4.4 改變半同軸狀金屬孔洞之間的排列
4.4.2 半同軸狀金屬孔洞之間的垂直位移
圖 4.4.2.1 所示為垂直方向位移的正方形晶格半同軸狀次波長金屬孔洞陣列，在實驗 中我們設計並且製作水平位移量（wy）為 0µm，50µm，100µm，200µm，以及 300µm 的樣品。
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圖 4.4.2.1 垂直方向位移的正方形晶格半同軸狀次波長金屬孔洞陣列；黃 色的部分為孔洞（空氣），黑色的部分為金屬。
圖 4.4.2.2 為水平方向位移的正方形晶格半同軸狀次波長金屬孔洞陣列穿透率對應 頻率圖，在圖中我們可以發現穿透率峰值集中在 TE11模的截止頻率附近，並且隨著水平 方向位移量改變而隨之變化。
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圖 4.4.2.2 垂直方向位移的正方形晶格半同軸狀次波長金屬孔洞陣列穿透 率對應頻率圖。
為了更清楚的觀察穿透率峰值隨著垂直方向位移量增加的變化，因此將圖 4.4.2.2 中的穿透率峰值頻率對應垂直方向位移量做圖。圖 4.4.2.3 顯示當水平位移量增加時，穿 透率峰值的頻率會往低頻的方向移動。而圖 4.4.2.4 則顯示當垂直位移量增加時，穿透率 峰值的半高寬會隨位移量上升而變大。由於在樣品中金屬孔洞的週期固定為 800µm，所
以表面電漿子的影響是固定的，因此主要的影響因素來自於局域性波導共振。由於組成 半同軸狀次波長金屬孔洞中的兩個扇形金屬波導之間的垂直距離改變，因而使得電磁波 在金屬孔洞的入射端以及出射端的反射係數產生改變 ，並且導致穿透率峰值頻率以及半 高寬隨之變化。
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圖 4.4.2.3 垂直方向位移的正方形晶格半同軸狀次波長金屬孔洞陣列穿透 率對應垂直位移量圖。
圖 4.4.2.4 垂直方向位移的正方形晶格半同軸狀次波長金屬孔洞陣列穿透 率峰值半高寬對應水平位移量圖。
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第五章 結論
在上述的實驗中，我們發現在兆赫波段中由於金屬接近於完美導體，因此其穿透率 現象不僅僅只受到表面電漿子的影響，局域性波導共振在兆赫波段的次波長金屬孔洞陣 列的異常光穿透率現象上亦為極其重要的因素。因此我們設計不同晶格常數以及週期的 次波長半同軸狀金屬孔洞陣列，藉以觀察由週期性表面所耦合的表面電漿子以及單一金 屬波導中的局域性波導共振次兩種機制對於異常光穿透率現象的影響。
在改變晶格常數的實驗中，我們發現具有異常光穿透率峰值主要位於金屬孔洞的截 止頻率附近，當我們增加孔洞之間的距離時，其穿透率峰值會往低頻移動並且在低於截 止頻率後會快速的變小。此外，我們以時域有限差分法模擬在穿透率峰值時金屬表面上 電場的分佈情形，並且我們發現在表面電漿子耦合頻率大於或是接近於截止頻率時，穿 透率峰值主要受到局域性波導共振的效應所影響，而當表面電漿子耦合頻率小於並且遠 離截止頻率時，在金屬表面上出現明顯的散射電場，並且此時我們發現相對應的歸一化 穿透率則小於 1。
在改變樣品厚度的實驗中，我們發現當樣品的厚度增加時，其第一組峰值的頻率幾 乎是不隨厚度改變，並且將其和最低模態的共振腔條件互相比較可得知，此峰值最主要 是來自於局域性波導中所假設的最低階波導模態在其中的 Fabry-Perot 共振現象；而我們 還發現第一組峰值的值會隨厚度上升而降低，這最主要是因為在兆赫波段下且金屬片厚 度遠大於兆赫波的衰減常數時，當金屬片厚度增加時原本在其金屬孔洞中共振的消逝波 因來回共振的距離增加使得衰減變大。而第二組峰值則會隨著金屬厚度改變而變，因此
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我們將其與次低階的共振腔條件互相比較發現，其峰值頻率的改變雖然和預測的有誤差，
但是趨勢向相同的。
最後，在實驗中我們將半同軸狀金屬孔洞中的兩個扇形金屬波導之間的排列做改變，
並且發現當兩個扇形金屬波導的位移量增加時，其穿透率峰值的頻率會隨之往低頻的方 向移動，而其穿透率峰值的半高寬值也會隨之增加。然而造成這個現象的最主要原因為 組成半同軸狀次波長金屬孔洞中的兩個扇形金屬波導之間的距離改變，因而使得電磁波 在金屬孔洞的入射端以及出射端的反射係數產生改變 ，並且導致穿透率峰值頻率以及半 高寬隨之變化。由以上三大類的實驗結果及分析我們認為在兆赫波段下造成異常光穿透 率的主因為有限厚度的金屬孔洞陣列中的局域性波導共振現象，雖然模擬中顯示類似於 表面電漿子的電場分佈在金屬表面上，但對照於歸一化穿透率時則發現其對異常光穿透 率並無貢獻，因此在兆赫波段中的異常光穿透率主要以局域性波導為主。
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