Local-field and multipole effects in second-harmonic generation from gold nanoparticles and nanodimers

摘要

Nanoscale variations in the local fields and material properties can enable higher-multipole (magnetic-dipole and electric-quadrupole) contributions to the nonlinear response in addition to electric-dipole contributions.More-over, the local-field distribution in the structure is important to achieve favorable interaction with the locally- varying nonlinearity. Local-field enhancement is particularly important for nonlinear optical effects.Extremelysmall features of a few nm, such as nanogaps between two particles, are expected to be particularly beneficialfor field localization and enhancement. Here, we provide evidence of multipole interference in polarized second-harmonic generation from arrays of L-shaped gold nanoparticles. We also prepare T-shaped gold nanodimersand vary the size of the nanogap between their vertical and horizontal bars. Surprisingly, the second-harmonic signals do not decrease trivially with increasing gap size, because the gap region is nearly centrosymmetric, thereby forbidding second-order effects. Instead, asymmetric local fundamental field distributions along the dimer perimeter are favorable, in accordance with the symmetry rule.