Researchers in Poland and Italy have developed photo-responsive materials for vat photopolymerization that allow 3D printed components to dynamically alter the intensity and polarization of a laser beam.
The materials incorporate azobenzene-based molecules whose orientation changes when exposed to polarized visible light. This creates a temporary difference in refractive index, known as photoinduced birefringence. The team achieved birefringence values of up to 2.5 × 10⁻⁴ and demonstrated optical modulation over 60,000 on-off cycles.
Published in Advanced Composites and Hybrid Materials, the study brings together researchers from Wrocław University of Science and Technology, the Italian National Research Council’s Istituto Nanoscienze, Scuola Normale Superiore and the University of Pisa. The components were produced using stereolithography (SLA) and digital light processing (DLP).
Most photo-responsive optical materials have previously been limited to thin films and other planar configurations. While these systems can change their optical properties in response to light, their geometry restricts their use in more complex photonic architectures.
The researchers say 3D printing could move these optical functions beyond flat films and distribute them across more complex component geometries.
Using molecular alignment to control light
The primary printable material combined Disperse Red 1, or DR1, with E-Shell 600 photocurable resin.
DR1 is an azobenzene-based nonlinear optical molecule that can switch between a straight, stable trans configuration and a bent, metastable cis configuration. Visible light drives repeated transitions between these two forms.
Before illumination, the DR1 molecules are randomly oriented inside the cured resin. The printed material is therefore approximately optically isotropic and does not significantly change the polarization of a laser passing through it.
A polarized pump beam alters this arrangement. Repeated molecular switching gradually reduces the number of molecules aligned with the pump polarization and increases alignment perpendicular to it. This produces an anisotropic molecular structure and a corresponding difference between the material’s refractive indices in different directions.
The researchers placed the printed material between two polarizers with perpendicular axes. Without the pump beam, a 637 nm input laser was blocked by the second polarizer. Activating the pump changed the beam’s polarization as it passed through the material, allowing light to reach the detector.
Switching off the pump caused the molecular alignment to relax and the transmitted signal to decline. Some residual birefringence remained after illumination stopped.
Separating curing from molecular degradation
UV exposure during photopolymerization had a significant influence on the optical response.
Samples exposed for less than 0.5 minutes produced birefringence values of around 5 × 10⁻⁵. Increasing exposure to 1.7 minutes raised the maximum response to approximately 2.5 × 10⁻⁴.
Between one and 40 minutes of UV exposure, the measured birefringence remained close to 2 × 10⁻⁴. This corresponded to a UV dose range of approximately 0.6 to 23 J cm⁻².
Raman spectroscopy showed resin reaching about 50% conversion during the first two minutes. The highest birefringence was therefore achieved during an early stage of polymer-network formation rather than after complete curing.
The researchers suggest that the developing network restricts molecular movement enough to preserve light-induced alignment. However, prolonged UV exposure begins to degrade the DR1 molecules.
Broadening of a Raman peak associated with DR1 appeared after exposure periods longer than five minutes, which the researchers say may indicate the onset of molecular degradation. After 120 minutes, the dye’s maximum absorbance had fallen to approximately 60% of its original value.
At longer exposure times, declining DR1 absorption limited the optical response. Curing and molecular degradation did not occur at the same rate, leaving a broad interval in which the material retained strong birefringence.
Within this dose range, the researchers could adjust printing parameters for different geometries and resolutions without substantially changing birefringence.
Printing spatially controlled optical responses
The team first printed uniform disks using a Sharebot Andromeda SLA system. Modulating the pump beam caused the intensity of the laser transmitted through the disk to rise and fall.
Dynamic birefringence remained near its maximum at modulation frequencies up to approximately 50 Hz before declining at higher frequencies.
A disk tested with a 10 Hz pump continued to modulate the input laser after 60,000 on-off cycles. The modulation amplitude varied by approximately 8% from its average value over the test.
Researchers then printed a segmented disk containing four quadrants with thicknesses ranging from 420 to 600 µm. The different thicknesses were designed to produce spatially varying phase retardation because the optical effect depends on the distance traveled through the photo-responsive material.
The 420 µm quadrant generated a phase shift of 13.3° ± 0.2°, while the 520 µm section produced a shift of 14.0° ± 0.2°. According to the study, this represents phase control on the order of 10⁻² degrees per micrometer through printed thickness.
DLP samples were also manufactured using a DR1-acrylate derivative embedded in a bisphenol A ethoxylate dimethacrylate matrix. These included a segmented disk and a multi-material structure containing transparent and photo-responsive regions.
A wedge-shaped component further demonstrated that transmitted intensity varied with the thickness of the printed geometry.
Component-level optical control
Together, the disk and wedge tests show that the optical response can be controlled both by the pump beam and by the thickness of the printed part.
However, the study remains a component-level demonstration. It does not validate a complete optical computer, communications network or augmented-reality system.
Long-term stability and the relationship between photopolymerization, optical performance and mechanical properties remain to be assessed. The researchers identify these areas as priorities for future work, alongside post-processing and the integration of additional photo-responsive molecules into photocurable resins.
Further testing is needed before the materials can be used in practical polarization optics or reconfigurable photonic devices.
From optical attenuation to phase control
The study extends earlier work by researchers at NEST, Scuola Normale Superiore and the University of Pisa on DLP 3D printed photochromic polymers. Those materials reversibly switched between colored and transparent states, allowing printed components to modify light transmission, store rewritable patterns and perform basic arithmetic and logic operations. Francesca D’Elia, Lorenzo Lavista, Andrea Camposeo and Dario Pisignano contributed to both studies.
Broader research into 3D printed photonics has identified optical clarity, mechanical robustness, printing throughput and the limited availability of multifunctional photopolymers as continuing challenges. The current study adds dynamic birefringence and a multi-material demonstration to the available range of printed optical functions.
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Feature image shows a series of photographs of the transmitted input beam spot at various time instants, upon switching the pump beam on and then off. Image cropped, via Szukalski et al.
