Identification of multiphoton induced photocurrents in metal-insulator-metal junctions

Illumination of metal-insulator-metal junctions with ultrashort laser pulses and multiphoton electron excitation in the top electrode leads to the injection of electrons into the backside electrode. Time resolved photocurrent spectroscopy shows that the carrier injection is not instantaneous but occurs with an effective lifetime of about 30 fs. The observation of an effective lifetime reveals that photon-assisted tunneling is negligible. It is shown that three-photon induced internal photoemission and two-photon induced tunneling of excited electrons are the dominating transport mechanisms.