Meta-Fibers: Merging Optical Fibers and Nanophotonics Through 3D Nanoprinting

<i>Synopsis:</i> Here, we will demonstrate how the combination of optical fibers and nanostructures creates a new category of fiber-integrated devices – meta-fibers, achieved by leveraging 3D nanoprinting. Essential results are high-NA holographic meta-lenses for optical trapping, multifocus generation via single-point phase-engineering, achromatic metasurface-based lenses, and dielectric ring-like gratings for boosting incoupling efficiencies.<br/><br/><i>Introduction:</i> The development of nanostructures on fiber end faces offers a promising approach for unlocking new functionalities across various fields, such as biophotonics, quantum technologies, and optical sensing. Conventional top-down fabrication methods usually fail as the fiber geometry is complementary to that of planar substrates (e.g., wafers). In this presentation, we show on several examples that 3D nanoprinting using direct laser writing circumvents this bottleneck, allowing to implement intricate nanostructures on the end face of optical fibers.<br/><br/><i>Optical trapping with single fiber:</i> Effective light focussing is crucial for numerous applications, while optical fibers have limitations due to the divergence of the emitted light. Here, we used 3D nanoprinting to integrate ultra-high NA holographic meta-lenses onto functionalized single-mode fibers, achieving record-high NA of up to 0.9 with diffraction-limited spots. This breakthrough allowed us to optically trap microspheres and bacteria with individual single-mode fibers, overcoming a significant limitation of fiber optic.<br/><br/><i>Achromatic light focusing:</i> An open issue in the field of telecommunication is achromatic light focusing, for which no satisfactory solution exists so far. By integrating a nano-printed metasurface-based lens onto a fiber, we have been able to achromatically and efficiently focus the output light of a SMF-28 over the entire telecommunications range. Key is the use of nanopillars of different heights, representing a unique degree-of-freedom provided by the nanoprinting process that is crucial to wavelength independent light focussing.<br/><br/><i>Boosting in-coupling efficiency</i>: Conventional commercial fibers suffer from low NS, restricting light incoupling to small angles. We have solved this problem by integrating axial-symmetric ring-type nanogratings onto the core of SMF-28, improving the incoupling efficiency at near-grazing incidence by more than four orders of magnitude. Additionally, incoupling at preselected angles and across large angular intervals by employing an optimization procedure was additionally shown. This approach overcomes the low NA limitation of commercial fibers and can lead to advancements in the field of fiber optics.<br/><br/><i>Summary:</i> We have demonstrated that meta-fibers represent a new category of fiber-integrated devices that offer unprecedented applications for fiber optics. With the implementation of versatile application strategies, we anticipate broader use of meta-fibers in various fields, including quantum technology, bioanalytics, and nanophotonics.