Screening of the bias field in terahertz generation from photoconductors

We include screening of the bias field by the space charge induced by carrier transport and by the generated terahertz radiation in a model describing the far-field electromagnetic radiation from a large-aperture biased photoconductor triggered by an ultrashort optical pulse. At high fluences, space-charge screening is shown to produce bipolar terahertz waveforms. However, the dominant saturation mechanism is screening of the bias field by the radiated terahertz field.