Centimeter-Scale Curing Depths in Laser-Assisted 3D Printing of Photopolymers Enabled by Er³⁺ Upconversion and Green Light-Absorbing Photosensitizer

Photopolymer resins used in stereolithographic 3D printing are limited to penetration depths of less than 1 mm. Our approach explores the use of near-infrared (NIR) to visible upconversion (UC) emissions from lanthanide-based phosphors to initiate photopolymer crosslinking at a much higher depth. This concept relies on the use of invisibility windows and non-linear optical effects to achieve selective crosslinking in photopolymers. SLA resin formulation capable of absorbing light in the visible region (420–550 nm) was developed, in order to take advantage of efficient green-UC of Er³⁺/Yb³⁺ doped phosphor. NIR-green light UC shows versatility in enhancing curing depths in laser patterning. For instance, a structure with a curing depth of 11 ± 0.2 mm, cured width of 496 ± 5 µm and aspect ratios of over 22.2:1 in a single pass via NIR-green light UC. The penetration depth of the reported formulation approached 39 mm. Therefore, this technique would allow curing depths of up to 4 cm. Moreover, it was also demonstrated that this technique can initiate cross-linking directly at the focal point. This shows the potential of NIR-assisted UC as a low-cost method for direct laser writing in volume and 3D printing.