In a non-equilibrium electron–hole plasma photoexcited in GaAs within 10 fs, we observe how a plasmon pole builds up on a femtosecond time scale. To this end, the dynamics of the complex dielectric function throughout the mid-infrared is probed with uncertainty-limited temporal resolution
by means of ultrabroadband THz spectroscopy. We show that collective effects, such as plasma oscillations, exhibit a delayed onset. This observation may be linked to the ultrafast formation
of dressed quasiparticles in the many-body system. The approximate time scale for these phenomena is found to be of the order of the inverse plasma frequency. Thus, our findings support quantum kinetic models of the temporal evolution of the Coulomb interaction after ultrafast carrier generation in semiconductors. These theories have been co-pioneered by R. Zimmermann and his coworkers around the beginning of the last decade.