Light-field control of real and virtual charge carriers

Light-driven electronic excitation is a cornerstone for energy and information transfer. In the interaction of intense and ultrafast light fields with solids, electrons may be excited irreversibly, or transiently during illumination only. Asthe transient electron population cannot be observed after the light pulse isgone, it is referred to as virtual, whereas the population that remains excited is called real(1,)(4). Virtual charge carriers have recently been associated with high-harmonicgeneration and transient absorption(5-)(8), but photocurrentgeneration may stem from real as well as virtual charge carriers(9,14). However, a link between the generation ofthe carrier types and their importance for observables oftechnological relevance is missing. Here we show that real and virtual charge carriers can be excited and disentangled in the optical generation of currents in a gold-graphene-gold heterostructure using few-cycle laser pulses. Depending on the waveform used for photoexcitation, real carriers receive net momentum and propagate to the gold electrodes, whereasvirtual carriersgenerate a polarization response read out at the gold-graphene interfaces. On the basis ofthese insights, we further demonstrate a proof of concept of a logic gate for future lightwave electronics. Our results offer a direct meansto monitor and excite real and virtual charge carriers. Individual control over each type of carrier will markedly increase the integrated-circuit design space and bring petahertz signal processing closer to reality(15-16).

Automated summary

Light-driven electronic excitation plays a crucial role in energy and information transfer. Researchers have discovered that virtual and real charge carriers can be excited and disentangled in a gold-graphene-gold heterostructure, offering new possibilities for lightwave electronics.