Red-Sensitive Photopolymerizable Glass for Fabrication of Holographic Optical Elements

Water-resistant photopolymerizable glass, which represents a photopolymerizable hybrid sol–gel (PHSG) material, has emerged as a compelling platform for holographic optical elements fabrication (HOEs), owing to its exceptional mechanical and thermal robustness with high holographic recording performance. Until now, PHSG compositions have been limited to blue/green-sensitive systems, despite significant interest in extending sensitivity into the red spectral region. Here, we report the first holographic photopolymerizable glass with sensitivity to red light, achieved through the incorporation of methylene blue (MB) as a photosensitizer and N-phenylglycine (NPG) as a co-initiator, with an optimal MB/NPG ratio of 1:150. This PHSG composition is capable of volume transmission grating recording across a spatial frequency range of 300–2200 lines mm⁻¹, achieving diffraction efficiencies up to 80% in 123 µm thick layers. Beyond holographic gratings, the material also enables the storage of modulated light patterns, demonstrated through the successful recording of a square dot array with a 5 µm feature size. The recorded structure operates as a diffractive optical element, reconstructing predefined diffraction patterns with high fidelity. These results establish a new capability for PHSG, as holographic patterning under red-light exposure, and represent a step toward the development of panchromatic photopolymerizable glass, enabling the fabrication of full-color HOEs.