Advanced light manipulation and waveguiding in plasmonic nanostructured optical fibers.
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Ghimire, Indra Mani, 1985-
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Conventional optical fibers are well-known for their efficient light guiding mechanism. However, the dielectric properties of core and cladding materials (e.g.- doped silica and silica glasses) limit the functionality of the optical fibers. Therefore, the optical properties of the fibers, such as phase, polarization state, amplitude, mode profile are fixed and cannot be altered once after the fiber drawing fabrication. The advent of new fabrication technologies for optical nanostructures such as plasmonic and metasurfaces allows us to overcome these limitations by tailoring those optical properties for advanced light manipulation and the development of novel optical fiber devices. In this dissertation, I have reported four main projects on integrating plasmonic and metasurface nanostructures into an optical fiber for novel optical functions. In the first project, I demonstrated in-fiber polarization-dependent color filters by nanopatterning asymmetric metallic metasurface array on the end-facet of polarization-dependent photonic-crystal fibers. The asymmetric cross-typed nanoslit metasurface arrays are fabricated on the core of the optical fiber using a focused ion beam milling technique. Highly polarization- and wavelength-dependent transmission with transmission efficiency ~ 70 % in the telecommunication wavelength observed by launching light into two orthogonal linear polarization states of the fiber. In the second project, I have extended the use of the focused ion beam milling technique to fabricate Berry phase metasurfaces on conventional single-mode optical fiber. A focusing effect has been observed in these metasurface-optical fibers, leading to the development of in-fiber metalens. The third project involves the integration of plasmonic nano-circuits on the facet of polarization-maintaining photonic crystal fiber (PM-PCF) and panda-shaped PM-optical fibers. A Yagi-Uda antenna-coupled low loss plasmonic slot waveguide is directly integrated on the optical fiber by direct milling technique. We demonstrated efficient coupling of light from the fiber core to the plasmonic slot waveguide. The light is then propagated in the plasmonic slot waveguide via the propagation of surface plasmon polaritons and emitted to the far-field in the output antenna located in the cladding. We further extend the design of the complex circuit by integrating multi-channels plasmonic waveguide with different waveguide lengths, polarization splitters, and optical directional coupler (ODC) onto the optical fiber. This project first proof-of-concept demonstration on developing an ultra-compact plasmonic network on the tip of optical fiber for advanced optical communications applications. The final project consists of the study of optical modulation by incorporating vanadium dioxide (VO2) nanocrystals into the air holes of anti-resonant hollow-core photonic crystal fibers (ARHCF). Efficient optical modulation is observed by inducing the insulator-to-metal phase transition of VO2 at different temperatures.