MvL-Kolloquium: Prof. Dr. Ursula Keller

The basic motivation for our work is to understand how charge is transported on an atomic spatial and attosecond time scale. Strong-field ionization in the dipole approximation (i.e. tunnel ionization) is much faster than the group delay of the electron wavepacket (i.e. Wigner delay), whereas photoemission from atoms can typically be described by the Wigner delay. The measured photoionization delays from Ag, Au, and Cu surfaces critically depend on the available final bulk states. With resonant transitions, we have observed enhanced bulk emission and delays well described by transport. However, with no final bulk states, surface emission dominates with no significant contribution from transport. Femtosecond charge transport modulation driven by a transient electric field in the petahertz regime has been observed in diamond and can be explained by the dynamical Franz-Keldysh effect (DFKE). State-of-the-art numerical calculations reveal that intra-band transitions dominate the response over inter-band transitions.