Multi-exciton processes in ultrafast nonlinear optical spectroscopy

Ultrafast nonlinear optical spectroscopy is a powerful tool for probing photo-induced processes such as exciton dynamics and interactions. Recent advances enable complete control over the interaction with the excitation pulses, allowing a full separation of the measured signals according to their order of nonlinearity. This capability provides unprecedented access to multi-exciton states and their dynamics. In this work, we review these developments in both coherently and action-detected spectroscopy, focusing on the signatures of multi-exciton effects. Among these, the most prominent process is exciton–exciton annihilation. By examining the spectra from a combined theoretical and experimental perspective, we identify which variants of current nonlinear spectroscopy are best suited to measure exciton transport and interaction, as well as the structure and properties of multi-exciton states.