自從1998 年T. W. Ebbesen 等人分別在Natrue 以及Physical
Review B 期刊上發表次波長金屬孔洞陣列的穿透光異常增強現象，
並且引起之後廣泛相關的研究。另外也可以在次波長金屬狹縫周圍放置週期性的微結構用來得到較高的光穿透效率，而擁有高穿透效率的次波長結構就可以利用作為近場光儲存，近場顯微鏡和光學元件。
利用表面結構對於次波長狹縫得到較高的光穿透效率而言，其傳
遞過程可以分成三個步驟：藉表面微結構使得入射光耦合到金屬表面傳遞，耦合到表面的光傳遞經過金屬狹縫，藉著出射面微結構使的在出射面的光再次的散射到自由空間中。明顯的，當入射面有越多的能量耦合到表面，就會得到更高的穿透效率。雖然已經有很多對金屬奈米狹縫的光穿透效率增強機制的研究，但是他們所得到的穿透效率依然很低，一部份的理由就是表面微結構激發的能量在經過其他表面結構的時候產生散射，產生能量的散失。
在此，我們利用CDEW 以及FDTD 演算法去分析奈米金屬狹縫
的光學特性和金屬表面奈米結構的散射效應。藉此，我們求得一高轉換效率及低散射的奈米結構並利用此結構去做為奈米狹縫的光穿透效率增強機制，使得奈米金屬狹縫的穿透效率高達27%。

Since Ebbesen et al. reported the observation of enhanced transmission of light through nano hole arrays in an optically opaque metallic film, this observations have
stimulated a large body of research interesting. Beside a nano hole array, it was found that that a nano aperture surrounded by small periodic corrugations on the entrance
plane of a metallic film can also perform a large transmission enhancement. This kind of device thus holds an immense potential for use in applications, where both high throughput and high resolution are required, such as near-field data storage, near-field microscope and photonic crystal coupler.
It has been discussed that the transmission process through a trench-surrounded slit can be separated into three independent steps: coupling in, transmission through
the aperture and coupling out. It is straightforward that the more free-space light is converted into surface waves and then coupled into the nano-scaled aperture, the
higher the transmission is. Although many researches focused on enhancing the transmission through a nano-scaled metallic slit, the transmission is still too low for
practical applications. Part of the reason for this is that some of the energy is lost during the propagation toward the aperture due to scattering by the surface
corrugations.
In this thesis, we analyze the optical properties of a nano-scaled meatallic slit and the scattering effect of the patterned trench structure on the entrance plane using both CDEW and FDTD method. A trench with low scattering loss was designed. An overall transmission over 27% (Normalized by 6.5μm Gaussian beam) through a nano-scaled slit can be achieved by bordering the low scattering loss trenches.

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