共查询到19条相似文献,搜索用时 156 毫秒
1.
提出了用时谐声散射场的远场信息来可视化三维可穿透目标的一种指示器样本方法,它是通过析取一个指示器函数在包含可穿透目标的某个样本区域中的支集来实现这种可视化的,其中,这个指示器函数在可穿透目标的内部和外部有显著不同的取值.这个算法的一个特别吸引人的性质是不需要关于障碍物的任何几何和物理的先验信息,并且只需要散射场在某个有限孔径中若干个入射和测量方向上的远场信息,即可得到可穿透目标的一个很理想的可视化.数值算例保证了这个可视化算法是有效和实用的
关键词:
声散射
反问题
物形反演 相似文献
2.
研究了从声散射场的远场分布的信息来再现声阻抗障碍物形状的反问题,建立了求解这类反问题的一种非线性最优化模型,并提出了数值实现该非线性最优化模型的一种两步调整迭代算法.两步过程的应用使在确定未知障碍物形状的非线性最优化步中未知函数的个数达到了最少,而在调整迭代过程中,通过利用前一迭代步所得重构信息,使重构精度得到了相当大的改进.所建立的反演算法的一个特别吸引人的性质是,只需要远场分布的一个Fourier系数即可对未知声阻抗障碍物作几何物形的设别.对大量具有各种几何形状的二维障碍物的数值算例保证了本算法是实用和有效的. 相似文献
3.
研究了从声散射场的远场分布的信息来再现声阻抗障碍物形状的反问题,建立了求解这类反问题的一种非线性最优化模型,并提出了数值实现该非线性最优化模型的一种两步调整迭代算法.两步过程的应用使在确定未知障碍物形状的非线性最优化步中未知函数的个数达到了最少,而在调整迭代过程中,通过利用前一迭代步所得重构信息,使重构精度得到了相当大的改进.所建立的反演算法的一个特别吸引人的性质是,只需要远场分布的一个Fourier系数即可对未知声阻抗障碍物作几何物形的设别.对大量具有各种几何形状的二维障碍物的数值算例保证了本算法是实用和有效的. 相似文献
4.
5.
研究了从声散射场的远场分布的信息来再现半空间三维目标几何特征信息的反问题,提出了求解这种非线性不适定问题的一种简单快速算法。该方法的一个特别的性质是只需要通过一个不适定的线性方程组求解,即可简单快速地得到半空间中三维目标几何特征信息的一个清晰的像,而且目标的个数和边界条件的类型等几何与物理的先验信息在本算法中都是不需要的。数值算例保证了本算法的有效性。 相似文献
6.
基于原子蒸汽池中铯里德伯原子的电磁感应透明光谱在微波场作用下的Aulter-Towns效应,测量了无芯射频识别标签线形散射单元的近场散射微波电场二维空间分布,空间分辨率可达到亚微波波长.实现了射频电场极化方向与线形散射体标签夹角的有效分辨.电磁仿真软件的仿真结果与实验测量符合得很好.本研究提供了一种测量微波电场近场测量的新方法,对无芯射频识别标签的散射单元设计和标定以及电子电路的电磁辐射测量具有重要的意义. 相似文献
7.
目标散射场全息重建方法研究 总被引:7,自引:1,他引:6
阐述了基于边界元方法(BEM)的散射场近场声全息,这种方法可以由测量的散射近场数据重建散射体表面散射场并预报整个散射场。文中首先推导了这种方法及相关的散射场分离方法的基本公式,然后通过数值仿真分析了不同全息图面的不同对全息重建结果的影响、奇异值滤波方法的作用、散射场分离方法的可行性等问题。 相似文献
8.
9.
提出一种新时域交替隐式有限差分(ADI-FDTD)算法格式. 传统ADI-FDTD算法的 场量步进方程涉及周围若干网格的较多场量,导致两个区域的步进方程处理较困难:一个是 邻近完全匹配层(PML)和散射场交界区,另一个是邻近连接边界区. 特别是后者,考虑入射 波影响需对场量所在区域判断,根据不同情况对原有方程进行修正,一维和二维散射问题 相对简单,可三维问题修正式有数十种之多而几乎无法完成. 本方法基于分裂场形式的ADI- FDTD技术,使得散射场区和PML吸收层区的表达形式完全一致,从而忽略两者差别.另
关键词:
时域交替隐式有限差分算法
电磁散射 相似文献
10.
首先讨论了自由空间中一维周期结构近-远场外推的Floquet模方法,即对FDTD计算所得散射近场中输出边界上散射场进行级数展开,求出各阶Floquet模的复数幅值,再求得远区场。接着介绍了求解一维周期结构远区散射场的周期Green函数方法,即根据FDTD计算所得散射近场中输出边界上散射场,求得等效面电磁流后,再借助周期Green函数进行外推得到远区场。两种方法均仅用一个周期单元内的散射近场进行外推。计算结果验证了上述两种方法外推的有效性。 相似文献
11.
J. Muller 《Journal of Quantitative Spectroscopy & Radiative Transfer》2011,112(7):1162-1169
The detection of surface waves through scanning near-field optical microscopy (SNOM) is a promising technique for thermal measurements at very small scales. Recent studies have shown that electromagnetic waves, in the vicinity of a scattering structure such as an atomic force microscopy (AFM) tip, can be scattered from near to far-field and thus detected. In the present work, a model based on the finite difference time domain (FDTD) method and the near-field to far-field (NFTFF) transformation for electromagnetic waves propagation is presented. This model has been validated by studying the electromagnetic field of a dipole in vacuum and close to a dielectric substrate. Then simulations for a tetrahedral tip close to an interface are presented and discussed. 相似文献
12.
Efficient cardiac diffusion tensor MRI by three-dimensional reconstruction of solenoidal tensor fields 总被引:2,自引:0,他引:2
Tensor tomography is being investigated as a technique for reconstruction of in vivo diffusion tensor fields that can potentially be used to reduce the number of magnetic resonance imaging (MRI) measurements. Specifically, assessments are being made of the reconstruction of cardiac diffusion tensor fields from 3D Radon planar projections using a filtered backprojection algorithm in order to specify the helical fiber structure of myocardial tissue. Helmholtz type decomposition is proposed for 3D second order tensor fields. Using this decomposition a Fourier projection theorem is formulated in terms of the solenoidal and irrotational components of the tensor field. From the Fourier projection theorem, two sets of Radon directional measurements, one that reconstructs the solenoidal component and one that reconstructs the irrotational component of the tensor field, are prescribed. Based on these observations filtered backprojection reconstruction formulae are given for the reconstruction of a 3D second order tensor field and its solenoidal and irrotational components from Radon projection measurements. Computer simulations demonstrate the validity of the mathematical formulations and demonstrate that a realistic model of the helical fiber structure of the myocardial tissue specifies a diffusion tensor field for which the first principal vector (the vector associated with the maximum eigenvalue) of the solenoidal component accurately approximates the first principal vector of the diffusion tensor. A priori knowledge of this allows the orientation of the myocardial fiber structure to be specified utilizing one half of the number of MRI measurements of a normal diffusion tensor field study. 相似文献
13.
A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing. 相似文献
14.
《Comptes Rendus Physique》2012,13(8):837-852
We show, through several selected case studies, the potential benefits that can be obtained by controlling the polarization states of the exciting and scattered radiations in a Raman scattering experiment. When coupled with polarization control, Raman spectroscopy is thus capable of providing extra information on the structural properties of the materials under investigation. The experimental examples presented in this work are taken from the area of both conventional, i.e., far-field, as well as from near-field Raman spectroscopy. They cover topics such as the stress tensor measurement in strained semiconductor structures, the vibration mode assignment in pentacene thin films and the Raman scattering tensor determination from near-field measurements on azobenzene monolayers. The basic theory necessary for modelling the far- and near-field polarized Raman responses is also given and the model efficiency is illustrated on the experimental data. 相似文献
15.
S. Grésillon R. Lecaque L. Williame J.C. Rivoal 《Applied physics. B, Lasers and optics》2006,84(1-2):167-173
Simultaneous topographical and near-field optical imaging have been performed on single gold particles of diameters close
to 12 nm. The optical source is a linearly polarized laser diode operating at λ=780 nm away from the plasmon resonance of
the particles. The experimental optical image is recorded with an apertureless scanning near-field optical microscope (ASNOM)
operating in transmission mode. It is compared to the components of the polarized scattered electric field around a single
cluster calculated using Mie formalism. We show that the tip used in the experiments is sensitive to the axial component of
the scattered field, thus allowing us to obtain the amplitude and the phase of the local field. Our derivation brings out
new information, usually shaded when applied to an ensemble of particles. In particular, the dipole model widely used to describe
the scattered field by a spherical particle is not suitable to describe the three components of the scattered field in the
near zone. Our results are of interest for fundamental studies of the optical properties of single metal clusters and the
control of local phenomena such as enhancement, extinction, etc.
PACS 07.79.Fc; 78.67.Bf; 78.35.+c 相似文献
16.
S. V. Sukhov 《Optics and Spectroscopy》2003,95(3):464-469
Based on the integral equation method, the boundary problem of classical optics of finding the strength of the electromagnetic field scattered by a probe positioned in the near zone of a plane surface is solved in the context of application in near-field optical microscopy of scattering. Only the dipole component of the scattered field was taken into account in the calculations. The obtained theoretical expressions were compared to the known experimental data on scanning the surface of SiC. Good agreement between the experimental and the theoretical results was obtained outside the region of the resonant interaction of the probe of a near-field microscope and the surface under study. 相似文献
17.
入射声波激励下非均匀流体介质内部散射声场的重建方法对超声层析成像具有重要意义。以往采用矩量法求解,但该方法全域离散形成的复数满秩矩阵规模随着分辨率与计算精度的提高而急剧增大,对算力具有很高的要求,一定程度上限制了其在实际中的应用。为克服上述缺陷,本文以逐层离散、逐层计算为核心思想,以声散射基本公式与近场声全息理论为基础,推导出逐层计算非均匀流体介质内部散射声场的理论公式并给出对应的几何离散模型。为验证该方法的可行性,以矩量法为参照,对同样的介质模型进行介质内部声场重构仿真。结果表明,逐层算法不仅可以有效地重建非均匀流体介质内部散射声场,且大幅度减小了求解规模。 相似文献
18.
Theoretical and experimental investigations demonstrate the prospects of near-field microwave location of malignant tumors
in biological tissue. The theoretical analysis is performed in terms of the theory of near-field location developed for layered
structures. The experimental verification of the theory is accomplished by sounding an aqueous medium with a controllable
permittivity. The tumor contrast is calculated based on the ideas of the dielectric properties of healthy and affected human
tissues. The reason for and the degree of background contrasts as applied to near-field measurements are studied experimentally.
An optimal measuring scheme for minimizing the masking effect of background contrasts is proposed. The 2D images of a tumor-simulating
contrast object immersed in the aqueous medium at different depths are obtained. The prospects of near-field diagnostics for
subsurface temperature measurement are discussed. 相似文献
19.
《声学学报:英文版》2017,(2)
In a non-free field,the existence of disturbing sources would destroy the free-field condition required by near-field acoustic holography,and confuse the identification of target source due to the scattering effect caused by disturbing sound falling on the target source.To totally remove the influence of disturbing source,a method for recovering and reconstructing the target sound field in a non-free field was proposed based on the equivalent source method and the pressure-velocity measurement.In the proposed method,the sound field separation technique was firstly used to separate the mixed sound field into the outgoing field coming from the target source and the incoming field coming from the disturbing source.Then the scattered component contained in the outgoing field was calculated by using the disturbing sound and the surface impedance of the target source.Finally,the sound field radiated by the target source in a free field was obtained by subtracting the scattered component from the outgoing field and was further used to reconstruct the sound field of the target source.The proposed method was numerically and experimentally testified and the results of both the simulation and the experiment demonstrated the validity of the proposed method. 相似文献