Because light of different wavelengths is shaped and refracted differently, lenses located on the end faces of optical fibers can experience strong chromatic aberration. Chromatic aberration can lead to a shift in the focal point and blurred imaging over a range of wavelengths, hindering these lenses, which are used for applications that include endoscopy for medical diagnostics. To remedy these aberrations, an international research collaboration optimized an optical glass fiber so that light of different wavelengths could be focused with extreme precision. The researchers designed and nanoprinted a 3D achromatic, diffractive metalens on the end face of a single-mode fiber. The 3D achromatic metalens, which the researchers refer to as an achromatic metafiber, has a lens diameter of 100 µm. It has a numerical aperture (NA) of 0.2, which is significantly higher than achromatic lenses previously used on fiber end faces. The high NA helps the achromatic metafiber to achieve better resolution, compared with previously developed achromatic lenses. “For ideal light shaping and achromatic focusing, we realized an ultrathin polymer-based lens, which consists of a complex design of geometric structures in the form of nanopillars,” said professor Markus Schmidt, head of the Fiber Photonics Department at Leibniz Institute of Photonic Technology. “This structure was printed directly on the tip of a 3D-printed hollow tower structure on one of the end faces of a commercial optical fiber.” According to Schmidt, in this way, optical fibers can enable light to be focused very efficiently on a focal point, leading to the generation of images with high resolution. The unlocked height degree of the 3D achromatic metalens largely increased the upper bound of the time-bandwidth product to 21.34, expanding the modulation range of the group delay from −8 to 14 fs. In addition, the subwavelength nanopillar meta-atoms of the achromatic metalens exhibited strong birefringence, capable of imprinting a hyperbolic lens profile via the geometric phase. As a result, the achromatic metafiber enabled achromatic and polarization-insensitive focusing across the near-infrared telecommunication wavelength band ranging from 1.25 to 1.65 µm — the entire single-mode domain of commercially used fibers. “It is remarkable that the individual nanopillars have different heights ranging from 8.5 to 13.5 µm, Schmidt said. “This allows the different wavelengths of light to be focused on a single focal point.” Principle of an achromatic metafiber used for achromatic focusing and imaging. An achromatic metalens located on top of a 3D-printed hollow tower (used for fiber-beam expansion) was interfaced with a single-mode fiber via 3D laser nanoprinting. Inset: An enlarged 3D nanopillar meta-atom (height: H, length: L, width: W), the height of which offers a large modulation range of group delay. Courtesy of Nature Communications (2022). DOI: 10.1038/s41467-022-31902-3. The researchers demonstrated the focusing efficiency of the achromatic metafiber using fiber optic confocal imaging. They showed that a fiber with achromatic meta-optics created in-focus, sharp images with spatial resolution up to 4.92 µm under broadband light illumination. They achieved convincing image quality with high image acquisition efficiency and high image contrast at different wavelengths. The focus positions remained almost constant, even at different wavelengths. The researchers also used the achromatic metafiber for fiber-optic confocal scanning imaging without a standard microscope, demonstrating that an ultracompact, ultrabroadband confocal endoscopic system could potentially be established using their approach. “Since the developed nanostructured metalens is extremely small and flat, a fiber optic design with achromatic optics at the top offers the potential to further advance miniaturized and flexible endoscopic imaging systems based on fiber technology and to enable even more gentle, minimally invasive examinations,” Schmidt said. In addition to endoscopic imaging, the researchers envision that the achromatic metafiber could be useful in laser-assisted therapy and surgery, deep tissue imaging, fiber optic communications, fiber sensor technology, and fiber lasers. The research team included scientists from Monash University, Ludwig Maximilian University of Munich, Pohang University of Science and Technology (POSTECH), Abbe Center of Photonics, POSCO-POSTECH-RIST Convergence Research Center for Flat Optics and Metaphotonics, Otto Schott Institute of Material Research, and Imperial College London, in addition to the Leibniz Institute of Photonic Technology. The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-31902-3).