Third-harmonic generation from arrays of sub-wavelength metal apertures

We measure third-harmonic generation (THG) from arrays of sub-wavelength metal apertures in transmission using fundamental input at 800 nm. Samples with different aperture spacings, sizes, and shapes are used. Strong angular dependence of THG is observed, with maxima located at incidence angles corresponding to extraordinary optical transmission (EOT) for the fundamental. We demonstrate an anomalous scaling of TH intensity with aperture size, where at different EOT peaks, the TH may either increase or decrease with aperture size. The aperture shape is also shown to have a strong effect on TH output.