Single-photon-assisted two-photon polymerization

Light-based additive manufacturing (AM) has revolutionized the fabrication of complex three-dimensional objects offering a cost-effective and high-speed alternative to traditional machining. Single-photon polymerization is a key process in this advancement, providing rapid printing time, albeit with limited resolution in the range of tens of micrometers. Two-photon polymerization provides sub-micrometer resolution but is accompanied by a tradeoff of prolonged printing times. We propose combining single- and two-photon absorption to benefit from the dual capabilities, allowing for faster printing time while maintaining high resolution. In this study, we employ a continuous-wave blue light source to pre-sensitize a photocurable resin by single-photon absorption followed by a tightly focused femtosecond laser beam to provide the required energy reaching the polymerization threshold for solidifying the resin through two-photon absorption. We first investigate the impact of pre-sensitization by blue light illumination followed by irradiation with a focused femtosecond laser beam and find that the voxel growth dynamics are markedly altered. Specifically, pre-sensitization by blue light lowers the polymerization threshold power of the femtosecond laser beam and increases the speed of voxel growth. We then exploit this effect by building a custom two-photon 3D printer in which the blue light pre-sensitization is created in a light-sheet configuration. We report successfully printed 3D objects for which the average femtosecond laser power was reduced by 28.6 % and the exposure time was reduced by a factor of two compared with polymerization performed only with the femtosecond laser beam. Additionally, a 63 % improvement in axial voxel size is attained through blue light-sheet pre-sensitization. Our theoretical analysis based on a diffusion-free model suggests that the mechanism of the pre-sensitization is oxygen depletion followed by a single-photon background latent polymerization.