Ultrafast Carrier Dynamics in Graphite

Optical pump-probe spectroscopy with 7-fs pump pulses and a probe spectrum wider than 0.7 eV reveals the ultrafast carrier dynamics in freestanding thin graphite films. We discern for the first time a rapid intraband carrier equilibration within 30 fs, leaving the system with separated electron and hole chemical potentials. Phonon-mediated intraband cooling of electrons and holes occurs on a 100 fs time scale. The kinetics are in agreement with simulations based on Boltzmann equations.

Figure 1

(a) Idealized linear band structure of graphene, red arrow indicating pumped and probed direct optical transitions. (b) Schematic of temporally evolving electronic occupation probabilities after optical excitation and under the influence of carrier-phonon scattering (indicated by ${\tau }_{c\mathrm{\text{−}}\mathrm{ph}}$), assuming a thermalization towards zero chemical potential. (c) The same for persistent carrier densities ($\mu \ne 0$). (d) Spectrally integrated DT as a function of pump-probe delay: experiment (open circles) and numerical fit (solid line). Dash-dotted line: Cross correlation of pump and probe pulses. Inset: Linear dependence of the maximum transmission change on the absorbed pump fluence.

Figure 2

(a) Spectrally resolved differential transmission (DT) plotted as a function of probe photon energy $E_{\mathrm{pr}}$ and short and long pump-probe delays (excitation density $5\times {10}^{20}{\mathrm{cm}}^{-3}$). (b) DT transients at $E_{\mathrm{pr}}=1.24\mathrm{eV}$ (solid), 1.55 eV (dash-dotted) and 1.77 eV (dashed) for short and long delays. Inset: Decay time ${\tau }_2$ vs $E_{\mathrm{pr}}$.

Figure 3

Top: Optical pump spectrum. Bottom: Experimental DT spectra (solid black lines) for fixed time delays of (top to bottom) 20, 40, 80, 150, 250, and 1000 fs, together with fits to the model described in the text (gray lines) and best-fit DT spectra assuming a zero chemical potential (dashed lines).

Figure 4

(a) Time-dependent carrier temperature (black circles) and chemical potential (gray triangles) derived from the fit to the data of Figs. 2, 3. (b) Carrier temperature and (c) chemical potential calculated with instantaneous (dashed lines) and without (solid lines) phonon decay; dash-dotted lines represent a simulation with $\mu =0$.