金屬奈米結構之光學感測

Metal Nanostructures for Optical Sensing

宋明哲(Ming-Je Sung)

指導教授 ： 黃鼎偉

國立臺灣大學/電機資訊學院/資訊工程學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.00752

摘要

本論文共提出三種以金屬奈米結構所設計的光學折射率感測器，分別是以有限元素法分析單顆壓花式矽奈米殼柱，與金屬球殼奈米柱呈六角狀排列產生的表面電漿場共振現象。此外我們也提出利用Fabry-Pérot共振腔原理設計液體微流道，並以傳輸矩陣法計算共振角。三種金屬奈米結構目標都是希望對待測物質的光學折射率變化產生高靈敏度感測。
前兩個研究題目:設計單顆壓花式矽奈米殼柱，以及將金屬球殼奈米柱作六角狀排列，目的是在金屬與待測介電物質間產生極大的表面電漿共振電場增強效應。我們也一併針對入射波長與角度、殼厚、介電物質的改變所產生的表面電漿共振現象作分析。由於表面電漿場對於介電材料折射率變化會產生明顯的共振波長位移，我們成功的利用不同形狀的奈米柱與改變奈米柱的排列方式，激發強烈的表面電漿共振場，達成折射率變化感測。
首先，我們設計壓花式矽包覆銀奈米殼柱。在 633 nm波長下，矽與銀的介電係數分別為 與 ，由於兩種材料的介電係數實部幾乎相抵銷，因此會造成矽與銀的介面產生極大的表面電漿等效折射率，此意味著在此介面的表面電漿波具有較低的相速度，因此會造成明顯的電場能量累積，同時若壓花式矽奈米殼柱週期設計使得入射光與銀矽介面的表面電漿波達成相位匹配，此結構就能激發較強的表面電漿共振電場，對待測物質的折射率變化具有較靈敏的感測能力。
更進一步地，我們也設計金屬奈米球殼包覆介電材料，並呈現六角狀的排列，我們期望不僅球殼與介電材料的介面產生表面電漿共振電場，奈米柱間也形成明顯的共振電場，以達成此六角狀排列奈米柱結構對待測物折射率變化有明顯的感測效果。我們並分別討論此結構對0°、15°、30°三種不同的波源入射角，所激發表面電漿共振電場的不同，經由共振波長的比較，得知15°波源入射角條件下的表面電漿共振電場，是0°與30°兩種條件的線性組合，因此整個呈六角狀排列的奈米柱結構可視為線性材料。
我們更進一步提出利用Fabry-Pérot共振腔原理設計液體微流道，就製作方法而言，可在玻璃基板鍍30-60 nm厚度金屬作為反射面，與另一塊經過深蝕刻的玻璃基板黏合後形成數百μm 的Fabry-Pérot共振腔，形成微流道型式的液體折射率感測器，由於此設計可視為多層膜結構，因此我們利用傳輸矩陣法計算共振角。並利用在大角度( 80 deg)的共振條件下，以增加光在共振腔內，上下基板間來回反射的行進距離，感測靈敏度達1000 deg/refractive index unit (RIU)，將近是先前研究文獻的四倍。此外，經由適當的共振腔厚度設計，可有效的縮小共振頻譜的半高寬至0.01°。因此，所設計的Fabry-Pérot微流道感測器角度解析度(sensing resolution )可達 RIU–1。
本文提供了多種利用金屬奈米結構達成光學折射率感測的設計。

關鍵字

金屬奈米 ； 光學感測

並列摘要

Here, to our knowledge, three kinds of metal nanostructures for optical sensing were reported. Firstly, a plasmon field enhancement in silver-core–protruded-silicon shell nanocylinder illuminated with light at 633 nm is presented. Secondly, a surface plasmon resonance in a hexagonal nanostructure formed by seven core-shell nanocylinders is also analyzed by using finite-element method. Finally, a high sensitivity Fabry-Pérot resonator based micro-fluidic sensor resonating at a large angle is numerically studied by use of transfer matrix method.
We show, to the best of our knowledge, the first simulation result of the strong plasmonic field coupling and enhancement at the Ag/Si interface of a silver core/protruded silicon shell nanocylinder by using the finite-element method. The strong plasmon field, with a slow effective phase velocity accumulated at Ag/Si interface, which results from the large effective index of the surface plasmon due to the nearly identical real parts with opposite signs of the permittivities of silver and silicon at 633 nm is analyzed. When the silicon shell has shallow protrusions of proper periodicity to meet the phase matching condition between the incident light and the surface plasmon wave at the Ag/Si interface, a higher scattered electric field and a higher sensitivity to the refractive index change of the surrounding medium can be achieved. Furthermore, a feasible implementation of the core-shell nanocylinder design concept is studied and discussed.
A hexagonal nanostructure formed by seven core-shell nanocylinders filled with different dielectric cores is also investigated. The surface plasmon resonance in such a hexagonal nanostructure under conditions of different illumination wavelengths, dielectric cores, angles of incidence, and thicknesses of silver shells is studied by use of finite element method. Simulation results show that the resonant wavelength is redshifted as the dielectric constant and the size of the core increase. The peak resonant wavelength and the local field enhancement are approximately proportional to the radius of the dielectric core. Additionally, the surface plasmon field excited by TM-polarized light at the incident angle of θ = 15° is exactly a linear combination of those excited at incident angles of θ = 0° and 30°, confirming the linear nature of the surface plasmon resonance in a nanostructure formed by linear media.
In addition, a novel refractive-index (RI) sensor which is based on a Fabry-Pérot resonator consists of a micro-fluidic channel with one of the interface formed by a metal film is reported. A large resonant angle ( 80 deg) was used to enhance the interaction length and thus the angular sensitivity can be more than 1000 deg/refractive index unit (RIU), which is five times higher than the sensitivity that has ever been achieved in previous studies. Meanwhile, the angular dip width was very narrow (< 0.01 deg) and can be adjusted by changing the thickness of the resonant cavity. Moreover, the corresponding sensing resolution which is higher than RIU–1 can be achieved. Finally, the effects of the angular positioning inaccuracy and finite bandwidth of incident light source upon the proposed sensor in practice were also studied.
Three different kinds of metal nanostructure for optical sensing were successfully demonstrated in this thesis.

並列關鍵字

metal nanostructures ； optical sensing

參考文獻

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