A modified spiral antenna termed a "sunflower spiral antenna" which consists of smooth section and zigzag section is proposed to improve the axial ratio. The analysis of the current distribution along the arm is carried out by an integral equation method. It is revealed that the zigzag section of the sunflower spiral considerably contributes to the suppression of standing wave near the arm end, especially at lower frequencies after the establishment of the first mode radiation in terms of current band theory. It can be emphasized that the sunflower spiral achieves a wide band characteristic of the axial ratio without deteriorating inherent radiation pattern, power gain and input impedance of a conventional round spiral antenna.

A set of integral equations is derived for a coupling problem in which a wire is energized by an electromagnetic wave through an aperture of a narrow slot. In order to facilitate the numerical treatment, the kernels of the integral equations are simplified so that neither derivatives nor integrals are contained. Typical examples, including a finite and an infinite wire are analyzed using the present integral equations. The currents on the wires and the axial distributions of the slot magnetic currents are demonstrated. It is found that the current induced on the infinite wire is characterized by a traveling wave which does not decay and a wave which dies out rapidly as it travels along the wire.

A new method of exciting a two-wire spiral antenna from two off-center sources is proposed in order to obtain the polarization diversity. It is found both numerically and experimentally that as the locations of two sources are shifted from the center of the spiral towards the arm ends, the radiation wave changes from a right-hand to a left-hand circularly polarized wave, by way of a stage in which a linearly polarized wave is produced. On the basis of the obtained results, the frequency characteristics of a spiral antenna with two fixed sources are investigated. It is found that the spiral antenna radiates a right-hand circularly polarized wave at a frequency of f" and a left-hand circularly polarized wave at a frequency of 2f".

Hallen's type integral equation derived by Mei for a curved wire antenna is applied to a system composed of many arbitrarily bent wires with arbitrary excitation. The simplification of kernels 2 and 3 of Mei's original equation is shown, and coupled integral equations with kernels in which neither derivatives nor integrals appear are derived concerning current and scalar potential for the bent wires. An analysis of bent wires with a junction is presented.

A pair of Hallen-type integral equations is formulated for general coupling problems of wires excited through slots in a conducting screen. The number of wires and slots, and their orientation can be chosen arbitrarily in these integral equations. The integral equations have simplified kernels in which neither derivatives nor integrals are contained. The electric and magnetic scalar potentials are also derived to analyze electromagnetic coupling between slots and wires of bent configuration. Results of the magnetic current in the slot and the electric current on the wire are given for a few typical cases.

The beam-propagation method (BPM) is applied to the evaluation of the loss of S-shaped step-index waveguides. It is demonstrated that the BPM enables us to analyze S-shaped bends which have an offset core and a trench section in the cladding at the outer side of the bend.

Analysis of the propagation of circularly symmetric fields is made using the finite-difference beam-propagation method. After testing the accuracy of this method, we analyze the guided-mode transmission of connected fibers whose core radii are different. The propagation behavior of the unguided-mode field generated at the junction is revealed using a transparent boundary condition.

A Drude-critical points (D-CP) model for considering metal dispersion is newly incorporated into the frequency-dependent FDTD method using the simple trapezoidal recursive convolution (TRC) technique. Numerical accuracy is investigated through the analysis of pulse propagation in a metal (aluminum) cladding waveguide. The TRC technique with a single convolution integral is found to provide higher accuracy, when compared with the recursive convolution counterpart. The methodology is also extended to the unconditionally stable FDTD based on the locally one-dimensional scheme for efficient frequency-dependent calculations.

A periodic array of InSb spheres on a substrate is numerically analyzed at terahertz frequencies. The incident field is shown to be coupled to the substrate due to the guided-mode resonance. The effect of the background refractive index on the transmission characteristics is investigated for sensor applications.

A plasmonic black pole (PBP) consisting of a series of touching spherical metal surfaces is analyzed using the finite-difference time-domain (FDTD) method with the periodic boundary condition. First, the wavelength characteristics of the PBP are studied under the assumption that the PBP is omnidirectionally illuminated. It is found that partial truncation of each metal sphere reduces the reflectivity over a wide wavelength range. Next, we consider the case where the PBP is illuminated with a cylindrical wave from a specific direction. It is shown that an absorptivity of more than 80% is obtained over a wavelength range of λ=500 nm to 1000 nm. Calculation regarding the Poynting vector distribution also shows that the incident wave is bent and absorbed towards the center axis of the PBP.

A grating consisting of a periodic array of InSb-coated dielectric cylinders on a substrate is analyzed at THz frequencies using the frequency-dependent finite-difference time-domain method based on the trapezoidal recursive convolution technique. The transmission characteristics of an infinite periodic array are investigated not only at normal incidence but also at oblique incidence. The incident field is shown to be coupled to the substrate due to the guided-mode resonance (GMR), indicating the practical application of a grating coupler. For the sensor application, the frequency shift of the transmission dip is investigated with attention to the variation of the background refractive index. It is found that the shift of the dip involving the surface plasmon resonance is almost ten times as large as that of the dip only from the GMR. We finally analyze a finite periodic array of the cylinders. The field radiation from the array is discussed, when the field propagates through the substrate. It is shown that the radiation direction can be controlled with the frequency of the propagating field.

The finite-difference beam-propagation method is applied to the analysis of hollow slab waveguides (HSWs). The attenuation constants for the TE0 and TE1 modes are evaluated and compared with those obtained by the perturbation theory. The propagating field and differential power loss in the transition from a straight HSW to a bent HSW are revealed and discussed.

The body-of-revolution finite-difference time-domain method (BOR-FDTD) based on the locally one-dimensional (LOD) scheme is extended to a frequency-dependent version for the analysis of the Drude and Drude-Lorentz models. The formulation is simplified with a fundamental scheme, in which the number of arithmetic operations is reduced by 40% in the right-hand sides of the resultant equations. Efficiency improvement of the LOD-BOR-FDTD is discussed through the analysis of a plasmonic rod waveguide and a plasmonic grating.

The current distribution along a square spiral arm is determined by using Mei's integral equation. The data concerning radiation pattern, power gain, axial ratio and input impedance are presented with good agreement between theoretical and experimental results over a frequency range of 3 to 7 GHz. It is revealed that similar decay in the current distribution, which is necessary to keep wide-band characteristics of the radiation, is obtained in spite of the change of frequency. The data show that the input impedance is nearly a pure resistance, and that, when the radiation pattern in the spiral plane becomes almost omnidirectional, phase of the radiation field varies linearly with the change of the azimuth.

A backfire helical antenna (BHA) is numerically analyzed to form a circularly polarized conical beam. The conical beam whose radiation is null in the helical axis direction is obtained using the second mode operation of the BHA. The characteristics of monofilar, bifilar and quadrifilar BHA's are compared and discussed. It is found that a second-mode bifilar BHA radiates a circularly polarized conical beam over a frequency range of 1 : 1.4. The use of the first mode of a bifilar BHA is also investigated to form a quasi-conical beam which has the weak radiation in the helical axis direction.

Two plasmonic band-bass filters are analyzed: one is a grating-type filter and the other is a slit-type filter. The former shows a band-pass characteristic with a high transmission for a two-dimensional structure, while the latter exhibits a high transmission even for a three-dimensional structure with a thin metal layer.

The locally one-dimensional finite-difference time-domain (FDTD) method in cylindrical coordinates is extended to a frequency-dependent version. The fundamental scheme is utilized to perform matrix-operator-free formulations in the right-hand sides. For the analysis of surface plasmon polaritons propagating along a plasmonic grating, the computation time is significantly reduced to less than 10%, compared with the explicit cylindrical FDTD method.

A bent-waveguide-based multimode interference (MMI) demultiplexer is designed for the operation at 0.85- and 1.55-µm wavelengths using the three-dimensional semi-vectorial beam-propagation method. First, it is shown that the use of a straight MMI waveguide results in a long coupler length of more than 1000µm for wavelength demulitplexing. To reduce the coupler length, we next introduce a bent MMI waveguide. Bending with a radius of 1500µm leads to a coupler length of less than 200µm. After designing two output waveguides connected to the MMI section, we finally choose a coupler length to be 175µm for efficient demultiplexing properties. Consequently, an output power of more than 90% can be obtained, leading to a low insertion loss of 0.34dB at both 0.85- and 1.55-µm wavelengths. The demultiplexer achieves small polarization dependence, i.e., less than 2dB difference in contrast and 0.02dB difference in insertion loss.

Pure bend loss of a fiber with a trench section is calculated by the alternating-direction implicit finite-difference method. The dependence of the loss on the trench location is evaluated. The mechanism of the oscillatory behavior of the loss is discussed in terms of a modal approach in a dielectric slab waveguide.

In this paper, we applied the circuit synthesis theory of filters to the design of transmission-type metasurface cells and arbitrarily designed the amplitude and phase of the transmission and reflection by adjusting the resonant frequency and coupling coefficient. In addition, we successfully designed the phase of the unit cell by using the frequency conversion of filter theory. Moreover, we designed a refractive transmission-type metasurface plate with a novel cell structure that reacts to both polarizations. The prototype operated at the desired refraction angle, confirming the design theory.