Ultrafast conductivity measurements in CVD graphene

We study carrier dynamics in CVD graphene films on Al$_{2}$O$_{3 }$using time-resolved THz spectroscopy (TRTS). Excitation with a 50fs, 800nm pulse produces a conductivity change $\Delta \sigma $which we measure as a decrease in transmission of an ultrafast THz pulse. Ongoing work seeks to investigate TRTS in a magnetic field to directly probe scattering rates of photogenerated carriers. At present we report zero field measurements. We observe different recovery dynamics for low ($\Delta \sigma <<\sigma _0 )$ and high ($\Delta \sigma >\sigma _0 )$ pump powers. For pump fluence $\varphi <10^{12}\mbox{photons cm}^{-2}$ ($\Delta \sigma <<\sigma _0 )$ we find$\Delta \sigma \propto \varphi $ and observe a nearly exponential decay $\Delta \sigma \propto e^{-t/\tau }$with decay time $\tau \approx 2\mbox{ps}$. At higher powers ($\Delta \sigma >\sigma _0 ) \quad \Delta \sigma $ is sublinear in $\varphi $, and the decay rate decreases, with $\tau \approx 4\mbox{ps}$at $\varphi \approx 5\cdot 10^{13}\mbox{photons cm}^{-2}$. Graphene's unusual conductivity relation, $\sigma \propto \sqrt n _{,}$ predicts the observed behavior, since $\Delta \sigma \propto \sqrt {n_0 +n_{photo} } -\sqrt {n_0 } $ is approximately linear for $n_0 >>n_{photo} $, while $\Delta \sigma \propto \sqrt {n_{photo} } $for $n_{photo} >>n_0 $. Here $n_0 $ and $\sigma _0 $ are the equilibrium carrier density and conductivity in these p-type films. At high pump powers we also observe a rapid initial recovery on $\sim $500fs timescale which is not described by this simple model.