Second harmonic generation in plasmonic nanostructures: A double dipolar resonant antenna design

Second harmonic generation (SHG) is an important tool for the study of plasmonic nanostructures and can open up important sensing applications since the second harmonic (SH) signal has an increased sensitivity to nanostructures shape and environment in comparison with the linear scattering. SHG from plasmonic centrosymmetric nanostructures is controlled by two main properties. First, SHG is forbidden in the bulk of centrosymmetric structures in the dipolar approximation, meaning that nanostructures made of metals like gold and silver generate a SH signal coming mainly from their surfaces, where the symmetry is effectively broken. Second, because of the same symmetry reason, a dipolar excitation at the fundamental frequency induces SH modes with vanishing dipole moments. In the case of plasmonic dimers, with the large intensity enhancement occurring in the gap, the SH emission is limited due to out-of-phase SH sources with destructive interferences in the far-field region. Consequently, new strategies must be developed to overcome this limitation.