一维无序扰动周期结构中局域长度的对称等价变换

以一维周期结构光谱对称性为基础，提出了无序扰动周期结构有关局域长度的一个新的变换关系：对称等价变换，并用数值计算加以了验证.该等价变换描述了不同无序度的结构对不同频率光子局域能力之间的等价关系，为无序结构中光子局域性质的进一步研究提供了一个新的工具. 关键词： 局域长度 无序扰动周期结构 对称等价变换     

单负色散材料光子晶体的带隙特性研究

针对单负色散材料构成的光子晶体的带隙特性进行了详细地研究.首先采用传输矩阵法分析计算了该结构的带隙产生与材料介电常数、磁导率色散特性间的关系,然后计算了带隙宽度和中心频率受入射场极化方式、入射方向、介质材料厚度比例缩放和厚度无序的影响程度,最后分析并对比了单负色散材料和普通材料构成的渐变周期光子晶体的传输特性.结果证明单负色散材料可以在光子晶体中引入一种受结构参数影响很小的带隙,同时和普通材料相比,在渐变周期结构中可以有效地拓宽Bragg带隙宽度,改善传输特性.     

含单负材料三明治结构的电磁隧穿特性

在理论分析的基础上,结合实验研究了单负材料/正折射率材料(或单负材料)/单负材料三明治结构的电磁隧穿性质.这种结构的电磁隧穿来源于入射平面波与结构的表面或体极化激元的耦合,从理论上讨论并计算了正入射情况下这种结构中的极化激元的色散关系.发现:由单负材料作为边界而正折射率材料为中间层的三明治结构具有体极化激元,这种极化激元的共振频率随着中间层厚度的增加向低频移动;由不同性质的单负材料构成的三明治结构具有两条色散曲线,这两条色散曲线随着中间层厚度的增加而简并.随后基于L-C传输线技 关键词： 单负材料 极化激元 隧穿 L-C传输线')" href="#">L-C传输线     

多孔硅材料光学Anderson局域的研究

本文基于Anderson局域效应和有限时域差分(FDTD)计算方法,对具有一定无序性的多孔硅材料内光的横向局域进行了理论分析,考察了不同无序度、孔洞排列的不同方式对硅材料光学性质的影响。计算结果表明,多孔硅内光的行为和多孔硅的结构参数有很大关系。当结构参数合适时,光束可以呈现比较明显的横向局域效果。适当改变材料中孔道的排列方式,可以有效的调节光在材料中的传播方式与局域效果。设计了具有"边墙"效果的孔洞排列方式,不仅能够固定光斑的位置,而且还可实现光束的局域增强效果。     

DNA分子链的电子局域性质及电导的研究

将一维随机二元固体模型应用于DNA分子链，利用传输矩阵方法来研究系统电子态的局域性质并进而讨论系统的导电性质.对一个链长为50000个碱基对的DNA序列，数值分析了局域长度和电导随碱基对的摩尔百分数、本征能量和无序度的变化关系.结果表明，系统的局域长度和电导强烈地依赖于能量，在能带中心部分局域长度大于边沿部分.无序度也在一定程度上影响着局域长度，双方成反向变化的关系.对有限长度的DNA分子链，局域长度体现出明显的对碱基对摩尔百分数的依赖关系，对正常成分比例的随机DNA序列，在所有能量范围内系统的态都是局域的，系统的电导很小，系统呈现绝缘体行为.仅当一种碱基对在序列中所占比例很小时，系统中可以发现与特定分立能量值相对应的“扩展态”存在，处在这些态下的系统有较大的电导，但这些扩展态是不稳定的，在热力学极限之下会消失. 关键词： DNA分子链 电子局域 局域长度 电导     

一维长程关联无序系统中的电子态

利用傅里叶滤波法在一维Anderson无序系统中产生了具有幂律谱密度公式s(q)∝q^-p形式的长程关联随机能量序列，并利用传输矩阵方法计算了系统中引入了长程关联后的局域长度，同时应用负本征值理论对系统中的电子态密度进行了分析，并分别把计算结果与系统中不具有长程关联时的局域长度与电子态密度进行了比较.结果表明，长程幂律关联的引入对电子态的性质产生了很大的影响，当关联指数p≥2.0时，在系统能带中心范围内发生了部分局域态向退局域态的转变，而同时电子态密度也发生了很大的变化，出现了六个范霍夫奇点，系统的能带范围也相应地得到展宽. 关键词： 无序系统 长程关联 局域长度 电子态密度     

两类单负材料组成的一维半无限光子晶体反射谱

运用传输矩阵方法和Bloch定理计算了两类单负材料组成的一维半无限光子晶体反射谱,与有限多层结构通带中振荡的反射谱相比,半无限结构的反射谱曲线是光滑的,是有限周期结构反射谱振幅取平均的结果,可用来估计通带中的反射率.研究半无限结构的反射谱,有利于分析带隙的位置和宽度,结果表明,由两类单负材料组成的光子晶体中,不仅存在零有效位相带隙,还存在角度带隙,尤其发现高频处的Bragg带隙也是一个全方位带隙.     

复合局域共振梁结构带隙研究

该文将周期局域共振梁结构中的基体结构由单一材料发展成多相材料,建立了周期复合局域共振梁结构。由共振单元在单胞中的排列位置,考虑了三种构型,并利用传递矩阵方法计算了在弹性基底作用下这些结构产生的局域共振带隙性质。结果表明,与局域共振梁结构相比,复合局域共振梁结构产生的局域共振带隙对应的频率范围更宽,禁带范围内弹性波的衰减峰值和衰减均值更高。除此之外,该文还分析了局域共振结构、单胞尺寸、基体材料和局域共振结构中的阻尼对弹性波在空间中传播和衰减特性的影响,为满足特定的隔声隔振要求的结构设计提供了依据。     

活性无序介质中泵浦功率对局域模发射的影响

基于具有局域-非周期-类结构的活性无序介质中的复杂局域模与合适泵浦光的相互作用模型,研究活性无序介质中泵浦功率对局域模发射的影响,定性解释了泵浦功率对局域模发射的影响.结果表明,局域-非周期-类结构的无序介质的透射谱只与介质的结构有关,而与介质的损耗和增益无关,不同的激发模具有不同的泵浦功率阈值.     

损耗型单负材料双层异质结构的反常场局域

通过计算损耗型单负材料双层异质结构中的透射、反射和吸收随损耗系数的变化,对单负材料双层异质结构基于损耗的光学性质调控进行了研究。首先,对损耗型单负材料单层结构的透射行为进行了计算,给出了单层结构随损耗系数增大的非单调透射的变化图像。接着,计算了损耗型单负材料双层异质结构中的透射、反射和吸收随损耗系数的变化曲线,并分析了不同频率下透射与损耗系数之间的关系。研究结果表明,在远离隧穿频率处,随损耗系数的逐渐增大,异质结的透射呈现出先减小后增大的变化。计算的电磁场强度分布图像表明,双层异质结中透射与损耗系数之间为非单调关系,起源于损耗型单负材料界面处的反常场局域效应,即随着损耗系数的增大,场局域先减弱后增强。     

The mechanism of the formation of passbands of electromagnetic waves in one-dimensional ordered or disordered structures containing single-negative metamaterials

We propose a mechanism of the formation of band structures of electromagnetic waves in one-dimensional periodic or nonperiodic systems containing single-negative metamaterials. The formation of passbands in these periodic systems is attributed to two factors, i.e., the existence of localized modes and ‘phase’ matching condition. It is completely different from the Bragg scattering. For the wave transmission in disordered systems, the two factors also play an important role. If they are satisfied, there are abnormal transmission behaviors of electromagnetic waves. It implies that the formation of passbands may not depend on periodic structures.     

Experimental and numerical study of omega type bianisotropic metamaterials combined with a negative permittivity medium

We report on the transmission properties of the omega (Ω) type metamaterials. Transmission through the periodic Ω-only and Ω-wire metamaterials are studied experimentally and numerically. A resonance band gap is observed for the periodic Ω medium around the resonance frequency of the single Ω unit cell. A transmission band is observed below the resonance band gap, when the periodic Ω structure is embedded in a negative permittivity medium composed of thin metallic wire arrays. We also studied the effect of periodicity on the transmission spectra of Ω type metamaterials.     

Photonic gap vanishing in one-dimensional photonic crystals with single-negative metamaterials

The properties of photonic band gap in one-dimensional photonic crystals composed of single-negative metamaterials are studied theoretically. Our study shows that the photonic gap will vanish at a certain incident angle when both the phase-match and impedance-match conditions are satisfied simultaneously, suggesting that the bandwidth and location of the photonic gap are strongly dependent on the incident angle and polarization. However, the photonic gap will not vanish and may become insensitive to the incident angle when the two match conditions cannot be met. Our study also shows that losses in metamaterials have little effect on the properties of the photonic gap.     

Wave propagation in double-period quasi-regular one-dimensional photonic crystals composed of single-negative metamaterials

In this paper, we theoretically study the propagation of electromagnetic waves in one-dimensional double-period quasi-regular structures consisting of negative-permittivity and negative-permeability metamaterials. A Drude-type dispersive response for both permittivity and permeability of single-negative metamaterials is considered. From the numerical results performed by transfer matrix method and effective medium theory, it is found that double-period photonic crystals can exhibit an omnidirectional gap at given microwave frequency. Such an omnidirectional gap is insensitive to incident angle and polarization, and is invariant upon the change of scale length. It is also found that this gap exists in all double-period sequences, and it is independent of structure sequence. Moreover, electromagnetic field distributions in this structures show that the fields are localized at each interface of two media. These results can lead to further applications of quasi-regular structures.     

Omnidirectional reflectance gaps and resonant tunneling effect in a one-dimensional photonic crystal consisting of two metamaterials

We investigate the properties of electromagnetic wave propagating in a one-dimensional photonic crystal (PC) consisting of two metamaterials with different dispersive model. The reflection gaps of metamaterials multilayer system are independent of the incident angle. Not only TE wave but also TM wave, the omnidirectional reflection gaps exhibit the same behavior with different incident angle for metamaterials as double negative material. We also observed that the frequency regimes of zero-transmission bands are different for TE and TM wave with the same incident angle, when one of metamaterials is the permittivity negative (ε < 0) and the other is the double negative. Correspondingly, we show that the result can be act as an efficient polarization splitter. At last, we discuss the resonant tunneling effect. If the total reflection condition is satisfied, the resonant tunneling effect is enhanced as the incident angle increases, even though the propagation wave is evanescent wave in the single layer medium.     

Flexural wave motion in beam-type disordered periodic systems: Coincidence phenomenon and sound radiation

This paper deals with flexural wave motion in uniform beam-type periodic systems whose repeating units are identical finite beams with multiple beam-length disorders. A general expression derived for the propagation constants has been employed to study its variation with frequency for a beam system having 4-span disordered repeating units. This is helpful in understanding flexural wave motion in disordered periodic beams. Free flexural waves have been studied as wave groups consisting of a large number of harmonic components of different wavelengths, phase velocities and directions. Phase velocities have been computed and plotted for different frequencies in the propagation zones in which the free waves progress without attenuation. This has been found to be useful in understanding and predicting the coincidence phenomenon in disordered periodic beams under convected pressure field loading. The excitation of wave groups in disordered periodic beam-type systems by a slow (subsonic) convecting pressure field can include fast (supersonic) moving flexural wave components which can radiate sound. It has been pointed out that sound radiation from a disordered periodic beam (or plate) can be quite different as compared to that from a periodic beam under similar convected pressure field loading.     

基于波传播法的周期复合板振动带隙衰减特性研究

运用波传播法对有限和无限周期对边简支复合板的振动带隙衰减特性进行了研究.在建立相邻板结构边界连续方程的基础上, 分别运用传递矩阵和Bloch定理建立了有限和无限周期复合板的耦合运动方程, 并详细对比分析了有限和无限周期复合板带隙衰减特性的关联关系.研究表明: 周期板结构的振动带隙频率范围与激励方式和激励位置是相关的, 若周期复合板在宽度方向按某阶模态进行线激励, 则该激励下的振动带隙与无限周期复合板在该阶模态下的振动带隙是一致的; 若周期板在点激励作用, 则该点激励下的振动带隙是参与振动的各阶模态振动带隙的交集. 此外, 还进一步研究了结构阻尼对振动衰减带隙的影响. 关键词： 周期复合板 带隙衰减特性 波传播法 结构阻尼     

Some Features of the Electrodynamics of Anisotropic Metamaterials

We study the features of propagation of electromagnetic wave fields in anisotropic metamaterials composed of lattices of the so-called split-ring resonators embedded in a dielectric substrate. It is shown that strongly slowed waves can exist in such metamaterials if some components of the tensor of effective magnetic permeability are negative. The presence of such waves explains the experimentally observed effect of wave propagation through cutoff waveguides filled with an artificial medium composed of split-ring resonators.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 2, pp. 152–158, February 2005.     

Wave localization in randomly disordered multi-coupled multi-span beams on elastic foundations

In this paper, the wave propagation and localization in randomly disordered periodic multi-span beams on elastic foundations are studied. For two kinds of beams, i.e. the multi-span beams on elastic foundations with periodic flexible and simple supports, the transfer matrices between two consecutive sub-spans are obtained by means of the continuity conditions. The algorithm for determining all the Lyapunov exponents in continuous dynamic systems presented by Wolf et al. is employed to calculate those in discrete dynamic systems. The localization factor characterizing the average exponential rates of growth or decay of wave amplitudes along the disordered beams is defined as the smallest positive Lyapunov exponent of the discrete dynamical system. The localization length that represents the distance of elastic waves propagating along the disordered periodic structures is defined as the reciprocal of the smallest positive Lyapunov exponent, i.e. the localization factor. For the two kinds of disordered periodic beams on elastic foundations, the numerical results of the localization factors are presented and analysed by comparing them with the results of the beams without elastic foundations to illustrate the effects of the elastic foundations on the wave propagation and localization. The effects of the disorder of span-length and the dimensionless torsional and linear spring stiffness on the localization factors are discussed. Moreover, the localization lengths are also calculated and discussed for certain structural parameters in disordered periodic structures. It can be observed from the results that ordered periodic multi-span beams have the characteristics of the frequency passbands and stopbands and the localization of elastic waves can occur in disordered periodic systems: the localization degree of elastic waves is strengthened with the increase of the coefficient of variation of the span-length. The influences of the elastic foundations on the wave propagation and localization are more complicated. Generally speaking, in lower-frequency regions the elastic foundations have pronounced effects on the spectral structures, but in higher-frequency regions the effects are negligible. The localization degree increases as the torsional spring stiffness increases. The linear spring has few effects on the spectral structures in higher-frequency regions, but in lower-frequency regions it has prominent effects. The larger the disorder degree, the shorter the non-dimensional localization length.     

Study on localization of plane elastic waves in disordered periodic 2-2 piezoelectric composite structures

Considering the effect of mechanic-electric coupling, the propagation and localization of plane elastic waves in disordered periodic layered piezoelectric composite structures are studied. The transfer matrix between two consecutive unit cells is obtained by means of the continuity conditions and the expression of the localization factors in disordered periodic structures is presented by regarding the variables of mechanical and electrical fields as the elements of state vectors. As examples, numerical results of localization factors are presented and discussed. It can be seen from the results that ordered periodic structures possess the properties of frequency passbands and stopbands and the phenomenon of wave localization in disordered periodic structures is observed, and the larger the coefficient of variation is, the larger the localization factor or the stronger the degree of wave localization is. The characters of wave propagation and localization are very different for different sorts of piezocomposites or different structural sizes, and even for same sorts of piezocomposites and same structural sizes the characters of wave propagation and localization are also very different for different non-dimensional wavenumbers. We may design different piezocomposites or adjust the structural sizes to control the characters of wave propagation and localization.