Fiber Photonics

achieving broad nonlinear frequency conversion, with suspended core fibers (SCFs) being particularly effective due to their high modal field concentration and precise dispersion control. However, their small core sizes, typically a few micrometers, pose significant challenges for light incoupling, resulting in a low and unstable coupling that often requires complex high numerical aperture bulk optics that are both costly and difficult to integrate. This work addresses this key challenge by introducing the concept of nonlinear metafibers. By implementing tailored metalenses directly on the end faces of SCFs using advanced 3D nanoprinting, we demonstrate alignment-free and highly robust coupling of broadband ultrashort pulses into small-core SCFs. This first demonstration of a nonlinear metafiber achieves full all-fiber integration, eliminating the need for bulky external optical components and facilitating broadband soliton-based SCG. The flexibility of this novel approach, which effectively overcomes a fundamental problem in nonlinear photonics, has broad applicability in various fields including quantum technology and life sciences. In addition, the concept extends beyond SCFs to other fiber types and on-chip waveguides, paving the way for new opportunities in nonlinear photonics and integrated optics. This study establishes nonlinear metafibers as a transformative platform with the potential to advance applications in which efficient, compact, and robust nonlinear photonic systems are critical.