Layer-by-layer assembly is used to produce thin films of J-aggregates of cyanine dye molecules with an absorption constant of 10(6) cm(-1). The films display high reflection and absorption at resonance (see Figure, similar to 50 nm films) and nanoscale features that make them well suited for use in strong-coupling optoelectronic applications, enabling the creation of exciton-polariton optoelectronic devices that operate at room temperature.