Enhanced resonant absorption in dye-doped polymer thin-film cavities for water vapour sensing

We propose a novel and sensitive water vapour sensing scheme based on enhanced resonances in absorbing ultrathin-film cavity systems. In particular, we experimentally and numerically demonstrate the water vapour sensing ability of such cavity systems, which comprise ultrathin poly(vinyl alcohol) films doped with Rhodamine 6G dye molecules coated on silver substrates. Due to coupling between molecular absorption and Fabry-Perot resonances excited in the ultrathin-film cavity, the studied structure exhibits highly enhanced resonant absorption at certain wavelengths under certain illumination conditions. Upon exposure to water vapour (relative humidity from 10% to 73%) the resonant features show much larger spectral shifts (∼60 nm) than those (∼20 nm) reported in most previous studies and a three-fold enhancement of the absorption reduction (>40%) as compared to that in an un-doped reference sample. In addition, the proposed ultrathin-film system demonstrates hysteresis-free sensing response to water vapour within a wide RH range (5-75%). Together with its flexibility of tuning sensing performance by adjusting dye-doping concentration, the absorbing ultrathin-film system provides an effective and versatile platform towards further development of practical water vapour sensing.