Microcavity exciton-polaritons are bosonic quasiparticles that result from the hybridization of excitons and modes of a confined electromagnetic field in a regime known as strong light-matter coupling. Having a low effective mass, polaritons can undergo condensation, the macroscopic occupation of the lowest energy and momentum state. Two-dimensional (2D) perovskites are promising candidates for polariton condensation due to their high exciton binding energies, low non-radiative recombination rates and strong oscillator strengths. However, despite their optimal optoelectronic properties, there are no reports of room temperature polariton condensation in 2D perovskites and only one unreproduced report at low temperature. In this study, we systematically examine the interplay between the emission from the exciton reservoir and the population of the lower polariton. We gain insights on how the spectral features of the emission of 2D perovskites affect polariton relaxation and onto one of the mechanisms making polariton condensation challenging in 2D perovskites.
In organic and organic/inorganic hybrid materials, the role of the exciton reservoir (i.e., uncoupled excitons) towards populating exciton-polariton states —emerging when an excitonic transition strongly couples with a microcavity optical mode— is not well understood. Here, we identify many-body processes in the exciton reservoir by probing the time-resolved nonlinear photoluminescence of polaritons in an organic dye (1,6,7,12-bay-substituted perylene-diimide derivative) and a Ruddlesden-Popper (PEA)2PbI4 perovskite. We observe that, in the dye, exciton-exciton annihilation is hindered in the strong light-matter coupling regime and, in (PEA)2PbI4, Auger recombination obstructs the population of exciton-polaritons via radiative pumping by the emission of the exciton reservoir.
Coherent nonlinear spectroscopy offers us a window into the system-bath interactions in materials. Specifically, the spectral lineshapes can reveal the nature and dynamics of the environmental fluctuations surrounding the system of interest. Here we will discuss how stochastic non-equilibrium exciton dynamics manifest in the peculiar lineshapes and how they provide mechanistic insights into the nature of exciton-phonon and exciton-exciton interactions in nanostructured derivatives of metal halide perovskites. Despite the success of such classical optical probes in unveiling the many-body physics in materials, we will elaborate on the ambiguities still present in the resultant photophysical models that stem primarily due to the high excitation intensities used in the measurements. We will also discuss alternative experimental methodologies based on quantum entangled photons, which may offer superior signal to noise ratio and thus enabling the measurement of many-body interactions at close to single photon excitation densities.
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