  We perform a femtosecond-THz optical pump-probe spectroscopy to investigate
the cooling dynamics of hot carriers in quasi-free standing bilayer epitaxial
graphene. We observed longer decay time constants, in the range of 2.6 to 6.4
ps, compared to previous studies on monolayer graphene, which increase
nonlinearly with excitation intensity. The increased relaxation times are due
to the decoupling of the graphene layer from the SiC substrate after hydrogen
intercalation which increases the distance between graphene and substrate.
Furthermore, our measurements do not show that the supercollision mechanism is
related to the cooling process of the hot carriers, which is ultimately
achieved by electron-optical phonon scattering.
