Nanoparticle surface electromagnetic fields studied by single-particle nonlinear optical spectroscopy

Polarization-resolved second harmonic generation (SHG) measurements were performed on solid gold nanosphere (SGN) dimers at the single-particle level. The results indicated that single-particle SHG measurements could be used to quantify the localized electromagnetic surface fields that result from excitation of inter-particle localized surface plasmon modes. For several dimers, the polarization-resolved measurements revealed that the surface fields localized between SGNs in a dimer were chiral. Quantitative analysis of SHG line shapes obtained from single-particle continuous polarization variation (CPV-SHG) experiments confirmed that the chirality originated from non-zero magnetic-dipolar contributions to the SGN dimer nonlinear optical response. Correlation of SEM images and single-particle SHG measurements obtained for several SGN dimers, as well as lithographically generated nanostructures, were used to demonstrate the structure sensitivity of the CPV-SHG method.