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24 February 2009 Graphene under optical pumping: nonequilibrium distributions, population inversion, and terahertz lasing
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We study nonequilibrium carriers (electrons and holes) in an intrinsic graphene at low temperatures under farand mid-infrared optical pumping in a wide range of its power densities. The energy distributions of carriers are calculated using a quasiclassic kinetic equation which accounts for the energy relaxation due to acoustic phonons and the radiative generation-recombination processes associated with thermal radiation and the carrier photoexcitation by incident radiation. It is found that the nonequilibrium distributions are determined by an interplay between weak energy relaxation on acoustic phonons and generation-recombination processes as well as by the effect of pumping saturation. Due to the effect of pumping saturation, the carrier distribution functions can exhibit plateaus whose width increases with increasing pumping power density. The graphene steady-state conductivity as a function of the pumping power density exhibits a pronounced nonlinearity with a sub-linear region at fairly low power densities. As shown, at certain pumping power density the population inversion as well as the dynamic negative conductivity can take place in terahertz and far-infrared frequencies, suggesting the possibility of utilization of graphene under optical pumping for optoelectronic applications, in particular, lasing at such frequencies.
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A. Satou, M. Ryzhii, F. T. Vasko, T. Otusji, and V. Ryzhii "Graphene under optical pumping: nonequilibrium distributions, population inversion, and terahertz lasing", Proc. SPIE 7211, Physics and Simulation of Optoelectronic Devices XVII, 721119 (24 February 2009);

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