Tunable metafibers: remote spatial focus control using 3D nanoprinted holograms on dual-core fibers.

The generation of tunably focused light at remote locations is a critical photonic functionality for a wide range of applications. Here, we present a novel concept in the emerging field of Metafibers that achieves, for the first time, fast, alignment-free, fiber-integrated spatial focus control in a monolithic arrangement. This is enabled by 3D nanoprinted intensity-sensitive phase-only on-fiber holograms, which establish a direct correlation between the intensity distribution in the hologram plane and the focus position. Precise adjustment to the relative power between the modes of a dual-core fiber generates a power-controlled interference pattern within the hologram, enabling controlled and dynamic focus shifts. This study addresses all relevant aspects, including computational optimization, advanced 3D nanoprinting, and tailored fiber fabrication. Experimental results supported by simulations validate the feasibility and efficiency of this monolithic Metafiber platform, which enables fast focus modulation and has transformative potential in optical manipulation, high-speed laser micromachining, telecommunications, and minimally invasive surgery.