  We report on the first experimental observation of graphene optical emission
induced by the intense THz pulse. P-doped CVD graphene with the initial Fermi
energy of about 200 meV was used, optical photons was detected in the
wavelength range of 340-600 nm. Emission started when THz field amplitude
exceeded 100 kV/cm. For THz fields from 200 to 300 kV/cm the temperature of
optical radiation was constant, while the number of emitted photons increased
several dozen times. This fact clearly indicates multiplication of
electron-hole pairs induced by an external field itself and not due to electron
heating. The experimental data are in a good agreement with the theory of
Landau-Zener interband transitions. It is shown theoretically that Landau-Zener
transitions are possible even in the case of heavily doped graphene because the
strong THz field removes quasiparticles from the region of interband
transitions during several femtoseconds, which cancels the Pauli blocking
effect.
