  Spontaneous optical emission of graphene irradiated by intense single-cycle
terahertz pulses was investigated experimentally and explained theoretically.
We found that emitted photons are polarized predominantly perpendicular to the
electric field of the terahertz pulse, which proves that the terahertz field
not only heats the electrons, but also creates a strongly nonequilibrium
momentum distribution. Comparison of the measured optical spectrum and
polarization anisotropy with the results of numerical modeling allowed us to
estimate a momentum isotropization time for electrons in graphene to be ~25 fs
and roughly reconstruct the distribution function evolution in k-space.
