Variable dose slicer for refractive index engineering in two-photon polymerization

TL;DR

This paper tackles the challenge of leveraging variable degree of conversion ($DC$) in two-photon polymerization to engineer refractive index ($RI$) distributions in 3D microstructures. It introduces an open-source voxel-based slicer that maps 3D $RI$ profiles to printing power, coupled with a robust two-immersion refractive-index calibration using digital holographic microscopy to achieve high accuracy ($\approx 10^{-4}$–level) and precision in $RI$ estimates. The authors demonstrate practical workflows, including grayscale lithography, structure rotation, and real 3D $RI$ phantoms (biological cell analogues and C. elegans), showing repeatable, stable RI control and potential for metrology and photonic applications. The work provides a comprehensive toolbox for 3D $RI$ engineering in TPP and paves the way for widespread adoption of variable-$DC$ printing in microfabrication and bio-imaging phantoms, with implications for metamaterials, GRIN optics, and digital twins for manufacturing and imaging. Overall, the approach integrates software, metrology, and advanced printing to expand the functional capabilities of existing TPP platforms.

Abstract

In two-photon polymerization (TPP), the degree of conversion (DC) of the resin has an effect on a broad range of material properties like refractive index (RI) or stiffness. Heterogeneous DC can substitute material doping and multimaterial structures, and outright enable structure designs not possible otherwise. However, obtaining variable DC in the polymer, typically achieved by implementing a variable exposure dose, is held back due to the lack of software support for fabrication and measurement techniques for validation, adding up to a high barrier of entry. This work presents two major breakthroughs in (3+1)D TPP: design freedom and variable-DC fabrication, that are provided by an open-source slicer, as well as calibration methodology for determination of the RI for any DC. Application examples include grayscale lithography, control over writing direction and trajectories, as well as bio-mimicking microphantoms with carefully engineered 3D RI. Results of the RI calibration demonstrate excellent repeatability, accuracy and stability of variable-DC structures. Supported by in-depth metrological analysis, the goal is to popularize variable DC printing within TPP community and to get more out of the existing TPP systems and workflows. In summary, this work provides a complete toolbox for 3D RI engineering and sets the stage for new inventions enabled by point-wise dose control.

Figures (7)

• Figure 1: Structure design for RI calibration of the material. a) Half-section view of the step structure, where the shell printed with high DC protects the resin chamber, that can be polymerized to any degree. The proposed geometry introduces $\Delta$d = 15µ m height difference between the resin and surrounding medium. b) Scanning electron microscopy image of the step with marked regions of interest (ROI) used for analysis. c) Phase map and cross-section of the step structure filled with liquid resin, immersed in 1.50 RI medium and measured by DHM.
• Figure 2: Refractive index values of IP-Dip2 resin cured with various powers, calculated at 633n m wavelength and 25℃. Error bars represent expanded uncertainty (k=2).
• Figure 3: Refractive index change of IP-Dip2 over time for four laser power values. The same sample has been measured 1, 38, 112 and 303 days after the fabrication. Error bars represent expanded uncertainty (k=2).
• Figure 4: Line width calibration of IP-Dip2 resin printed with various laser powers. a) Scanning electron microscopy image of suspended individual lines. Image area indicated by the dashed line is averaged in the direction indicated by the arrow to reduce noise, and then the width of the individual lines is evaluated at half maximum (inset). b) Voxel width calibration plot with the error bars indicating standard deviation from 10 measurements.
• Figure 5: 3D refractive index engineering using the proposed slicer. a) Biological cell phantom design surrounded by the woodpile structure that introduces adjustable scattering. b-c) Optical diffraction tomography reconstruction of the 3D RI of the cell target, shown as the XY and XZ cross-sections. d-e) Corresponding cross-sections of the design (d) and optical diffraction tomography reconstruction (e) of the printed C. elegans worm phantom, where all internal structures are visible purely due to the RI contrast modulated via printing laser power. Dataset for the C. elegans worm is provided in the repository.