Bright-dark exciton splitting in lead halide perovskite crystals accessed via quantum beats in photon echoes

TL;DR

This study uses polarization-sensitive photon echo spectroscopy in magnetic fields to measure exciton fine structure and bright-dark splittings in lead halide perovskite crystals, revealing robust quantum coherences at cryogenic temperatures.

Contribution

It introduces a novel application of photon echo spectroscopy to precisely determine exciton energy splittings and coherence properties in bulk perovskite crystals.

Findings

Bright-dark exciton splitting of 0.46 meV measured

Quantum beats persist for 20-50 ps at 2 K

Dephasing caused by localization potential fluctuations

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, which is hidden by inhomogeneous broadening. Exciton quantum beats observed in both Faraday and Voigt geometries offer a precise probe of the energy splittings among the four 1$s$ exciton states, enabling determination of the fine structure and bright-dark splittings. Application of this technique to bulk mixed halide perovskite crystals FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ reveals a bright-dark exciton splitting of $\Delta_\mathrm{X}=0.46~$meV, along with electron and hole Land\'{e} $g$ factors $g_\mathrm{e}=3.38$ and $g_\mathrm{h}=-1.14$, 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 $\sim0.06~$meV caused by localization potential fluctuations, while dephasing in the bright exciton inner doublet originates from a small zero field splitting of $\sim0.035~$meV due to anisotropic potentials.