Nonresonant Local Fields Enhance Second-Harmonic Generation from Metal Nanoislands with Dielectric Cover

Abstract

We study second-harmonic generation from gold nanoislands covered with amorphous titanium oxide (TiO2) films. As the TiO2 thickness increases, the plasmon resonance of the nanoislands shifts away from the second-harmonic wavelength of 532 nm, diminishing the resonant enhancement of the process at this wavelength. Nevertheless, the second-harmonic signal is enhanced by up to a factor of 45 with increasing TiO2 thickness. This unexpected effect arises from the scaling of local fields at the fundamental wavelength of 1064 nm—which is at the far tail of the resonance—due to a change in the dielectric environment of the nanoislands.