Fig. 3. Strong coherent phonons and ultrafast polaron formation.

(A) Pseudo color map of the TR profile of Cs₂AgBiBr₆ SC. (B) Top: The TR spectrum of Cs₂AgBiBr₆ SC monitored at 1-ps delay upon 500-nm excitation. Bottom: The transformed TA spectrum of Cs₂AgBiBr₆ SC using the Kramers-Kronig relations. (C) The recovery from the blue shift of the TR spectra at early delay time. (D) The TR kinetics of Cs₂AgBiBr₆ SC probed near the DE resonance at 440 nm in TRS measurement. (E) The terahertz transmission kinetics of Cs₂AgBiBr₆ PC in TRTS measurement. (F) Strong optical and acoustic modes coupled to the electronic dynamics probed near the isochromatic point (~2.83 eV). At room temperature (292 K), only the optical mode is obvious. At low temperature (78 K), the acoustic mode becomes apparent. Blue and red lines, fitted curves using Eq. 3; green line, extracted optical mode oscillations at 78 K; yellow line, extracted acoustic mode oscillations at 78 K.