  Being an assembly of identical upright helixes, 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 spectrums 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, the deviation from the ratio
of helical pitches being explainable at least in part be- cause the bulk
refractive index of zinc selenide decreased a little with wavelength. We
concluded that CSTFs can be fabricated to function as circular-polarization
filters in the mid-wavelength infrared regime.
