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Abstract

Efficient operation of electronic nanodevices at ultrafast speeds requires
understanding and control of the currents generated by femtosecond bursts of
light. Ultrafast laser-induced currents in metallic nanojunctions can originate
from photo-assisted hot electron tunneling or lightwave-induced tunneling. Both
processes can drive localized photocurrents inside a scanning tunneling
microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM
with atomic spatial resolution. Femtosecond laser excitation of a metallic
nanojunction, however, also leads to the formation of a transient thermalized
electron distribution, but the tunneling of thermalized hot electrons on time
scales faster than electron-lattice equilibration is not well understood. Here,
we investigate ultrafast electronic heating and transient thermionic tunneling
inside a metallic photoexcited tunnel junction and its role in the generation
of ultrafast photocurrents in STM. Phase-resolved sampling of broadband THz
pulses via the THz-field-induced modulation of ultrafast photocurrents allows
us to probe the electronic temperature evolution inside the STM tip, and to
observe the competition between instantaneous and delayed tunneling due to
nonthermal and thermal hot electron distributions in real time. Our results
reveal the pronounced nonthermal character of photo-induced hot electron
tunneling, and provide a detailed microscopic understanding of hot electron
dynamics inside a laser-excited tunnel junction.