Achromatic THz wave plate composed of stacked parallel metal plates with a pillar array

We propose an achromatic wave plate in the high THz frequency region based on the artificial medium. It is composed of the stacked parallel metal plates with a pillar array, which bring in the controllable birefringence in an effectively lossless medium. We experimentally demonstrate that the artificial medium operates as the quarter wave plate from 2.05 to 3.26 THz.