Nonlinear Plasma Dipole Oscillations in Spheroidal Metal Nanoparticles
  The theory of nonlinear dipole plasma oscillations generated in a metal spheroidal nanoparticle by a laser-wave field has been developed. Approximate (to within the cubic term) analytic expressions for the nanoparticle dipole moment have been obtained in the case where the laser field is oriented along the spheroid rotation axis. 
P. M. Tomchuk and D. V. Butenko
cond-mat.mes-hall physics.plasm-ph quant-ph