The polarization state is one of the most important properties of an electromagnetic (EM) wave. Full control of the polarization state of an EM wave is a promising promotion for figuring out many practical engineering problems in designing antennas, astronavigation, communication and radar target recognition. Thus, a lot of effective efforts have been devoted to longing for full domination of the propagation of EM waves.^[1–3] Conventional devices of controlling polarization are mainly designed by using Faraday effects^[4] and are based on optical activity crystals. However, these conventional approaches usually require bulky configurations, and it is extremely inconvenient for practical engineering applications.

Since metasurface was proposed, it has drawn a great deal of attention due to its unique physical characteristics and flexible controlling of EM waves which are unattainable in natural materials.^[5] Metasurfaces have been considered as an effective method to realize perfect lens, invisible cloak, polarizer, terahertz communication and optoelectronic devices.^[6–10] Recently, polarization converters based on metasurfaces have attracted much more attention due to manipulating the polarization of EM wave easily, and different types of polarization converters based on anisotropic and chiral metamaterials have been reported.^[11–17] These reported polarization converters can be miniaturized effectively, but the narrow bandwidth and high insertion loss have still been shackles of the practical applications. To release from such shackles, great efforts have been made to develop novel polarization converters. For instance, a highly efficient polarization converter with a wide bandwidth based on the chiral metasurface was proposed, which consists of cells with four twisted anisotropic structure pairs in four-fold conversion symmetry.^[16] Lourdes et al. proposed a broadband circular polarization converter, which is formed by four-layer frequency selective surface based on chapped rings bisected by a metal clip.^[17] These novel polarization converters can effectively expand the bandwidth and enhance the polarization efficiency, but the thickness values are still too large and the structures are complicated. Thus, polarization converters based on a metasurface with a high conversion efficiency and wide bandwidth are in great demand.

In this paper, we propose an ultra-wideband and high efficient linearly reflective polarization converter on the basis of the metasurface which is formed by square-shaped resonators with intersectant diagonal. According to the phase differences between the two reflected coefficients at u-polarized and v-polarized incidences, we present that the square-shaped particles with intersectant diagonal are competent to rotate linear polarized EM waves into their orthogonal polarization at four different resonant frequencies, which are generated by magnetic responses. Induced surface current distributions are investigated to illustrate the physical mechanism of polarization conversion. The measured and simulated results are in good agreement with each other. The polarization conversion efficiency is higher than 90% from 7.57 Ghz to 20.46 GHz with a relative bandwidth of 91.2%. Compared with the results previously reported in Refs. [14]–[22], this anisotropic metasurface-based one has a wide bandwidth, high efficiency and simple geometry, which means that it is convenient for practical engineering application.

The unit cell of the proposed polarization converter based on anisotropic metasurface consists of a square-shaped resonator with intersectant diagonal and metallic ground sheet separated by a dielectric substrate as shown in Fig. 1(a). The geometrical parameters of the unit cell are given by a = b = 4.5 mm, c = 3.5 mm, p = 11.5 mm, θ = 45°. In addition, the thickness d of dielectric substrate is 3 mm, and the relative dielectric constant ε_r and loss tangent δ are 2.2 and 0.001, respectively.

Fig. 1.

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	Fig. 1. (a) Unit cell of the proposed polarization converter, (b) intuitive image of y-to-x polarization conversion in the polarization converter.

From Fig. 1(a), it can be seen that the proposed metasurface is an anisotropic structure, and it has a pair of mutually perpendicular symmetric axes u and v along ±45° directions with respect to the y axis, respectively. Thus, the proposed polarization converter can be regarded as an anisotropic homogeneous metamaterial layer mounted on a metallic ground sheet with a relative dispersive permeability tensor and a relative permittivity. The metasurface anisotropic axis can be marked by u, v axes as shown in Fig. 1(b).

By using the commercial software CST Microwave Studio, we can investigate the performance of our design. A y-polarized wave E_i is assumed to be incident normally on the polarization converter. Owing to the anisotropy of the unit structure, the reflected wave generally consists of both cross-polarized and co-polarized components. Under the y-polarized incidence, we define the cross- and co-polarized reflections as r_xy = |E_xr|/|E_yi| and r_yy = |E_yr| |E_yi|, respectively. In addition, the polarization-conversion ratio (PCR) is defined as .

The simulated reflection coefficients, together with the PCR, are shown in Fig. 2. It is shown that within the frequency range from 7.57 GHz to 20.46 GHz, the cross-polarized ratio r_xy is approximately higher than −1 dB, while the co-polarized ratio r_yy is reduced by more than 10 dB. According to the data curve of r_yy shown in Fig. 2(a), there are four trough values while the co-polarized ratio r_yy is better than 30 dB, which means that almost all energies of the incident y-polarized EM wave at four key frequencies (8.07, 12.27, 18.09, and 20.31 GHz) can be converted into the x-polarized one.

Fig. 2.

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	Fig. 2. Simulated results of polarization conversion pattern: (a) reflection coefficients ryy and rxy, (b) polarization conversion ratio.

Figure 2(b) gives the simulated magnitudes of PCR versus frequency. It can be seen clearly that in the frequency range from 7.57 GHz to 20.46 GHz, PCR is all greater than 0.9 and achieves 1 at the four resonant frequencies. Thus, we can conclude that the proposed polarization converter can effectively convert a y-polarized incident EM wave into its cross-polarized wave with a large bandwidth, which spans a frequency interval of 12.89 GHz.

To gain a physical insight into the cause of ultra-wideband polarization conversion, we also consider the normal incident y-polarized EM wave E_i. The EM waves can be decomposed into u- and v- component respectively as shown in Fig. 1(b). Thus, the incident plane EM wave and the reflected wave can be expressed respectively as follows:

Due to the anisotropy of the unit cell, there is a phase difference Δφ between the u- and v- components of the reflected wave. Depending on the frequency, Δφ can take the arbitrary value within [−180° 180°]. It implies that all polarization states including linear polarization, circular polarization and elliptical polarization can be realized for the reflected wave. When Δφ = 0, no polarization occurs. When Δφ = ± 90° and r_u/r_v = 1, the y-polarized EM wave converts into circular polarized wave. Incident y-polarized EM wave converts into its cross polarization wave when r_u = r_v = 1 and Δφ = ± 180°, while two perpendicular components can be expressed as E_u = E_v in respect that u and v axes are along the ±45° directions with respect to the y axis, respectively. Hence, the synthetic fields for E_ru and E_rv will be along the x direction, and a 90° polarization rotation is obtained.

To validate the polarization conversion performance of the proposed polarization converter, we carry out numerical simulations of the reflected amplitudes and phase differences between u-polarized and v-polarized incidences versus frequency. The simulation results are presented in Fig. 3. As shown in Fig. 3(a), the reflected amplitude at u-polarized incidence is approximately equal to that at v-polarized incidence. From Fig. 3(b), we can observe that the phase difference Δφ is roughly 180° in a frequency range from 7.57 GHz to 20.46 GHz, further confirming the competence of polarization conversion in this broad frequency scope. Furthermore, the phase difference Δφ at the four resonant frequencies of 8.07, 12.27, 18.09, and 20.31 GHz are 180°. Thus, we can conclude that the spectrum of 180° phase hysteresis corresponds only to the frequency scope with a high cross-polarized efficiency, and the four resonant frequencies are in good agreement with that based on analyzing the reflection coefficients r_yy, as shown in Fig. 2(a). It is further confirmed that the y-polarized EM wave is almost converted into its cross-polarized ones at the four resonant frequencies.

Fig. 3.

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	Fig. 3. Simulated results of polarization at u-polarized and v-polarized incidences versus frequencies: (a) reflected amplitude, (b) phase difference.

To better understand the physics mechanism of polarization rotation, we present the surface current distributions on front and back layers at resonance frequencies of 8.07, 12.27, 18.09, and 20.31 GHz under y-polarized EM waves which pass through the substrate along the z direction. As shown in Fig. 4, the instantaneously induced surface current distribution along the resonator at 8.07 GHz is similar to that at 18.09 GHz. Besides, the resonator has the similar instantaneously induced surface current distributions at 12.27 GHz and 20.31 GHz. The surface currents can produce induced currents on the bottom metal layer at four resonant frequency points. According to the directions of induced currents on the bottom metal layer, we can judge whether the resonance is electric or magnetic at four resonant frequencies. In Fig. 4, the surface currents along the square-shaped resonator with intersectant diagonal are all antiparallel to the induced currents on the bottom metal layer, forming current loops in the dielectric substrate. Thus, the resonances at 8.07, 12.27, 18.09, and 20.31 GHz can be considered as magnetic resonances. To further clarify the physical mechanism of magnetic resonance, we take the resonance at 8.07 GHz as an example to be analyzed. As shown in Fig. 4(a), the induced magnetic H is along the v axial direction under the incident y-polarized EM wave. We can observe that the magnetic field component in the x axial direction H_x is perpendicular to the electric field E, and there is no cross-coupling between them. Hence, we can conclude that the magnetic field component H_x never leads to the y–x polarization conversion. Besides, the magnetic field component in the y axial direction H_y is parallel to the electric field E_i, leading to the cross polarization with y–x polarization rotation. Similarly, for the magnetic resonances at 12.27, 18.09, and 20.31 GHz, the magnetic field components H_y are also parallel to the electric field E_i, and the cross polarization with y–x polarization rotations can be generated.

Fig. 4.

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	Fig. 4. Surface current distributions on front and back layers at resonant frequencies: (a) and (b) 8.07 GHz, (c) and (d) 12.27 GHz, (e) and (f) 18.09 GHz, and (g) and (h) 20.31 GHz.

In order to verify the proposed polarization converter, a sample containing 26 × 26 unit cells is fabricated by printed circuit board techniques, which covers an area of about 300 mm × 300 mm, and the photographs of fabrication are shown in Figs. 5(a) and 5(b). The schematic illustration of the measurement setup is shown in Fig. 5(c), in which two identical linearly polarized standard-gain horn antennas are set symmetrically with an angle of 6° relative to the normal. Two horn antennas acting as transmitter and receiver are connected to the Agilent E8363B Agilent network analyzer. A normal incident y-polarized EM wave in a frequency range from 6 GHz to 22 GHz is used as an excitation source. The cross- and co-polarization reflections are measured when the receiving horn antenna is rotated by 0° and 90°, respectively. According to the complementary relations of and , the PCR can be calculated. Figure 5(d) gives the measured and simulated results, which indicate that the experiment results are in good agreement with numerical predications.

Fig. 5.

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	Fig. 5. Photograph and measurement of the fabricated polarization converter: (a) the fabricated polarization converter, (b) local amplification of the photo, (c) schematic illustration of measurement setup, (d) comparison between simulated and measured results.

In this work, we propose a high-efficiency ultra-wideband reflective cross-polarization converter based on anisotropic metasurface. The physics mechanism is illustrated by simulating the reflected amplitudes and phase differences between u-polarized incidence and v-polarized incidence, as well as the instantaneously induced surface current distributions on front and back layers at four resonant frequencies. The simulated and measured results are in good agreement with each other, indicating that the incident linearly polarized wave is converted into its cross polarization in the frequency range from 7.57 GHz to 20.46 GHz with a relative bandwidth of 91.2%, and PCR within this band is greater than 90%. Compared with the previous result, this one has wide bandwidth, high efficiency and simple geometry, which make it possess potential applications in the microwave, terahertz and even optic regimes.