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We present a numerical study on the intraband optical conductivity of hotcarriers at quasi-equilibria in photoexcited graphene based on thesemiclassical Boltzmann transport equations (BTE) with the aim of understandingthe effects of intrinsic optical phonon and extrinsic coulomb scattering causedby charged impurities at the graphene--substrate interface. Instead of usingfull-BTE solutions, we employ iterative solutions of the BTE and thecomprehensive model for the temporal evolutions of hot-carrier temperature andhot-optical-phonon occupations to reduce computational costs. Undoped grapheneexhibits large positive photoconductivity owing to the increase in thermallyexcited carriers and the reduction in charged impurity scattering. Thefrequency dependencies of the photoconductivity in undoped graphene having highconcentrations of charged impurities significantly deviate from those observedin the simple Drude model, which can be attributed to temporally varyingcharged impurity scattering during terahertz (THz) probing in the hot-carriercooling process. Heavily doped graphene exhibits small negativephotoconductivity similar to that of the Drude model. In this case, chargedimpurity scattering is substantially suppressed by the carrier-screeningeffect, and the temperature dependencies of the Drude weight and optical phononscattering governs the negative photoconductivity. In lightly doped graphene,the appearance of negative and positive photoconductivity depends on thefrequency and the crossover from negative photoconductivity to positive emergesfrom increasing the charged impurity concentration. This indicates the changeof the dominant scattering mechanism from optical phonons to chargedimpurities. Our approach provides a quantitative understanding of non-Drudebehaviors and the temporal evolution of photoconductivity in graphene.

Intrinsic and extrinsic effects on intraband optical conductivity of hot carriers in photoexcited graphene