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Abstract
Controlling of electromagnetic wave radiation is of great importance in many fields. In this work, a hybrid metasurface (HMS) is designed to simultaneously reduce the microwave reflection and the infrared emission. The HMS is composed of the metal/dielectric/metal/dielectric/metal configuration. The reflection reduction at microwave frequencies mainly results from the phase cancellation technique, while the infrared emission reduction is due to the reflection of the metal with a high filling ration in the top layer. It has been analytically indicated that reflection reduction with an efficiency larger than 10 dB can be achieved in the frequency band of 8.2-18 GHz, and this has been well verified by the simulated and experimental results. Meanwhile, the designed HMS displays a low emission performance in the infrared band, with the emissivity less than 0.27 from 3 to 14 μm. It is believed that our proposal may find the application of multispectral stealth technology.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Controlling of electromagnetic wave radiation is highly important in civil and military fields. As one of the most concerned applications, reducing electromagnetic wave reflection from an object has been widely investigated in the stealth technology [1]. This is usually achieved using absorbing materials which dissipate the incident energy and transform it into heat. The absorbing materials are fabricated in the forms of the coatings filled with carbon materials [2–7] and magnetic powers [8–11], or the sandwiched structures comprising resistive sheets [12–15] or frequency selective surfaces [15–18]. Additionally, lowering infrared emission is another desired aspect in the stealth technology. According to the law of Kirchhoff, low emission means the high electromagnetic reflection, and thus good conductive materials such as metals and semiconductors are usually used to achieve this goal. Obliviously, it is difficult to realize the materials with simultaneously reduced microwave reflection and infrared emission because of the completely opposite working principles. The thin films with a low emissivity in the infrared band have been attempted to cover the absorbing materials [19,20], but this will deteriorate the absorption performance.
Since the wavelengths at microwave frequencies are much larger than the infrared wavelength, the materials with frequency dispersive response can be actually used to achieve the radar-infrared stealth goal. It has been indicated that artificially structured metasurfaces offer a promising way to manipulate the electromagnetic wave radiation [21–25]. Particularly, electromagnetic response of the metasurfaces is frequency selective and thus suitable for designing the radar-infrared stealth structures. For example, the hybrid metasurfaces (HMSs) comprising different-sized artificial inclusions have been reported to achieve large microwave absorptivity and low infrared emissivity simultaneously [26–28].
In this work, we propose an alternative scheme to reduce the microwave reflection and infrared emission simultaneously. For this purpose, a HMS is designed by stacking two specially designed metallic arrays. The low emission in the infrared band is attributed to reflection of the metal in the surface layer, similar to the principle in [26–28]. Nevertheless, the microwave reflection reduction mainly arises from the phase cancellation [29–33] rather than absorption. By designing thicknesses of the dielectric between metal layers and geometry parameters of the artificial inclusions, broadband reflection at microwave frequencies is achieved and well verified by the simulation and experiment. Simultaneously, the emissivity of the designed HMS is less than 0.27 from 3 to 14 μm. Additionally, the non-absorptive principle will not lead to temperature rising for the proposed HMS even if under the higher power conditions. This is highly desired for the infrared stealth application.
2. Structure design
As mentioned in Section 1, the principle of microwave reflection reduction here is the phase cancellation, while the low infrared emission arises from the high-efficiency reflection of the HMS and mainly depends on the filling ration of metal in the top layer. Due to the distinct operation principles at the two frequency bands, two different-sized artificial structures are therefore combined to achieve the goal of this work. In addition, the simple structures are chosen for ease of design and fabrication in the design.
The structure component of the unit cell for the designed HMS is illustrated in Fig. 1, which has a metal/dielectric/metal/dielectric/metal configuration. The middle metal layer is composed of the cut wire with the length of a and width of b, and the square patches with the size of c are periodically arranged with the gap width of g to form the top metal layer. Since the size of the square patches is much smaller than the microwave wavelength of interest, the incident wave can efficiently pass through the top metal array [15]. The thicknesses of the two dielectric layers are h1 and h2, respectively. The repeated periods of the unit cells in x- and y-axis directions are defined as p. Throughout this work it is assumed that the electromagnetic wave is normally incident on the HMS.
Fig. 1 Schematic of the anisotropic unit cell. (a) Cut wires spaced from the backed metal by a dielectric layer. (b) Square patches etched on a dielectric layer. (c) Unit cell created by combining the cut wires and square patches.
The numerical simulations were performed by the commercial software, CST Microwave Studio [34]. In the simulations, the metal has an electric conductivity of 5.8 × 107 S/m and the dielectric layers have a relative permittivity of 4.3(1-j0.025). The gap width between the square patches in the top layer is g = 0.1 mm. This is subject to the fabrication precision. The other geometry parameters of a, b, c, h1, h2 and p are designed as 5.4, 1.1, 0.9, 1.5, 0.7 and 6 mm, respectively. Since the cut wires are arranged along the x-axis direction, the corresponding frequency response is anisotropic under differently polarized wave incidences, as shown in Fig. 2. The amplitude, especially at resonance frequencies, is somewhat less than unity for two different polarizations because of absorption, which can be easily understood using the equivalent circuit method [35]. This is actually desired from the bi-static scattering reduction point of view and has been discussed in our previous work [36].
Fig. 2 (a) Reflection amplitude and (b) phase of the anisotropic unit cell under x- and y-polarized wave incidences.
Because of the anisotropic configuration of the unit cell, the phase difference can be produced by rotating the unit cell with an angle of 90° as another kind of elementary cells. In this case, the phase difference between the two kinds of unit cells can be calculated from the simulated results in Fig. 2(b) and is presented in Fig. 3(a). The phase difference from 8.2 to 18 GHz is located in the range of 180° ± 37°, as denoted by the gray area in Fig. 3(a), and thus the reflection reduction larger than 10 dB can be approximately achieved (without considering the amplitude influence here) in this frequency band [29–33]. Also, three reflection reduction peaks are expected to be gained at 8.5, 12.2 and 16.7 GHz where the phase differences are close to 180°. Note that the absorption also results in the reflection reduction for the designed HMS, but the attribution is relative small, not exceeding that of the structures comprising by the single unit cells alone, as shown in Fig. 2(a).
Fig. 3 (a) Phase difference between the two kinds of unit cells. The values were calculated from the reflection phases shown in Fig. 2 (b) and the gray filling area defines the range of 180° ± 37°. (b) Pattern of the cut wires in the designed HMS. The top square patch array is not shown for conciseness and the axis defines polarization of the incident wave.
The reduction level near the backscattering direction is determined by the phase differences from different kinds of unit cells, but the power distribution in the spatial angle domain is of dependence of the arrangement of the elementary cells, and can be accurately manipulating with the concept of coding technique [37–39]. From the point of view of the low scattering in the whole angle domain without a strong peak in some direction, the diffuse reflection is desired here. To achieve this goal, we have used a fast design method [40]. The finally designed pattern of the cut wires is shown in Fig. 3(b), where the top square patch array is not shown for conciseness. The HMS is composed of 8 × 8 super-cells and each kind of super-cells is constructed by 4 × 4 elementary cells.
3. Results and discussion
The simulated reflection reduction spectra of the HMS under normal incidence with different polarization angles are shown in Fig. 4(a). The polarization angle is defined by the axis direction in Fig. 3(b). It can be observed that the reduction spectra agree well with each other except for slight reduction level differences at some frequencies, showing a polarization-insensitive performance under normal incidence. Additionally, one can further found that the reflection reduction larger than 10 dB is achieved in the frequency bands of 8.4-17.2 GHz, 8.4-17.0 GHz, 8.4-16.9 GHz, 8.4-16.9 GHz and 8.5-17 GHz for different polarization angles (φ = 0°, 20°, 45°, 70° and 90°), respectively. This is close to the approximately predicted bandwidth of 8.2-18 GHz in section 2. In addition, three distinct reduction peaks in the reflection spectra can be observed as expected.
Fig. 4 Simulated reflection reduction spectra of the HMS under (a) normal incidence with the polarization angles of 0°, 20°, 45°, 70° and 90°, and (b) TE and (c) TM waves with the incident angles of 0°, 15°, 30° and 45° in the x-z plane.
The reflection reduction performance of the HMS under different incident angles is also simulated. The simulated results for TE and TM waves with the incident angles of 0°, 15°, 30° and 45° are presented in Figs. 4(b) and 4(c), respectively. For the sake of simplicity, the incident plane is selected in the x-z plane. It can be found that the reflection reduction performances are varied as the incident angle increase for both TE and TM waves, but the variation for TE wave is more obvious than that of TM wave. For example, the reflection under TE wave incidence has been larger than −10 dB from 9.1 to 11.9 GHz when the incident angle increases to 45°, while the reflection less than −10 dB can be achieved all the same in a broad frequency band for TM wave.
To further illustrate the scattering reduction performance of the HMS, the three-dimensional scattering patterns at 9, 13 and 16 GHz have been monitored for the polarization angle of φ = 0° as examples and are presented in Figs. 5(a), 5(b) and 5(c), respectively. It can be found that the scattered field is uniformly distributed in the space angular domain, forming the diffuse reflection. Figures 5(d), 5(e) and 5(f) clearly indicate that the incident power is redirected in other directions and thus the backscattering is significantly reduced.
Fig. 5 Three-dimensional scattering patterns (a, b, c) and RCS in the x-z plane (d, e, f) at 9, 13 and 16 GHz.
To confirm the predicted reflection reduction performance of the HMS, one prototype with a size of 192 × 192 mm2 has been fabricated using the printed circuit board technique. The dielectric layers are FR4 boards and the metal is copper films with a thickness of 0.017 mm. The aperiodic cut-wire and periodic square patch arrays etched on the FR4 boards were fabricated independently, as shown in Figs. 6(a) and 6(b), respectively. The prototype was then obtained by sticking them together.
Fig. 6 Photographs of the fabricated (a) cut-wire and (b) square patch array layers. Inset is a larger version of the square patch array.
The experimental measurement of reflection properties at microwave frequencies for the prototype was performed using the free space method in a microwave anechoic chamber. The measurement system is based on an Agilent 8720ET network analyzer with a pair of broadband horn antennas working in the frequency range of 8-18 GHz. In the measurement, the reflection from a metal plate with the same size as the prototype is first measured for the sake of normalization. The reflection spectra under differently polarized wave incidences were measured by rotating the prototype direction with respect to the antennas. The measured results under normal incidence with the polarization angles φ = 0° and 90° are shown in Figs. 7(a) and 7(b), respectively, and compared with the simulated reduction. One can see that the measured results show an acceptable reproduction of the broadband reduction feature from the simulations for both polarizations, except that the reduction bands slight shift towards high frequencies. This discrepancy is mainly due to the permittivity difference between the simulated and the actual values as well as the fabrication tolerance.
Fig. 7 Measured reflection reduction spectra of the HMS under normal incidence with the polarization angles of (a) 0° and (b) 90°. The simulated results are also plotted to provide a comparison.
The emissivity of the HMS is difficult to be obtained by the simulation due to the high requirement for the hardware, so we only give an experimental discussion here. According to the law of Kirchhoff, the emissivity equals to the absorption rate when the material is at the condition of thermal equilibrium. Therefore, the infrared emissivity could be obtained by measuring the reflection spectra of the prototype. The reflection measurement in the infrared band of 3-14 μm was performed using Vertex 70. The emissivity was then calculated from the measured reflectivity and is shown in Fig. 8. In the measurement, the reflection spectra were measured from three arbitrarily different regions. As expected, the emission rates of the three different regions are all less than 0.27 in the whole band of 3-14 μm, showing a low emission behavior. The emissivity can be also estimated by an empirical formula ε = εmfm + εd(1-fm) [26,27], where ε is the emissivity, f is the filling ration of metal in the top array, and the subscripts m and d denote the metal and the dielectric substrate, respectively. The emissivity of copper is about 0.09 and 0.955 for the FR4 board [26]. Based on these values, the estimated emissivity of the HMS is about 0.25, much close to the measured results.
The emissivity can be further lowered by increasing the filling ration of metal in the top layer of the HMS. Nevertheless, this may be not the optimal solution for reducing the radiative intensity. The low emission reduces heat loss and thus results in temperature rising under the conditions of weak heat conduction and convection. As a consequence, we further propose the advanced HMSs with the wavelength or angle selective emission performance in the infrared band. It is believed that this can be achieved by integrating with the infrared metamaterial absorbers [41–47] or multilayer films [48,49].
4. Conclusion
In summary, a HMS has been designed to reduce the reflection at microwave frequencies and the infrared emission simultaneously. The HMS is constructed by stacking two specially designed arrays and the overall thickness is about 2.2 mm. The reflection reduction in the microwave band results from the phase cancellation instead of absorption, while the infrared emission reduction is due to the high reflection of the top metal layer. A prototype has been fabricated using the print circuit board technique. The broadband reduction performance with the efficiency larger than 10 dB has been well verified by the simulated and experimental results. Also, the HMS displays a low emission performance with the emissivity less than 0.27 in the band from 3 to 14 μm. These results make one believe that our study provides an alternative effective way to achieve the multispectral stealth.
Funding
National Natural Science Foundation of China (61501497, 61510502, 61501503 and 61331002).
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