Quantitative Analysis of Dipole and Quadrupole Excitation in the Surface Plasmon.pdf

J. Phys. Chem. C 2008, 112, 20233–20240 20233 Quantitative Analysis of Dipole and Quadrupole Excitation in the Surface Plasmon Resonance of Metal Nanoparticles Fei Zhou, Zhi-Yuan Li,* and Ye Liu Laboratory of Optical Physics, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China Younan Xia Department of Biomedical Engineering, Washington Uni Versity in St. Louis, St. Louis, Missouri 63130 Recei Ved: August 8, 2008; ReVised Manuscript ReceiVed: October 16, 2008 Metal nanoparticles have received increasing attention for their peculiar capability to control local surface plasmon resonance (SPR) when interacting with incident light waves. In this article, we calculate the optical extinction spectra of silver nanocubes with the edge length ranging from 15 to 200 nm by using the discrete dipole approximation method. An increasing number of SPR peaks appear in the optical spectra, and their positions change when the nanocube size increases. We have developed a method to quantitatively separate the contributions of the individual dipole component and quadrupole component of the optical extinction cross sections. This allows us to specify unambiguously the physical origin of each SPR peak in the spectra. We have also extensively analyzed the distribution patterns of electric fields and electric charges within and around the silver nanoparticle. These patterns clearly show the dipole and quadrupole excitation features at the SPR peaks.