The ability to steer electrons using the strong electromagnetic field of light has opened up the possibility of controlling electron dynamics on the sub-femtosecond timescale. In dielectrics and semiconductors, various light-field-driven effects have been explored, including high-harmonic generation and sub-optical-cycle interband population transfer. In contrast, much less is known about light-field-driven electron dynamics in narrow-bandgap systems or in conductors, in which screening due to free carriers or light absorption hinders the application of strong optical fields.
Graphene is a promising platform with which to achieve light-field-driven control of electrons in a conducting material because of its broadband and ultrafast optical response, weak screening and high damage threshold. We have recently shown that a current induced in monolayer graphene by two-cycle laser pulses is sensitive to the electric-field waveform, that is, to the exact shape of the optical carrier field of the pulse, which is controlled by the carrier-envelope phase, with a precision on the attosecond timescale. Such a current, dependent on the carrier-envelope phase, shows a striking reversal of the direction of the current as a function of the driving field amplitude at about two volts per nanometre. This reversal indicates a transition of light–matter interaction from the weak-field (photon-driven) regime to the strong-field (light-field-driven) regime, where the intraband dynamics influence interband transitions.
We show that in this strong-field regime the electron dynamics are governed by sub-optical-cycle Landau–Zener–Stückelberg interference, composed of coherent repeated Landau–Zener transitions on the femtosecond timescale. Time permitting, we will show another type of quantum path interference in multiphoton emission of electrons from nanoscale tungsten tips, where the admixture of a few percent of second harmonic radiation can suppress or enhance the emission with a visibility of 98%, depending on the relative phase of fundamental and second harmonic.
Dr. Peter Hommelhoff is professor of physics at Friedrich Alexander University Erlangen-Nuremberg (FAU). He studied physics at TU Berlin and ETH Zurich, where he obtained his diploma in 1999. In 2002 he completed his PhD in T. W. Hänsch's atom chip group at Ludwig Maximilian University Munich. From 2003 to 2007 he was a postdoctoral fellow in M. Kasevich's group at Stanford University and subsequently became head of a Max Planck Research Group at Max Planck Institute for Quantum Optics in Garching/Munich. From there, he went to FAU in Erlangen in 2012, where he is also associated with the Max Planck Institute for the Science of Light. Dr. Hommelhoff has received, amongst others, a Lynen Fellowship of the Alexander-von-Humboldt Foundation, a Trimble Postdoctoral Fellowship, and an ERC Consolidator Grant.