Spectroscopic signatures of biexcitons: A case study in Ruddlesden-Popper lead-halides

Exciton-exciton interactions are fundamental to the light-emitting properties of semiconductors, influencing applications from lasers to quantum light sources. In this study, we investigate the spectroscopic signatures and binding energy of biexcitons in a metal halide two-dimensional Ruddlesden-Popper structure, which is known for hosting distinct excitonic resonances (X1 and X2) with unique lattice coupling. These multiple resonances enable the formation of both self- and cross-coupled biexcitons (mixed biexcitons). Using three spectroscopic techniques-photoluminescence and two variations of two-dimensional electronic spectroscopy (2DES)-we map coherent one-quantum and two-quantum correlations to gain deeper insight into the biexciton characteristics. While PL spectroscopy is hindered by spectral broadening and reabsorption, 2DES provides a more accurate characterization, revealing multiple biexciton states and uncovering the mixed biexciton species arising from exciton cross coupling. These findings highlight the importance of advanced spectroscopic approaches in accurately determining biexciton binding energies and offer new perspectives on many-body interactions in exciton-polarons within layered perovskites.