Modeling sculptured thin films as platforms for surface-plasmonic-polaritonic optical sensing

Two different types of sculptured thin film (STF), namely columnar thin films (CTFs) and chiral STFs, were theoretically investigated with a view to their potential application as optical detectors based on surface-plasmon-polariton (SPP) waves. It was envisaged that the species to be detected infiltrates the void regions in between the nanowires which make up the STFs. The constitutive and morphological parameters of infiltrated STFs were estimated via the inverse Bruggeman homogenization formalism, using experimentally determined data from uninfiltrated CTFs. These parameters were then input to the forward Bruggeman homogenization formalism to compute the relative permittivity dyadic of infiltrated STFs. By solving the corresponding boundary-value problem for a modified Kretschmann configuration, the characteristics of SPP waves excited at the interface of an STF and a thin metallic layer were explored as functions of the refractive index of the fluid infiltrating the STF and the morphological parameters of the STFs. Attention was focussed on multiple SPP modes - which offer greater sensing opportunities than do single SPP modes - that were excited in the case of chiral STFs. The sensitivity of the optical response of STFs to infiltration was found to bode well for the implementation of STFs as SPP-based optical sensors.