Planarized Sidewall-Integrated Metafibers for Enhanced Evanescent Field Interaction via Long-Range Resonant Near-Field Coupling

Metafibers, optical fibers integrated with metasurfaces, have recently emerged as a transformative platform for miniaturized fiber-optic components with advanced functionalities. However, existing metafibers are largely limited to fiber tip integration, where metasurfaces interact dominantly with localized guided modes, thus failing to exploit the inherent potential of fiber sidewalls for long-range evanescent field interaction. Here, we propose a novel metafiber paradigm—integrating metasurfaces on planarized fiber sidewalls—to fully harness the evanescent field mechanism. To this end, we develop a robust in situ fabrication method via a fiber-substrate planarization strategy, enabling direct, large-area, and high-fidelity metasurface patterning on side-polished fibers (SPFs). Numerical simulations reveal that long-range resonant near-field coupling drives cumulative amplification of the evanescent field within the metasurface layer, leading to significant enhancement of both linear and nonlinear optical responses. As a proof of concept, we present a nonlinear metafiber by integrating a 1-cm-long gold nanorod-based metasurface onto an SPF, demonstrating a high-performance saturable absorber (SA). This metafiber SA enables all-fiber ultrafast lasers across all soliton regimes with ultralow mode-locking thresholds. This work establishes a universal methodology for planarized sidewall-integrated metafibers, reshaping the landscape of metafibers, and provides a versatile lab-on-fiber platform for enhanced linear and nonlinear optics.