Ultra-broadband asymmetric transmission and linear polarization conversion based on terahertz metamaterials

In this paper, a terahertz (THz) metamaterial (MM) is designed to achieve ultra-broadband, highly efficient polarization conversion and asymmetric transmission (AT) for linearly polarized (LP) wave. The MM structure is composed of two orthogonal metallic sub-wavelength grating layers and a sandwiched cross-shaped array layer, in which the orthogonal grating layers are used as polarization filters to enhance AT parameters and the cross-shaped structure functions as the polarization converter. Owing to the coupling between two different cut-wires (CWs) and the excitation of higher-order plasmon mode in the cross-shaped structure, the polarization conversion bandwidth is greatly expanded. Results show that the polarization conversion rate (PCR) of the proposed MM structure can exceed 0.99 from 0.41 to 2.38 THz. The AT parameters are above 0.64 in the range of 0.50-2.41 THz with a relative bandwidth of 131.27%, and the near-perfect AT effect with AT parameter in excess of 0.8 occurs at two bands, from 0.53 to 1.48 THz and from 1.80 to 2.06 THz, with the relative bandwidth of 94.53% and 13.47%, respectively. Comparisons of our results with other published works prove that our proposed MM can provide relatively wider bandwidth and higher AT parameter. Due to the excellent performance, the proposed MM has potential applications in designing THz diodes.