Broadband Ultra-Thin High-Efficiency Linear Polarizer Based on Metasurfaces

Abstract

In this paper, an ultra-wideband (UWB) polarizer with high performance based on the metasurface is designed and demonstrated, which is composed of a dielectric substrate with a double-gap circular pattern and metal film. Multiple strong resonance points enable the design to achieve the conversion from incident linearly polarized waves to cross-polarized waves at 6.49–11.64 GHz, with a fractional bandwidth of 56.8% and a corresponding polarization conversion rate (PCR) of 90%. The PCR remains above 90% at 6.49–11.52 GHz when the electromagnetic wave is obliquely incident between 0° and 30°. Furthermore, the surface current distribution of the polarizer is discussed to explain the physical mechanism. The sample is fabricated for microwave validation. Compared with previous reports, the proposed polarizer has a larger bandwidth and higher efficiency and is expected to be used in microwave communications, antennas, radar cross section reduction, and other fields.

1. Introduction

Metamaterials (MMs), as artificially designed sub-wavelength materials, exhibit exotic characteristics compared with natural materials [1]. Over the past few decades, MMs have rapidly gained the attention of researchers owing to their unusual response to electromagnetic (EM) waves. The permittivity and permeability can be designed arbitrarily, which provides the possibility of using peculiar functional devices such as negative refraction [2], cloaks [3,4], holograms [5], images [6], perfect lenses [7], perfect absorption [8], sensors [9], and antennas [10]. The metasurface (MS) is a two-dimensional structure of the MMs. By designing the microstructure, the EM wave can be manipulated artificially. Among them, the control of EM wave polarization has aroused increasing interest [11,12,13,14,15,16,17,18]. However, the previously proposed MS devices suffer from narrow bandwidth and low efficiency, which limits their practical applications. To solve this problem, researchers have successively proposed a series of wideband high-efficiency polarization conversion devices [19,20,21]. For example, a transmissive-type anisotropic metadevice, consisting of stacks of bilayer gear-like metallic patterns, is proposed to manipulate the polarization state of electromagnetic waves. It can simultaneously achieve nondispersive cross-polarization conversion for both linearly and circularly polarized incident waves in a broadband frequency region. However, they are still generally limited due to the large device thickness. In recent years, researchers have proposed many ultrathin polarizers operating in the microwave band [22,23]. For example, an ultrathin linear-to-cross-polarization transmission MS converter [24] with a thickness of 0.8 mm (0.0235λ) was proposed that gives near-unity PCR at 8.8 GHz. Moreover, a tunable wideband reflective cross-polarization converter [25] with a T-shaped carved-hollow array based on the metasurface was designed, with a PCR greater than 80% and a fractional bandwidth of 40%. Similarly, a reflective broadband linear polarizer [26] composed of metallic disks and asymmetric arc metallic wires was presented, and its PCR value can be maintained over 90% in the range of 11.2–20.3 GHz when the incident angle varied from 0° to 40°. However, the operating bandwidth and efficiency need to be further improved. In addition, ultra-thin and wide-angle manipulated EM waves have also been extended to terahertz and near-infrared applications [27,28]. Therefore, the design of ultra-thin, UWB, and high-performance polarization devices still faces a series of challenges.

In this work, we propose an ultra-thin, UWB, and high-performance linear polarizer based on the MS. The thickness of the polarizer is only 3 mm (0.09λ). The device converts linearly polarized (LP) waves to orthogonally polarized waves. By decomposing the electric field, the working principle is explained. The surface current distribution at three response points is also detected to analyze the physical mechanism. The experimental results show that the PCR is higher than 90% within 6.49–11.64 GHz under the normal incidence of EM waves. When the oblique incidence angle is between 0° and 30°, the PCR can still reach 90% at 6.49–11.52 GHz. Since the proposed polarization device is ultra-thin and efficient, it is easier to integrate into other systems.

2. Design, Simulation, and Experiment

In general, the reflective polarizer has a multi-layer structure, including the top layer of MS, the intermediate layer, and the bottom metal film. The interference of the transmitted wave between the dielectric and metal layers forms the final reflected waves. Therefore, we can adjust the phase and amplitude of the reflected waves by adjusting the structure of the metal layer and the thickness of the dielectric layer.

We propose an ultra-broadband linear polarizer with high efficiency based on the design concept proposed above. The designed polarizer comprises three layers, as depicted in Figure 1a. A deformed double-gap circular metal pattern makes up the top layer of this structure. The intermediate substrate layer is FR-4 with a relative permittivity of 4.3 and a loss tangent of 0.025. A metal film forms the bottom layer to block EM waves. The metal layers are copper with a conductivity of 5.8 × 10⁷ S/m and a thickness of 0.035 mm. The front view of the polarizer is depicted in Figure 1b. The optimized geometries of the polarizer displayed in Figure 1a are: r₀ = 3.1 mm, r₁ = 1.4 mm, t = 3 mm, p = 7 mm, w₁ = 2.3 mm, w₂ = 1.0 mm, w₃ = 0.4 mm, and w₄ = 0.1 mm. Figure 1c shows a photograph of the partially fabricated sample, and Figure 1d presents the experimental environment and setup.

To study the EM response of this polarizer, we simulate the designed MS using CST 2019. The infinite model structure is numerically analyzed by setting the Floquet boundary on the x and y axes. In order to better understand the UWB polarizer, for x-polarized incident waves, we define $r_{xx}=\left|E_{xr}/E_{xi}\right|$ and $r_{yx}=\left|E_{yr}/E_{xi}\right|$ as the reflectance of the co-polarization and cross-polarization, respectively. The corresponding subscripts i and r indicate the incident and reflected EM waves, respectively. Here, the PCR and polarization extinction ratio (PER) are utilized to describe the polarization conversion performance and the polarization stability of the polarizer, respectively. The PCR and PER are defined as [29]:

$$PC{R}_x={\left|{\mathit{r}}_{yx}\right|}^2/\left({\left|{\mathit{r}}_{yx}\right|}^2+{\left|{\mathit{r}}_{xx}\right|}^2\right),$$

(1)

$$PER=10\lg \left({\left|{\mathit{r}}_{yx}\right|}^2/{\left|{\mathit{r}}_{xx}\right|}^2\right),$$

(2)

3. Results and Discussion

First of all, we investigate the cross-polarization conversion performance under x-polarized incident waves. Figure 2a,b show the simulated reflectance and the corresponding PCR and PER, respectively. We can clearly see that the simulated r_yx is above −1 dB at 6.60–11.22 GHz, and the corresponding r_xx is below −10 dB, with a relative bandwidth of 53.2%. In addition, the reflected waves resonate strongly at 6.93 GHz, 8.57 GHz, and 11.23 GHz, corresponding to a PCR of 100%. The presence of resonance points explains cross-polarization conversion, while more resonance points leads to a larger conversion bandwidth. Figure 2b indicates that the calculated PCR is greater than 90% within 6.49–11.64 GHz. Meanwhile, the PER can reach 15.2 dB, 17.9 dB, and 14.8 dB at the three resonance points, respectively. The above results prove that the designed polarizer can efficiently transform LP waves into their orthogonal polarized counterparts in the broadband range.

Assuming that the incident EM waves (E_i) propagate along the x axis, the reflected waves will propagate along the y axis. The electric field can be decomposed along the u and v directions. The u-v axis is derived by turning the x-y axis 45° counterclockwise. Figure 3a shows a schematic diagram of the electric field decomposition. Therefore, the incident and reflected electric fields can be expressed as [30]:

$$\begin{array}{ll}{\mathit{E}}_i& =\overrightarrow{x}{\mathit{E}}_0\\ & =\mathit{u}{E}_{iu}{e}^{j\varphi }+\mathit{v}{E}_{iv}{e}^{j\varphi }\end{array}$$

(3)

$${\mathit{E}}_r=\mathit{u}{r}_{uu}{E}_{iu}{e}^{-j\left(\varphi +{\varphi }_{uu}\right)}+\mathit{v}{r}_{vv}{E}_{iv}{e}^{-j\left(\varphi +{\varphi }_{vv}\right)},$$

(4)

where r_uu and r_vv represent the reflectance of the u axis and v axis, respectively, and ϕ_uu and ϕ_vv stand for the reflection phase of the u axis and v axis, respectively. The converter can exhibit anisotropic properties attributed to the asymmetry of the structure. When r_uu = r_vv = 1 and Δφ = ϕ_uu − ϕ_vv = 180° are satisfied, the complex electric field of the reflected waves (E_r) is located along the y axis, indicating that the polarization orientation of the reflected wave is converted to the orthogonal orientation of the incident wave. Consequently, for x-polarized incident waves, Formula (4) can be further stated as:

$$\begin{array}{ll}{\mathit{E}}_r& =\mathit{u}{r}_{uu}{E}_{iu}{e}^{-j\left(\varphi +{\varphi }_{uu}\right)}+\mathit{v}{r}_{vv}{E}_{iv}{e}^{-j\left(\varphi +{\varphi }_{uu}-\pi \right)}\\ & =\left(\mathit{u}{E}_{iu}-\mathit{v}{E}_{iv}\right)e^{-j\left(\varphi +{\varphi }_{uu}\right)}\\ & =\overrightarrow{y}{\mathit{E}}_0{e}^{-j\left(\varphi +{\varphi }_{uu}\right)}\end{array}$$

(5)

According to Figure 3b, we can observe that co-polarized reflection amplitudes (r_uu, r_vv) are almost equal and close to 1, while the corresponding phase difference is 180° in the operating frequency range. This indicates that the polarizer achieves the conversion from x-polarized incident waves to y-polarized reflected waves.

The surface current distributions are monitored at 6.93 GHz, 8.57 GHz, and 11.23 GHz resonance points to explain the physical mechanism of the polarizer. Figure 4a,c exhibit the current distributions at 6.93 GHz and 11.23 GHz for the top and bottom layers, respectively. We note that the current distributions in the top and bottom metal layers are reversed, and a current loop is formed in the middle dielectric layer to excite the magnetic dipoles. For x-polarized incident waves (the electric field E is along the x direction and the magnetic field H is along the y direction), the magnetic field H₀ excited by a magnetic dipole can be decomposed into a set of orthogonal magnetic field components H_x and H_y. H_y along the y direction is parallel to the original magnetic field H, and H_x along the x direction is orthogonal to H. This allows the x-polarized incident waves to be transformed into its orthogonal y-polarized waves. Figure 4b demonstrates the surface current distributions at 8.57 GHz. Different from the previous physical mechanism, electric resonance is the leading cause of polarization conversion, and the analysis is the same as above. In summary, interaction between surface currents generates magnetic resonance (electrical resonance). A component of the induced magnetic field H₀ (induced electric field E₀) is perpendicular to the original magnetic field (electric field), thus converting the incident LP waves into the orthogonally polarized waves. Simultaneously, the multiple resonances extend the bandwidth.

To validate our proposed linear polarization converter, a sample of 25 × 25 cells with dimensions of 175 mm × 175 mm is fabricated. The reflectance of the fabricated polarizer is measured in an anechoic chamber using an analyzer (R&S ZNB/40) connected to two standard gain horn antennas. It should be noted that the sample should be placed vertically, and the angle between the two horn antennas should be within 5° [31]. A horn antenna is placed horizontally to emit x-polarized EM waves. Another horn antenna is placed horizontally and vertically to measure the co-polarized and cross-polarized reflectance (r_xx and r_yx), respectively.

A comparison of the simulated and measured results is given in Figure 5. According to the measured reflectance r_xx, we can observe that the three resonant frequencies (at 6.85 GHz, 9.84 GHz, and 11.52 GHz) are below −20 dB at 6.04–12.23 GHz. Furthermore, the cross-polarized reflectance r_yx is approximately consistent with the simulated curve in such a frequency range. Similarly, we can view that the PCR calculated from the measured reflectance (r_xx, r_yx) largely coincides with the PCR calculated from the simulations. The measurement error can be attributed to the following two aspects. Firstly, an infinite plane is simulated in the CST; however, the actual measured sample is finite, which leads to edge diffraction effects. Secondly, the dielectric constant of the substrate in the processed sample is slightly different from the simulated model. Therefore, the stated reasons lead to a discrepancy between the simulated and experimental results.

4. Parameter Analysis

Parameter analysis is then performed to reveal the geometric influence of the designed broadband metasurface. Figure 6a shows that the bandwidth of the polarization conversion grows gradually as the radius of the outer ring r₀ increases, and the PCR decreases sharply when r₀ exceeds 3.1 mm. However, with an increase in the inner ring radius r₁, the bandwidth of polarization conversion progressively becomes narrower, as indicated in Figure 6b. A change in the outer ring radius r₀ and inner ring radius r₁ affects the inductive and capacitive effects in the u-v direction, resulting in a change in the corresponding polarization phase, thus affecting the performance of polarization conversion. Next, we focus on the effect of the opening slit on PCR, as depicted in Figure 6c,d, where the PCR remains almost unchanged with an increasing slit width for both the outer and inner ring slits. The outer and inner ring slits w₁ and w₄ are located in the horizontal and vertical directions, respectively, and the decomposed components along the u-v direction are unchanged, so they have little effect on the polarization conversion performance. Moreover, we can also explore the impact of different incident angles and polarization angles on conversion efficiency, as shown in Figure 7. From Figure 7a, the PCR exceeds 90% in the range of 6.49–11.64 GHz when the oblique incidence angle is 0°–30°, and the PCR decreases with an increase in the polarization angle, as displayed in Figure 7b. This is primarily due to the fact that the designed polarizer is not centrosymmetric. From the foregoing analysis, we can conclude that the polarizer shows good conversion performance after selecting the appropriate geometric parameters. Finally,

Table 1. Performance comparison with previous works.

Paper	Frequency Range	Fractional Bandwidth	PCR
[24]	8.8 GHz	/	1
[32]	9.65–14.16 GHz	38%	80%
[25]	10.29–15.46 THz	40%	80%
[33]	8–12 GHz	40%	90%
[26]	11.3–20.2 GHz	53.4%	85%
This work	6.49–11.52	56.8%	90%

gives a performance comparison between the proposed polarizer in this paper and the previous works.

5. Conclusions

To summarize, we propose an ultra-thin, UWB, and high-efficiency linear polarizer. The thickness of the polarizer is only 3 mm (0.09λ). The UWB of the polarizer is derived from three EM resonances. Numerical analysis and measured results show that the PCR is greater than 90% at 6.49–11.64 GHz, and the corresponding fractional bandwidth reaches 56.8%. Moreover, the PCR is higher than 90% in the range of 6.49–11.52 GHz when the oblique incidence angle is 0°–30°. Finally, the physical mechanism of the polarizer is analyzed and the effect of geometric parameters on PCR is investigated. The converter designed in this paper can be applied in wireless communication and RCS reduction.