  We experimentally observe the fast relaxation and relatively slow
recombination dynamics of photogenerated electrons/holes in an epitaxial
graphene-on-Si heterostructure under pumping with a 1550-nm, 80-fs pulsed fiber
laser beam and probing with the corresponding terahertz (THz) beam generated by
and synchronized with the pumping laser. The time-resolved electric-field
intensity originating from the coherent terahertz photon emission is
electro-optically sampled in total-reflection geometry. The Fourier spectrum
from 1.8 to 5.2 THz agrees well the pumping photon spectrum. This result is
attributed to amplified emission of THz radiation from the graphene sample
stimulated by the THz probe beam, and provides evidence for the occurrence of
negative dynamic conductivity in the terahertz spectral range.
