Ultrafast electron dynamics in platinum and gold thin films driven by optical and terahertz fields

We investigate the ultrafast electron dynamics triggered by terahertz and optical pulses in thin platinum and gold films by probing their transient optical reflectivity. The response of the platinum film to an intense terahertz pulse is similar to the optically induced one and can be described by a two-temperature model with a 20% larger electron-phonon coupling for the terahertz-driven dynamics compared to the optically induced one, ascribed to an additional nonthermal electron-phonon coupling contribution. Surprisingly, gold films exhibit a much smaller terahertz pulse-induced reflectivity change and with a sign opposite to the optical case. We explain this remarkable observation with field emission of electrons due to Fowler-Nordheim tunneling, enabled in samples with thicknesses below the structural percolation threshold, where nanostructuring promotes near-field enhancement. Our results provide a fundamental insight into the ultrafast processes relevant to modern electro- and magneto-optical applications.