Electric-Field-Induced Second-Harmonic Generation in Serrated Nanogap Arrays

Asymmetric plasmonic nanoparticles can be used to generate and control the spatial distribution of electric fields at the nanoscale in order to efficiently generate second-harmonic light and control its polarization response. Electric-field-induced second-harmonic generation (EFISH) allows for the optical modulation of second-harmonic light using an external, applied electric field. In our experiments, we fabricated novel asymmetric gold nanogaps and demonstrated that they produced second-harmonic light with a conversion efficiency on the order of 10$^{-11}$. Three plasmonic geometries were fabricated to create unique electric field gradients on a length scale of the order of 100nm. Finite-difference time-domain (FDTD) simulations and experimental extinction spectra of the nanogaps were performed, and the nanogaps were found to have broad plasmonic resonances at 800nm. The plasmons were excited with a horizontally polarized ultrafast Ti:Sapphire laser at 800nm. PMMA was then deposited into the nanogaps, and we found that the PMMA red-shifted the plasmon resonance and reduced the SHG conversion efficiency due to absorption by the PMMA. Future experiments are planned with additional centrosymmetric, non-centrosymmetric and ferroelectric materials.