Saturation of the enhanced second harmonic generation (SHG) based on the localized fields confined in a nanoscale periodic, GaAs-filled, plasmonic structure is presented. The SHG saturates at high pump intensities due to three-photon absorption and the resultant free-carrier absorption. Free carriers are monitored by photoluminescence (PL). A fifth power dependence of the PL is observed at 297 K, which results from the three photon absorption along with a transition from nonradiative carrier recombination at the defected GaAs surface at low excitation to bimolecular recombination at high carrier concentrations.