Researchers from the University of Stuttgart have developed a new reliable method of coating 3D printed lenses with anti-reflective coatings.
Called low-temperature thermal atomic layer deposition (ALD), the approach is capable of coating multi-lens systems as small as 600 microns in diameter and helps minimize light loss due to reflections between lens interfaces. According to the team, the innovation will have major implications for 3D printing high-performance optical systems that rely on multiple microlenses.
“Our new method will benefit any complex 3D-printed optical system that uses multiple lenses,” said Harald Giessen, lead author of the study. “However, it is particularly useful for applications such as miniature fiber endoscopes, which require high quality optics and are used for imaging in less than ideal lighting conditions.”
The need to eliminate reflections
In optical systems, a small amount of light energy is lost each time light crosses a lens-air boundary due to reflection. This phenomenon is particularly apparent in multi-lens systems as losses can accumulate very quickly, so anti-reflective coatings are a necessity if we are to preserve image quality.
Large and simple lenses, such as those used in cameras, are coated before being assembled into the final product. One of the most common methods is sputtering, which is a physical vapor deposition process. Unfortunately, we cannot use the same conventional techniques for tiny 3D printed lens systems, as they typically feature more complex monolithic geometries with hard-to-reach cavities and overhangs.
“We have been working on 3D-printed micro-optics for several years and are always striving to improve and optimize our manufacturing process,” adds Giessen. “It was the next logical step to add anti-reflective coatings to our optical systems to improve the image quality of complex lens systems.
There are traditional thin film deposition processes that can actually be used to apply anti-reflective coatings to 3D printed geometries, but they often require high temperatures. The resins used in two-photon polymerization are generally stable up to 200°C, so the team sought to develop an ALD technique that works at just 150°C.
Low temperature thermal atomic layer deposition
The low temperature ALD technique works by exposing a 3D printed part to a gas containing the molecular precursors of an anti-reflective coating. Since gas molecules are free to move and diffuse, they can seep into hollow cavities and overhangs of a complex structure, successfully forming a homogeneous thin layer. By varying the precursor gas and depositing additional layers, the thickness, refractive properties, and reflective properties of the coating can be fine-tuned to create custom 3D printed lenses.
The team tested their ALD coating method with a set of miniature lens samples 3D printed on a Quantum Nanoscribe X system. The results indicated that the coatings were indeed effective, reducing the broadband reflectivity of flat substrates in the visible wavelengths to less than 1%.
In the future, the researchers believe they can also adapt the process to deposit other thin films such as chromatic filters directly onto 3D-printed micro-lenses.
“We applied ALD for the first time to fabricate anti-reflective coatings for complex 3D-printed micro-optics,” said Simon Ristok, first author of the paper. “This approach could be used to make new types of extremely fine endoscopic devices that could enable new ways to diagnose – and perhaps even treat – disease. It could also be used to make miniature sensor systems for autonomous vehicles or high-quality miniature optics for augmented/virtual reality devices such as goggles.
Further details of the study can be found in the article titled “Atomic layer deposition of conformal anti-reflective coatings on complex 3D printed micro-optical systems”.
Two-photon polymerization is undeniably the cutting edge technology when it comes to micro-optical 3D printing systems. Scientists from the University of Friborg have also used Nanoscribe 3D printers in the past, fabricating glass silica microstructures with a resolution of only tenths of a micrometer. Using a “Glassomer” polymer-based resin, the team 3D printed objects with a surface roughness of 6 nanometers, well below the 40-200 nanometers seen in many other glass parts.
Somewhere else, UpNano, a Vienna-based 2PP 3D printer manufacturer, recently launched two new resins for use with its 3D printing technology. Named UpBlack and UpOpto, the photopolymer materials enable the printing of non-transmissible black and translucent parts, respectively. The materials can be used to 3D print entire optical systems, including components such as housings and lenses.
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The featured image shows researchers using a microscope to acquire images of a 600-micron 3D-printed lens system. Photo via University of Stuttgart.