Deeply penetrating waves in lossy media

The incidence of an inhomogeneous plane wave on the interface between two lossy media is analyzed. The analytical expressions of the incidence angle of the phase vector, for which the transmitted wave has the phase or the attenuation vector parallel to the interface, are obtained. The transmitted wave with the attenuation vector parallel to the interface is physically interpreted, finding a wave in a lossy medium without attenuation away from the interface. The same effect appears at the interface between a lossless medium and a lossy one.     

Dyadic Green's functions and electromagnetic local density of states

A formal proof to relate the concept of electromagnetic local density of states (LDOS) to the electric and magnetic dyadic Green's functions (DGF) is provided. The expression for LDOS is obtained by relating the electromagnetic energy density at any location in a medium at uniform temperature T to the electric and magnetic DGFs. The appropriate boundary conditions governing the DGFs are obtained and it is seen that the two types of DGFs are electromagnetic duals of each other. With this the concept of LDOS is also extended to material media. The LDOS is split into two terms—one that originates from the energy density in an infinite, homogeneous medium and the other that takes into account scattering from inhomogeneities. The second part can always be defined unambiguously, even in lossy materials. For lossy materials, the first part is finite only if spatial dispersion is taken into account.     

Including dispersion and attenuation directly in the time domain for wave propagation in isotropic media

When sound propagates in a lossy fluid, causality dictates that in most cases the presence of attenuation is accompanied by dispersion. The ability to incorporate attenuation and its causal companion, dispersion, directly in the time domain has received little attention. Szabo [J. Acoust. Soc. Am. 96, 491-500 (1994)] showed that attenuation and dispersion in a linear medium can be accounted for in the linear wave equation by the inclusion of a causal convolutional propagation operator that includes both phenomena. Szabo's work was restricted to media with a power-law attenuation. Waters et al. [J. Acoust. Soc. Am. 108, 2114-2119 (2000)] showed that Szabo's approach could be used in a broader class of media. Direct application of Szabo's formalism is still lacking. To evaluate the concept of the causal convolutional propagation operator as introduced by Szabo, the operator is applied to pulse propagation in an isotropic lossy medium directly in the time domain. The generalized linear wave equation containing the operator is solved via a finite-difference-time-domain scheme. Two functional forms for the attenuation often encountered in acoustics are examined. It is shown that the presence of the operator correctly incorporates both, attenuation and dispersion.     

衰减材料对高增益相对论速调管自激振荡的抑制

在高增益相对论速调管放大器中,自激振荡严重影响器件的正常工作并导致脉冲缩短。通过粒子模拟研究了高次模式自激振荡的激励机制,并提出在器件内加入微波衰减材料来抑制该类自激振荡。详细研究了衰减材料的电导率和几何参数与高次模式衰减常数的关系,并开展了微波衰减体的冷测实验。冷测表明微波衰减体对高次模式的衰减达11.25 dB,其性能满足器件实验要求,可以用于高增益相对论速调管的热腔实验。     

Low-loss surface modes and lossy hybrid modes in metamaterial waveguides

We show that waveguides with a dielectric core and a lossy metamaterial cladding (metamaterial-dielectric guides) can support hybrid ordinary-surface modes previously only known for metal-dielectric waveguides. These hybrid modes are potentially useful for frequency filtering applications as sharp changes in field attenuation occur at tailorable frequencies. Our results also show that the surface modes of a metamaterial-dielectric waveguide with comparable electric and magnetic losses can be less lossy than the surface modes of an analogous metal-dielectric waveguide with electric losses alone. Through a characterization of both slab and cylindrical metamaterial-dielectric guides, we find that the surface modes of the cylindrical guides show promise as candidates for all-optical control of low-intensity pulses.     

Numerical analysis of propagation characteristics of electromagnetic wave in lossy left-handed material media

The propagation characteristics of electromagnetic wave in lossy left-handed materials (LHM) are studied using finite-difference time-domain (FDTD) method base on auxiliary differential equation (ADE) technology. The LHM medium is realized with lossy Drude models for both the negative electric permittivity and the negative magnetic permeability. The discretized ADE-FDTD equations are derived in detail. The incident wave used in the simulation is a multiple cycle m-n-m pulses source. The term of Poynting's vector E_xH_y was calculated. These numerical results demonstrate conclusively that the phase velocity direction of electromagnetic wave propagation and the direction of the Poynting vectors are anti-parallel in LHM. The amplitude of electric field is reduced with the enhancive distance of LHM slab. It is also demonstrated that the energy of electromagnetic wave in the LHM slab is obviously attenuated, and the attenuation of energy becomes stronger with the angular plasma frequency ω_p increasing. These results indicate that LHM stealth is effective in theory, and reasonable selection of the large negative index of refraction can greatly enhance its effectiveness.     

Damping rates of waves in a switched magnetoplasma medium:longitudinal propagation

The interaction of a circularly polarized electromagnetic wave with a switched-on magnetoplasma medium is considered. A static magnetic field in the direction of propagation is assumed to be present, resulting in longitudinal propagation. The incident wave splits into three waves whose frequencies are different from that of the incident wave. It is shown that these waves ultimately damp out if the plasma is even slightly lossy. The damping of the waves is interpreted in terms of their attenuation with distance and decay with time as they propagate in the lossy plasma. The attenuation-length and decay-time constants of the waves are obtained, and their dependence on the incident-wave frequency and the gyrofrequency is examined. Optimum parameters for an experiment to detect these waves are suggested     

时间反演聚焦经颅磁声电刺激仿真与实验研究*

无创脑神经调控技术是生物医学领域的研究热点,经颅磁声电刺激是利用静磁场和声场的耦合而产生的感应电场作用于神经组织,对大脑的目标位置进行刺激和调控的一项技术。颅骨的存在使超声在传播过程中发生相位畸变和幅值衰减,聚焦区域偏离,难以实现精准聚焦。该文基于时间反演法,模拟颅内点声源发射脉冲以及超声传播过程,计算各个阵元接收到的时间差,按照后到先发的原则发射脉冲进行聚焦刺激。与传统相控阵聚焦相比,焦点偏移现象基本得到解决,焦域横向、纵向分辨率均有所提高,提高了声束聚焦精度和感应电场峰值。通过搭建实验平台,将两种聚焦方法所测得的声场归一化处理,验证了时间反演法能补偿焦点偏移,并通过实验证实了超声换能器声场和产生感应电场分布存在较高的一致性。基于真实颅脑结构的虚拟点源时间反演聚焦可以实现无创、精准、灵活的经颅磁声电刺激,有助于推动精准神经调控技术的发展。     

Stochastic model in microwave propagation

Further experimental results of delay time in microwave propagation are reported in the presence of a lossy medium (wood). The measurements show that the presence of a lossy medium makes the propagation slightly superluminal. The results are interpreted on the basis of a stochastic (or path integral) model, showing how this model is able to describe each kind of physical system in which multi-path trajectories are present.     

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.     

均匀带电非理想介质球对外围另一个非理想介质球壳的放电

通过一个非理相介质球对外围另一个介质球放电过程的研究,剖析了各边界面上的面电荷分布,系统耗损的焦耳热与产一磁场等问题。     

海洋可控源三维电磁响应显式灵敏度矩阵的快速算法

借助电场耦合势三维有限体积法与直接求解技术,研究建立了一套海洋可控源三维电磁响应显式灵敏度矩阵(或称为Fréchet导数)高效算法.首先,利用Yee氏交错网格和有限体积法对电场混合势Helmholtz方程进行离散处理,建立与移动源电磁场正演模拟相对应的大型代数方程组,再应用直接法得到的逆矩阵和三维线性插值技术事先确定插值算子和投影算子,并利用投影算子与各个发射源离散向量的乘积计算多发射源电磁响应,极大地提高了多发射源电磁场正演模拟效率.在此基础上,根据块状模型和像素模型中异常体电导率分片常数分布特征,将电导率摄动产生的一次散射电流场表示成Yee氏剖分网格上散射电流元的叠加,由投影算子与散射电流元的离散向量的乘积快速计算出电场强度与磁场强度的显式灵敏度矩阵.最后,通过数值计算检验算法的有效性,并通过块状模型与像素模型分别研究海洋可控源电磁响应特征.     

Source identification in time domain electromagnetics

In this paper, we introduce two numerical methods to get an electric source that gives a specified electromagnetic field some time after the source starts emitting. The first one is called the Time Reversal Method (TRM) and originates from acoustics and the second one is called Linear Combination of Configuration Fields (LCCF) and consists in constructing and solving a linear problem in order to find a possible required source. In this paper, we show on 1D and 2D examples that the last method gives lower relative errors when compared with the time reversal method.     

Spatial,temporal, and thermal contributions to focusing contrast by time reversal in a cavity

The accuracy of wave focusing by time reversal depends on a quantity termed ‘contrast ratio’ that measures the amplitude of a localized peak of velocity relatively to background noise. A comprehensive expression for the contrast ratio in a lossy cavity is derived by modal decomposition of the wave field. This expression accounts for the effects of the mechanical and the dimensional properties of the cavity, the bandwidth of the excitation signal, the number of sources, and the duration of time reversal window. The expression can predict the characteristics of a given process such as the long-time saturation and the single-channel time reversal limit. The expression also models the effect of temperature variations on focusing accuracy and shows that thermal drift exhibits two regimes. In the first regime, small temperature variations have little effect on contrast. The second regime is characterized by a rapid deterioration of contrast. Experimental measurements show close agreement with the theory.     

Numerical time reversal of waves

A numerical time reversal of waves is proposed instead of the conventional time reversal of wave procedure used in underwater acoustics. In the numerical method, the test sound source and the receiving arrays are used, as in the conventional method, but the transmission of the received signals after their time reversal into the same medium, as well as the measurement of the field obtained in this way at the point of the test source, is replaced by computations. To use the proposed technique for obtaining the same results as those provided by the conventional time reversal of waves, the teset source should be placed at different depths. A simplified numerical algorithm with the test source operating at a single depth is proposed and justified. This version of the time reversal of waves is successfully applied to the experiment in the Barents Sea. In contrast to the conventional method, the proposed technique allows one to study the stability of the sea medium with currents.     

Exact analytical treatment of noise initiation of SBS in the presence of loss

An exact analytical expression for the Stokes power generated by stimulated Brillouin scattering in a lossy medium is derived for the first time. The approach is based on a distributed, fluctuating source model in the undepleted pump regime, previously only rigorously applied to lossless media. A comparison with known approximations is performed.     

Radiation from a short vertical dipole in a metal-backed rod array

Based on an effective medium approach, we investigate the problem of radiation of a short vertical electric dipole embedded in a metal-backed dielectric rod array. We obtain the radiated electromagnetic field in the form of a Sommerfeld integral, and calculate the near and far electric fields for both lossless and lossy rod arrays. The characteristic equation for the guided modes is derived and solved. The guided modes are found to be slow waves and their propagation constants and modal distributions are extracted. All theoretical results are compared with full-wave simulations.     

Density matrix of a quantum field in a particle-creating background

We examine the time evolution of a quantized field in external backgrounds that violate the stability of vacuum (particle-creating backgrounds). Our purpose is to study the exact form of the final quantum state (the density operator at the final instant of time) that has emerged from a given arbitrary initial state (from a given arbitrary density operator at the initial time instant) in the course of evolution. We find a generating functional that allows one to obtain density operators for an arbitrary initial state. Averaging over states of the subsystem of antiparticles (particles), we obtain explicit forms of reduced density operators for the subsystem of particles (antiparticles). Analyzing one-particle correlation functions, we establish a one-to-one correspondence between these functions and the reduced density operators. It is shown that in the general case a presence of bosons (e.g., gluons) in the initial state increases the creation rate of the same type of bosons. We discuss the question (and its relation to the initial stage of quark–gluon plasma formation) whether a thermal form of one-particle distribution can appear even if the final state of the complete system is not in thermal equilibrium. In this respect, we discuss some cases when pair-creation by an electric-like field can mimic the one-particle thermal distribution. We apply our technics to some QFT problems in slowly varying electric-like backgrounds: electric, SU(3) chromoelectric, and metric. In particular, we analyze the time and temperature behavior of the mean numbers of created particles, provided that the effects of switching the external field on and off are negligible. It is demonstrated that at high temperatures and in slowly varying electric fields the rate of particle-creation is essentially time-dependent.     

Thermal Near Field and the Possibilities of Its Use for In-Depth Temperature Diagnostics of Media

We discover experimentally the effect of an electromagnetic thermal near field of a lossy medium. The effective depth d_eff of the layer in which the received signal is formed is found to be less than the skin-layer depth d_sk and is a function of the size D of the receiving antenna and its height h above the medium surface. The dependence d_eff(D,h) is obtained from measurements of the emission from a temperature-stratified water medium at a wavelength of 31 cm using a specially developed, electrically small antenna. The results of experimental studies of radiophysical parameters of the antenna are presented. We propose to use measurements of the dependence of the received emission on D and h as a new source of information on the in-depth temperature distribution. Methods for solving the corresponding inverse problems are developed, and the first results of retrieval of the subsurface temperature profile of a water medium are obtained.     

Localizing impulse sources in an open space by time reversal with very few transducers

Localizing impulse point sources is a problem of major practical importance for numerous applications in security or equipment monitoring. It is difficult to solve when posed in a strongly congested propagation medium. This paper concerns the case where, in an open space, obstructing bodies are of a sufficient size and number to impede reception of certain direct paths from the source to the receivers. They produce reflected or diffracted paths. A low number of point receivers is used, 2-5, depending on the case. This fits practical constraints one meets in the field. The localization principle being time reversal, the aim is therefore to model the time reversed signal propagation from the receivers. From a direct signal obtained from measurements or computer simulations, the reversed propagation computation is made in the frequency domain or in the time domain. Despite the low number of receivers, which we would expect not to give good refocusing of the reversed wave, we are able in each case to localize the source with a conveniently chosen criterion, based on the time shortness of the signal. The advantage of this technique is its simplicity and speed: in a time formulation, a unique computation allows the localization. This result opens the way to economical measurement techniques for localizing impulse sources in congested or disturbed media, as long as a propagation model allowing integration of refraction, diffraction and reflection effects is available.