ANALYSIS OF A METAL DISC-TYPE SURFACE WAVE SPLITTER USING THE CYLINDRICAL SEMI-IMPLICIT FDTD METHOD AND ITS EXPRIMENTAL STUDY

A metal disc-type surface wave splitter utilizing the propagation characteristics of spoof surface plasmon polaritons is analyzed using the cylindrical semi-implicit finite-difference time-domain (FDTD) method. We design the metal disc-type splitter with a center hole and radially placed gratings in the microwave region. Numerical and experimental results are in good agreement with each other. It is also shown that the computation time of the semi-implicit FDTD method is significantly reduced to less than 20% of the explicit counterpart, while maintaining the accuracy

Establishment of implicit and hybrid-implicit-explicit FDTD methods in cylindrical coordinates and development of a metal disc-type THz surface wave splitter

研究成果の概要 (和文) : 本研究では、円筒座標系FDTD法の計算効率改善のため、陰的な局所的一次元(LOD)法を導入した。また、周回方向の離散化のみに陰解法を用いるハイブリッド型半陰的円筒座標系FDTD法を開発した。以上の手法により、円筒型デバイスの計算時間が1/3～1/10に低減され、当初の目的が達成された。これらの手法を金属円盤形テラヘルツ表面波分割器の設計に応用し、GHz帯でデバイスを作成した。8～12.5GHzで放射パターンの測定をしたところ、計算結果と極めて良い一致を示し、設計の妥当性が確認できた。また、周波数分割の振る舞いが観察され、表面波分割の基礎的知見が得られた。研究成果の概要 (英文) : In this project, we have introduced the locally one-dimensional scheme to improve the efficiency of the finite-difference time-domain (FDTD) method in cylindrical coordinates. In addition, we have developed a semi-implicit FDTD method in cylindrical coordinates, in which the implicit scheme is adopted only in the circumferential direction. Using these technique, the computation time is successfully reduced to 1/3 to 1/10 of that of the traditional explicit FDTD method in cylindrical coordinates. We have applied these methods to the design of a metal disc-shaped surface wave splitter and fabricated one at GHz frequencies. The radiation patterns were measured and compared with the numerical results obtained from the developed FDTD method in cylindrical coordinates. It has been found that the measured results agree well with the numerical results, demonstrating the validity of the design. We have also observed the behavior of the frequency splitting

Development and design of a dielectric optical waveguide with reduced bend loss

研究成果の概要 (和文) : 屈折率差の大きい強導波路が屈曲した場合の損失を明らかにするために、円筒座標系フルベクトル虚軸ビーム伝搬法に強調した増幅係数を導入する手法を開発した。これを用いて、埋め込み型シリコン細線導波路のコア位置が伝搬特性に及ぼす効果を明らかにした。特にコアの縦横比が屈曲損に及ぼす効果を検討した。結果として、界分布のピーク値から５％に当たる位置に空気界面からの埋め込み量を選ぶと、効果的に屈曲損を低減できることを明らかにした。また、コア側壁のラフネスがどの程度屈曲損を増加させるかを明示した。加えて、直交座標系で屈曲導波路を解析する際に多用されている変換屈折率法に関して、より適切な変換法を明らかにした。研究成果の概要 (英文) : The imaginary-distance procedure with an enhanced amplification factor is applied to the Yee-mesh-based implicit beam-propagation method in the cylindrical co-ordinate system. The developed technique is adopted to study the effect of core location in a buried Si-wire waveguide with various aspect ratios. Reduced bend loss is achieved when the air-cladding interface is placed at the position where the field amplitude decays to 5% of its peak. In comparison with the results obtained from the transformed index method in the rectangular co-ordinate system, we find the transformed technique based on an exponential function is superior to that obtained from a linear function.We finally investigate the effect of the sidewall roughness on the propagation behavior. It is found that the bend loss becomes largest when the correlation length is around 100 nm, and that the polarization crosstalk remains rather small, compared with the crosstalk generated with a tilted sidewall