一维压电Fibonacci类准周期声子晶体传输特性

基于传输矩阵法研究了一维压电Fibonacci类准周期声子晶体的传输特性, 比较了一维Fibonacci序列压电准周期声子晶体与非压电准周期声子晶体以及压电周期性声子晶体的透射性. 计算结果表明:弹性波通过一维准周期结构压电声子晶体时与周期性声子晶体一样会有带隙的出现, 且发现具有压电性的Fibonacci序列准周期声子晶体禁带宽度发生了展宽. 进一步讨论了入射角度对固定频率下声子透射系数的影响,结果表明一维压电Fibonacci序列准周期结构声子透射性依赖于入射角度的选取.     

Surface electromagnetic waves at the interface between two dielectric superlattices

The electrodynamic properties of the interface between two periodic dielectric superlattices are studied. It is shown that the interface may serve as a guide for an electromagnetic wave whose field decays exponentially on both sides of the plane of the interface. The field and power flux distributions, as well as the frequency dependence of the field penetration depth, are studied.     

Acoustic-electromagnetic slow waves in a periodical defective piezoelectric slab

Coupled slow waves, slow acoustic waves, and electromagnetic waves are simultaneously achieved based on a piezoelectric material, in which a line defect is created within a honeycomb lattice array of cylindrical holes etched in a LiNbO_3 slab. Finite element simulations in frequency domain and time domain demonstrate that a highly localized slow mode is obtained in the defect. Owing to the piezoelectricity of LiNbO_3, acoustic and electromagnetic waves are coupled with each other and transmit along the line defect. Therefore, in addition to a slow acoustic wave, an electromagnetic wave with a group velocity even lower than conventional acoustic waves is achieved.     

Linear and nonlinear vibrations and waves in optical or acoustic superlattices (photonic or phonon crystals)

A model of a dielectric or an elastic superlattice is proposed which describes quite simply the frequency spectrum of electromagnetic or acoustic waves. The frequency band spectrum of a one-dimensional lattice consists of minibands, which narrow down with increasing frequency (so that the forbidden bands in the spectrum broaden with increasing frequency). An elementary analysis of the spectrum of a one-dimensional lattice reveals the presence of many forbidden frequency bands in this case as well. It is shown that dynamic equations for superlattices can be generalized to the nonlinear case, leading to equations of the type of the nonlinear Schrödinger equation for the lattice. Soliton excitations are described and the particle-like dynamics of solitons is demonstrated. Local vibrations near point defects of different complexity in superlattices are studied and graphically illustrated. The existence of Bloch oscillations of a wave packet in a superlattice in a homogeneous external field is discussed.     

Acoustic phonon transmission spectra in piezoelectric AlN/GaN Fibonacci phononic crystals

We study the acoustic-phonon transmission spectra in periodic and quasiperiodic (Fibonacci type) superlattices made up from the III-V nitride materials AlN and GaN. The phonon dynamics is described by a coupled elastic and electromagnetic equations within the static field approximation model, stressing the importance of the piezoelectric polarization field in a strained condition. We use a transfer-matrix treatment to simplify the algebra, which would be otherwise quite complicated, allowing a neat analytical expressions for the phonon transmission coefficients. Numerical results, for the normal incidence case, show a strike self-similar pattern for both hexagonal (class 6 mm) and cubic symmetries crystalizations of the nitrides.     

Generation of one-photon states by a quantum dot

A technique for preparation of a one-photon wave packet through action of a classical electromagnetic field on a semiconducting quantum dot is proposed. We demonstrate that the Coulomb repulsion between charge carriers allows one to select the frequency, amplitude, and duration of an electromagnetic pulse so that one electron will transfer from an upper size-quantized level of the valence band to a lower size-quantized level of the conduction band with a probability close to unity. As a result of radiative recombination of the produced electron-hole pair, exactly one photon is emitted (a one-photon wave packet). This source of one-photon states can be used in quantum systems of data transmission and in quantum computers. Zh. éksp. Teor. Fiz. 112, 1257–1272 (October 1997)     

Acoustic Imaging Frequency Dynamics of Ferroelectric Domains by Atomic Force Microscopy

We report the acoustic imaging frequency dynamics of ferroelectric domains by low-frequency acoustic probe microscopy based on the commercial atomic force microscopy. It is found that ferroelectric domain could be firstly visualized at lower frequency down to 0.5 kHz by AFM-based acoustic microscopy. The frequency-dependent acoustic signal revealed a strong acoustic response in the frequency range from 7kHz to 10 kHz, and reached maximum at 8.1 kHz. The acoustic contrast mechanism can be ascribed to the different elastic response of ferroelectric microstructures to local elastic stress fields, which is induced by the acoustic wave transmitting in the sample when the piezoelectric transducer is vibrating and exciting acoustic wave under ac electric fields due to normal piezoelectric effects.     

Attenuation of electromagnetic wave propagation in sandstorms incorporating charged sand particles

A theoretical approach for predicting the attenuation of microwave propagation in sandstorms is presented, with electric charges generated on the sand grains taken into account. It is found that the effect of electric charges distributed partially on the sand surface is notable. The calculated attenuation is in good agreement with that measured in certain conditions. The distribution of electric charges on the surface of sand grains, which is not easy to measure, can be approximately determined by measuring the attenuation value of electromagnetic waves. Some effects of sand radius, dielectric permittivity, frequency of electromagnetic wave, and visibility of sandstorms on the attenuation are also discussed quantitatively. Finally, a new electric parameter is introduced to describe the roles of scattering, absorption and effect of charges in attenuation.     

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.     

Coupled thermal-optic effects and electrical modulation mechanism of birefringence crystal with Gaussian laser incidence

We study the Gaussian laser transmission in lithium niobate crystal(LiNbO3) by using the finite element method to solve the electromagnetic field's frequency domain equation and energy equation. The heat generated is identified by calculating the transmission loss of the electromagnetic wave in the birefringence crystal, and the calculated value of the heat generated is substituted into the energy equation. The electromagnetic wave's energy losses induced by its multiple refractions and reflections along with the resulting physical property changes of the lithium niobate crystal are considered.Influences of ambient temperature and heat transfer coefficient on refraction and walk-off angles of O-ray and E-ray in the cases of different incident powers and crystal thicknesses are analyzed. The E-ray electrical modulation instances, in which the polarized light waveform is adjusted to the rated condition via an applied electrical field in the cases of different ambient temperatures and heat transfer coefficients, are provided to conclude that there is a correlation between ambient temperature and applied electrical field intensity and a correlation between surface heat transfer coefficient and applied electrical field intensity. The applicable electrical modulation ranges without crystal breakdown are proposed. The study shows that the electrical field-adjustable heat transfer coefficient range becomes narrow as the incident power decreases and wide as the crystal thickness increases. In addition, it is pointed out that controlling the ambient temperature is easier than controlling the heat transfer coefficient. The results of the present study can be used as a quantitative theoretical basis for removing the adverse effects induced by thermal deposition due to linear laser absorption in the crystal, such as depolarization or wave front distortion, and indicate the feasibility of adjusting the refractive index in the window area by changing the heat transfer boundary conditions in a wide-spectrum laser.     

Influence of a magnetically permeable surface layer on transient electromagnetic field of a vertical magnetic dipole on a two-layer conducting earth

The transient electromagnetic field, in the frequency domain, on a two-layered conducting earth is reexamined when the usually neglected magnetic permeability contrast is considered. It is shown that for two-layered earth model, where the upper is permeable, the electromagnetic induction response is modified over the nonpermeable case. It is obtained as a summation of waveguide modes plus contributions from branch-cuts in the complex plane of the longitudinal wave number. The results from this paper can be used to evaluate numerical solution of other more complicated forward modeling algorithms. It can also be used in developing fast and simple interpretation tools, which can deliver quick estimation of the earth structure at the field.     

Wannier-Stark states in finite superlattices

Individual Wannier-Stark states are resolved in a current experiment over a wide electric-field range for a 5 and 4 period finite superlattice utilizing a hot-electron transistor. The observed field dependence of the tunneling transmission through the various states directly resembles the progressive localization of the wave functions. The basic transport through Wannier-Stark states in short-period superlattices is identified to be coherent. By tuning the Wannier-Stark state splitting with electric field into the optical phonon energy, the opening of new LO-phonon mediated transport paths is observed.     

压电条SH声辐射场研究

具有一定宽度和厚度的无穷长压电条置于均匀各向同性半空间上, 压电条在电激励下产生机械振动从而在整个空间产生声场. 将压电条取为6mm晶系, 其对称主轴沿长度方向. 针对这 种压电换能器结构, 深入研究了SH波的激发和辐射特性. 首先将压电条中的声场展开为傅里 叶级数, 而将压电条下的无界半空间中的声场展开为傅里叶积分, 然后根据边界条件得到了 整个空间中声场的求解方案, 通过数值模拟计算了声场的分布特性, 并和传统方法进行了定 量的对比和分析. 最后采用最陡下降法(鞍点法)研究并得到了压电条声辐射场的远场近似表 达式, 分析了声场的指向性规律. 结果表明只有当频率f和压电条宽度a的乘积 fa<1 kHz·m 时, 传统方法得到的结果是可靠的，当频率升高时，压电条内应力和位移呈振荡式分布，频率愈高振荡愈激烈，且与传统方法的差异愈大.本文对声学微系统与声传感器件的研究具有 重要意义. 关键词： 压电条 6mm晶系 表面位移 指向性因子     

THz电磁波在时变非磁化等离子体中的传播特性研究

本文建立了时变非磁化等离子体平板的一维模型,并采用时域有限差分(FDTD)方法对太赫兹(THz)电磁波在时变等离子体中传播时的反射、透射系数及吸收率进行了计算.然后根据计算结果分析了时变等离子体的上升时间、电子密度、温度以及等离子体平板厚度等参数对不同频段THz波在等离子体中传播特性的影响.分析结果表明:THz波在时变等离子体中传播时,其反射系数受等离子体电子密度和上升时间的影响较大;而吸收率则随着上升时间的减小、电子密度及平板厚度的增加而增大;此外,THz电磁波能够穿透量级为1020m-3的高密度等离子体层,可以作为再入段飞行器通信以及高密度等离子体诊断的理想工具.     

一维有限超晶格的电子态与透射问题的转移矩阵方法研究

采用转移矩阵方法,研究了一维有限超晶格的电子态与透射问题.计算了一维有限超晶格含单个缺陷层或少量缺陷层的透射谱和波函数,以及当电子被束缚在一维有限超晶格中电子的本征值和相应的定态本征函数.给出的方法对于研究电子通过任意排列的一维有限超晶格的输运具有普适性.     

压电体超晶格中的新型极化激元

对压电体超晶格中电磁波的传播情况进行了研究，通过压电效应，压电体超晶格中的纵振动声波将引起横向电极化，这一极化会与某一特定波段的电磁波强烈耦合，从而出现通常离子晶体中所没有的新型极化激元，产生非布拉格反射引起的光学禁带。     

Dynamics in a tightly-focused end-pumped solid-state laser near degenerate cavity configurations

We numerically study the dynamics of a tightly-focused end-pumped solid-state laser near the degenerate cavity configurations by using the Collins integral together with a rate equation. We find that the field and gain adjust their distributions to exhibit the temporal rather than spatiotemporal instabilities in spite of the coupling of many transverse modes. These instabilities are attributed to the frequency locking but phase unlocking of the degenerate Laguerre-Gaussian modes. The phase portrait is plotted in the three dimensional space with gain, real part, and imaginary part of the field because of similar dynamics for every transverse point. Both of the portrait and the Poincare map show the peculiar features. Moreover, the route to chaos is the mixed effect of period-doubling and quasi-period as the cavity length is tuned.     

Inhomogeneous magnetostriction states in uniaxial ferromagnetic films

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H _c, different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k=k _c≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate. Fiz. Tverd. Tela (St. Petersburg) 41, 665–671 (April 1999)