However, most devices of this kind have been limited by low resonance quality factors (Q) or low coupling efficiencies when coupling the plasmons and the fiber-guided light waves. Consequently, the signal-to-noise ratio (SNR) of refractive index change detection has lagged far behind the free-space optics or side-coupling waveguide counterparts, preventing fiber-tip SPR devices from meeting the requirements of real applications where the target concentrations are often low.

A team of scientists, led by Prof. Tian Yang from Shanghai Jiao Tong University, has reported remarkable progress in device design, fabrication technology and SNR of surface plasmon polariton (SPP) sensing devices on the end-facets of single-mode optical fibers. This work has been published in Light: Advanced Manufacturing, titled "A quasi-3D fano resonance cavity on optical fiber end-facet for high signal-to-noise ratio dip-and-read surface plasmon sensing."

Based upon their previous work on SPP crystal microcavities, the researchers used a hybrid structure that produces Fano resonance between the SPP cavity and a Fabry-Pérot etalon. The SPP crystal comprises periodic nano-slit arrays in a thin gold film. The SPP cavity consists of an on-SPP-band region which sits at the center and aligns with the fiber core, and a within-SPP-bandgap region, which sits in the surroundings. The on-SPP-band region converts the normally incident light to guided waves on a second-order SPP band through the grating coupling effect. Finally, the fiber-guided light wave is converted to an oscillating SPP surface wave on the aqueous solution side of the SPP cavity, using the F-P etalon as an intermediary so that high Qs and high coupling efficiencies can be achieved.