Researchers from the University of Stuttgart have developed a new, reliable method of coating 3D printed lenses with anti-reflective coatings.
Dubbed 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 to minimize the light lost due to reflections between lens interfaces. According to the team, the innovation will have major implications for the 3D printing of high-performance optical systems that rely on multiple microlenses.
“Our new method will benefit any 3D printed complex optical system that uses multiple lenses,” said Harald Giessen, lead author of the study. “However, it is especially useful for applications such as miniature fiber endoscopes, which require high-quality optics and are used for imaging under less-than-ideal lighting conditions.”
The need to eliminate reflections
Within optical systems, a small amount of light energy is lost every time light passes through a lens-air border due to reflection. This phenomenon is especially apparent in multi-lens systems as the losses can compound very rapidly, so anti-reflection coatings are a necessity if we want to preserve image quality.
Large and simple lenses, such as the ones used in cameras, are coated before they’re assembled into the final product. One of the most common methods is sputtering, which is a physical vapor deposition process. Unfortunately, we can’t use the same conventional techniques for tiny 3D printed lens systems because 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 always strive to improve and optimize our fabrication process,” adds Giessen. “It was a logical next step to add anti-reflective coatings to our optical systems to improve the imaging 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 typically 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 to an anti-reflective coating. Since the gas molecules are free to move around and diffuse, they can infiltrate the hollow cavities and overhangs of a complex structure, successfully forming a homogenous thin coat 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 Nanoscribe Quantum X system. The results indicated that the coatings were indeed successful, slashing broadband reflectivity of flat substrates in visible wavelengths to under 1%.
Moving forward, 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 to the fabrication of antireflection coatings for 3D printed complex micro-optics for the first time,” said Simon Ristok, first author of the paper. “This approach could be used to make new kinds of extremely thin endoscopic devices that might enable novel ways of diagnosing — and perhaps even treating — 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 paper titled ‘Atomic layer deposition of conformal anti-reflective coatings on complex 3D printed micro-optical systems’.
Two-photon polymerization is undeniably the leading technology when it comes to 3D printing micro-optical systems. Scientists from the University of Freiburg have also utilized Nanoscribe 3D printers in the past, fabricating glass silica microstructures with a resolution of just a few tenths of a micrometer. Using a ‘Glassomer’ polymer-based resin, the team 3D printed objects with a surface roughness of 6 nanometers, much less than the 40-200 nanometers seen in many other glass parts.
Elsewhere, UpNano, a Vienna-based manufacturer of 2PP 3D printers, recently launched two new resins for use with its 3D printing technology. Named UpBlack and UpOpto, the photopolymer materials enable the printing of non-transmitting black and translucent parts, respectively. The materials can be used to 3D print entire optical systems, including components such as casings and lenses.
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Featured image shows the researchers using a microscope to acquire images of a 600 micron 3D printed lens system. Photo via University of Stuttgart.