An X-Band Reflective Active Polarization Conversion Metasurface

Abstract

In this paper, an active polarization conversion metasurface (APCM) operating in the X-band (8–12 GHz) is presented. Active unit cells consist of three metal layers and two substrate layers. Anisotropic metal patches, positive-intrinsic-negative (PIN) diodes, and bias lines are integrated into the surface layer to form a chiral structure. A simple and efficient polarization conversion function is realized in this designed structure. The operating mode is switched between linear-to-linear (LTL) polarization conversion and linear-to-circular (LTC) polarization conversion by switching the loaded PIN diode. In the LTL polarization conversion state, the polarization conversion ratio of APCM is over 90% in X-band. Reflected wave circular polarization axis ratio <3 dB in 8–12 GHz when in LTC state. The proposed structure is verified by experimental measurements.

1. Introduction

As an essential property of electromagnetic (EM) waves, polarization is an important basic parameter other than the amplitude, frequency, and phase. It describes the vector characteristics of electromagnetic waves, that is, the time-varying trajectory characteristics of the electric field vector on the propagation cross-section [1]. When electromagnetic waves irradiate the target, a variable polarization effect is discovered. The polarization state of the scattered wave will change relative to the incident wave, and there is a specific mapping transformation relationship between the two. It is closely related to the physical properties of the target, such as posture, size, structure, and material, so the target can be regarded as a polarization converter [2]. The polarization of the reflected EM wave carries the information of the target, which has great application potential in wireless communication and microwave imaging. It also improves the ability of target detection, anti-interference, classification, and recognition.

Two-dimensional metasurfaces are periodic arrangements of subwavelength-sized artificial unit cells with physical properties beyond the limits of natural materials. Different media such as high permittivity ceramics, plasmonic, bulk Dirac semimetal, heat-sensitive materials, etc., are used in the design of metasurface structures to achieve different functions [3,4,5,6,7]. Recently, metasurfaces have been gaining increasing attention due to their remarkable ability in polarization conversion. It also has sound application effects in full-polarization invisibility and radar cross section (RCS) reduction [8,9,10]. However, the performance of a metasurface depends on its unit cell structure, and it is difficult to change once the structure is fabricated. Immutability of properties limits applications of such metasurfaces.

Active metasurfaces integrated with active components can dynamically adjust the performance to obtain different effects [11,12,13]. Diodes, MEMS, graphene, phase change materials, etc., are commonly used active components [14,15,16,17]. The literature [18] reported a polarization conversion in the THz regime based on MEMS. The transmissive and reflective polarized conversion surfaces based on PIN diodes have been presented [19,20]. Arbitrary editing of reflected wave polarization is even possible [21]. Active polarization conversion metasurfaces (APCM) have been widely applied. A transparent APCM based on PIN diodes is used to tune between circular polarization (CP) and linear polarization (LP) in antennas [22]. Communication links are established by a graphene-based metasurface [23]. By using the signal programable APCM, the information on near-field distributions can be modulated [24,25]. However, reviewing the above literature, the polarization conversion can only achieve the effect of linear-to-linear (LTL) polarization conversion. To realize complex functions, more components are integrated (shown as

Table 1. The number of APCM active components used in the existing literature.

Literature	Number of Active Components
[10]	4
[12]	12
[16]	3
[20]	4
This paper	2

), which makes the state switching inflexible. The multi-layer unit cell structure and complex control network make metasurfaces inconvenient and limit the application. Therefore, the research direction of active polarization conversion metasurfaces is simple structure, few electronic components, and high switching rate.

In this paper, an active polarization conversion metasurface for X-band loaded PIN diodes is designed. The phase and amplitude responses can be modulated by controlling the state of the PIN diode. With different applied bias voltages, the resonant state of the unit is changed due to the variety of PIN diodes. This allows the APCM to switch between linear-to-linear (LTL) and linear-to-circular (TLC) polarization conversion. The bias network is cleverly arranged according to the characteristics of the unit cell structure. A chiral structure consists of bias lines and metal patches, which are connected via holes. Meanwhile, only two diodes are integrated into each unit. The fewer components used, the easier polarization control is achieved. The designed surface can realize efficient polarization conversion with a simple structure, easy fabrication, and fewer integrated active devices. Experimental tests verify the effectiveness of the designed structure.

2. Design of APCM

Figure 1 illustrates the principle of the APCM. The APCM is placed on the XOY plane, and the electromagnetic wave is incident along the +z direction. The metasurface is anisotropic and its optical axes are along the x and y-directions, respectively. To explain the APCM principle, the incident and reflected wave can be decomposed into x and y components. When the device is illuminated with a linearly polarized wave E_i is along the x or y direction, the incident EM wave and reflective EM wave can be denoted as

$$\begin{array}{cc}{E}_i=\left[\begin{array}{c}{E}_x^i\\ E_y^i\end{array}\right]& E_r=\left[\begin{array}{c}{E}_x^r\\ E_y^r\end{array}\right]\end{array}$$

(1)

$$E_r=R\cdot E_i=\left[\begin{array}{cc}{R}_{xx}& R_{xy}\\ R_{yx}& R_{yy}\end{array}\right]\left[\begin{array}{c}{E}_x^i\\ E_y^i\end{array}\right]$$

(2)

where E denotes the electric field, the subscripts i and r indicate the incidence and the reflection of EM waves. To study the performance of the designed polarization converter, we first define R_xx = E_xr/E_xi and R_xy = E_yr/E_xi to denote the co-polarized and cross-polarized reflection coefficients, respectively. Thus, R_xx and R_yy are the co-polarization reflection coefficients of x- and y-polarized waves, while R_yx and R_xy are the cross-polarization reflection coefficients. Because of the anisotropy topology, the polarization converter reveals a different response in the x and y components, thus affecting the polarization of the reflective waves. When ∆φ_yx = arg(R_y) − arg(R_x) = 90° and |R_x| = |R_y|, the reflected wave is circularly polarized. Whereas, when ∆φ_yx = 180° or 0°, the reflected wave is linearly polarized. The others are elliptically polarized waves.

As shown in Figure 2, an APCM structure is designed. The 3D schematic of the unit cell is presented in Figure 2b, which consists of three substrate layers and two metal layers. The structure of the top metal patch is similar to ‘x’, which has symmetry and has the same conversion effect for the incident EM waves of x- and y-polarization. The chiral structure comprises the metal patch and bias line, which has a perfect polarization conversion effect. PIN diodes are used to redistribute the surface resonant current (SRC) to control the polarization conversion state.

The top anisotropic metal patch is printed on the dielectric substrate, FR4 (ε_r = 4.4, tanδ = 0.02). The bias networks are printed on another side of the same substrate. Two via holes are used to conduct the bias voltage. Two PIN diodes (BA585) are integrated into the central gap of the anisotropic metal patch. The bottom substrate is FR4 with one-side metal cladding. An F4B (ε_r = 2.65, tanδ = 0.001) layer is arranged between these two FR4 substrates in Figure 2b. The detailed parameters of the unit cell structure are given in

Table 2. Parameters of the APCM unit cell.

Parameter	Size (mm)	Parameter	Size (mm)
l1	10.89	h1	0.04
l2	1.67	h2	0.5
w1	2	h3	2.5
w2	3.25	p	9.8

.

3. Simulation and Analysis

The APCM unit cell structure shown in Figure 2b is simulated using CST Microwave Studio. The boundary conditions in the x- and y-directions of the CST unit cell are set as periodic boundaries, and the +z-direction is set as open boundaries. The EM wave is vertically incident along the −z-direction. The polarization angle is set to be along the +x-direction. The frequency range is 8 GHz to 12 GHz. The on–off states of diodes are represented using resistors; 100 Ω representing the diode on state and 3000 Ω representing the diode off state. The simulated reflection coefficients of the structure are shown in Figure 3, which show the amplitude. Figure 3a shows the co-polarization and the cross-polarization amplitude response when the diode is off, and Figure 3b presents the diode on state. R_xx-S and R_yx-S represent the simulation results of R_xx and R_yx. Figure 4 shows the reflection phase of the cell structure obtained from the simulation of the co-polarization channel and the cross-polarization channel, and the phase difference ∆φ of the x- and y-polarization components.

As illustrated in Figure 3a, when the diode is switched off, the resonance point is at 10.64 GHz; cross-polarization conversion occurs at 9.19–12 GHz, where R_xx is less than −15 dB and R_yx is more than −1 dB. When the diode is switched on, cross-polarized reflection coefficient R_xx and co-polarized reflection coefficient R_xy are almost equal, as shown in Figure 3b.

To further investigate the performance of polarization conversion for the diode on-off state, we calculated the polarization conversion ratio (PCR) for linear polarization conversion and the axial ratio (AR) for circular polarization conversion, respectively. The PCR is obtained as PCR = R_xy²/(R_xy² + R_xx²), and AR is obtained as follows [26]:

$$\sin 2\epsilon =\frac{2\left|E_x\right|\left|E_y\right|\sin \mathrm{\Delta }\varphi }{|E_x{|}^2+|E_y{|}^2}$$

(3)

$$AR=\tan \epsilon$$

(4)

The calculated PCR and AR are presented in Figure 5. We clearly see that the relative bandwidth with the PCR over 80% reaches whole X-band and the PCR is over 90% in 9.4–10 GHz. When the diode is off, and AR < 3 dB in 8.2–12 GHz, a circular polarized wave is obtained.

To explain the physical principle of APCM, the resonance phenomenon of the unit cell under two polarization conversion modes is analyzed. When the electric field of the incident electromagnetic wave is in the x- or y-direction, the resonance is excited on the cell to produce a surface resonant current (SRC). Figure 6 shows the SRC distribution in two states. The red arrow marked in the figure points to the SRC direction of the frequency point. In LTC polarization conversion mode, the unit cell SRC at 8.25 GHz is shown in Figure 6a,b. When the diode is on, the SRC generated at the middle rectangular metal patch flows to the small rectangular metal patches on both sides, resulting in reverse SRC. Thus, the reflected wave has equal amplitude polarization components in both X and Y directions. Figure 6c,d shows the unit cell SRC at 10.64 GHz in LTC polarization conversion mode. The SRC opposite to the direction of the incident electric field is generated only at the middle metal patch, so the cross polarized electromagnetic wave is reflected. Therefore, the PIN diode controls the current distribution and realizes the switching polarization conversion state.

4. Fabrication and Measurement

To experimentally validate the configurable polarization conversion of the proposed device, a sample consisting of 30 × 30-unit cells was fabricated using printed circuit board (PCB) technology, which was measured in a standard microwave chamber. Two BA585 PIN diodes were soldered on each unit cell and two bias lines were printed on the opposite sides of the surface. Figure 7 presents a photograph of the fabricated sample.

The experiment setup is presented in Figure 8a, the sample and feeding source are fixed on a rotatable table. Two 8–12 GHz dual-polarized horn antennas are employed as the transmitter and receiver. Antennas are, respectively, connected to the ports 1 to 4 of a vector network analyzer. The sample was placed on the front of the two horn antennas. A DC power source was used to supply the bias voltage for the diodes. The centers of the sample and the horn antennas were fixed at the same height. In order to avoid mutual interference, the position of the receiving antenna is 10 cm ahead of the transmitting antenna, and the height difference between the two antennas is 8 cm, as shown in Figure 8b. Absorbing materials were used to avoid the influence of the scattered EM wave on experimental results.

We first calibrated antennas. Then we measured the co- and cross-polarized reflection coefficients, under the bias voltage of 9 V (PIN diode off) and 13.4 V (PIN diode on). The measured and simulated reflection coefficients R_xx and R_yx for bias voltages of 9 V and 13.4 V are illustrated in Figure 9. R_xx-M and R_yx-M represent the measurement results of R_xx and R_yx. The experimental results demonstrate the reconfigurable polarization conversion of the proposed device. A linear cross-polarized converter is obtained for the bias voltage 9 V, and circularly polarized converter is obtained for the bias voltage 13.4 V. The measured resonant frequency of R_xx reflection coefficients is illustrated in Figure 9a, which is less than the simulation result. We observed that the measured reflection coefficients were slightly different from the simulation. This difference may be caused by tolerances in the PCB fabrication, the component errors from the PIN diodes and in realistic measurements the converter may introduce more parasitic and coupling effects.

5. Conclusions

The novelty of this paper was to propose an X-band reflective APCM structure that is efficient, simple, and flexible. The switching function between LTL and LTC polarization conversion states was achieved by controlling the PIN diode. The state of the diode was changed by the bias voltage so that the surface current was redistributed. A new resonance state was generated to obtain different polarization conversion effects. To simplify the structure, only two PIN diodes were used in this design, and the bias line between the two cells was fused with the surface metal patch. A sample was fabricated and measured. The measurement results verify the switchability of APCM polarization conversion. The designed APCM may find potential applications in polarization target feature identification, information communication, etc.