Third-Harmonic Generation Enhancement by Film-Coupled Plasmonic Stripe Resonators

Because of their ability to strongly localize and enhance optical fields, plasmonic nanostructures have the potential to dramatically amplify the inherent nonlinear response of materials. We illustrate the impact of this plasmonic interaction by investigating the third-harmonic generation (THG) from a system of film-coupled nanostripes operating at 1500 nm. Both the film and the stripes are gold, separated by a nanoscale layer of aluminum oxide (Al₂O₃) grown using atomic layer deposition. This nanoscale junction, with an ultrasmooth interface, forms a waveguide cavity resonator with a large and controllable electric field enhancement, whose plasmon resonance can be tuned independently by changing the stripe width. We study experimentally the dependence of THG on the field enhancement by varying the gap size between the stripe and the metal film while simultaneously maintaining a fixed plasmon resonance. The experiments are supported with numerical simulations in which nonlinear contributions of the dielectric spacer layer and the metal are considered. Enhancements of the THG of nearly 5 orders of magnitude with respect to a bare metal film are measured experimentally for the smallest gap sizes, with a trend similar to that found in the numerical simulations.