Tunneling is a peculiar quantum phenomenon with no classical counterpart. It plays an essential role for strong field phenomena in atoms and molecules interacting with intense lasers. Processes such as high-order harmonic generation are driven by electron dynamics following tunnel ionization.

While this has been widely explored, the behavior of electrons under the tunneling barrier, though equally significant, has remained obscure. The understanding of laser-induced strong field ionization distinguishes two scenarios for a given system and laser frequency: the multiphoton regime at rather low intensities and tunneling at high intensities.

However, most strong-field experiments have been carried out in an intermediate situation where multiphoton signatures are observed while tunneling is still the dominant process.