The polarization state is an important basic property of THE electromagnetic wave and researchers have made great efforts to manipulate it.^[1,2] In recent years, metamaterials,^[3,4] which have numerous intriguing artificial electromagnetic responses, have been proposed to control the polarization state. Some examples include birefringent metamaterial,^[5–7] anisotropic metamaterials,^[8–10] and chiral metamaterials.^[11,12] Anisotropic or birefringent metamaterials have been designed to achieve cross polarization conversion and circular polarization, while chiral metamaterials have been proposed to realize cross polarization conversion and asymmetric transmission. Metasurfaces, which are essentially two-dimensional (2D) metamaterials,^[13–15] can be used to control the polarization state in subwavelength range. Therefore, miniaturized polarization converters can be achieved by using metasurfaces. The polarization conversion working in terahertz and visible wavelength range has been widely examined. For instance, a bilayer plasmonic transmission-type metasurface operating at visible frequencies experimentally yields a conversion efficiency of 17% for anomalous refraction.^[16] A compact broadband circular polarization analyzer in the meso-field has demonstrated that the light of different handedness is focused into a subwavelength confined spot or a ring-shaped intensity profile.^[17] An ultrathin, terahertz quarter-wave plate based on planar babinet-inverted metasurface can convert linear polarization into circular polarization.^[18] A concept of degenerated image dipole array is reported to realize anomalous light bending with high efficiency in nearly the entire visible band.^[19] However, the narrow bandwidth of operation has restricted the practical applications of these polarization converters in microwave band. In general, the bandwidth can be broadened by using different techniques including stacking multilayer structures,^[20] multiple plasmon resonances,^[21] and plasmon hybridizations.^[22] For instance, the double-head arrow metasurface^[23] expands bandwidth of polarization conversion, but the conversion efficiency is just 50% within 3-dB bandwidth. The average conversion efficiency of anchor-shaped metasurface^[24] is 80% within 3-dB bandwidth. The plasmon hybridization in C2-symmetric ring/disk cavity can be used to expand the bandwidth of the polarization converter, but the conversion efficiency is not very high. However, comparing with that of reflection-type metasurface, the cross-polarization conversion efficiency of the transmission-type metasurface^[25] is fundamentally limited to below 25%. Therefore, the conversion efficiency and bandwidth of polarization conversion still need to be improved simultaneously to realize a practically useful device.

In this work, we propose a metasurface design to achieve ultra-broadband and high-efficiency polarization conversion simultaneously. The metasurface is composed of an array of unit cell resonators, each of which combines an H-shaped structure and two rectangular metallic patches. Three plasmon resonance modes are excited in the unit resonator, which modifies the phase distribution of the reflected waves, allowing the polarization states to be manipulated as desired. Simulations and experimental results show that the conversion efficiency is over 90% from 15.7 GHz to 37.5 GHz, which is 82% of the central frequency for both x and y polarized waves. Additionally, the proposed metasurface can also convert the linearly polarized wave into a circularly polarized wave. At 14.43 GHz and 40.95 GHz, the linearly incident wave is converted into a left handed or right handed circularly polarized wave respectively.

The anisotropic metasurface has the potential to achieve cross polarization conversion. Furthermore, wideband polarization conversion can also be achieved by using multiple plasmon resonances. Individually cut wire resonators support multi-order dipolar resonances; an individual H-shaped structure and a rectangular metallic structure can be seen as cut wire resonators in a particular direction. Combining these two structures yields a metasurface that can support multiple plasmon resonances while giving the designer more controls over device characteristics. The unit cell structure of the proposed metasurface is shown in Fig. 1(a). It can be divided into two parts: one part consists of two rectangular metallic patches, and the other part consists of an H-shaped structure that lies between the metallic patches. As shown in Fig. 1(b), the structure of the proposed metasurface is symmetric across v axis, which is the axis along the 45° direction with respect to the y direction. Hence, the metasurface is an anisotropic structure. The geometrical parameters of the proposed metasurface are given by p = 3.9 mm, l = 3.8 mm, g = 1.2 mm, d = 0.2 mm, w = 0.2 mm, a = 1.7 mm, and b = 1.1 mm. The dielectric spacer parameters are permittivity = 2.62 and loss tangent tan δ = 0.001. The thickness of the substrate is 1.45 mm, and a metallic sheet is placed on the back side of the substrate to ensure that almost all of the incident power is reflected.

Fig. 1.

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	Fig. 1. (a) Proposed structure of the metasurface. (b) Intuitive image of y-polarized incident wave rotated to x-polarized reflection wave.

A 410 mm×410 mm sample of the metasurface as shown in Fig. 6(a), is fabricated to further verify the design through actual measurements. The measurement setup is shown in Fig. 6(b). In this setup, two horn antennas are used: one is to radiate electromagnetic (EM) waves onto the sample, and the other is to receive the reflected waves from the sample. The two horn antennas are kept at the same vertical distance from the center of the sample to ensure that the wave can be reflected off the sample and received by the receiving antenna. The two horn antennas support TM and TE modes on different sides. When the radiating horn antenna is fixed, the receiving horn antenna can be placed on its longer and shorter sides, to measure the cross- and co-polarization conversion respectively. The measured cross- and co-polarization conversion results along with the simulation results are plotted in Fig. 6(c). The measured results are in agreement with the simulation results, verifying the concept behind the proposed metasurface. The measured bandwidth of the cross-polarization conversion is 21.4 GHz, which matches very closely with the 21.8 GHz predicted through simulations. There is a small difference between the measured and simulated results. In our simulations, the unit cell boundary conditions in CST Microwave Studio are set on the four lateral boundaries and open add space boundaries are set to be along the incidence direction. Hence, the metasurface is assumed to be infinite and the port covers the total metasurface. Thus, the incidence wave can be totally reflected with 180° phase difference along the u axis and v axis, meaning that the reflective wave will have a higher reflection in cross-polarization and smaller reflection in co-polarization. However, in the actual experiment, the metasurface is finite, which cannot guarantee the 180° phase difference in the entire reflective wave, especially when the wave is reflected from the edge of the metasurface. Therefore, the measured cross-polarization reflection is not so high as that shown in the simulation results, and the measured co-polarization reflection is not so small as in that indicated in the simulation results either. Furthermore, the error from its fabrication may also cause the difference between the measured and simulated results. The measured PCR indicate that the metasurface has a high-polarization conversion efficiency, which matches well with the simulated PCR as shown in Fig. 6(d).

Fig. 6.

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	Fig. 6. (a) A fabricated sample of the metasurface, (b) schematic plot of measurement setup, (c) the simulated and the measured cross- and co-polarization reflections, and (d) the simulated, and the measured PCR.

In this work, we demonstrate an ultra-broadband and high-efficiency polarization converter by using an innovative metasurface design. By combining structures in the unit cell resonator to cause different plasmon resonance modes, the phase difference of the reflected wave along the v axis and the u axis is designed to be nearly 180° over a wide bandwidth. This leads to cross-polarization conversion from 15.7 GHz to 37.5 GHz with a conversion efficiency of over 90%, and a bandwidth that is 82% of the central wavelength for both x and y polarized waves. At 14.43 GHz and 40.95 GHz, the phase differences between the reflected waves along the v axis and the reflected waves along the u axis are −90° and 90° respectively. Thus, the x(y)-polarized incident wave is converted into a left (right)-handed and a right (left)-handed circularly polarized wave respectively.