448 Progress In Electromagnetics Research Symposium 2005, Hangzhou, China, August 22-26 Plasmon Resonances of Spherical and Ellipsoidal Nanoparticles Jiunn-Woei Liaw Industrial Technology Research Institute, Taiwan M. K. Kuo and C. N. Liao National Taiwan University, Taiwan Abstract Surface plasmon resonance (SPR) of a single metallic nanoparticle is analyzed and simulated via multi- multipole method for 3D problems. The excitation, at optical frequencies, of the SPR leads to an extremely strong field in the vicinity of the nanoparticle. Numerical results indicate that a red shift of SPR is induced for an ellipsoidal nanoparticle, compared to a spherical one. Two structures of core-shelled spherical nanoparticles are also studied; one is a nanosphere of Ag shell with an oxide core, and the other one is Ag core with an oxide shell (ZrO 2 or SiO 2 ). Numerical results illustrate the SPR of these two core-shelled structures are quite different from each other and different from that of a solid one. It suggests that one can manipulate the optical response on demand by tuning the core/shell ratio and the permittivity of shell or core. Introduction Optical properties of plasmon resonant metallic nanoparticles are of great interest owing to the ability of controlling optical fields on the nanometer scale. The phenomenon can be utilized to manipulate the light for versatile applications, e.g. nanosensors [1], photocatalyst [2] etc. Recently, non-spherical nanoparticles as well as core-shelled nanoparticles have attracted lots of attention, since they provides new varieties to tune the optical properties [3] by changing not only the shape, size and material of the core, but also the thickness and the material dielectric of the shell. There are two main structures of the core-shelled nanoparticles having been synthesized in the last decade by chemists, namely Au shell with a silica core (SiO 2 ) [4] and Au or Ag core with an oxide shell (e.g. ZrO 2 or TiO 2 ) [5]. In the paper, the effects of ellipsoidal shapes and of the core/shell ratio on the plasmon resonance of the above-mentioned nanostructures will be studied by using muti-multipole expansion (MMP). Theory MMP was originally developed for systems with piecewise homogeneous, isotropic, and linear media [6, 7] and has been applied successfully for computing electromagnetic field in many applications. According to the MMP proposed by Hafner [7], the electric and magnetic fields can be expanded with respect to several centers, and each series can then be truncated to finite terms. Thus, the electric and magnetic fields, in the exterior region of all scatterers, can be expressed as:              E = q  α=1 N  n=1 n  m=−n (a mn,α M mn,α + b mn,α N mn,α ) H = k jωµ q  α=1 N  n=1 n  m=−n (a mn,α N mn,α + b mn,α M mn,α ) (1) where q = ℓ × p is the total number of the expanding centers, and ℓ and q are the numbers of the scatterers and the numbers of the centers per each scatterer, respectively. For the fields in the interior region of the any one of scatterer, the expressions are exactly the same as (1), except that q = p, i.e. only the centers of the particular scatterer are needed in the expansion of interior fields, and the bases functions M mn and N mn should be replaced by the appropriate ones.Coefficients a and b have to be determined to accommodate the incident fields and boundary conditions of scatterers. Basis functions M and N are defined as