Chiral sculptured thin films for circular polarization of mid-wavelength infrared light

Being an assembly of identical upright helices, a chiral sculptured thin film (CSTF) exhibits the circular Bragg phenomenon and can therefore be used as a circular polarization filter in a spectral regime called the circular Bragg regime. This has been already demonstrated in the near-infrared and short-wavelength infrared regimes. If two CSTFs are fabricated in identical conditions to differ only in the helical pitch, and if both are made of a material whose bulk refractive index is constant in a wide enough spectral regime, then the center wavelengths of the circular Bragg regimes of the two CSTFs must be in the same ratio as their helical pitches by virtue of the scale invariance of the frequency-domain Maxwell postulates. This theoretical result was confirmed by measuring the linear transmittance spectra of two zinc-selenide CSTFs with helical pitches in the ratio 1:7.97. The center wavelengths were found to be in the ratio 1:7.1, and the deviation from the ratio of helical pitches is explainable at least in part because the bulk refractive index of zinc selenide decreases a little with wavelength. We concluded that CSTFs can be fabricated to function as circular polarization filters in the mid-wavelength infrared regime.