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標題: | 週期性孔洞對次波長圓環穿透率影響之研究 Research and Development on the Transmission of Periodic Hole Structure based Sub-wavelength Annular Aperture |
作者: | Chun-Hung Weng 翁郡鴻 |
指導教授: | 李世光 |
關鍵字: | 貝索光束、次波長圓環,次波長聚焦,雷射加工,光子井,side lobes, Bessel beam,subwavelength annular aperture,subwavelength focus,laser machining,photon sieves,side lobes, |
出版年 : | 2014 |
學位: | 碩士 |
摘要: | 傳統光學中,使用短波長與增加透鏡數值孔徑可以增加空間解析度,但相對的,焦深受到這兩項參數所影響。貝索光束之次波長聚焦能力可以突破繞射極限,使光學系統中焦深與空間解析度的連動關係受到改善。近年來,Ebbesen等人發現金屬周期性結構產生表面電漿效應,並使穿透效率增加,且可透過此結構讓光束具有指向性。本研究團隊過去幾年陸續提出不同結構的次波長圓環結構,其中包含利用拉伸中空毛細管材料形成單圓環結構光學頭,可產生具有次波長尺度之光學頭。但因中空光學頭尺寸不易控制,亦曾改用二氧化矽基板,於其上沉積金屬層,並利用聚焦離子束於金屬層製作次波長圓環結構來形成貝索光束。而應用於微影製程與雷射加工上時,可製作出高深寬比之結構;但前述形成貝索光束的方法仍有side lobes能量過高與穿透能量較低兩個問題,這兩個問題使得加工時能量超過加工閥值之side lobes也會被記錄下來,形成除了主光束以外之同心圓環,也因此產生加工不精確的潛在問題。
本研究基於前述研究,確認要降低side lobes與main lobe的比值,可以考量降低光束side lobes的能量,或是改善貝索光束main lobe的穿透能量。本研究嘗試運用孔洞排列來構成近似於次波長圓環的次微米結構,以替代次波長圓環(SAA)。換言之,運用孔洞排列來構成近似於次波長圓環之所謂「類次波長圓環(PSAA)」,來形成無基板金薄膜上之孔洞,以求降低side lobes或增加類次波長圓環結構的透射能量乃是本論文研究的重點。 本論文以製作可產生低side lobes光束與可增加貝索光束的能量之結構為目標,由孔洞尺寸、孔洞間距與圓環間距各項參數開始選擇,討論製作無基板結構之材料、方法與各種規格優劣,將矽晶圓一面蒸鍍金薄膜,另一面使用背蝕的方式製作方形孔洞,由其他區域支撐金屬薄膜。而本研究使用有限時域差分法電磁模擬軟體,對次波長結構各種參數進行模擬,討論其最佳之設計方式。接著為次波長結構出光特性之探討,同時使用自製橫向顯微系統進行光強量測實驗;討論其聚焦效應,並重建光束以求得完整光束外觀,可方便與模擬進行比對,且驗證此結構具有長焦深聚焦能力,與形成的光束性質之改善,而希望能夠整合於次波長微影製程與雷射加工平台。實驗結果確認本論文所開發的PSAA結構,可以產生較過往SAA結構更符合雷射加工需求的貝索光束,也為進一步開發次世代雷射加工平台中,最重要的光學頭元件打下良好的進階基礎。 In traditional optical system, either shorter wavelength incident light or higher numerical aperture lens can lead to spatial resolution improvement. However, the depth of focus also depends on the above-mentioned two parameters. Subwavelength focus of Bessel beam breaks free the strong interaction among depth of focus and focus spot size. In recent years, Ebbesen et al. discovered that surface plasmon generated by periodic subwavelength aperture on metallic film can lead to unexpected large transmission and directional beaming effect. Our research group proposed different configuration of subwavelength apertures. Preliminary studies include using tapered capillary tube to produce annular aperture on optical head, which generated subwavelength size spot by the aperture. However, due to the difficulty in controlling the size of the optical head, we also tried to create the optical head by first sputtering gold on SiO2 substrate and then manufactured subwavelength annular aperture (SAA) by using focus ion beam (FIB). This SAA structure was found to produce Bessel beam and was applied to produce high aspect ratio structure. However, the problems associated with high side lobes energy and low transmission efficiency remain in the above-mentioned design approaches. It is to be noted that the part of the side lobes with energy higher than that of the machining threshold will be recorded. The concentric circles apart from main lobe caused by the high side lobes energy thus caused imprecise machining, which limited further advancement of Bessel-beam based laser micromachining. Based on the understanding generated from prior researches, it is confirmed that reducing the energy ratio between the side lobes and the main lobe can be achieved either by reducing the side lobes energy or by enhancing the Bessel beam main lobe transmission energy. Based on these preliminary studies on different structures, constructing pseudo-subwavelength annular aperture (PSAA) using hole array arranged in a way to mimic SAA on simple gold film was adopted as the main focus of this thesis with an attempt to improve transmission efficiency and lower the side lobes of the Bessel beam emitted. Taking the goal of using PSAA to lower side lobes and to improve transmission, we started from choosing parameters related to the size and distance of the holes and distance of rings of PSAA. Gold film was sputtered on one side of silicon wafer, and the other side of wafer used back-etch method to manufacture square holes. The non-etched remaining area supported the thin gold film. This study used finite-difference time-domain (FDTD) method to do the electromagnetic simulation. Different parameters associated with subwavelength annular aperture were identified to fabricate the structure. Moreover, light intensities and Bessel beam profiles at different distances from the PSAA was measured by using a modified microscope so as to verify the simulation and inspect the properties enhancement in Bessel beam. Finally, possibilities of using these PSAA structures to create a subwavelength laser machining system were examined. The experimental results obtained from the Bessel beam generated by the PSAA structures developed in this thesis confirmed the superiority of the newly created Bessel beam for laser micromachining. The PSAA structures developed appeared to form a good advancement platform for next-generation laser micromachining needs. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/55118 |
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顯示於系所單位: | 工程科學及海洋工程學系 |
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