Modeling Chiral Sculptured Thin Films as Platforms for Surface-Plasmonic-Polaritonic Optical Sensing

Biomimetic nanoengineered metamaterials called chiral sculptured thin films (CSTFs) are attractive platforms for optical sensing because their porosity, morphology, and optical properties can be tailored to order. Furthermore, their ability to support more than one surface-plasmon-polariton (SPP) wave at a planar interface with a metal offers functionality beyond that associated with conventional SPP-based sensors. An empirical model was constructed to describe SPP-wave propagation guided by the planar interface of a CSTF-infiltrated with a fluid which supposedly contains analytes to be detected-and a metal. The inverse Bruggeman homogenization formalism was first used to determine the nanoscale model parameters of the CSTF. These parameters then served as inputs to the forward Bruggeman homogenization formalism to determine the reference relative permittivity dyadic of the infiltrated CSTF. By solving the corresponding boundary-value problem for a modified Kretschmann configuration, the characteristics of the multiple SPP modes at the planar interface were investigated as functions of the refractive index of the fluid infiltrating the CSTF and the rise angle of the CSTF. The SPP sensitivities thereby revealed bode well for the implementation of fluid-infiltrated CSTFs as SPP-based optical sensors.