Properties of PVA/AA photopolymers at very low spatial frequencies

Abstract

Polyvinyl/acrylamide (PVA/AA) photopolymers are often used as holographic recording materials. Depending on each particular application, different spatial frequencies can be recorded. The only limitation appears when high spatial frequencies are stored (over 2500 lines/mm the diffraction efficiencies achieved are very low). This cut-off spatial frequency is due to the finite size of polymer chains. On the other hand, in the case of very low spatial frequencies, monomer diffusion times are highly increased and so there are important distortions in the shape of the gratings stored. In order to characterize the hologram formation in these types of materials, many parameters must be taken into account. One of the most important parameters to measure is the diffusion of the molecules inside the layer. In this sense, when very low spatial frequencies are recorded, component diffusion and thickness variations can be determined more accurately. In previous works, we have characterized the photopolymer at the zero spatial frequency limit, where diffusion does not take place, measuring the refractive index variations and the shrinkage. In this work we have recorded different long period gratings (at very low spatial frequencies diffusion starts to be important) and we have analyzed the profiles formed in the material during exposure and the refractive index distributions. This study has been carried out measuring the transmission and reflection orders of diffractive gratings. Furthermore, to obtain a deeper insight into the profile of the gratings recorded and especially into the sharpness of their edges, we have obtained numerically the diffraction-efficiency values fitting a Fermi-Dirac function to each profile.