Resonant absorption in dielectric thin films for humidity sensing

Abstract

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.

Efficient Plasmonic Gas Sensing Based on Cavity-Coupled Metallic Nanoparticles

“…This RH sensitivity is highly improved as compared to most of the recently developed nanoscale humidity sensors, e.g., 0.05 dB/% RH of the dye-doped ultrathin-film sensor, 22,29 0.07 dB/ % RH of the nanofiber-based plasmonic sensor, 30 and 0.09 dB/ % RH of the plasmonic gap resonance sensor. 31 What is worth noting is that the remarkable RH sensitivity has little relevance to the geometry of NPs, but is highly dependent on the initial NP−mirror distance.…”

“…We note that such a large humidity-induced intensity reduction allows for an averaging RH sensitivity of 0.12 dB/% RH over a wide RH range (5%-75%), which is a remarkable sensitivity improvement as compared to other nanophotonic humidity sensors, e.g. 0.05 dB/% RH of the dye-doped ultrathin-film sensor [26,32], 0.07 dB/% RH of the nanofibre-based plasmonic sensor [33] and 0.09 dB/% RH of the plasmonic gap resonance sensor [34]. Moreover, it is worth noting that since the scattering intensity of the cavity-coupled NP system shows a nonlinear change over the whole RH variation, at certain RH ranges the sensitivity can be even higher.…”