  Multidimensional coherent spectroscopy directly unravels multiply excited
states that overlap in a linear spectrum. We report multidimensional coherent
optical photocurrent spectroscopy in a semiconductor polariton diode and
explore the excitation ladder of cavity polaritons. We measure doubly and
triply avoided crossings for pairs and triplets of exciton-polaritons,
demonstrating the strong coupling between light and dressed doublet and triplet
semiconductor excitations. These results demonstrate that multiply excited
excitonic states strongly coupled to a microcavity can be described as two
coupled quantum-anharmonic ladders.
