Nanoscale Composition Mapping and Depth-Resolved Diffraction in Photopolymer Holographic Gratings.

The performance of holographic photopolymers, one of the most promising material platforms for addressing the critical needs in emerging holographic applications, is governed by spatial variations of chemical composition, which have rarely been assessed at length scales commensurate with the grating pitch. Here, the chemical specificity of AFM-IR, an infrared technique capable of nanoscale resolution, is leveraged to map the local composition of a holographic grating recorded in a thiol-ene/polyurethane photopolymer film. Quantitative analysis enabled by calibration of photopolymer (1584 cm-1) to polyurethane (1532 cm-1) intensity ratios in flood-cured films with systematically varied photopolymer loading reveals variations of the grating local peak-to-valley concentration (averaged through the grating thickness) of ≈1.7% g/g. In addition, the grating structure was probed at different depths by measuring the grating angular diffraction efficiency upon sequential, layer-by-layer material removal with a cryo-microtome. The evolution of the diffraction peak intensity and position with depth (i.e., upon thinning of the grating) indicates through-thickness heterogeneity in both the refractive index modulation and the grating geometry. In combination, these approaches offer a framework for assessing the composition of holographic gratings and for understanding their performance.