Researchers at the University of Stuttgart have developed a low-temperature atomic layer deposition (ALD) technique for applying antireflective (AR) coatings to lens surfaces in multi-lens systems. The technique could reduce reflectivity and enhance transmission in complex, 3D-printed micro-optics systems consisting of lenses as small as 600 µm. As a manufacturing technique for optical elements, 3D printing has many advantages, including the capabilities to create freeform surfaces such as aspheric lenses and to print multi-lens systems in perfect alignment. 3D-printed multi-lens systems are printed in a single process that includes any hollow parts and undercuts in the system. Conventional coating methods, such as sputtering or directed plasma etching, cannot be used to apply AR coatings to multi-lens systems, because a directed coating beam cannot reach the spaces between the individual lenses. The ALD technique developed by the Stuttgart team, which is designed for use with 3D-printed polymer materials, enables the AR coating to be deposited on every surface of a microlens in one step. University of Stuttgart researchers used a microscope to acquire tilted-view images of a 600-µm-diameter doublet lens system that was 3D-printed on a 1- × 1-cm² glass slide. The doublet lens system is visible as the small dot in the center of the glass slide. The coin is included for scale. Courtesy of Moritz Flöss, University of Stuttgart. During the ALD process, the 3D-printed lens system is exposed to a gas that contains the molecular building blocks of the AR coating. The gas molecules move freely into the hollow parts of the 3D-printed structure, forming a thin, homogeneous layer over all exposed surfaces of the lens. Four alternating layers of titania (TiO2) and silica (SiO2) make up the coating. By adding successive layers and varying the precursor gas, the researchers can tune the thickness and properties of the layers to form sequences of high- and low-refractive index coatings or other AR coating designs. The researchers can additionally tune thickness of the layers to decrease the broadband reflectivity of coated flat substrates in the main part of the visible wavelength spectrum (450 to 650 nm) to below 1%. The ALD process is compatible with the low glass transition temperature of the polymer materials used in 3D printing. It works at 150 °C, a temperature that is low enough to be used for 3D-printed lenses, which are typically stable up to about 200 °C. The researchers characterized the properties of the AR coatings based on transmission measurements taken on coated and uncoated 3D-printed test samples. They also tested the ALD coating technique with a 3D-printed, double-lens imaging system that was 600-µm wide. In addition to reducing reflectivity, the AR coating was found to increase the transmission through six interfaces from about 74% to about 90%. A resolution test target viewed by the 3D-printed double-lens imaging system demonstrated about 20% higher intensity compared to a target viewed by an uncoated lens. “To print the double-lens system, we used a Nanoscribe Quantum X microfabrication system that enables unprecedented surface smoothness for 3D-printed lenses,” researcher Simon Ristok said. “We showed that our ALD coatings significantly reduced reflectivity and, conversely, enhanced transmission for this multi-lens system.” In complex multi-lens systems, some light is lost at each lens-air interface due to reflection. These losses add up, making AR coatings essential. “We have been working on 3D-printed micro-optics for several years and always strive to improve and optimize our fabrication process,” professor Harald Giessen, who led the research, said. “It was a logical next step to add AR coatings to our optical systems to improve the imaging quality of complex lens systems. Our new method will benefit any 3D-printed complex optical system that uses multiple lenses. 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 AR coating technique could lead to high-quality, 3D-printed microlens systems for a range of applications. “We applied ALD to the fabrication of antireflection coatings for 3D-printed complex micro-optics for the first time,” Ristok said. “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.” The researchers plan to combine the AR coatings with complex optical systems that comprise more than two lenses, and to create advanced coating designs with additional layers, which will further decrease reflection losses for specific wavelengths. The researchers believe that 3D printing of micro-optics and ALD of AR coatings are both well suited for rapid prototyping or small-series production. Reducing the processing time could make both approaches suitable for large-scale production. The team said it is open to collaborating with researchers who would like to incorporate AR coatings into 3D-printed optical systems. The research was published in Optical Materials Express (www.doi.org/10.1364/OME.454475).