Sensitivity enhancement in panchromatic photopolymers for holography using a mixture of visible-light photoinitiators

Abstract

Photopolymers usually present lower energetic sensitivity, and to narrower wavelength range, than other more conventional recording materials for holography such as dichromated gelatins. We have previously overcome some of these disadvantages using a mixture of visible light photoinitiators. In present communication we describe a new aqueous photopolymer containing the monomers, methylenbisacrylamide and zinc acrylate, the initiators 4,5- diiodosuccinyfluoroscein (2ISF) and methylene blue (MB), and the coinitiator sodium p-toluensulfinate. This formulation exhibits a dramatic enhancement of the energetic sensitivity upon radiation with 514 or 633 nm light (Ar or He-Ne laser respectively), with regard to the same mixture but with only one dye, reaching maximum diffraction efficiencies of 15 - 20% with 15 - 60 mJ cm-2. This enhancement is explained through a more efficient photogeneration of initiator radicals by the ground-state formation of on ion- pair complex between cationic (MB) and anionic (2ISF) chromophores, the proximity of which must favor electron transfer between groups. This must compensate the observed absorbance decrease at 514 nm, with regard to the absorbance of 2ISF alone in the same medium (maximum absorption at 490 nm). A clear absorbance increase at 633 nm, with regard to the absorbance of MB alone (maximum absorption at 660 nm), must also favor the photopolymerization.