Comparison of Birefringent Metamaterials and Meanderline Structure as   Quarter-Wave Plates at Terahertz Frequencies
  We have fabricated a quarter-wave plate from a single layer birefringent metamaterial. For comparison, an appropriately scaled double layer meanderline structure was fabricated. At the design frequency of 639 GHz, the metamaterial structure achieves 99.9% circular polarization while the meanderline achieves 99.6%. The meanderline displays a larger bandwidth of operation, attaining over 99% circular polarization from 615 - 743 GHz, while the metamaterial achieves 99% from 626 - 660 GHz. However, both are broad enough for use with CW sources making metamaterials a more attractive choice due to the ease of fabrication. Both samples are free standing with a total thickness of 70um for the meanderline structure and a mere 20um for the metamaterial highlighting the large degree of birefringence exhibited with metamaterial structures. 
Andrew C. Strikwerda, Kebin Fan, Hu Tao, Daniel V. Pilon, Xin Zhang,   Richard D. Averitt