Revealing the Transmission Hologram's Refractive Index Profile Stored in a Photohydrogel: A Structural Characterization

Ensuring the structural stability and optical performance of holographic gratings in photopolymerizable hydrogels is essential for their integration into advanced photonic and biosensing platforms. Here, we investigate unslanted volume transmission holograms recorded in acrylamide‐based photohydrogels and establish how grating bending and internal attenuation govern diffraction efficiency. By fitting a physical model to experimental data, we report, for the first time to our knowledge, a three‐dimensional visualization of refractive index profiles in stored holograms, offering insights that are otherwise inaccessible with conventional imaging techniques. By using this tool, this work also reveals the role of radiant exposure and spatial frequency in defining recording quality, achieving diffraction efficiencies up to 75%, among the highest reported for hydrogel‐based holographic materials. In addition, as a novel feature, an LED curing process has been implemented for this type of material that significantly enhances the mechanical and optical stability of the gratings in aqueous environments. This work provides a new characterization methodology for holographic systems and offers guidelines for developing high‐performance holograms in hydrogel materials for use as reliable detection devices.