Precise on-Fiber Plasmonic Spectroscopy Using a Gradient-Index Microlens

Plasmonic spectroscopy has emerged as a powerful technique for interrogation of light in the subwavelength regime. Realization of active plasmonic elements on optical fibers can simplify the optical design, allow for performing remote tasks and thus extend the scope of plasmonic utilization. However, the optimization of fiber optics for improving the plasmonic excitation condition has been overlooked. Here we present a plasmonic spectroscopy platform that combines a gradient-index microlens and a single-mode optical fiber. This design ensures large-area and uniform illumination in normal incidence for the excitation of surface plasmon resonance. We demonstrate precise plasmonic spectroscopy with a fiber-optic platform by patterning a nanohole array onto the facet of a gradient-index microlens with our template transfer technique. The experimental spectra are almost identical to the simulated results under ideal conditions, with accurate resonance wavelength (<; 10 nm deviation), high sensitivity (5 nm/RIU deviation), narrow linewidth (about 5 nm deviation), and diverse shifting patterns. This platform provides a fiber-optic avenue of precise plasmonic spectroscopy for remote interrogation.