Electron Transmission through a Smooth Tunnel Barrier in High-Frequency Field

The photon-assisted transmission of an electron through a smooth one-dimensional barrier that simulates the potential in the lower energy subband of the quantum point contact is numerically studied. The dependence of the transmission coefficient on energy shows shoulder-like features, that frequency dependence transforms into maxima. The features are caused by transitions into a channel with the energy close to the top of the barrier when one, two or three photons are absorbed, and their position does not depend of the amplitude of the high-frequency field. With decreasing frequency, the photon-induced steps disappear, but the derivative of the transmission coefficient on energy shows two peaks placed at points lower and higher average height of the barrier on the value equal the amplitude of oscillations. The discovered features can be observed experimentally upon irradiation of a semiconductor structure with a quantum point contact at frequencies 0.01-1.7 THz.