Tunable Out-of-Plane Excitons in 2D Single-Crystal Perovskites

Hybrid organic–inorganic perovskites have emerged as very promising materials for photonic applications, thanks to the great synthetic versatility that allows tuning their optical properties. In the two-dimensional (2D) crystalline form, these materials behave as multiple-quantum-well heterostructures with stable excitonic resonances up to room temperature. In this work strong light–matter coupling in 2D perovskite single-crystal flakes is observed, and the polarization-dependent exciton–polariton response is used to disclose new excitonic features. For the first time, an out-of-plane component of the excitons is observed, unexpected for such 2D systems and completely absent in other layered materials such as transition-metal dichalcogenides. By comparing different hybrid perovskites with the same inorganic layer but different organic interlayers, it is shown how the nature of the organic ligands controllably affects the out-of-plane exciton–photon coupling. Such vertical dipole coupling is particularly s...