ULTRAFAST FIELD-DYNAMICS IN LARGE-APERTURE PHOTOCONDUCTORS

We investigated field screening mechanisms in large-aperture GaAs phot oconductors, using an ultrafast pump-probe terahertz technique. After photoexcitation the bias field decreases to an intensity-dependent val ue as a result of near-field screening of the bias field. For longer d elays the field exhibits an intensity-dependent decrease that results from a space-charge field caused by transport-induced charge separatio n. These measurements support recent theoretical results that the domi nant saturation mechanism that limits terahertz output from large-aper ture 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.