Spectral signatures of exciton-polarons in two-dimensional hybrid lead-halide perovskites*

Two-dimensional hybrid organic inorganic perovskites (2D-HOIPs) feature a previously unexplained excitonic fine-structure in their absorption and emission spectra. We here show using advanced optical probes that features within this fine-structure correspond to exciton-polarons of distinct polaronic nature. We first successfully reproduce their absorption spectrum by supposing distinct excitonic states with contrasting couplings to the lattice. This hypothesis is then unambiguously confirmed by high-resolution resonant impulsive Raman spectroscopy which reveals the polaronic nature of each excitons within this fine structure. We then show that their distinct polaronic nature drives the relaxation dynamics down this excitonic manifold. We also show that these effects protect the excitons from many-body interactions hinting at their importance in polaritonic microcavities based on these materials. Finally, we also observe stable biexcitons in these materials up to room temperature. Our conclusions are of broad fundamental importance to this class of materials which, due to the high degree of tunability of their crystalline structure, offer an ideal system on which to test models of exciton-polaronic effects.