Experimental Characterization of Chiral Uniaxial Bianisotropic Composites at Microwave Frequencies

This paper presents an experimental procedure for retrieving the effective constitutive parameters of chiral materials. Unlike past research that primarily deals with isotropic materials, this study considers a lossy uniaxial bianisotropic slab with a nonzero chirality along its axial direction. First, plane-wave scattering off the uniaxial slab in a free-space environment is studied analytically. This forward analysis gives insight into the problem and the choice of proper independent measurements required for the inverse process, i.e., retrieving the slab constitutive parameters from its S-parameters. Based on this analysis, three sets of co-polarized and cross-polarized S-parameters are required, including both the transmission and reflection coefficients of the slab. Given the measured scattering data, the complex permittivity, permeability, and chirality tensors are determined numerically using the results of the analytic study. To test the performance of the new retrieval method, an array of 2 × 56 long, metallic helices is designed and fabricated for operation at C-band. Having the same handedness, the helices are closely spaced and held in parallel to each other in a wooden frame in order to create an effective uni-axial chiral medium. A conventional transmission/reflection setup measures the array scattering parameters, which are fed into the retrieval process to obtain the effective parameters. The measured parameters well model the array scattering response, exhibiting a significant averaged chirality of ~0.4 over 5.5-8.7 GHz and a plasmonic behavior at ~7.1GHz.