Tuning Photoinduced Terahertz Conductivity in Monolayer Graphene:   Optical Pump Terahertz Probe Spectroscopy
  Optical pump-terahertz probe differential transmission measurements of as-prepared single layer graphene (AG)(unintentionally hole doped with Fermi energy $E_F$ at $\sim$180 meV), nitrogen doping compensated graphene (NDG) with $E_F$ $\sim$10 meV and thermally annealed doped graphene (TAG) are examined quantitatively to understand the opposite signs of photo-induced dynamic terahertz conductivity $\Delta\sigma$. It is negative for AG and TAG but positive for NDG. We show that the recently proposed mechanism of multiple generations of secondary hot carriers due to Coulomb interaction of photoexcited carriers with the existing carriers together with the intraband scattering can explain the change of photoinduced conductivity sign and its magnitude. We give a quantitative estimate of $\Delta\sigma$ in terms of controlling parameters - the Fermi energy $E_F$ and momentum relaxation time $\tau$. Further, the cooling of photoexcited carriers is analyzed using super-collision model which involves defect mediated collision of the hot carriers with the acoustic phonons, thus giving an estimate of the deformation potential. 
Srabani Kar, Dipti R. Mohapatra, Eric Freysz, A. K. Sood
cond-mat.mtrl-sci