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We report on the first experimental observation of graphene optical emissioninduced by the intense THz pulse. P-doped CVD graphene with the initial Fermienergy of about 200 meV was used, optical photons was detected in thewavelength range of 340-600 nm. Emission started when THz field amplitudeexceeded 100 kV/cm. For THz fields from 200 to 300 kV/cm the temperature ofoptical radiation was constant, while the number of emitted photons increasedseveral dozen times. This fact clearly indicates multiplication ofelectron-hole pairs induced by an external field itself and not due to electronheating. The experimental data are in a good agreement with the theory ofLandau-Zener interband transitions. It is shown theoretically that Landau-Zenertransitions are possible even in the case of heavily doped graphene because thestrong THz field removes quasiparticles from the region of interbandtransitions during several femtoseconds, which cancels the Pauli blockingeffect.

Optical emission of graphene and electron-hole pair production induced by a strong THz field