以有限差分時域法分析次波長結構之穿透與散射特性

Abstract

本篇論文採用有限差分時域法模擬研究以金屬為主的次波長元件。我們使用D ru d e 色散模型模擬金屬，並使用具有色散的單軸異向性完美匹配層作為相對應的計算空域吸收邊界。我們旨在研究次波長狹縫的異常光學透射現象。先前曾有文獻指出當p-極化的光波入射於一個具有單狹縫的金屬薄膜，並且在金屬表面製造凹槽陣列，在透射的頻譜上將會出現劇烈的峰值。在光波的入射面製造凹槽可以增強穿透率，而在光波的出射面製作凹槽可以調整透射光的發射方向。我們計算透射頻譜並觀察每個要素包括金屬薄膜的厚度、狹縫寬度、凹槽深度、凹槽週期以及凹槽的數目對頻譜的影響，並且嘗試著不對稱的排列凹槽。我們也檢查了由完美導體或是真實金屬製成的的薄膜，對於穿透特性的影響。我們嘗試設計這些結構，針對完美導體，理論上可以得到相對於入射光高達1 9 2 倍的穿透效率。這些現象可以用狹縫中的共振模態解釋。我們也探討了其他週期性結構的穿透特性，包括週期性排列的介電質和導體圓柱，以及探討得以造成表面增強拉曼光譜的粗糙金屬表面的電磁特性。

In this research, a two-dimensional finite-difference time-domain (FDTD) analysis is applied to model and study several subwavelength metallic structures. The Drude model for metallic material dispersion is implemented into the FDTD algorithm along with the dispersive uniaxial perfectly matched layer (UPML) as the absorbing boundary condition for the computational domain. This model is employed to study the extraordinary optical transmission of subwavelength slits. Previous literature has shown that when p-polarized wave impinging upon a corrugated metal thin film with a single slit, sharp peaks present on the transmission spectra at wavelengths much larger than the slit width. While the grooves in the input side of the thin film can enhance the transmission, grooves in the output side can shape the emerging radiation pattern. Here we calculate the transmission spectra and scan every factor that controls the transmission, including film thickness, slit width, groove depth, groove period, and the number of grooves. We also test the asymmetric groove patterns. The difference in transmission properties between the perfect electric conductor (PEC) thin film and the metal thin film is examined. We try to design the structure, and the maximum transmission enhancement up to 192 is observed for the PEC case. The resonance modes inside the slit can be used to explain these phenomena. The transmission properties of other periodic structures including periodic arrays of cylinders made of dielectric and metal are also analyzed. The electromagnetic effects on rough metallic surface for surface enhanced Raman scattering are studied.