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arXiv · 1701.06314

Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene

S. Aeschlimann·R. Krause·M. Chávez-Cervantes·H. Bromberger·A. Al-Temimy·C. Coletti·A. Cavalleri·I. Gierz
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Abstract

The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photo-excited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization, and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy and in-plane momentum. We first show that the rapidly-established quasi-thermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photo-excited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photo-control experiments and optoelectronic device applications.

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S. Aeschlimann, R. Krause, M. Chávez-Cervantes, H. Bromberger, A. Al-Temimy, C. Coletti, A. Cavalleri, I. Gierz. 2017-01-23. Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene. https://doi.org/10.1103/physrevb.96.020301

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