Electromagnetic Wave Propagation Close to Microstructures Studied by Time and Phase-Resolved THz Near-Field Imaging

We demonstrate microscopic mapping of electromagnetic waves close to metal microstructures with sub-ps temporal and sub-wavelength spatial resolution by pulsed THz near-field imaging. The inherent phase-sensitivity of this technique allows mapping wavefronts of propagating modes and the measured amplitude distributions provide information on field concentration and localization close to the structures. Using this approach we investigate wave propagation through a sub-wavelength aperture, as well as the formation of traveling and standing surface waves along a metal microwire.     

Propagating eigenmodes for plane waves in a uniaxial bianisotropicmedium and reflection from a planar interface

A time-harmonic electromagnetic plane wave propagating in a uniaxial bianisotropic medium is considered. Forward- and backward-propagating eigenmodes are identified, together with their dispersion equations. The eigenmodes consist of two pairs with different phase velocities, and the two components of each pair correspond to the forward and backward modes. The propagating modes are used to calculate the reflection coefficient matrix at an interface between a vacuum and a uniaxial bianisotropic half-space. Numerical results for such a reflection are presented as a function of the direction of the optical axis     

A modified conical cavity for submillimeter wave applications

The conical cavity has been modified for use in electron beam devices. In this new cavity resonator a short radial line section couples the beam hole with the conical region providing a suitable interaction gap. Operated in a higher order TM0p0mode the cavity size is large compared to wavelength making it attractive for millimeter or submillimeter applications. In this mode, both Q0and Rshuntare large. For analysis, the modified conical cavity can be considered as a composite consisting of a circular waveguide at cutoff, a radial waveguide, and a shorted "quasi-wedge" guide. Resonant frequencies are found by impedance matching at a transverse plane in the cavity. UnloadedQand Rshuntare found by calculating total energy stored and power loss along with an equivalent voltage across the gap. An experimental cavity was tested. This cavity was scaled forXand Ku bands and operated in TM0p0modes from p=5 to p=9. Measured results agreed well with theory. At the p=9 mode, the radial dimension of the cavity was 4.2 λ. Design curves are presented for the modified conical cavity without beam hole.     

高功率横流CO2激光器的锥面反射镜谐振腔

根据针-板放电5kW横流CO2激光器的结构特点,设计并实验研究了直角锥面镜作为全反射镜的谐振腔。在菲涅耳-基尔霍夫理论的基础上,引入柯林斯公式并考虑增益影响,推导了锥面谐振腔的衍射积分方程,并进行了相应的模拟计算。研究结果表明,在高功率横流CO2激光器中锥面反射镜与平面输出镜相配合使用组成稳定腔,获得了TEM30的低阶模输出;由于锥面反射镜受加工精度的影响中心残留有加工盲孔,导致TEMmn(m=0,n≠0)的模损耗较大不易耦合输出;而镜面中心场分布很弱的其他低阶模因损耗小,则会优先输出。     

Modes In Radial Wave Beam Resonators

Resonant modes in ring shaped resonators formed by a reflector strip bent into a circle are discussed here. These modes are derived by superimposing two radially propagating wave beams, one converging toward the resonator axis, the other one diverging from this axis. The condition for resonance leads to an integral equation whose eigenfunctions describe the field distribution at the reflector strip and whose eigenvalues determine the diffraction loss due to the fraction of energy bypassing the reflector. The approximations made in deriving the radial beam mode system are equivalent to those used for the derivation of the axial beam mode system in a Fabry-Perot resonator. Within the limits of these approximations the kernel of the integral equation for a ring resonator is of the same form as the kernel of the integral equation for a parallel strip resonator. If in the radial case the reflector strip is also curved within the axial planes with a radius of curvature equal to the diameter of the reflector ring, and if the width of the reflector strip is sufficiently large, the axial field distribution of the modes is described by Gauss-Hermite functions. The Q of the ring resonator is determined by the diffraction loss and by reflection loss caused by the finite conductivity of the reflector. Formulas for the corresponding Q-values are derived. A numerical evaluation shows that in the microwave region Q-values of the order of 10/sup 6/ are feasible.     

Mode-selective properties of a metal-clad-dielectric-slab waveguide for integrated optics

Attenuation and phase constants are calculated for a metal-clad-dielectric-slab waveguide and the results show the behavior of the propagating modes as a function of cladding material and dielectric. Analysis indicates that the dielectric properties influence the attenuation and cutoff frequencies whereas the metal cladding influences primarily the attenuation of propagating modes. Variation of the attenuation and phase constant around a center frequency show surface-wave properties of the TM0and TM1modes.     

Plane Waves Propagating in Chiral Plasma and Chiral Ferrite Media

Plane wave propagation in chiral plasma and chiral ferrite media is studied in kDB coordinate system. General wave equations and characteristic equations of plane waves propagating along an arbitrary direction in chiral plasma and in chiral ferrites are derived in simple formulations respectively. Four wavenumbers and their corresponding dispersion characteristics are resulted for propagation both along and normal to the biasing magnetic field. When plane wave with negative helicity propagates along the biasing magnetic field in chiral ferrites, backward waves emerge. However backward waves occur with both positive and negative helicities when propagating along the biasing magnetic field in chiral plasma.     

用轴棱锥设计贝塞尔-高斯谐振腔

本文从较直观的角度出发,把贝塞尔光理解为一对趋向于传播轴线和离开轴线的锥形波的叠加,将腔两端的镜面理解为用于改变光场位相的器件,讨沦用轴棱锥设计谐振腔以产生贝塞尔光,我们发现在所设计的腔中振荡的光是受钟形函数调制的锥形波,其模式与贝塞尔一高斯光相似.并且我们所得到的结论与福克斯一厉数值迭代法验证的结果是一致的.     

New Leaky Surface Waves in Anisotropic Metal-Diffused Optical Waveguides

Propagation of guided waves in anisotropic metal-diffused optical waveguide is investigated. Two-dimensional guide-mode dispersion curves are computed and classified for a metal-diffused waveguide with arbitrary optic-axis orientation in various diffusion depth. It is found that a new leaky surface wave exists in the region where the refractive index is above the cutoff value, not below it. Typical values of decay constant are about 5 dB/cm for the wave propagating along X axis on 128/spl deg/ rotated Y-cut LiNbO/sub 3/, and 35 dB/cm for the wave propagating along the direction making an angle of 70/spl deg/ to X axis on Y-cut plane LiNbO/sub 3/. We were able to observe the leaky surface waves experimentally.     

Optical resonators in the approximation of Gauss

A general theory of optical resonators based on the concept of complex point-eikonal is presented. The analysis is limited to the approximation of Gauss. The modes of resonance of open resonators formed by two spherical mirrors facing each other have been obtained in previous works by fitting the wavefronts of Gaussian beams to the mirror surfaces. This method becomes complicated when the resonator contains focusing elements or dielectric slabs. The approach proposed in this paper is more straightforward than previous approaches dealing with this problem and is applicable to resonators containing anisotropic media. The round-trip point-eikonal is first evaluated on the basis of the conventional laws of Gaussian optics. The presence of apertures with Gaussian transmissivity, and of lasers with quadratic transverse variation of the gain, are accounted for by introducing complex round-trip point-eikonals. The modes ψm(x) of the resonator are obtained from a power series expansion of the Green function of a mode-generating system related to the round-trip point-eikonal S of the resonator. The resonance frequencies and the round-trip losses are given by simple and general formulas. The mode fields are described by Hermite-Gauss functions with complex arguments, explicitly in terms of S. For resonators that do not contain apertures, the wavefronts are the same for all the modes and they are plane at planes of symmetry. In reciprocal resonators, clockwise and counter-clockwise modes have the same losses and resonant frequencies, but different mode pattents. These modes are shown to be mutually adjoint. Adjoint modes, with < ψ^†m, ψn> = 0 if m ≠ n, are useful to evaluate the response of optical resonators to incident fields. Results applicable to resonators containing nonorthogonal astigmatic elements are also discussed.     

平面波在任意磁化方向旋磁媒质中传播时的极化特性

在铁氧体基片平面电路和发射天线中必须研究平面波在任意磁化方向旋磁媒质中的传播。这时平面波的特征波为ＴＥ波,其磁通密度、磁场、电通密度、电场的极化是不相同的,磁场的极化方向为左、右椭圆极化。     

Poynting's vector and the wavefronts near a plane conductor

An equation is derived for the time average of Poynting's vector near a plane conductor, which shows that the intensity vector is approximately normal to the wavefronts if one only excludes those regions for which the electric field changes very rapidly in the direction normal to the plane conductor.     

Twist Modes in Magnetoplasma-Filled Circular Waveguides

The properties of the axially symmetric normal modes of a circular waveguide containing an axially magnetized gas or solid-state plasma (i.e., the so-called Faraday configuration) are examined. Of particular interest is the fact that transverse electric fields demonstrate a characteristic twisting wave motion rather than the more familiar rotating motion of the circularly polarized TE/spl deg//sub 11/ limit modes or the undulating motion of the normal modes of an empty waveguide. Modes demonstrating this unique wave motion are termed "twist" modes. Within a restricted range of magnetic field, twist modes divide into evanescent (TE-limit) modes and low-loss propagating (TM-limit) modes. Since wavelengths of propagating modes depend on the axial B field, twist modes in solid-state magnetoplasmas such as InSb may find applications in magnetically tunable millimeter and submillimeter devices.     

Coupling of vias in electronic packaging and printed circuit board structures with finite ground plane

A full wave method is presented for modeling and analyzing multiple interactions among vertical vias in densely packaged integrated circuits and printed circuit board with ground plane of finite extent. In such structures, the TEM mode in the planar structure is excited and can propagate and cause interaction of waves among vias. Reflections will also occur at the edges of the finite ground plane. The electromagnetic analysis methodology is an extension of the previous methodology in analyzing multiple scattering among vias for infinite ground plane . The analysis is based upon the cylindrical wave mode expansion of the magnetic field Green's function, the Foldy-Lax multiple scattering formalism and utilizing the resonator modes of a circular cavity. The circular resonator modes are transformed into cylindrical waves onto the cylindrical via structures. Numerical results illustrate the physics of the underlying resonance scattering problems. We consider the cases of a) two coupled active vias of differential mode and b) two coupled vias of common mode. Results are also illustrated for ground plane resonance and the effects of shorting vias on such resonance. The effects of off-centering and the presence of idle vias are also illustrated.     

Ohmic effects in quasioptical resonators

Several properties of the Fabry-Perot-type open resonator used in the quasioptical gyrotron (QOG) and the quasioptical induced resonance electron cyclotron (IREC) maser are derived. The electric fields of the normal modes are given for the general case of the resonator axis tilted with respect to the direction perpendicular to the electron beam axis. The ohmic quality factor and the power dissipated in the mirrors are derived, as is the energy stored in the resonator. The time dependence of the mirror heating, relevant for pulsed experiments, is also derived. The formulae are applied to an example of current relevance, the quasioptical IREC maser resonator.     

Fabry-Perot resonators in uniaxially anisotropic media

This paper investigates the resonant modes of Fabry-Perot interferometers with end reflectors embedded in linear, homogeneous, uniaxially anisotropic media. It is shown that for almost all practical cases the resonant modes consist of transverse electric (TE) modes and transverse magnetic (TM) modes relative to the direction of the optic axis in the medium. Two distinct integral equations are derived which together with Maxwell's equations are sufficient to perform a detailed analysis of these modes. However, since the results of the analysis of Fabry-Perot resonators in isotropic regions are available and well understood, the approach of the paper is to reduce a given "anisotropic resonator" to two corresponding equivalent "isotropic resonators": one for determining the TE modes and the other for determining the TM modes. The equivalent isotropic resonator for the TE modes has the same geometry as the actual resonator in the anisotropic medium. The geometry of the equivalent isotropic resonator for the TM modes is derivable in a very simple manner from the geometry of the actual resonator in the anisotropic medium and from the specified orientation of the direction of the optic axis in the medium. The well-known results for resonators embedded in isotropic media may henceforth be applied to determine the resonant modes of the anisotropic resonator.     

Orthogonality properties of transverse eigenmodes of phase conjugate optical resonators

The orthogonality properties of the transverse eigenmodes of optical resonators which have phase conjugate mirrors at both ends are derived. As in conventional resonators and also resonators with only one phase conjugate mirror, it is shown that the transverse eigenmodes are essentially biorthogonal, a relation which is satisfied between the set of modes propagating in one direction around the resonator and the adjoint set of modes propagating in the reverse direction.     

Intrinsic modes in a wedge-shaped taper above an anisotropicsubstrate

Intrinsic modes yield exact solutions away from the tip in a wedge-shaped taper with penetrable boundaries. Unlike adiabatic modes, they are uncoupled and pass smoothly through the cutoff transition. This model has been generalized to accommodate stratified multiwave substrates and weakly range-dependent environments. Here, a wedge-shaped taper above an anisotropic substrate is considered. In the spectral representation of intrinsic modes, the plane wave reflection coefficient from the substrate depends on the orientation of the optic axis, which, in turn, affects the cutoff condition and the direction of the leakage field in the substrate. Since the phase propagation vector and average power flow vector are usually nonparallel in anisotropic media, the field in the substrate is substantially different from that in isotropic cases     

空时域求解左手介质中逆切仑科夫辐射

在空时域中利用推迟势推导了左手介质材料中的逆切仑科夫效应,并对比经典的正常切仑科夫效应进行了讨论。给出了逆切仑科夫辐射波矢锥角公式、背向辐射能量锥角公式和辐射物理图像。     

涡旋电磁波无线通信技术的研究进展

It is known from electromagnetic momentum that electromagnetic waves can carry Spin Angular Momentum (SAM) related to polarization and Orbital Angular Momentum (OAM) related to the trajectory of the Poynting vector. When OAM is not zero, the wave-front electric field distribution of the electromagnetic wave is vortex-like and has the characteristic of propagating along the axial direction. Therefore, this electromagnetic wave is aptly named vortex electromagnetic wave. Based on the mathematical model of the plane electromagnetic wave field, the researchers introduce a Fourier factor that uses the topological charge (also called mode) of the OAM as a parameter to describe the field of the vortex electromagnetic wave. Therefore, the wave-front of the vortex electromagnetic wave with a “polarization” pattern associated with topological charge, the use of polarization patterns of vortex electromagnetic waves in different modes can further increase the spectrum effect of the wireless communication system. Studies show that although it is feasible to generate "planar" vortex electromagnetic wave beams from Uniform Circular Array (UCA) arrays in an open environment, to obtain modal multiplexing gain, and it is necessary to explore vortex electromagnetic wave beams based on orthogonal phase sequences distributed on a unit circle in the complex plane. At the same time, the paper also investigates the current research status of compatibility between OAM and Multiple Input Multiple Output (MIMO) systems in the field of radio frequency.