Functional Integration Of Micro/nano Optical Waveguides For Device Applications

With a variety of unique advantages such as compact size, low power consumption and strong optical confinement, micro/nano optical waveguides have wide application potential in optical communication, optical information processing, optical sensing and so on. They also provide mature platforms for fundamental researches on nanophotonics. Nowadays, among the large group of micro/nano optical waveguides, micro/nano optical fiber, semiconductor nanowire (NW), integrated waveguide and metal surface plasmon polariton waveguide are frequently employed to construct different optical devices in diverse researches and applications, featuring specific properties of their own. However, single kind of material will probably limits functionality expansion and performance improvement of optical devices. For example, the indirect bandgap makes the fabrication of pure-silicon-based integrated light sources a challenging task. Therefore, designs and fabrications of optical devices usually demand functional integration of different optical materials for certain purposes. Base on the above mentioned consideration, we researched the functional integration of micro/nano optical waveguides and relevant device applications, including graphene integration on micro/nano optical fibers and semiconductor NWs, and flexible integration of silicon waveguides and freestanding NWs. We successfully demonstrated ultrafast all-optical modulators based on saturable absorption of graphene, and photonic devices based on silicon waveguide-NW integrated photonic circuits.In the first chapter of this work, we review the background of micro/nano optical waveguides and the research advances of their functional integration with typical research achievements.In the second chapter, we demonstrate the graphene integration on micro/nano fibers. We firstly introduce the theoretical modal analysis, fabrication and optical transportation characterization of micro/nano fibers, and then the optical properties and mechanical exfoliation fabrication of graphene, followed with the compositing method for graphene and micro/nano fibers. Subsequently we show the nonlinear saturable absorption property of graphene-composited microfibers and the application in ultrafast all-optical modulation. Benefiting from the large partial evanescent field on the surface of microfibers and longer light-graphene interaction distance, these composite structures exhibits much more significant saturable absorption effect compared with the normal incidence of light onto graphene. Using the pump-probe measurement, we obtained an ultrafast dynamic response time of 2.2 ps and realized ultrafast all-optical modulation. Finally we investigate the photoluminescence properties of the graphene coated on nanofibers.In the third chapter, we report the graphene integration on zinc oxide (ZnO) NW. We firstly introduce the optical waveguide properties and relevant applications of ZnO NW, and then demonstrate the fabrication, characterization and device application of the graphene-composited ZnO NWs. From the characterization results of Raman spectroscopy and electron microscopy, we found that the graphene were coated in the NWs smoothly and tightly, maintaining intact crystal structures. Measurement results of linear optical waveguide absorption suggested a remarkably strong interaction between graphene and the evanescent field on the NW surface. From calculations and all-optical modulation experiments, we found that with a larger refractive index, ZnO NWs are promising in reducing the power of the switch light.In the fourth chapter of this work, we investigate the flexible functional integration of silicon waveguides and freestanding NWs. First we introduce the fabrication method and optical properties of cadmium sulfide (CdS) NWs and rare earth doped optical nanofibers. And then we demonstrate the near-field optical coupling between silicon waveguides and CdS NWs at telecommunication band, with coupling efficiencies up to nearly 100%. Base on this coupling structure, we fabricated hybrid Mach-Zehnder interferometers and racetrack resonators, where the NWs presented significant modulation effect 4 times stronger than the silicon waveguides in all-optical modulation. Finally, integrating Er3+/Yb3+ co-doped tellurite glass nanofibers into silicon ring resonators, we demonstrate on-chip light sources at telecommunication band.The fifth chapter summarizes the abovementioned works and provides prospects.Functional integrations of micro/nano optical waveguides can promise functionality expansions and performance improvements for optical devices. We selected different materials and structures to compensate each other base on their own advantages for the functional integrations. Hybrid optical devices integrating graphene on micro/nano fibers and NWs, and silicon waveguide-NW hybrid photonic circuit were fabricated and investigated. For the first time, we realized ultrafast all-optical modulation based on the carrier relaxation time of picoseconds on graphene, and highly efficient coupling between silicon waveguides and freestanding NWs with several demonstrations of relevant hybrid integrated devices, exhibiting a series of novel features and advantages. These results can offer new insight for the development of optical devices based on micro/nano waveguides.