Ultra-broadband and high-efficiency reflective polarization rotator based on fractal metasurface with multiple plasmon resonances

A fractal metasurface-based ultra-broadband and high conversion efficiency reflective polarization rotator (PR) is proposed. The linearly y-/x-polarized (LP) incident electromagnetic (EM) wave can be rotated over 90 degrees after reflection by the proposed metasurface. The unit cell of the proposed PR is designed based on the fractal cut-wire resonator. Compared with a typical cut-wire resonator exciting three resonances, the fractal one can generate an extra resonant frequency. An ultra-broad bandwidth with a polarization conversion ratio greater than 0.9 (PCR>0.90) can be obtained from 8.00 to 23.00 GHz (a fractional bandwidth of 96.8%) at normal incidence, which is realized by the combination of the four resonances. The physical working mechanism and the polarization conversion performance of the metasurface are analyzed by surface current and E-field distributions, simulations and explained based on numerical calculations and plasmon resonance modes. Good agreement between the experiments and simulations is obtained. Compared with traditional PRs and other published metasurface-based PRs, the proposed metasurface can achieve an ultra-broad band and a high efficiency with a sub-wavelength size of the unit cell. The use of fractal topologies provides a novel and efficient method to further upgrade resonance characteristics in the design of PR, and the scalable geometry of the design shows its potential applications at optical, terahertz, and microwave frequencies.