次波長週期結構之異常吸收現象

Abstract

次波長週期結構在電磁波的研究領域中一直扮演一個重要的角色。當結構為週期性排列時具有可調式共振，以及次波長結構的週期是小於入射波長，可以用等效介質的觀念來探討物理現象，所以在設計元件方面有很高的自由度，所設計出的元件能擁有一般自然界無法擁有的特性，如負折射、後退波等等，這些被稱為超常材料。次波長週期結構在光學方面有許多有趣的物理現象，如異常穿透和異常吸收，而次波長週期結構會有這些物理特性都是來自於共振。 本論文目的在探討電磁波在次波長週期結構之異常吸收行為，重點在利用共振機制使得不易吸收的金屬產生異常高吸收，分成第三部分來探討。第一部份是固定凹槽深度改變凹槽的週期，探討在不同週期下的吸收情形。由於腔體共振使得電場強度在凹槽內部明顯地增強，發現表面挖很淺週期凹槽的時候會有異常全吸收，並嘗試利用等效介質理論去解釋其異常現象。而當h=30 nm的時候，發現高吸收的波長會隨週期增加先藍移後紅移。第二部分是在小週期(d=10 nm)與大週期(d=600 nm)下改變凹槽深度，探討在不同深度下的吸收情形，發現在波長大於600 nm之後會有Fabry-Perot共振所造成高吸收，在大週期下嘗試用MIM的波導去近似。比較特別是在大週期下，還會有Wood異常現象所造成的吸收。並且在小週期下，成功利用等效介質得到一樣的現象。第三部分改變形狀為正弦曲面探討週期與深度改變對吸收的影響，發現要有異常全吸收，週期與深度之間必須要搭配適當，而且對不同材料的搭配也不同。

Subwavelength periodic structure plays an important role in the electromagnetic science. Caused by periodic arrangement of structures, structures themselves are equipped with tunable resonance. Because the period of subwavelength structure is less than the incident wavelength, it can explore to the physical phenomena by the way of effective medium concept. With these properties, the design objects possess high degrees of freedom which can create physical characteristics that cannot be found in nature material, including the negative index of refraction and backward wave. These are called metamaterial. Subwavelength periodic structures have many interesting optical phenomena, such as extraordinary optical transmission and extraordinary optical absorption, and these properties are caused by resonance. This thesis aims to investigate the behavior of extraordinary optical absorption of the subwavelength periodic structure, and focus on the resonance mechanism. It lets the metal which does not absorb easily have extraordinary high absorption by resonance. The investigation has three parts. First, it is fixed at the groove height and varies the groove period to investigate the absorption situation in the different periods. Due to the cavity resonance, the electric field strength is significantly enhanced inside the groove. The structure with shallow grooves has abnormally total absorption, and it tries to use the effective medium theory to explain these anomalies. When the groove height is fixed at 30 nm, the wavelength of the high absorption is first blue-shift then red-shift with increasing of the groove period. Second, it is fixed at the groove period(d=600 nm and d=10 nm) and varies the groove height to investigate the absorption situation in the different heights. Found that when the wavelength is greater than 600 nm there is high absorption caused by Fabry-Perot resonance. In the greater period it tries to use MIM waveguide to approximate the absorption curve. Particularly, there is a Wood anomaly caused by absorption in the greater cycle. In the smaller period, we successfully use effective medium to simulate the same phenomenon. Third, it changes the groove shape from rectangle to sinusoidal surface. It investigates to absorption effect by changing period and height. It found that if one wants to have total absorption, the ratio of period and height must be precise in the different materials.