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Abstract
We propose a radar-infrared bi-stealth rasorber that not only provides broad microwave absorptivity and low infrared emissivity but also possesses a microwave transmission window at low frequency. It is composed of three functional layers, which are carefully designed to independently control the infrared emission, microwave absorption, and transmission, respectively. The structure exhibits broadband (8.1–19.3 GHz) and high-efficiency (>90%) absorption. A transmission window appears at low frequency with a transmission peak of 80% at 2.68 GHz. The thermal emissivity of the structure is about 0.27 in the atmosphere window, which is close to that of metal. Moreover, the total thickness of the proposed structure is only 3.713 mm. The low-infrared-emissivity, high-microwave-absorption and frequency-selective-transmission properties promise it will find potential applications in various stealth fields.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metasurfaces, composed of artificial periodical unit cells, have attracted considerable attentions due to their excellent performances in manipulating electromagnetic (EM) waves. Various functional devices based on metasurface have been proposed to realize absorbers [1–3], rasorbers [4–6], multi-spectrum stealth structures [7–9]. Flexibility and transparency are then integrated in those absorbers by using flexible material [10] or indium tin oxide (ITO) [11–13]. However, once the structure was fabricated, the properties of them cannot be changed anymore. In terahertz regime, switchable absorbers were realized by adopting tunable elements into the design such as vanadium oxide (VO2) [14], or graphene [15], whose conductivities can be tuned by thermal, optical or chemical potential excitation. Generally, the tunable absorbers in microwave regime are achieved by combining PIN diodes [16] into the structure, or injecting liquid water [17,18] in the design. However, in some real applications, such as the radome of stealth aircraft, the signal communication property is disabled for these structures.
Frequency selective rasorber (FSR), combining the advantages of absorber and frequency selective surface (FSS), which can transmit EM waves in specific frequency range with good out-of-band absorption performance, has been reported in the literature [19–21]. The rasorbers typically cascade one lossy layer above the lossless FSS layer, where the top layer is used for broadband absorption and the bottom for narrow-/wideband transmission. Lumped resistors [22–24] or resistive sheets [5,21] are typically used to realize broadband absorption. Narrowband transmission can be obtained by a square [25,26] or a slot [27] geometry. By using multilayer FSS, wideband transmission can be achieved with a considerable thickness [28–30]. Thus the rasorber, being a combination of absorption and band-pass property, can be divided into three types according to the relative transmission window with respect to the absorption band: rasorbers having transmission band at higher frequency [31], rasorbers having transmission band at lower frequency [32], and rasorbers having transmission band in the middle of absorption band [33]. However, All the FSR mentioned above are not going to work in radar and infrared simultaneously to meet the needs of the modern complex military applications.
Recently, several metasurface structures have been proposed to achieve radar-infrared bi-stealth [34–39], which however cannot meet the application with the communication requirement. Here, we propose a radar-infrared bi-stealth rasorber with frequency-selective-transmission, high-microwave-absorption, and low-infrared-emissivity, simultaneously. It exhibits a strong absorptivity over 90% from 8.1 to 19.3 GHz and appears a transmission window at 2.68 GHz with a transmission efficiency of 80%. The IR emissivity can achieve 0.27 in the region from 8 to 14 μm. Simulation results accord well with the experimental results. It is worth mentioning that the total thickness of the proposed structure is only 3.713 mm. It is noted that Prof. Ma’s group has demonstrated a transparent transmission-selective radar-infrared bi-stealth structure with a low thermal emissivity of 0.52 and a thickness of 33 mm [40].
2. Modeling, simulation, and discussion
Our aim is to design a multifunctional structure that not only provides broad microwave absorptivity and low infrared emissivity but also possesses a microwave transmission window at low frequency. To achieve this complex purpose, three functional layers are carefully designed, including an infrared shielding layer (IRSL), a radar absorption layer (RAL) and a frequency selective transmission layer (FSTL). Specifically, the IRSL is designed with low infrared emissivity, which is responsible for infrared stealth. The RAL is designed with broadband microwave absorption, which is responsible for radar stealth. The FSTL possesses a transmission window at microwave frequency, which is responsible for communication.
In order to achieve radar and infrared bi-stealth with a microwave transmission window, the key factor is to design the IRSL with high microwave transmission. As we know, a periodic metal patch could be regarded as a low-pass filter, which transmits waves below its resonant frequency. Generally, a lower IR emissivity can be obtained with a higher filling rate of metal patch. When the filling rate of metal patch is 100%, a continuous metal sheet is formed with the lowest IR emissivity. However, the continuous metal sheet will strongly reflect the radar wave. Therefore, the low IR emissivity implies a proper occupancy of metal sheet on top of the structure.
Metamaterial absorbers have been attracted great attention in the past decade [1–3], which usually have a metal back plate, thus resulting in a complete reflection. To achieve broadband absorption and possesses a microwave transmission window, the metal layer of the metamaterial absorber can be replaced by a band-pass FSS. The FSTL can transmit EM waves in a specific frequency range and act as a continuous metal sheet in the absorption band. We have to recall that the transmitting frequency is away off the resonance frequency of the RAL. Hence, the period of the FSTL should be larger than that of the RAL. However, a larger period may form a grating lobe, degrading the stability as the incident angle increases. In order to deal with this problem, we decrease the period of FSTL and equate it to that of the RAL by loading capacitors on the bottom circle loop at where the electric field is maximal.
The proposed multifunctional structure is shown in Fig. 1(a), and the unit cell is shown in Fig. 1(b). The IRSL is composed of a periodic copper patch with square size a=0.45 mm and gap width w=0.05 mm. The thickness of the copper is t=0.035 mm with a conductivity of 5.8×107 S/m. The RAL is made up of double ring resonators lumped with resistors as shown in Fig. 1(c). The corresponding structural parameters are r1 = 2.2 mm, r2=3.65 mm, w1=w2=0.6 mm, g1 = g2 = 0.5 mm, R1 = 900 Ω, and R2 = 220 Ω. A circle-loop hybrid resonator has been used to form the FSTL, where lumped capacitances are symmetrically embedded, as shown in Fig. 1(d). The optimized dimensions of the FSTL are as follows: r3=3.5 mm, w3=1 mm, w4=0.4 mm, g3=0.8 mm and C = 0.82 PF. Both the RAL and the FSTL are printed on a 0.254 mm thick dielectric substrate F4B-2, which are separated by a 3.1 mm thick PMI foam. The dielectric constant of F4B-2 is 2.65(1 − j0.002). The period of the structure is p=11 mm. The total thickness of the structure is 3.713 mm.
Fig. 1. (a) Schematic of the multifunctional structure. (b) Side view of the unit cell. (c) Top view of the radar absorption layer. (d) Top view of the transmission-type FSS.
Full-wave simulations are carried out to validate the proposed multifunctional structure. Periodic boundary conditions are set in both the x and y directions. Excitations propagate along z direction. The structure is illuminated by the TE-polarized wave with the H field along x-axis and the E field along the y-axis. The absorptivity can be calculated by A=1-|S11|2-|S21|2, where |S11|2 and |S21|2 are reflectivity and transmissivity of the structure, respectively. Figure 2(a) shows the reflectivity and transmissivity of the designed rasorber. We can see that the absorptivity is higher than 90% from 8.1 to 19.3 GHz, and there is a transmission peak of 80% at 2.68 GHz. Similar results are also obtained for TM-polarized wave at normal incidence due to the symmetry of the structure.
Fig. 2. (a) Simulated absorption and transmission spectra of the structure under normal incidence. (b) Simulated absorption when the RAL unit consists of a single inner ring with resistors, single outer ring with resistors, and double rings with resistors. (c) The transmission and absorption with the change of a. (d) The transmission with the change of c.
The mechanism of the proposed multifunctional structure can be understudied by revealing the role of the three functional layers. The RAL consists of the outer ring and inner ring, which play major roles in the absorption of lower and higher bands, respectively. As illustrated in Fig. 2(b), only inner ring or outer ring cannot achieve an efficient broadband absorption. When they are combined, an ultra-broadband absorption is obtained. In order to achieve relative low IR emissivity and high microwave transmission, the geometric size of the IRSL unit needs to be carefully designed. Figure 2(c) shows the transmission and absorption spectrum of the multifunctional structure when the patch size of the IRSL is varied. We can see that the absorption bandwidth gradually moves toward to the high frequency and the transmission peak is almost unaffected with the decrease of a. For the FSTL, the transmission peak can be tunable at will with the change of capacitance value. It is noted that the absorption is not affected when the capacitance value is changed.
To get physical insight into the absorption mechanism of our structure, the surface current and power loss density distributions at different frequencies are investigated in Fig. 3. As shown in Fig. 3(a), at the transmission frequency of 2.68 GHz, the current mainly focuses on the bottom circle loop, which results in a very small power loss and a high transmission. At the resonance frequency 10 GHz, as illustrated in Fig. 3(b), the current is enhanced on the top double rings while anti-parallel current was found to be on the bottom, thus resulting in the formation of magnetic resonance and a strong power loss. Figure 3(c) shows that the mode at 18 GHz, which behaves similarly to Fig. 3(b) except that the surface current flows in the opposite direction. Therefore, the broadband absorption originates from magnetic resonance excited by the incident wave, in which anti-parallel currents are induced between RAL and FSTL.
Fig. 3. Distributions of the surface current on the RAL (in the first row) and the FSTL (in the second row), the power loss density on the RAL (in the third row) at different resonance frequency of: (a) 2.68 GHz, (b) 10 GHz, (c) 18 GHz.
3. Experimental verification
In order to experimentally validate the performance of the proposed multifunctional structure, a 330 mm×330 mm sample with 30×30 elements was fabricated by using the standard photolithography technique. We first choose a 0.254 mm thick dielectric substrate F4B-2 covered with copper on both sides. The top surface and bottom surface are fabricated as IRSL and RAL, respectively. Then, a 0.254 mm thick dielectric substrate F4B-2 covered with copper on one side is chosen to fabricate FSTL. The two dielectric substrate are separated by a 3.1 mm thick PMI foam. Finally, they are bonded together by glue with the thickness of 0.05 mm. The measurement setup is shown in Fig. 4(a). A pair of broadband horn antennas working in the frequencies of 2–18 GHz are connected with the Agilent E8363C vector network analyzer to measure the transmission and reflection of the structure. Figures 4(b) and 4(c) show the comparison between the simulated and measured results. We can see that the simulated results and measured results are basically consistent, which validates the proposed design methodology. The difference between measurement and simulation may attribute to the parasitic effects of packaging, the effect of adhesive between layers and fabrication tolerance.
Fig. 4. (a) Schematic diagram of the measurement setup. (b) Comparison between simulated and measured S11 of the proposed rasorber. (c)The simulated and measured S21 of the proposed rasorber.
We next examine the infrared stealth performance of the proposed structure. The surface emissivity of the IRSL can be evaluated by an empirical formula [34,35]
where $\varepsilon $ is the IR emissivity of the IRSL, ${\varepsilon _m}$ (less than 0.1) is the IR emissivity of copper metal, ${\varepsilon _\textrm{s}}$ (less than 0.9) is the IR emissivity of substrate F4B-2, and f is the area percentage of the metal part. With f=81%, the IR emissivity of the IRSL is evaluated less than 0.27, which is much lower than the emissivity of the substrate (generally larger than 0.8). To experimentally validate the design, we measured the thermal images of the sample and reference. Both sample and reference are obtained by cutting two 66 × 33 mm2 area from the original IRSL. When the top surface is IRSL (which means that the bottom surface is RAL), sample is obtained. When the top surface is RAL (which means that the bottom surface is IRSL), reference is obtained. Figure 5(a) shows the experimental setup of the infrared measurement. Sample and reference are placed on a hot plate, and an IR camera (FLIR T1050sc) operating at 8-14 µm are utilized to capture the thermal images. When the hot plat is heated to 80°C, the thermal images of reference and sample are demonstrated in Figs. 5(b) and 5(c), respectively. We can see that the measured temperature of reference and sample are 76.7 °C and 51.5 °C, respectively. The designed multifunctional structure presents good infrared stealth property.Fig. 5. (a) Schematic diagram of infrared testing device and heating plate. (b) Thermal IR images of the sample without IRSL at a heating furnace. © Thermal IR images of the structure with the IRSL.
4. Summary
In conclusion, a transmission-type radar absorbing structure with low infrared emissivity was designed, fabricated, and measured. A radar absorption layer and a frequency selective transmission layer are adopted to achieve broadband absorption efficiency higher than 90% in 8.1-19.3 GHz, as well as a transmission peak of 80% at 2.68 GHz. Moreover, the infrared shielding layer exhibits a low IR emissivity of 0.27 in the atmosphere window of 8-14 µm. Both simulated and experimental results show that our proposal has potential application for multispectral stealth and communication integration technology.
Funding
National Natural Science Foundation of China (51972046, 52021001).
Disclosures
The authors declare no conflicts of interest.
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