Ultrafast transport of electrons in GaAs:: Direct observation of quasiballistic motion and side valley transfer -: art. no. 085211

Temporally and spatially resolved femtosecond electron dynamics in GaAs is investigated, tracing ultrafast modifications of the Franz-Keldysh absorption spectrum. A complex heterostructure design allows for a study of unipolar transport as well as a nanometer scale definition of layers for both carrier injection and probe of the propagating electron distribution. Transit times through the structure are directly measured for electric fields between 7 kV/cm and 180 kV/cm comparing room temperature operation to results for T-L=4 K. For low lattice temperatures, quasiballistic electron motion with an average velocity of 5.4x10(7) cm/s is observed over distances as large as 300 nm. Realistic Monte Carlo simulations are in excellent agreement with our observations.