We propose a novel design of broadband plasmonic nanoantenna that is suitable for fluorescence and Raman enhancement. The structure consists of a gold nanoring and bowties at the center. We numerically investigate the near field and far field performance by employing the finite-difference time-domain method. High Purcell enhancement and large SERS are demonstrated in a record wide spectral bandwidth of 700 nm based on a single emitter-antenna configuration. Moreover, unlike a traditional antenna design, the proposed nanoantenna has low heat generation and high field enhancement at the gap simultaneously, when operating off resonance.
In this paper, we demonstrate a device that is capable of generating an ultrashort (sub-nanosecond) high power microwave pulse by means of passive pulse compression in a compact reverberant cavity. The long duration input pulse into the cavity is created using time-reversal techniques, which allows the waveform to contain the inverse profile of the cavity phase distortion. When fed back into the cavity, the wave focusing at the output port results in a com-pressed ultrashort pulse with enhanced peak amplitude. We experimentally demonstrate a pulse compressor consisting of a 0.0074 m3 cavity capable of generating a 130 picosecond pulse from an input waveform of 300 nanosecond duration with the peak gain of up to 19 dB.
An accurate and efficient computational approach is presented for analyzing radiation characteristics of large antenna arrays with radome. This approach is based on the hybrid finite element-boundary integral-multilevel fast multipole algorithm (FE-BI-MLFMA). Unlike the conventional single-domain FE-BI-MLFMA, the whole domain of the antenna array with radome is separated into many disconnected domains. A large free space area unavoidable in the single-domain FE-BI-MLFMA is eliminated in this multi-domain FE-BI-MLFMA formulation, thus the number of unknowns is greatly reduced in the presented multi-domain FE-BI-MLFMA approach. Different from the single-domain FE-BI-MLFMA, many integral equations are required in this multi-domain FE-BI-MLFMA. The numerical experiment shows that the presented multi-domain FE-BI-MLFMA is more efficient than the single-domain one while maintaining the same accuracy. A whole complicated system of a slotted-waveguide array with radome mounted on an aircraft is analyzed to further demonstrate the generality and capability of the presented multi-domain FE-BI-MLFMA.
An equivalent-circuit model for a reconfigurable unit cell is proposed. This circuit model facilitates fast prediction of scattering parameters and dispersion analyses of a reconfigurable periodic structure. The cutoff frequencies obtained using equivalent-circuit models are in excellent agreement with those from measurements and full-wave numerical simulations. The proposed circuit model is then modified to include non-ideal, commercial RF FET switches. The effect of such a switch in each state, On or Off, is modeled by a frequency-dependant impedance, derived from the scattering parameters of the switch. The proposed technique can be used to analyze a reconfigurable periodic structure with any type of switches. For the structure with 24 unit cells considered here, the equivalent circuit model is about five orders of magnitude faster than full-wave simulations.
In this work, numerical analysis of nonlinear ferromagnetic problems is presented using the three-dimensional time-domain finite element method (TDFEM). Formulated with the secondorder nonlinear partial differential equation (PDE) combined with the inverse Jiles-Atherton (J-A) vector hysteresis model, the nonlinear problems are solved in the time domain with the Newton-Raphson method. To solve the ordinary differential equation (ODE) representing the magnetic hysteresis accurately and efficiently, several ODE solvers are specifically designed and investigated. To improve the computational efficiency of the Newton-Raphson method, the multi-dimensional secant methods, aka Broyden's methods, are incorporated in the nonlinear TDFEM solver. A nonuniform time-stepping scheme is also developed using the weighted residual approach to remove the requirement of a uniform time-step size during the simulation. The capability and the performance of the proposed methods are demonstrated by various numerical examples.
We apply the method of the Broadband Green's Functions with Low wavenumber extraction (BBGFL) to calculate band diagrams in periodic structures. We consider 2D impenetrable objects placed in a 2D periodic lattice. The low wavenumber extraction is applied to the 2D periodic Green's function for the lattice which is used to formulate the surface integral equation. The low wavenumber extraction accelerates the convergence of the Floquet modes expansion. Using the BBGFL to the surface integral equation and the Method of Moments gives a linear eigenvalue equation that gives the broadband (multi-band) solutions for a given point in the first Brillouin zone. The method only requires the calculation of the periodic Green's function at a single low wavenumber. Numerical results are illustrated to show the computational efficiency and accuracy of the method. Because of the acceleration of convergence, an eigenvalue problem with dimensions 49 plane wave Floquet modes are sufficient to give the multi-band solutions that are in excellent agreement with results of the Korringa Kohn Rostoker (KKR) method. The multiband solutions for the band problem and the complementary band problem are also discussed.
In this work, a numerical solution of nonlinear ferromagnetic problems is formulated using the three-dimensional time-domain finite element method (TDFEM) combined with the inverse Jiles-Atherton (J-A) vector hysteresis model. After a brief introduction of the J-A constitutive model, the second-order nonlinear partial differential equation (PDE) is constructed through the magnetic vector potential in the time domain, which is then discretized by employing the Newmark-β scheme, and solved by applying the Newton-Raphson method. Different Newton-Raphson schemes are constructed and compared. The capability of the proposed methods is demonstrated by several numerical examples including the simulation of the physical demagnetization process, the prediction of the magnetic remanence in the ferromagnetic material, and the generation of higher-order harmonics.
This paper presents a new non-rare-earth axial-field magnetic variable gear (MVG). By real-time changing the numbers of permanent magnet (PM) pole-pairs in the input and output rotors, the gear ratio becomes controllable. The key is to propose a new stationary ring integrated with magnetizing windings in such a way that various PM pieces can be independently magnetized to form different pole-pair numbers. After introducing the unique features of the non-rare-earth PM material aluminum-nickel-cobalt (AlNiCo), the proposed topology and design principle are discussed. By using finite element analysis, the electromagnetic performances of the proposed MVG under different gear ratios are analyzed. In particular, the corresponding torque transmission capability is assessed, and the influence caused by the introduction of the magnetizing windings is discussed. Hence, the validity of the proposed MVG can be verified.
The finite-element method (FEM) is applied to solve the EEG forward problem. Two issues related to the implementation of this method are investigated. The first is the singularity due to the punctual dipole sources and the second is the numerical errors observed near the interface of different tissues. To deal with the singularity of the punctual dipole sources, three source modeling methods, namely, the direct, the subtraction and the Saint Venant's methods, are examined. To solve the problem of numerical instability near the interface of different tissues, a modification on the Saint Venant's method is introduced. The numerical results are compared with analytical solution in the case of the multilayer spherical head models. The advantages of the proposed method are highlighted.
A Circularly Polarized (CP) high efficiency wide band Reflectarray (RA) antenna is designed for Ka-band using cross bow-tie elements. The reflected wave phase curve is obtained by anti-clockwise bowtie rotation. The linear phase curve with complete 360° degree is obtained when left-hand circularly polarized (LHCP) is incident normally in unit cell environment. The proposed method provides high gain, high aperture efficiency, wideband axial ratio (AR), in circularly polarized bow-tie RA using multiple copies of unit cell to form 25*25 antenna array. Before designing RA, the unitcell is analyzed, for oblique incidence to predict its bandwidth. The proposed antenna provided good performance in terms of Half Power Beam width HPBW, Side Love Level (SLL), cross polarization, gain bandwidth and AR bandwidth. A 25*25 bow-tie RA antenna provides the highest aperture efficiency of 57%, HPBW of 9.0 degrees, SLL -19 dB, cross polarization -27 dB. A 1-dB gain bandwidth of 32.5%, 3-dB gain bandwidth of 51.4% and 1.5-dB AR bandwidth of 32.9% while 3-dB AR bandwidth of 48.7% is achieved in simulation. These results are validated through fabricated cross bow-tie RA, and the measurements make good agreement with simulation results.