Mapping the dynamics of energy relaxation in exciton–polaritons using ultrafast two-dimensional electronic spectroscopy

Exciton–polaritons are quasiparticles that are formed by strong interactions between light and electronic transitions of matter. Polariton states exhibit the characteristics of both photons and the matter transitions, which leads to photophysical and chemical properties distinct from those observed in pure matter states, such as enhanced energy transport and altered chemical reactivity and conductivity. Critical to understanding how these exciting phenomena are enabled is understanding the underlying photophysical mechanisms of the interactions between polaritonic states and the associated energy relaxation pathways. Ultrafast spectroscopic techniques, such as transient absorption spectroscopy, have been increasingly utilized to interrogate the rapid relaxation dynamics of these partly light-like, short-lived states, albeit with limitations and ambiguities. In this review, we discuss how two-dimensional electronic spectroscopy, an ultrafast technique that has been underemployed in the studies of exciton–polaritons thus far, can offer detailed insights into the primary photophysical events of energy relaxation in exciton–polaritons that are not accessible in transient absorption, through the analysis of off-diagonal cross peaks and line shapes.