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Research on the Optical Properties and Applications of Micro&Nano Metallic Coupling Structures

Author: FuQiang
Tutor: MingHai
School: University of Science and Technology of China
Course: Optics
Keywords: metallic nano-structures surface plasmon polaritons coupling lithography surface enhanced Raman scattering (SERS) sensing fluorescence enhancement fluorescence quenching
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Type: PhD thesis
Year: 2013
Downloads: 79
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Abstract


The surface plasmon resonance of metallic nano-structures has two important characteristics:focalized electrical field and near-field enhancement. Because of these two unique optical characteristics owned by metal, there are significant and wide applications, such as biochemical sensor, surface enhanced Raman scattering (SERS), enhanced nonlinear optics, metal enhanced fluorescence (MEF), etc. This dissertation foucus on the optical properties of plasmonic coupling system made of metallic nano-structures of different morphology. We also investigated their appliciations in optical sensing, SERS and fluorescence imaging from an experimental viewpoint. Our work has potential significance to exploit low cost, real-time, free label and highly sensitive optical sensor chip and to develop simple-production, high-contrast fluorescence imaging technique.The main research works and conclusions are described as follows:1. We synthesized various metallic nano-structures of different shapes by the chemical method which is of low cost and large yield. These nano-structures included gold nanorods and god(?)@silver bimetallic nanorods which are synthesized by seed-mediated growth, silver nanocubes and silver nanowires synthesized by polyol reduction method. We also inscribed the sinusoidal metal grating by two beam interference and thermal evaporation technology;2. The optical properties of gold nanorods and gold@silver bimetallic nanorods are investigated based on Gans theory. In our experiment, we find that the lognitudinal localized surface plasmon resonance (LLSPR) peak of gold nanorods are red-shifted when the dielectric constant of surrounding medium or the aspect ratio of gold nanorods was increased. When silver nano-shell is coated on the surface of gold nanorods, we find that the optical sensing characteristics are significantly affected by thickness of silver nano-shell. When the thickness of silver nano-shell is5nm, the refractive index sensitivity (RIS) and the figure of merit (FOM) can achieve the best balance, and the RIS is321nm/RIU, the FOM is4.0, the latter is74%higher than that of uncoated gold nanorods;3. We experimentally investigated the novel optical properties of the chemically- synthesized silver nanocubes and lithography-inscribed metal grating by the surface enhanced Raman spectroscopy (SERS). The result shows that the intensity of SRES peak can be immensely enhanced by the dual-layer coupling structure. We compared the SERS intensity collected from the single-layer structure composed of only silver film or one dimensional silver grating or silver nanocubes. We find that the intensity of rhodamine6G (Rh6G) at1650cm-1collected from dual-layer is196times higher than that from single-layer substrate such as the silver grating. The SERS intensity of dual-layer substrate composed of silver nanocubes and silver grating is also larger than the dual-layer substrate composed of silver nanocubes and silver film. The former intensity is35940a.u and the latter is2548a.u.;4. We experimentally investigated the contrast enhancement of fluorescence image by the coupling of surface plasmons. We design the "sandwich" structure composed of silver nanocubes (or silver nanoparticles or plystyrene spheres), PMMA thin film which is doped with rhodamine B (RhB) and silver film We study the plasmonic characteristic of this "sandwich" structure by the fluorescence emission from the RhB. We find that when the thickness of PMMA film is50nm, the plasmonic coupling effect will give rise to the quench of fluorescence, but when the thickness of PMMA film is110nm, the plasmonic coupling effect will lead to the enhancement of fluorescence. Whether the fluorescence is quenched or enhanced, the contrast of fluorescence image of metallic nano-structure can be increased. For example, the quenching effect can increase the contrast as high as1.25times that of bright-filed image, the enhancement effect can increase the contrast as high as one time that of bright-filed image. For the non-metal plystyrene spheres, the contrast of fluorescence image is unchanged because there is no plasmonic coupling effect between the silver film and polystyrene spheres.The innovations of this thesis are shown as following:1. We find the thickness of silver nano-shell can influence the performance of optical sensor made of gold nanoparticles.. The experimental result shows when the gold nanorods with aspect ratio3.0is coated with silver shell of5nm thickness, the RIS of the sensor can reach321nm/RIU. And the FOM can reach4.0which is74%higher than that of uncoated gold nanorods; 2. We present a new type dual-layer structure composed of silver nanocubes and silver grating which can highly increase the SERS intensity. The SERS intensity collected from this dual-layer structure is196times larger than that from the single-layer structure composed of only one dimensional silver grating. The intensity of the Raman signals from this dual-layer structure is also14times larger than that from the dual-layer structure composed of silver nanocubes and silver film. The result is helpful for exploiting low cost, simple construction, good repeatability and highly sensitive SERS sensor or detection chip;3. We present the "sandwich" structure composed of metal-dielectric-metal which can increase the contrast of fluorescence image. We find that the thickness of dielectric layer can control the quenching or enhancement of the fluorescence. We also find that whether the fluorescence is quenched or enhanced, the contrast of fluorescence image of metallic nano-structure can be increased. The quenching effect can increase the contrast as larger as1.25times that of bright-fifed image, and the enhancement effect can increase the contrast as large as one time that of corresponding bright-filed image.

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