利用過渡金屬氮化物電漿子超穎介面增強二次諧波產生

Abstract

過渡金屬氮化物(Transition Metal Nitrides, TMNs)是一種新穎電漿子材料，為了克服以往常使用的電漿子材料¬的缺點，像是金、銀等傳統電漿子材料的熔點較低，在高溫或是強光照射下容易會因為變形而失去表面電漿子共振，或是鋁在大氣環境下會有氧化的問題，而過渡金屬氮化物具有高熔點、化性穩定、高硬度、低成本等優點，因此非常適合在嚴苛環境下作為電漿子材料。故我們嘗試將過渡金屬氮化物應用在非線性光學領域，透過表面電漿子共振可產生局域強場的特性，增強二維材料的非線性光學現象。因為表面電漿子共振的局域強場集中在金屬材料表面，我們選用單層二硫化鉬(Molybdenum Disulfide, MoS2)做為非線性材料，能夠在原子層級的厚度產生極強的二次諧波產生(Second Harmonic Generation, SHG)。 在本研究中我們將比較貴金屬(金、銀)以及過渡金屬氮化物(氮化鈦、氮化鉿)作為電漿子材料應用在非線性光學的差異。首先我們選用具有偏振性的光柵設計作為共振結構，可更清楚的探討表面電漿子共振的影響，並透過時域有限差分(Finite-Difference Time-Domain, FDTD)的模擬方法設計出光柵結構，因為主要是使用摻鈦藍寶石脈衝雷射作為激發光，其波長為800奈米，我們將結構的共振波長設計於此處，實驗上我們量測其反射光譜，兩者能夠得到相近的結果。接著我們先分析本實驗中的非線性材料－單層二硫化鉬(MoS2)於的基本光學性質，再將材料乾轉印至設計好的奈米光柵共振結構上，過程中所有樣品的單層二硫化鉬與光柵方向都是對齊的，避免受到其他因素影響。接著分別探討貴金屬與過渡金屬氮化物的光柵結構所產生的影響，並對其做統計處理，最後兩者皆得到100倍上下的增強倍率。因此我們能夠利用過渡金屬氮化物，達到與貴金屬相近的效率，然而同時對於環境有更好的適應力，故相信過渡金屬氮化物在非線性光學領域也有很好的發展價值。 故在本篇論文中，我們將過渡金屬氮化物作為電漿子材料去增強單層二硫化鉬的二次諧波產生響應，探討局域強場在原子尺度下非線性的光物質交互作用，並利用其高熔點與化學穩定性克服傳統電漿子材料所面臨的問題，將過渡金屬氮化物的應用拓展至非線性光學等領域。

Transition Metal Nitrides (TMNs) are emerging alternative plasmonic materials. TMNs can overcome the weakness of traditional plasmonic materials, such as the low melting points of gold or silver that the surface plasmon resonance might lose due to its deformation after high irradiation or at high temperature or the oxidation problem of aluminum. As plasmonic materials, TMNs are suitable for extreme environments because of these advantages, e.g., high melting point, chemical stability, and mechanical hardness. Therefore, we tried to apply TMNs in the region of nonlinear optics, enhancing the nonlinear optical phenomena via localized strong field property of surface plasmon resonance. We choose molybdenum disulfide (MoS2) as nonlinear materials because it can exhibit strong second harmonic generation (SHG) within atomic thickness. In this research, we compared the responses of noble metals, e.g., gold and silver, and TMNs, e.g., titanium nitride and hafnium nitride, applied in nonlinear optics as plasmonic materials. First, to study the effects of surface plasmon resonance clearly, we choose the grating design, which is polarization-dependent, as our resonant structure. Because we use Ti:Sapphire pulsed laser, which wavelength is at 800 nm, as the excitation light, we set the resonant wavelengths of plasmonic structures at 800 nm by the finite-difference time-domain (FDTD) method. We can get similar outcomes experimentally by measuring the reflectance of the structures. After analyzing the basic properties of the nonlinear material in this research – monolayer MoS2, we transferred it onto designed resonant grating structures. The direction of monolayer MoS2 and the grating structure are aligned during the dry transfer process to avoid unwanted effects. And then, we studied the results of the grating structures made of noble metals and TMNs statistically. Both of them can reach an enhancement factor of around 100. TMNs can achieve similar optical efficiency to noble metals while having better environmental compatibility. We believe TMNs are promising in the field of nonlinear optics. In this thesis, we enhanced the SHG of monolayer MoS2 using plasmonic TMNs and studied the nonlinear light-matter interaction via localized strong electromagnetic fields at the nanoscale. In the meantime, the high melting point and chemical stability of TMNs could overcome the problems of traditional plasmonic materials. We’ve extended the applications of TMNs to the fields of nonlinear optics.