Holographic characteristics of two different films using methyl violet dyes in polyvinyl alcohol matrices

摘要

Holographic characteristics of two different films using methyl violet dyes in polyvinyl alcohol matrices are reported.One of the films, Film A, contains only methyl violet in the matrix. The other film, Film B, contains methyl violet,triethanolamine and acrylamide in the matrix. Diffraction efficiencies of the two films were measured and compared.Mechanisms of holographic recording and the functions of the dyes are discussed. The films were 8 μm thick.Holographic gratings were recorded in the films by two intersecting YVO laser beams at 532 nm. The recordingintensity of each beam was 25 mW, 50 mW and 100 mW, and the beam diameter was 2.25 mm. The spatial frequencyof the grating was 653 line/mm. During recording, intensity of the diffracted light was simultaneously measured using aHe-Ne laser at 633 nm. Regarding Film A, diffraction efficiency reaches a peak at a specific exposure time. This meansthat holographic grating grows according to exposure time in low exposure, but vanishes when over-exposed. In brightregions of the interference fringe on the film, the dyes are bleached. Changes of absorption and refractive index due tobleaching would result in the holographic grating. Regarding Film B, the diffraction efficiency was saturated in highexposure, and the maximum diffraction efficiency was not strongly dependent on the concentration of methyl violet. Inthe bright regions of the interference fringe on the film, excited methyl violet and triethanolamine cause polymerizationof acrylamide. Changes in the refractive index due to the polymerization build the holographic grating.