Abstract
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 $7\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm}$ and $180\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm}$ comparing room temperature operation to results for ${T}_{L}=4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For low lattice temperatures, quasiballistic electron motion with an average velocity of $5.4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.3em}{0ex}}\mathrm{cm}∕\mathrm{s}$ is observed over distances as large as $300\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Realistic Monte Carlo simulations are in excellent agreement with our observations.
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