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Efficient operation of electronic nanodevices at ultrafast speeds requiresunderstanding and control of the currents generated by femtosecond bursts oflight. Ultrafast laser-induced currents in metallic nanojunctions can originatefrom photo-assisted hot electron tunneling or lightwave-induced tunneling. Bothprocesses can drive localized photocurrents inside a scanning tunnelingmicroscope (STM) on femto- to attosecond time scales, enabling ultrafast STMwith atomic spatial resolution. Femtosecond laser excitation of a metallicnanojunction, however, also leads to the formation of a transient thermalizedelectron distribution, but the tunneling of thermalized hot electrons on timescales faster than electron-lattice equilibration is not well understood. Here,we investigate ultrafast electronic heating and transient thermionic tunnelinginside a metallic photoexcited tunnel junction and its role in the generationof ultrafast photocurrents in STM. Phase-resolved sampling of broadband THzpulses via the THz-field-induced modulation of ultrafast photocurrents allowsus to probe the electronic temperature evolution inside the STM tip, and toobserve the competition between instantaneous and delayed tunneling due tononthermal and thermal hot electron distributions in real time. Our resultsreveal the pronounced nonthermal character of photo-induced hot electrontunneling, and provide a detailed microscopic understanding of hot electrondynamics inside a laser-excited tunnel junction.

Femtosecond Thermal and Nonthermal Hot Electron Tunneling inside a Photoexcited Tunnel Junction