Here we present a novel fabrication approach that allows implementing sophisticated planar nanostructures with deep subwavelength dimensions on fiber end faces by electron beam lithography. Specifically we planarize the end faces of fiber bundles such that they are compatible with planar nanostructuring technology, with the result that fibers can be treated in the same way as typical wafers, opening up the entire field of nanotechnology for fiber optics. To demonstrate our approach we have implemented densely-packed arrays of gold nanotrimers on the end face of 50 cm long standard single mode fibers, showing resonances that arise due to the interplay of surface lattice resonance and individual timer response at infrared wavelengths that overlap with the single mode regime of typical telecommunication fibers. Refractive index sensing experiments suggest sensitivities of about 390 nm/RIU representing the state-of-the-art for such device type. Due to its unique capability of making optical fibers compatible with planar nanostructuring technology, we anticipate our approach to be applied in numerous fields including bioanalytics, telecommunications, nonlinear photonics, optical trapping and beam shaping.