Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta2NiSe5

Femtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ 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 $\mathrm{\ensuremath{\Gamma}}$ point of the Brillouin zone. The optical response saturates at a critical excitation fluence ${F}_{C}=0.30\ifmmode\pm\else\textpm\fi{}0.08$ mJ/${\mathrm{cm}}^{2}$ due to optical absorption saturation. This limits the optical excitation density in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.