Dynamic analysis of refractive index evolution and diffraction properties during single-photon polymerization of photopolymers for micro-optical applications

Additive manufacturing of gradient-index (GRIN) optics and diffractive optical elements (DOEs) by photopolymerization requires quantitative knowledge of the spatiotemporal refractive index dynamics. We combine time-resolved refractometry based on total internal reflection (TIR) at a prism interface with in-situ diffraction on a phase grating written into an acrylate photopolymer by a digital light processing (DLP) projector. The refractive index follows an S-shaped evolution governed by oxygen inhibition, autoacceleration and vitrification (Δn = 0.036±0.003). In the thin-grating regime, a Fourier-optics model reproduces the oscillating diffraction orders and their substructures caused by the dead zones of the DLP pixels. The extrema of the 0th order mark the optical path difference in steps of half a wavelength and thus link the diffracted intensities to the evolving index distribution. The sequence of deviations from the model indicates a progressive broadening of the polymerized lines into the nominally unexposed regions.