Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta${}_2$NiSe${}_5$

Abstract

Femtosecond time-resolved mid-infrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta${}_2$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 Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C=0.30\pm 0.08$~mJ/cm${}^2$ due to optical absorption saturation. This limits the optical excitation density in Ta${}_2$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 Ta${}_2$NiSe${}_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.