Broadband and wide-angle reflective linear polarization converter for terahertz waves

Polarization control of electromagnetic waves has wide applications in the field of communications, imaging, and remote sensing. Recent designs of periodic two-dimensional devices or metasurfaces employed for polarization control are limited in efficiency, bandwidth, and allowable incidence angle. This is attributed to high dissipation in the dielectric material used and to less-optimal device configuration. We propose and experimentally validate a reflective linear polarization converter metasurface with high efficiency, wide bandwidth, and wide acceptance angle in the terahertz regime. Our device is composed of three layers: an array of oriented metallic T-shaped resonators, cyclic olefin copolymer (COC) as a low loss dielectric layer, and a ground plane. For the normal and 45 degrees incidence angles, a fabricated sample shows a bandwidth of 95% and 100%, with the average polarization conversion ratio above 80%, covering a frequency range of 0.38-1.07 and 0.36-1.08 THz, respectively. The wide-angle stability is attributed to a phase difference between a single resonance along the T-shaped resonator and a smooth phase response in the low-loss COC dielectric layer. For broad bandwidth performance, a resonator arm extending to adjacent unit cells introduces the fundamental resonance at a lower frequency, while the packed unit cell size shifts the grating lobe onset to a higher frequency. These design aspects can significantly improve the performance of other metasurfaces operating in any frequency range.