Tailoring Second-Harmonic Generation in Single L‑Shaped Plasmonic Nanoantennas from the Capacitive to Conductive Coupling Regime

We investigate the efficiency of second-harmonic generation (SHG) over the transition from capacitive to conductive coupling in orthogonal L-shaped dimer gold antennas. By tuning both the gap and antenna length, the bonding and antibonding resonances are individually addressed. Results on the intensity and polarization of SHG are compared quantitatively with microscopic numerical simulations taking into account the nanoscale nonlinear surface dipole distribution, elucidating the interplay between symmetry at macroscopic and microscopic levels and optical resonance effects. Microscopic modeling reveals strong cancellations of nonlinear dipoles by capacitive coupling in plasmonic nanogaps, resulting in only small changes in SHG efficiency despite large local field enhancement in the gap. Experimentally, irreproducible polarization properties are obtained in a range of parameters associated with strong optical near fields in the gap of the antennas, which is interpreted as a consequence of nanoscopic asymmetries inherited from the fabrication process. Our results demonstrate that nanoscopic defects can either strongly impact the nonlinear optical emission or have a barely detectable influence depending on the excited optical resonance and associated optical near-field distribution. These results provide useful design rules to optimize the design of nonlinear plasmonic nanostructures