The flat tip of an optical fiber is a unique and unconventional platform for micro and nanotechnologies. The small cross-section and large aspect ratio of the fiber provide an inherently light-coupled substrate that is uniquely suited to remote, in vivo and in situ applications. However, these same characteristics challenge established fabrication technologies, which are best suited to large planar substrates. This review presents a broad overview of strategies for patterning the flat tip of an optical fiber. Techniques discussed include self-assembly, numerous lithographies, through-fiber patterning, hybrid techniques, and strategies for mass manufacture, while the diverse applications are discussed in context throughout.

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Advances in nanoscale fabrication provide access to the smallscale phenomena that underpin fi elds of research such as plasmonics and subwavelength optics. [ 1 ] The choice of substrate for these nanostructures can defi ne the scope of their utility. Nanostructures on an optical-fi ber facet obtain the advantages of a miniaturized substrate that can be interrogated remotely and deployed in vivo and in vitro. However, fabrication on fi ber facets is challenging, because of their unusually small 125 μ m diameter and their large aspect ratio. To date, demonstrations of fi ber-facet nanopatterning have used interference lithography (IL), [ 2 ] electron-beam lithography (EBL) [3][4][5] and focused ion-beam lithography (FIB) [ 6 , 7 ] as fabrication techniques. The transfer of EBL nanostructures to fi ber facets has also been demonstrated, [ 8 , 9 ] achieving 25 nm feature separations. However, IL can produce only limited geometries, while EBL and FIB are unsuited to high-throughput fabrication.In contrast, nanoimprint lithography (NIL) provides largearea, high-resolution nanofabrication at low cost and highthroughput. New platforms that have been explored for NIL include mask aligners, [ 10 ] roll-to-roll imprinting, [ 11 ] optical-fi ber lengths [ 12 ] and optical-fi ber-facets (OFF-NIL). To date, OFF-NIL has demonstrated periodic diffraction features with dimensions 250 nm, [ 13 ] and 630 nm, [ 14 , 15 ] and nanorods for surface-enhanced Raman scattering (SERS) with diameters ∼ 110 nm. [ 16 ] All of these demonstrations of fi ber-facet nanopatterning have been limited to single-fi ber processing. Given the typical facet diameter of 125 μ m, there is clear opportunity for increased throughput via parallel OFF-NIL with large-area molds. Fiber-ribbons would seem an appropriate candidate for parallel fabrication, [ 17 ] however these are restricted in the type and number of fi bers available, and require specialized stripping and cleaving tools. Additionally, the cleaved ribbon-facets are restricted to a single plane.In this paper, we demonstrate an elegant, low-cost, highly accessible platform for imprinting arrays of optical-fi ber facets.We show that by segmenting optical-fi bers into short lengths and loading them into separate U-grooves, many fi bers can be imprinted in parallel. Over-sizing these U-grooves enables the fi bers to independently slide back-and-forth, and to self-align once they contact the mold, eliminating the need for critical proximity control during imprinting. Using this platform, we have achieved sub-15 nm feature separations across fi ber arrays. Further, using this system with large-area molds eliminates the need for critical lateral alignment. The demands on the imprinting platform are thus reduced to providing coarse, uniaxial translation, allowing us to demonstrate a compact, portable module for fi ber-array imprinting. Finally, self-alignment uniquely accommodates non-planar molds, allowing us to employ a biological nanotemplate.Arrays of optical-fi bers were created using arrays of ...

The technique of oblique angle deposition has been extended to the fabrication of nanostructured metal coatings on the tips of standard silica optical fibers by thermal evaporation. The coatings are initiated as metal island films, which grow into extended rodlike structures as the deposition continues. The nanorod coatings demonstrate excellent surface-enhanced Raman scattering performance with variability of less than 10% as shown by direct measurements off the fiber tip with thiophenol as a test analyte. However, in the remote sensing configuration, the nanorod structures perform no better than thin metal island films. This appears to be mainly due to reduced transmission when nanorod lengths exceed ~100 nm. Moreover, the variability of remote measurements is increased to 18%. This is believed to be due to variations in coupling efficiency.

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