複合式銀-金圓型孔隙之次波長聚焦特性研究

Abstract

我們提出了結合金、銀的複合式金屬圓形孔隙的電漿透鏡。透過結構中多個不同寬度變化的圓形孔隙使入射波通過電漿透鏡後達到相位的調變以形成聚焦。透過雙層金屬複合的結構可以縮小結構聚焦的半高寬及提高結構的穿透率。 在本論文中，我們主要著重在透過改變奈米圓形孔隙的幾何結構參數優化並縮小聚焦的半高寬，包括: 金屬層厚度、比例及奈米圓形孔隙寬度，使其能夠產生理想的次波長聚焦。模擬以及實驗結果顯示此結構的次波長聚焦、穿透率等基本聚焦特性與結構的幾何參數有極大相關性。為了呈現此結構在次波長聚焦的特性，我們進行了近場量測分析。在掃描式近場顯微鏡實驗中，透過不同高度位置改變的量測，驗證與模擬得到相符的結果。在入射波長633 nm，量測複合式銀-金圓形孔隙電漿透鏡後110 nm的高度上可觀察到半高寬為285 nm的次波長聚焦。透過此研究，我們了解次波長聚焦的機制以及結構幾何參數的影響，對未來設計出更高解析度的電漿元件有極大的幫助。

We propose a metal plasmonic lens, Ag-Au hybrid circular nanoslits. By different widths of circular nanoslits to achieve the phase modulation of incident wave so that it can focus behind the plasmonic lens. Through the two metal combined nanostructure can reduce the FWHM of focus and improve the transmittance. In this thesis, we focus on reducing and optimizing the FWHM of focus by geometry tuning of circular nanoslits including metal thickness, ratio of metal and circular nanoslits width to generate ideal narrow subwavelength focusing. The results of simulation and experiment show that the focusing properties including subwavelength focusing, transmittance are dependent on the structural geometry. In order to rendered the sub-wavelength focusing characteristics, we conducted a near-field measurement analysis. In the scanning near-field microscope experiments, we measure optical near field at different height position to get consistent results with simulation. At incident wavelength 633 nm, the focus is at height 110 nm above Ag-Au hybrid circular nanoslits and reaching the subwavelength focusing with FWHM = 285 nm. Through this study, we investigate the mechanism and geometrical influence of sub-wavelength focusing. It provides the design rules of plasmonic lens for excellent resolution in the future.