Low-cost and highly accessible technology based on radially polarized beam-excited plasmonic microfiber for label-free Raman detection

A low-cost and highly accessible method for high sensitivity label-free Raman spectroscopy based on a cylindrical metalized microfiber internally excited via a radially polarized beam (RPB) has been developed. A silver (Ag)-pyramid coated microfiber was designed to be matched with the RPB. Simulation results show that the enhancement of the electric-field intensity of the cylindrically metalized microfiber excited by the RPB can reach a factor of 2.5 × 10³, which is much higher than that of linearly polarized beam (LPB) excitation. In experiments, the RPB is directly generated in the optical fiber by converting the LPB to an RPB using electrically controllable, low cost acoustic-optical technology. An Ag-pyramid coated microfiber with ∼9.6 μm diameter was prepared using a simple Ag-mirror reaction. The Raman intensity for malachite green (MG) detected by the Ag-pyramid coated microfiber under RPB excitation was 5 times stronger than that under LPB excitation. The Raman sensitivity for MG in the case of RPB excitation was as low as 10^-10 M, a concentration level which was not detectable for LPB excitation. The sensor system gave high repeatability and reliability and should find application in high sensitivity label-free detection in optical sensing with the advantages of low-cost and ease of operation.