Double-stacked hyperbolic metamaterial waveguide arrays for efficient and broadband terahertz quarter-wave plates.

We demonstrate how it is possible to achieve weak dispersion in the phase delay between two orthogonal polarization states by using double-stacked hyperbolic metamaterial (HMM) waveguide arrays. The weak dispersion in the phase delay originates from the different signs of phase delay from the two different HMM waveguide arrays. The condition of dispersion-free phase delay for the transmitted waves has been theoretically derived from the transmission matrix as the propagation characteristic of the HMM waveguide is involved. We further reveal that the designed double-stacked HMM waveguide array can function as an efficient quarter-wave plate that enables the conversion of linearly polarized light to circularly polarized light within a broad frequency band. In addition, the bandwidth over which the degree of linear polarization is nearly unity and over which the angle of linear polarization is kept at approximately 45° is basically consistent with the phase bandwidth. This offers a promising approach for developing a practical polarization converter in the terahertz domain.