| Owing to their shape-and size-dependent optical properties know as localized surface plasmon resonance (LSPR), metal nanoparticles are finding use in a range of emerging applications such as biological and chemical sensing, surface-enhanced Raman spectroscopy, biomedicine and nanophotonics. To date, LSPR excitation in metal nanoparticles is commonly realized using free-space irradiation. Because of the small extinction cross section of a single nanoparticle and the relatively large irradiation area of a free-space light beam, the efficiency of photon-to-plasmon conversion is rather limited (usually less than1%). In addition, to redirect a light beam to a nanoparticle in free space, bulky components such as prisms or objectives are often required, making it difficult to realize nanoparticle-based photonic devices with miniaturized sizes and low operation optical powers. And at the same time, metal nanoparticles usually suffer from large losses because of metal’s intrinsic absorption and nanoparticles" radiative scattering, which significantly broaden the plasmonic resonance linewidths of single metal nanoparticles, severely deteriorating the performance of metal nanoparticles in applications such as high-sensitivity biological and chemical sensing and surface-enhanced Raman spectroscopy. Therefore, the development of a highly efficient and compact approach for LSPR excitation in metal nanoparticles, and also a method for the dramatic reduction of plasmonic resonance linewidths of single metal nanoparticles are the current critical issues faced by the research fields of LSPR in metal nanoparticles. With this regard, we propose to integrate metal nanoparticles and one-dimensional optical waveguides (optical micro-/nanofibers), including Au nanorods embedded polymer nanofibers and Au nanorods surface-deposited optical microfibers. for the efficient and compact excitation of LSPR in Au nanorods, and also for the dramatic reduction of localized plasmonic resonance linewidths of single Au nanorods. Furthermore, we successfully apply these hybrid plasmonic-photonic structures to optical sensing, and demonstrate the dynamical tuning of plasmonic resonance wavelength of Au nanorods.In the first chapter of the work, we briefly review the backgrounds of optical micro-/nanofibers and LSPR in Au nanorods. and also the research progress in hybrid metal nanoparticle-optical micro-/nanowaveguide structure.In the second chapter of the work, we mainly introduce the optical properties of single optical micro-/nanofibers. Firstly, we briefly introduce the fabrication of silica and polymer micro-/nanofibers. Secondly, based on theoretical analysis and numerical calculations, we investigate the waveguiding properties of single optical micro-/nanofibers. which offer many fascinating properties such as tight optical confinement and large fraction of evanescent fields. At the same time, we introduce methods for launching light into or out of single optical micro-/nanofibers. Thirdly, we introduce methods for micro-/nanofiber functionalization, including surface modification, doping, and electron-beam activation. Finally, we introduce micromanipulation technique used to assemble micro-/nanofibers into desired structures or patterns. Above research backgrounds provide theoretical and technical basis for our following research on optical micro-/nanofiber-Au nanorod hybrid structures.In the third chapter of the work, with the use of waveguiding polymer nanofibers embedded with Au nanorods, we demonstrate a highly efficient approach to photon-to-plasmon conversion in Au nanorods. The nanofibers are directly drawn from a polyacrylamide (PAM) solution containing Au nanorods. which are uniaxially aligned along the long axes of the fibers. When light is coupled into and guided through a single nanofiber. LSPR in the embedded Au nanorods could be efficiently excited with a photon-to-plasmon-conversion efficiency as high as70%for a single nanorod at its longitudinal plasmonic resonance wavelength. The highly efficient waveguiding excitation approach demonstrated here may open up new opportunities for developing Au-nanorod-based photonic components and devices with miniaturized sizes, high compactness, and low optical power consumption.To demonstrate this capability, we also apply the Au-nanorod-embedded waveguiding PAM nanofibers to optical relative humidity (RH) sensing. We first investigate the spectral shift of LSPR of a single embedded Au nanorod when exposed to different levels of humidity. The sensitivity of the single Au nanorod is estimated to be～0.19nm/%RH. which is1order of magnitude higher than that of bare Au nanoparticles. Also, we realize intensity-dependent RH sensing using PAM nanofibers containing multiple Au nanorods by measuring the intensity of light output. The sensor offers a sensitivity of～0.07dB/%RH and an estimated resolution better than1%RH with a response time of110ms and an operation optical power as low as500pW.In the fourth chapter of the work, we demonstrate dramatic reduction in plasmonic resonance linewidths of single Au nanorods by coupling them with optical microfibers. The LSPR modes of Au nanorods couple with whispering gallery modes of optical microfibers when Au nanorods are deposited on the surface of optical microfibers, which results in dramatic modulation in the scattering spectra of single Au nanorods. When the diameter of a silica microfiber goes down to1.46μm,there is only one main peak exists in the scattering spectrum of an Au nanorod with a linewidth of about3.4nm. providing a15-fold spectral narrowing as compared with linewidths of uncoupled single Au nanorods. Also, there is about30-time increase in the scattering intensities of coupled Au nanorods, which is of great importance in the enhancement of light-matter interactions. Moreover, based on PAM microfiber coupled Au nanorods, we demonstrate dynamical tuning of plasmonic resonance wavelength of Au nanorods by controlling the environmental RH, with a spectral tunability as large as～40nm.Finally, in the fifth chapter, we provide a brief summary of our work, the innovations, and future research plans. The optical micro-/nanofiber-Au nanorod hybrid structures demonstrated here provide a new platform for "photonic-plasmonic" research at nanoscale, and show great potential in applications such as high-sensitivity biological and chemical sensing, surface enhanced Raman spectroscopy, plasmonic lasing. and optical modulations. |
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