Numerical study of the optical properties of silver nanostructures with different topologic shapes
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The collaborative oscillation of electrons in noble metal nanostructures results in a surface plasmon resonance that makes them useful for various applications including chemical and biological sensors, plasmonic waveguides, and enhanced spectroscopy. In this dissertation, we study the morphology dependent properties of the surface plasmon resonance, and the discrete dipole approximation method is used to investigate the optical properties of Ag nanostructures with different topologic shapes, such as the cylindrical nanorods, needle-shaped nanorods, period-shaped nanorods, L-shaped nanorods, Y-shaped nanorods, parallel-nanorod structures, U-shaped nanostructures, and the helical nanostructures. For the cylindrical nanorods, the extinction spectra depend on both the aspect ratio (length/diameter) and the diameter. For short nanorods, the longitudinal mode plasmon peaks red shift linearly with aspect ratio. For larger aspect ratios, multipolar plasmon peaks appear in the extinction spectra which also red shift linearly with aspect ratio. Comparing to nanorods with a simple shape (cylinder and needle), irregular nanorods shows many distinct plasmon resonances over a large spectra range. More hot spots are observed for the nanorods with more defects. These results show that defects or imperfections in Ag nanorod structures could drastically change the optical properties, generate extra hot spots for surface enhanced spectroscopy, and have different enhanced field distribution for future plasmonics applications. A U-shaped nanostructure has more hot spots than a parallel-nanorod structure; and at the longitudinal mode incidence, the electric field enhancements are much larger than those of the parallel-nanorod structure. These results could be used to engineer U-shaped nanostructures for specific plasmon applications. Helical nanostructures are another good structure to tune the plasmon peak and arrange the electric field distribution. The plasmon peak and the electric field distribution can be tuned not only by the structural parameters but also by the polarizations of the incident light, especially the circular polarizations. In addition, the origins of the plasmon modes are also investigated. The transverse modes are due to the electron oscillations perpendicular to the cross sections and the longitudinal plasmon peaks are due to the charge oscillations along the arc length of the helix and are mainly determined by the effective dipole length.