Gap Plasmon Enhanced Metasurface Third-Harmonic Generation in Transmission Geometry

The influence of gap plasmons in the third-harmonic generation of annular and H-shaped aperture arrays has been investigated. Surprisingly, the maximum third harmonic in the annular ring structures is not from the smallest gap, but rather appears for a gap of 14 nm. We interpret this result in terms of resonant plasmonic antenna design to match scattering and absorption cross sections. Unlike the annular aperture, continuous gold apertures, like the H-structure, achieve 0.45% conversion efficiency for 2 mW/μm2 CW pump power of a 100 fs pulse, which is 9 times larger than rectangular apertures we have reported in the past. Nonlinear scattering theory has been used to estimate the transmitted third-harmonic signal and is in generally good agreement with the experimental results.