Plasmon spectra of nanospheres under a tightly focused beam

We study the modification of the far-field cross sections and the near-field enhancement for gold and silver nanospheres illuminated by a tightly focused beam. Using a multipole-expansion approach we obtain an analytical solution to the scattering problem and provide insight on the effects of focusing on the optical response. Large differences with respect to Mie theory are especially found when the nanoparticle supports quadrupole or higher-order resonances. (C) 2008 Optical Society of America.