We present new high-resolution measurements of transient time-domain photoconductivity in ErAs:InGaAs superlattice nanocomposites intended for THz photoconductive switches and photomixers using a pure optical pump-probe method. We developed a model, using separate photocarrier trapping, recombination, and thermal reactivation processes, which very accurately fits the measurements. The measured material structures all exhibit a slow secondary decay process, which is attributed to thermal reactivation of the trapped carriers, either into the conduction band, or into high-energy defect states. We examined the influence of superlattice structure, dopants, DC bias, and temperature. Analysis shows that all of the THz energy produced by the photocarrier trapping and decay processes are at frequencies less than 1 THz, while the reactivation process only serves to create a large portion of the bias power dissipated. Energy higher than 1 THz must be created by a fast generation process or band-filling saturation. This allows pulsed THz generation even from a long-lifetime material. Pure optical pump-probe measurements are necessary to expose slow material processes, and eliminate the influence of electrical terminals and THz antennas. These measurements and modeling of THz photoconductive devices are necessary in order to optimize the output spectrum and power. © 2014 AIP Publishing LLC.