Role of local fields and defects in the nonlinear response of metal nanostructures

We show that local fields associated both with overall structural features and with unintended defects can be important in the second-order nonlinear response of metal nanostructures. We first consider noncentrosymmetric T-shaped gold nanodimers with nanogaps of varying size. The reflection symmetry of the T-shape is broken by a small slant in the mutual orientations of the horizontal and vertical bars, which makes the sample chiral and gives rise to a different nonlinear response for left- and right-hand circularly-polarized fundamental light. Measurements of achiral and chiral second-harmonic signals as well as the circular-difference response exhibit a nontrivial dependence on the gap size. All results are explained by considering the distribution of the resonant fundamental field in the structure and its interaction with the surface nonlinearity of the metal. We also prepared arrays of ideally centrosymmetric circular nanodots. Second- and third-harmonic generation microscopies at normal incidence were used to address polarization-dependent responses of individual dots. Both signals exhibit large differences between individual dots. This is expected for second-harmonic generation, which must arise from symmetry-breaking defects. However, similar results for third-harmonic generation suggest that both nonlinear responses are dominated by strongly localized fields at defects.