Ultrafast carrier dynamics and photoconductivity in gated graphene probed by THz pump-THz probe spectroscopy

The unique properties of graphene due to its gapless linear electronic dispersion and high mobility of charge carriers make it very promising for applications in high speed optoelectronic and photonic devices. To this extent it is essential to understand the relaxation dynamics and cooling mechanisms of photogenerated carriers in graphene. Here we report on carrier dynamics in a gate-tunable graphene device using THz pump-THz probe spectroscopy. At low temperatures we observe that the relaxation dynamics is characterized by a double exponential decay that evolves into a single exponential decay at higher temperatures. Time-resolved THz conductivity measurements at different carrier densities indicate the presence of two components with opposite contributions to the photoconductivity. The two components evolve on distinct timescales consistent with the relaxation dynamics. This suggests that the THz-induced current in graphene is carried by two distinct modes at low temperatures.