Tuning Photoinduced Terahertz Conductivity in Monolayer Graphene: Optical Pump Terahertz Probe Spectroscopy

摘要

Optical pump-terahertz probe differential transmission measurements ofas-prepared single layer graphene (AG)(unintentionally hole doped with Fermienergy $E_F$ at $\sim$180 meV), nitrogen doping compensated graphene (NDG) with$E_F$ $\sim$10 meV and thermally annealed doped graphene (TAG) are examinedquantitatively to understand the opposite signs of photo-induced dynamicterahertz conductivity $\Delta\sigma$. It is negative for AG and TAG butpositive for NDG. We show that the recently proposed mechanism of multiplegenerations of secondary hot carriers due to Coulomb interaction ofphotoexcited carriers with the existing carriers together with the intrabandscattering can explain the change of photoinduced conductivity sign and itsmagnitude. We give a quantitative estimate of $\Delta\sigma$ in terms ofcontrolling parameters - the Fermi energy $E_F$ and momentum relaxation time$\tau$. Further, the cooling of photoexcited carriers is analyzed usingsuper-collision model which involves defect mediated collision of the hotcarriers with the acoustic phonons, thus giving an estimate of the deformationpotential.