Effects of pump pulse propagation and spatial distribution of bias fields on terahertz generation from photoconductive antennas

Through theoretical analysis and numerical simulation, we show that bipolar terahertz pulses generated from photo-excited small-gap or mid-gap photoconductive (PC) antennas actually stem from a unique combination of spatially inhomogeneous bias field and propagation effects of pump pulses through the PC if the influence of fast carrier recombination on terahertz radiation is excluded. This finding provides new insights on the formation of bipolar terahertz pulses, instead of the traditional view based on the Drude-Lorentz model and the space-charge screening effect. We find large-aperture PC antennas always emit unipolar terahertz pulses rather than bipolar ones because the bias fields in this case can be considered homogeneous. We also show that the saturation of the peak amplitude of terahertz pulses at high pump fluence can be correctly predicted using our model without invoking space-charge screening.