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Abstract
In this paper, a metamaterial structure with radar and infrared (IR) compatible stealth characteristics is designed based on the principle of plasmonic absorbing structure (PAS). Due to the lack of reports on PAS-based IR radar compatible stealth, this article combines PAS and IR frequency selective surfaces to achieve the desired purpose. Through mathematical modeling and dispersion engineering of the unit cell proposed, a PAS with ultra-wideband wave absorption is realized. The low emissivity of the IR atmospheric window band is realized by means of the simulation and analysis of the IR frequency selective surface with different indium tin oxide (ITO) occupation ratios. The absorptivity of designed structure is higher than 90% from 4GHz to 28.6GHz, and the emissivity of the IR atmospheric window is only 0.3. The experience of the fabricated sample is consistent with the theoretical analysis and the simulation. Our method enriches the implementation strategies of radar-IR compatibility and has reference significance for multi-spectrum compatible stealth.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The traditional radar-IR stealth-compatible strategy is to use chemical methods to make mixtures [1,2] or add infrared coatings on microwave absorbing materials [3]. After the method of designing electric and magnetic resonators to be coupled to the external electric field and magnetic field to achieve perfect wave absorption [4] was proposed, the research on how to broaden the wave absorption bandwidth has never stopped. Radar-IR stealth-compatible has been greatly developed with the help of the booming development of metamaterial absorbers (MAs). Adding an infrared shielding layer (IRSL) in front of the radar absorption layer (RAL) can simultaneously achieve low infrared emission and microwave absorption in a wide frequency band. The working principle of the radar absorbing layer is mainly the bandwidth expansion based on the single frequency point resonance [5–10]. In addition, replacing the dielectric substrate with a magnetic material can also broaden the operating frequency band [11–13]. Superimposing multiple resonators in the incident direction of electromagnetic waves can achieve the combination of absorbing frequency bands [14–20]. In addition to the above principles, the use of the diffuse reflection of the metasurface [21–23], the destructive interference of electromagnetic waves [24], and the use of a physical layer which can realize the functions of IRSL and RAL [25] can all achieve the purpose of IR-radar compatible-stealth.
Recently, PAS can achieve complete control of dispersion [26]. The plasmon mentioned in this article is a surface wave mode excited on the surface of air and artificial structures in the microwave band [27]. Compared with the planar metamaterial microwave absorber [21,22], PAS has the advantages of achieving customized absorption bandwidth and ultra-wideband absorption of low frequency coverage. Besides, it has the advantages of high efficiency, light weight, easy manufacturing, and flexible design [28–34]. In the microwave band, the energy of incident electromagnetic waves can be efficiently constrained and consumed at the metal-medium interface by artificially designing sub-wavelength deep structures. Up to now, there have been large quantity of researches on PAS and many impressive results have been achieved. Metal strips in PAS are completely horizontal [35], bended in three-dimensional space [36], and PAS is placed on a resistive frequency selective surface [37] and magnetic absorbing film [38], which can broaden the absorbing bandwidth of PAS. However, PAS-based infrared radar compatibility has not been reported.
In this paper, a radar-IR compatible metamaterial based on PAS has been proposed, as shown in Fig. 1. In terms of microwave, we firstly designed a general model of unit cells and gave their mathematical expressions. The dispersion relationship of unit cells of different lengths is used to help design a structure that meets the required absorption bandwidth. Combining the obtained unit cells together can obtain an ultra-wideband absorbing structure that is polarization-independent covering low frequencies from 4GHz to 28.6GHz. The absorbing mechanism was analyzed and discussed. In terms of infrared, we designed a high duty cycle frequency selective surface that can transmit microwaves and reflect infrared. The emissivity of the infrared atmospheric window measured experimentally is 0.3. The combination of frequency selective surface and PAS effectively achieves the purpose of radar-IR compatibility. The test results of processed samples show the consistency of theory and experiment.
Fig. 1. (a) Functional structure diagram of the overall structure. (b) IRSL (The upper layer of the structure). (c) RAL (The lower part of the structure).
2. Design and methods
Figure 2(a) displays the structure of a metal wire plated on the dielectric. The metal in the model is copper, and the conductivity is 5.8×10^7 S/m. The permittivity of the dielectric used is 4.3(1-j0.025). The height of metal wire and dielectric are $h\; $and $w $, respectively. Twenty-nine such structures with different lengths compose the schematic shown in Fig. 2(b). The modeling is carried out in the commercial software CST. The selected solver is the Time Domain Solver. The boundary conditions are electromagnetic boundaries. The incident wave is along the -z-direction, the y-direction is the electrical boundary (Et=0), the x-direction is the magnetic boundary (Ht=0), and the z-direction is the free space (open add space). The wave source is a waveguide port. After the Cartesian coordinate system is established with the center of the bottom ITO film as the coordinate origin, the 29 structures can be expressed by mathematical formulas. The left and right parts in Fig. 2(a) are symmetrical, and the structure in Fig. 2(b) is symmetrical about the plane a.
Fig. 2. (a) The structure diagram of a unit cell after a symmetric operation. (b) The combination of the rectangular structure obtained after the symmetrical operation of unit cells and the bottom ITO backplane.
The obtained schematic is arranged as a periodic unit along the horizontal and vertical directions to obtain the functional metamaterial (Fig. 3(a)). As shown in Fig. 3(b), the length of the periodic unit is a, the width is b, the height is c, and the thickness of the dielectric is d/2.
After the unit cells described in Table 1 are symmetrical twice, the entire structure can be obtained. When λ takes different values, unit cells of different lengths are obtained, and their dispersion relationship is shown in Fig. 4(a). The reason for the cutoff frequency is that electromagnetic waves will be strongly absorbed at this frequency. When the following parameters are given: a = b=10mm, c=10mm, d=0.8mm, k=0.3mm, w=0.3mm, h=0.2mm, simulations are carried out. The results obtained are shown in Fig. 4(b). The results show that there is a strong absorption between 4GHz and 28.6GHz, and the absorption efficiency is above 90%. It is worth mentioning that the results under the polarization state of TE and TM are exactly the same, which shows that the structure has the characteristics of polarization insensitivity.
Fig. 4. (a) Dispersion relationship of different length metal wires. (b) The absorption curve of designed RAL (TE and TM polarization are the same).
Table 1. The mathmatical description of unit cells
In order to analyze the in-depth mechanism of microwave absorption by the schematic, we added monitors for different frequency points in the simulation process. The surface current distribution diagrams of RAL at 5GHz, 10GHz, 15GHz, 20GHz and 25GHz are shown in Fig. 5(a)-(e). The distribution of surface current is mainly on the single metal strip at low frequencies such as 5GHz and 10GHz, and the induced current generated at this time is relatively large. As the frequency increases, the distribution of surface current is no longer limited to a certain metal strip. The metal strips plated on the entire structure will respond to incident electromagnetic waves from the outside to generate induced currents. Due to the large distribution range of the induced current, its value will drop slightly at this time as shown in Fig. 5(c)-(e). The induced current generated will be consumed on the metal strip with resistance. Figure 5(f)-(j) show the distributions of energy loss at different frequencies. At low frequencies such as 5GHz and 10GHz, only 4 resonance points are excited. The area where the dielectric loses energy is also limited to the vicinity of the resonance point. The position of the resonance point moves toward the shorter metal line at 10 GHz. When the frequency is 15GHz, the resonance point starts to increase. The loss area of the dielectric begins to appear beyond the resonance point. From Fig. 5(h), we can see that 6 resonance points are excited. The number of resonance points is further increased, and the loss area on the dielectric is also wider at 20GHz. Resonance occurs at the location of almost all lengths of metal strips, and most areas of the dielectric are dissipating energy at 25GHz. The comprehensive analysis concludes that the designed structure absorbs the incident electromagnetic energy through the resonance between the metal strip and the dielectric.
Fig. 5. Surface current of RAL at (a) 5 GHz, (b) 10 GHz, (c) 15 GHz, (d) 20 GHz, (e) 25 GHz. Distribution graph of energy loss at (f) 5 GHz, (g) 10 GHz, (h) 15 GHz, (i) 20 GHz, (j) 25 GHz.
When the size of the ITO patched on the IRSL changes, the resulting reflection curve is shown in Fig. 6(a). It can be concluded from the curve that the smaller e is, the stronger the microwave transmission capability of the frequency selection surface is, and the duty cycle of ITO will also become smaller, leading to an increase in infrared emissivity. Through comprehensive consideration, the value of e is determined to be 0.45mm. The gap between adjacent ITO squares is 0.1mm. The value of the surface resistance of ITO film is 6Ω/sq. We choose polyethylene terephthalate (PET) as the substrate of ITO. The simulation results show that the thinner the PET, the better the microwave transmission effect on the surface, as shown in Fig. 6(c). Finally, it is determined that the size of the ITO patched on the infrared frequency selective surface is 0.45mm, and the thickness of the PET substrate is 0.05mm. The frequency selective surface was added to PAS for simulation and was compared with the result without IRSL, as shown in Fig. 6(d). There is little difference between the two, and the absorption rate in the frequency band of interest is still above 90%
Fig. 6. (a) ITO dimensioning on IRSL. (b) The reflectivity of the structure to microwaves when e takes different values. (c) Reflection curve when the thickness of PET changes. (d) Absorption rate comparison chart with or without IRSL.
3. Experimental validation
In order to verify the absorption performance of the metamaterial in the microwave frequency band, we tested the processed samples in the microwave anechoic chamber, as shown in Fig. 7(d). The size of the sample is 360mm*360mm, as shown in Fig. 7(b). Firstly, we use numerical control technology to cut out 66 long rectangular bases made of Fr4. On each long rectangular substrate, 32 notches are cut to form grooves for splicing into a solid structure. A reflective backplane of ITO is placed under the three-dimensional structure, and a frequency selection surface pasted with ITO is placed on it. Due to the wide range of the microwave tested, we used 5 pairs of antennas working in different frequency bands of 2-4, 4-8, 8-12, 12-18, 18-30GHz to conduct experiments. The experimental results are shown in Fig. 7(e), showing a high degree of consistency with theoretical simulations.
Fig. 7. (a) Metal-plated long rectangular strip. (b) Three-dimensional structure based on plasma absorption. (c) Combination of three-dimensional structure and upper and lower functional layers. (d) Microwave anechoic chamber for testing. (e) Comparison chart of absorption rate under simulated and experimental conditions.
Since the reflection of infrared occurs in the range of several microns on the surface of the material, only the upper IRSL needs to be tested when testing the emissivity. The emissivity meter and the FTIR spectrometer are utilized to test the IRSL, and the test results are shown in Fig. 8. It can be seen that the average value of infrared emissivity is 0.3, and the emissivity at each frequency point also fluctuates around 0.3.
Fig. 8. (a) Infrared emissivity measured by emissivity meter. (b) Infrared spectrum measured by FTIR spectrometer.
4. Conclusion
To sum up, it is feasible to combine PAS with broadband microwave absorption and IRSL with infrared frequency selection function. After modeling and chromatic dispersion analysis of unit cells of different lengths, an absorption of more than 90% with a bandwidth of 24.6 GHz in the range of 4 to 28.6 GHz is achieved. By simulating the reflection curves of different sizes of ITO patches and different thicknesses of PET, the best parameters are obtained to realize the function of infrared frequency selection surface. The test results of the processed sample are also consistent with the results of the analysis and the simulation. The strategy we proposed has reference value in terms of polarization insensitivity, multi-spectrum compatibility, and stealth technology, etc.
Funding
National Natural Science Foundation of China (12004437); Natural Science Basic Research Program of Shaanxi Province (2020JQ-471).
Disclosures
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Compatible stealth design of infrared and radar based on plasmonic absorption structure”.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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