  Using optical-pump terahertz-probe spectroscopy, we study the relaxation
dynamics of photoexcited carriers in graphene at different temperatures. We
find that at lower temperatures the tail of the relaxation transients as
measured by the differential probe transmission becomes slower, extending
beyond several hundred picoseconds at temperatures below 50K. We interpret the
observed relaxation transients as resulting from the cooling of the
photoexcited carriers via phonon emission. The slow cooling of the photoexcited
carriers at low temperatures is attributed to the bulk of the electron and hole
energy distributions moving close enough to the Dirac point such that both
intraband and interband scattering of carriers via optical phonon emission
becomes inefficient for removing heat from the carriers. Our model, which
includes intraband carrier scattering and interband carrier recombination and
generation, agrees very well with the experimental observations.
