Polarized second-harmonic-generation spectroscopy for Au nanorods arrayed on SiO2 substrates at localized surface plasmon resonances

Polarized second-harmonic-generation (SHG) spectroscopy was studied for Au nanorods arrayed on fused silica substrates at localized surface plasmon (LSP) resonances. The geometry of the nanorods is centrosymmetric and electric dipolar second-order nonlinearities are forbidden for such a geometry. However, second-order nonlinearities are permitted for rods with sizes comparable to light wavelengths by taking advantage of the field retardation effect across the rods. In the study, we demonstrated SHG by photoexciting nanorods at an oblique incidence in two optical configurations. Independent of the configuration, efficient SHG was obtained when the excitation light was polarized in the LSP direction. The SHG waves were p-polarized, irrespective of the configuration and the excitation light polarization. The polarization behaviors were well reproduced by using numerically simulated field distributions. This comprehensive study classified the roles of the field retardation in the second-order nonlinearities of Au nanorods.