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From coherent electron trajectory control in graphene to attosecond-fast charge transfer at graphene-based interfaces

Date and Time: 
August 26, 2019 - 2:00pm
Location: 
Shriram 262

From coherent electron trajectory control in graphene to attosecond-fast charge transfer at graphene-based interfaces

Christian Heide

Department of Physics
University of Erlangen–Nuremberg (FAU), Germany
e-mail:Christian.heide@fau.de

 

Graphene with its exceptional optical and electronic properties is a unique material for light-field controlled electron dynamics inside of a (semi-)metal. Its Dirac cone dispersion relation is ideally suited to drive intraband currents and interband transitions by the optical field of phase-controlled few-cycle laser pulses [1, 2, 3]. In particular, when the influence of the intraband dynamics to the interband transition cannot be neglected, these combined dynamics turn into a novel non-perturbative light-matter interaction regime. Based on the coupled nature of the intraband and interband processes, we observe repeated coherent Landau-Zener transitions between valence and conduction band separated by around half an optical period of ~1.3 fs. Because of the extremely fast dynamics, fully coherent Landau-Zener-Stückbelberg (LZS) interferometry takes place. Moreover, we could show complex electron trajectory control in the graphene plane by tailoring the polarization state of the driving laser pulses. That way we can manipulate subsequent LZS interferences [3].
In order to utilize graphene for electronics, built-in functional interfaces are crucial, such as p-n junctions or Schottky contacts. In the second part of the talk, we combine graphene with the bulk material silicon carbide and show that the charge transfer across the graphene-SiC solid-state interface takes place within 300 attoseconds, which is the fastest charge transfer observed across a solid-state interface [4]. To reveal the attosecond dynamics, we use saturable absorption in graphene as an intrinsic clock to determine how long an excited state stays excited before charge transfer and thermalization depopulates this state. This process and its time scales are fully supported by numerical simulations, which also take various thermalization decay channels into account. The root cause for the extremely fast charge transfer time in this peculiar solid-state interface lies in the combination of the employed materials: the atomically thin graphene, with photo-excited electrons right next to the interface, and a 3D semiconductor, ideally suited to receive excited electrons without any backlash.
[1] T. Higuchi, C. Heide, K. Ullmann, H. B. Weber, P. Hommelhoff, Nature 2017, 550, 224–228.
[2] C. Heide, T. Boolakee, T. Higuchi, H.B. Weber, P. Hommelhoff, 2019, New J. Phys. 21 045003.
[3] C. Heide, T. Higuchi, H. B. Weber, P. Hommelhoff, Phys. Rev. Lett. 2018, 121, 207401.
[4] C. Heide, M. Hauck, T. Higuchi, J. Ristein, L. Ley, H. B. Weber, P. Hommelhoff, under rev. at Nature photonics.