Polaritons are hybrid light-matter quasiparticles that condense into a quantum fluid at sufficiently high densities. Although two-dimensionnal hybrid lead-halide perovskites seem to be the ideal material platform to observe polaritons condensation, reports of quantum fluidics in these systems remain scarce. Here, using coherent non-linear spectroscopy, we provide insights into the mechanisms behind this reluctance to condensation. These insights provide guidance to the design of photonic systems to steadily achieve polariton condensation using these promising materials.

By using nonlinear coherent spectroscopy, we identify bound Frenkel biexcitons in a model polymeric semiconductor and find, unexpectedly, that excitons with interchain vibronic dispersion reveal intrachain biexciton correlations and vice versa. Moreover, we relate the biexciton binding energy to molecular parameters quantified by quantum chemistry, including the magnitude and sign of the exciton-exciton interaction the intersite hopping energies. This work shows how multidimensionnal spectroscopy can be used to gain insights into many-body physics despite the complexity of the material hosting them. This required a careful theoretical treatment of coherent dynamics to distinguish them from incoherent artefacts, which we provide in supplementary information.

Quantum dephasing is a universal phenomenon in which the quantum system loses coherence over time due to interactions with the surrounding environment. In this article, we introduced a stochastic scattering model to account for the dephasing effect on spectral line shapes through the interaction with uncorrelated excitations as a co-evolving, non-stationary environment. This non-stationary stochastic approach is generally applicable outside spectroscopy in areas like quantum technologies.

We dive deeper into the rich many-body physics at the heart of excitons, primary photoexcitations, in two-dimensional lead-halide perovskites. Their polaronic nature, already a product of exciton-phonon interactions, is revealed to also impact their mutual interactions. Using non-linear coherent spectroscopy, we reveal that the polaronic cloud surrounding excitons shield them from inelastic scattering, making this interaction a thousand times weaker than other non-polaronic two-dimensionnal semiconductors. This observation is a new important insight into polaron many-body physics where insights from simulations remain scarce.

We believe these results to be the first direct evidence of the polaronic nature of excitons in 2D-HOIPs: each line of their excitonic fine structure corresponds to a distinct exciton-polaron. This many-body paradigm is central in understanding the properties of primary photoexcitations in these materials and motivated widespread theoretical efforts in exciton-polaron physics.