Storage Optimization of Transmission Holographic Gratings in Photohydrogels

The development and optimization of holographic materials represent a great challenge today. These materials must be synthesized according to the characteristics that are desirable in photonic devices whose application is the object of investigation. In certain holographic sensors and biosensors, it is essential that the recording material be stable in liquid media. Furthermore, the holographic gratings stored in them must have temporal and structural stability, so that they can act as transducers of the analytical signal. Therefore, it is essential to optimize its storage in terms of the chemical composition of the material and the optical parameters of recording. This work focuses on the study of the storage optimization of unslanted transmission volume phase holograms in photohydrogels based on acrylamide and <i>N</i>,<i>N'</i>-methylenebis(acrylamide). Hydrogel matrices, also composed of acrylamide and <i>N</i>,<i>N'</i>-methylenebis(acrylamide), with different degrees of cross-linking were used and analyzed by scanning electron microscopy and UV-visible spectroscopy. The best results in terms of diffraction efficiency were reached for hydrogel matrices with an acrylamide/<i>N</i>,<i>N'</i>-methylenebis(acrylamide) molar ratio between 19.9 and 26. This relationship was also optimized in the incubator solution used to incorporate the components necessary for the formation of the holograms in the hydrogel matrices. The maximum diffraction efficiency, about 35%, was achieved when using an incubation solution with an acrylamide/<i>N</i>,<i>N'</i>-methylenebis(acrylamide) molar ratio of 4.35. The influence of the physical thickness of the hydrogel layers, the intensity, and the exposure time on the diffraction efficiency was also investigated and optimized. In addition, the behavior of the hologram was analyzed after a washing stage with PBST. A simple model that considered the effects of bending and attenuation of holographic gratings was proposed and used to obtain the optical parameters of the holograms.