Surface Plasma Enhanced Stimulated Emission In ZnO Whispering Gallery Microcavity

Zinc oxide (ZnO) nano materials is regarded as as the third generation wide bandgap semiconductor and becomes the research focus of nano science and technology. It became an important candidate of short wavelength optoelectronic functional materials and devices. It has significant advantages in UV laser with low threshold, high quality factor due to its wide band gap of 3.37 EV and high exciton binding energy of 60 MeV at room temperature. Over the past decade, researchers pay a great deal of attention on UV semiconductor photoelectric characteristics, especially the characteristics of laser. Many top journals such as Science Phys., Rev. Lett., Adv. Mater., Nano Lett., and so on have made a lot of reports on this topic. There are three types of oscillation modes for ZnO ultraviolet laser generation:The first one is random laser based on the scattering among grain boundary and the random formation of positive feedback; the second one is Fabry Perot laser based on the reflection of the end surface of the micro-rod structure; the third one is whispering gallery mode (WGM) laser based on the total reflection formed in micro cavity wall. ZnO micro nanorods, pipe, plate and other crystal structure has good optical quality and high refractive index to ensure the efficient formation of inner total reflection optical loop gain. It thereby greatly reducing the loss due to the light scattering and transmission, and can be used as a good WGM cavity. Therefore, WGM laser has an important application in cavity quantum electrodynamics, optical fiber communication and sensing fields. Many reports have showed that ZnO WGM laser has excellent quality. The physical process and the corresponding application have been studied. It is a very meaningful research topic on how to improve the quality factor, increase the laser intensity, and reduce the optical loss of the micro cavity as well as lower lasing threshold.It has been greatly developed since the localized surface plasmon resonance (LSPR) was first applied in sensor technology in the eighties of the last century. After the development of 20 years, its research content has covered nano plasmonic focusing, Plasmon-exciton coupling effect, the nano waveguide, SERS, nano switch, LSPR sensor, surface plasmon laser, plasmon resonance energy transfer, photothermal therapy, super-resolution imaging below the diffraction limit, optical logic operation and negative exponential refractive index materials and so on. In recent years, with the development of micro/nano machining technology, nano material preparation technology, nano near-field characterization techniques and effective electromagnetic simulation tool, various local optical effect and devices research related to surface plasmon polaritons have attracted wide attention and rapid development. It becomes the research hotspots. In 2009, a team in the United States experimentally demonstrate the amplified stimulated emission in the golden nanoparticles ball (nature,2009,460:1110) and metal film system (nature,2009,461:629) by the the plasma metal surface, which attracted the scholars’widespread attention on the this topic. At present, studies show that metal surface plasmon significantly enhances the optical properties of the semiconductor material. Significant improvements have also been made in PL enhancement of micro/nano ZnO material based on metal surface plasma. Many researches have indicated that the UV emission of ZnO thin films can be greatly enhanced through plasmon polariton resonance (SPR) effect after combining nanostructures or metal nanoparticles or appropriate metal film. However, study on ZnO stimulated radiation enhancement based on surface plasmon resonance enhancement effect is rather limited, it is very meaningful to carried out this work. This research aims to use ZnO micro/nanostructures as whispering gallery modes of micro cavity and enhance the stimulated emission of ZnO based on surface plasmon resonance effect. Thus the the performance of WGM multimode laser and single mode laser from micro/nano structure ZnO can be improved. Different enhancement mechanism of Au Ag and Pt is discussed, respectively. It provides technical support to realize UV laser with high performance and high output power. It also provide experimental basis understand the underline physical mechanism better. The main research works are carried out as follows:1. The composite microcavity is constructed by combining ZnO microtube with Au nanoparticles. The UV emission was enhanced obviously with decrease of the visible emission as the Au nanoparticles were decorated on the ZnO microtube surface. The enhanced stimulated emission from the Au-decorated whispering-gallery microcavity was further investigated systematically through a series of optimization of the sputtering time to control the Au nanoparticle size and the corresponding optical behaviors.10-folds enhancement of the whispering-gallery mode lasing was obtained and defect emission of ZnO was significantly depressed after the Au nanoparticles decoration.2. A comb-like ZnO structure with hexagonal prism microrod array was fabricated to design the coupled whispering-gallery-mode cavities without any complicated micromanipulation. Lasing spectral behavior and coupling interaction between ZnO microteeth were experimentally investigated. The evolution process from multi-modes, double-modes to single-mode lasing were investigated systematically in experiment just by moving the pumping points along the wedge interval to change flexibly the coupling distance based on Vernier effect. This approach provides a general strategy for lasing mode modulation and single-mode operation.3. The optical gain in ultraviolet wavelength region was improved by sputtering Pt nanoparticles onto the ZnO microcomb.17-fold PL enhancement was obtained through optimizing the sputtering time. Based on the SPR coupling conditions,7-fold enhancement WGM lasing and single mode lasing from the coupled microteeth of ZnO microcomb was obtained and the defect emission was depressed. These results demonstrated the plasmon-coupled single mode lasing with stronger intensity, higher Q factor, lower threshold, narrower full width at half maximum and higher side-mode suppression ratio.4. Square and hexagonal ZnO microrods were prepared by a vapor phase transport method. Ag nanoparticles were synthesized through hydrothermal methods and attached to the ZnO microrods by pin coating method. The UV whispering gallery mode laser has been significantly enhanced. Ag nanoparticles were further used to construct the LED devices of Ag/ZnO/GaN and 10 fold EL enhancements were obtained. The mechanism of ultraviolet luminescence enhancement was investigated.