Surface Plasmon Resonances in Oriented Silver Nanowire Coatings on Optical Fibers

Silver nanowires 1-3 mu m in length and diameters of 0.04-0.05 mu m were synthesized by a polyol process and deposited on a single mode optical fiber with the Langmuir-Blodgett technique. For nanowire surface coverage of similar to 40% and partial orientation of their long axis obtained by controlling the deposition parameters, the optical properties of the nanowire coating become identical to those of a uniform metal coating obtained by sputtering or evaporation. Excitation of the nanowires by the polarized evanescent field of fiber cladding modes at near-infrared wavelengths near 1.5 mu m results in surface plasmon-like resonances in the transmission spectrum of the optical fiber. The polarization-dependent loss (PDL) spectrum of the tilted fiber Bragg grating used to excite the cladding modes shows a pronounced characteristic dip indicative of a plasmon resonance for radially polarized light waves and complete shielding of light for azimuthally polarized light. The PDL dip shifts at a rate of 650 nm/(refractive index unit) when the surrounding refractive index is changed, a 10-fold increase compared to uncoated fiber gratings and similar to that of uniform metal coated gratings. The advantage of the nanowire approach is to provide a much increased contact surface area for biomolecular recognition-based immunosensing.