超短THz脉冲在随机散射介质中的传播

用Monte-Carlo方法模拟了超短THz脉冲在随机散射介质中的传播.根据Mie理论计算出随机散射介质的散射系数和各向异性因子，研究了随机散射颗粒及介质厚度的大小对透射脉冲的影响.结果表明在Mie散射范围内，在相同的浓度下，颗粒尺寸越小，散射介质越厚，THz散射越严重，对透射脉冲的影响越大.散射同时降低了THz脉冲在随机散射介质中的成像分辨率. 关键词： 超短THz脉冲 随机散射介质 Mie理论 Monte-Carlo方法     

非线性光学 非线性光散射、振荡与谐波产生

O437 2004053491 超宽频带THz脉冲在随机散射介质中传播的理论研究=Theoretical study of propagation of ultra-wideband THz pulse in random scattering media[刊,中]/陈敏(中科院上海原子核研究所,上海(201800)),肖体乔…∥光子学     

半透明介质内激光脉冲链传输过程的叠加机理

根据光波在介质内的散射、折射特点,分析了脉冲链在参与性介质内经界面反射和介质散射后的叠加机理.导出了脉冲链入射非散射性半透明平板时透射比和反射率的解析解.利用间断有限元法和时间平移叠加法计算了脉冲链在一维散射性介质平板内传输的透射比和反射率.结果表明由于介质内部入射光与介质的相瓦影响,脉冲链受界面的反射和内部介质的散射,分裂成一系列时滞不同的子光波.经过不同的光学行程后,子光波叠加成离开平板的反射光波和透射光波.叠加效果随介质散射系数、脉冲宽度和脉冲时间间距发生变化.通过调节脉冲链的脉冲宽度和脉冲时间间距可得到不同波形的反射波和透射波.     

随机粗糙面激光脉冲散射四阶统计特征

依据基尔霍夫近似,推导出一维高斯随机粗糙面四阶统计特性解析式,并获得脉冲波入射下粗糙面双点双频四阶矩互相关函数.数值计算不同入射角,不同高度起伏和相干长度,随机粗糙面双点双频四阶矩互相关函数随相干频谱带宽频差,散射角的变化情况.计算结果表明,高度起伏和相干长度对粗糙面散射四阶统计特的影响很大,粗糙面越光滑,在镜反射方向有最大的四阶矩散射峰值和小的相关带宽.随着粗糙度增加,随机粗糙面脉冲散射的四阶矩的相干分量减弱,展宽现象明显;而相干带宽频差越大,四阶矩衰减越快. 关键词： 四阶矩统计特征 双点双频互相关函数 随机粗糙面 脉冲波散射     

散射介质中层间杂质检测效率的影响因素及分析

散射介质中层间杂质检测是一个非常重要的研究课题.改进现有的Monte Carlo方法,模拟大量光子在散射介质中的传输,得到入射光强、杂质埋藏深度、介质折射率、介质散射系数和各向异性因子对光学透反射成像法检测层间杂质效率的影响规律.结果表明,入射光强、杂质埋藏深度和介质折射率对透反射成像检测结果均有影响,且影响规律相似.增加入射光强、减小杂质埋藏深度或减小介质折射率均可提高反射光成像的检测效率;增大入射光强、减小介质折射率、减小介质散射系数或增大各向异性因子均可提高透射光成像的检测效率.这些规律对散射介质中层间杂质检测具有一定指导意义.     

太赫兹波在雾中的多重散射特性

太赫兹(THz)波作为微波和毫米波的延伸,它所提供的通信带宽和容量远大于毫米波。在随机介质中传播时,不但会发生时域和空域的形变,介质中的粒子还会对入射波发生散射,这些都会使得脉冲信号发生衰减。根据Mie理论与随机离散分布粒子的波传播与散射理论,计算了THz波信号入射下雾滴粒子的消光系数。结合雾滴粒子谱分布,得到了雾媒质的平均体系散射特性,采用蒙特卡罗法得到了平流雾对THz信号的多重散射特性,计算了THz波段信号对平流雾的透过率与反射率,分析了THz波段信号的前向、后向散射特性随散射角的分布。结果表明,低能见度大气环境中,雾对THz波产生的吸收和衰减不容忽视。相关研究结果对THz在大气传输、通信等方面的应用具有重要意义。     

超短脉冲激光在二维非均匀介质内的瞬态辐射传输

随着超短脉冲激光的快速发展,吸收散射性介质内的瞬态辐射传输引起了广泛的关注.本文采用最小二乘有限元法模拟了超短脉冲激光局部入射条件下,具有高散射核的二维非均匀介质内的瞬态辐射传输.研究了不同边界位置上反射和透射信号随时间的变化情况.结果表明,对于具有高散射核的非均匀介质,能够揭示散射核位置的双峰现象可能在早期的反射信号中发生,早期的反射信号比后期的、经过介质衰减后信号更重要.因此,在利用短脉冲激光进行光谱分析和成像等技术中,人们应该重视早期反射信号的测量.     

宽频带光束非线性热像效应的实验研究

实验研究宽频带脉冲光束通过非线性介质时的非线性热像的形成过程,揭示了光束入射功率和脉冲宽度、介质厚度等因素对热像光强和位置的影响. 结果表明,与单纵模激光束一样,在宽频带脉冲光束情形下,非线性介质上游元件的模糊斑调制也可导致在下游共轭位置形成热像. 热像光强随入射光束功率的增加而增加;若脉冲能量一定,则热像光强随脉冲宽度的增加而减弱;若脉冲峰值功率一定,则热像光强随脉冲宽度的增加而增强. 关键词： 高功率激光 热像 宽带光束 小尺度自聚焦     

雾天气中THz与红外的传输特性

基于HITRAN数据库,对比了大气主要成份分子对THz波段的吸收.从Mie理论出发,结合雾滴粒子谱分布,分析了不同波长信号雾滴粒子群的平均体系散射特性.采用蒙特卡罗方法,得到了红外波段和THz波段雾天气的透过率、反射率,以及前向、后向散射强度.结果表明:THz波段内,频率越低,透过率越高,反射率越低,后向散射强度越小;相同传输距离下,THz波段比红外波段具有更好的透射能力;相同散射角下,红外波段信号前向散射强度大于THz波段.该研究结果对THz技术在大气空间中的应用具有重要意义.     

基于编码超表面的太赫兹宽频段雷达散射截面缩减的研究

本文设计了一种柔性, 非定向低散射的1bit编码超表面, 实现了太赫兹宽频带雷达散射截面的缩减. 这种设计基于对“0”和“1”两种基本单元进行编码, 其反射相位差在很宽的频段范围内接近180°, 为一种非周期的排列方式, 该电磁超表面使入射的电磁波发生漫反射, 从而实现雷达散射截面的缩减. 全波仿真结果表明, 在垂直入射条件下, 编码超表面的镜像反射率低于-10 dB的带宽频段范围为1.0-1.4 THz, 该带宽内超表面相对同尺寸金属板可将雷达散射截面所减量达到10 dB以上, 最大缩减量达到19 dB. 把柔性编码表面弯曲在直径为4 mm的金属圆柱面上, 雷达散射截面的所减量高于10 dB以上的带宽频段范围为0.9-1.2 THz, 仍然可实现宽频带缩减特性. 总之, 编码超表面为调控太赫兹波提供一种新的途径, 将在雷达隐身、成像、宽带通信等方面具有重要的意义.     

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).     

相对折射率趋近于1的颗粒Mie散射与反常衍射的分析比较

利用Ｍｉｅ散射与反常衍射理论的数值计算，对球形颗粒Ｍｉｅ散射和反常衍射作了大量详细的计算和分析比较，提出了相对折射率趋近于１的颗粒的反常衍射近似替代Ｍｉｅ散射的判据。     

Electromagnetic-wave scattering from chiral spheres in chiral media

Summary The scattering of electromagnetic waves in chiral (optically active) media from chiral spheres is studied. Mie-scattering techniques are used to find the exact solution for plane-electromagnetic-wave scattering from a chiral sphere of arbitrary size in an infinitely extended chiral medium of arbitrarily different permeability, permittivity, and chirality, and the scattering and extinction efficiencies for chiral spheres in chiral media are derived. Special cases of achiral exterior medium-chiral sphere and achiral exterior medium-achiral scatterer are considered and in the latter case well-known results of Mie scattering are recovered. Simplified results for small spheres are also found for the limit of Rayleigh scattering.     

Terahertz Radiation Transport in Photonic Glasses

Multiple scattering of electromagnetic (EM) waves arises in disordered media with a refractive index varying on the scale of the wavelength. The diffusion approximation is a powerful tool to treat multiple scattering as a photon random walk, neglecting resonant phenomena. However, as the light intensity varies on a scale much smaller than the transport mean free path, resonances may occur in media formed by finite-size scatterers and break the diffusion approximation. The energy and phase velocity are very useful tools to reveal the onset of the resonant transport regime. In this paper the study of the propagation of terahertz (THz) waves through 3D random media by employing terahertz time-domain spectroscopy (THz-TDS) is addressed. Specifically, measurements of the electric field transmitted by samples of different thicknesses made of 1 mm diameter silica spheres dispersed in a paraffin matrix at different filling fractions are reported. This investigation has provided an accurate measurement of the EM field phase and, hence, information on the radiation propagation velocity that has enabled the first observation of a photonic glass at the THz range.     

Effects of Scattering on THz Spectra of Granular Solids

Experimental studies of granular solids have shown that significant scattering effects restrict the accurate determination of material absorption in the terahertz (THz) region. The present work investigates the grain size dependent scattering contribution on the extinction spectra of Ammonium Nitrate, flour and salt between 0.2 to 1.2 THz using THz time-domain spectroscopy. The scattering contribution can be estimated by applying Mie theory for spherical grains. The approach essentially separates the independent contributions of true absorption and scattering losses and thus determines the total extinction for different grain sizes of various materials. The separation of the intrinsic material absorption from scattering losses shows that the frequency dependence in weakly absorbing materials is predominantly particle size dependent. Consequently, that range of THz frequencies cannot be used to differentiate granular solids having no intrinsic absorption.     

Backscattering of circularly polarized pulses

Using numerical simulations of vector radiative transport, we examine time-resolved backscattering of circularly polarized plane waves normally incident upon a slab containing a random distribution of latex spheres in water. For large spheres the effect of polarization memory occurs a short time after first-order scattering and before depolarization. It is the result of successive near-forward-scattering events that maintain the incident wave's helicity. For moderately large scatterers it exhibits a simple dependence on the anisotropy factor. For larger spheres or those with higher refractive indices, it also depends on complicated angular and polarization characteristics of backscattering given by Mie theory.     

A new proof of Mie effect in light scattering

In detecting particles on a substrate, two-white-spots phenomenon was observed. This gives an illusion that light scattering is caused by two spheres, in fact it is just caused by one sphere. This phenomenon makes particle detection by light scattering much more complicated. Analysis on this phenomenon shows that Mie effect is the reason of formation of two white spots. This experiment demonstrates the images of Mie effect, and provides a strong evidence of theory of Mie effect.     

Photon density wave for imaging through random media

The passage of a photon density wave through random media has been investigated extensively for medical imaging based on the diffusion approximation. In this paper, the photon density wave is studied based on the exact time-dependent vector radiative transfer theory. Both continuous and pulse photon density waves are analysed in a plane parallel medium using Mie scattering and the discrete ordinates method. The photon density wave shows superior properties over regular waves in several aspects. It has a narrower angular spectrum and maintains the original pulse shape. It also preserves the degree of polarization and increases the cross-polarization discrimination. These properties of a photon density wave suggest its potential for improving imaging. Thus, we apply the photon density wave to an imaging problem and show that it improves the quality of the images compared to other conventional imaging techniques.