Chiral metamaterials: A tool for THz polarization control

Metamaterials, i.e. engineered composite materials structured in sub-wavelength building blocks, attract nowadays continuously increasing attention. This is mainly due to the unique properties of those materials, which entail new possibilities for the manipulation of electromagnetic (EM) waves. With metamaterials one is able to achieve permittivity and permeability values ranging from very large to negative, combined negative permittivity and permeability leading to negative index of refraction, unusual anisotropic response, giant chirality values in chiral metamaterial structures, etc. The giant chirality values achievable in chiral metamaterials, orders of magnitude larger than the chirality of natural chiral materials, empowers chiral metamaterials with unique possibilities for the control of electromagnetic wave polarization. Such a control is extremely useful in the THz regime, where there is a serious lack of polarization control components. In this talk we present a variety of chiral metamaterials operating in the THz regime and we demonstrate, both theoretically and experimentally, the THz-polarization control capabilities of those metamaterials. The main structures discussed are shown in Fig. 1 [1]. These structures are planar designs based on the bi-layer conductor configuration, i.e. the chiral response derives from the electromagnetic coupling of a pair of conducting elements (along z-direction on Fig. 1), mutually twisted as to break the mirror symmetry of the structures along propagation direction. The structures are of micrometer length scale, they have been fabricated by UV lithography in a flexible polyimide substrate, they have been characterized by FTIR spectroscopy and analyzed theoretically and computationally by the finite element method. The results show in all cases extremely large optical activity, large circular dichroism and negative refractive index response for both left- and right-handed circularly polarized waves [1]. Incorporating properly in the structures a photoconducting material (e.g. Si), which can be transformed from an insulating to a conducting state using photo-excitation, we show that one can achieve dynamically tunable or switchable chiral response, demonstrated as switchable optical activity or switchable transmitted wave ellipticity [2]. Thus the structures can offer a great tool for the realization of dynamic polarization control components, such switchable polarization filters, modulators, wave plates and others, extremely valuable in the THz regime.