Measurements of Ultrafast Dynamics in GaAs Crystals using Time-resolved X-Ray Diffraction

Abstract

We present initial results of picosecond time resolved diffraction from GaAs(111) crystals using plasma generated, ultrafast x-ray pulses. Laser excitation of the GaAs transfers electron population from the valence into the conduction band. Such an excitation typically produces a hot electron gas in the conduction band that thermalizes with the lattice via phonon emission with a time constant of about 2 ps.[1, 2] However, upon transfer of a few percent of all valence electrons, the cohesive energy binding the atoms in the crystal lattice changes, which can lead to a rapid modification of the crystal structure.[3-8] This process, which does not require the thermalization of electrons and phonons, can proceed on femtosecond timescales. Both effects, which substantially disturb the equilibrium crystal structure, are in principle detectable by ultrafast x-ray diffraction, as are other effects such as melting and shock wave propagation.