  We report the first experimental observation of an Excitonic Optical Tamm
State supported at the interface between a periodic multilayer dielectric
structure and an organic dye-doped polymer layer. The existence of such states
is enabled by the metal-like optical properties of the excitonic layer based on
aggregated dye molecules. Experimentally determined dispersion curves, together
with simulated data, including field profiles, allow us to identify the nature
of these new modes. Our results demonstrate the potential of organic excitonic
materials as a powerful means to control light at the nanoscale, offering the
prospect of a new alternative type of nanophotonics based on molecular
materials.
