  Metafibers expand the functionalities of conventional optical fibers to
unprecedented nanoscale light manipulations by integrating metasurfaces on the
fiber tips, becoming an emerging light-coupling platform for both nanoscience
and fiber optics communities. Mostly exploring the isolated bare fibers,
current metafibers remain as proof-of-concept demonstrations due to a lack of
standard interfaces with the universal fiber networks. Here, we develop new
methodologies to fabricate well-defined plasmonic metasurfaces directly on the
end facets of commercial single mode fiber jumpers using standard planar
technologies and provide a first demonstration of their practical applications
in the nonlinear optics regime. Featuring plug-play connections with fiber
circuitry and arbitrary metasurfaces landscapes, the metafibers with tunable
plasmonic resonances are implemented into fiber laser cavities, yielding
all-fiber sub-picosecond (minimum 513 fs) soliton mode locked lasers at optical
wavelengths of 1.5 micrometer and 2 micrometer, demonstrating their unusual
polarimetric nonlinear transfer functions and superior saturation absorption
responses. Novel insights into the physical mechanisms behind the saturable
absorption of plasmonic metasurfaces are provided. The nanofabrication process
flow is compatible with existing cleanroom technologies, offering metafibers an
avenue to be a regular member of functionalized fiber components. The work
paves the way towards next generation of ultrafast fiber lasers, optical
frequency combs, optical neural networks and ultracompact "all-in-fibers"
optical systems for sensing, imaging, communications, and many others.
