Femtosecond carrier dynamics in graphite

We present a comprehensive report of pump-probe reflectivity and transmission measurements on highly oriented pyrolytic graphite with 50 fs time resolution. The experiments trace the generation, relaxation, and recombination of nonequilibrium carriers in a quasi-two-dimensional semimetallic solid over a wide range of experimental parameters. The fluence of excitation at hν=2.0 eV was varied between 10^-6 and 10^-2 J/cm², below the threshold for optical damage, while probe pulses in the photon energy range 1.5<hν<4.0 eV were used. On a subpicosecond time scale we observe a strong, initial, broadband absorption saturation caused by state filling by a hot, dense π-band electron population, which recovers with a fluence- and probe-wavelength-dependent time constant as the carriers cool and recombine in less than 1 ps. Later dynamics reflect the generation and diffusion of heat in the lattice, and are consistent with previous picosecond reflectivity measurements.