Highly sensitive optical sensor for hydrogen gas based on a polymer microcylinder ring resonator

A highly sensitive platform is demonstrated for hydrogen gas (H₂) sensing based on a polymer microcylinder ring resonator (PMRR) obtained by an optical fiber coated with an inner nanofilm of amorphous palladium (Pd) and an outer polymer layer of polydimethylsiloxane (PDMS) permeable to H₂. The sensing scheme is based on monitoring the spectral shifts of high-quality optical resonances called whispering gallery modes (WGMs) that propagate in the vicinity of the outer rim of the PDMS layer without being affected by the absorption and scattering losses caused by the Pd nanofilm. WGMs are excited by a single-mode tapered optical fiber evanescently coupled to the PMRR. The observed reversible spectral shifts of the WGMs are induced by changes in the diameter of the PDMS layer caused by expansion or contraction of the Pd nanofilm exposed to varying concentrations of H₂. Maximum spectral shift sensitivity of 140 pm/% H₂, a minimum response time of 95 s, and minimum limit of detection of ∼60 ppm were measured for sensors prepared with different thicknesses of the amorphous Pd nanofilm and tested in the H₂ concentration range up to 1%, having nitrogen gas (N₂) as a carrier. Experiments were also conducted with Pd nanofilms annealed in air or N₂ atmosphere after the deposition. In both cases, smaller sensitivities were observed due to the formation of larger grains within the film, resulting in slower diffusion and reduced solubility of H in the Pd layer. The impacts of oxygen gas and humidity on sensor performance were also studied.