Biexcitons in Ruddlesden-Popper Metal Halides Probed by Nonlinear Coherent Spectroscopy

Abstract

Excitons and their correlated complexes underpin the rich photophysics of quantum-confined semiconductors. Among these, biexcitons -- bound states of two electrons and two holes -- provide a sensitive probe of Coulomb correlations, exciton-exciton interactions, and the role of the dielectric environment. In Ruddlesden-Popper metal halide materials (RPMHs), strong quantum and dielectric confinement stabilize excitons with binding energies of hundreds of meV, creating an ideal platform for multi-exciton phenomena. Conventional linear spectroscopies, such as photoluminescence and transient absorption, reveal biexciton signatures but suffer from spectral congestion and reabsorption artifacts. Two-dimensional coherent spectroscopies, particularly two-quantum (2Q) multidimensional techniques, uniquely access multi-exciton coherences and provide unambiguous estimates of biexciton binding energies. This minireview surveys the spectroscopic evidence for biexcitons in RPMHs, highlights the advantages of nonlinear multidimensional approaches, and situates biexciton physics within the broader context of excitonic materials, including GaAs quantum wells, quantum dots, and transition-metal dichalcogenides. By emphasizing the interplay of exciton-exciton annihilation, excitation-induced dephasing, and biexciton formation, we argue that multidimensional coherent spectroscopy offers the most reliable pathway to disentangle many-body interactions in quantum-well derivatives of metal-halide perovskites.