  We demonstrate experimentally an enhanced delayed fluorescence in tetracene
single crystals strongly coupled to optical modes in open cavities formed by
arrays of plasmonic nanoparticles. Hybridization of singlet excitons with
collective plasmonic resonances in the arrays leads to the splitting of the
material dispersion into a lower and an upper polariton band. This splitting
significantly modifies the dynamics of the photo-excited tetracene crystal,
resulting in an increase of the delayed fluorescence by a factor of four. The
enhanced delayed fluorescence is attributed to the emergence of an additional
radiative decay channel, where the lower polariton band harvests long-lived
triplet states. There is also an increase in total emission, which is
wavelength dependent, and can be explained by the direct emission from the
lower polariton band, the more effcient light out-coupling and the enhancement
of the excitation intensity. The observed enhanced fluorescence opens the
possibility of effcient radiative triplet harvesting in open optical cavities,
to improve the performance of organic light emitting diodes.
