Many-Body Exciton-Polariton Quantum Dynamics in Ruddlesden-Popper Metal-Halide Optical Microcavities

We probe the quantum dynamics of exciton-polaritons in the prototypical Ruddlesden-Popper metal halide (PEA)₂PbI₄ (PEA = phenylethylammine) by means of two-dimensional coherent electronic spectroscopy (2DES), over the temperature range 5–300 K, and as a function of excitation density. We analyze the coherent optical lineshape to extract the homogeneous linewidth, which is governed by optical dephasing of the third-order mesoscoptic polarization generated by a time-ordered, phase-matched femtosecond pulse sequence. We quantify excitation-induced dephasing (EID), which is the contribution to the dephasing due to multi-particle correlations that lead to elastic Coulomb scattering, by analysis of the homogeneous linewidth as a function of polariton density, and by the density dependence of the real and imaginary parts of the complex lineshape. Excitons in (PEA)₂PbI₄ films exhibit clear signatures of EID, with a homogeneous linewidth that increases linearly with excitation density, and by evidence of phase shifts of the real and imaginary spectra with respect to the low-density pure dephasing limit [1]. However, comparison of EID parameters with respect to covalent 2D semiconductors such as single-layer transition-metal dichalchogenides reveals that polaronic effects due to the ionic nature of the lattice screen the multi-particle correlations by orders of magnitude with respect to non-ionic semiconductors. In the optical microcavity, such signatures of polariton EID are amplified with respect to the bare excitons, and the spectral structure becomes more intricate, with cross-peak structure that differs with respect to that of the bare exciton. We model the optical lineshapes via quantum-dynamical simulations [2], and find that while the polariton dispersion relation reflects the exciton spectral fine structure, which we ascribe to polaronic effects that lead to distinct exciton polarons with binding energies that differ by ~35 meV [3] due to specific lattice dressing [4], which modulates the permittivity, the strength of multi-particle interactions is substantially higher in the microcavity. We hypothesize that this arises from diminished polaronic screening of polaritons with respect to excitons due to long-range interactions mediated by the photon component. We discuss these findings in the context of mechanisms for polariton condensation in this class of materials.<br/> <br/>[1] A.R. Srimath Kandada <i> et al</i>., J. Chem. Phys. <b>153</b>, 164706 (2000).<br/>[2] S.A. Shah <i>et al.</i>, arXiv:2210.16355 <b>[quant-ph]</b>.<br/>[3] S. Neutzner <i>et al</i>., Phys. Rev. Materials <b>2</b>, 064605 (2018).<br/>[4] F. Thouin <i>et al</i>., Nature Materials <b>18</b>, 349-356 (2019).