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Abstract

Femtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta₂NiSe₅ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence F_C=0.30±0.08 mJ/cm² due to optical absorption saturation. This limits the optical excitation density in Ta₂NiSe₅ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta₂NiSe₅ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.