Second-Order Nonlinear Optical Properties of Plasmonic Nanostructures

We review our work on second-order nonlinear optical properties of plasmonic nanostructures. In order to achieve the required non-centrosymmetry of the structures, our samples consist of arrays of L-shaped nanoparticles and T-shaped nanodimers. The samples are investigated by polarization-dependent second-harmonic generation to address the tensorial nonlinear response. We show that the response can be strongly modified by symmetry-breaking defects and other deviations of the samples from ideal. Nonlinear sources localized to defects can also give rise to higher-multipolar emission. The defect problem is overcome with a recent and significant improvement in sample quality, allowing the dipole limit of the nonlinear response to be reached. This achievement opens the path towards plasmonic metamaterials with tailorable nonlinear properties. As a demonstration of this possibility, we modify the nonlinear response by the mutual arrangement of the L-shaped particles in the array. We will also summarize our numerical boundary-element method to describe the nonlinear response of nanoparticles.