A miniature platform tolerant antenna is presented which is suitable for low frequency applications. A Split Ring Resonator (SRR) antenna loaded with lumped capacitances is proposed. The antenna is compact, low profile and easy to fabricate. It has a maximum dimension of λ/9 and -10 dB bandwidth of 1%. Miniaturized artificial magnetic conductor surfaces (AMCs) are designed using capacitance loaded metal patches with individual elements measuring just λ/70. Placing the SRR above the AMC improves the bandwidth to between 1.5 and 3.5% dependent on the overall size of the AMC and produces a platform tolerant antenna measuring 0.11λ×0.17λ×0.019λ. The performance of the antenna over an AMC with and without vias is studied and discussed. The AMC mounted antenna's performance in free space and over a ground plane is also compared.
A passive DC magnetic concentrator is designed with transformation optics (TO) and realized by meta-materials. The passive DC magnetic concentrator, based on space compression transformation, can greatly enhance the magnetic field in a free space region, which can be used for e.g. improving the sensitivity of magnetic sensors and increasing the efficiency of wireless energy transmission. The magnetic property of the medium obtained by TO is extremely anisotropic. To solve this, we use magnetic meta-materials made of alternated high-permeability ferromagnetic (HPF) materials and high-temperature superconductor (HTS) materials. We optimize our structure by conducting simulations using the finite element method (FEM), and experimentally demonstrate a strong, 4.74-time enhancement of the DC magnetic field by our meta-material magnetic concentrator. We also demonstrate that a simplified structure with only HPF materials working at room temperature could still give 3.84-time enhancement of the DC magnetic field. The experimental results are in good agreement with the numerical simulations based on FEM.
High-Q inductors are realized on a 3-8 Ω•cm silicon substrate in the buildup of BCB/Cu. Anisotropic wet etching is utilized to remove the silicon in the cavities underneath the spirals from the backside. Examples of 3.5-turn spiral inductors with and without cavity are compared, and their parameter extractions are accomplished with an equivalent circuit model. Compared to the inductor without cavity, the measured peak quality factor of a 8.19-nH inductor with cavity increases from 24 at 0.8 GHz to 39 at 2.5 GHz by 67%, and the inductor with cavity has a wider bandwidth using the same equivalent model. The inductors utilizing this technique have a potential wide application in hand-held RF modules either as part of an off-chip device or as an integrated passive in a silicon interposer.
A model of solar cell embedding quantum dots in the intrinsic layer of a p-i-n solar cell has been presented. With proper selection of material, size and fractional volume, quantum dots can provide an intermediate band between the valence and conduction bands of the matrix material, which will absorb photons with energy lower than the original bandgap to absorb more incident photons in the otherwise unused spectral irradiance. The design approach to acquire the highest efficiency of the conventional p-i-n solar cell is presented as a benchmark. Quantum dots are then embedded in the intrinsic region of the reference solar cell to improve its efficiency. InAs is chosen to implement the quantum dots, to be embedded in the p-i-n solar cell made of GaAs. With a more packed arrangement of QD's from that in the literatures, the simulation results shows that the efficiency of the conventional GaAs p-i-n solar cell can be increased by 1.05%.
In this paper, we investigate electromagnetic problems for nanoscale antennas by using a boundary integral equation method with fast inverse Laplace transform. The antennas are designed for realizing ultrafast and high-density magnetic recording. Characteristics of nanoscale antennas are discussed in terms of eigenmodes and time domain responses of electric fields. Our computational method is highly efficient and the computational cost can be reduced by selecting coarse time step size and performing parallel computation.
The idea of transformation inside a null-space region is introduced for the first time, and used to design a novel DC magnetic compressor that concentrates DC magnetic flux greatly and behaves as a DC magnetic funnel. The proposed device can be used as a passive DC magnetic lens to achieve an enhanced DC magnetic field (e.g. 7.9 times or more depending on the size and other parameters of the compressor) with a large gradient (e.g. 400T/m or more) in free space. After some theoretical approximation, the proposed device can be easily constructed by using a combination of superconductors and ferromagnetic materials. Numerical simulations are given to verify the performance of our device. The proposed method (use a null-space region as the reference space) can be extended to reduce the material requirement when designing other devices with transformation optics.
This paper proposes an ecient method to obtain ISAR images of multiple targets ying in formation. The proposed method improves the coarse alignment and segmentation of the existing method. The improved coarse alignment method models the ight trajectory using a combination of a polynomial and Gaussian basis functions, and the optimum parameter of the trajectory is found using particle swarm optimization. In the improved segmentation, the binary image of the bulk ISAR image that contains all targets is constructed using a two-dimensional constant false alarm detector, then the image closing method is applied to the binary image. Finally, the connected set of binary pixels is used to segment each target from the bulk image. Simulations using three targets composed of point scattering centers and the measured data of the Boeing747 aircraft prove the eectiveness of the proposed method to segment three targets ying in formation.
A novel dual frequency microstrip antenna with low radar cross section (RCS) is proposed in thispaper. Compared with the traditional microstrip antenna, the novelmicrostrip antenna loaded complementary split-ring resonators (CSRRs) has a low RCS. The novel and traditional dual-frequency microstrip antennas with the center frequency of 3.4 GHz and 5 GHz are designed and fabricated. The results demonstrate that the monostatic RCS of the proposed antenna has beenwell reduced. The RCS reduction at 5 GHz is as much as 17 dB. Besides,in the case of the φ-polarized incident wave, the RCS reduction can be achieved in the angular ranges of -90°≤θ≤+90° in xoz-plane and yoz-plane. At the same time, the CSRRs cause no obvious deterioration in radiation performance.
In this paper, we propose a stochastic approach for the analytical analysis of the multicarrier multipactor discharge occurring in high-power vacuum microwave devices, in which electric fields are not homogeneously distributed. We indicate that the statistical behavior of large amount of secondary electrons in the process of a multipactor discharge can be well described by the probabilistic random walk and Levy walk theory. Based on the derived probability density of the lateral diffusion of secondary electrons in homogeneous fields, the multicarrier multipaction in inhomogeneous fields can be analytically computed with significantly enhanced efficiency. As a demonstration, the accumulation of secondary electrons of a multicarrier multipaction in a rectangular waveguide supporting TE10 mode is given. The theoretical results comply well with the results achieved by the time-consuming particle simulation, the slope difference of which is less than 0.8%, while only costs one-order less computational time. To the best of our knowledge, this is the first time that the probability density of the lateral diffusion of secondary electrons during a multipaction is theoretically derived. This density depicts the physical picture of the statistical movement of secondary electrons during the process of a multicarrier multipactor, which can be widely used in the areas of high-power electronics and electromagnetism.
We present the design of 3-D metamaterial stacked arrays for efficient conversion of electromagnetic waves energy into AC. The design consists of several vertically stacked arrays where each array is comprised of multiple Split-Ring Resonators. The achieved conversion efficiency is validated by calculating the power dissipated in a resistive load connected across the gap of each resonator. Numerical simulations show that using stacked arrays can significantly improve the efficiency of the harvesting system in comparison to a flat 2-D array. In fact, the per-unit-area efficiency of the 3-D design can reach up to 4.8 times the case of the 2-D array. Without loss of generalization, the designs presented in this work considered an operating frequency of 5.8 GHz.
This paper presents a dual-band tunable bandpass filter with independently controllable dual passbands based on a novel asymmetric λ/4 resonator pair with shared via-hole ground. Because two separated passbands can be independently generated by the two λ/4 resonators with different electric lengths, the asymmetric λ/4 resonator pair can realize flexible passband allocation when it is utilized to design dual-band filters. Two varactors are placed at the two open circuit ends of the asymmetric λ/4 resonator pair to control the two dominant resonant frequencies, respectively. A prototype tunable dual-band filter with Chebyshev response is designed and fabricated. The measured results are in good agreement with the full-wave simulated ones. The results show that the first passband varies in a frequency range from 0.88 GHz to 1.12 GHz with the 3-dB fractional bandwidth of 5.1%-6.4%, while the second passband can be tuned from 1.5 GHz to 1.81 GHz with the 3-dB fractional bandwidth of 5.4%-6.4%.
This work demonstrates an all-optical slow light Time Division Multiplexing (TDM) structure based on photonic crystals (PCs). The structure shows good ability of divide time domain signal into repetition time slots signal by four tunable group velocity waveguides from 0.006*c to 0.248*c where c is the velocity of light in the vacuum at the center wavelength of 1550 nm and over a bandwidth 4.52 THz with group velocity dispersion below 10 2 ps2/km. New high efficiency Y-type directional coupling output can get larger than ~1.4 times intensity and ~93% loss improvement which are comparable to conventional output device. The proposed PCs waveguide structure is leading the way to achieve the TDM application and has good capability to extend the application of the optical communication and optical fiber sensors systems.
Using the Green's dyad technique based on cuboidal meshing, we compute the electromagnetic field scattered by metal nanorods with high aspect ratio. We investigate the effect of the meshing shape on the numerical simulations. We observe that discretizing the object with cells with aspect ratios similar to the object's aspect ratio improves the computations, without degrading the convergency. We also compare our numerical simulations to finite element method and discuss further possible improvements.
A metasurface for Radar Cross Section (RCS) reduction is proposed. The surface is composed of the same type of metamaterial units with different geometric dimensions, leading to various reflection phases under the incidence of plane waves. By carefully choosing the phase distributions, diffusion will be produced for the reflected waves which may redistribute the scattering energy from the surface toward all the directions, and hence it can be applied as the coating of metallic targets with ultra-low RCS. Both the simulated and experimental results have validated the proposed method.
A scattered radar signal by a jet engine inlet involves engine information of the target. The jet engine RCS due to the engine inlet is therefore significant in radar target recognition. Accordingly, several accurate analyses of the jet engine RCS have been reported. Reduction of the jet engine RCS is also required for stealth technology. We suggest a new approach for jet engine RCS reduction that involves the insertion of many small corrugations into the jet engine inlet. The proposed jet engine structure is assumed to be a circular waveguide cavity with many corrugations. The structure is analyzed by combining a mode matching technique with a scattering matrix analysis. The proposed closed-form RCS solutions of the corrugated engine structure are validated with a MoM simulation using FEKO. The proposed closed-form solution has numerical efficiency and rapid convergence. Using the characteristics of the solution, we simply apply a genetic algorithm (GA) to optimize the structure of the corrugations in terms of minimizing the RCS.
The time-domain shielding effectiveness of planar conductive thin screens excited by a transient electric-line source is studied in detail by means of an approximate semi-analytical formulation based on a Cagniard-De Hoop approach. Such a formulation allows for easily deriving and discussing several definitions of time-domain shielding effectiveness, recently introduced in the literature. Comparisons with results obtained numerically through an exact canonical double inverse Fourier transform are provided which furnish a benchmark to discuss the advantages and limits of the proposed approximate formulation.
Organic solar cells (OSCs) have recently attracted considerable research interest. For typical OSCs, it is highly desirable to have optically thick and physically thin thickness for strong light absorption and efficient carrier collection respectively. In the meantime, most organic semiconductors have short exciton diffusion length and low carrier mobility [1-3]. As a consequence, the active layers of OSCs are generally thin with a thickness of few hundred nanometers to ensure the efficient extraction of carriers, hence limiting the total absorption of incident light. Optimizing both the optical and electrical (i.e. multi-physical) properties of OSCs is in demands for rationally designed device architectures. Plasmonic nanomaterials (e.g. metallic nanoparticles [4-6], nanorods [7, 8], nanoprisms [9, 10], etc.) have recently been introduced into different layers of multilayered solar cells to achieve highly efficient light harvesting. The multilayered solar cells structures commonly have active layer, carrier (electron and hole) transport layer and electrode (anode and cathode). Through the localized plasmonic resonances (LPRs) [11-16] from metallic nanomaterials, very strong near-fields will be generated, which can provide a large potential for enhancing optical absorption in the multilayered OSCs. Besides the optical effects, it has been reported that metallic nanomaterials can modify the morphology, interface properties as well as the electrical properties of OSCs which will significantly modify the performances of OSCs [17-23]. In this article, the effects of various optical resonance mechanisms and the theoretical studies of the multi-physical properties of OSCs will be reviewed. Meanwhile, the experimental optical and electrical effects of metallic nanomaterials incorporated in different layers of OSCs will be studied. The morphology and interface effects of metallic nanomaterials in the carrier transport layers on the performances of OSCs will also be described.
This paper proposes a method to estimate human exposure to electromagnetic field radiation in a variable and highly multiscale problem. The electromagnetic field is computed using a combination of two methods: a rigorous time domain and multiscale method, the DG-FDTD (Dual Grid Finite Difference Time Domain) and a fast substitution model based on the use of transfer functions. The association of these methods is applied to simulate a scenario involving an antenna placed on a vehicle and a human body model located around it. The purpose is to assess the electromagnetic field in the left eye of the human body model. It is shown that this combination permits to analyse many different positions in a fast and accurate way.
In this paper, indirect microwave holographic imaging of concealed ordnance is demonstrated. The proposed imaging technique differs from conventional microwave imaging methods in that it does not require the direct measurement of the complex field scattered from the imaged object but mathematically recovers it from intensity-only scalar microwave measurements. This brings the advantages of simplifying the hardware implementation and significantly reducing the cost of the imaging system. In order to demonstrate the ability of the proposed technique to reconstruct good quality images of concealed ordnance, indirect microwave holographic imaging of a metallic gun concealed in a pouch is carried out for airport security imaging applications. It is demonstrated that good resolution amplitude and phase images of concealed objects can be recovered when back-propagation is applied.
Previous invisibility cloaks were based on metamaterials, which are difficult for practical realization in visible light spectrum. Here we demonstrate a unidirectional invisibility cloak in visible light spectrum. By using water as the effective material and separated into several regions by glass sheets, a simplest and cheapest invisible device is realized. This device can hide macroscopic objects with large scale and is polarization insensitive. Owing to simple fabrication and easily acquisitive materials, our work can be widely applied in our daily life.