The noteworthy optoelectronic properties of graphene stimulated an intense research activity in the past decade, among which it is of particular technological importance the design of photodetectors with breakthrough specifications . Any progress in this direction relies on the understanding of fundamental physical processes such as photoexcitation and carrier relaxation (see e.g. [2,3]), and their impact on graphene's conductivity. In this work , we use optical-pump THz-probe spectroscopy to investigate the THz photoconductivity of a graphene system where the equilibrium carrier density is varied over a wide range by a drop cast ionic gate. The experimental results are interpreted using a theoretical framework which includes the calculations of the nonequilibrium carrier distribution following photoexcitation and of the THz conductivity of the photoexcited system. We find that, as the equilibrium carrier density increases, the photoconductivity changes sign from positive to negative and, at the same time, the dominant electronic heating channel changes from interband to intraband. We highlight the key role that, in this transition, is played by the temperature- and density-dependent electronic screening of the impurities which limit the dc conductivity. Our results are also an evidence of graphene supporting the so-called carrier multiplication process, which is of substantial technological relevance for the optimization of light-harvesting devices.
A.T. and M.P. are supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 696656 “Graphene Core1”.
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