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TRTS measurements were performed using an all-air plasma-based system providing single-cycle, coherently detected THz pulses continuously spanning the range of 1 to 20 THz as shown in figs. S12 and S13. Broadband THz light was generated by a two-color laser plasma by focusing 800-nm, 35-fs pulses with an energy of 1 mJ and their second harmonic into dry air (23). The reflected THz transients were detected with an air-biased coherent detection scheme using the generated second harmonic of the 800-nm sampling beam in a biased plasma as a phase-sensitive probe of the instantaneous THz field (27). No compensation for the group delay dispersion between 800- and 400-nm pulses was performed, so the peak THz field strength was limited to ~20 kV/cm. A near-infrared pump pulse excites charge carriers at the band edge at a center wavelength of ~795 nm (1.56 eV), a fluence of 200 μJ/cm2, and a pump-probe delay τ. Optical pump–THz probe experiments were performed at normal incidence reflection from the 〈111¯〉 facet of a large MAPI crystal (size, ~1 cm3) of rhombic dodecahedral habit. Other facets showed similar results and are shown in the Supplementary Materials. Time- and energy-resolved transient THz reflectivity maps were obtained by Fourier analysis of the reflected THz pulses without photoexcitation, Eref(ω), and with photoexcitation, Epump(ω,τ), at a delay time τ. Differential reflectivity was defined as Δr(ω,τ)r0(ω)=(EpumpEref)/Eref and was complex-valued with full amplitude, as shown in the main text, and phase information is given in the Supplementary Materials. Measurements were performed at 295 K in the tetragonal phase of MAPI under dry-purged gas conditions with no evidence of surface degradation during the map acquisition time of ~20 hours.

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