  We theoretically investigate tunneling through free-space or dielectric
nanogaps between metallic nanocontacts driven by ultrashort ultrabroadband
light pulses. For this purpose we develop a time-dependent quasiclassical
theory being especially suitable to describe the tunneling process in the
non-adiabatic regime, when this process can be significantly influenced by the
photon absorption as the electron moves in the classically forbidden region.
Firstly, the case of driving by an ideal half-cycle pulse is studied. For
different distances between the contacts, we analyze the main solutions having
the form of a quasiclassical wave packet of the tunneling electron and an
evanescent wave of the electron density. For each of these solutions the
resulting tunneling probability is determined with the exponential accuracy
inherent to the method. We identify a crossover between two tunneling regimes
corresponding to both solutions in dependence on the field strength and
intercontact distance that can be observed in the corresponding behaviour of
the tunneling probability. Secondly, considering realistic temporal profiles of
few-femtosecond pulses, we demonstrate that the preferred direction of the
electron transport through the nanogap can be controlled by changing the
carrier-envelope phase of the pulse, in agreement with recent experimental
findings and numerical simulations. We find analytical expressions for the
tunneling probability, determining the resulting charge transfer in dependence
on the pulse parameters. Further, we determine temporal shifts of the outgoing
electron trajectories with respect to the peaks of the laser field in
dependence on the pulse phase and illustrate when the non-adiabatical character
of the tunneling process is particularly important.
