  We investigated negative photoconductivity in graphene using ultrafast
terahertz techniques. Infrared transmission was used to determine the Fermi
energy, carrier density and mobility of p-type CVD graphene samples.
Time-resolved terahertz photoconductivity measurements using a tunable
mid-infrared pump probed these samples at photon energies between 0.35eV to
1.55eV, approximately one half to three times the Fermi energy of the samples.
Although interband optical transitions in graphene are blocked for pump photon
energies less than twice the Fermi energy, we observe negative
photoconductivity at all pump photon energies investigated, indicating that
interband excitation is not required to observe this effect. Our results are
consistent with a thermalized free carrier population that cools by
electron-phonon scattering, but inconsistent with models of negative
photoconductivity based on population inversion.
