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We theoretically examine the effect of carrier-carrier scattering processes(electron-hole and electron-electron) on the intraband radiation absorption andtheir contribution to the net dynamic conductivity in optically or electricallypumped graphene. We demonstrate that the radiation absorption assisted by thecarrier-carrier scattering can be stronger than the Drude absorption due to thecarrier scattering on disorder. Since the intraband absorption of radiationeffectively competes with its interband amplification, this can substantiallyaffect the conditions of the negative dynamic conductivity in the pumpedgraphene and, hence, the interband terahertz and infrared lasing. We find thethreshold values of the frequency and quasi-Fermi energy of nonequilibriumcarriers corresponding to the onset of negative dynamic conductivity. Theobtained results show that the effect of carrier-carrier scattering shifts thethreshold frequency of the radiation amplification in pumped graphene to highervalues. In particular, the negative dynamic conductivity is attainable at thefrequencies above 6 THz in graphene on SiO2 substrates at room temperature. Thethreshold frequency can be decreased to markedly lower values in graphenestructures with high-k substrates due to screening of the carrier-carrierscattering, particularly at lower temperatures.

Carrier-carrier scattering and negative dynamic conductivity in pumped graphene