由分裂共振環所構成的超穎材料具有異常的電磁共振現象，且對周圍環境折射率變化的偵測非常敏感。將超穎材料的外觀尺度設定在微米等級，能夠使其共振頻率發生在兆赫波。因此，使用超穎材料開發可調控式兆赫波元件是非常合適的。常見調控超穎材料共振頻譜的方法可以藉由液晶、微機電製程、半導體材料、電子元件與金屬氧化物所達成。由於液晶具有大的折射率異向性的特性，在這五個方法中，利用液晶製做可調控式超穎材料元件具有低成本、製程容易、多控性。因此本論文將結合液晶與具有超穎材料的軟性基板研究它們之間的物理機制，並發展可調控式超穎材料元件、兆赫波帶阻濾波器、被動式兆赫波調控頻率器與兆赫波頻率隔離器。
本論文題目為『沉積在軟性基板上超穎材料的電磁共振』。主要工作包含四個部分，簡列如下:
(1) 第一部分工作主題為『利用沉積在聚二甲基矽氧烷的超穎材料偵測具有相似折射率的有機溶劑』。由過去文獻中了解超穎材料可感測有機溶劑，由於超穎材料為人造的週期結構並且對元件周圍環境折射率的改變非常敏感，所以適合做為兆赫波感測器。但有機溶劑在具有相似折射率情況下，兆赫波頻譜就會有相同的共振峰值，無法分辨出有機溶劑的種類。本實驗是用簡單製作的聚二甲基矽氧烷樣品盒搭配超穎材料並利用酒精稀釋甲苯溶劑當待測物。實驗的結果發現，在兆赫波頻譜下，酒精、0.5 wt%甲苯溶劑與1.0 wt%的甲苯溶劑擁有相似折射率，利用低濃度甲苯溶劑，檢驗元件對甲苯溶劑的敏感性。低濃度甲苯，會讓聚二甲基矽氧烷元件中的超穎材料溶脹，因此使超穎材料尺度上有些微變化。聚二甲基矽氧烷的溶脹，會使訊號頻譜移動，且甲苯溶劑濃度越高，越會讓頻譜產生紅移。藉此方法偵測相似折射率且低濃度的有機溶劑，最低可檢測0.5 wt%的甲苯溶劑，具有開發出檢測低濃度有機氣體的潛力。
(2) 第二部分工作主題為『利用具有兆赫波超穎材料的液晶彈性體薄膜開發可連續調控的光強度調製器』。本工作利用具有快速冷卻系統的熱蒸鍍機以及金屬遮罩將超穎材料沉積於液晶彈性體薄膜。使用藍光雷射彎曲液晶彈性體薄膜量測具有超穎材料之液晶彈性體薄膜在不同彎曲角度下的兆赫波頻譜。實驗結果發現，具有超穎材料之液晶彈性體薄膜在共振頻率處的穿透率會隨彎曲角度的增加而增加。當薄膜彎曲角度為0度 (90度)時，超穎材料在共振頻率處的穿透率為0.0036 (1)，開關比為277。透過實驗發現彎曲角度隨著光強度的增強遞增，超穎材料的光譜波峰的透射率隨之增加，進而導致兆赫波訊號的衰減。在恆定的兆赫波頻率並可連續調控兆赫波穿透強度的機制，可以開發出大開關對比度的強度調製器，應用於兆赫波影像、不同頻率的帶阻濾波器與6G通訊系統中的頻率隔離器。
(3) 第三部分工作主題為『利用具有超穎材料的液晶盒開發出被動式兆赫波頻率調控濾波器』。此部分的工作為利用具有超穎材料塑膠基板封液晶盒，樣品內的液晶會因為配向方向不同而感受到不同折射率變化。本實驗的液晶盒為將配向方向與分裂共振環缺口夾角0度、45度與90度，並量測兆赫波頻譜的變化。實驗發現共振頻率會隨著配向方向與分裂共振環缺口夾角的增加而增加，最大的頻率調控偏移量為18 GHz。由模擬結果可知，共振頻率的增加是因為液晶雙折射的特性所引起的，而在兆赫波下，液晶的折射率異向性為0.15。因此具有分裂共振環結構液晶盒可以通過配向膜不同的配向方向，被動式調控共振頻率。可以應用在兆赫波通訊、兆赫波感測器與兆赫波影像上。
(4) 第四部分工作主題為『利用具有超穎材料不同厚度的液晶盒，找出對兆赫波可偵測最大的頻譜移動量』。液晶雙折射的特性可製作調控式超穎材料元件。
但從文獻中了解，用液晶調控折射率，難以達到較大的頻譜移動率。是因為液晶盒內配向層的錨定力有限，太厚或太薄的液晶層都難使兆赫波具有較大的頻譜移動量。太厚的液晶盒會容易造成液晶不整齊的排列，兆赫波會感受到液晶較小的雙折射率。太薄的液晶盒會使兆赫波感受的ne與no非常接近，因此頻譜移動量小。因此本實驗將研究液晶層厚度對超穎材料共振頻率偏移率的影響。液晶層厚度從310 m增加到1487 m，找出對兆赫波最佳化的頻譜調控率。實驗發現，波長效應與配向效應互相影響下，厚度為710 m的液晶盒具有最大的頻率調控偏移量為21 GHz。

Metamaterials, which comprise arrays of split-ring resonators, are sensitive to the refractive indices of the media that surround the metamaterials. The resonance frequencies of metamaterials can be set in the terahertz regime using microscale SRRs. Therefore, the metamaterials are ideal for use in terahertz devices. The resonance spectra of metamaterials can be tuned by liquid crystals (LCs), micro electro mechanical systems, semiconductor materials, electronic devices and metal oxides. Tunable terahertz metamaterials based on LCs have the advantages of high reliability, low cost and ease of fabrication. This thesis studies the effect of soft substrates on the electromagnetic resonance of metamaterials, and presents continuously tunable terahertz intensity modulators and terahertz band-stop filters.
This thesis entitled “Electromagnetic resonance of metamaterials deposited on soft substrates” includes four works, and the four works are listed as follows”
(1) The title of the first work is “Detection of organic solutions with an identical refractive index using metamaterials deposited on polydimethylsiloxane films.” A simple method for detecting organic solutions with a similar refractive index using metamaterials that are deposited on polydimethylsiloxane films is proposed. Pure toluene is diluted with pure alcohol for making up toluene solutions with concentrations of 0.5 wt% and 1.0 wt%. The experimental results from terahertz time-domain spectroscopy depict that pure alcohol and the toluene solutions have a similar refractive index in the terahertz region. The spectra of the metamaterials that are exposed to the toluene solutions are redshifted from that of the metamaterial that is exposed to pure alcohol, and the resonance frequencies of the metamaterials decrease with the increase in the concentrations of the toluene solutions. This result arises from the fact that the swelling of the polydimethylsiloxane films in the toluene solutions extends the arms of the split-ring resonators, changing the dimensions of the metamaterials. The minimum detectable concentration of the metamaterials is less than 0.5 wt% when they are exposed to the toluene solutions. The metamaterials deposited on the polydimethylsiloxane films can detect the organic solutions with a similar refractive index, so they have the potential to sense organic gases with low concentrations.
(2) The title of the second work is “Continuously tunable intensity modulators with large switching contrasts using liquid crystal elastomer (LCE) films that are deposited with terahertz metamaterials.” A LCE film is successfully deposited with a terahertz metamaterial using thermal evaporation via a programmed electronic shutter and high-efficiency cooling system. The transmittance of the metamaterial at its resonance frequency is monotonically increased from 0.0036 to 1.0 as a pump beam bends the LCE film, so the metamaterial has a large switching contrast of 277 at the frequency. The monotonic increase in the resonance transmittance arises from the constant resonance frequency of the metamaterial at the transmittance modulation, and depicts that the metamaterial-deposited LCE film can continuously tune the transmitted intensity of a terahertz beam. The metamaterial-deposited LCE film has potential in developing continuously tunable intensity modulators with large switching contrasts for the application of terahertz imaging and terahertz communication. Therefore, the thermal evaporation expands the application of metamaterials and improves their optical properties.
(3) The title of the third work is “Passively tunable terahertz filters using LC cells coated with metamaterials.” Liquid crystal cells that are coated with metamaterials are fabricated in this work. The LC directors in the cells are aligned by rubbed polyimide layers, and make angles θ of 0°, 45° and 90° with respect to the gaps of the split-ring resonators of the metamaterials. Experimental results display that the resonance frequencies of the metamaterials in these cells increase with an increase in θ, and the cells have a maximum frequency shifting region of 18 GHz. Simulated results reveal that the increase in the resonance frequencies arises from the birefringence of the LC, and the LC has a birefringence of 0.15 in the terahertz region. The resonance frequencies of the metamaterials are shifted by the rubbing directions of the polyimide layers, so the LC cells coated with the metamaterials are passively tunable terahertz filters. The passively tunable terahertz filters exhibit promising applications on terahertz communication, terahertz sensing and terahertz imaging.
(4) The title of the fourth work is “Effect of thicknesses of LC layers on shift of resonance frequencies of metamaterials.” A LC layer that is too thick exhibits a small terahertz birefringence due to the limited long-range force of the alignment layers that exert on it. A LC layer that is too thin has a small terahertz birefringence due to its invisibility to incident terahertz waves. Therefore, a LC layer may have a large terahertz birefringence at a specific thickness. It is well known that the birefringence of a LC layer dominates the shift of the resonance frequency of the metamaterial which is imbedded into the LC layer. As a result, this work studies the effect of the thicknesses of LC layers on the shift of the resonance frequencies of metamaterials. Liquid crystal layers with various thicknesses that range from 310 um to 1487 um are deposited on terahertz metamaterials, and each of the layers is aligned by two polyimide layers that are rubbed in a direction. The terahertz metamaterials have a maximum frequency shifting range of 21 GHz as 710-μm-thick LC layers with mutually orthogonal rubbing directions are deposited on them. The maximum frequency shifting range arises from the competition between the long-range force of the polyimide layers and the interaction between the LC layers and their incident terahertz waves.

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