Terahertz chiral structures with large optical activity

Metamaterials are subwavelength man-made plasmonic or dielectric structures designed to realize specific effects on the electromagnetic radiation interacting with the material. Here we aim to rotate the polarization of an incident terahertz beam using chiral metamaterials, while suppressing the circular dichroism which induces ellipticity of the output beam. An incident linearly polarized wave can be decomposed into left-circularly and right-circularly polarized waves, and the difference in propagation phase will result in rotation of the plane of polarization. Since chiral structures couple electric and magnetic fields, they are often implemented in complex geometries such as spirals or bi-layered plasmonic structures, which can achieve carefully balanced responses to the two fields. The important feature of the bi-layered plasmonic structure is the cross-coupling between the resonances of the two layers. It is precisely this coupling between the layers that induces currents in the structures that are mutually dependent producing chirality within the structure. By coupling a metallic structure to its complement, we are able to achieve strong transmission in the region of maximum polarization rotation, and relatively low ellipticity of the output state. Three different structures were fabricated for this work that will be referred to as: the plain crosses, crossed arrowheads, and crossed arcs, pictured in the figure below. The terahertz responses of the structures were compared using terahertz time-domain spectroscopy and numerical simulations using CST Microwave Studio software