Phase-assisted multi-material two-photon polymerization for extended refractive index range

Two-photon polymerization (TPP) is a 3D-printing technique enabling fabrication of structures in a submicrometer scale. With almost no shape limitation, structure's functionality is mainly defined by a fabrication resin used in the curing process. Recent advances in TPP attempt to incorporate multiple materials into the printing process; however, due to required nano-scale accuracy, maintaining common coordinate system between subsequent materials poses the biggest challenges in the process. In this paper, a novel multi-material TPP procedure is introduced based on transport of intensity equation phase imaging method for alignment purposes. The proposed method utilizes hardware already present in most TPP fabrication setups, delivering a complete and cost-free solution for multi-material fabrication. As a result, the position inaccuracy reaches less than 0.5μm. For demonstration purposes, a multi-material structure is designed, fabricated, and verified in terms of geometry and refractive index distribution accuracy. The multi-material object is made out of two polymers across two development cycles effectively expanding accessible refractive index range.