Fig. 4.

Tunneling currents driven by THz and optical fields together. a Pictorial description of an electromagnetically applied time-dependent potential across a quantum barrier. A THz field and an added optical field with specific time delay together drive the tunneling current. b Tunneling current density calculated by using the Simmons formula (barrier height of 2.2 eV and gap size of 2 nm), describing a non-vanishing optical current under the THz field. Note the log-scale plot of the tunneling current density that shows the extremely nonlinear behavior of the optically induced current