Quantification of Exciton Fine Structure Splitting in a Two-Dimensional Perovskite Compound

Applications of two-dimensional (2D) perovskites have significantly outpacedthe understanding of many fundamental aspects of their photophysics. The optical response of2D lead halide perovskites is dominated by strongly bound excitonic states. However, acomprehensive experimental verification of the excitonfine structure splitting and associatedtransition symmetries remains elusive. Here we employ low temperature magneto-opticalspectroscopy to reveal the excitonfine structure of (PEA)2PbI4(here PEA is phenyl-ethylammonium) single crystals. We observe two orthogonally polarized bright in-plane freeexciton (FX) states, both accompanied by a manifold of phonon-dressed states that preservethe polarization of the corresponding FX state. Introducing a magneticfield perpendicular tothe 2D plane, we resolve the lowest energy dark exciton state, which although theoreticallypredicted, has systematically escaped experimental observation (in Faraday configuration)until now. These results corroborate standard multiband, effective-mass theories for theexcitonfine structure in 2D perovskites and provide valuable quantification of the finestructure splitting in (PEA)2PbI4