Resonant absorption in dielectric thin films for humidity sensing

We have designed and fabricated novel humidity sensors based on thin-film systems capable of sustaining optical resonances in subwavelength thick absorbing dielectric coatings on reflecting surfaces, as a result of coupling between molecular absorption and Fabry-Perot resonances. Specifically, our experiments demonstrate that a strong resonance can be observed in the reflection spectrum of a 105 nm thick Rhodamine 6G-doped polyvinyl alcohol (PVA) coating on a silver surface under certain illumination conditions; this resonance shows an excellent spectral and temporal sensitivity to the change of environmental humidity. In addition, we also demonstrate the humidity sensing potential of a 47 nm thick titanium dioxide coating on an aluminium substrate as a more robust system for practical implementation.