Local-field and multipole effects in second-harmonic generation from optical metamaterials

We present evidence of local-field and multipole effects in second-harmonic generation from optical metamaterials consisting of arrays of metal nanoparticles and nanodimers with a nanogap. Second-harmonic generation from T-shaped nanodimers is not driven primarily by strong gap fields, instead symmetry of the local fundamental field distribution in the structure plays a key role. Interference between dipolar and higher multipolar (magnetic-dipole and electric-quadrupole) sources is obtained by comparing polarized second-harmonic responses in reflection and transmission.