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

Applying pump and probe differential reflection (ΔR) and transmission (ΔT) 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{}{τ}{}1 ns. At a pump fluence of 800 {μ}J cm^-2 a relatively sharp charge-carrier front was observed, with high velocities of 14×10⁵ cm/s at a delay time τ≈20 ps, decreasing as v∝τ^-2/3 to 2×10⁵ cm/s at τ≈350 ps. The arrival times τ of the carriers at a fixed sample thickness depend on the fluence of the pump pulses F such as τ∝1/F0.45. 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.