Raumzeitliche Dynamik optisch angeregter Elektron-Loch-Plasmen in Galliumarsenid

Abstract

Applying pump and probe differential reflection and transmission 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 surface for delay times 20 ps< t < 1 ns. As an additional parameter we vary the lattice temperature in the range of 4 K < TL < 300 K. At a lattice temperature of TL = 300K and a pump fluence of 800 mJ/cm2 a relatively sharp charge-carrier front was observed, with high velocities of 14 x 10 5 cm/s at a delay time t = 20 ps, decreasing as v ~ t 2/3 to 2 x 105 cm/s at t = 350 ps. The arrival times t of the carriers at a fixed sample thickness depend on the fluence of the pump pulses F such as t ~ 1/F 0.45. At low lattice temperatures TL we found a similar time dependence of the expansion velocities v, but here a faster transport with a stronger density dependence is observed. The experimental results are discussed in the framework of diffusive transport with a strongly density- and carrier temperature -dependent diffusivity D. The data can be described consistently with the assumption of Fermi pressure as the dominating driving force for plasma expansion.