Tunneling processes induced by terahertz electric fields.

Tunneling processes induced by terahertz frequency electric fields havebeen investigated.A drastic enhancement of the tunneling probabilityhas been observed by increasing the frequency ω atωτ(e)≫ 1 whereτ(e) is the tunneling time.For a given constant tunneling rate an increase offrequency by a factor of seven leads to a drop of the requiredelectric field strengthby three orders of magnitude.It is shown that the enhancement of tunneling ionization at terahertz frequencies is due to the factthat electrons can absorb energy from the radiation field during tunnelingreducing the effective width of the tunneling barrier.