Optical near field spectral analysis of single metal nanoparticles

The scattering and absorption properties of ellipsoidal geometry (spheres, rods, and disks) metal nanoparticles using modified long wavelength approximation method (MLWA) is presented. The validity of this technique is verified by comparison to the exact solution (Mie theory). For spherical particles, up to 16-nm diameter of gold and 8-nm diameter of silver, we confirm that our approach yields an exact correspondence with Mie theory, and gives an approximation error of less than 15% for gold and silver particles with diameters approaching 40nm and 18nm respectively. For core/shell particles by varying the relative dimensions of core and shell, the optical resonance of these nanoparticles can be precisely and systematically varied over a broad region ranging from the near-ultraviolet to the mid-infrared. These making core/shell nanoparticles attractive as functional materials for many applications. A model for core/shell nanoparticles is presented to investigate shell thickness effects and gives an approximation error of less than 9.49% for silica-gold particles with diameters approaching 36-40 nm.