Chiral sculptured thin films as integrated dual-modality optical sensors

Chiral sculptured thin films (CSTFs) are well-suited to optical-sensing applications because their multiscale porosity and optical properties can be tailored to order. Two independent modalities of optical sensing were considered. For both modalities, the analytes to be sensed are assumed to fully penetrate the void regions of the CSTF and thereby give rise to measurable changes in the macroscopic optical responses of the CSTF. The first modality is based on the excitation of multiple surface-plasmon-polariton (SPP) waves at the planar interface of a CSTF and a metal film, while the second is based on the spectral shift in the circular Bragg phenomenon (CBP). We considered a CSTF with a central twist defect of 90°. Our numerical studies revealed a CSTF coated with a thin layer of metal of appropriate thickness can simultaneously support the excitation of multiple SPP waves and the CBP, with both phenomenons being independently sensitive to the refractive index of a fluid which infiltrates the void regions of the CSTF. Accordingly, an integrated dual-modality optical sensor may be envisaged which harnesses both modalities of sensing simultaneously. Such an optical sensor offers the potential to detect more than one type of analyte at a time, with increased sensitivities and/or specificities.