Bernhard Bohn1 Thomas Simon1 Moritz Gramlich1 Alexander Richter1 Lakshminarayana Polavarapu1 Alexander Urban1 Jochen Feldmann1

1, Ludwig-Maximilians-Universität München, Munich, , Germany

Perovskite nanocrystals have emerged as an interesting material for light-emitting and other optoelectronic applications. Excitons are known to play an important role in determining the optical properties of these nanocrystals and their energetic levels as well as quantization properties have been extensively explored. Despite this, there are still many aspects of perovskites that are still not well known, e.g. the homogeneous and inhomogeneous linewidths of the energetic transitions, quantities that cannot be directly extracted by linear absorption optical spectroscopy on nanocrystal ensembles. Here, we present temperature-dependent absorption and four-wave mixing (FWM) experiments on thick methylammonium lead iodide (MAPI) perovskite nanoplatelets exhibiting bulk-like absorption and emission spectra. Dephasing times T2 of excitons are determined to lie in the range of several hundreds of femtoseconds at low temperatures. This value enables us to distinguish between the homogeneous and inhomogeneous contribution to the total broadening of the excitonic transitions. These turn out to be predominantly inhomogeneously broadened at low temperatures and homogeneously broadened at room temperature. Furthermore, we find excitonic quantum beats, which allow for the determination of the exciton binding energy and we extract EB = 25±2 meV in the low temperature regime, in good agreement with other reports.