Chiral planar waveguide for guiding single-mode backward wave

Single-mode backward wave is shown to be guided in a planar dielectric waveguide with a strong chiral core. The significant difference of such a waveguide from the traditional one is the guidance of single-mode backward wave, without using negative permittivity and/or negative permeability. In the design, we generalize the idea of total internal reflection to the chiral medium and make a numerical analysis on the reflection with oblique incidence. We deduce rigorously a general solution of incident wave on the boundary of two arbitrary chiral magneto-electric media. We observe that the impedance matching can eliminate the coupling between two eigenwaves in chiral media. With strong chiral core and the matched impedance with cladding, one eigenwave becomes a backward wave and can be guided without transferring to the other eigenwave. If a single-mode propagation condition is satisfied, we will get single-mode backward guided wave. A special interface has been designed to prevent the forward wave entering the waveguide from the source.     

各向异性超常材料中倒退波的传播研究

研究了完全各向异性超常材料中的倒退波传播现象,得到了在材料本征轴和传输轴成任意角度情形下倒退波形成的条件,分析了超常材料的介电张量和磁导率张量、电磁波的偏振方式对倒退波形成和传播的影响. 在此基础上,进一步分析了几种不同色散曲线关系的各向异性超常材料中倒退波的产生情况,获得了电磁波波矢和坡印亭矢量(能流)夹角的具体表达式和倒退波传播的一般性结论. 此外,还研究了近零介电常数超常材料中倒退波的传播特性,发现在此类超常材料中倒退波只能是完美倒退波. 关键词： 超常材料 负折射 倒退波 各向异性     

Stress reduction in planar waveguide using polymer top layer

Planar optical waveguides consisting of layers from different materials created at elevated temperatures usually exhibit substantial stresses. By controlling the layer thickness of polymeric top layer on planar waveguide structures, it is possible to use very thin layers for stress compensation, significantly reducing required deposition times. It is possible to reduce birefringence within planar device by controlling top polymer layer thickness with thermal expansion coefficient greater than silica or PMMA.     

Optical wave propagation in photonic crystal metamaterials

Metamaterials that provide negative refraction can be implemented in photonic crystals (PhCs) through careful design of the devices. Theoretically, we demonstrate that the dispersion can be altered to achieve negative refraction. This can be done through engineering the geometry of the device as well as selecting appropriate materials. The PhC also demonstrates slow light that facilitate sensing chemicals or biological agents. Using metallic materials such as gold nano-particle enables PhCs to guide optical waves in desired pathways. Also using magnetic materials such as highly doped n-GaAs, we can tune the band gap by changing magnetic field. The simulated results are consistent with some of the previously reported experimental results and give us guidance for future experiments.     

Electromagnetic wave propagation in a rectangular waveguide partially loaded with magnetized planar ferrite

The propagation of electromagnetic waves in a rectangular waveguide containing a slab of transversely magnetized planar ferrite is investigated. Numerical results indicate a large differential phase shift on application of small magnetic fields when the easy plane is oriented normal to the waveguide axis. This suggests the possibility of application in ‘Remanence Operation’ at high frequencies. Work partially supported by CSIR (India).     

Ultrasensitive terahertz/infrared waveguide modulators based on multilayer graphene metamaterials

This paper studies and classifies the electromagnetic regimes of multilayer graphene‐dielectric artificial metamaterials in the terahertz/infrared range. The employment of such composites for waveguide‐integrated modulators is analysed and three examples of novel tunable devices are presented. The first one is a modulator with excellent ON‐state transmission and very high modulation depth: >38 dB at 70 meV graphene's electrochemical potential (Fermi energy) change. The second one is a modulator with extreme sensitivity towards graphene's Fermi energy ‐ a minute 1 meV variation of the latter leads to >13.2 dB modulation depth. The third one is a tunable waveguide‐based passband filter. The narrow‐band cut‐off conditions around the ON‐state allow the latter to shift its central frequency by 1.25% per every meV graphene's Fermi energy change.     

The influence of backward wave transmission on quantitative ultrasonic evaluation using Lamb wave propagation

In view of the various novel quantitative ultrasonic evaluation techniques developed using Lamb wave propagation, the influence of an important related phenomenon, backward transmission, is investigated in this paper. Using the discrete layer theory and a multiple integral transform method, the surface displacement and velocity responses of isotropic plates and cross-ply laminated composite plates due to the Lamb waves excited by parabolic- and piston-type transmitting transducers are evaluated. Analytical expressions for the surface displacement and velocity frequency response functions are developed. Based on this a large volume of calculations is carried out. Through examining the characteristics of the surface displacement and velocity frequency response functions and, especially, the different propagation modes' contributions to them, the influence of the backward wave transmission related to quantitative ultrasonic nondestructive evaluation applications is discussed and some important conclusions are drawn.     

Sensitivity enhancement in optical waveguide sensors using metamaterials

In this paper, we show negative differential resistance (NDR) in CdSe quantum dot/MEH-PPV based nanocomposite multi-layer heterostructures at room temperature. The four-layer structure exhibited a maximum peak-to-valley ratio of current of 1190 at room temperature, while two-layer structures show a value of 4. Two-, three- and four-layer structures are studied. Each device configuration exhibits different kind of negative differential resistance. The possible mechanism is explained on the basis of tunneling phenomena.     

A general method for analyzing the multilayer planar waveguide

A general method for analyzing the multilayer planar optical waveguide is presented. This method can also be used to predict the evolution of a wave propagating in the multibranch planar waveguide. The numerical simulation results show that the prediction is correct.     

磁光平面波导的单向传播特性

表面磁等离子体(surfacemagnetoplasmons,SMPs)是一种在电介质和偏置磁场作用下磁光材料界面处传播的近场电磁波.其独特的非互易传播特性引起了大量科研工作的关注,但在具体的波导结构设计上仍存在很多问题.本文研究了一种银-硅-磁光材料的3层平面波导结构,SMPs在磁光材料和硅的界面处传播,发现在特定的频率范围内,SMPs的基模及高阶模式均具有正向或反向的单向传播特性.分别计算了旋磁与旋电材料平面波导的色散方程,研究了硅层厚度与外加磁场对能带结构及SMPs单向传播区域的影响,发现无论是旋磁或旋电材料的结构,硅层厚度的增加使高阶模式使高阶模式出现在更低的频率位置,使单向传输带宽变小甚至消失,外加磁场的变大使磁光材料的能带结构频率增大的同时带隙中也引入了高阶模式.计算了2种磁光材料平面波导的正向和反向的单向传播带宽宽度,发现旋磁材料YIG的单向SMPs模式出现在GHz波段,最大单向带宽可达到2.45 GHz;旋电材料InSb的单向SMPs模式出现在THz波段,最大单向带宽达到3.9 THz.     

650 GHz正弦波导返波管的模拟计算

提出了一种利用正弦波导慢波结构设计的太赫兹频段大功率返波管的新方案。3维粒子模拟研究结果显示:在20 kV的工作电压和5 mA的驱动电流下,该返波管在645.9 GHz频率处将具有1.55 W的峰值功率输出,互作用效率为1.55%,频谱纯净,输出模式为TE10模。通过在15～25 kV之间进行电压调谐,可获得80 GHz的调谐带宽,且峰值输出功率都在1 W以上。     

650 GHz正弦波导返波管的模拟计算

提出了一种利用正弦波导慢波结构设计的太赫兹频段大功率返波管的新方案。3维粒子模拟研究结果显示:在20 kV的工作电压和5 mA的驱动电流下,该返波管在645.9 GHz频率处将具有1.55 W的峰值功率输出,互作用效率为1.55%,频谱纯净,输出模式为TE10模。通过在15～25 kV之间进行电压调谐,可获得80 GHz的调谐带宽,且峰值输出功率都在1 W以上。     

Derivative free method for computing modes of multilayer planar waveguide

A technique to solve generalized dispersion equation of multilayer planar waveguide has been demonstrated to obtain all the expected guided modes. The solution is based on the derivative free method for computing the zeros of an analytical function in complex plane. The derivative free method extracts the roots which are very close to actual zeros of the function. Roots are further refined using the robust iteration method to achieve the desired accuracy. Application of the proposed method has been verified by solving the modes of a variety of structures including lossless structure, leaky structure, quantum well waveguide, active waveguide, ARROW waveguide and metal clad waveguide. The method is efficient and computes all modes of planar waveguide with high accuracy.     

Optical terahertz wave generation in a planar GaAs waveguide

We report generation of terahertz (THz) radiation in a planar 61-microm-thick GaAs waveguide with a TM0 propagation mode, achieved by phase-matched difference frequency mixing. The THz output was centered near 2 THz and had 1 microW average power. As a pump source we utilized both the signal and the idler outputs of a near-degenerate type II synchronously pumped optical parametric oscillator operating near 2 microm with the average powers of 250 and 750 mW, correspondingly.     

Broadband planar left-handed metamaterials using split-ring resonator pairs

In this paper, we showed that split-ring resonator (SRR) pairs can be used as broadband planar left-handed metamaterials (LHMs). Simulations were carried out for one layer of infinite LHM slab using SRR pairs. The results showed that by carefully adjusting dimensions of the SRR pairs, magnetic and electric resonances can be coexistent at some frequency ranges and in the frequency range where there are both negative magnetic and electric responses, there is a broad LH band. Equivalent circuits for the magnetic and electric resonance were offered to give a qualitative and quantitative explanation of the LH behaviors of LHMs using SRR pairs.     

The study of multilayer planar optical waveguide structure with nonlinear cladding

A novel method for analyzing the multilayer and multibranch optical waveguides with nonlinear cladding is presented. This method can also be used to analyze the multiple-quantum-well (MQW) waveguide structure. Numerical simulation results show that our analyses are reasonable.     

Hilbert space inverse wave imaging in a planar multilayer environment

Most diffraction tomography (DT) algorithms use a homogeneous Green function (GF) regardless of the medium being imaged. This choice is usually motivated by practical considerations: analytic inversions in standard geometries (Cartesian, spherical, etc.) are significantly simplified by the use of a homogeneous GF, estimating a nonhomogeneous GF can be very difficult, as can incorporating a nonhomogeneous GF into standard DT algorithms. Devaney has circumvented these issues by developing a purely numerical DT inversion algorithm [A. J. Devaney and M. Dennison, Inverse Probl. 19, 855-870 (2003)] that is independent of measurement system geometry, number of frequencies used in the reconstruction, and GF. A planar multilayer GF has been developed for use in Devaney's "Hilbert space" algorithm and used in a proof-of-principle nondestructive evaluation (NDE) experiment to image noninvasively a flaw in an aluminum/copper planar multilayer medium using data collected from an ultrasonic measurement system. The data were collected in a multistatic method with no beamforming: all focusing through the multilayer was performed mathematically "after-the-fact," that is, after the data were collected.     

Light propagation in a planar dielectric slab waveguide with step discontinuities

The common matrix formalism for multi-layer systems is generalized to describe the optical field propagation along an arbitrary cascade of step discontinuities in a planar dielectric waveguide. The reflection and transmission properties of the step cascade are modelled by rigorous operator relations in a Hilbert-space calculus. In this way the complete propagation problem is reduced to two basic transformations. TE- and TM-polarized fields are treated simultaneously.     

Uniform transverse-electric fundamental wave in a symmetric linear multilayer optical waveguide

In the context of a symmetric linear multilayer optical waveguide, a special transverse-electric fundamental (TE(0)) wave is analyzed that has identical amplitudes in the guide core. The guided structural parameters, such as the core thickness, the number of wells, and the ratio of the well thickness to the periodic length, must have specific relationships to one another to form this kind of uniform TE(0) wave. These relationships are investigated, and some field profiles of the uniform TE(0) wave are plotted.     

Theoretical models for designing a 220-GHz folded waveguide backward wave oscillator

In this paper,the basic equations of beam-wave interaction for designing the 220 GHz folded waveguide(FW)backward wave oscillator(BWO) are described.On the whole,these equations are mainly classified into small signal model(SSM),large signal model(LSM),and simplified small signal model(SSSM).Using these linear and nonlinear one-dimensional(1D) models,the oscillation characteristics of the FW BWO of a given configuration of slow wave structure(SWS) can be calculated by numerical iteration algorithm,which is more time efficient than three-dimensional(3D)particle-in-cell(PIC) simulation.The SSSM expressed by analytical formulas is innovatively derived for determining the initial values of the FW SWS conveniently.The dispersion characteristics of the FW are obtained by equivalent circuit analysis.The space charge effect,the end reflection effect,the lossy wall effect,and the relativistic effect are all considered in our models to offer more accurate results.The design process of the FW BWO tube with output power of watt scale in a frequency range between 215 GHz and 225 GHz based on these 1D models is demonstrated.The 3D PIC method is adopted to verify the theoretical design results,which shows that they are in good agreement with each other.