  We study spatial coherence properties of a system composed of periodic silver
nanoparticle arrays covered with a fluorescent organic molecule (DiD) film. The
evolution of spatial coherence of this composite structure from the weak to the
strong coupling regime is investigated by systematically varying the coupling
strength between the localized DiD excitons and the collective, delocalized
modes of the nanoparticle array known as surface lattice resonances. A gradual
evolution of coherence from the weak to the strong coupling regime is observed,
with the strong coupling features clearly visible in interference fringes. A
high degree of spatial coherence is demonstrated in the strong coupling regime,
even when the mode is very excitonlike (80%), in contrast to the purely
localized nature of molecular excitons. We show that coherence appears in
proportion to the weight of the plasmonic component of the mode throughout the
weak-to-strong coupling crossover, providing evidence for the hybrid nature of
the normal modes.
