▎ 摘　　要

The Dirac fermion, a high-mobility electron in the Dirac cone of monolayer graphene, has significant potential for use in the terahertz probing technique. For undoped graphene, ultrafast terahertz conductivity relaxation is mostly driven by electron-acoustic phonon supercollision coupling. The decay time of this coupling can be increased to tens or hundreds of picoseconds by decreasing the temperature. However, for chemical vapor deposition (CVD)- grown graphene, which exhibits negative photoinduced terahertz conductivity, there is currently no consensus on the dominant aspects of the terahertz conductivity relaxation process on time scales of less than 10 ps. In this study, the competition between electron-acoustic and optical-acoustic phonon coupling processes during the cooling of CVD graphene was systematically investigated. We experimentally verified that the ultrafast disorder-assisted optical-acoustic phonon interaction plays a key role in ultrafast terahertz conductivity relaxation. Furthermore, the ultrafast cooling process was found to be robust under different pump wavelengths and external temperatures, and it could be modulated by substrate doping. These findings are expected to contribute to a possible cooling channel in CVD graphene and to increase hot electron extraction efficiency for the design of graphene-based photoconversion devices.