Ultrafast optically induced tunneling in narrow metallic gaps from the time dependent density functional perspective

In this work, using the time-dependent density functional theory, we address the electron tunneling triggered by short (single-cycle and several-cycle) optical pulses in narrow metallic gaps under conditions relevant for actual experiments. We identify photon-assisted tunneling with one-photon, two-photon, and higher-order photon absorption, and we discuss the effect of the tunneling barrier, applied bias, and strength of the optical field on transition from photon-assisted tunneling (weak optical fields) to the optical field emission at strong optical fields. The numerical single-electron calculations and an analytical strong-field theory model are used to gain deeper insights into the results of the time-dependent density functional theory calculations. Additionally, our parameter-free calculations allow us to retrieve and explain recent experimental results on optically induced transport in narrow metallic gaps under an applied dc bias.