Purely Longitudinal Guidance in Dielectric Waveguide Array

Rigorous analysis shows that independent longitudinally guided TE and TM modes are possible in dielectric waveguide array (DWA) as a two dimensionally periodic medium. Especially, purely longitudinally guidance is also possible and investigated in detail.     

Transverse Propagation in Dielectric Waveguide Array

Transverse propagation eigen equations for dielectric waveguide array (DWA) are derived and eigen solutions can be uncoupled into TE and TM modes. Numerical results shows that much stronger space harmonic interaction exists so that a series of interesting physical effect of canonical two-dimensionally periodic (2DP) medium can obviously be observed in DWA and related devices can be predicted, designed and realized much easier.     

A General Approach to Guidance in General Two-Dimensionally Periodic Medium

Based on Floquet theorem and Maxwell equations, eigenvalue problem for general two-dimensionally periodic (2DP) medium is rigorously solved. The dispersion relation for piecewise uniform (PU) 2DP medium is derived in most general analytic formulations to suit all kinds of Dielectric Waveguide Array (DWA) configurations. Numerical results for a particular rectangular DWA are compared and shown to be in exact agreements with those obtained by previous different way, but recent method is much more universal and with much better simplicity and accuracy.     

Longitudinal Wave Propagation in Dielectric Waveguide Array

Based on the established rigorous theory for two-dimensionally periodic (2DP) medium, Longitudinal propagation in Dielectric Waveguide Array (DWA) was analyzed in detail. Firstly, given longitudinal wave vector (k _z), Brillouin dispersion relations between transversal wave vectors were analyzed. Interactions between space harmonics were also studied. Secondly, on condition that transversal wave vector k _x equaled k _y, dispersion relations between longitudinal and transversal wave vectors were analyzed in detail. Because of interactions between space harmonics increasingly getting stronger with k _z or modulation coefficients increasing, stop-bands could be distinctly displayed and thus longitudinal propagation in DWA could be comprehensively analyzed. The applications about longitudinal propagation in DWA were also discussed.     

绝缘微堆质子直线加速器关键技术研究进展

基于绝缘微堆技术的直线加速器由于其能够实现较高的粒子加速梯度,尤其在质子加速及肿瘤治疗领域的优势得到高度关注。目前该种加速器处于研发阶段,有一系列技术和工程问题有待解决。介绍了课题组在过去的两年里围绕建立一台1 MeV质子注入器原型样机在固态脉冲功率系统、绝缘微堆及质子束源等方面取得的研究进展。实现了耐压梯度接近20 MV/m的环形绝缘微堆样品,样品内径30 mm,外径50mm,厚度15mm,基本达到设计要求;固态脉冲功率系统实现了光导开关多路稳定工作模式,开关直流偏置耐压达到20kV,采用激光二极管触发同步系统在15路同步时实现了低于1ns的抖动,输出300kV的电压脉冲,输出电压脉冲宽度10ns;进行了低能质子加速束流动力学的初步分析和模拟工作,模拟结果表明采用微堆结构可以实现质子束的有效加速和传输。     

The Convergence of the Eigen Equation for Dielectric Waveguide Array

The eigen equation for Dielectric Waveguide Array (DWA) established by [1] was discussed. The convergence of the eigen equation was also analyzed in detail. Numerical results for a particular rectangular DWA was presented. Compared with those obtained by previous different ways, faster convergence for the eigen equation was reached easily. The characteristics of the eigen equation for DWA was particularly useful in computer propramming and engineering.     

The Amplitude of Longitudinal Characteristic Modes in Dielectric Waveguide Array

It's not adequate to just obtain eigenvalue for solving eigenvalue problem. Since the eigenvalues associated with longitudinal characteristic modes in dielectric waveguide array (DWA) has been studied, the paper concentrates on the corresponding eigenvector, namely, the amplitude of these characteristic modes. Through numerical study of the amplitude of longitudinal guided TE and TM modes in DWA, space harmonic (SH) coupling and energy conversion effect have been discovered. The paper not only contribute to deep understand the mechanism of SH interaction, but also do good to analyze the resonance, radiation problem in DWA.     

介质壁加速器加速场建立过程中波传输分析

 介绍了介质壁加速器(DWA)的原理和几种可能实现的结构。通过对多层介质圆柱的平面波电磁散射的研究,用FORTRAN语言编写程序计算和分析了DWA加速管三层介质柱体结构的平面波电磁散射的散射宽度与几何结构参数、材料参数的关系,用以优化设计介质壁加速管结构。计算结果表明：当加速管材料和等势环介电常数一定时,平面波电磁散射宽度随半径增大而增大;当加速管内外径一定时,加速管材料和等势环介电常数增大时散射宽度变化不明显,但最小散射宽度显著减小。当加速管半径和材料一定时,总能找到使散射宽度达到最小的等势环介质厚度。     

介质壁加速腔加速结构优化

为了在介质壁加速器中增大轴向加速电场, 提高加速梯度的同时抑制径向电场对束包络的扩张, 提出了在每个加速电极上添加金属栅网结构。采用基于粒子云网格方法的电磁粒子模拟软件对不加栅网与添加栅网的电极结构进行了数值仿真, 分析了不同结构下加速管道中的电场分布和束包络变化。通过实验对比了两种不同结构下经过相同的加速长度获得的粒子能量。结果表明:添加金属栅网结构相对于不加栅网的金属小孔式结构, 轴向加速电场强度提高20%, 同时径向电场得到有效抑制;栅网结构下, 被加速的粒子束在自由漂移空间中的径向发散基本得到抑制;在相同的加速长度下加速H3+粒子, 栅网结构得到的能量增益提高了一倍。     

Femtosecond laser direct writing of TiO2 crystalline patterns in glass

Crystalline TiO₂ was induced three dimensionally inside a glass sample by 800-nm, 250-kHz femtosecond laser irradiation. Micro-Raman spectra and X-ray diffraction analysis indicated that the laser-induced crystals were monophase TiO₂ rutile. A periodic structure consisting of TiO₂ rutile crystalline lines was inscribed in the glass sample by continuously moving the focal point of the laser beam. This technique may be useful for fabricating integrated optical devices in glasses.     

High quantum efficiency of intersubband transitions in coherent tunneling of electrons through asymmetric double-barrier structures

A converging perturbation series that can be summed analytically has been obtained for intersubband transitions of electrons coherently tunneling through the middle of a dimensionally quantized level in an asymmetric double-barrier structure in a high-frequency terahertz electric field. The possibility of a substantial increase in tunneling current accompanied by either absorption or emission of a photon has been demonstrated. The quantum efficiency of radiative transitions between dimensionally quantized levels can be up to 66%. Zh. éksp. Teor. Fiz. 112, 237–245 (July 1997)     

Electromagnetic simulation study of dielectric wall accelerator structures

Two types of dielectric wall accelerator (DWA) structures, a bi-polar Blumlein line and zero integral pulse line (ZIP) structures were investigated. The high gradient insulator simulated by the particle in cell code con rms that it has little in uence on the axial electric field. The results of simulations using CST microwave studio indicate how the axial electric field is formed, and the electric eld waveforms agree with the theoretical one very well. The in uence of layer-to-layer coupling in a ZIP structure is much smaller and the electric eld waveform is much better. The axial of the Blumlein structure's electric field has better axial stability. From both of the above, it found that for a shorter pulse width, the axial electric field is much higher and the pulse stability and delity are much better. The CST simulation is very helpful for designing DWA structures.     

Nature of the magnetic order in the charge-ordered cuprate La1.48Nd0.4Sr0.12CuO4

Using polarized neutron scattering we establish that the magnetic order in La(1.48)Nd(0.4)Sr(0.12)CuO(4) is either (i) one dimensionally modulated and collinear, consistent with the stripe model or (ii) two dimensionally modulated with a novel noncollinear structure. The measurements rule out a number of alternative models characterized by 2D electronic order or 1D helical spin order. The low-energy spin excitations are found to be primarily transversely polarized relative to the stripe ordered state, consistent with conventional spin waves.     

Spontaneous quantum gravity: A soluble model

We present a model of gravity in which the Planck mass is generated spontaneously by a similar mechanism that is responsible for the tension in a recent membrane model of strings. The action describes very floppy fluctuations of the physical space in some large flat embedding space. The coupling constant is dimensionless but the fluctuations are so violet that they produce spontaneously a mass which plays the role of the dimensionally transmuted coupling constant and can be identified with the Planck mass.     

Dimensional renormalization and factorization

The problem of formulating a high-energy factorization explicitly in terms of dimensionally renormalized operators and coefficient functions is analyzed in the context of deep-inelastic scattering in renormalizable scalar theories. The coefficient functions that emerge are found to be the finite parts of dimensionally continued on-shell amplitudes, and are readily amenable to explicit computation. As a byproduct, an explicit forest formula emerges for the mass-singularity poles of on-shell amplitudes in renormalizable theories. The extension to gauge theories is briefly discussed at the leading twist level. The method is compared to the alternative approach to factorization whereby a finite hard part is defined by factorizing off mass-singularities.     

Simulation and experimental study of the solid pulse forming lines for dielectric wall accelerator

Two types of pulse forming lines for dielectric wall accelerator (DWA) were investigated preliminarily. By simulation with CST Microwave Studio, the results indicate the pulse forming process, which can help to understand the voltage wave transmission process and optimize the line parameters. Furthermore, the principle of the pulse forming process was proved by experiments and some excellent pulse waveforms were obtained. During the experiments, the Blumlein line and zero integral pulse (ZIP) forming line, constructed with aluminum foil, poly plate and air gap self-closing switch, were tested. The full width at half maximum (FWHM) of the waveform is 16 nanoseconds (BL) and 17 nanoseconds (ZIP line),and the formed pulse voltage amplitude is 5 kV (BL) and +2.2 kV/ - 1.6 kV (ZIP line). The experiments result coincides well with the simulation.     

Long-range antigravity

We consider a theory in which fermionic matter interacts via longe-rage scalar, vector and tensor fields. In order not to be in conflict with experiment, the scalar and vector couplings for a given fermion must be equal, as is natural in a dimensionally reduced modell. Assuming that the Sun is not approximately neutral with respect to these new scalar-vector charges, and if the couplings saturate the experimental bounds, then their strength can be comparable to that of gravity. Scalar-vector fields of this strength can compensate for a solar quadropole moment contribution to Mercury's anomalous perihelion precession.     

Dark-state imaging for two-dimensional mapping of a magnetic field

By imaging the dark states that are due to coherent population trapping in Na atomic vapor, we have succeeded in depicting a spatially inhomogeneous magnetic field. Highly resolved dark lines represent cross sections of the surfaces of constant magnetic fields, and they agree well with the predicted hyperfine Zeeman splitting and the two-photon selection rules of the Na atom. Mapping was made two dimensionally in real time, but extension to three dimensions is straightforward. Furthermore, unlike the previous techniques based on optical pumping, this method can be used for any magnetic-field directions.     

Dimensional reduction by Monte Carlo

A Monte Carlo method for mapping a field theoretical or statistical system to a new theory embedded in a space-time of lesser dimensionality is presented. Typically, the critical properties of the dimensionally reduced system depend upon the details of the mapping. As an example, the two-dimensional Ising model is mapped to a one-dimensional Ising model with long-range forces and a phase transition. Systems with long-range interactions and known exponents can thus be constructed with this procedure.     

Directionally anisotropic Si nanowires: on-chip nonlinear grating devices in uniform waveguides

Computational studies are used to show that the crystalline structure of Si causes the waveguide Kerr effective nonlinearity, γ, to vary by 10% for in-plane variation of the orientation of a silicon nanowire waveguide (SiNWG) fabricated on a standard silicon-on-insulator wafer. Our analysis shows that this angular dependence of γ can be employed to form a nonlinear Kerr grating in dimensionally uniform SiNWGs based on either ring resonators or cascaded waveguide bends. The magnitude of the nonlinear index variation in these gratings is found to be sufficient for phase matching in four-wave mixing and other optical parametric processes.