“…12,13 In particular, as a critical step towards graphene terahertz lasers, the possibilities of terahertz wave amplifications by graphene have attracted increasing research interest. Non-equilibrium carrier behaviors in optical pumped graphene have been studied theoretically and experimentally, [14][15][16][17][18][19][20][21] which showed population inversion in graphene is possible, and several graphene based terahertz laser concepts have also been proposed. [22][23][24][25] However, due to the ultrafast carrier relaxation dynamics, population inversion in graphene lasts for only a short time, 26,27 signifying a monolayer graphene may be not ideal for a terahertz laser gain material.…”