The buildup of photoconductivity in an archetypal polar semiconductor GaAs was monitored on the timescale of carrier-carrier and carrier-lattice correlations, with sub-50 fs temporal resolution provided by ultra-broadband transient terahertz spectroscopy. Two relevant timescales are identified: the transition in photoconductivity from a regime of extremely weak screening to a screened Drude plasma was observed on the ∼280 fs timescale, followed by the gradual increase and saturation of electron mobility on a timescale of ∼160 fs. The Drude photoconductivity picture following carrier photoinjection was found to stabilize only on the timescale corresponding to several lattice and plasma oscillations.

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