Exciton polarons in Ruddlesden Popper metal halides - lessons from coherent spectroscopy

Two-dimensional Ruddlesden-Popper metal halides (2D-RPMHs) are materials composed of quasi-2D layers of metal-halide octahedra separated by long (~1nm) organic cationic layers. The latter facilitate electron and hole quantum confinement within the metal-halide layers resulting in a quantum-well like structure. Properties of excitons (electron-hole bound states) in such structures are characterized by strong binding energy (>200 meV) arising from the dynamically screened Coulomb interactions. We have experimentally observed that polaronic effects arising from the lattice dressing of the carriers, are not only active but that they fundamentally define excitons in 2D-RPMHs. We thus refer to such excitons as the exciton-polarons, with properties that are measurably distinct from those of free excitons in semiconductors. We will discuss the quantum dynamics of exciton-polarons and provide spectroscopic insights into the peculiar phonon-phonon, exciton-phonon and exciton-exciton interactions. We will present our perspective on how the coherent optical response of 2D-RPMHs can be effectively rationalized within the "exciton-polaron" framework, in which lattice dressing of photo-carriers constitute an integral component of excitonic wavefunction, with consequences on exciton recombination dynamics and diffusion.