Doping hybrid photopolymerisable glass with BODIPY photosensitiser: an efficient approach to improve UV resistance

The development of photosensitive materials with improved performance and extended functionality is crucial for advancing holographic technologies. This study presents the modification and characterization of a recently reported photopolymerisable glass composition, focusing on achieving an optimal balance between maximizing photosensitivity of the material during recording and minimizing the impact of UV light on the performance of the recorded holographic structure. Building on these findings, the ultimate goal is to develop UV-resistant holographic optical elements for wavefront sensing in space applications. The research approach is based on introducing a photosensitiser with weaker UV absorption than the typically used in this material - Irgacure 784 dye. In this paper, photopolymerisable glass is synthesized using a sol-gel process and doped with a 2-component photoinitiating system, i.e. diiodo-BODIPY(I2BDP) as a photoinitiator and N-phenyl glycine (NPG) as a coinitiator, which had previously demonstrated resistance to UV-A light exposure of up to 22 J/cm² . The photosensitivity of the material doped with I₂BDP was found to be lower compared to the original composition doped with Irgacue 784. Therefore, the main challenge was to enhance the material’s photosensitivity without causing an excessive increase of UV absorption, which can adversely affect its environmental stability. To identify the optimal I₂BDP concentration, the performance of the material was studied by testing the optical properties of volume holographic transmission gratings, such as exposure sensitivity, refractive index modulation and diffraction efficiency, for different dye concentrations. Additionally, the shelf-life of the material after a prolonged exposure to UV light was assessed by examining the layers for cracks and monitoring diffraction efficiency of the transmission gratings over time, confirming its suitability for practical space-based applications. Absorption spectra of the material were also taken before and after holographic recording and after UV exposure to monitor absorption changes of the compositions due to the changing I₂BDP concentration at different stages of the process. In conclusion, this study presents an efficient approach in the development of UV-resistant photosensitive materials for holography, highlighting the successful integration of a BODIPY dye that strikes an effective balance between photosensitivity and reduced UV absorption.