Toshiaki Kitamura

All-optical logic gates containing a two-mode nonlinear waveguide

We propose a new element containing a two-mode nonlinear waveguide consisting of Kerr-like material for all-optical logic circuits. The proposed element, incorporated into the Mach-Zehnder interferometer, functions like a phase shifter. Although the optical waves propagating in the proposed element are monochromatic and identical in polarization, its function is insensitive to the relative phase difference between the incident beams. Using this element, we can realize various logic functions. In this paper, we demonstrate such logic circuits as inverters, distributors, and AND gates.

Analysis of cylindrical microstrip lines utilizing the finite difference time-domain method

This paper presents full-wave analysis of the cylindrical striplines and the cylindrical microstrip lines utilizing the finite difference time-domain method in a circular-cylindrical coordinate system. Numerical results on the normalized characteristic impedance are in good agreement with the available results. Furthermore, the junctions of the coaxial transmission line and the cylindrical microstrip line are analyzed to show the usefulness of this method. >

A dual-plane comb-line filter having plural attenuation poles

In this paper, a dual-plane comb-line filter having plural attenuation poles is proposed. We investigate the filtering characteristics from both experiments and numerical simulations by means of the finite-difference time-domain method. It is shown that this filter has attenuation poles just above, as well as below the passband and that intersections between the curves of input susceptance of the even and odd modes agree with the attenuation-pole frequencies. Furthermore, it is demonstrated that, by changing the position of the metal pin, which connects two resonators, we can change the input susceptance of the odd mode alone and, hence, regulate the attenuation-pole frequencies.

Study of the influence of grounding for microstrip resonators

The influence of ground terminal shape on the resonant frequency of a microstrip resonator is investigated through numerical simulations and experiments. Resonators are one of the basic components of very small high-dielectric stripline filters, named the laminated-planar filter. The resonant frequencies are calculated by means of the finite-difference time-domain (FDTD) method and compared with experimental results. It is also shown that the resonant frequency is directly related to the square root of its line inductance when the resonator is regarded equivalently as a series LC circuit.

A Balanced Multi-Layered CRLH TL with Defected Parallel Plates

This paper studies multi-layered composite right-/left-handed transmission lines (ML CRLH TL). To realize wideband response from left-handed passband to right-hand passband without band gaps and other undesired parasitic resonances, a balanced ML CRLH TL with defected parallel plates is newly proposed and demonstrated theoretically by using FEM-based EM simulator. A wide passband from 2.0 GHz to 9.5 GHz was attained with balanced condition, which is useful for UWB applications.

Proposal of a new folded comb-line filter

We propose a new type of comb-line filter that is suitable for miniaturization utilizing stratifying technology. It consists of a couple of composite resonators. Each resonator is composed of two sections of a microstrip and a coplanar waveguide fabricated on the microstrip ground plane which are connected through a piece of wire at an open end to set up a new folded comb-line filter. The filtering characteristics are investigated through experiments as well as numerical simulations by means of the FD-TD method. It is shown that the filtering characteristics can be much improved by removing part of the barrier in the coplanar-line portion of the resonator to alter the coupling.

Multistage dual-plane comb-line filter excited by conductor-backed coplanar waveguide

This paper proposes a novel multi-stage dual-plane comb-line filter having plural attenuation poles just below and above the passband. As this filter has such a structure such that two quarter-wavelength step impedance resonators are placed face-to-face beyond the substrate on each filter block, the mutual coupling between adjacent blocks are small and the optimisation on each block can be done independently. High performance filters up to 4-stages are demonstrated both theoretically and experimentally.

Study on nano-aperture with metallic nano-spheres for near-field optical disk

There is an optical diffraction limit for the recording density of conventional optical recording systems. There has been a lot of recent interest in the field of optical storage technologies for the recording methods that are based on near-field optical principles because they have the potential to overcome the limitation by using the localized optical near-field for writing and reading recorded marks. Many types of nano-apertures and nano-antennas have been investigated to achieve the high throughput of the optical near field. It is also well-known that the near-field is observed when the incident light is illuminated to a metallic nano-sphere. This study focuses on a ridged-circular nano-aperture with metallic nano-spheres for a near-field optical disk. The analysis is accomplished by a three-dimensional FDTD method, which combines the motion equations of the free electrons. This method can be applied to the analysis of various plasmonic devices and also handle Drude dispersion easily. First, nearfield distributions through the nano-aperture are analyzed and it is shown that the transmitted light intensity increases by adding the nano-spheres near the ridged-circular nano-aperture. It is thought to be due to the surface plasmon resonance between two adjacent nano-spheres. Phase distributions of the nearfield are also analyzed and the relevance to the near-field enhancement is investigated. Next, the near-field optical disk structures are analyzed and the readout characteristics are studied. The structure is composed of the ridged-circular nano-aperture with nano-spheres and the recording layer that is sandwiched between the two protective layers. The dependency of the readout characteristics on the recorded-mark size is studied and the crosstalk when using the plural recorded marks is also analyzed and the advantage for using the nano-spheres for the near-field optical disk nano-apereture is confirmed through FDTD simulations.

Investigation on near-field optical disk using rectangular nano-aperture with ledges

The recording density of optical storage systems has been increased by using the shorter wavelength of laser light and the higher numerical apertures of the objective lenses. However, the optical diffraction limit restricts these methods to get the higher recording density. Near-filed optical recording is one of the promising techniques because it has the potential to overcome this limitation. Many types of nano-apertures and nano-antennas have been proposed in order to obtain near-field of high emission intensity and a small spot size. In this study, the three-dimensional optical disk structure utilizing a metallic nano-aperture with ledges is analyzed by using FDTD method. The motion equations of free electrons have been installed to analyze the metallic material. First, the nano-aperture with ledges is analyzed and the near-field through the aperture and the far-field scattering patterns are studied. It is demonstrated that the narrow beam width can be achieved by adding the ledges on the aperture. Next, the readout characteristics of the near-field optical disk are investigated. The structure is composed of the rectangular nano-aperture with ledges and the recording layer that is sandwiched between the two protective layers. The dependency of the characteristics on the recorded-mark size is studied and the crosstalk characteristics between adjacent recorded marks are investigated when the plural recorded marks are arranged. It is shown that the crosstalk can be reduced by using the proposed aperture. The readout characteristics of the continuously arranged recorded-marks are also studied and the minimum recorded-mark size that can be distinguished is discussed.

FDTD analysis of metal-insulator-metal waveguide resonant cavity with metallic nano-cylinders

Surface plasmon polaritons (SPPs), which carry the electromagnetic energy along the metal-dielectric interface, have received much attention in the field of the photonic devices because they have potential to realize nanoscale photonic devices with the dimension smaller than the diffraction limit. There are mainly two types of configurations that are composed of metal-insulator-metal (MIM) and insulator-metal-insulator (IMI). Among of these, MIM configurations have been attracting much interest because they can realize strong field localization and their fabrication process is simpler and easier compared with the IMI ones. Many types of MIM plasmonic devices have been proposed so far, such as ring resonators, Bragg gratings, splitters and demultiplexers, filters utilizing cascaded transverse cavities. Analytical modeling of resonant cavities for MIM waveguide junctions has also been investigated. In this study, MIM waveguide resonant cavities that include metallic nano-cylinders in them are investigated through FDTD analysis. In the FDTD method, special handling of the metallic material is required because the permittivity is dispersive and has a negative value in the optical frequency. To overcome this difficulty, the motion equations are introduced into the FDTD method in order to evaluate the conducting current. The MIM waveguide consists of silver and air. The fundamental mode of the MIM waveguide is excited at the input port and the frequency characteristics of the power transmittance through the output port are investigated. It is shown that the drastic change of the power transmittance characteristics occur by slightly changing the location of the nano-cylinders inside the cavity. The field pattern inside the cavity is analyzed and the association with the power transmittance characteristics is demonstrated. It is shown that a wide variety of filtering characteristics can be obtained by arranging the nano-cylinders properly.