  Room temperature Terahertz stimulated emission and population inversion in
optically pumped graphene is reported. We experimentally observe fast
relaxation and relatively slow recombination dynamics of photogenerated
electrons/holes in an exfoliated graphene on SiO2/Si substrate under pumping
with a 1550-nm, 80-fs pulsed fiber laser beam and probing with the
corresponding terahertz beam generated by optical rectification in a nonlinear
electro optical sensor. The time resolved electric field intensity originating
from the coherent terahertz photon emission is electro-optically sampled in an
total-reflection geometry. The comparison of terahertz electric fields
intensities measured on SiO2/Si substrate and that one from graphene clearly
indicate that graphene sheet act like an amplifying medium. The Emission
spectra agrees relatively well the pumping photon spectrum and its dependency
on the pumping power shows a threshold like behavior, testifying the occurrence
of the negative conductivity in the THz spectral range and the population
inversion. The threshold pumping intensity > 5*10^6 W/cm^2 is in a good
agreement with simulations.
