2013/04/04 by Isabella Gierz, Jesse C. Petersen, Matteo Mitrano +8 · 1 citation
Engineering · Materials Science · Physics and Astronomy · #2D Materials and Applications #Graphene research and applications #Plasmonic and Surface Plasmon Research #cond-mat.mes-hall #cond-mat.mtrl-sci
paper · pdf · doi:10.1038/nmat3757
published as Nature Materials 12, 1119 (2013) · 16 pages, 7 figures
arxiv created 2013/04/04 · openalex publication_date 2013/10/04 · arxiv updated 2014/09/02 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a semiconducting bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy and momentum dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme ultra-violet (EUV) pulses at 31.5 eV photon energy to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. While excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find indication for carrier multiplication, questioning the applicability of graphene for light harvesting. Time-resolved photoemission spectroscopy in the EUV emerges as the technique of choice to assess the suitability of new materials for optoelectronics, providing quantitatively accurate measurements of non-equilibrium carriers at all energies and wavevectors.