Terahertz-pulse emission by laser excitation of surface plasmons in a metal grating

Metallic nanostructures have been prepared by depositing thin layers of gold onto a fused-silica transmission grating. Excitation with 800-nm femtosecond laser pulses in air results in the emission of terahertz radiation with an angle-dependent efficiency and a third-order dependence on input power. Nanostructures consisting of metal nano-pyramids on glass as well as silver nano-pyramid arrays covered in gold also exhibit terahertz emission. We will show that the source of the terahertz radiation in all of these cases is surface-plasmon- assisted multiphoton ionisation and ponderomotive acceleration in the evanescent laser field.