Broad-band and high-efficiency polarization converters around 1550 nm based on composite structures

Broad-band and high-efficiency polarization converter is an imperative component in communication systems, but its functionality often clashes with the constraint of materials. Herein we theoretically and numerically demonstrate that a broad-band and high-efficiency 90° polarization rotator around 1550 nm can be realized using an ultrathin and geometry-optimized composite structure. Based on simulation results, the reflection efficiency and operation bandwidth is up to ≈80% and ≈300 nm, respectively, for the 90° polarization rotator. With similar concept, we also demonstrate a quarter-wave plate with an efficiency of 94% and bandwidth of 110 nm. The electric filed distribution indicates that the conversion behaviors are caused by the strong magnetic coupling in the designed composite structure. Furthermore, the polarization ellipticity properties are investigated to further understand the broad-band effect of the proposed polarization convertors.