Optical detection of tunneling ionization.

We have experimentally detected optical harmonics that are generated due to a tunneling-ionization-induced modulation of the electron density. The optical signature of electron tunneling can be isolated from concomitant optical responses by using a noncollinear pump-probe setup. Whereas previously demonstrated tools for attosecond metrology of gases, plasmas, and surfaces rely on direct detection of charged particles, detection of the background-free time-resolved optical signal, which uniquely originates from electron tunneling, offers an interesting alternative that is especially suited for systems in which free electrons cannot be directly measured.