Bright‐Dark Exciton Splitting in Lead Iodide Perovskite Crystals Accessed via Quantum Beats in Photon Echoes

ABSTRACT Understanding the fine structure of excitons is crucial for optoelectronic and quantum photonic applications of lead halide perovskites. It is demonstrated that polarization‐sensitive photon echo spectroscopy in magnetic field provides a powerful method to access coherent exciton dynamics and reveal their energy level structure hidden by inhomogeneous broadening. In particular, exciton quantum beats in Faraday and Voigt geometries offer a precise probe of the energy splittings among the four 1 exciton states. Applying this technique to bulk mixed halide perovskite crystals reveals a bright‐dark exciton splitting of meV, along with electron and hole Landé factors and , respectively. The quantum beats persist on timescales of 20–50 ps, demonstrating remarkably robust spin and optical coherences at cryogenic temperature of 2 K. The decay of the quantum beats of the outer doublet is governed by dephasing due to dispersion of the bright‐dark splitting of meV caused by localization potential fluctuations, while dephasing in the bright exciton inner doublet originates from a small zero field splitting of meV due to anisotropic potentials.