Reducing Haze of Holographic Gratings Recorded in a Two-Stage Photopolymer

The haze developed during the recording of holographic gratings was investigated and mitigated in a typical two-stage holographic photopolymer system where a high-refractive-index acrylate (1,3-bis(phenylthio)-2-propyl acrylate, i.e., BPTPA) was utilized as a writing monomer. An acrylate writing monomer (1,3-bis(phenylthio)-2-propyl urethane ethyl acrylate, i.e., BPTPUA) was proposed and synthesized to achieve a lower interaction parameter (χ) between the matrix and the writing monomer. Confirmed by theoretical predictions and experimental photolithography results, the formulation incorporating BPTPUA exhibited a significant improvement in miscibility between the two phases as compared to the analogue compound without a urethane, BPTPA. The higher miscibility led to dramatically lower haze of holographic gratings, due to which the quality of angular playback curves from holographic gratings improved from being strongly distorted to being well-matched with the Kogelnik coupled wave theory. Additionally, a higher light intensity reduced the haze remarkably due to the potential kinetic restriction for phase separation and lower molecular weight of the photopolymer formed. The molecular weight decrease at higher light intensity was confirmed experimentally, and the trend was fit by a theoretical calculation of the kinetic chain length. Using the fitted relationship, the free energy change of mixing was calculated and suggested that a higher light intensity improved the miscibility between the photopolymer and the matrix thermodynamically. Moreover, a reactive matrix was introduced to mitigate the haze development by forming covalent bonds between the two phases. With contributions from covalent attachment and BPTPUA as the writing monomer, haze as low as 0.5% was achieved in a 50 μm recording medium at an extremely high light intensity (200 mW/cm²) for holographic grating.