Mechanisms of THz generation from silver nanoparticle and nanohole arrays

We study THz pulses generated from plasmonic metal nanostructures under femtosecond illumination of near-IR light. We find two regimes of excitation, according to the order of the dependence of the THz fluence on the incident near-IR intensity: less then second order at low intensities, changing to approximately fourth order for higher intensities. These regimes are most likely associated with two THz generation mechanisms: optical rectification, and the ponderomotive acceleration of ejected electrons. These data provide evidence that both mechanisms can be at work in the same experiment.