Large two-beam coupling effect in poly(methylmethacrylate) doped with hemicyanine dye

Unexpected asymmetric energy exchange (two-beam coupling) between two writing beams in two-wave mixing experiments was observed in poly(methylmethacrylate) doped with hemicyanine dye. It was found that photobleaching of hemicyanine dyes was responsible for the formation of refractive index gratings. Instability of the laser-based optical system could be the cause of the phase shift between the interference pattern of the writing beams and the refractive index gratings, such a phase shift being a necessary condition for the two-beam coupling.