1. Ultrafast Carrier Dynamics in Graphene M. Breusing, N. Severin, S. Eilers, J. Rabe and T. Elsässer Conclusion information about carrier distribution with10fs time resolution Carrier equalibration / formation of Fermi-Dirac distribution within first 100fs Carrier optical phonon scattering with time const. of about 150fs substrate influences observably the carrier distribution, but not the cooling by phonon scattering Motivation Graphene - building block for future nanostructured electronic devices (FET, analog GHz-THz applications) Optical application (e.g. saturable absorber) carrier relaxation - dominant limit for high frequency application Semi-metal –> tendency towards metals or semiconductors is still an open issue influence of supporting media for monolayer important Sample Preparation / Analysis Graphene on Muscovite (Mica) Spectrum of laser source offering bandwidth of 0.6eV Decrease of sharp spectral features in  T/T indicate carrier equilibration Spectra for various delays of both sample kinds; in red: best fit assuming Fermi-Dirac distribution Extracted carrier Temperature (T) and chem. potential (µ) within the first ps. Phonon scattering reduces T within the first 300fs; simultaneously µ rises, but reaches different values Different kinds of samples (with / without water-film) Spectrally integrated transients and fits of transmission change for sample with water film (blue / green) and without (black / red) Inset shows linear dependence on added energy Spectral and time resolved transmission change (  T/T) Shift to lower energies for longer delays clearly visible abs. after t D … Pump-Probe Spectroscopy Two delayed ultrashort laserpulses Probe detects pump induced sample changes Absorption changes (  ) depend on carrier distribution (f e,f h ) Graphite on Oxidized Silicon  ph G-band  ph D‘-band  ph D-band Spectrally resolved  R/R, simulated and fitted by Fresnel equations combined with transfer matrix method, assuming Fermi-Dirac distribution Temperature (T) drops within first 200fs, chem. potential (µ) rises coevally, but returns to zero within first ps (1) Sample structure; the well defined oxidized layer induces relevant multiple reflections and thereby Fabry-Perot oscillations in reflected light (2) spectrally integrated reflection change (  R/R) for thick graphite (blue) and graphene (black), corrected for substrate contributions (3) Sample analysis by Raman spectroscopy – single D‘ peak indicates single layer graphene, absence of, for idealized graphene forbidden, D peak high crystal quality (1) (2) (3) Properties Graphene M. Breusing et al., Phys. Rev. Lett. 102 (2009) 3 layers of graphene (two dimensional carbon lattice) Brillouin zone of graphene, showing conical bands centered at K and K‘ Tips of conduction and valence band cones touch each other at E F =0eV, making graphene a semi-metal Pump-Probe Set-Up Focal spot diameter  8µm Lock-in detection Time resolution 10fs Carrier dynamic simulation for graphene based on Bloch- Boltzmann- Peierls equations 3 cases assumed: no varying µ (dash-dotted), istantaneous phonon decay (dashed) and infinite phonon lifetime (solid)