Sub-femtogram sensing by engineering evanescent field on a single serially grafted waveguide

The intensity of the evanescent field (EF) is vital for the sensitivity and limit of detection (LoD) of optical fiber or waveguide EF sensors. The fusion of ultra-highly sensitivity, simplicity, and cost effectiveness is still a major research field of sensing system despite the advances achieved during the last decades. In this study, we engineered EF by coating a high-refractive-index layer on a polymer waveguide that was serially grafted onto optical fibers. The simulation results show that the proportion of EF intensity can be significantly increased by 2–3 orders of magnitude compared with that of the non-coated waveguide. As a proof-of-concept, we fabricated Ta 2 O 5 coated optical fiber-waveguide-fiber (OFWF) sensors via laser-induced waveguide self-writing and electron beam deposition techniques. The absorbance- and fluorescence-based sensing properties of the coated OFWF were investigated by spectral measurement of rhodamine B aqueous solutions. Owing to large EF ratio and high adsorption capacity of the sensor, we achieved a spectral sensing LoD of 9.5 × 10 −16 g/ml with absolute mass of 0.4–0.5 fg.