This Letter presents a double-layer structure combining a cracked cross meta-surface and grating surface to realize arbitrary incident linear terahertz (THz) wave polarization conversion. The arbitrary incident linear polarization THz wave will be induced with the same resonant modes in the unit cell, which results in polarization conversion insensitive to the linear polarization angle. Moreover, the zigzag-shaped resonant surface current leads to a strong magnetic resonance between the meta-surface and gratings, which enhances the conversion efficiency. The experimental results show that a more than 70% conversion rate can be achieved under arbitrary linear polarization within a wide frequency band. Moreover, around 0.89 THz nearly perfect polarization conversion is realized.

In this Letter, we demonstrate a linear polarization conversion of transmitted terahertz wave with double-layer meta-grating surfaces, which integrated the frequency selectivity of a split ring resonator metasurface and the polarization selectivity of a metallic grating surface. Since the double-layer can reduce the loss, and the Fabry–Perot like resonant effect between the two layers can improve the conversion efficiency, this converter can rotate the incident y-polarized terahertz wave into an x-polarized transmitted wave with relatively low loss and high efficiency. Experimental results show that an average conversion efficiency exceeding 75% from 0.25 to 0.65 THz with the highest efficiency of 90% at 0.43 THz with only 2 dB loss has been achieved.

A polarization-insensitive, square split-ring resonator (SSRR) is simulated and experimented. By investigating the influence of the asymmetrical arm width in typical SSRRs, we find that the variation of the arm width enables a blue shift of the resonance frequency for the 0° polarized wave and a red shift of the resonance frequency for the 90° polarized wave. Thus, the resonance frequency for the 0° polarized wave and the resonance frequency for the 90° polarized wave will be identical by asymmetrically adjusting the arm width of the SSRR. Two modified, split-ring resonators (MSRRs) that are insensitive to the polarization with asymmetrical arm widths are designed, fabricated, and tested. Excellent agreement between the simulations and experiments for the MSRRs demonstrates the polarization insensitivity with asymmetrical arm widths. This work opens new opportunities for the investigation of polarization-insensitive, split-ring resonator metamaterials and will broaden the applications of split-ring resonators in various terahertz devices.