Optical Materials Forming Tightly Polymerized Voxels during Laser Direct Writing

Routine TPL studies of functional microstructures often demand strict conditions of resolution, form, and stability. This results in a tradeoff between resolution and mechanical stability in finely featured structures. Herein, the authors demonstrate a method to improve the mechanical stability of structures fabricated from a urethane acrylate photoresist, that is, prone to mechanical failure. In this work the authors show that the unique characteristics of two-photon-induced polymerization (TPP) can be exploited to form tightly polymerized voxels (voxels, or volume pixels, are the building blocks of a microstructure). This approach leads to stable microstructures without a sizeable trade-off in terms of resolution. This is achieved by adding a co-sensitizer and radical quencher pair to the photoresists sensitized with a two-photon absorbing dye. The co-sensitizer leads to high degrees of polymerization, while the radical quencher limits polymerization at the periphery of the voxel. The formation of tightly polymerized voxels has been validated experimentally from line dimensions of the structure and their degree of polymerization. The authors have tested the sensitizer-radical quencher combination in another photoresist and demonstrated the adaptability of this concept to tune different photopolymer systems.