  The recent development of light-based 3D printing technologies has marked a
turning point in additive manufacturing. Through photopolymerization, liquid
resins can be solidified into complex objects. Usually, the polymerization is
triggered by exciting a photoinitiator with ultraviolet (UV) or blue light. In
two-photon printing (TPP), the excitation is done through the non-linear
absorption of two photons; it enables printing 100-nm voxels but requires
expensive femtosecond lasers which strongly limits their broad dissemination.
Upconversion nanoparticles (UCNPs) have recently been proposed as an
alternative to TPP for photopolymerization but using continuous-wave lasers.
UCNPs convert near-infrared (NIR) into visible/UV light to initiate the
polymerization locally as in TPP. Here we provide a study of this multi-photon
mechanism and demonstrate how the non-linearity impacts the printing process.
In particular, we report on the possibility of fine-tuning the size of the
printed voxel by adjusting the NIR excitation intensity. Using gelatin-based
hydrogel, we are able to vary the transverse voxel size from 1.3 to 2.8 {\mu}m
and the axial size from 7.7 to 59 {\mu}m by adjusting the NIR power without
changing the degree of polymerization. This work opens up new opportunities for
speeding up the fabrication while preserving the minimum feature size with
cheap light sources.
