High-energy sensitivity enhancement in panchromatic photopolymers for holography using a mixture of visible-light photoinitiators

A new aqueous photopolymer containing the monomers acrylamide, N,N'-methyle nbisacrylamide and zinc acrylate, the initiators 4,5-diiodosuccinylfluoresc ein (2ISF) and methylene blue (MB), and the coinitiator sodium p-toluenesul phinate is described. This formulation exhibits a clear enhancement of the high-energy sensitivity upon irradiation with 514 or 633 nm light (Ar+ lase r or He-He laser respectively), with respect to the same mixture but with o nly one of the two dyes, reaching maximum diffraction efficiencies of 15-20 % with 15-60 mJ cm(-2). This enhancement is explained by the more efficient photogeneration of initiator radicals by the ground-state formation of an ion-pair complex between cationic (MB) and anionic (2ISF) chromophores. Thi s must compensate the observed decrease in the absorbance of the mixture of the two dyes at 514 nm, with respect to the absorbance of the same medium but with only 2ISF (maximum absorption at 490 nm). A clear absorbance incre ase at 633 nm, with respect to the absorbance of this system with only MB ( maximum absorption at 660 nm), must also favour photopolymerization.