Probing the origin of highly efficient Third Harmonic Generation in plasmonic nanostripes

Nonlinear generation of optical fields has enabled many exciting breakthroughs in light science, paving ways for many applications ranging from sensing, communication to optical processing and light sources. However, the origin of large nonlinear response enhancement observed in plasmonic structures are not fully understood. Here we investigate the third harmonic generation process using hybrid plasmonic resonators formed by a sub-5 nm dielectric layer between a gold film and patterned gold nanostripes. By examining different dielectric spacer materials, we achieved a maximum enhancement factor of more than six orders of magnitude, which represents a nonlinear conversion efficiency of 8.78×10^-4%. From the spectral analysis of the same structure with various spacer gap materials, stripe widths and excitation wavelengths, we conclude that the nonlinear response results primarily from the embedded dielectric spacer layer. This conclusion is further strengthened by the bandwidth analysis of third harmonic response spectra from stripe compared with substrate. Our results illustrate the promise of using plasmonic gap modes for future efficient on-chip nonlinear devices.