Direct observation of Fermi-pressure-driven electron-hole plasma expansion in GaAs on a picosecond time scale

Abstract

Applying pump and probe differential reflection $(\ensuremath{\Delta}R)$ and transmission $(\ensuremath{\Delta}T)$ of femtosecond light pulses for either co- or counterpropagating pump and probe geometries, a direct time of flight method with submicrometer resolution is presented. With this technique we study the density-dependent transport of photogenerated carrier plasmas perpendicular to the surface of GaAs samples for delay times 20 ps\ensuremath{\leqslant}\ensuremath{\tau}\ensuremath{\leqslant}1 ns. At a pump fluence of 800 \ensuremath{\mu}J ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ a relatively sharp charge-carrier front was observed, with high velocities of $14\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm/s at a delay time $\ensuremath{\tau}\ensuremath{\approx}20$ ps, decreasing as $v\ensuremath{\propto}{\ensuremath{\tau}}^{\ensuremath{-}2/3}$ to $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm/s at $\ensuremath{\tau}\ensuremath{\approx}350$ ps. The arrival times $\ensuremath{\tau}$ of the carriers at a fixed sample thickness depend on the fluence of the pump pulses F such as $\ensuremath{\tau}\ensuremath{\propto}{1/F}^{0.45}.$ The results are discussed in the framework of diffusive transport with a strongly density-dependent diffusivity D. The data can be described consistently with the assumption of Fermi pressure as the dominating driving force for plasma expansion.