基于亚波长结构实现高效发光器件的研究

Many techniques have been developed to enhance the efficiency of light-emitting devices. For examples, resonant cavities can enhance light extraction by increasing photon density of states (DOS). Photonic crystal structures can enhance light extraction by recovering the guided modes, which are responsible for losses in light-emitting devices. Recently, surface plasmon polariton (SPP) has been shown to improve the emission efficiency of light-emitting devices. Apart from surface-enhanced Raman scattering, localized surface plasmon resonance can also induce surface-enhanced emission, owing to the size of "hot" spots, where the incident fields are strongly enhanced. The larger the SPP-DOS and electric field, the higher is the spontaneous emission rate into the SPP mode. The emission enhancement is realized if the resultant SPP mode re-emits. On the other hand, semiconductor nanoribbons and nanowires fabricated from bottom-up approaches have the natural advantages of inherent optical cavities with smooth surfaces, as well as single crystallinity of high quality. Moreover, electrically-driven single emitters based on individual nanoribbons and nanowires have been realized. Therefore, introduction of SPP-mediated enhancement to the emission efficiency of nanowire/nanoribbon-based emitters for practical applications is extremely interesting. Here we investigate SPP-mediated emission enhancement by virtue of subwavelength structures, as well as single crystal emitters. The system we investigated is composed of one-dimentional, two-dimentional, or three-dimentional subwavelength structures and two- or one- dimentional emitters. At the same time, we investigate the light-emitting behavior of dye-doped PMMA in order to get clear physics of subwavelength structures. The dispersion relation of the designed system should be calculated first. Theoretical work will be employed further by means of simulation softwares based on finite-element method, and finite-difference time-domain method to get large SPP-DOS and high out-coupling efficiency. Experimentally, the one/two/three-dimensional metallic subwavelength structures can be fabricated by use of micro/nano-stations, e.g. EBL, FIB, and so on. We will combine theory with experiment to realize the control of subwavelength propagion and other optical properties, and to explore the potential applications in high efficient nano-emitters.

谐振腔结构和光子晶体结构有利于提高发光器件的发光效率，表面等离激元也被证实可以用来提高发光效率，也因此成为这些年来的研究热点，除了表面等离激元，局域等离谐振也能被用来增强发光效率。从下至上法制备的半导体纳米线/带因为其高的晶体质量、具有光滑表面的谐振腔结构，已经被人们用来做成纳米激光器。如果将这种高效纳米发光材料的发光效率进一步提高，则会推进其走向实际应用的步伐。我们引入亚波长结构来实现这一目的，在理论上设计并计算出这一体系的色散关系，进一步借助于有限元和有限差分方法深入研究这一系统。实验上利用EBL、FIB、SEM、AFM等微/纳加工技术制备与表征一维、二维和三维金属亚波长纳米结构，最终实现对半导体纳米线/带的发光性能以及亚波长光传播的调控，探索其在高效纳米发光器件上的应用。

从光与物质相互作用这一基本物理问题出发，深入系统地研究了金属参与亚波长结构的基本物理特性，如增强透射、偏振旋转、波导传播、谐振腔、耦合谐振、场增强、能带结构等。理论上，通过解析编程、有限元法以及时域有限差分法等模拟计算亚波长结构的光子能带结构、模式特征、场强分布、能量分配、耦合出射等特性；实验上经由材料生长及制备、EBL与FIB等先进加工手段、纳米线与二维材料的转移技术、微纳工艺、以及具有光谱分辨、空间分辨、时间分辨、角度分辨、偏振分辨等的精密光学测量，实现发光器件的制备、表征以及对其性能的全方位认识。分析比较实验与理论结果，逐步调整、改进和优化亚波长结构设计以及材料性能，为发光器件效率的提高总结并提取了物理规律，为高效发光器件的研制提供了有效指导。. 通过表面等离激元能带结构设计，利用带边高能态密度增强荧光发射，并获得最优化的结构参数，如光栅凹槽的占空比为1/4时带隙最大，增强效果最佳；当F-P腔模式与表面等离激元带边模式相耦合时，自发辐射谱的半高宽最小，放大式自发辐射的阈值也最低；通过局域表面等离谐振与表面等离激元相耦合等耦合谐振机制可实现出射偏振的可控旋转；研究了ZnO纳米线的F-P、WGM谐振行为及其与周围环境的依赖关系，如SiO2、Si、graphene、graphite、Au、Ag、Al等基板，观测到ZnO纳米线在UV波段的激子发光增强达到3个数量级，进一步结合亚波长结构，可实现对表面等离激元模式的传播控制与发光性能调控。对纳米线腔谐振规律的掌握与控制将促进高效光源以及低阈值电驱动亚波长可调激光器的研制。其它相关工作包括：高品质耦合谐振器、表面增强拉曼散射、以及纳米材料介电函数及传播模式的有效折射率测量等。