A Novel Reconfigurable Metasurface with Coincident and Ultra-Wideband LTL and LTC Polarization Conversion Functions

In this paper, a novel reconfigurable metasurface (NRM) with coincident and ultra-wideband linear-to-linear (LTL) and linear-to-circular (LTC) polarization conversion functions is proposed. The unit of the proposed NRM consists of superstrate, air layer, metal patch, substrate and metal ground successively. By loading micro-electromechanical system (MEMS) between two adjacent metallic via, the proposed NRM can realize LTL and LTC polarization conversion efficiently. Numerical simulation results reveal that the bandwidth of LTL and LTC polarization conversion are coincident from 5.7 GHz to 23.5 GHz (fractional bandwidth of 122%). It is worth mentioning that few metasurface in existing literature can achieve highly coincident and ultra-wideband LTL and LTC polarization conversion functions. Finally, measurement results are in accordance with simulation results.

Although there are many researches about the reconfigurable polarization conversion based on metasurface, wider bandwidths still need further study. To date, in existing reconfigurable polarization converters, the common bandwidth of the two different polarization conversion states is narrow, which limits their application.
In this paper, a NRM with coincident and ultra-wideband LTL and LTC polarization conversion functions is proposed. Two kinds of current loops are realized by loading switching device in the strong induced current part. Compared with the previous polarization converter, the proposed NRM only loads one MEMS of each unit but can achieve highly coincident and ultra-wideband LTL and LTC polarization conversion functions. The NRM has advantages of multi-polarization, ultra-wideband, fewer electronic devices and high efficiency.

Design and Simulation
Based on our previous studies [26], [30], we found that by loading switching device in the strong induced current part, two kinds of current loops and polarization reconfiguration can be realized. The proposed NRM is based on an ultra-wideband single polarization converter [15]. The MEMS switch is loaded between two adjacent metallic vias where the induced current is strong, multipolarization can be realized by controlling MEMS switch.
The unit of the NRM consists of superstrate, air layer, metal patch, substrate and metal ground successively, as shown in Fig. 1(a). The same material (ε r = 2.2 and tan δ = 0.0009) is used in substrate and the superstrate. The actual conditions for loading MEMS switch should be satisfied. The distance g 2 should be greater than 1 mm, and the air layer h 2 should be at least 1 mm, as shown in Fig. 1(b). At the same time, the bandwidth of LTL and LTC polarization conversion should be coincident as far as possible. The unit structural parameters are given in Tab. 2. Radant RF MEMS switch RMSW200HP is selected as the intended object. In order to facilitate simulation, it is simplified into a metal bridge structure. According to the literature [30], [34], this method is feasible. When the switch is on, MEMS switch is simplified into a metal bridge, as shown in Fig. 1(b). When the switch is off, the MEMS switch is simplified as nothing, as shown in Fig. 1(c). The yellow parts represent copper and blue parts represent substrate. parameter Tab. 2. The unit structural parameters of the NRM (mm).
Electromagnetic simulation software Ansoft HFSS is used to simulate the proposed NRM. We set the x-polarized waves as the incident waves. We use R xx = e rx /e ix , R yx = e ry /e ix , φ xx = δ rx -δ ix and φ yx = δ ry -δ ix to represent the coefficient of co-polarized reflections, the coefficient of cross-polarized reflections, the phases of co-polarized reflections and the phases of cross-polarized reflections, respectively.
When the MEMS switch is on, the amplitudes of R yx is much larger than R xx in the extremely wide frequency band, as shown in Fig. 2(a). R yx is very close to 1 at frequencies of 6.2 GHz, 8.8 GHz, 12.4 GHz, 16.2 GHz, 21.5 GHz and 23.5 GHz, indicating that x-polarized incident waves are converted into y-polarized reflection waves efficiently. When the MEMS switch is off, the amplitudes of x-polarized reflection waves and y-polarized reflection waves are very close, as shown in Fig. 2 Black and blue lines represent reflected phase, red lines represent reflected phase difference in Fig. 2(c) and 2(d). The reflected phase difference between x-polarized incident waves and y-polarized reflection waves keeps approximately Δφ = 90º ± 2k  180° from 5 GHz to 23 GHz, as shown in Fig. 2(c) and 2(d).
Therefore, when the switch is off, linearly polarized incident waves can be rotated into circularly polarized reflection waves. The PCR (Polarization Conversion Ratio) is calculated as (1) and Axis Ratio (AR) is defined as (2) [35].
The PCR of the NRM is shown in Fig. 3(a). When the switch is on, the PCR is higher than 90% ranging from 5.7 GHz to 23.8 GHz, and the fractional bandwidth is 122.7%. Hence, x-polarized incident waves are converted into their orthogonal polarized reflection waves. When the MEMS switch is off, the AR of circularly polarized reflection wave is shown in Fig. 3(b). The 3dB bandwidth is 5.6 GHz to 23.5 GHz, and the fractional bandwidth is 123%.
According to the above analysis, the bandwidth of the two different polarization states is highly coincident. Therefore, the proposed NRM could achieve LTL and LTC polarization conversion from 5.7 GHz to 23.5 GHz, the fractional bandwidth is 122%.

Results and Discussion
In order to analyze the physical mechanisms of the designed NRM better, the electric field is decomposed into u-v coordinate system, as shown in Fig. 1(b) and 1(c). We define R uu to represent the reflection coefficient of u-to-upolarization, R vu to represent the reflection coefficient of v-to-u-polarization, R uv to represent the reflection coefficient of u-to-v-polarization and R vv to represent the reflection coefficient of v-to-v-polarization, respectively [30].
There are the simulated reflection coefficients, as shown in Fig. 4(a) and 4(b). It reflects that R uu ≈ R vv ≈ 1, R uv ≈ R vu ≈ 0, indicating all the reflected waves are co-polarized reflections. Furthermore, the co-polarized reflection coefficient is very close to 1, which indicates that the proposed NRM is highly efficient in the polarization conversion process. When the frequency is higher than 23 GHz, cross-polarized components appear in the reflected waves.
We define φ uu to represent the reflection phases of uto-u, and φ vv to denote v-to-v the reflection phases of polarization conversions. Δφ' denotes the corresponding phase difference between φ uu and φ vv . The simulated reflection phase is exhibited in Fig. 4(c) and 4(d). When the MEMS switch is on, Δφ' fluctuates around ±180°, as shown in Fig. 4(c). At this moment, u-polarization and v-polarization compose y-polarized reflected waves, indicating that the proposed NRM rotates linearly polarized incident wave to its orthogonal polarization.
In order to understand the circular polarization characteristics better, we analyze the elliptical polarization. χ represents the elliptical angle. a is the long axis and b is the short axis of an ellipse: According to previous analysis, the reflected amplitudes of u and v axis are equal. Since the coordinate axis of When the switch is on, reflection coefficient is shown in (a) and phase is shown in (c). When the switch is off, reflection coefficient is shown in (b) and phase is shown in (d).
u and v are orthogonal, the AR has been obtained as the following formula.
If the reflected wave is circular polarization, the AR must be less than 3 dB in engineering. According to (5), Δφ' should satisfy [71º+ 180º× k,109º+ 180º× k], in which k is integer. When the switch is off, Δφ' fluctuates around 90° and -270° from 5.6 GHz to 23.5 GHz, as shown in Fig. 4(d). The reflected waves are circular polarization. And the result is in accordance with the result that is obtained under x-y coordinates. Fig. 4(c) and 4(d), the two curves of φ uu are almost the same, but the two curves of φ vv are obviously changed due to the on-off of the switch. It indicates that it has no impact on u-polarized waves regardless of whether the switch is on or off, while it has the impact on the v-polarized waves. Therefore, the induced current distribution based on v-polarized incident waves should be further analyzed.

Compared with
Under v-polarized incidences, the induced current distributions at the resonant frequencies (6.2 GHz, 8.8 GHz, 12.4 GHz, 16.2 GHz, 21.5 GHz and 23.5 GHz) are drawn successively, as shown in Fig. 5(a) and Fig. 5(b). With the increase of frequency, it is obvious that the distribution of the induced current gradually flows through the metallic via and switch to a small square metal patch. In addition, when the switch is on, the induced current mainly distributes around the switch. When the switch is off, the induced current mainly distributes around the metallic via and the long side of L-shaped metal patch, indicating that the onoff of the switch can change the distribution of the induced current significantly.
The intensity of resonance is stronger, indicating that the higher PCR will be realized. As shown in Fig. 5(a) and Fig. 5(b), the amplitude of "switch-on" induced current is about twice as much as the "switch-off" induced current, which corresponds to the PCR reduces half of switch on after switching off, as shown in Fig. 3(a). Therefore, the proposed NRM can achieve LTL and LTC polarization conversion in one metasurface simultaneously, which is consistent with the original conclusion.

Experimental Results
To further validate this design, the NRM device was fabricated. As shown in Fig. 6(a), the practical sample is composed of (30  30) units, and the overall structural size of the sample is 270 mm  270 mm  6.5 mm. The air layer is supported by 1 cm thick nylon gasket at four corners of the sample. To simplify manufacturing, a metallic connection bridge is used to take the place of the MEMS switch in this experiment. According to [30], this method is reliable. Space wave method [36] was used to test the sample in the microwave anechoic chamber, as shown in Fig. 6(b). Above all, we define the horizontal orientation as the xpolarization, and the vertical orientation as y-polarization. The transmitting and receiving horn antenna are placed in x-polarization. R xx and φ xx were measured respectively. Next, the transmitting horn antenna is placed in y-polarization, and R xy and φ xy were measured respectively.
Based on the measured results, we can calculate the PCR and AR, according to (1) and (2). Owing to the limitation of experimental conditions, the proposed NRM is measured less than 18 GHz. The comparison of measurement and simulation is shown in Fig. 6(c) and (d). Although limited by the frequency of measurement, the trend of simulated and measured curves is roughly same. From the experimental results, we can find that the proposed NMR  can achieve LTC polarization conversion efficiently, and LTL polarization conversion with the PCR is greater than 90%. Because the experimental measurement is not an ideal state and because of the accuracy of fabrication, there is discrepancy among measurement and simulation results. Hence, the validity of the simulation is verified by the experimental measurements.

Conclusion
We proposed a novel reconfigurable metasurface. When the switch is on, the proposed polarization converter could achieve LTL polarization conversion ranging from 5.6 GHz to 23.5 GHz with the polarization conversion ratio over 90%. When the switch is off, it could achieve LTC polarization conversion ranging from 5.7 GHz to 23.8 GHz with the AR less than 3 dB. To further validate the mechanism, the electromagnetic wave was decomposed into u-v direction and the distribution of induced current was analyzed, which can testify the reconfigurable feature of it.
It is worth mentioning that the bandwidth of LTL and LTC polarization conversion are highly coincident from 5.7 GHz to 23.5 GHz (fractional bandwidth of 122%). The measured results are in accordance with simulated results. The designed NRM has potential application in the dynamic modulation of antenna polarization.