Photonic Properties And Sensing Applications Of Surface Plasmon Polaritons In Low-dimensional Metal Structures

Owing to their unique capabilities of confining and manipulating light on the deep-subwavelength scale by converting photons into surface plasmon polaritons, low dimensional metal nanostructures have been attracting considerable attention from science communities. As typical zero- and one-dimensional nanostructures, due to their remarkable photonic properties, metal nanoparticles and nanowires have inspired a variety of potentials ranging from ultra-compact optoelectronic circuits, plasmon lasers, biochemical sensors, nonlinear optics, to quantum electrodynamics research.Metal nanowires serve as fundamental building-blocks for propagating SPP-based components and devices because of their remarkable properties including large surface-to-volume ratio, strong evanescent field, high mechanical stability, deep-subwavelength confinement, and relatively low propagation loss. However, photonic properties of metal nanowires have not been thoroughly investigated. For example, waveguiding properties of substrate-supported metal nanowires have not been adequately studied, and sensing properties of metal nanowires in liquid environment have yet to be investigated.As typical localized surface plasmon resonance-based zero-dimensional nanostructures, metal nanoparticles have the unique ability to concentrate the optical field within deep-subwavelength regions in all three dimensions, which enables large local electric field enhancement. However, plasmonic nanoparticles often suffer from both radiative and non-radiative losses, which severely shorten the plasmon lifetime and subsequently broaden the resonance linewidth and deteriorate their performance in many applications such as biochemical sensing. Therefore, narrowing the line width of the plasmon resonance is one of the current critical issues for plasmonic community.With this regard, photonic properties and sensing applications of metal nanowires and nanoparticles are investigated. The thesis is organized as follows:In chapter one, principles of propagating surface plasmon polaritons and localized surface plasmon resonance, research background and typical applications of metal nanowires and nanoparticles are introduced.In chapter two, based on numerical simulation, photonic properties and sensing applications of low dimensional metal nanostructures are investigated. Firstly, photonic properties of metal nanoparticles, including near-field modal profile and far-field spectrum are investigated. Meanwhile, plasmon linwidth, lifetime, Q factor, sensitivity and figure of merit are discussed as well. Secondly, Single-mode plasmonic waveguiding properties of metal nanowires with dielectric substrates are investigated using a finite-element method. Au and Ag are selected as plasmonic materials for nanowire waveguides with diameters down to 5-nm-level. Basic waveguiding properties, including propagation constants, power distributions, effective mode areas, propagation distances and losses are obtained at the typical plasmonic resonance wavelength of 660nm. Compared to that of a freestanding nanowire, the mode area of a substrate-supported nanowire could be much smaller while maintaining an acceptable propagation length. The dependences of waveguiding properties on geometric and material parameters of the nanowire-substrate system are also provided. These results may provide valuable references for waveguiding dielectric-supported metal nanowires for practical applications. Finally, we theoretically demonstrate a plasmonic nanosensor, using Au-nanowire waveguide to measure the refractive-index changes in aqueous solutions. Based on finite element method simulations, waveguiding and sensing properties of Au nanowires for plasmonic sensing in liquids are investigated, with Au nanowire diameter down to 10 nm. A plasmonic nanowire Mach-Zehnder interferometer is proposed to measure the phase shift introduced by the index changes of surroundings. We find that, for a typical Au nanowire with 100-nm diameter, the calculated sensitivity is as high as 5.5π/(μm·RIU), and the sensitivity can be increased by reducing the nanowire diameter. Besides, for reference, the dependences of sensing properties on geometric parameters and different liquid environment of are also investigated. The nanowire plasmonic sensing scheme proposed here represents a high-sensitivity nanosensor with ultra-small footprint, and may open new opportunities for miniaturized sensing platform based on highly confined 1-D waveguiding plasmons,In chapter three, growth and characterization method of typical low dimensional nanostructures are introduced. Meanwhile, the dark-field microscopy for light scattering imaging and spectroscopy of single metal nanoparticles is introduced as well.In chapter four, we demonstrate a dramatic reduction in plasmon resonance linewidth of a single Au nanorod by coupling it to a whispering gallery cavity of a silica microfiber. With fiber diameter below 6μm, strong coupling between the nanorod and the cavity occurs, leading to evident mode splitting and spectral narrowing. Using a 1.46-μm-diameter microfiber, we obtained single-band 2-nm-line-width plasmon resonance in an Au nanorod around a 655-nm-wavelength, with a quality factor up to 330 and extinction ratio of 30 dB. Compared to an uncoupled Au nanorod, the strongly coupled nanorod offers a 30-fold enhancement in the peak intensity of plasmonic resonant scattering. The coupling system presented here may open new opportunities for pushing the limits of plasmon-based techniques and inspiring better plasmonic devices such as ultrasensitivity nanosensors and ultralow-threshold plasmon lasers.In chapter five, a brief summary and an outlook regarding the future development are presented.