Fig. 1: Ground-state absorption and transient absorption spectra of CsFAMA thin films.

a Elliott model fit to the ground-state absorption spectrum. The dashed line denotes the continuum absorption, in case excitons are fully screened (ξ = 1). The divergence indicated at 1.82 eV implies that parabolic band dispersion is invalid for E > 1.82 eV. b TA spectra recorded at 5 ps delay time for carrier densities varying from 1.5 × 10¹⁷ to 3.5 × 10¹⁸ cm⁻³. The pump wavelength was 550 nm (photon energy ~2.25 eV). PIA-H represents the photo-induced absorption at the high-energy part of the TA spectra. Inset: normalized TA spectra for carrier densities of 1.5 × 10¹⁷ and 3.5 × 10¹⁸ cm⁻³, respectively. c Plot of the amplitude of the PIA-H signal at E = 1.82 eV versus the carrier density. The error bars indicate standard errors. The red solid line is a linear fit to the data. The dash-dotted line is a power law fit with ∝N^1/3. Inset: normalized TA spectra for a carrier density of ~2 × 10¹⁸ cm⁻³, generated by different pump photon energies.