Ultrafast Relaxation of Excited Dirac Fermions in Epitaxial Graphene Using Optical Differential Transmission Spectroscopy

Dong Sun, Zong-Kwei Wu, Charles Divin, Xuebin Li, Claire Berger, Walt A. de Heer, Phillip N. First, and Theodore B. Norris

Phys. Rev. Lett. 101, 157402 – Published 6 October 2008

Abstract

We investigate the ultrafast relaxation dynamics of hot Dirac fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission spectroscopy. We observe differential transmission spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, the electron cooling is determined by electron-acoustic phonon scattering, found to occur on the time scale of 1 ps for highly doped layers, and 4–11 ps in undoped layers. The spectra also provide strong evidence for the multilayer structure and doping profile of thermally grown epitaxial graphene on SiC.

Received 15 March 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.157402

Authors & Affiliations

Dong Sun¹, Zong-Kwei Wu¹, Charles Divin¹, Xuebin Li², Claire Berger², Walt A. de Heer², Phillip N. First², and Theodore B. Norris^1,*

¹Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109-2099, USA
²School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

^*Author to whom correspondence should be addressed. tnorris@eecs.umich.edu

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Vol. 101, Iss. 15 — 10 October 2008

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Images

Figure 1
Sample structure and energy dispersion curves of doped and undoped graphene layers. The sample has a buffer layer (green) on the SiC substrate followed by 1 heavily doped layer (red) and approximately 20 undoped layers (blue) on top. The Fermi level is labeled with a dashed line (brown) lying 348 meV (from the later data) above the Dirac point of the doped graphene layer and passing through the Dirac point of the undoped graphene layers. The blue solid line shows the transitions induced by the 800-nm optical pump pulse; the three dashed lines correspond to probe transitions at different energies with respect to the Fermi level (discussed in the text).Reuse & Permissions
Figure 2
DT spectrum normalized by transmission (T). (a) DT spectrum on epitaxial graphene at 10 K, with $500 − μ W$ 800-nm pump (photon fluence of $1.6 \times 10^{14} photons / {cm}^{2}$ per pulse), at probe delays of 10, 5, 2, 1, 0.5 ps, and background (50 ps before the pump arrives). The arrows at 1.78 and $2.35 μ m$ indicate the DT zero crossings. (b) DT time scan of the two probe wavelengths marked in (a) at the red ( $1.85 μ m$ ) and blue side ( $1.75 μ m$ ) of the $1.78 μ m$ DT zero crossing. (c) Time scan of the two probe wavelengths marked in (a) at the red ( $2.40 μ m$ ) and blue side ( $2.25 μ m$ ) of the $2.35 μ m$ DT zero crossing. In all figures, the dashed line (brown) marks where the DT signal is zero. The DT tails in (b) and (c) are simply fitted by a sigmoidal curve.Reuse & Permissions
Figure 3
Temperature dependent DT spectrum. (a) DT time scans at temperature 10, 30, 50, 77, 130, and 180 K with $500 μ W$ pump at 800 nm and $2.25 μ m$ probe. The DT time scans were fit with a sigmoidal curve to show more clearly the behavior of the zero crossings. In the inset the DT zero-crossing points at different temperatures are marked with different colors. (b) DT time scans at temperatures of 10, 77, and 287 K with 1 mW pump at 800 nm and $1.57 μ m$ probe. (c) DT time scans at temperatures of 77 and 290 K with 1 mW, 800 nm pump and $2.4 μ m$ probe. In all figures, the dashed line (brown) marks where the DT is zero.Reuse & Permissions
Figure 4
DT signal simulation. (a) Simulated transmission curves at different electron temperatures. In the inset, the transmission curves at low electron temperature are shown expanded for frequencies around the two DT zero crossings. (b) Simulated $DT / T$ curves at different electron temperatures with lattice temperature at 10 K. In the inset, the $DT / T$ curves for low electron temperatures are expanded in the vicinity of the two DT zero crossings. Both figures share the same legend.Reuse & Permissions

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