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For many of the envisioned optoelectronic applications of graphene it iscrucial to understand the sub-picosecond carrier dynamics immediately followingphotoexcitation, as well as the effect on the electrical conductivity - thephotoconductivity. Whereas these topics have been studied using variousultrafast experiments and theoretical approaches, controversial and incompleteexplanations have been put forward concerning the sign of thephotoconductivity, the occurrence and significance of the creation ofadditional electron-hole pairs, and, in particular, how the relevant processesdepend on Fermi energy. Here, we present a unified and intuitive physicalpicture of the ultrafast carrier dynamics and the photoconductivity, combiningoptical pump - terahertz probe measurements on a gate-tunable graphene device,with numerical calculations using the Boltzmann equation. We distinguish twotypes of ultrafast photo-induced carrier heating processes: At low(equilibrium) Fermi energy ($E_{\rm F} \lesssim$ 0.1 eV for our experiments)broadening of the carrier distribution involves interband transitions -interband heating. At higher Fermi energy ($E_{\rm F} \gtrsim$ 0.15 eV)broadening of the carrier distribution involves intraband transitions -intraband heating. Under certain conditions, additional electron-hole pairs canbe created (carrier multiplication) for low $E_{\rm F}$, and hot carriers(hot-carrier multiplication) for higher $E_{\rm F}$. The resultantphotoconductivity is positive (negative) for low (high) $E_{\rm F}$, whichoriginates from the effect of the heated carrier distributions on the screeningof impurities, consistent with the DC conductivity being mostly due to impurityscattering. The importance of these insights is highlighted by a discussion ofthe implications for graphene photodetector applications.

The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies