Femtosecond Optical-Field-Driven Currents in Few-Nanometer-Size Gaps with Hot Electron Injection into Metallic Leads

Current nanoscale optoelectronic devices can reach femtosecond response times by exploiting highly nonlinear light-matter interactions. Shaping of the field waveform of few-cycle optical pulses allows one to control electron emission from nanotips and nanoparticles as well as to drive electron transport in ≳10 nm wide plasmonic gaps. In this work, we address the less explored optically induced electron transport in much narrower, 1-2 nm metallic gaps of interest in many practical situations such as in light-wave-driven scanning tunneling microscopy or in transduction between electrons and photons for optoelectronic applications. Using the time-dependent density functional theory, model calculations, and semi-classical electron trajectories derived from an analytical strong-field model, we bring robust evidence that the sub-cycle bursts of photoemitted electrons might cross the gap prior to the change of the sign of the optical field and thus without experiencing quiver motion. This leads to a characteristic carrier-envelope phase dependence of the net electron transport. Most importantly, we show that in the optical field emission regime, continuous acceleration of electron bursts moving in the gap by an optical field results in high electron energies. The electron current in a narrow-gap nanocircuit is then associated with hot electron injection into the metallic leads characterized by a non-thermal post-injection energy distribution. This is in contrast with electron transport through wide gaps dominated by low-energy electrons. Our results contribute to the design of optoelectronic devices operating on femtosecond temporal and nanometer spatial scales.