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Phys Rev Lett. 2012 Apr 20;108(16):167401. Epub 2012 Apr 16.

Femtosecond population inversion and stimulated emission of dense Dirac fermions in graphene.

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Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.


We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.

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