Fabrication of refractive index grating into azo-dye-containing polymer films by irreversible photoinduced bleaching

We report on the direct fabrication of refractive index gratings by irreversible photoinduced bleaching (photobleaching) into azo-dye-containing polymer films with a large second-order optical nonlinearity. Gratings were formed into azo-dye-doped or attached polymer films upon exposure to an interference pattern of two laser beams from second harmonic light at 532 nm of a mode-locked Nd: yttrium–aluminum–garnet laser. To investigate whether the refractive index grating is formed into the azo-dye-containing polymer films by the two-beam interference exposure, IR spectroscopy was examined. Absorption strength from an azo bond in the exposed polymer film reduced selectively as compared with that of unexposed film. Although surface relief structure with a few nanometers depth on the polymer film was observed by using atomic force microscopy, the modulation depth of the grating was very small. So, we considered that the effect of the relief structure to diffraction efficiency obtained here is negligible. Furthermore, the grating could not be erased by light. From these results, we confirmed that the grating obtained here was mainly caused by photobleaching, not by photoinduced birefringence and/or optically induced surface relief grating. In addition, highly efficient gratings, 20% or more, could be formed by using the azo-polymer film.