With their strong optical absorption coefficients, long carrier lifetimes, tunable material dimensionalities and remarkable defect tolerance, hybrid organic-inorganic perovskites (HOIPs) have emerged as solution-processed semiconducting materials for real-world light-harvesting and light-emitting applications. These unique material systems comprised of distinct organic and inorganic sublattices exhibit new photophysical properties that are not present in traditional semiconductors. In this talk, I will present our time-resolved spectroscopic investigation revealing the weak vibrational and electronic coupling between the two sublattices in HOIPs, by selective vibrational excitation in the infrared and electronic probing near the bandgap of the materials. In addition, the organic and inorganic sublattices with disparate atomic masses and bond strengths render 2D HOIPs as strongly acoustic-impedance-mismatched superlattices, which result in strongly reduced cross-plane acoustic phonon velocities and the signature of coherent phonon transport at the ultrathin limit. I will further describe our first study of the anisotropic optical dielectric constants of a family of layered perovskites. In particular, the strong exciton resonance leads to a unique hyperbolic dispersion in the visible range, which was previously only realized by artificially fabricated metamaterials.
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