Optical Resonance And Field Enhancement Effects Of Symmetry Broken Metallic Nanostructures

Metallic nanostructures support surface plasmon (SP) resonance. SP in the opticalfrequency range can confine electromagnetic energy. Metallic nanostructures with SPresonance can reduce effective wavelength of light, thus break the diffraction limit, andopen a way to sub-wavelength optics.It can also control the nature of light intensity andlight transmission at the nanoscale, to achieve optical information transmission andprocessing, realizing new type of optical waveguide devices. SP resonance can generatestrong electric field enhancement which can be applied in spectroscopy and sensingprobe. This thesis focuses on symmetry broken metallic nanostructures and their opticalresonance and field enhancement properties.Using open source software Meep based on the finite-difference time-daman(FDTD) algorithm and commercial software Comsol based on finite element method(FEM) to simulate the electromagnetic response of metallic nanostructures, we obtainits extinction spectra and field enhancement, and analyze the coupling mechanism of theSP resonance modes in these structures.The crescent tip structures are used to analyze the resonance characteristics. Bychanging the size of the crescent structure, the dielectric constant of the surroundingmedium and a series of other parameters of the crescent structure, we calculate theextinction spectrum, and analyze the resonance characteristic. From the electric fieldintensity distribution, we can intuitivly identify the coupling of the resonance modes.Our results show that by using a concentric crescent-disk structure, the tip mode and theslit structure can couple to improve the SP resonance. In this crescent-disk structure, thecrescent and the slit quadrupole (second-order) resonance modes can be matched,leading to improve surface electric field enhancement. By moving the position of thedisk to form a crescent slit, the multi-pole resonance modes of the crescent slit and thecrescent can be well-matched. Changing the position and size of the disk and thedielectric constant of the slit, we find that the changes of the resonance peaks appearsimilar.For a given geometry of nanocrescent, the optical resonance and enhancement areconstants. It is desirable to tune this resonance and enhancement without altering thegeometry of the nanocrescent. Herein, we numerically show how the resonance at thetip of a nanocrescent can be proximally tuned by incorporating a nanorod in the vicinity.