Highly efficient SERS inside microstructured optical fibres via optical mode engineering

Microstructured optical fibres (MOFs) offer versatile engineering of the internal microstructure geometry to provide large surface areas and aspect ratios with outstanding mechanical properties which, when functionalised with metal nanoparticles, serve as exceptional substrates for surface enhanced Raman spectroscopy (SERS) due to the large electromagnetic fields generated in the vicinity of the metal surface. A new MOF template based on a 126 times 1mum hole structure with a small 2 mum silica core, where a larger overlap between the guided modes and the silver particles surrounding the core was expected. Using aminothiophenol as the target molecule, the SERS spectra obtained show a clear difference in the signal strength for the two excitation geometries with the signal from the core ~10 times stronger than that from the air hole cladding, highlighting the importance of the long interaction lengths. Significantly, owing to the greatly improved overlap between the excitation beam and the metal nanoparticles, the small core substrates exhibit SERS signals 2 times 10³ times stronger than the large core structure. The paper discussed the importance of MOF template design and modal overlap engineering to maximise the electromagnetic enhancement factor thus improving the efficiency of the fibre sensors. The optimised substrates have already shown potential for use as remote optical fibre sensors where the SERS active region is confined to the tip of a longer empty MOF.