Plasmonic enhancement of second harmonic generation on metal coated nanoparticles.

Second Harmonic Generation (SHG) is a widely used tool to study surfaces. Here we investigate SHG from spherical nanoparticles consisting of a dielectric core (radius 100 nm) and a metallic shell of variable thickness. Plasmonic resonances occur that depend on the thickness of the nanoshells and boost the intensity of the Second Harmonic (SH) signal. The origin of the resonances is studied for the fundamental harmonic and the second harmonic frequencies. Mie resonances at the fundamental harmonic frequency dominate resonant effects of the SH-signal at low shell thickness. Resonances excited by a dipole emitting at SH frequency close to the surface explain the enhancement of the SHG-process at a larger shell thickness. All resonances are caused by surface plasmon polaritons, which run on the surface of the spherical particle and are in resonance with the circumference of the sphere. Because their wavelength critically depends on the properties of the metallic layer SHG resonances of core-shell nanoparticles can be easily tuned by varying the thickness of the shell.