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Abstract: White-light broadband chirp-compensated spectral interferometry is applied to fully probe the optical response of strongly coupled excitons (Xs) and surface plasmon polaritons (SPPs) in J-aggregate/metal hybrid nanostructures at field level. Under impulsive excitation, amplitude and spectral phase of the sample reflectivity are measured with high precision and the time structure of the electric field emitted by the hybrid modes of the nanostructures is accurately reconstructed. Quantitative description of strong X-SPP coupling is precisely obtained by fitting both measured spectra and phases simultaneously to a Fano lineshape model.