Strong Carrier Temperature Dependence of Complex THz Conductivity of Photo-excited Graphene due to electron-phonon coupling

Ultra-broadband THz conductivities of doped monolayer graphene on PET substrate after photo-excitation at various optical pump fluence are investigated. THz conductivity corresponds to the intra-band transition of hot carriers decreases and deviates from the Drude-type frequency dependence as the pump fluence increases, indicating the broader carrier-energy distribution and energy dependent scattering. However, we found that the semi-classical Boltzmann transport theory with dominant carrier scattering mechanisms such as charged impurity and intrinsic optical phonon scattering with e-ph coupling strength from Density functional theory within GGA approximation failed to explain the hot carrier temperature dependence of THz conductivity spectra. This result suggests that the underestimation of e-ph coupling by DFT calculation and the importance of e-e interaction on the e-ph coupling.