Theoretical investigation into the optimized design of a durable OFSPR hydrogen sensor based on a PdY alloy

Hydrogen sensing technology by definition necessitates high accuracy, rapid response time, and durability. Thin film Pd has demonstrated excellent use in this field owing to large sensitivity and fast detection time. Interaction with hydrogen causes a crystallographic phase transition of the Pd lattice resulting in expansion. Subsequently repeated hydrogen loading cycles increases mechanical stress on the Pd lattice and thus leads to delamination of the hydrogen sensitive layer. By alloying Pd with Y, it is possible to mitigate the unwanted phase transition thereby significantly improving durability. We present the first optical fibre surface plasmon resonance (OFSPR) hydrogen sensor based on a multilayer Ag/SiO₂/PdY deposited on the unclad core of a silica optical fibre. In this submission, we investigated the spectral influence of fibre numerical aperture in addition to Ag and SiO₂ thickness within the multilayer. Sensor sensitivity and figure of merit were found to reach a maximum when a fixed Ag thickness was paired with a set of corresponding SiO₂ thicknesses. We demonstrate that changing the thickness of one of these layers alters the optimal thickness of the other. We present a figure by which an array of optimal sensing structures can be determined. The largest sensor figure of merit in this study was found to be 0.062732, and was produced using Ag = 50nm, and SiO₂ = 70nm. This sensor operates with sensitivity of 17.57nm to 4% hydrogen, detection accuracy of 0.014282nm^-1, and operated at a spectral centre of 524.09nm.