Chiral Sculptured Thin Films for Circular Polarization of Mid-Wavelength Infrared Light

TL;DR

This paper demonstrates that chiral sculptured thin films can be designed to act as circular-polarization filters in the mid-wavelength infrared range, based on scale invariance principles and experimental validation with zinc-selenide CSTFs.

Contribution

The study confirms the scale invariance of the circular Bragg regime in CSTFs and demonstrates their potential for mid-infrared polarization filtering through theoretical and experimental analysis.

Findings

Center wavelengths of Bragg regimes scale with helical pitch ratios

Experimental measurements align with theoretical predictions

Refractive index variation affects wavelength ratios slightly

Abstract

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.