Copper complex-based photoinitiator for high resolution two-photon polymerization

Two-photon polymerization (TPP) has numerous advantages in the additive manufacturing of 3D polymer micro and nanostructures. Although mature commercial photoinitiators are already available, the development of new photoinitiators with high sensitivity and good performance to enhance the spatial resolution of writing is still needed. In this context, metal-based photoinitiators that offer the advantage of long-living excited state lifetimes, have been reported to efficiently initiate single-photon polymerization at near-UV wavelengths. Here, we investigated, for the first time, the two-photon polymerization performance of a copper complex photoinitiator (G1) and we compare it to some commercial photoinitiators such as Phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO) and 2-isopropylthioxanthone (ITX). In combination with pentaerythritol triacrylate (PETA), the copper complex G1 showed higher resolution and sensitivity than BABO and ITX allowing higher writing rates. The two-photon absorption cross-section of G1 was determined by femtosecond Z-scan technique. Polymer linewidths with 120 nm were obtained and woodpile structures with a period down to 350 nm were successfully fabricated. The formulation containing G1 was successfully applied to the fabrication of structural colors.