We investigated field screening mechanisms in large-aperture GaAs photoconductors, using an ultrafast pump-probe terahertz technique. After photoexcitation the bias field decreases to an intensity-dependent value as a result of near-field screening of the bias field. For longer delays the field exhibits an intensity-dependent decrease that results from a space-charge field caused by transport-induced charge separation. These measurements support recent theoretical results that the dominant saturation mechanism that limits terahertz output from large-aperture photoconductors is near-field screening of the bias field because the space-charge field develops on a much longer time scale than that of the terahertz pulse. (C) 1997 Optical Society of America.
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