Refractive index characterization and modeling of polymerization-dependent refractive index variations in two-photon polymerization resins

Two-photon polymerization (2PP) is a cutting-edge technique for fabricating precise micro- and nanostructures, with applications in photonics, biomedical engineering, and micro-optics. A critical factor influencing the optical performance of 2PP-fabricated structures is the refractive index (RI) of the printed parts. This study analyzes the refractive indices of three resins developed by UpNano GmbH . Using a Pulfrich refractometer, dispersion curves of the polymerized resins were measured across the visible to near-infrared spectrum (450 nm – 1550 nm), and temperature-dependent RI behavior was characterized for both liquid and polymerized resins (15°C – 50°C). The RI of the polymerized resins ranges from 1.496 to 1.567. What we believe to be a novel model was developed to account for periodic RI fluctuations inherent in 2PP-printed parts due to voxel-based polymerization patterns. The model fits experimental data well and provides additional insights into the degree of polymerization within printed samples. These findings not only enhance understanding of the optical properties of 2PP-fabricated structures but also suggest opportunities for tailoring materials for gradient-index optics and other advanced optical applications.