電磁共振效應用於材料光外耦合增益之研究

Using Electromagnetic Resonance Effects to Develop the Techniques of Material Light-Out-Coupling Enhancement

曾奕鈞(Yi-Chuan Tseng)

指導教授 ： 陳學禮

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2016年)

https://doi.org/10.6342/NTU201603617

摘要

材料的光外耦合技術在近期相當吸引科學界的注目，尤其在於提升材料的非線性光學現象，例如表面增益拉曼散射。相對於線性的彈性碰撞而雷利散射之光子數目來說，非彈性碰撞而產生的拉曼散射之光子數量相當貧乏。因此提高材料的光外耦合，可以有效地提高得以偵測到的光子數量，進而擴展拉曼檢測的應用範疇。光外耦合技術亦被提出應用於螢光材料，透過材料的光外耦合技術，得以大幅提升材料的發光效率。在過往的研究中，光外耦合技術多以金屬奈米結構提升侷域電磁場強度進而達到提高發光效果為主。然而，這些金屬奈米結構都容易對於發光材料產生摻雜、驟熄等影響；除此之外，金屬奈米結構亦有本身對於入射光發生吸收以及其產生侷域電磁場範圍不足等缺點。高折射率的介電奈米材料會對於特定波長的入射電磁波產生磁性共振，誘發其本身內部及向鄰近材料延伸的高電磁場熱區。本論文之研究主要即是利用此磁性共振產生的高電磁場熱區提高材料的光外耦合效應。
我們發現一個透過非金屬結構磁偶極共振產生大面積電磁場熱區進而提升基板拉曼訊號的方法。透過設計高折射率之矽奈米粒子的尺寸，使得入射電磁波在奈米粒子的內部產生磁性共振。我們發現良好分散的矽奈米粒子透過其本身磁性共振下凝聚於粒子內部的高電磁場，得以使矽奈米粒子單位體積下的拉曼增益達到近六萬倍。由非金屬奈米粒子產生的強電磁場熱區不僅存在於奈米粒子內部及周圍，亦延伸至與奈米粒子接觸的下方基板內部。我們透過實驗，將覆蓋率低於百分之零點三的矽奈米粒子佈局於氮化鎵及碳化矽基板表面；即觀察到此二基板之拉曼散射訊號明顯的增強超過百分之五十。我們在對照的實驗之中，將具有侷域性表面電漿共振行為的金屬奈米粒子佈局於相同的基板表面，基板的拉曼散射訊號則無明顯的增益情形。我們觀察到相對於金屬奈米結構產生的侷域性表面電漿共振，由非金屬奈米共振腔產生的磁偶極共振現象可以更加有效率的增加位於共振腔下方之待測分子的拉曼散射訊號。相對於金屬奈米結構，非金屬共振腔的磁偶極共振亦展現出較大的有效增益拉曼散射範圍。此由非金屬結構產生的磁偶極共振具有相當良好的潛力進而發展次世代的非金屬電磁增益技術。
為了穩定地獲得石墨烯，化學氣相沉積大面積的碳原子於金屬表面已經廣泛地被接受為一個有效的方式。因此，及時非破壞性地監測金屬基板表面的石墨烯亦成為了重要的考量。雖然得以透過光學上的拉曼光譜進行監測，但石墨烯於金屬表面僅能產生相當微弱的拉曼訊號。我們將具有特定尺寸、符合入射電磁波磁偶極共振的矽奈米粒子佈局於透過化學氣相沉積的石墨烯銅箔上方，巧妙位於石墨烯的位置引發了一個相當強烈的電磁場區域。利用模擬，我們發現此位於石墨烯位置高達超過一百二十倍的高電磁場強度來自矽奈米粒子磁偶極共振的位移電流與下方銅箔內部的自由載子之耦合。除了位於石墨烯之高強度的電磁場，我們還觀察到矽奈米粒子亦有效率地在石墨烯內部產生相當大範圍的電磁場熱區。我們在實驗上證明了透過此高強度、大範圍的電磁場熱區，銅箔上方的石墨烯之拉曼訊號可以被大幅地增益超過兩百倍。此外，實驗上亦觀察到矽奈米粒子對於原始石墨烯的拉曼訊號幾乎沒有影響；覆蓋於石墨烯上方的矽奈米粒子也可以輕易地被移除。本論文同時呈現了應用此方式透過拉曼測定位於不同晶粒尺寸銅箔基板上方之不同石墨烯的品質；驗證了此方法可以達到及時非破壞性地進行拉曼訊號增益之應用。
由於矽塊材典型的非輻射再結合特性，其具有相當低的量子效率；相對於具有直接能隙的材料，矽塊材被一般認定為「暗」材料。在本論文中，透過上述磁偶極共振發生時的矽奈米粒子會於粒子內部產生強烈的電磁場，我們發現此電磁場可以誘使九十至一百二十奈米直徑之矽奈米粒子發生聲子協助放光現象。在這個尺寸下，矽奈米粒子不具有量子限域效應。藉由模擬，我們發現設定於矽奈米粒子內部的點光源在放光光譜上的峰值位置符合其磁偶磁共振的發生位置。因此我們進而設計了一個可以提升矽奈米粒子之磁偶極共振的奈米共振腔基板。透過實驗上將矽奈米粒子放置於此基板上方，觀察到磁偶極共振波長下的矽奈米粒子明顯地放光。估計此時矽奈米粒子的量子轉換效率達到百分之二點四。實驗上我們亦透過降低矽奈米粒子的尺寸改變其磁偶極共振發生的波長，進而在不同波長下觀察到矽奈米粒子的發光藍移現象。這個實驗提供了一個清楚的證據指出由磁偶極共振產生於矽奈米粒子內部的強烈電磁場可以誘發其聲子協助放光現象。根據我們的調查，在本論文之前尚未有文獻提出矽材料在這個尺寸下得以輻射出可見光。因此，我們建議，磁性共振下的矽奈米材料相當具有潛力應用於開發下一世代的「全光程矽光子學」。

關鍵字

矽奈米粒子 ； 磁偶極共振 ； 表面增強拉曼散射 ； 石墨烯 ； 矽發光

並列摘要

Recently, techniques of material light-out-coupling have attracted considerable attention in the scientific community, especially, to enhance the material light-out-coupling in nonlinear optical phenomenon such as surface enhanced Raman scattering. In previous literatures, the main methods to achieve the material light-out-coupling are by employing the electromagnetic field generated from metal nanostructures. However, generally, the metal nanostructures might dope and quench the fluorescent materials. In addition, the metal nanostructures might also absorb light to reduce the emission efficiency of the fluorescent materials. Dielectric materials having high refractive index can be induced a magnetic resonance for the incident electromagnetic wave having a specific wavelength. The magnetic resonance can induce an intense electromagnetic field hot zone inside the dielectric materials. The hot zones of intense electromagnetic field can be generated not only within the dielectric materials but also around them, even within the nearby materials. The main studies in this thesis are enhancing the material light-out-coupling by using the intense electromagnetic field generated from the magnetic resonance.
We found that the large area of electromagnetic field hot zone induced through magnetic dipole resonance of metal-free structures can greatly enhance Raman scattering signals. The magnetic resonant nanocavities, based on high-refractive-index silicon nanoparticles (SiNPs), were designed to resonate at the wavelength of the excitation laser of the Raman system. The well-dispersed SiNPs that were not closely packed displayed significant magnetic dipole resonance and gave a Raman enhancement per unit volume (REV) of 59347. The hot zones of intense electric field were generated not only within the nonmetallic NPs but also around them, even within the underlying substrate. We observed experimentally that gallium nitride (GaN) and silicon carbide (SiC) surfaces presenting very few SiNPs (coverage: <0.3%) could display significantly enhanced (>50%) Raman signals. These nonmetallic NPs displaying magnetic dipole resonance were more effective at enhancing the Raman scattering signals from analytes that were underlying, or even far away from, them. This application of magnetic dipole resonance in metal-free structures appears to have great potential for use in developing next-generation techniques for Raman enhancement.
Chemical vapor deposition (CVD) large-area crystal carbon atom on metal surface to stably obtain graphene is widely accepted as an effective method, thereby, in-situ and non-destructive characterizing the properties of the graphene on a metal substrate became a considerably important issue. Raman spectroscopy is very useful for identifying the properties of graphene, unfortunately, the as-grown graphene on a metal substrates provides a very weak Raman signal. In this thesis, we arrange the SiNPs having a specific dimension and an internal magnetic dipole resnance upon the CVD-as-grown graphene/Cu foil and cleverly give rise to a dramatically intense electromagnetic field around the graphene. The SiNPs provide an efficient electromagnetic hot zone in a large area within the graphene. We also demonstrated the electromagnetic field in the graphene can be employed to enhance its Raman signals (up to 206 times). We experimentally observed this approach can enhance the original Raman signal of the CVD-as-grown graphene under causing extremely low influence. Moreover, the coated SiNPs on the CVD-grown graphene can also be easily removed without destroying the graphene. We believe this interesting and valuable Raman signal enhancement approach would be very useful for in-situ and non-destructive measuring the characteristics of the CVD-as-grown graphene on Cu foil.
Due to the “bulk-scale” Si is a typical material of nonradiative recombination, the “bulk-scale” Si which has quite low quantum efficiency is generally considered as a “dark” material in comparison to direct bandgap semiconductors. In this thesis, we found the intense electric fields generated through magnetic resonance inside SiNPs can induce phonon-assisted light emission from "bulk-scale" Si (90 to 120 nm). The radiative emissions from spherical SiNPs were demonstrated in the scale without quantum confinement effect (90-120 nm). We designed a nanocavity substrate which can provide an environment to generate strong magnetic resonance inside the coating SiNPs. By placing the SiNPs upon the nanocavity substrate, we experimentally observed the emission intensity of SiNPs on the nanocavity substrate is obviously stronger than the SiNPs on the Si wafer with the coating SiO2 layer substrate and bare Si wafer. We attribute the results to the enhanced electric field accompanied from the enhanced magnetic resonance inside the SiNPs induces the more intense phonon-assisted light emission phenomenon. We also estimated an internal quantum efficiency for the phonon-assisted light emission of the SiNPs (diameter of 90 nm) placed on the nanocavity is 2.4%. The experimental results provide an apparent evidence to illustrate the strong electromagnetic field caused from magnetic resonance would induce phonon-assisted light emission from Si. Worth to note, to the best of our knowledge, Si emission visible light from the size has not been reported previously. We suggest that this concept, based on magnetic resonance, should be very applicable in the development of next-generation techniques for all-optical processing Si photonics.

並列關鍵字

silicon nanoparticle ； magnetic dipole resonance ； surface enhanced Raman scattering ； graphene ； silicon light emission

參考文獻

[1] S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, C. M. Soukoulis, Science 2004, 306, 1351.

[2] N. Liu, H. Guo, L. Fu, S. Kaiser, H. Schweizer, H. Giessen, Nat. Mater. 2008, 7, 31.

[3] C. Enkrich, M. Wegener, S. Linden, S. Burger, L. Zschiedrich, F. Schmidt,

J. F. Zhou, T. Koschny, C. M. Soukoulis, Phys. Rev. Lett. 2005, 95, 203901.

[4] C. Bohren, D. Huffman, Absorption and Scattering of Light by Small Particles Wiley, New York, 1983.