奈米金屬線結構在光電元件之研究與應用

Optical Properties of Metallic Nanowires and Their Applications in Photonic Devices

許勝裕(Shen-Yu Hsu)

指導教授 ： 李明昌 ； 魏培坤

國立清華大學/電機資訊學院/光電工程研究所/博士(2009年)

https://doi.org/10.6843/NTHU.2009.00379

摘要

This dissertation contains the studies and applications for three kinds of nanoscale metallic wire structures. The first one is the studies of subwavelength dielectric-loaded surface plasmon polariton (DLSPP) waveguides by using a near-field scanning optical microscopy combined with a high numerical aperture (N.A.) optical microscope. We verified the existence of anti-symmetrical mode single-mode in the DLSPP waveguide and observed different propagation properties of SPPs when the light source is launched at different positions near the waveguide entrance. The second one is the polarization studies of subwavelength Au nanoslits. A giant form-birefringence larger than one is induced due to surface plasmon polaritons (SPPs) in gold nanoslit arrays. The last one is the applications of Au Nanowires array for enhancing the light extraction of light emitting devices.
The optical fiber tip which is widely used in near field scanning optical microscopy (NSOM) was employed to generate the surface plasmon wave (SPW) at the entrance of the DLSPP waveguide. Comparing with conventional end-fire coupling, the near-field has a higher spatial resolution and more broadband angular spectrum (in Fourier space). Besides, a high N.A. microscope can observe the radiation of DLSPP waveguide, and the optical images were collected by charge coupling devices (CCD). Previous research reported that there are symmetric and antisymmetric modes supported in the insulator-metal-insulator (IMI) waveguide. This thesis is the first one to experimentally confirm the antisymmetric modes. Furthermore, the NSOM and high N.A. microscope were employed to excite and observe, respectively, the bending DLSPP waveguides (L-shape). The results show the SPW can make a 90o turn in the bending waveguides at the special excitation region.
Large phase differences between transverse electric (TE) and transverse magnetic (TM) waves were investigated in plasmonic nanoslit arrays. The phase of the TE wave shifts ahead because of its low propagation constant. On the other hand, the phase of the TM wave is retarded due to the propagation of surface plasmons. The opposite phase shift forms a giant birefringence. The results show that its magnitude is dependent on the width of nanoslits. The birefringence magnitude was ~1 for 300-nm-wide nanoslits and up to ~2.7 for 100 nm ones (the world record was 1 in 2008). Besides, the spectroscopic measurements indicate that waveplates made of gold nanoslits have large bandwidths.
This dissertation also demonstrates the extraction efficiency enhancement of organic light emitting devices by fabricating Au nanowires on the indium tin oxide (ITO) anode. For the aluminum-tris-(8-hydroxyquinoline) (Alq3) base devices, a 100 nm-width and 450 nm-period gold nanowire array increases light extraction up to 46% from glass substrate and 80% from the organic layer. Such metallic nanowire arrays can double the brightness with small absorption, only 10% lower than ITO-coated glass. In addition, the colors extracted by the nanowires can be selected by the period of nanowire array. We extracted blue to red light extraction by using single Alq3-based device.

關鍵字

近場光學 ； 表面電漿波導 ； 雙折射率 ； 有機光電元件

並列摘要

本論文的主題為奈米金屬波導之研究與應用，在內文中探討了三種不同光在奈米金屬波導中的傳播行為與光學特性。首先，我利用近場光學顯微術結合洩漏模式光學顯微術來研究光在介電負載的表面電漿波導的模態。接下來，我們發現當光傳透過奈米尺度的金屬狹縫，會引發巨大雙折射性質。這個特別的光學特性，無論在我們理論的計算或是實驗的驗證中都擁有很好的一致性，同時我們也得到一個巨大雙折射率差高達2.7(2008年的世界紀錄是1)。最後在本論文中，我們利用週期性的奈米金線的區域化表面電漿共振引發的高散射能力去增加有機發光元件的出光效率。
首先，我們利用近場光學的光纖探針去產生表面電漿子波，進而激發數百奈米寬的通道式光學電漿子波導。相較於傳統的直接耦合激發，近場激發不僅具有較高的空間解析度，而且在富氏空間具有較寬的角度頻譜。在此，漏模式光學顯微術幫助我們直接觀察光在介電負載的表面電漿波導傳播的情形，同時也利用電荷耦合元件取得影像。在1986年的文獻中首度指出，在介電質-金屬-介電質類型的波導中存在著對稱性與非對稱性的傳播模態。到目前為止，只有對稱模態被人類所觀察且應用，但是非對稱模態至今只在理論的計算上被實現，本論文是第一個在實驗上證明非對稱模態。在研究中，一些特別的干涉現象成功地被發現，而且這發現在實驗上證明了非對稱性的傳傳播模態存在。除此之外，這個近場激發的裝置也被應用來研究表面電漿波導的轉彎特性。結果顯示只要在特定的位置激發，被激發的表面電漿子波的確可以在L型波導中作垂直的轉彎。
除此之外，在研究中我們發現當一道橫向電場跟一道橫向磁場的光同時穿透過一個週期性的奈米狹縫，此兩道光可以造成一個相位差。橫向電場的光相位會超前橫向磁場的相位，因為橫向磁場會引發表面電漿，使得傳播常數變大造成相位延遲，而這個相位延遲便會形成光學的複折射性質。在研究中，我們發現這個複折射性質跟狹縫寬度有直接的相關，當狹縫寬度為300奈米時，引發的複折射係數為~1，但是當狹縫寬度增少至100奈米時，所引發的複折射係數高達~2.7。此外，在光譜的量測中，利用奈米金狹縫所製成的1/4波長板具有很大的使用頻寬。
最後在這本論文中，利用週期性的奈米金線引發的區域化表面電漿共振效應所產生的高散射能力，增加發光元件的出光效率達一倍的亮度。除此之外，本研究還發現，藉由調變金線的週期還可以控制不同的顏色的光被取出，在這類型的元件中，從藍色到紅色皆可以順利地被取出。針對於Alq3系列的元件，以100奈米寬，450奈米週期的週期性金線陣列，可以從玻璃基板中多取出46%的光子，從有機材料中取出80%的光子。

並列關鍵字

Near field optics ； Surface plasmon waveguide ； Birefrigence ； Organic light emitting device, OLED

參考文獻

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