Ultra-Wideband and Lightweight Electromagnetic Polarization Converter Based on Multiresonant Metasurface

Polarization-control devices have attracted considerable interest, however, most of the polarization converters operating at lower frequencies have a heavy design and narrow bandwidth which limits their practical applications. Here we report a simple design of an ultra-wideband and lightweight polarization converter for applications in the S- and C-bands. The proposed converter is designed based on a metasurface structure with the dielectric layer modified to hollow structure to obtain a lightweight design even working at such low frequency. Theoretical analysis and simulation results indicate that the converter can convert the orthogonal polarization transformation of reflected wave. Furthermore, the measurement results show good agreement with the simulation results. The proposed polarization converter can achieve a polarization conversion ratio above 90% in an ultra-wide frequency range from 2 to 8.45 GHz due to multi-resonance modes. These performances are going beyond state of the art in terms of bandwidth and lightweight design, thus it can be applied in various applications in the operating bands.

tion converter is used to control the EM polarization. Con-23 ventional polarization converters are achieved by using either 24 The associate editor coordinating the review of this manuscript and approving it for publication was Zhongyi Guo . birefringent crystals based on Faraday effects [1], optical 25 gratings [2], or employing the Brewster [3], [4]. Therefore, 26 the conventional designs are usually heavy-weight, difficult 27 in miniaturization, and narrow bandwidth, which limits their 28 practical applications [5], [6]. Since then, many efforts have 29 been made to overcome the disadvantage of the conven-30 tional polarization converters [7], [8], [9], [10], [11], [12], 31 [13], [14], [15], [16], [17], [18]. Among the proposed meth-32 ods, using metamaterials (MMs) is considered as a potential 33 approach to developing a new class of miniaturized and wide-34 band polarization converter [15], [16], [17], [18]. 35 Metasurface has proved as an effective method to real- 36   polariton couplers [24] due to its advantages such as ultrathin 39 thickness and scalable property. Normally, a metasurface is 40 formed by arranging unit cells into a two-dimensional pattern 41 at a surface or interface, therefore, they are considered as  respectively. The metallic pattern plays as a resonator which 84 is composed of two opposite arcs connected by a strip at 85 the center ( Fig. 1(b)). The top and bottom layers of unit cell 86 are made of copper, which has a thickness of 0.035 mm and 87 an electric conductivity of 5.96 × 10 7 S/m. The geometrical 88 parameters of the unit cell are shown in Table 1. It is worth 89 to mention that a thicker dielectric contributes to a lower 90 radiative Q-factor and can therefore enhance the operational 91 bandwidth [26]. However, a unit cell with actual (solid) and 92 thick substrate normally is heavy, therefore, the airgap is 93 introduced here as a solution to achieve the wideband and 94 lightweight characteristics at the same time.

95
To evaluate the performance of the proposed polar-96 ization converter, the commercial CST Microwave Stu-97 dio 2015 software is performed using a frequency-domain 98 solver. The unit cell boundary conditions are assigned to the 99 x-and y-directions, while open conditions are applied to the 100 z-direction. 101 In general, the working principle of a linear polariza-102 tion converter is that it converts most of the incident wave 103 into the reflected wave with a rotating 90 • in polarization 104 (cross-polarization). However, there is an amount of reflected 105 wave that does not change the polarization after the reflec-106 tion (co-polarization); therefore, cross-reflection (r xy ) and 107 co-reflection (r yy ) coefficients are used to represent the co-108 and cross-polarization reflected components. Assuming that 109 incident EM wave is y-polarized wave with electric field E iy , 110 then the co-and cross-reflection coefficients are defined as 111 r xy = |E rx |/|E iy |, and r yy = |E ry |/|E iy |, where |E rx | and |E ry | 112 are the magnitude of the electric field of the reflected wave 113 components along x-and y-axes, respectively.

134
Assuming that the incident wave is polarized along the y-axis 135 and propagates perpendicular to the top layer of the polariza-136 tion converter along the z-direction. As shown in 3(a), the 137 incident electric field (E i ) and reflected electric field (E r ) 138 of the EM wave can be decomposed into two mutually per-139 pendicular components in the uv-coordinate system as Eqs. 2 140 and 3, respectively.
where r uu and r vv are the reflected coefficients along the u-144 axis and v-axis, which can be defined as r uu = |E ru |/|E iu |, 145 and r vv = |E rv |/|E iv |.

146
Since a low loss dielectric (tanδ = 0.025) is used for the 147 polarization converter, the energy loss of the incident EM 148 wave can be neglected; Therefore, the amplitude of r uu and 149 r vv can be assumed to unit (|r uu | = |r vv | = 1). Also, due 150 to the proposed polarization converter is an asymmetrical 151 structure, it can be considered as an anisotropic material 152 with dispersive relative permittivity and permeability results 153 in a phase difference ( ϕ) between r uu and r vv [27]. Then, 154 we have the relation between r uu and r vv as: r vv = r uu e j ϕ . 155 Thus, Eq. 3 can be rewritten as Eq. 4.
When | ϕ| = 180 • + 2kπ (k is integer), combine Eqs. 3 158 and 4 we have E ru = r uu E iu and E rv = −r uu E iv . It means that 159 either E ru or E rv is opposite to their corresponded incident 160 direction, leading to the synthetic reflected electric field (E r ) 161 is rotated exactly 90 • compared to the incidence as depicted 162 in inset of Fig. 3(a). 163 To verify this theoretical analysis, we simulate the mag-164 nitude and phase difference of the reflection of u-and 165 v-components for the converter under normal incidence. 166 As shown in Fig. 3(a), the co-reflection amplitudes of u-167 and v-polarized waves, r uu and r vv , are nearly equal 1 in an 168 ultra-wide bandwidth from 2 to 8.45 GHz as we theoretically 169 predicted. Moreover, the cross-reflection coefficient between 170 u-and v-polarizations, r uv and r vu , are nearly zero in the 171 band which confirms no cross-polarization exists at u-and 172 v-polarizations. Fig. 3(b) gives the phase difference ϕ 173 between r uu and r vv over frequency. As shown in Fig. 3b  In additional, to better qualitatively verdict the polarization 179 state of EM wave, ellipticity (η) and polarization azimuth 180 angle (θ) for the y-polarize and normal incidence also are 181 investigated, which are determined as Eqs. 5 and 6 [28], [29]. 182 where |p r | = |r xy |/|r yy | and ϕ is the phase difference 185 between r xy and r yy . Theta is the rotation angle of the electric 186 VOLUME 10, 2022  , which typically exhibits the feature of magnetic 218 resonance. However, the excited magnetic fields are created 219 by these loops is antiparallel and therefore cancel each other. 220 Moreover, the currents in two arm of arcs both flow to the 221 up-left ( Fig. 5(b)), while the direction of currents are with 222 the opposite direction ( Fig. 5(d)), displaying the character-223 istic of electric resonance. The synthetic electric field is the 224 same direction with the incident electric field at the resonant 225 frequency of 2.98 GHz. It indicates that the magnetic reso-226 nance is mainly contributed to this resonant frequency [31]. 227 However, the synthetic electric field is the opposite of the 228 incident electric field at 7.83 GHz (Fig. 5(d)), indicating that 229 the excited electric field is stronger than the incident electric 230 field. This phenomenon proves that an electric resonance 231 is excited at 7.83 GHz [31]. It is can be concluded that 232 the overlap of the four polarization rotation resonances is 233 responsible for the ultra-wideband polarization conversion of 234 the proposed polarization converter.

235
To further investigate the contribution of the airgap layer 236 on the conversion performance of the proposed polarization 237 converter, we simulate the PCR of the converter at different 238 values of the airgap thickness. As shown in Fig. 6, the polar-239 ization converter without the airgap shows four separated 240 and narrow peaks in the band from 0 to 12 GHz with a low 241 performance. By introducing the airgap, the PCR level of 242 the peaks are improved significantly. Moreover, the lowest 243 resonances are nearly unchanged when the thickness of the 244 airgap varies from 4 to 14 mm. In contrast, the other reso-245 nances move to lower frequency band with the increase in 246 the thickness of airgap forming a wide band PCR spectrum. 247 However, with the airgap above 10 mm, the resonances are 248 too close to each other results in the PCR bandwidth of 249 the proposed polarization converter shrank. The effects of 250 reported that the EM absorption is caused by an extra reso-283 nance between the metallic ground and metasurfaces [34]. 284 We also calculate the average simulated PCR on the work- the working band, respectively; PCR(θ) is average simulated 291 PCR under incident angle of θ.
292 Fig. 7(b) shows the average PCR of the polarization 293 converter under TE polarization in the ultra-wideband 294 from 2 GHz to 8.45 GHz as a function of the incident angle. 295 The achieved average PCR of the polarization is higher than 296 90% with the incident angle up to 30 • , and it is still main-297 tained a higher 85% for a wide incident angle up to 38 • . 298 This result confirms that the proposed polarization converter 299 exhibits a stable polarization conversion with respect to vari-300 ations in the incidence angle. Fig. 8 illustrates the photo of the fabricated polarization con-303 verter and the measurement setup for the fabricated prototype 304 as a device under test (DUT). The proposed converter is 305 fabricated by a standard printed circuit board (PCB) manu-306 facturing process which consists of an 11 × 11 unit-cell that 307 corresponds to the dimensions of 244.2 × 244.2 mm 2 . The 308 unit cell parameters of the fabricated sample are the same in 309 Tab. 1. To prevent our fabricated sample from any possible 310 degradation, oxidation, or corrosion, the PCB tinning which 311 is the process of coating copper metasurface with thin layer of 312 tin, is carried out and the result of the sample surface after the 313 tinning process is shown in Fig. 8 (a). The measurement of the 314 DUT is performed using a vector network analyzer (Rohde 315 and Schwarz ZNB20) associated with two standard-gain horn 316 antennas as transmitter (Tx) and receiver (Rx) to determine 317 the reflection coefficients. The DUT is located 0.5 m away 318 from the antennas to satisfy a far-field condition. Due to the 319 measurements are performed in the free space, the measure-320 ment system is calibrated with this condition to embed the 321 effect of the environment. The measured PCR of the DUT 322 is determined from the co-and cross-reflection coefficients 323 as described in Eq. 1. The measurements are taken in the 324 frequency range from 1 to 10 GHz. The detailed description 325 of the measurement setup can be found in [35].      Table 2, the proposed polarization converter operates at 358 lowest frequency band with highest fractional bandwidth and 359 has the most lightweight design among them. 361 We present an ultra-wideband and high efficiency polariza-362 tion converter for S-and C-bands applications. By intro-363 ducing a hollow dielectric layer to a simple metasurface 364 resonator, the polarization converter shows a very lightweight 365 design even working at a relatively low frequency. Firstly, 366 theoretical analysis and simulation are carried out to explain 367 the working mechanism of the polarization converter. Then, 368 the polarization converter is fabricated and the experimental 369 results are carried out with good agreement with the simula-370 tion results. The results show that the proposed polarization 371 converter converts linear-polarized incident EM waves into 372 its cross-polarized reflective counterparts with high polariza-373 tion conversion with a polarization conversion ratio above 374 90% in an ultrawide frequency range from 2 to 8.45 GHz. 375 This operating frequency band corresponds to a relative band-376 width of 123.4% and entirely covers both the S-and C-bands. 377 Compared with the available broadband designs, the pro-378 posed polarization converter shows excellent performance in 379 terms of simple and lightweight design and ultrawide band-380 width. The advantages demonstrate great potential for appli-381 cations in microwave communications, imaging systems, and 382 remote sensors.