Effective properties of siloxane containing imbedded metal helices as a chiral microwave-absorbing material

We have carried out microwave measurements, accompanied by a theoretical analysis, of the effective permittivity and permeability of a chiral composite microwave-absorbing material consisting of a siloxane matrix with imbedded metal-microhelices. Absorptivities and material constants of effective chiral materials (containing either helices or chiral-material inclusions) have recently been obtained in the form of a single resonance in the frequency variable. Our experiments are the first ones to show two resonances in one and the same measurement (first since the 1957-1960 studies of microwave optical activity by Tinoco et al., see e.g. J. Phys. Chem. 61, 1196), which leads to important advances in the physical interpretation of the origin of the resonance absorptivity: determination of the resonance order of a standing wave spanning the overall length of the helix in steps of half-wavelengths (thereby also obtaining the orders of the previously observed single resonances), and a quantitative theoretical prediction of the resonance values of the effective permittivity and permeability, based on a single-scattering model by Bohren and Huffman as adapted to helices, in good agreement with the measurements.