光子晶体表面光波导的特性研究

采用平面波展开法和时域有限差分法,研究了光子晶体表面光波导的色散及光传输特性。研究结果表明,表面模的出现及其色散特性与表面介质柱的半径相关,色散曲线的斜率保持单调变化,并在最低或最高频率附近展现低群速度频率区。由于光子带隙和全内反射的共同影响,光场将沿晶体表面高效率地传输。研究结果为探索在光子晶体外部控制光传输具有重要的理论意义。     

"Ideal" optical delay lines based on tailored-coupling and reflecting, coupled-resonator optical waveguides

We present a design of "ideal" optical delay lines (i.e., constant amplitude and constant group delay over the desired bandwidth). They are based on reflection from coupled-resonator optical waveguides (CROWs). The inter-resonator coupling coefficients are tailored and decrease monotonically with the distance from the input to realize all-pass Bessel filters. The tailored coupling coefficients result in a frequency-dependent propagating distance which compensates for the group velocity dispersion of CROWs. We present a simple formalism for deriving the time-domain coupling coefficients and convert these coefficients to field coupling coefficients of ring resonators. The reflecting CROWs possess a delay-bandwidth product of 0.5 per resonator, larger than that of any kind of transmitting CROW. In the presence of uniform gain, the gain enhanced by slow light propagation and the constant group delay result in efficient and dispersion-free amplifiers.     

JEMS-FDTD软件在电磁脉冲区域传播数值模拟中的应用

电磁脉冲区域传播数值模拟是电磁环境效应分析的重要环节, 面临空间尺度巨大、多辐射源、复杂地貌等技术挑战。本文介绍了三维时域全波电磁模拟并行软件JEMS-FDTD的研制进展。在大规模并行计算FDTD方法的基础上结合自适应网格技术, 研制了一种适应于大区域电磁脉冲传播的高效时域全波计算方法。并基于该技术实现了千km2级城市电磁脉冲区域传播的数值模拟, 获取了全空间的时域电磁场信息, 验证了软件在电磁脉冲区域传播仿真应用中的可行性。     

磁化等离子体光子晶体的FDTD分析

磁化等离子体光子晶体是磁化等离子体和介质(真空)构成的人工周期性结构.本文用磁化等离子体的分段线形电流密度卷积(PLCDRC)时域有限差分(FDTD)算法分析了磁化等离子体光子晶体特性.分析了磁化等离子体参数对电磁带隙的影响.从时域的角度分析了高斯脉冲在磁化等离子体光子晶体中的传播过程，给出了时域反射和透射波形.从频域的角度给出了磁化等离子体光子晶体的电磁反射系数和透射系数，并对结果进行了分析. 关键词： 磁化等离子体 光子晶体 时域有限差分法     

Exact solution of Maxwell's equations for optical interactions with a macroscopic random medium

We report what we believe to be the first rigorous numerical solution of the two-dimensional Maxwell equations for optical propagation within, and scattering by, a random medium of macroscopic dimensions. Our solution is based on the pseudospectral time-domain technique, which provides essentially exact results for electromagnetic field spatial modes sampled at the Nyquist rate or better. The results point toward the emerging feasibility of direct, exact Maxwell equations modeling of light propagation through many millimeters of biological tissues. More generally, our results have a wider implication: Namely, the study of electromagnetic wave propagation within random media is moving toward exact rather than approximate solutions of Maxwell's equations.     

时间反演脉冲电磁波在双负材料中传播特性研究

利用全时域电磁仿真技术, 对比研究了时间反演脉冲电磁波和脉冲电磁波透过Smith结构双负材料后的时域波形、脉宽压缩以及材料内部空间电场峰值强度分布等时域传播特性. 结果表明：时间反演脉冲电磁波在透过双负材料后, 在初始源激励处表现出良好的时间和空间聚焦特性. 更重要的是, 在双负材料内部, 观测到了电场峰值强度减弱、截面电场峰值强度趋向均衡分布等新型的物理现象. 这些物理现象对发展能够承受大功率新型的双负材料电子器件及其应用系统很有参考研究价值.     

Slowdown of Group Velocity of Light Using Coherent Population Oscillation

We investigate the reduction of the group velocity propagation resulting from the steep change of the refractive index by the coherent population oscillation in erbium-doped optical fibre. The largest delay is measured to be about 8.75ms corresponding to a group index of 1.312×10^6. The time delay or advancement depends on the pump intensity. Influences of the ion density on the fractional delay and the group velocity reduction of light propagation are studied. Based on our discussion the optimization parameters should be selected in order to obtain more appropriate time delay and the slowdown of group velocity.     

Time-domain analysis of second-harmonic generation of primary Lamb wave propagation in an elastic plate

Within the second-order perturbation approximation,this paper investigates the physical process of generation of the time-domain second harmonic by a primary Lamb wave waveform in an elastic plate.The present work is performed based on the preconditions that the phase velocity matching is satisfied and that the transfer of energy from the primary Lamb wave to the double frequency Lamb wave is not zero.It investigates the influences of the difference between the group velocities of the primary Lamb wave and the double frequency Lamb wave,the propagation distance and the duration of the primary Lamb wave waveform on the envelope shape of the time-domain second harmonic.It finds that the maximum magnitude of the envelope of the second-harmonic waveform can grow within some propagation distance even if the condition of group velocity matching is not satisfied.Our analyses also indicate that the maximum magnitude of the envelope of the second-harmonic waveform is kept constant beyond a specific propagation distance.Furthermore,it concludes that the integration amplitude of the time-domain second-harmonic waveform always grows with propagation distance within the second-order perturbation.The present research yields new physical insight not previously available into the effect of generation of the time-domain second harmonic by propagation of a primary Lamb wave waveform.     

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.     

A new perspective to the attributes of lightning electromagnetic field above the ground

The characteristic parameters of lightning electromagnetic field, which include the peak value, time derivatives, power flow density and energy density, are very important for lightening electromagnetic field shielding, remote sensing of lightning and modeling of discharge channel. Different from issued results, which operate the lightning electromagnetic field only at some discrete points near the ground, the electromagnetic field and its derivatives at every point in the whole space are calculated by a modified finite difference time-domain (FDTD) approach and are illustrated with groups of plots in this paper. The spatial attribute of these parameters is investigated, and their propagation processes over finite conductivity ground are time-sampled and analyzed. Moreover, the propagation effects of lightning electromagnetic field and its time derivatives over different finite conducting ground are also analyzed in the entire area. In the region of ground, the conclusions gotten from our data are fairly consistent with the consequences from the published articles. Meanwhile, some novel phenomena are detected from our results in the atmosphere, which has never been involved in reported articles. Furthermore, the method employed has been tested and compared with the issued experiment data and the results from theoretical approach firstly.     

Tunable pulse delay in an anisotropic metamaterial slab

We show that large and tunable pulse delays can be obtained at propagation through an anisotropic metamaterial slab. The pulse delay depends not only on frequency but also on the incident/propagation angle and polarization of the electromagnetic field, the last two parameters being much easier to tune than the frequency of the incident field. Although there is a trade-off between large pulse delay values and large tuning ranges, the pulse delay can be modified several times by changing the incidence angle. The results apply to a wide frequency range, from the visible to the THz spectrum.     

基于群速度失配的超声兰姆波二次谐波的时域测量方法

考虑到发射和接收换能器对超声兰姆波时域二次谐波信号所带来的不可避免的影响,提出一种基于基频与二倍频兰姆波群速度失配的超声兰姆波二次谐波的时域测量方法。当基频与二倍频超声兰姆波的相速度匹配而群速度失配时,在超声兰姆波传播过程中所发生的二次谐波信号,在时域上可与源于斜劈换能器的二次谐波信号相分离。采用仅源自于基频兰姆波的时域二次谐波的积分振幅,定量描述兰姆波二次谐波的发生效率。以铝板中传播的兰姆波为例,给出了时域二次谐波的具体测量过程。本文提出的测量方法放宽了超声兰姆波二次谐波的测量条件,且扣除了换能器对二次谐波信号所带来的影响,所测得的二次谐波信号完全来自于基频兰姆波时域信号的二次谐波发生效应。      

Research of electromagnetic field in different length left-handed material slab

Finite-difference time-domain （FDTD） method is used to simulate the propagation of electromagnetic wave in left-handed material slab （LHMs） with cold plasma model The effects of optical propagation in the left-handed material compared to convex lens are discussed. The wider the LHMs is, the stronger electric field of focusing image in left-handed material slab is confirmed by the simulation with various slab length. However, the outer image point location would slightly moves to the LHMs side when the length of LHMs is reduced.     

Second Harmonic Generation of Lamb Wave in Numerical Perspective

The influences of phase and group velocity matching on cumulative second harmonic generation of Lamb waves are investigated in numerical perspective.Finite element simulations of nonlinear Lamb wave propagation are performed for Lamb wave mode pairs with exact and approximate phase velocity matching,with and without group velocity matching,respectively.The evolution of time-domain second harmonic Lamb waves is analyzed with the propagation distance.The amplitudes of primary and second harmonic waves are calculated to characterize the acoustic nonlinearity.The results verify that phase velocity matching is necessary for generation of the cumulative second harmonic Lamb wave in numerical perspective,while group velocity matching is demonstrated to not be a necessary condition.     

基于弹性模量检测骨疲劳的超声导波方法研究

研究早期诊断骨疲劳的方法是当前骨质评价方面的研究热点之一. 本文对不同弹性模量下长骨中超声导波的传播特性进行了理论分析和仿真研究.首先, 通过数值计算得到导波在管状长骨中的理论解析解.然后对管状长骨进行了时域有限差分(FDTD) 仿真, 并验证了它与理论解析解的一致性, 同时得到长骨中不同模式导波群速度、 中心频率和衰减与弹性模量的关系.研究结果表明各个导波模式的群速度和中心频率均随弹性模量的增加而增加, 而衰减随弹性模量的增加而减小.说明超声导波的传播特性参量可以反映长骨弹性模量的变化, 从而为长骨的早期疲劳诊断提供理论依据.     

Beam and phase effects in angle-beam-through-transmission method of ultrasonic velocity measurement

The accuracy of a plane wave approximation for phase velocity measurements in isotropic and anisotropic material using the angle-beam-through-transmission method has been investigated numerically and experimentally. In this method the velocity is measured in different propagation directions as a function of incidence angle. The effect of two factors on the measurement accuracy have been discussed: intrinsic phase shift of the transmitted signal through a fluid-solid interface and beam diffraction due to the finite beam size of receiver and transmitter. It is shown that the interface-induced phase shift can introduce an error in time delay measurements of the shear wave after the first critical angle and that this time delay error can be accurately corrected for. Numerical results obtained by a time-domain beam model show that except at the critical angles, the finite width of the transmitter and receiver only affects the amplitudes of the transmitted signals and has almost no effect on the measured zero-cross time delay; therefore the plane wave approximation for obtaining phase velocity from the measured time delay data by this method and the plane wave interface-induced phase correction are fully applicable.     

高空核爆电磁脉冲穿透电离层数值计算

 电离层对电磁脉冲传播的影响可以用一个频域的电流密度来描述,在时域则等效为一附加的电流密度和电导率。推导了附加电流密度和电导率在时域的表达式,它们是电离层参数和作用电场的函数,并以差分方程的形式给出。高空核爆电磁脉冲(HEMP)的计算是在时域进行的,将该电流密度和电导率计算方法应用于HEMP产生和传播的自洽计算中。作为算例,计算了100 km高空核爆电磁脉冲产生和向上传播的3种情况,并对计算结果进行了比对分析。计算结果表明:电离层时域计算方法与核爆电磁脉冲计算方法的结合是合理有效的,爆点上方考虑电离层影响的HEMP要比不考虑电离层影响的结果小得多。     

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.     

Resonant electromagnetic soliton in a dense plasma

A study is made of the propagation of an electromagnetic wave along a constant magnetic field B₀ in a high-density plasma under cyclotron resonance conditions when the interaction between the electrons and the wave is strongly nonlinear. It is found that the relativistic reduction in the electron gyrofrequency in the field of the electromagnetic pulse stabilizes the wave. A solution is obtained in the form of an envelope soliton and it is shown that its group velocity is considerably reduced.State University, Rostov-on-Don. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1994.     

NOVEL ANALYTIC THEORY FOR ACTIVELY MODE-LOCKED FIBER LASER

The time-domain ABCD matrix formalism is based on the propagation of a Gaussian pulse which is characterized by the pulsewidth and chirp. In this paper, the time-domain ABCD matrix of th SPM (self phase modulation) is perfected so as to acquire more information. With the time-domain ABCD matrices of amplitude modulation, filtration from the optical bandpass, group velocity dispersion and SPM, the detailed characteristics of actively mode-locked fiber laser are described and the steady-state solutions are derived for two simple cases. The application of the formalism is also helpful to understand the mechanism of mode-locking in an actively mode-locked fiber laser.