  The strong coupling of a dense layer of molecular excitons with
surface-plasmon modes in a metal gives rise to polaritons (hybrid light-matter
states) called plexcitons. Surface plasmons cannot directly emit into (or be
excited by) free-space photons due to the fact that energy and momentum
conservation cannot be simultaneously satisfied in photoluminescence. Most
plexcitons are also formally non-emissive, even though they can radiate via
molecules upon localization due to disorder and decoherence. However, a
fraction of them are bright even in the presence of such deleterious processes.
In this letter, we theoretically discuss the superradiant emission properties
of these bright plexcitons, which belong to the upper energy branch and reveal
huge photoluminescence enhancements compared to bare excitons. Our study
generalizes the well-known problem of molecular emission next to a metal
interface to collective molecular states and provides new design principles for
the control of photophysical properties of molecular aggregates using
polaritonic strategies.
