1. Introduction

Reducing reflection from a target illuminated by electromagnetic waves is of great importance in stealth technology. To achieve this goal, there are generally two strategies: shaping the physical geometry and using radar absorbing materials (RAMs). By changing the geometries of targets to guide the reflected wave to other directions, radar scattering signal could be reduced, which may result in the compatibility of aero- and hydro-dynamics [1]. On the other hand, RAMs have attracted extensive attention in the last decades due to their good compatibility and universality [2]. They attenuate microwaves by dielectric, magnetic, or Ohmic loss. Among them, dielectric RAMs have low efficiency [3] and a large thickness [4]. By contrast, magnetic RAMs may achieve better impedance match, showing a strong absorption and large bandwidth to thickness ratio [5]. Contemporary aircrafts, possessing high-stealth and high-speed simultaneously, requires refractory RAMs that can work for high temperature and remain stable when temperature changes. However, magnetic RAMs cannot work for high temperature since their magnetic responses gradually deteriorate with rising temperature and completely vanishes at above Curie temperature [6]. To overcome this defect, insulated ceramics filled with semiconducting ceramics or carbon materials are reported [7], which results in a large thickness [8] and sensitive absorption with the fluctuation of environment temperatures [9].

Emerging metamaterial [10] and metasurface [11–13], consisting of periodic or non-periodic arrays of subwavelength metal/dielectric particles, provides a powerful tool to control the propagation of electromagnetic waves, leading to a plethora of unprecedented devices, e.g., invisible cloaks [14,15], meta-lens [16], metamaterial absorbers [17,18], polarization converter [19–21], etc. Since magnetic materials are not required, metamaterial absorbers have been extended to high-temperature by employing Ohmic sheet [22], single layer frequency selective surface (FSS) [23–25], and double layer FSS [26]. These metamaterial absorbers require high loss in the periodical metallic layer or the dielectric [25,26], leading to a large thickness [22] or a narrow bandwidth [24]. Recently, a new concept of coding metasurface has been reported to control the scattering pattern by elaborately designing the coding sequences [27,28]. Based on polarization conversion [29–33], broadband electromagnetic backward scattering reduction has been experimentally verified [34–36], which were applied at low temperature maturely. While at high temperature, there is little research on reflection reduction with broadband polarization conversion [37,38]. By drilling holes in the aluminum nitride ceramic, all-dielectric metasurface has been designed to achieve high-temperature reflection reduction [37], which may be hard to meet the mechanical requirements in practical engineering applications. In contrary, all-metallic metasurface also has been designed to achieve high-temperature reflection reduction [38], which works in 8∼17GHz with a large thickness of 9 mm.

Here, we provide an alternative scheme to achieve broadband reflection reduction at high temperature focused on C-band. The proposed structure consists of a top Ag metasurface and a bottom continuous Ag layer separated by Al₂O₃ ceramics. Based on genetic algorithm (GA) of commercial software CST, we derived a high-effective polarization conversion structure. The structure can effectively convert linearly polarized waves into cross-polarized waves in the frequency range of 4.3∼7.3 GHz, with a polarization conversion ratio (PCR) above 90%. The significant bandwidth expansion is attributed to the three electric/magnetic resonances generated in a simple cut-wire unit. By arranging the polarization conversion units in chessboard layout with 0/1 form, a low specular reflection below -10dB from 4.3 GHz to 7.3 GHz is obtained. Experimental results agree well with simulation results, which shows that the proposed structure remains stable when temperature changes from room temperature to 500°C.

2. Design, simulation and analysis

Figure 1(a) conceptually demonstrates the proposed structure, in which the normal incident waves are scattered into four beams far away from the specular direction, thus achieving a broadband reflection reduction. This function is achieved based on polarization conversion. The polarization converter is usually composed of a top metasurface and a metallic ground, which is separated by a dielectric layer [29–31]. In order to meet high temperature, the metasurface is Ag and the dielectric is Al₂O₃ ceramics. The dielectric constant of Al₂O₃ ceramics is 9.7, and the loss tangent is 0.017. The metasurface generally consists of asymmetric resonators [39]. Without loss of generality, we choose the metal cut-wire as the unit cell due to its simple structure with distinctive anisotropy characteristics [40–41], as shown in Fig. 2(a). According to GA, we derive the optimal geometric parameters as l=9.55 mm, w=2.86 mm, p=15 mm, and t=4 mm. After arranging the polarization conversion units in chessboard layout with 0/1 form, a low specular reflection below -10dB from 4.3 GHz to 7.3 GHz is obtained, as shown in Fig. 1(b). It can be seen that the experimental result agrees well with the simulated result.

 

Fig. 1. (a) Conceptually demonstration of the refractory structure with broadband reflection reduction. (b) Simulated and measured results under normal incidence.

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Fig. 2. (a) The working principle of polarization conversion. (b) The reflected amplitudes and phase difference. (c) The simulated co-polarization (r_yy) and cross-polarization (r_xy) reflection coefficients. (d) The calculated PCR.

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The broadband reflection reduction is achieved based on a polarization conversion. Figure 2(a) schematically demonstrates the working principle of polarization conversion. The normally incident y-polarized wave can be decomposed into two perpendicular components as ${\vec{E}_i} = ({\hat{u}{E_{iu}} + \hat{v}{E_{iv}}} ){e^{j\phi }}$. Accordingly, the reflected wave can be expressed as ${\vec{E}_r} = (\hat{u}|{{r_u}} |{e^{j{\phi_u}}}{E_{iu}} + \hat{v}|{{r_v}} |{e^{j{\phi_v}}}{E_{iv}} ){e^{j\phi }}$. Here, u-v coordinate system is obtained by rotating the x-y coordinate axis by 45°. r_u and r_v represent the reflected coefficients along the u- and v-axis, respectively. Due to the asymmetric of the structure, there is a phase difference ($\Delta \phi = |{{\phi_u} - {\phi_v}} |$) between r_u and r_v. If $|{{r_u}} |\approx |{{r_v}} |$ and $\Delta \phi = {180^ \circ }$, the synthetic field E_r will be along the x-direction, as illustrated in Fig. 2(a). In other words, the oscillation direction of the reflected field is rotated by 90° relative to incident field, thus achieving a 90° polarization conversion. Figure 2(b) shows the reflection amplitudes and phase difference when the incident plane wave is polarized along the u- and v-axis, respectively. It can be seen that the reflection amplitudes are nearly equal to one and the phase difference is roughly 180° from 4.3 to 7.3 GHz, leading to an ultra-broadband and high-efficiency polarization conversion. In particular, a nearly 100% polarization conversion ($\Delta \phi = {180^ \circ }$) can be reached at the resonance frequencies of 4.54 GHz, 5.63 GHz, and 7.15 GHz.

Furthermore, we calculated the co-polarization (r_yy) and cross-polarization (r_xy) reflection coefficients, as shown in Fig. 2(c). It can be seen that the co-polarization is lower than 0.3, and the cross-polarization is higher than 0.9 across a wide frequency range of 4.3-7.3 GHz, which implies that the illuminated y-polarized wave has been efficiently converted into x-polarized wave. The polarization conversion ratio (PCR) is often introduced to evaluate the efficiency of the polarization conversion, which is defined as PCR = r²_xy/(r²_xy+ r²_yy). The calculated PCR is illustrated in Fig. 2(d). We can see that PCR is above 90% from 4.3 to 7.3 GHz. In particular, the PCR reaches 100% at the resonance frequencies of 4.54 GHz, 5.63 GHz, and 7.15 GHz.

To understand the physical mechanism of the proposed polarization converter, we then simulated the surface current distributions on the top metasurface and bottom layer at resonant frequencies of 4.54 GHz, 5.63 GHz and 7.15 GHz, respectively, as shown in Fig. 3. Since the surface currents along the top layer are antiparallel to those on the background sheet at 4.54 GHz and 5.63 GHz, which results in a magnetic resonance. In contrast, the surface currents along the top layer are parallel to those on the background sheet at 7.15 GHz, which results in an electric resonance. The three resonances play an important role in achieving high efficiency and broadband polarization conversion. Actually, the broadband property is attributed to the superposition of multiple PCR peaks.

 

Fig. 3. Surface current distributions on the metasurface and bottom metallic ground at different resonance frequencies. (a) 4.54 GHz. (b) 5.63 GHz. (c) 7.15 GHz.

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To understand how magnetic and electric response contribute to polarization conversion, the schematic diagram of magnetic/electric field decomposition is demonstrated in Fig. 3(a)-(c). It is noted that magnetic/electric response may cause one component of reflected magnetic/electric field along the u- or v-axis emerging a π phase difference, while the other component keeps in phase relative to the incident field. As shown in Figs. 3(a) and 3(b), magnetic resonance causes a π phase difference between H_rv (H_ru) and H_iv (H_iu), and keeps H_ru (H_rv) in phase relative to H_iu (H_iv), thus leading to a synthetic magnetic field rotated by 90°. In contrast, as shown in Fig. 3(c), electric resonance causes a π phase difference between E_ru and E_iu, and keeps E_rv in phase relative to E_iv, thus leading to a synthetic electric field rotated by 90°. Due to the overlap of three polarization rotation resonances, a broadband polarization conversion is achieved.

Figure 2(a) shows the element “0”, and element “1” can be obtained by rotating a 90° of element “0”. The phase difference between element “0” and element “1” is π. After arranging the two elements in chessboard layout as shown in Fig. 1(a), specular reflection will be greatly suppressed. For a metasurface with N×N elements, the scattered far-field can be expressed as:

The chessboard metasurface makes the normally incident plane wave be scattered to four diagonal directions where φ=45°, 135°, 225°, 315°, thus the reflected beams along the specular direction are cancelled out. The angle between each beam with the z-axis is expressed as [42]:

Figure 4 shows the scattered patterns of the proposed structure at different frequencies. Compared to the PEC with a strong specular reflection, the normally incident waves have been scattered into four beams with the proposed structure, leading to a significant reduction of reflection in the specular direction. For quantitatively observation, the scattering patterns at φ=45° (or 135°) plane is shown in Fig. 4(f)-(h). We can derive the scattering angle θ about 21.9°, 17.8° and 14.1° at the frequency of 4.54 GHz, 5.63 GHz, and 7.15 GHz, respectively, which is in good agreement with the theoretical value (21.5°, 17.2° and 13.5°) calculated by Eq. (2).

 

Fig. 4. Simulation results of scattering patterns under normal incidence. (a), (e) PEC at 4.54 GHz. (b), (f) Metasurface at 4.54 GHz. (c), (g) Metasurface at 5.63 GHz. (d), (h) Metasurface at 7.15 GHz. The second row corresponds to the scattering pattern in the φ=45° plane.

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3. Fabrication and experiment

To experimentally validate the performance of our structure, a 180 mm ×180 mm sample composed of 2×2 equal-sized lattices was fabricated by using the screen printing technology, as shown in Fig. 5(a)-(d). First, a 300-mesh screen (see Fig. 5(a)) was used to print the metasurface pattern on the surface of Al₂O₃ ceramic. A squeegee is moved toward the end of the mesh at a uniform speed to spread the silver paste over the mesh, and a reverse stroke causes the screen touch the dielectric instantly, therefore forming the pattern on the substrate surface as the screen springs back after the blade has passed. Second, the sample was dried at 120°C for 10 min, as shown in Fig. 5(b). Finally, the sample was sintered at 900°C for 3 hours, as shown in Fig. 5(c). Figure 5(d) demonstrates the final processed sample.

 

Fig. 5. Fabrication process of the sample.

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Figure 6(a) shows the experimental setup, which consists of a Vector Network Analyzer (Agilent 8720ET), a pair of horn antennas (one is transmitter and the other is receiver), and a high temperature heating furnace surrounded by an absorbing environment. Figure 6(b) shows the measured reflection at 25°C, 300°C and 500°C, respectively. We can see that the measured result at 25°C is in good agreement with the simulated result, showing an excellent specular reflection suppression feature in a broadband range. When the temperature rises from 25°C to 300°C even 500°C, there is almost no change in the reflection spectrum, which validates the stable performance at a wide temperature range.

 

Fig. 6. (a) Experimental setup; (b) The measured results at 25°C, 300°C and 500°C.

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From Fig. 6(b) we can see that the reflection spectrum presents a slight redshift with the increase of temperature. This may be attributed to the change of the permittivity of Al₂O₃ ceramics. Specifically, the permittivity of Al₂O₃ ceramics increases with the increase of temperature. We simulated the reflection spectrum of the structure when the permittivity of the dielectric layer is varied from 9.7 to 11, as shown in Fig. 7. Obviously, the variation trend of the simulation results is consistent with the measured results, which confirms our judgement.

 

Fig. 7. Simulated reflection spectrum when the permittivity of the dielectric layer is varied.

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In addition to normal incidence, we also examine the performance at oblique incidence. Figure 8 shows the measured reflection spectrum of the structure under different incident angles. We can see that the performance decreases slightly with fluctuations as the incident angle increases. On balance, our structure keeps a good angle stability.

 

Fig. 8. Measured reflection spectrum under different incident angles.

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4. Summary

In summary, we have designed, fabricated, and measured a refractory metamaterial with a broadband reflection reduction. Different from the existing high-temperature absorption coating, we utilized polarization conversion to suppress the backward scattering. Experimental results demonstrate a low specular reflection below -10dB ranging from 4.3 GHz to 7.3 GHz, which is in good agreement with the simulated results. Furthermore, the stable performance has been experimentally validated in a wide temperature range from room temperature to 500°C. The proposed structure is easy to be fabricated and the performance is insensitive to temperature, which can be further extended to fractal metasurface [43] and dual-band metasurface [44], hence are promising for many applications.