随机取向团簇自然气溶胶光学特性

Mineral dust particles are the second highest emitted aerosol type by mass. Due to changes in particle size, composition, and shape that are caused by physical processes and reactive chemistry, optical properties vary during transport, contributing uncertainty in the calculation of radiative forcing. Hematite is the major absorbing species of mineral dust. In this study, we analyzed the extinction cross sections of nigrosin and hematite particles using cavity ring‐down aerosol extinction spectroscopy (CRD‐AES) and have measured particle shape and size distributions using transmission electron microscopy. Nigrosin was also used in this study as a spherical standard for absorbing particles. The size‐selected nigrosin particles have a narrow size distribution, with extinction cross sections that are described by Mie theory. In contrast, the size distribution of size‐selected hematite particles is more polydisperse. The extinction cross sections were modeled using Mie theory and the discrete dipole approximation (DDA). The DDA was used to model more complex shapes that account for the surface roughness and particle geometry. Of the four models used, Mie theory was the simplest to implement, but had significant error with a 26.1% difference from the CRD‐AES results. By increasing the complexity of the models using the DDA, we determined that spheroids had a 14.7% difference, roughened spheres a 12.8% difference, and roughened spheroids a 11.2% difference from the experimental results. Using additional parameters that account for particle shape is necessary to model the optical properties of hematite particles and leads to improved extinction cross sections for modeling aerosol optical properties.

Scattering characteristics of atmospheric haze particles, main ingredients particles of PM2.5 were studied, ammonium sulfate and carbonaceous aerosol particles were taken as typical example. By the T matrix algorithm, the non-spherical particles of ammonium sulfate and carbonaceous aerosol particles were analyzed, including the ellipsoid particles(major to minor axis ratio, a/b=1/2,1/3,2/1), cylindrical particles(diameter to length ratio, D/L=1/2,1/3,1/1,2/1) and chebyshev particles(polynomial n=2,deformation parameter =-0.1,0.1). Corresponding scattering, absorption and extinction efficiency factor changing with the shape and the size parameter were given. Study results show that, for ammonium sulfate particles, if size parameter was less than 1, efficiency factor tendencies of different shape particles were similar, if size parameter was greater than 1, scattering and extinction efficiency factor increased with the size parameter and then decreased with oscillation, and absorption of ammonium sulfate particles were weak. For carbonaceous aerosol particles, if size parameter was less than 2, efficiency factor tendencies of different shape particles were similar, if size parameter was greater than 2, efficiency factor increased with the size parameter and soon decreased slowly and smoothly, and carbonaceous aerosol particles had strong absorption of light. The analyzing results can be used for optical monitoring and detection of haze particle,improve the ability of science and technology supported environmental pollution prevention.

In regions of deep tropical convection, ice particles often undergo aggregation and form complex chains. To investigate the effect of the representation of aggregates on electromagnetic scattering calculations, we developed an algorithm to efficiently specify the geometries of aggregates and to compute some of their geometric parameters, such as the projected area. Based on in situ observations, ice aggregates are defined as clusters of hexagonal plates with a chainlike overall shape, which may have smooth or roughened surfaces. An aggregate representation is developed with 10 ensemble members, each consisting of between 4-12 hexagonal plates. The scattering properties of an individual aggregate ice particle are computed using either the discrete dipole approximation or an improved geometric optics method, depending upon the size parameters. Subsequently, the aggregate properties are averaged over all geometries. The scattering properties of the aggregate representation closely agree with those computed from 1000 different aggregate geometries. As a result, the aggregate representation provides an accurate and computationally efficient way to represent all aggregates occurring within ice clouds. Furthermore, the aggregate representation can be used to study the influence of these complex ice particles on the satellite-based remote sensing of ice clouds. The computed cloud reflectances for aggregates are different from those associated with randomly oriented individual hexagonal plates. When aggregates are neglected, simulated cloud reflectances are generally lower at visible and shortwave-infrared wavelengths, resulting in smaller effective particle sizes but larger optical thicknesses.

We review recent progresses in light scattering for non-spherical particles. Special attention is paid to cluster of spheres in order to improve our understanding of interplanetary dust particles. For scattering by non-spherical particles the discrete-dipole approximation (DDA) has widely been used in many scientific fields. However mainly due to the requirements of large computing memory and long computing time, the applicability of this theory is practically limited for rather small particle compared with wavelength. In order to overcome this practical problem, i.e., the large particle can not be calculated by the DDA, we recently developed the a 1-term method, which is a modification version of the DDA where the dipole polarizability is determined by the first term of scattering coefficient in Mie theory. Accuracy of this method is tested by comparing the solutions by the a 1-term method with those by modal analysis, which gives the analytical solutions for cluster of spherical monomers. According to the error analysis mentioned above, the applicabilities of the a 1-term method are established as follows. The maximum size parameter of the monomer in the cluster is 1 and the total size parameter of the cluster can exceed X ∼ 100 when the N ∼ 106 dipoles are used. We show the extinction efficiencies and asymmetry factors for cluster of spheres whose size parameter is larger than the wavelength, e.g., the volume equivalent size parameter X is larger than 30. Finally we summarize the applicabilities of DDA, T-Matrix, modal analysis and the a 1-term method. The a 1-term method can partly fulfill a gap where both DDA and the ray tracing technique based on geometrical optics can not be applied when the target is cluster. However for the target which has edges remains to be problematic. This would be the topic which should be focused on future research.

A simulation model for estimation of the optical properties of spherical and spheroid nano plasmonic geometry coupled with organic media has been studied using discrete dipole approximation (DDA) technique. Silver (Ag) and aluminium (Al) metal nano particles (NPs) are used as a plasmonic element surrounded by organic photovoltaic environment [poly (3-hexyl thiophene)] P3HT-(phenyl-C61-buryricacid methyl ester) PCBM blend. The analysis done in the current article depicts considerable influence of shape, aspect ratio and size on the plasmon resonance that facilitates improvement in the performance of nano plasmonic organic system in the useful spectral domain (250–750 nm). Discretisation of dipoles have been done in the DDA and compared with Mie theory to maintain the accuracy of DDA technique. It has been established that the oblate shape Ag and Al NPs with size of 80 nm (both) with aspect ratio 0.35 and 0.20 are the optimized parameter to enhance the optical properties in an organic surrounding. The optical properties are studied in terms of wavelength dependent extinction spectrum and resonant electric field distribution. Broadband electromagnetic field coupling under the plasmonic effect has led to considerable improvement in the photon absorption inside the organic environment that has been supported by the enhanced J–V curve for different nano geometries. Results of the present article can be used as a guiding principle for the plasmonic light trapping devices to improve their optical properties and electrical properties.

On the radiative properties of soot aggregates – Part 2: Effects of coating

How machine learning approaches are useful in computing the optical properties of non-spherical particles across a broad range of size parameters?

Polymer dispersed liquid crystal droplets: methods of calculation of optical characteristics

MATLAB package for discrete dipole approximation by graphics processing unit: Fast Fourier Transform and Biconjugate Gradient

Optical properties of gold nanoshells on monodisperse silica cores: Experiment and simulations

Near-infrared (NIR) absorbing plasmonic nanoparticles enhance photothermal therapy of tumors. In this procedure, systemically delivered gold nanoparticles preferentially accumulate at the tumor site and when irradiated using laser light, produce localized heat sufficient to damage tumor cells. Gold nanoshells and nanorods have been widely studied for this purpose, and while both exhibit strong NIR absorption, their overall absorption and scattering properties differ widely due to their geometry. In this paper, we compared the photothermal response of both nanoparticle types including the heat generation and photothermal efficiency. Tissue simulating phantoms, with varying concentrations of gold nanoparticles, were irradiated with a near-infrared diode laser while concurrently monitoring the surface temperature with an infrared camera. We calculated nanoshell and nanorod optical properties using the Mie solution and the discrete dipole approximation, respectively. In addition, we measured the heat generation of nanoshells and nanorods at the same optical density to determine the photothermal transduction efficiency for both nanoparticle types. We found that the gold nanoshells produced more heat than gold nanorods at equivalent number densities (# of nanoparticles/ml), whereas the nanorods generated more heat than nanoshells at equivalent extinction values at the irradiance wavelength. To reach an equivalent heat generation, we found that it was necessary to have ∼36× more nanorods than nanoshells. However, the gold nanorods were found to have two times the photothermal transduction efficiency than the gold nanoshells. For the nanoparticles tested, the nanoshells generated more heat, per nanoparticle, than nanorods, primarily due to their overall larger geometric cross-section. Conversely, we found that the gold nanorods had a higher photothermal efficiency than the gold nanoshells. In conclusion, the ideal choice of plasmonic nanoparticle requires not only per particle efficiency, but also the in vivo particle targeting ability under study.

This Account provides an overview of the methods that are currently being used to study the electromagnetics of silver and gold nanoparticles, with an emphasis on the determination of extinction and surface-enhanced Raman scattering (SERS) spectra. These methods have proven to be immensely useful in recent years for interpreting a wide range of nanoscience experiments and providing the capability to describe optical properties of particles up to several hundred nanometers in dimension, including arbitrary particle structures and complex dielectric environments (adsorbed layers of molecules, nearby metal films, and other particles). While some of the methods date back to Mie's celebrated work a century ago, others are still at the forefront of algorithm development in computational electromagnetics. This Account gives a qualitative description of the physical and mathematical basis behind the most commonly used methods, including both analytical and numerical methods, as well as representative results of applications that are relevant to current experiments. The analytical methods that we discuss are either derived from Mie theory for spheres or from the quasistatic (Gans) model as applied to spheres and spheroids. In this discussion, we describe the use of Mie theory to determine electromagnetic contributions to SERS enhancements that include for retarded dipole emission effects, and the use of the quasistatic approximation for spheroidal particles interacting with dye adsorbate layers. The numerical methods include the discrete dipole approximation (DDA), the finite difference time domain (FDTD) method, and the finite element method (FEM) based on Whitney forms. We discuss applications such as using DDA to describe the interaction of two gold disks to define electromagnetic hot spots, FDTD for light interacting with metal wires that go from particle-like plasmonic response to the film-like transmission as wire dimension is varied, and FEM studies of electromagnetic fields near cubic particles.

The radiative properties of bubbles or particles embedded in an absorbing medium are investigated. We aim first to determine the conditions under which absorption by the surrounding medium must be accounted for in the calculation of the efficiency factors by comparing results from Mie theory and the far-field and near-field approximations. Then, we relate these approximations for a single particle to the effective radiation characteristics required for solving the radiative transfer in an ensemble of scatterers embedded in an absorbing medium. The results indicate that the efficiency factors for a spherical particle can differ significantly from one model to another, in particular for large particle size parameter and matrix absorption index. Moreover, the effective scattering coefficient should be expressed based on the far-field approximation. Also, the choice of the absorption efficiency factor depends on the model used for estimating the effective absorption coefficient. However, for small void fractions, absorption by the matrix dominates, and models for the absorption coefficient and efficiency factor are unimportant. Finally, for bubbles in water, the conventional Mie theory can be used between 0.2 and 200 mum except at some wavelengths at which absorption by water must be accounted for.

Optical and radiative-transfer properties of mixed atmospheric aerosols

We design and fabricate optically driven micromachines which find the applications in microfluidics, manipulation of biological cells, microchemistry etc. The micromachines are driven by tightly focused laser beams. To aid the design of micromachines, we employ a number of modeling methods such as the Mie theory, the point matching method, finite difference methods, discrete dipole approximation (FDFD), sometimes in combination, to simulate the light scattering from the individual components. The micromachines prototypes are fabricated using a two-photon polymerization process.