Sculptured-thin-film spectral holes for optical sensing of fluids

A novel class of fluid sensors is proposed based on monitoring the optical properties of multi-section chiral sculptured thin films (STFs) that function as spectral reflection holes. Using a nominal model that treats a chiral STF as a two-phase composite material with locally biaxial dielectric properties, we predict that the presence of a fluid in the porous film results in a red-shift of the spectral holes. Several device operation modes are proposed, and their relative merits are compared. Proof-of-concept experiments with both circularly polarized and unpolarized incident light confirm the red-shift of the spectral holes, and demonstrate operation in a practical situation.