Optimization of Terahertz Metamaterials for Near-Field Sensing of Chiral Substances

We demonstrate metamaterials based on Archimedean spiral arrays that can be used to create elliptically polarized terahertz (THz) waves from a linearly polarized input wave. The operational frequencies of these metamaterials are defined by the geometry of the Archimedean spiral, and the correlation between geometry and operational frequency is studied experimentally as well as numerically. We demonstrate that the operational frequency can be tuned from 0.5 to 1.5 THz. The THz electromagnetic wave is strongly localized near the metamaterial surface. The field enhancement associated with this localization can be used to realize a high-sensitivity chirality sensor, and its potential is studied with full-wave simulations, in which a subwavelength 5-μm-thick layer is detectable. More importantly, it is also possible to distinguish the chiral effect of a single enantiomer in a 50:50 racemic mixture.