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, a chiral metamaterial (CMM) structure composed of pairs of vertical U-shape resonators of "twisted" arms is studied, and it is revealed 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 degrees for an unmatched bandwidth of 1 THz (relative bandwidth 80%), from a 130 mu m-thickness structure, while theoretical optimizations show that the rotation can reach 45 degrees. The enhanced chiral response of the structure is analyzed through an equivalent RLC circuit model, which also provides simple optimization rules for the enhancement of its chiral response. The proposed chiral design allows for easy fabrication via direct laser writing (DLW) and electroless metal plating, making the associated structures suitable candidates for polarization control applications.