivalent to the system of polydisperse spherical particles that have the same values of volume and surface area as nonspherical particles. The power law size distribution meeting these requirements has been obtained.     

Peculiarities of induced radiation and controlled dechanneling of relativistic particles in crystals under the Cherenkov—Vavilov extreme condition

Peculiarities of induced radiation and absorption of hard electromagnetic radiation are considered on the basis of the channeling of relativistic particles in crystals under the Vavilov—Cherenkov extreme condition. It is shown that the consistent inclusion of quantum recoil in the relativistic region makes it possible to produce a number of anomalous effects having no analogs in optics. In particular, it is predicted that it is possible to simultaneously cool the beam of fast particles during radiation and absorption and to create a tandem laser for which successive radiation of quasicharacteristic-radiation quanta at the normal and anomalous Doppler effect leads to the restoration of the initial state of the quantum system and to the multiple repetition of the radiation cycle. It is shown that the main difficulties preventing these effects from being produced directly in the X-and gamma-ray spectral regions are related to the necessity to use media with positive dispersion. It is considered that such dispersion can be produced by changing the effective susceptibility of the medium in the case of radiation diffraction.     

Scattering amplitude of absorbing and nonabsorbing spheroidal particles in the WKB approximation

This work is devoted to a theoretical study of scattering of light by absorbing and nonabsorbing oriented spheroidal particles in the Wentzel-Kramers-Brillouin (WKB) approximation. Within the framework of the scattering theory, we investigate the form factor and the scattering amplitude for this approximation. The Rayleigh-Gans-Debye theory (RGD), the diffraction approximation (DA), and the anomalous diffraction (AD) are treated as particular cases for nonabsorbing spheroids. To illustrate our formalism, we analyze some numerical examples.     

Photon adsorption and radiation by a nonequilibrium plasma of finite volume in lines with doppler broadening

Photon absorption in a plasma of finite volume is analyzed for a model of spherical geometry. The radiation of a plasma in a cylindrical volume is found in the approximation of two hemispheres.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 54–59, September, 1969.     

The fabrication and photo-induced melting of networked gold nanostructures and twisted gold nanorods

Fabrications of both networked Au nanostructured particles and twisted gold nanorods (TGNRs) suspended in water were accomplished via a laser ablation technique. They can be further purified by simple filtration. We conducted the photo-induced melting studies on these two types of nanostructures under both 532 and 1064 nm laser irradiation wavelengths. Both structural changes and their absorption spectral responses are reported. The TGNR sample contains particles with a wide distribution of aspect ratios. Its typical absorption spectrum shows a flat and continuously rising feature to the red of ca. 520 nm, which show a similarity with that of the suspended Au networks. The photo-induced melting processes corresponding to these two types of Au nanostructures demonstrate quite different spectral responses associated with their structural variations. Various observations on the wavelength- and laser power-dependence are presented. Generally, this type of photoannealing results in a formation of spherical shaped particles. Among them, we demonstrate that a spectral ‘hole’ burning can be uniquely achieved in the case of TGNRs suspension after 1064 nm low-fluence laser beam irradiation. Its contribution based on a selective melting will be addressed.     

煤烟聚集粒子的光散射特性研究(英文)

基于分形理论,用计算机模拟了由球形基本粒子构成的煤烟聚集粒子。利用离散偶极子近似方法(Discrete Dipole Approximation)研究了煤烟聚集粒子的散射特性,讨论了分形煤烟聚集粒子的散射强度随煤烟聚集粒子的分形结构、大小、相对折射率及入射波波长变化情况。     

A method for determining optical constants, dimensions, and concentrations of “soft” absorbing particles in the brightening band region

We investigate the possibilities of creating a method for estimating the optical constants, dimensions, and concentrations of “soft” absorbing particles by applying a theoretical analysis of the angular dependence of the intergrated indicatrix, overall characteristics of light scattering, and absorption on the phase shift and diffraction parameter of particles in the brightening band region. We show that using the investigated optical characteristics, it is possible to determine the unknown parameters of a suspension from experimental data. Institute of Biophysics, Siberian Branch of the Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, 660036, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 807–812, November–December, 1997.     

球形α-Fe2O3超微粒的红外光学特性和表面模吸收

用改进的水解法制备了大小均匀、形状规则的球状α-Fe₂O₃超微粒;分别测量了两种不同粒度超微粒的红外透射谱;观测到了球状α-Fe₂O₃超微粒的全部表面模;讨论了微粒的聚集对吸收的影响;并用柱状微晶来近似链状聚集解释了光谱中的两个展宽吸收带;介绍了将多原子立方结构超微粒的表面模理论推广应用到单轴晶体结构的α-Fe₂O₃超微粒的情况。对表面模吸收频率进行了计算,理论数据同实验结果 关键词：     

Anomalous light-induced drift of linear molecules

The sudden approximation in energy is used to derive analytic formulas that describe the anomalous light-induced drift (LID) of linear molecules absorbing radiation in the rovibrational transition nJ _i −mJ _f (n and m are the ground and excited vibrational states, and J _α is the rotational quantum number in the vibrational state α=m, n). It is shown that for all linear molecules with moderate values B≲1 cm⁻¹ of the rotational constant, anomalous LID can always by observed under the proper experimental conditions; temperature T, rotational quantum number J _i , and type of transition (P or R). The parameter γ=B[J _i (J _i +1)−J _f (J _f +1)] ν _n /2k _BT (ν _m −ν _n ) is used to derive a condition for observing anomalous LID: γ∼1 (k _B is the Boltzmann constant and ν _α is the transport rate of collisions of molecules in the vibrational state α and buffer particles at moderate molecular velocities , where is the most probable velocity of the buffer particles). For ν _m >ν _n anomalous LID can be observed only in P-transitions, while for ν _m <ν _n it can be observed only in R-transitions. It is shown that anomalous LID is possible for all ratios β=M _b /M of the masses of the buffer particles (M _b ) and of the resonant particles (M) and any absorption-line broadening (Doppler or homogeneous). The optimum conditions for observing anomalous LID are realized when the absorption line is Doppler-broadened in an atmosphere of medium-weight (β∼1) and heavy (β≫1) buffer particles. In this case, anomalous LID can be observed in the same transition within a broad temperature interval ΔT∼T. If the buffer particles are light (β≪1) or if the broadening of the absorption line is homogeneous, anomalous LID in the same transition can be observed only within a narrow temperature range ΔT≪T. Zh. éksp. Teor. Fiz. 115, 1664–1679 (May 1999)     

Free field measurements of the absorption coefficient for nonlocally reacting sound absorbing porous layers

A simple asymptotic approximation with two parameters (the plane wave reflection coefficient and a correction factor) has been derived by Brekhovskikh and Godin [Acoustics of Layered Media II. Point Sources and Bounded Beams (Springer, New York, 1992)], for the spherical field reflected by nonlocally reacting surfaces. In the present work, an expression for the correction factor is obtained for the case of a homogeneous and isotropic porous layer. A free field method for evaluating reflection and absorption coefficients with this approximation is presented. The evaluation of the absorption coefficient at normal incidence is performed on a porous layer on a rigid backing, and compared to measurements performed using a two microphone technique and an impedance tube.     

Absorption of electromagnetic radiation by a cylindrical metal particle

The cross section of the absorption of electromagnetic radiation by a cylindrical metal particle is calculated. The calculation is performed in the low-frequency limit, in which the contribution of eddy currents to the absorption dominates, and for comparatively small particles (with radii of ≈10 nm), which allows us to neglect the skin effect. The case when the mean free path of electrons in the metal bulk substantially exceeds the radius of the cylindrical particle is considered in detail. The specific absorption cross sections for spherical and cylindrical particles are compared.