By using optical pump-terahertz (THz) probe spectroscopy, the photoexcited terahertz conductivity dynamics of chemical vapor deposition grown graphene is investigated in different atmospheric environments. It is shown that the Fermi energy of doped graphene is engineered by oxygen adsorption and desorption, which is probed by transient THz conductivity measurement. We show that the ultrafast energy relaxation processes depend on Fermi energy (changed by environmental gas) and the density of excited carriers (changed by photo-excitation fluence). The rise process of the negative conductivity dynamics becomes less efficient upon decreasing the Fermi energy and/or increasing the pump fluence. All findings show that the Fermi energy of graphene engineered by environmental gas allows us to tune the ultrafast energy relaxation pathways in photoexcited graphene.
Available for download on Thursday, March 01, 2018