Real-time Observation of Phonon Anharmonicity in Hybrid Lead Halide Perovskite CH$_3$NH$_3$PbI$_3$

Inorganic-organic hybrid lead halide perovskites, consisting of the soft lead halide octahedral framework and the organic molecular cations, are among the key materials for the next generation photovoltatics. Theoretical studies predicted that both the inorganic and organic components are mechanically soft with profound dynamic disorder at room temperature, which can dominate the structural stability and even affect the carrier transport. We investigate ultrafast vibrational dynamics of CH$_3$NH$_3$PbI$_3$ under pre-resonant photoexcitation with a 1.5-eV light pulse, where the perturbation from photoexcited carriers is expected to be minimized. The optical pulse induces transient polarization anisotropy in the PbI$_6$ octahedral network as well as in the alignment of the methylammonium cations (MA$^+$). This is followed by coherent angular bending of the PbI$_6$ sublattice at 0.5 THz and libration and torsion of MA$^+$ at 4 and 8 THz. The frequencies of the PbI$_6$ angular bending and the hydrogen-bonded MA$^+$ torsion exhibit clear phonon downchirps, demonstrating in the real time the strong anharmonicities of their vibrational potentials that were predicted by previous theoretical studies.