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.
This Conference Presentation, Holographic photopolymers using thiol-ene chemistry was recorded at SPIE Photonics West 2022 held in San Francisco, California, United States.
Two-stage holographic photopolymers capable of high refractive index modulation (Δn) on the order of 10–2 enable the fabrication of a myriad of optical elements. While there are commercial products available that meet these requirements, researchers often want the flexibility to customize both the form factor of the samples as well as the mechanical and chemical properties for their specific applications. We present a novel high refractive index acrylate writing monomer in a low refractive index urethane matrix as a model material for customization for optical applications. We discuss the achievable Δn of this custom monomer, 1,3-bis(phenylthio)-2-propyl acrylate (BPTPA) in the urethane matrix as a function of solubility, along with a comparison to a commercially available high refractive index monomer, 2,4,6- tribromophenyl acrylate. Formulations with BPTPA exhibit a peak-to-mean Δn ≈ 0.029 in transmission holograms without any obvious deficiencies in transparency, color, or scatter. This writing monomer and the synthetic processes present a promising platform for the fabrication of high-performance holographic photopolymers for a wide range of research applications.
Holographic photopolymers capable of high refractive index modulations (Δn) on the order of 10-2 are integral for the fabrication of functional holographic optical elements (HOEs) for use in a range of optical applications. A novel high refractive index liquid writing monomer suitable for two-stage photopolymer systems was designed and synthesized. This monomer exhibits facile synthetic procedure, low viscosity, high refractive index as well as excellent solubility in a low refractive index urethane matrix. The solubility limit, refractive index change and reaction kinetics/conversion were studied against a commercial reference high refractive index monomer, 2,4,6-tribromophenyl acrylate (TBPA). Superior performance in solubility to TBPA is shown with similar reaction kinetics and final conversion as confirmed by realtime Fourier transform infrared spectroscopy (FTIR) and real-time monitoring of diffraction grating growth. We demonstrate the ability to load substantial amounts of these writing monomers enabling a straightforward path to higher achievable Δn values (peak-to-mean Δn ~ 0.03) without sacrificing optical properties (transparency, color or scatter) as validated through recording ~100% efficiency volume transmission holograms in sub-15 μm thick films.
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