Theoretical Study Of Local Light Field Enhancement And Fano Resonance Of Metal Nanostructures

Metal nanostructures support surface plasmon resonances which offer a way to the diffraction limit and open a route to sub-wavelength optics. They are expected to find applications in optical information transmission and processing, improving devices miniaturization and integration, promoting the development of modern information technology. Surface plasmon resonances are very sensitivity to the surrounding environment and can result in very high local electric field enhancement. They can be applied in surface enhancement spectroscopy, and biological and chemical sensing, which is expected to push the detection sensitivity to the level of single molecular level. In the thesis, we focused on the studies of enhancement and coupling characteristics of surface plasmon resonances, and designed several metal nanostructures. The innovative results are as follow:(1) Recently, many groups concentrated their interests on the sharp-tip and the narrow-gap nanostructures to enhance the local electric field. But, the tip radius and the gap width in such structures which decide the local electric field enhancement, are difficult to decrease infinitely which limits to the experimental level. So, it is important to find new methods enhancing the local electric field. In the thesis, we propose a nanodisk/nanocrescent nanostructures which contain gap and tip simultaneously, and calculated the change of its local electric field enhancement (LFE) factor with different structural parameters. In the structure, the quadrupole resonance modes of the nano-gap and the nanocrescent can match well which result in improving2to3order of magnitude of the LFE factor. Besides, the quadrupole resonance wavelengths can be tuned in the range of400-1000nm by change the size of structures.(2) The LFE factor of the quadrupole resonance modes can reach to about700times in the concentric nanodisk/nanocrescent nanostructure, but that of the dipole and octupole modes show no obvious enhancement. In addition, the LFE factor of the quadrupole resonance modes is not high enough for for single molecular detection. In the thesis, we then designed the non-concentric nanodisk/nanocrescent nanostructure, and calculated the LFE factor change with it structural parameters. In the structure, the resonance modes of the crescent gap are similar to that of the nanocrescent, so they can match well in all the multipole resonance modes. The LFE factors of the dipole and octupole modes of the structure can reach to700, which makes the resonant wavelengths of the non-concentric nanostructures change from the visible to near infrared regions. In addition, the LFE factor of the quadrupole reach to1400times, which is suitable for single molecular detection.(3) The coupling of surface plasmon structures can form Fano resonance, decrease the radiation loss of metal, and result in high LFE factor. Experiments confirmed that the sensitivity of the surface enhance Raman spectroscopy and biological sensing can be enhanced by using surface plasmon structures with Fano resonacne. Nanoring is a highly tunable nanostructure, which support multipolar dark plasmon modes. In the thesis, we proposed metallic conjoint nanoring structures of different size. The dark multipolar mode of big nanoring can be excited by the bright dipolar modes of the small one. The Fano resonance can be achieved by exciting the hexapole and octupole dark modes, which enhanced the tunability of Fano resonance. The Fano resonance would increasing the resonance sensitivity of nanoring structures which will broaden it application in high-sensitivity chemical and biological sensing.