Fig. 1. Pump-probe microscopy of Ta₂NiSe₅.

(A) Top: Crystal structure of Ta₂NiSe₅. The alternating chains of Ta and Ni atoms confer a quasi-1D nature to the material and independently host conduction and valence band states (8). Bottom: Schematic electronic band structure for a prototypical EI. Above the critical temperature (T_c), the material is close to the semiconductor-semimetal transition (portrayed here as a semiconductor of energy gap E_g). Below the T_c, the exciton binding energy (E_b) exceeds the single-particle gap (E_g), resulting in a macroscopic coherent state. (B) Schematic of the measurement setup at the location of the sample with the photoinduced transmissivity change (ΔT/T) signal. The white dot and dashed circle are examples of pixel regions over which we average the signal in our analysis. (C) Schematic diagram of a possible low-energy excitation structure for an EI in the presence of electron-phonon coupling. The phonon modes of the material hybridize with the phase mode, resulting in mixed phonon phase modes as long as the energy gap in the phase mode dispersion at k = 0 is smaller than the phonon energy E_ph,i. The phonon and phase content of the modes is represented as a color gradient from green (pure phase mode) to red (pure phonon mode). (D) Photoinduced transmissivity change (ΔT/T) as a function of the pump-probe delay time collected at the center of the pump region (inset) and its FT power density.