Scattering and propagation of terahertz pulses in random soot aggregate systems

Scattering and propagation of terahertz pulses in random soot aggregate systems are studied by using the generalized multi-particle Mie-solution(GMM) and the pulse propagation theory. Soot aggregates are obtained by the diffusion-limited aggregation(DLA) model. For a soot aggregate in soot aggregate systems, scattering characteristics are analyzed by using the GMM. Scattering intensities versus scattering angles are given. The effects of different positions of the aggregate on the scattering intensities, scattering cross sections, extinction cross sections, and absorption cross sections are computed and compared. Based on pulse propagation in random media, the transmission of terahertz pulses in random soot aggregate systems is determined by the two-frequency mutual coherence function. Numerical simulations and analysis are given for terahertz pulses(0.7956 THz).     

散射大气对脉冲紫外光传输时间特性研究

介绍基于椭球坐标系的单次散射近似模型基础上,定量分析脉冲紫外光在散射大气中传输特性,具体讨论了传输距离及能见度等因素对脉冲紫外光传输特性的影响。最后开展了脉冲紫外光展宽初步实验研究,其实验结果与单次近似结果符合较好,从而指导了紫外光信息传输系统的设计工作。     

Debye analysis applied to multiple reflections and attenuation of electromagnetic plane waves in a stratified sea substratum

Investigation of the wave problem related to multiple reflections and attenuation in marine controlled source electromagnetic is important for geophysical sounding of the sea substratum. An approach towards this goal is to study the scattering of obliquely incident plane waves by a four-layered earth: sea, sea bottom sedimentary rock, hydrocarbon reservoir, and lower sedimentary rock. The approach employs Debye theory in the frequency domain and assumes that each layer is a planar lossy medium. Explicit expressions show that the secondary waves related to the sea substratum layers multireflections carry useful information about their features.     

Optical characterization of metallic aerosols

Airborne metallic particulates from industry and urban sources are highly conducting aerosols. The characterization of these pollutant particles is important for environment monitoring and protection. Because these metallic particulates are highly reflective, their effect on local weather or regional radiation budget may also need to be studied. In this work, light scattering characteristics of these metallic aerosols are studied using exact solutions on perfectly conducting spherical and cylindrical particles. It is found that for perfectly conducting spheres and cylinders, when scattering angle is larger than 90° the linear polarization degree of the scattered light is very close to zero. This light scattering characteristics of perfectly conducting particles is significantly different from that of other aerosols. When these perfectly conducting particles are immersed in an absorbing medium, this light scattering characteristics does not show significant change. Therefore, measuring the linear polarization of scattered lights at backward scattering angles can detect and distinguish metallic particulates from other aerosols. This result provides a great potential of metallic aerosol detection and monitoring for environmental protection.     

Influence of complex component and particle polydispersity on radiative properties of soot aggregate in atmosphere

The effects of morphological structure, water coating, dust mixing and primary particle size distribution on the radiative properties of soot fractal aggregates in atmosphere are investigated using T-matrix method. These fractal aggregates are numerically generated using a combination of the particle-cluster and cluster-cluster aggregation algorithms with fractal parameters representing soot aggregate in atmosphere. The radiative properties of compact aggregate notably deviate from that of the branched one, and the effect of morphology changes on the radiative properties in wet air cannot be neglected. However it is reasonable to use realization-averaged radiative properties to represent that of the aggregates with certain morphology. In wet air, the scattering, absorption and extinction cross-section and symmetry parameter of soot aggregates coated with water notably increase with water shell thickness. The mixing structures of dust have little effect on radiative properties of aggregates, but the volume fraction of dust has an obvious effect on extinction, scattering and absorption cross-section of aggregates when the size parameters are above the Rayleigh limit. Although the primary particle size distribution of soot aggregate has mild effect on the scattering albedo and asymmetry parameter, the deviations of the extinction, scattering, absorption cross-section among the three size distributions are significant in this study. The size distribution has a significant effect on forward scattering of phase function, while the effect can be neglected as the size parameter approaches to the Rayleigh limit.     

Wave propagation in axion electrodynamics

In this paper, the axion contribution to the electromagnetic wave propagation is studied. First we show how the axion electrodynamics model can be embedded into a premetric formalism of Maxwell electrodynamics. In this formalism, the axion field is not an arbitrary added Chern–Simon term of the Lagrangian, but emerges in a natural way as an irreducible part of a general constitutive tensor. We show that in order to represent the axion contribution to the wave propagation it is necessary to go beyond the geometric approximation, which is usually used in the premetric formalism. We derive a covariant dispersion relation for the axion modified electrodynamics. The wave propagation in this model is studied for an axion field with timelike, spacelike and null derivative covectors. The birefringence effect emerges in all these classes as a signal of Lorentz violation. This effect is however completely different from the ordinary birefringence appearing in classical optics and in premetric electrodynamics. The axion field does not simple double the ordinary light cone structure. In fact, it modifies the global topological structure of light cones surfaces. In CFJ-electrodynamics, such a modification results in violation of causality. In addition, the optical metrics in axion electrodynamics are not pseudo-Riemannian. In fact, for all types of the axion field, they are even non-Finslerian.     

The parity function in optical waves propagation

We propose a generalization of the concept of parity as a continuous function and show that it is conserved in optical waves free propagation. Some properties are commented and an experimental implementation to optically generate a representation is suggested.     

Attenuation of electromagnetic wave propagation in sandstorms incorporating charged sand particles

A theoretical approach for predicting the attenuation of microwave propagation in sandstorms is presented, with electric charges generated on the sand grains taken into account. It is found that the effect of electric charges distributed partially on the sand surface is notable. The calculated attenuation is in good agreement with that measured in certain conditions. The distribution of electric charges on the surface of sand grains, which is not easy to measure, can be approximately determined by measuring the attenuation value of electromagnetic waves. Some effects of sand radius, dielectric permittivity, frequency of electromagnetic wave, and visibility of sandstorms on the attenuation are also discussed quantitatively. Finally, a new electric parameter is introduced to describe the roles of scattering, absorption and effect of charges in attenuation.     

飞秒激光脉冲在正色散固体材料中的自压缩

实验研究了正色散固体介质中的激光脉冲自压缩现象,证明了无需任何外加色散补偿情况下，固体透明介质中的自聚焦传输过程可使高功率飞秒激光脉冲实现时域脉冲压缩，并详细研究了输出脉冲的时域和频域特性随入射脉冲强度的演化规律.实验结果表明脉冲自压缩量随入射脉冲强度的增加呈递增趋势，然而当入射光强增大到足以引起超连续谱及锥形辐射产生时，脉冲时域形状会发生分裂.此外还发现发散光束入射情况下同样可以观察到脉冲自压缩现象. 关键词： 超短激光脉冲 脉冲压缩 非线性传输     

Chirping and spreading of ultrashort laser pulses in underdense plasmas

A linear theory on the propagation of ultrashort pulses including only a few cycles in underdense plasmas is presented. It is shown that the dispersion in plasmas causes severe distortions in the pulse shape, including pulse chirping and spreading. The analytical calculations coincide very well with those obtained by particle-in-cell (PIC) simulations. The upper limit of the peak amplitude of the pulses, above which the linear theory breaks down due to the setting in of nonlinear effects of both the relativistic electron-mass increase and ponderomotive force, is also examined by PIC simulations. At certain high amplitudes, it is found that the ultrashort laser pulses can propagate like solitons.     

激光在不同类型气溶胶中传输特性研究

激光在大气中的传输衰减特性是激光工程应用中需要考虑的一个重要问题.本文针对常用的1.06 μm和10.6μm激光,基于Mie散射理论计算了气溶胶粒子的单次散射参量;对于激光在气溶胶中多次散射传输衰减,建立了蒙特卡罗模拟计算模型,利用Matlab语言编制了相应的计算程序,计算分析了两种波长的激光分别在沙尘性、水溶性、海洋性和煤烟性四种不同类型气溶胶中透过率与传播距离、能见度的关系,并将蒙特卡罗方法和单次散射的计算结果进行了比较.结果表明,当能见度较低、气溶胶粒子反照率较高时,单次散射计算存在很大的误差,用蒙特卡罗方法更能揭示多重散射现象;煤烟性气溶胶对1.06 μm激光的传输衰减影响最大,沙尘性气溶胶对10.6 μm激光的传输衰减影响最大.     

Scattering of ultrashort laser pulses on “ion‐sphere” in dense plasmas

Scattering of ultrashort electromagnetic pulses on the dense strongly coupled plasma is under consideration in the frame of hard ion sphere model. The electron distribution inside the ion sphere is obtained from self‐consistent solution of the Shrodinger equation for bound electrons and the Poisson equation for free electrons. The electron density distribution is determined by plasma electron temperatures. The ion density of Al plasmas under consideration is of the order of 10²⁰–10²² cm^?3, the electron temperature changes between 54 and 816 eV. Dynamical polarizability of the hard sphere determining the scattering cross sections is calculated using the modified local plasma frequency approximation. The spectrum of scattering cross section has maxima in the vicinity of the mean plasma frequency. Dependencies of scattering probability on carrier frequency and pulse duration are analysed in detail. The transition of the total scattering probabilities from nonlinear time dependence at small times to standard linear ones with the increase of pulse duration is demonstrated.     

Uncertainties in measured and modelled asymmetry parameters of mineral dust aerosols

The error caused by the uncertainty in the refractive index in the determination of the asymmetry parameter g is studied for a variety of mineral dust aerosol samples at two different optical wavelengths. Lorenz–Mie computations for spherical model particles are compared with results based on laboratory-measured phase functions in conjunction with a commonly used extrapolation method. The difference between the g-value based on measurements and the g-value based on Lorenz–Mie simulations is generally on the same order of magnitude as the error caused by the uncertainty in the refractive index m. For larger effective radii the error in g related to the use of spherical model particles is even larger than that related to the uncertainty in m. This indicates that the use of spherical model particles can be among the major error sources in the determination of the asymmetry parameter of dust aerosols.     

The Role of Aerosols in Light Scattering in the Atmosphere

This paper addresses the important role of how aerosols play in light scattering in the earth’s atmosphere and how their characteristics can be retrieved. It is shown here, by referring to typical aerosol types, that more than a half of the atmospheric light is scattered by aerosols, and the aerosol optical properties can be estimated in terms of the scattering behavior. It is also confirmed that aerosol retrieval can be accurately determined by combining polarization and radiance data.Presented at the 7th International Workshop on Multiple Scattering Lidar/Light Experiments (MUSCLE7), July 21–23 1994, Chiba, Japan.     

Determination of the soot aggregate size distribution from elastic light scattering through Bayesian inference

Recovering the size distribution of aerosolized soot aggregates from multiangle elastic light scattering data requires the inversion of an integral equation, which is a mathematically ill-posed problem. This paper demonstrates how maximum a posteriori (MAP) inference can be used to stabilize the inversion by introducing prior information about the size distribution of the soot aggregates. Results show that the size distribution can be recovered using only simple smoothness and non-negativity priors if the aggregate number density is known, but otherwise it is necessary to specify additional information about the presumed distribution shape.     

Raman散射探测南京地区气溶胶光学特性

利用位于中国气象局南京综合观测基地的Rayleigh-Raman-Mie激光雷达Raman通道实测数据反演分析南京北郊气溶胶光学特性。在反演计算消光系数的过程中发现Raman信号波动很大、起伏很多,对我们反演结果带来了很大的困难,导致消光系数值波动范围达到-0.1~0.4km~(-1)。经过反复试验计算,用小波去噪解决这个问题,并将其反演结果与同一时刻同一仪器不同通道(Mie通道)反演结果进行对比,2011年3月30日晴天条件下二者消光系数都在0.05km~(-1)左右,它们的趋势具有很好的一致性,表明小波去噪能提高Raman散射反演气溶胶光学特性的准确性。     

散射介质中多重散射太赫兹脉冲的时域统计特性

对散射介质中多重散射太赫兹脉冲的时域统计特性进行了分析.给出了时间延迟量取不同值时电场的概率分布、散射场强度及其概率分布、散射场实部和虚部的概率分布.由于脉冲的宽带特性及瞬间特性,多重散射场概率统计分布与不同时间延迟量有关,太赫兹脉冲多重散射场的统计特性分布是非平稳过程.     

Effects of gas and soot radiation on soot formation in counterflow ethylene diffusion flames

Numerical study of soot formation in counterflow ethylene diffusion flames at atmospheric pressure was conducted using detailed chemistry and complex thermal and transport properties. Soot kinetics was modelled using a semi-empirical two-equation model. Radiation heat transfer was calculated using the discrete-ordinates method coupled with an accurate band model. The calculated soot volume fractions are in reasonably good agreement with the experimental results in the literature. The individual effects of gas and soot radiation on soot formation were also investigated.     

Radiation damage from EELS and NEXAFS in diesel soot and diesel soot extracts

Carbon NEXAFS and EELS spectra of soot, and NEXAFS spectra of soot extracts, are presented. The EELS spectra of solid soot particles from a TEM-EELS show fewer structures than the corresponding NEXAFS spectra obtained at two different synchrotron beamlines. We attribute radiation damage in the TEM-EELS to the failure at resolving structures of surface functional carbon groups in or on soot. NEXAFS spectra of soot extracts studied with a scanning transmission X-ray microscope show alterations during X-ray exposure, which can be explained by a simple chemical model where oxygen apparently reacts with the sample. When the same extract is studied in an ultrahigh-vacuum beamline, no such alterations are observed.