微奈米結構用於表面增益紅外吸收光譜與近場熱輻射之研究

Abstract

本論文主要討論兩個部分，第一部分是討論利用薄膜干涉和磁偶極共振兩种原理，開發一種具有大範圍電場熱區，製程簡單並且可重複使用的表面紅外光譜增益結構。相較於目前主流的利用金屬表面電漿共振式(surface plasmonic resonance, SPR)增益結構，其基板本身在紅外光波段就有很大的吸收，而待測物由於鍵結振盪造成的特徵吸收峰，則隱藏在基板吸收内，不易被觀察。就此缺點，本論文所開發的這種矽粒子二氧化矽氧化層共振增益基板，不利用金屬之電漿共振造成吸收增益，而是利用紅外波段下消光係數幾乎為零的二氧化矽氧化薄膜層和分散的矽粒子，利用光學薄膜建設性干涉和介電粒子磁偶極共振的手段，達到強電磁場增益的效果。因為基板本身吸收很低，所以對於觀察低濃度化合物的紅外特徵吸收峰訊號，有較高的訊號對比。最後對於可檢測待測物之要求而言，我們所開發的增益基板不需要與待測物進行吸附。目前主流研究中的表面紅外光譜增益基板需要待測物含有硫氫鍵結與金屬發生鍵結吸附，或是額外鍍一層單分子層與待測物進行化學吸附，將分子聚集在侷域數百平方奈米的電場熱區内，方可進行偵測。而我們所開發的增益基板，單一顆矽粒子電場熱區就可以達到數個平方微米的範圍，利用旋轉塗布的方式，可以使基板表面都覆蓋大範圍的電場熱區，所以不需要吸附待測物分子，只需滴在基板上即可檢測。 在實際檢測過程中，我們選用了三種待測物，分別是溴化十六烷基三甲銨(CTAB)、品紅(Fuchsin red)和羅丹明(R6G)三種不同濃度溶液，其中常被使用進行表面紅外光譜增益的研究之待測物是CTAB分子，本論文可以檢測比別人更低的分子數目為4.13×105。以及R6G分子，我們可以檢測的最低分子數目為2.08×106，同樣遠低於別人的檢測極限。並且我們也針對特定波長下，模擬出增益基板的最佳結構尺寸，熟知這種模擬方法，就可以任意調控電磁場增益的行爲，使其可以成爲一個實用性很高的表面紅外光譜增益基板。 第二部分是討論金屬間近場熱輻射之研究，總所周知，由於金屬的表面電場很低，遠低於介電材料的表面電場，而且熱放射率平均只有0.025，所以金屬的遠場熱輻射現象不明顯。且金屬的消光係數都遠大於其實部折射率，在近場的尺度下，難以激發出表面波的模態，傳遞近場熱輻射的能量也相遠較於介電材料偏低。目前主流研究近場熱輻射的材料，主要是天生具有表面波模態的介電材料，比如碳化矽(SiC)、二氧化矽(SiO2)等。近場熱輻射的原理是表面波行進在材料界面上，在垂直方向上以衰逝波(evanescent wave)的形式，通過接近或小於光波長大小的真空間距，與另一端材料界面的表面波模態耦合，進而傳遞近場能量，而金屬平膜表面並無表面波模態，所以難以耦合近場能量。 我們通過設計並製作金屬光柵結構，通過這種光學結構，激發出表面波模態，達到增强金屬之間近場熱輻射現象。同時我們用三維有限時域差分法 (three dimensional finite difference time domain, 3D-FDTD)模擬金屬光柵表面平面電場分佈情況，以及探討光柵的周期，寬度以及深度，三者對於共振波長之影響。在實驗上，我們架設一套溫度量測系統，記錄輸入功率和冷熱兩段溫度，擬合計算出熱輻射單位面積下的能量。通過和碳化矽晶片，玻璃片和金屬平膜比較，在材料兩端近場100奈米的空氣間距下，溫差20攝氏度下，計算出兩個金屬光柵結構之間所傳遞的熱輻射單位面積能量是1872 (W/m2)，遠大於金屬平膜之間的熱輻射單位面積能量為611 (W/m2)，説明金屬光柵表面確實可以增强兩個金屬界面的近場熱輻射能量，未來可以應用到金屬電子元件的散熱工程，以及熱電元件等相關應用。

This thesis contains two parts. The first part is the use of thin film interference and magnetic dipole resonance to develop a large-area hot zone of electric field making a simple and reusable surface enhanced infrared absorption (SEIRA) substrate. The current mainstream of metallic structures based surface plasmonic resonance (SPR) enhanced SEIRA substrate generally have a large absorption in IR resonant regime. Moreover, the characteristic absorption peak of analyte due to bond oscillation is usually hidden in the SEIRA substrate’s strong absorption regime. In view of the shortcoming, in this thesis, the silicon (Si) particle on cavity based SEIRA substrate uses both of constructive interference and magnetic dipole resonance to achieve strongly enhanced electromagnetic field. Since the absorption of the Si particle on cavity based SEIRA substrate is very low, there is a high signal contrast for observing the characteristic absorption peak signals of the low amount of molecular of analytes. Moreover, the analytes does not need to be adsorbed on the Si particle on cavity based SEIRA substrate. It is well known that SEIRA substrate in current mainstream research requires that analytes contain sulfur-hydrogen bonds or an additional adhesive layer that can link the analytical molecules within the regime of several tens square nanometers of electric field hot spots, which are generated by metallic SEIRA substrates. In contrast, the Si particle on cavity substrate possesses large area of electric field hot zone over several square micrometers regime. Therefore, it is not necessary to adsorb analytical molecules on our SEIRA substrates. In the experimental processes, we selected three kinds of commonly used analytes, namely cetyltrimethylammonium bromide (CTAB), magenta (Fuchsin red) and rhodamine (R6G). For CTAB molecules, we could detect a number of molecules 4.13×105, which is smaller than that of previously published papers. As well as R6G molecule, the lowest number of molecules we can detect is 2.08×106, which is also lower than the detection limit of previously published papers. And we also simulate the optimal size of Si particle and underlying cavity substrate for different specific wavelengths, making it become a highly practical SEIRA substrate. The second part of this thesis, we discuss the near-field thermal radiation between two metals. It is well known that the surface electric field of metal is much lower than that of of dielectric materials. For example, the thermal emissivity of gold film is ca. 0.025. And the extinction coefficient of metal is much larger than its real pat of refractive index. Above all, in the near-field scale, it is difficult to excite the modes of the surface wave and further transmit the energy by near-field thermal radiation. At present, the mainstream materials for the study of near-field thermal radiation is dielectric materials, such as silicon carbide (SiC) and silicon dioxide (SiO2), having strong surface waves. The near-field thermal radiant energy of metal is much lower than that of dielectric materials. The principle of near-field thermal radiation is that the surface wave in the form of an evanescent wave travels at the surface coupling with another surface wave at the other side of interface between a gap that is smaller than the wavelength of thermal radiation wave. However, the surface of metal film almost has no surface wave mode, it is very difficult to excite or couple the near-field radiation energy to another surface within near field regime. In this thesis, by designing and fabricating a metal grating structures, the structured metal can excite surface waves and enhance the electric field intensity between two metal surfaces that can further enhance the near-field heat radiation between two metal surfaces. At the same time, we used three dimensional-finite difference time domain (3D-FDTD) method to simulate the electric field distribution of the metal gratings with different periods, widths and depths for working at different specific resonant wavelengths. In the experiment, we set up a temperature measurement system to record the input power and the temperature of hot and cold ends that can be used to calculate the energy per unit area of thermal radiation. Compared with silicon carbide plate, glass, and flat Au film, the energy per unit area of thermal radiation that transmits between two metal grating structures was calculated to be 1872 (W/m 2) at a temperature difference of 20 °C in the air gap of 100 nm. At the same distance and temperature difference, the energy per unit area of thermal radiation between two flat Au films is 611 (W/m2) that indicates that the metal gratings can enhance the near-field thermal radiation energy between two interfaces. We suggest the near-field thermal radiation on structured metals can be applied to heat dissipation engineering of electronic chip or heat management of thermoelectric chip in the future.