In this paper, a reflective polarization convertor is proposed based on a sandwich structure consisting of an "I"-shaped resonator at the top, a medium layer of foam, and a metal layer at the bottom. The characteristics of ultra-wideband (UWB) and higher relative bandwidth (RB) have been achieved than previous structures. The structure effectively controls the polarization of electromagnetic waves in the terahertz band, which convert the incident linear-polarized (LP) wave to cross-polarized wave within 2.75-10.35THz, with a relative bandwidth of 116 and a polarization conversion ratio (PCR) greater than 90. In addition, this structure also converts the incident linear-polarized wave to circular-polarized wave in the range of 2.28-2.65THz with axial ratio (AR) less than 3 dB. The characteristics are also analyzed based on the surface current distribution. The reason of realizing UWB polarization conversion is explained by polarization decomposition method. Our structure could provide many potential applications in radar antenna, metamaterial lens, and other electromagnetic wave and optical fields.
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