Use of an elastic buffer layer for improved performance of a polymer microcylinder ring resonator hydrogen sensor

The impact of substrate on Pd nanofilm expansion in a Pd-H₂ system is investigated using polymer microcylinder ring resonator (PMRR) platform. Being a highly sensitive platform for H₂ gas detection, PMRR comprises of an inner sensitive Pd nanofilm and an outer PDMS layer coated on a standard optical fiber. Optical whispering gallery modes (WGMs) are excited in the rim of the outermost PDMS layer through evanescent field of a tapered fiber. H₂ molecules penetrating the H₂-sensitive Pd nanofilm through the PDMS layer cause reversible expansion in the PMRR. This translates into shifts in spectral positions of the WGMs that are observed with tapered fiber transmission spectroscopy. Two types of PMRRs were fabricated. In the first type, Pd nanofilm was directly deposited on the silica surface of an optical fiber. In the other one, a PDMS buffer layer was precoated between Pd nanofilm and the silica surface, with different thicknesses. It is demonstrated that, the use of a PDMS buffer layer yields higher radial expansion of the nanofilm during the interaction with H₂ gas. A 180-nm-thick Pd nanofilm coated on ∼2.5-μm-thick PDMS buffer layer showed at least 18% higher radial expansion compared to the case without buffer layer. Identical thickness of Pd nanofilm on a ∼3.5-μm-thick PDMS buffer layer showed 30% higher radial expansion. Numerical and analytical calculations were also performed confirming the experimental results. Among mechanical properties of the PDMS buffer layer, Poisson's ratio was found to be the most significant parameter affecting the expansion of the nanofilm.