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Breakthroughs in holographic optical devices frequently rely on advances in high-refractive-index photopolymer materials (HRIPs). While significant progress has been made in the pursuit of HRIPs, additional considerations have prevented broad application of photopolymerization-based materials for fabricating high-performance holographic gratings. To address the deficiency of suitable high-refractive-index monomers for holographic recording, our recent works were conducted from two main aspects, which are (1) monomer synthesis to improve the theoretical refractive index contrast between photopolymer and matrix (or binder), and (2) formulation manipulation for improving segregation degree between photopolymer and matrix (or binder). We have explored several synthetic approaches to obtain high-refractive-index acrylate monomers (nD=1.6) of high miscibility with matrix, multifunctional low-viscosity, high-refractive-index thiol-ene and thiol-yne monomers (nD=1.6). Combining with polyurethane matrix (binder) with a refractive index of 1.48, these monomers exhibit a high theoretical peak-to-valley index contrast of more than 0.12. To fully utilize the high theoretical index contrast, thiol-ene click chemistry in combination with a linear functionalized polymer binder was explored to achieve a high refractive index modulation(peak-to-mean) close to 0.04. Meanwhile, in the thiol-ene formulations, a variety of chemical modification methods, which can be readily translated into other material systems, were proposed, and studied to manipulate the rates of reaction and diffusion processes during holographic recording to optimize the refractive index modulation. The dramatic difference of achievable refractive index modulation in similar thiol-ene formulations with close theoretical index contrast was observed in such study. The difference underscores the importance of customized strategies and systematic formulation manipulation for achieving high-performance holographic photopolymers.
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