利用光柵耦合器達到高效率螢光萃取自奈米鑽石螢光激發之表面電漿極化子

Abstract

表面電漿極化子（Surface plasmon polaritons, SPPs）是在金屬與光交互作用後，由自由電子集體振盪形成被侷限於金屬與介電質間傳遞的電磁波形式，這樣的電磁波擁有比電子元件更高的操作速度並能傳遞於極微小的結構中，特別的是它能保有激發光子的許多量子特性，如單量子糾纏態(Quantum entanglement)、次泊松分布(Sub-Poissonian distribution)等；奈米鑽石（Nanodiamonds, NDs）擁有多種缺陷螢光，而其中的氮空缺中心（Nitrogen-vacancy center, N-V center）以擁有室溫下之穩定單光子（Single photon）光源特性備受矚目，當以這樣的量子光源作為表面電漿之激發光源時，將可產生出單表面電漿極化子（Single surface plasmon polaritons）。本研究以奈米鑽石NV中心螢光作為點光源在大尺寸金膜上自然激發出表面電漿子，在金膜上以光柵耦合器將表面電漿子轉化為遠場螢光，並以螢光偏振性與光譜分析，確認螢光訊號確實來自於奈米鑽石NV中心螢光激發之表面電漿子，這樣的光柵耦合器從實驗中可粗估得到1.2 %的螢光萃取效率，而模擬上得到在波長680 nm時有著34%的高轉換效率。本論文研究成果將讓我們能在大尺寸單晶金膜上鎖定特定奈米鑽石，對其激發出之表面電漿子進行螢光萃取，能便利及準確的篩選所需要之觀測目標。未來將可在大尺寸金膜上製作電漿子迴路，並以光柵耦合器來量測確認NV中心螢光激發而成的單電漿子源。

Surface plasmon polaritons (SPPs) are electromagnetic waves caused by the collective oscillations of free-electrons and propagate between a metal and a dielectric. They have higher operating speed than electric devices and propagate in subwavelength structures. Specially, they can preserve many quantum properties of the excitation photons, such as quantum entanglement and sub-Poissonian distribution. The nitrogen-vacancy center (N-V center) in a nanodiamond is a well-known single photon source. The single surface plasmon polaritons can be excited by a single N-V center. We demonstrate that using the grating couplers on the large-area single-crystalline gold film can efficiently convert the SPPs excited by the N-V centers in nanodiamonds to free-space light. In our experiment, we measure the polarization and the spectra of the fluorescence from grating couplers. It shows that the fluorescence from grating couplers must be converted from SPPs excited by the N-V centers. We can get the conversion efficiency of our grating couplers is 1.2 % in experiment and 34 % at 680 nm wavelength in simulations. Using the grating couplers, we can easily and conveniently do light extraction from the SPPs excited by the chosen nanodiamonds on the large-size gold film. In the future, we can design the plasmonic circuits on the large-size gold film.