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Even though optical fibers have reached great success in a multitude of areas, applications such as wide-field fiber-based endoscopy or random photon collection in quantum optics demand collecting light under comparable large angles. Here reaching satisfactory coupling efficiencies is difficult with fibers that have unstructured end faces due to small acceptance angles of commonly used step-index fibers. For instance the SMF-28 has as an NA of only 0.16 and therefore an acceptance angle of only 7°, thus leading to a diminishing light collection performance for incident angles >20°.
Here we report on a plasmonic nano-structure enhanced step-index fiber for the efficient collection of light at extremely large incident angles (> 30 °), reaching a regime that is inaccessible for fibers with plane faces. For the experimental demonstration, we have implement arrays of gold nano-dots (diameter: 480 nm, height: 40 nm) on the end face of a SMF-28 exactly at the location of the core via electron-beam lithography and measured the light collection efficiency at different incident angles under various circumstances including variations in incidence wavelength, array lattice constant and input polarization. The measurements show several orders of magnitude of improvement in light-coupling efficiency when using the nano-structure functionalized fiber for incident angles ranging from 30° to 80°, while fibers with plane end faces only show measurable coupling efficiencies up to angles of 25°. In addition to the mentioned improvement, we observe an additional local enhancement in coupling efficiency, which is located at around 40° to 60° and is associated with lattice constant of nano-dot array, i.e., with the -1st diffraction order. To analyze the impact of the nano-structure on the fiber-coupling efficiency from the theoretical perspective, we developed a toy model which includes the phase evolution of the incident beam across the region of the core mode at the location of the fiber end face and conducted full 3D numerical simulations on the basis of the Finite-Element method. Both the theoretical analysis and the data obtained from toy model indicate a significant enhancement of the coupling efficiency that solely originates from optical response of the gold nano-dot array.
Since bare step-index fibers severely suffer from poor incoupling efficiencies at large incident angles, particular bioanalytical applications such as fiber-based endoscopy or in-vivo Raman spectroscopy will greatly benefit from the presented concept of boosting fiber incoupling efficiencies via nano-structure functionalized fiber end faces. Our concept is of generic origin and is not restricted to nano-dot arrays, but rather paves the way towards high-performance fiber-based photonic devices that include sophisticated nano-structures such as dielectric metasurfaces in order to boost incoupling efficiencies even further. Due to its unique performance regarding light-collection efficiency at incident angles being out-of-reach for fibers with plane end faces, we strongly believe that our concept will generate great impact in various fields of research and applications, including biophotonics, green photonics and quantum technologies.
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