Stress-induced birefringence in 3D direct laser written micro-optics.

3D direct laser writing is a widely used technology to create different nano- and micro-optical devices for various purposes. However, one big issue is the shrinking of the structures during polymerization, which results in deviations from the design and in internal stress. While the deviations can be compensated by adapting the design, the internal stress remains and induces birefringence. In this Letter, we successfully demonstrate the quantitative analysis of stress-induced birefringence in 3D direct laser written structures. After presenting the measurement setup based on a rotating polarizer and an elliptical analyzer, we characterize the birefringence of different structures and writing modes. We further investigate different photoresists and the implications for 3D direct laser written optics.