基于非对称性等离激元结构的ZnO纳米线WGM激光器微型化研究

One-dimensional semiconductor nanowires have been widely investigated as miniaturized whispering gallery mode (WGM) nanolasers because they can perform both as optical cavities and gain materials. However, as diameter decreases, larger WG mode loss occurs in nanowires. Besides, the imperfect symmetry of cross section and direct contact with normal substrate can further induce the WG mode loss in nanowires. Thus, the miniaturization progress of nanowire-based WGM nanolasers is hindered. How to compensate the optical mode loss and enhance the mode confinement is vital for realizing the miniaturization of nanowire-based WGM nanolasers. In this project, we plan to design and fabricate the (metal nanoparticles or graphene-)ZnO nanowire-insulator-metal film plasmonic heterostructures. By using thin ZnO nanowires with good quality and perfect cross section, and by coupling with the surface plasmon (SP) in metal or graphene, the optical mode loss can be diminished or compensated in thin ZnO nanowires. By changing the type and thickness of metal film and insulator, by controlling the surface roughness of metal film, and by controlling the size of metal nanoparticles, the frequency and intensity of SP in metal can be adjusted. Based on the simulation and experimental results, we will investigate the influence of SP in metal and graphene on the WG mode in ZnO nanowires with diameters below subwavelength. The ultimate aim of this project is to realize single-mode WGM lasing with high quality factor and low threshold at room temperature in ZnO nanowires with diameters below subwavelength or even close to the diffraction-limit size, achieving the miniaturization of nanowire-based WGM nanolasers. Another aim of this project is to explain the coupling mechanisms in the plasmonic heterostructures. Our project is valuable for promoting the miniaturization of WGM nanolasers.

一维半导体纳米线既可作为光学微腔又可作为增益介质，被广泛用于研究微型WGM激光器。但随着纳米线直径减小，WG模式泄漏严重；另外，横截面对称性不完美、与衬底接触会进一步导致WG模式泄漏，使WGM激光器微型化受阻。基于此，本项目提出一种（金属颗粒or石墨烯-）ZnO纳米线-介质层-金属薄膜非对称性等离激元结构，采用高品质、横截面完美对称的小直径ZnO纳米线，利用金属、石墨烯的SP与纳米线耦合，来抑制、弥补小直径纳米线中WG模式的泄漏。通过改变金属薄膜和介质层的种类、厚度及表面粗糙度，控制金属颗粒的尺寸，来调节金属的SP频率和强度；理论结合实验研究金属、石墨烯的SP对直径小于亚波长的ZnO纳米线的WG模式的影响。本项目最终目的是利用金属、石墨烯的SP，在直径小于亚波长甚至接近衍射极限的ZnO纳米线中实现室温下低阈值、高品质因子的单模式WGM激光；揭示耦合机制，实现ZnO纳米线WGM激光器微型化。

半导体纳米线激光器的微型化对于光计算、信息存储等领域的发展具有重要意义。氧化锌（ZnO）纳米线由于其优异性能，是制备室温低阈值紫外纳米激光器的重要材料。然而随纳米线直径减小，光学限域能力降低，微腔效应退化。特别是对于分散在硅片等常规衬底上的纳米线，衬底引起的模式泄漏使微腔效应进一步退化。对于亚波长直径ZnO纳米线，回音壁模式（whispering-gallery-mode，WGM）探测不到，WGM激光器微型化受阻。为实现ZnO纳米线WGM激光器微型化，本项目从衬底结构设计入手，结合实验、计算和模拟展开了系统、深入的研究，取得了一系列成果： .（1）在SiO2/Si基底上制备Al2O3/Ag film hybrid结构作为衬底来替代常规衬底。Ag film采用粗糙薄膜结构，以此在结构中引入局域表面等离激元（localized surface plasmon, LSP）；通过调整Al2O3层厚度对LSP共振频率和强度进行调节，使其与ZnO激子能量共振匹配。.（2）系统研究了LSP对直径分布在10-450 nm之间的ZnO纳米线光学性能的影响。通过实验、理论计算和场分布模拟，发现LSP能显著增强亚波长尺寸ZnO纳米线的WGM谐振。最终在直径为165 nm的ZnO纳米线中探测到模式阶数NTE=1、折射率n=3.44的WGM激子极化激元模式，并实现了室温下低阈值、单模式WGM激子极化激元激光。该工作实现了ZnO纳米线WGM激光器的微型化。.（3）通过调控ZnO纳米线的直径，在声子伴线区域探测到强WGM激子极化激元模式。LSP显著增强了激子极化激元-纵光学声子相互作用，促进了激子极化激元的散射弛豫，对于实现低阈值激光器具有重要意义。.（4）LSP能显著增强激子激发速率及辐射复合发光速率，在80 K下探测到清晰的多种束缚激子跃迁峰，这预示着半导体束缚激子的研究可从液氦温度提升到液氮温度。.本项目的研究成果对于推进半导体纳米线WGM激光器微型化以及半导体激光器芯片的进一步发展具有重要意义。

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