Strong and Broadband Pure Optical Activity in 3D Printed THz Chiral Metamaterials

Optical activity (polarization rotation of light) is one of the most desired features of chiral media, as it is important for many polarization related applications. However, in the THz region, chiral media with strong optical activity are not available in nature. Here, we study theoretically, and experimentally a chiral metamaterial structure composed of pairs of vertical U-shape resonators of "twisted" arms, and we reveal that it demonstrates large pure optical activity (i.e. optical activity associated with negligible transmitted wave ellipticity) in the low THz regime. The experimental data show polarization rotation up to 25 (deg) for an unmatched bandwidth of 1 THz (relative bandwidth 80 %), from a 130 um-thickness structure, while theoretical optimizations show that the rotation can reach 45 (deg). The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which provides also simple optimization rules for the enhancement of its chiral response. The proposed chiral structures allow easy fabrication via direct laser writing and electroless metal plating, making them suitable candidates for polarization control applications.