Journal Article

Light-field-driven currents in graphene

Higuchi, Takuya, Ullmann, Konrad, Heide, Christian, Weber, Heiko B., Hommelhoff, Peter

2017

Overview

Light-field-driven control of electrons in a conductor is demonstrated by inducing a current by laser pulses in graphene that is sensitive to the carrier-envelope phase. A light on current advances in graphene A new field of ultrafast electronics has emerged in recent years that uses strong electromagnetic (light) fields to steer electrons. It is difficult to explore such effects in metals because of screening by the charge carriers, so mainly insulators and semiconductors have been studied in this area. However, different physics applies to graphene, a two-dimensional material in which only weak screening is present. Takuya Higuchi et al . have now generated electric currents in graphene with ultrafast light fields. The current is sensitive 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. The results open up new opportunities for creating light-field-driven petahertz electronics. 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 (less than 10 −15 seconds) timescale. In dielectrics and semiconductors, various light-field-driven effects have been explored, including high-harmonic generation 1 , 2 , 3 , 4 , sub-optical-cycle interband population transfer 5 and the non-perturbative change of the transient polarizability 6 , 7 . 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 6 , 8 . 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 9 , 10 . Here we show 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 (10 −18 seconds) 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 11 , composed of coherent repeated Landau–Zener transitions on the femtosecond timescale. Furthermore, the influence of this sub-optical-cycle interference can be controlled with the laser polarization state. These coherent electron dynamics in graphene take place on a hitherto unexplored timescale, faster than electron–electron scattering (tens of femtoseconds) and electron–phonon scattering (hundreds of femtoseconds) 12 , 13 , 14 . We expect these results to have direct ramifications for band-structure tomography 2 and light-field-driven petahertz electronics 8 .

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/301/1019/584 / 639/624/400/584 / 639/925/918/1054 / Conductors / Electric properties / Electromagnetic fields / Electromagnetism / Graphene / Humanities and Social Sciences / Interferometry / Lasers / letter / Light / Light absorption / multidisciplinary / Optical properties / Science / Semiconductors