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

TL;DR

This study investigates the dynamic changes in refractive index and diffraction during single-photon polymerization of photopolymers, combining experimental measurements and simulations to improve micro-optical 3D printing processes.

Contribution

It introduces a comprehensive experimental and simulation approach to analyze the optical property evolution during photopolymerization, aiding optimization of micro-optical element fabrication.

Findings

Refractive index change follows an S-shaped curve indicating different polymerization phases.

Diffraction patterns are influenced by pixel structure and dead zones of the DLP system.

Simulation confirms scattered light and radical diffusion cause delayed polymerization and reduced contrast.

Abstract

Photopolymerization enables the production of micro-optical elements, such as diffractive optical elements or GRIN optics. This process utilizes targeted spatial modulation of the refractive index, which is achieved through additive manufacturing. In this context, a thorough understanding of the dynamic processes during curing is essential in order to be able to accurately predict the optical function of the element. For this reason, this work investigates the kinetics and resulting optical properties of an acrylate-based photopolymer under UV irradiation using a DLP projection system. The experimental approach combines two measurement methods: On the one hand, the absolute change in the refractive index is determined in a time-resolved manner at the interface of a prism using a method based on total reflection. On the other hand, the formation of a phase grating in the volume of the polymer is monitored in real time by analyzing the diffraction orders of a coherent sample laser. The results show a characteristic S-shaped curve of the refractive index change, which reflects the phases of polymerization: oxygen inhibition, autoacceleration, and vitrification. Analysis of the diffraction patterns reveals complex intensity curves and substructures in the diffraction orders. These could be traced back experimentally and through simulations to the discrete pixel structure of the DLP projector and the existing dead zones. Furthermore, the simulation model developed on the basis of Fourier optics reproduces the experimental diffraction patterns and confirms the hypothesis that scattered light and radical diffusion lead to time-delayed polymerization in the theoretically unexposed areas. This results in a reduction of the refractive index contrast over time. This work thus provides parameters for the simulation and optimization of exposure strategies in 3D printing of micro-optics.