Abstract: Particles comprising a core surrounded by at least one shell wherein at least one of the core or one of the shells is a metal and at least one of the core or one of the shells is a dielectric material which exhibits a third order nonlinear response. The "shell" of a metal particle may be the adjacent region of a medium in which said particle is suspended, said medium having a third order nonlinear response. In a preferred embodiment, the particles are suspended in a dielectric medium that may be linear or may exhibit a third order nonlinear response. Enhancement of the effective .chi..sup.(3) of the particles and suspensions result from the concentration of the electric field in and around the particles at the plasmon resonance frequency of the metal.