Electromagnetic activity and absorption in microwave chiral composites

Abstract

The sense of rotation of the polarisation direction of an electromagnetic wave propagating in a chiral medium depends on the handedness of the medium. At certain frequencies, a phenomenon called the Cotton effect (in optics) occurs when the angle of rotation rapidly increases or decreases or even changes sign. This effect has been observed in artificial chiral composites fashioned by embedding chiral inclusions (miniature metallic helices) in an epoxy host medium in the 8–40 GHz frequency range. The ellipticity and rotation angle of the elliptically polarised transmitted wave and the power absorbed in nine samples (three volume concentrations with left, right and racemic samples containing helices of the same size) are measured using a free space set up. The handedness of the microstructure of a chiral material can be experimentally determined by studying the Cotton effect curves. The Cotton effect is observed in the frequency region where maximum electromagnetic power absorption occurs. At maximum power absorption, the characteristic frequency is 28.45 GHz and the ratio of the one turn length of the helix to average wavelength L/λc is 0.6037, an average value obtained for all nine samples. This is a useful parameter in the design of microwave chiral absorbers.