Grating microstructures in photoactive polymers by laser interference patterns

Abstract

The formation of grating microstructures in new photopolymer formulations are investigated by holographic recording. Holographic recording in cast polymer layers occurs in real-time and in-situ, meaning that no wet-chemical or post-thermal/photochemical processing are required. The hydrophilic photopolymers are based on acrylamide monomers, which are dissolved together with photoactive components, such as the sensitizing dye eosin and the new charge transfer agent diphenyl iodonium (DPI) salt with triethanolamine, in polyvinyl alcohol. The addition of DPI significantly increases the exposure sensitivity from about 35 to 12 mJ/cm 2, i.e. by a factor of about three, without affecting the maximum obtainable diffraction efficiency (DE). The influences on the storage mechanism by other additives, such as cross-linking agents and plasticizers are also investigated. The introduction of the cross-linking agent bisacrylamide stabilizes the formed structures against thermal and environmental relaxations. The introduction of plasticizers have a significant effect on the spatial modulation of the refractive index in the recorded grating patterns. Large enough refractive index modulations can be obtained, so as to yield very high DEs of about 90%. The angular-DE and thickness-DE responses closely follow the theoretical predictions, according to the coupled-wave theory. These photopolymer layers have potential in photonics applications, such as holographic optical elements and waveguide structures.