Optical sensor for hydrogen gas based on a palladium-coated polymer microresonator

We report an integrated optical sensor of hydrogen (H-2) gas employing an SU-8 polymer microdisk resonator coated with a palladium (Pd) layer and coupled to a single-mode optical waveguide. The sensing mechanism relies on the expansion in the Pd lattice due to palladium hydride formation in the presence of H-2. Strain induced in the microresonator then causes a red shift of the spectral positions of the resonator whispering gallery modes (WGMs) which is monitored using a tunable laser coupled to the waveguide. H-2 concentrations below the flammable limit (4%) down to 0.3% could be detected in nitrogen atmosphere at room temperature. For H-2 concentrations between 0.3 and 1%, WGM spectral positions shifted linearly with H-2 concentration at a rate of 32 pm/% H-2. Average response time of the devices was measured to be 50 s for 1% H-2. The proposed device concept can also be used to detect different chemical gases by using appropriate sensing layers. (C) 2015 Elsevier B.V. All rights reserved.