Spin-dependent exciton-exciton interactions in a mixed lead halide perovskite crystal

We investigate the two-pulse photon echo response of excitons in the mixed lead halide perovskite crystal in dependence on the excitation intensity and polarization of the incident laser pulses. Using spectrally narrow picosecond laser pulses, we address localized excitons with long coherence times T_2 ≈ 100ps. This approach offers high sensitivity for the observation of excitation-induced changes in the homogeneous linewidth Γ_2=2ħ/T_2 on the μeV scale. Through intensity-dependent measurements, we evaluate the increase of Γ_2 by 10 μeV at an exciton density of 10^17 cm^-3 being comparable with the intrinsic linewidth of 14μeV. We observe that the decay of the photon echo and its power dependence are sensitive to the polarization configuration of the excitation pulses, which indicates that spin-dependent exciton-exciton interactions contribute to excitation-induced dephasing. In cross-linear polarization, the decay is faster and its dependence on exciton density is stronger as compared to the co-polarized configuration. Using a two-exciton model accounting for different spin configurations we are able to reproduce the experimental results.