Abstract
Abstract We present optical-pump THz-probe spectroscopy to measure the ultrafast relaxation dynamics of quasi-particle on monolayer chemical vapor deposited (CVD) graphene, and the Fermi levels can be tuned in a wide range by applying gate voltage on the casted polymer electrolyte. As probing the photoexcited carriers around the Fermi level, it has shown several controversial results from the ultrafast studies on graphene grown on SiC. One is a positive pump induced differential transmission (PIDT), that is, a transient negative conductivity after photoexcitation, and the others is the extracted exponential decay times of PIDT depend on the pump fluence and the doping levels. To better understand the relaxation dynamics, we model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The simulation results indicate that graphene cannot be treated as semiconductor as usual; it is more like a zero bandgap material that gives rise to a fast thermalization among carriers after pumping and reaches a quasi-Fermi distribution described by a common Fermi level with a relatively high carrier temperature. It is also noted that the metallic temperature-dependent carrier scattering rate plays a significant role in the cooling process, leading to different observations of the relaxation dynamics. The carrier scattering rate shows two distinct behaviors with the Fermi level separated by a turning point at 91 meV. As approaching charge neutral point, the carrier scattering rate is dominated by charge impurities and decreases with increasing the doping levels due to the screening effect. Different scattering mechanism in two distinct regions leads to the change of relaxation dynamics and gives a clear new cooling control variable. The cooling rate can be changed by varying the doping levels providing means for a variety of new applications that rely on hot-carrier transport.