We demonstrate humidity sensors based on optical resonances sustained in sub-wavelength thick dye-doped polymer coatings on reflecting surfaces. As a result of coupling between dye molecular absorption and Fabry-Perot resonances in the air-coating-surface cavity, the absorption spectra of such thin-film structures show a strong resonant peak under certain illumination conditions. These resonances are sensitive to the structural and material properties of the thin-film, metal underlayer and ambient conditions and hence can be used for gas and vapor sensing applications. Specifically, we present our proof of principle experimental results for humidity sensing using a thin-film structure comprising Rhodamine6G-doped polyvinyl alcohol (PVA) films on silver substrates. Depending on the PVA film thickness, dye-concertation and angle of incidence, the resonant absorption peak can undergo either red-shift or blue-shift as RH level increases in the range 20% to 60%. Also, the absorption magnitude at certain wavelengths near to resonance show almost linear reduction which can be used as the sensing signal. Our simulation studies show a very good agreement with the experimental data. The spectral and temporal sensitivity of this thin-film structure is attributed to the changes in the thickness of the PVA layer which swells by absorbing water molecules
We prepared thin film colloidal crystals, namely, opals and Langmuir-Blodgett crystals, of different lattice constants
and coated them with 20, 50 and 100 nm thick gold films. Angle-resolved transmission spectra of these hybrid
nanostructures were obtained using linear polarized light over a spectral range of a photonic bandgap energy structure of
these colloidal photonic crystals. Up to 10 times transmission enhancement has been observed in specific spectral
intervals determined by the light coupling to surface plasmon polaritons in the metal film. At the same time these
hybrid nanostructures retain transmission attenuation bands inherited from a photonic crystal. The strong crosscorrelation
of the photonic bandgap and surface plasmon polariton properties was observed in the transmission spectra
of studied hybrids.
Heterogeneous 3-dimensional photonic crystals have been prepared by sandwiching self-assembled opal films and
forced-assembled Langmuir-Blodgett colloidal crystal films. Strong transformation of the transmission spectra of the
light traversing such hetero-crystals has been observed and interpreted in terms of the mismatch between eigenmodes of
these photonic crystals and subsequent strong scattering at the interface between these crystals. Direct measurements of
the spectra of forward scattered light have confirmed the peculiar character of the light scattering at the photonic crystal
interface.
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