Polariton induced transparency at low light level: polymer chains dispersed in a monomer single crystal.

Excitons of polydiacetylene chains, diluted in a 3BCMU monomer single crystal, turn at very low light intensity into a nonabsorbing polariton state once the polymer concentration exceeds 10{-4} in weight. The transition is evident from a Lorentzian absorption peak which is truncated at an optical density OD approximately 2 and is consistent with polariton coupling of light to eigenstates of the solid while Rabi coupling of individual chains to light is excluded. The saturated absorption is nearly independent of polymer content and crystal thickness and defines a surface region where photons are converted into polaritons, the corresponding electromagnetic eigenmodes of matter.