Low-frequency scattering by rectangular cylinders

An integral equation formulation is used to investigate potential problems associated with low-frequency scattering by both dielectric and perfectly conducting cylinders of rectangular cross section. Induced dipoles and scattering cross sections are obtained for 1) waves withbar{E}orbar{H}parallel to the axis, and 2) directions of propagation perpendicular and parallel to the broad side of the rectangle.     

High-Q resonanaces of surface waves in thin metamaterial cylinders

The 2D problem of excitation of a circular metamaterial cylinder by a filament source is numerically investigated. It is found that, when the relative permittivity and permeability are close to minus unity, high-Q resonances occur in cylinders of an electrically small diameter. Near- and far-field patterns are calculated. It is discovered that, under resonance conditions, a multilobe scattering pattern typical of superdirective antennas is formed. The influence of loss on the resonance characteristics is investigated.     

基于背景材料磁导率各向异性实现全介质双曲线超材料

提出一种基于Mie谐振实现全介质双曲线超材料的新方法。首先设计了由介质颗粒组成的背景材料磁导率各向异性双曲线超材料,通过模拟计算出有效磁导率和有效介电常数张量的主对角分量,表明该材料具有双曲性质。最后,通过点源平面透镜聚焦成像现象直观地验证其双曲色散特性。     

Low-frequency resonances in asymmetric external cavitysemiconductor lasers: theory and experiment

A simple adjustment of the “Lang and Kobayashi” delayed rate-equations which describe the dynamics of an external-cavity semiconductor laser is shown to predict low-frequency resonances in the output intensity spectrum of the system. Numerical simulations with control parameter values-driving current and feedback strength-corresponding to our experiments show excellent agreement with theory. A possible mechanism for the occurrence of low-frequency self-oscillations is also proposed     

海水包裹空心椭球体的低频散射场计算及分析

为了分析水下金属壳体目标的散射场,提出了一种基于移位算子时域有限差分方法的加速计算方法.利用该方法对海水、空气和金属三种媒质材料进行了建模,并计算了水下金属椭球状壳体目标低频散射场的空间分布和起伏海面对低频电磁波传播的影响.通过数值算例测试了所提方法的加速性能,可以得到接近线性的加速比和95%以上的加速效率.计算结果表明,海面会影响水下壳体目标散射场的空间分布,特别是对于电场的空间分布影响比较大,并且由于水下电导率的存在导致海面上方散射场的传播速度要比海面下方的快.     

Proper interpretation of source excitations for resolving ostensible excessive power gains of metamaterial resonances

The aim of this communication is to resolve the anomalous phenomenon of extremely large radiated powers of antennas embedded within metamaterial layers apparently achievable under resonances. The essence of the discussion rests on the law of power conservation and causality conditions. A slotted spherical dipole coated with two layers of homogeneous materials is chosen to illustrate the concepts, and the classical approach for solving boundary-value problems via separation of variables is used to treat the structure.     

数值研究一种基于腔共振和电共振的近红外双频段超材料吸收器

设计, 数值模拟和讨论了一种具有两个宽带和扁平的吸收带的超材料吸收器, 其中一个是腔共振吸收带, 另一个是电共振吸收带.电共振的吸收带由于空腔尺寸(d)或者介质层厚度(H)的增加而蓝移, 而腔共振吸收带则表现出红移的现象.同时, 电共振和腔共振吸收带可以通过优化吸收器的结构设计耦合为一个吸收带.最后, 数值模拟研究了入射角度的改变对电共振和腔共振吸收带的影响.利用不同波段的共振模式形成不同吸收带的方式提供了将双吸收带调制为单吸收带的可能性.     

Dual band monopole antenna based on metamaterial structure with narrowband and UWB resonances with reconfigurable quality

In this article, an Ultra Wide Band (UWB) monopole antenna based on Metamaterial (MTM) unit cell with reconfigurable feature has been developed. The proposed antenna covers 3.1–10.6 GHz for UWB applications and it has a reconfigurable narrow-band for L-band (1.27 GHz) and wireless applications. The gaps in Split Rings Resonator (SRR) element are made for the Left-hand capacitance and Ω-shape strip layer by four via junctions are used for Left-hand inductance. The antenna is printed on FR-4 low cost substrate with relative permittivity of 4.4 and thickness of 1.6 mm. The total size of the antenna is 40 mm × 40 mm. The simulation is carried out using HFSS commercial full-wave software. In addition, the experimental results are presented and compared with simulated results. The antenna gives a maximum peak gain of 6 dBi with Omni-Directional radiation pattern and high efficiency of more than 70%. By embedding four switches in Ω-shape strip layer, a reconfigurable antenna has been successfully designed for wireless applications with sufficient qualification. The monopole part covers the UWB spectrum and the CRLH is responsible for the controllable narrowband resonance. The simulation and experimental results are confirmed by the numerical results.     

Forming electrodes for bipolar beams shaped as strips and solid or hollow cylinders

The configuration of forming electrodes for strip and cylindrical bipolar beams is calculated. The method applied for strip beams is based on the exact parametric equations of equipotential surfaces. An analytic extension of the equation for a planar diode and a solution to Laplace’s equation are used in the case of cylindrical beams. The solution to Laplace’s equation is represented in an integral form. The results obtained are compared to approximate solutions to the corresponding problems. The decompositions describing the explicit equations of the zero equipotential surfaces in axially symmetric and plane systems are analyzed. The behavior of these decompositions makes it possible to extend the results to 3D problems for beams having more complicated shapes.     

A metamaterial based on a 2D two-element lattice of cylinders with right- and left-handed surface conductivity

A new method is proposed for calculating the effective constitutive parameters of small-period 2D lattices. A two-element lattice of resonance chiral cylindrical scatterers with the opposite signs of chirality is investigated. It is found that such a structure exhibits the properties of the Veselago medium in a narrow frequency band.     

金属圆管管壁厚度对瑞利波影响的研究

为了探究管道中瑞利波信号的产生、传播规律,基于热弹激发机制,建立了激光线源在偏心圆管侧面激发超声波的有限元模型,研究了在不同激发源厚度下及不同曲率半径下铝质圆管中瑞利波的产生、传播规律。结果表明,均匀薄管中,管壁厚度在一定程度上决定了所产生的超声信号的成分,并且在一定范围内,超声信号也会随着管壁厚度的变化而变化;在非均匀管道中,激发源处的管壁厚度对超声信号的产生有很大影响,能在一定程度上决定超声信号的成分。这一结果对激光超声在管状材料的无损检测中的进一步应用是有帮助的。     

Transient electromagnetic field propagation through infinite sheets, into spherical shells, and into hollow cylinders

Gaussian electromagnetic field pulses of several durations are propagated through infinite sheets into the interior of hollow cylinders and into the interior of spherical shells. The plates, spheres and cylinders are made of aluminum and contain no slots. The time history of the propagated pulses is computed. Finally, the time sequence of the electric field is calculated in the interior of a cylinder of finite length when connected at its ends by wires to a generator delivering a current pulse of Gaussian shape. The dimensions of the cavities are assumed to be sufficiently small so that resonances are not excited by the highest significant frequency contained in the shortest pulse considered. The numerical study is restricted to thin-walled aluminum shields 1/32 inch, 1/16 inch, 1/8 inch and 1/4 inch thick. The half-amplitude widths of the pulses employed lie in the range14 musec to2400 musec. It is shown that the resultant Gaussian pulse electric fields defined on the surface of the plates and cylinders are propagated with little diminution in amplitude. This is understandable due to the requirement that the tangential fields are continuous across the interfaces, and to the fact that skin effect is almost nonexistent at low frequencies. The incident (as contrasted to resultant) field pulse undergoes reflection at the boundary surface. Hence, the attenuation sustained by the incident field is great, since reflection is the chief mechanism of attenuation of fields at low frequencies. Thin spherical shells form effective magnetic shields. The electric field is small in the interior of thin-walled cylinders carrying extremely large transient currents.     

Low-frequency noise in n-GaN

Low-frequency noise has been investigated in the hexagonal polytype of n-type gallium nitride (GaN) with equilibrium electron concentration at 300 K n ₀⋍7×10¹⁷ cm⁻³. The frequency and temperature dependence of the noise spectral density S _I/I² was studied in the range of analysis frequencies f from 20 Hz to 20 kHz in the temperature range from 80 to 400 K. Over the entire temperature range the frequency dependence of the dark noise is close to S _I/I²∼1/f (flicker noise). The rather weak temperature dependence of the noise level is characterized by very high values of the Hooge constant α⋍5–7. These large α values indicate a rather low level of structural quality of the material. The effects of infrared and band-to-band illumination on low-frequency noise in GaN are studied here for the first time. The noise level is unaffected by illumination with photon energy E _ph<E _g (E _g is the band gap) even for a relatively high value of the photoconductivity Δσ⋍50%. Band-to-band illumination (E _ph⩾E _g) influences the low-frequency noise level over the entire investigated temperature range. At relatively high temperatures the influence of illumination is qualitatively similar to that of band-to-band illumination on low-frequency noise in Si and GaAs. At relatively low temperatures the influence of illumination on the noise in GaN is qualitatively different from the results obtained earlier for Si and GaAs. Fiz. Tekh. Poluprovodn. 32, 285–289 (March 1998)     

Low-frequency capacitance measurements

The letter proposes a technique for the fast determination at extremely low frequencies of the capacitance and leakage components of capacitors. The proposed method permits the measurement of C and G in only a few cycles. An apparatus is described employing this technique for the automatic measurement of m.o.s. capacitance as a function of frequency and bias voltage.     

Low-frequency subsurface antennas

The fields of a dipole in a semi-infinite conducting medium are discussed and experimental verification of theoretical approximations presented. From these results, concepts and criteria are developed to determine optimum, subsurface antenna configurations. Impedance and mutual coupling properties of such antennas are discussed in the companion paper appearing in this issue of these TRANSACTIONS, entitled, "Impedance Properties of Large Subsurface Antenna Arrays."     

Low-frequency reactance amplifier

A reactance amplifier has been constructed which is capable of low-noise amplification at audio and subaudio frequencies [1]. Stable voltage amplification of 20-30 db and noise figures as low as 0.16 db (50-K source resistance) have been observed for signal frequencies between 2 and 50 cps. The circuit configuration is a double sideband up-converter or modulator in which a band of frequencies from dc to several hundred cycles is used to modulate a 150-kc carrier. An analysis for noise and gain is presented which considers the average, fundamental, and second-harmonic components of the time-varying diode capacitance. The amplifier input is a signal at low audio frequency, while the output is composed of the sum and difference sidebands of the pump. Gain is realized through up-converter action associated with the frequency shift from signal frequency to near pump frequency; the sum and difference sidebands are further amplified in the output tank by the negative resistance resulting from the action of the component of time-varying capacitance at the second harmonic of the pump frequency. Gain and equivalent input noise resistance are discussed as functions of detuning in the passive output tank; measured and calculated results are compared. A scheme is also presented for stabilizing the amplifier at high values of negative resistance gain.     

Low-frequency gyrator realisation

A gyrator realisation consisting of a Hall generator and two negative-resistance circuits is described. The Hall generator is the basic antireciprocal device; the negative resistances are needed to cancel out the effect of the Hall-plate input and output resistances.     

偏振不敏感的九聚物太赫兹超材料

在实验上提出将九个开口谐振环以第一个谐振环的开口方向为基准按照涡旋状的轨迹排布,且依次将开口环逆时针扭转40,构成九聚物太赫兹超材料,并在理论上系统研究了该超材料的传输特性。将原本只具有单一谐振模式的开口谐振环按照类似于低聚物的结构排布方式构成周期单元结构时,利用结构末端引起的能量耦合来增强相邻谐振器间的能量耦合,便会有更多的谐振模式产生,最终实现偏振不敏感的效果。这种思路丰富了传统超材料离散结构的设计理念,为太赫兹超材料功能器件,如开关,调制器和滤波器等的设计提供了一种新的参考方案。     

Nanostructure resonances

Coherent electron propagation is discussed for a class of nanostructures in which a `T' arrangement of quasi-linear conductors has a closed cavity element at the bottom of the T and electrons are transmitted between the ends of the T bar. It is proposed that resonant transmission effects will occur when propagation from the upright T element into the bar is made weak, by a constriction or a gate potential. The transmission properties are investigated analytically in terms of the S matrix of the T structure, and resonance effects are found