Is multi-material 3D printing the future of microoptical imaging systems? On the macroscale, compound lenses are made of materials with different optical characteristics and are commonly used for the correction of chromatic aberrations in high-quality optical systems such as telescopes or microscope objectives. Scientists at the University of Stuttgart have now introduced a 3D-printed multi-material hybrid lens that corrects chromatic aberrations for microscale applications.
3D-printed apochromat lenses for
The invention of achromatic lenses can be traced back to the 18th century and became the pillar of almost all modern optical devices such as telescopes, camera lenses or microscopes. The basic idea behind these lenses is to improve the image quality by eliminating chromatic aberration. This imperfection in an optical image can be reduced by combining two materials with different dispersion in one lens design in such a way, that the focal points of different wavelengths coincide. While lenses with corrected chromatic aberration are widely used for macroscale applications, their fabrication on the microscale remains challenging.
Two-Photon Polymerization for near-perfect microoptics
Scientists at the University of Stuttgart are now well on their way to create near-perfect microlenses using Nanoscribe’s microfabrication technology. With the inherent design freedom of Two-Photon Polymerization (2PP), the researchers are able to 3D-print freeform lenses that can compensate not only the imperfections resulting from spherical lenses, but also for chromatic aberrations. In a first step, the scientists printed a hybrid refractive/diffractive microlens, merging the focal points of the two visible wavelengths 500 nm and 700 nm. To further compensate chromatic aberrations, the researchers incorporated two materials with different optical properties into the hybrid lens, printing the world’s first apochromatic refractive/diffractive microlens by Two-Photon Polymerization.
Refractive optics meets diffractive optics
To reduce chromatic aberrations, the researchers fabricated a hybrid refractive/diffractive lens with a diameter of 400 µm and exploited the different dispersion behavior of refractive and diffractive optics. While refractive lenses exhibit positive dispersion, meaning that their focal length increases with increasing wavelength, diffractive optics exhibit opposite behavior and focus light with longer wavelength first. Additive manufacturing such as direct laser writing is known for its design freedom, which allowed the scientists to combine a refractive and diffractive design in one single lens. This hybrid microlens brings red and blue light to the same focal distance, resulting in a significant increase in quality of the test object’s images compared to the aspherical refractive lens.
3D-printed apochromats with Nanoscribe’s photoresins
The team of the University of Stuttgart, gripped by ambition, looked for further improvements of the achromatic lens towards a nearly perfect apochromatic lens. This means that a third wavelength of 600 nm is corrected and brought into the focal plane of the two corrected wavelengths of the achromat lens.
Conventional macroscale doublets are made of glasses with different dispersion properties and can be tuned to bring different wavelengths to the same focal point. For the 2PP-printed apochromat, the scientists took inspiration from this classical approach and printed a doublet lens of materials with different optical properties. Nanoscribe’s functional IP Photoresin IP-n162 proved to be an excellent candidate for the researchers’ goal of printing an apochomatic doublet lens. The high refractive index material features strong dispersion and is used as the negative, concave-shaped part of the lens. On top of this first element of the lens, the scientists printed a convex-shaped lens with the integrated diffractive design of the refractive/diffractive hybrid lens.
This results in microlenses that perfectly correct chromatic aberrations for the visible wavelength range, paving the way for new improved microoptical systems.
If you would like to read more about this research project, you can find the complete scientific publication here: 3D printed hybrid refractive/diffractive achromat and apochromat for the visible wavelength range
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