兆赫波，介於電磁頻譜的可見光和微波之間，頻率範圍約為0.1THz~3THz，擁有其它電磁頻段所沒有的特殊性質，例如可以直接辨識生物分子和高資料容量運載的能力，近二十年來激起廣泛的研究，使得其產生和偵測技術突飛猛進，許多不同的應用相繼被開發出來。特別是兆赫波分子感測和影像技術，以及兆赫波通訊元件之開發，都是近期兆赫波科技中最迅速發展的領域之一。本論文設計和製作各種毫米尺寸的周期性金屬陣列，利用此結構同時實現兆赫波通訊以及分子感測元件。包含兩種兆赫波濾波器：高通濾波器(high pass filter)和帶阻濾波器(band rejection filter)，以及高靈敏度之氣體和奈米薄膜分子感測器。
透過簡單的UV光立體微影技術(Microstereolithography，uSL)，我們成功將UV聚合材料製作成尺寸可自由調控並且具高高寬比(High-aspect ratio)之三維圓柱陣列。基於表面電漿效應，我們實驗上首次證明：在TE極化入射下，僅僅一列和兩列之金屬圓柱即可實現兆赫波高通濾波器，實驗上觀察到該元件之最低截止頻率隨著金屬圓柱陣列之周期上升而下降，符合理論之預期。該元件具有製程簡單迅速(曝光時間短，元件之完成僅需數分鐘)、和可透過調整圓柱陣列周期而大幅調變截止頻率(圓柱週期每變化100um，截止頻率飄移100GHz)等優點。
利用多排數之金屬陣列，以TM極化入射，透過金屬圓柱間的強布拉格散射，我們也成功實現兆赫波帶阻濾波器，該元件之濾波消光比(extinction ratio)可大於23dB，並且截止頻帶(forbidden band)之中心頻率以及頻寬皆可透過調整金屬圓柱之尺寸(周期、直徑和圓柱排數)而產生明顯調變。實驗結果顯示當金屬在結構中之佔有率提升，截止帶頻寬會明顯增大，而圓柱週期主要是控制截止頻帶之中心頻率，實驗結果和FDTD理論計算結果具相當之吻合。
在分子感測方面，分別利用上述證明之元件做分子薄膜和蒸氣分子的感測。薄膜感測主要利用多排金屬圓柱產生的近場表面波干涉原理來實現，實驗上得到可感測的聚丙烯(polypropylene, PP)薄膜最小厚度差異可達到1.03um (~波長/544@520GHz)，這是目前利用光子晶體結構在兆赫波頻段下之薄膜偵測靈敏度是最高的。此外，我們利用該元件成功也分辨SiO2和ZnO兩種不同材料的奈米薄膜。在偵測揮發性蒸氣分子方面，主要利用金屬圓柱對周圍介質折射率的高敏感性作為偵測原理，由於折射率差異產生高通濾波器之截止頻率飄移，我們在實驗上證明不但可分辨不同濃度之甲醇蒸氣分子，最小辨識濃度差異可低於11.6%，相當於0.667(nano-mole/mm3)之氣體分子密度差異；也可成功分辨酒精和甲醇等不同氣體分子。該元件之最低可偵測氣體分子密度差異可達到0.67(nano-mole/mm3)，是目前兆赫波氣體偵測器之靈敏度最高。該實驗結果對未來應用於具揮發性危險液體以及生物薄膜的分析有著不錯的淺力。

Terahertz (THz) radiations are electromagnetic waves sandwiched between visible lights and microwaves and their frequencies are ranging from 0.1THz to 3THz. THz wave has several unique properties such as to directly identify biological molecules and with a high data capacity for communication application. Recently, THz technology grows rapidly and various THz applications have been extensively developed, which are attributed to the great advances of THz generation and detection techniques. Especially, THz molecular sensing, bio-imaging, as well as development of various active and passive devices for THz communication application attract lots of attentions in recent years. In the thesis, we designed and successfully manufactured various periodic metallic rod arrays (MRA) with millimeter sizes for THz photonic devices and molecular sensing applications. Based on this structure, we successfully demonstrate a multifunctional THz filter which enables to act as a high pass filter and a band rejection filter. In addition, we utilized the MRA to successfully demonstrate minute gas molecules and nano-films detections.
Firstly, we have successfully fabricated a three-dimensional dielectric cylindrical array with high-aspect ratios by using photo-polymers through a simple ultraviolet light microstereo-lithography (uSL) process. Based on surface plasmon effect, we demonstrated for the first time that the single and two rows of MRA are able to act as a THz high-pass filter under TE polarized excitation. The experimental cut-off frequency of the high pass filter is decreased when the period of the rod array becomes larger, and it is agree well with the theoretical result. The home-made THz high pass filter based on MRA has advantages including: the simple and fast fabrication due to the short UV exposure time, and with a high tunibility of cut-off frequency based on adjusting rod diameter. The maximum spectral shift of cut-off frequency is demonstrated over than 100GHz for 100um-long period variation.
Secondly, we have successfully demonstrated a THz band rejection filter utilizing the multi-rows of MRA under TM polarized excitation. Based on the strong scattering between the metallic rods, the obvious bandgap caused by Bragg reflections has been observed and its best power rejection ratio is demonstrated greater than 23dB. Additionally, the central frequency and bandwidth of the Bragg forbidden band enable to be tunable significantly by changing the dimension of MRA such as the period, rod diameter, and array-columns. The experimental results show that the forbidden bandwidth is significantly increased with the occupied ratio of the metallic rods in the device chip due to the large scattering cross section, and the central frequency of forbidden band can be controlled by the period of metallic rods. The experimental results and the FDTD theoretical calculations make well a consistence.
For molecular sensing applications, we have respectively demonstrated the minute vapors sensing and nanofilm detection utilizing the MRA operating at THz frequency ranges. For thin film sensing application, we have successfully demonstrated to sensitively detect the subwavelength-thick polypropylene film covered on the rod arrays based on the refractive index induced phase change, and the detection limit of the thickness variation can be down to 1um (~wavelength/544@520GHz) which is the highest sensitivity for thin film detection based on the THz photonic crystals devices. In addition, we also successfully identified two kinds of semiconductor nanofilm made by SiO2 and ZnO based on the MRA. For volatile vapors sensing application, we successfully identified various kinds and different concentrations of vapor molecules by using our THz high pass filter consisted of single row of metallic rods. The metallic rods in the THz high pass filter device are highly sensitive to the surrounded refractive indexes of materials, and it causes the spectral shift of the cut-off frequency while index change of the surrounding is generated. We have successfully both identified various concentrations of methanol vapors with the minimum detectable concentration variation of 11.6% equivalent to molecular density of 0.667 nano-mole/mm3, as well as distinguished different types of gas molecules such as alcohol and methanol. Based on the THz high pass filter device, the measured minimum detectable gas molecular density variation can reach 0.67nano-mole/mm3 which is the highest sensitivity of current THz gas sensors. The sensing results have potentials applied for future detection of dangerous gas leakage and biomolecular membrane.

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