Holographic photopolymer with high sulfur content for high refractive index modulation

The thiol-ene polymerization between a tetrafunctional thiol and a cyclic allylic sulfide (CAS) monomer is used to generate a crosslinked structure with high concentration of sulfur atoms, which allows for the manufacturing of volume phase holographic gratings with high refractive index modulation (▵n). The writing chemistry is dispersed in a cellulose acetate butyrate (CAB) matrix together with Irgacure^® 784 as free radical photoinitiator, making the system sensible to blue light. After the writing step, the performances of the grating can be further improved by means of a thermal treatment, which induces a strong enhancement of the refractive index modulation. This effect could be explained as a reorganization of the polymeric chains that leads to a marked separation between the writing chemistry and the polymeric binder and to local variations of the material density. This hypothesis is supported by different experimental evidences. Values of ▵n up to 0.0346 are reached, which are among the largest reported in the literature.