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Abstract
Optically transparent metamaterials with the performance of infrared radar compatible stealth have been designed and manufactured on the basis of the continuous in-depth research on single-band stealth technology. In this paper, metamaterials are designed through theoretical calculations and modeling simulations. The designed structure can achieve higher than 90% broadband (8.7-32 GHz) absorption at wide-angle (45 degrees), emissivity of 0.3 in infrared atmospheric window, and optical transparency. In addition, the material can be bent, which greatly expands its application scenarios. The experimental results are consistent with the theoretical calculation and simulation results.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterials are favored by many researchers because of their powerful ability to manipulate electromagnetic waves. Research on how metamaterials respond to electromagnetic waves in microwave [1,2], visible light [2–4], terahertz [5], and infrared [3,6] frequency bands has been ongoing. Due to the application of the indium tin oxide (ITO), microwave absorbers using metamaterials can exhibit the property of optical transparency [7–10]. It is possible to realize infrared stealth in the infrared band with the help of metamaterials [11–13]. In addition, the joint control of infrared and visible light [14] and the adjustment of infrared characteristics at high temperatures [15] can also be achieved. The combination of radar stealth and infrared stealth realized by the principle of microwave absorption in the military is both a difficult point and a hot spot. The traditional methods [16–19] bring about low infrared emissivity while significantly deteriorating the microwave absorption performance, and the impedance of the material does not match the free space.
By adding a frequency selective surface (FSS) in front of the microwave absorber, the shortcomings of traditional methods can be overcome while maintaining the compatibility of infrared and radar. A lot of research on the basic of the above has been completed [20–24]. It is worth mentioning that when transparent materials are used, infrared radar compatible absorbing materials with high optical transmittance can be obtained [25–27]. Since none of the above-mentioned materials are flexible, these metamaterials with sub-wavelength structures will be limited in application. The research and design of transparent infrared radar metamaterials with flexible properties are put on the agenda. Based on the realization of bendable infrared stealth [28] and bendable radar stealth [29,30], the flexible transparent infrared radar stealth target [30] is also realized. There is no doubt that when it has the characteristic of flexibility, metamaterials can be used where conformal is required. In Ref. [31]., the achievable effect is optical transparency, an absorption higher than 90% in the frequency range of 7.7 to 18 GHz with a 40-degree variation of the incident angle, and a low emissivity of 0.23 under the infrared atmospheric window.
In this paper, transparent polyethylene terephthalate (PET) and indium-tin-oxide (ITO) materials are used to design and fabricate a flexible metasurface, which is composed of (infrared shielding layer) IRSL and (radar absorbing layer) RAL, with broadband, wide-angle microwave absorption and low infrared emissivity (Fig. 1). For the infrared band, theoretical analysis and calculation methods are adopted to obtain the designed emissivity of the functional layer; for the microwave band, the CST software is used for simulation and optimization to select the structural parameters corresponding to the best results. Effects attained are an absorption higher than 90% in the range of 8.7-32 GHz and an emissivity of 0.3 in the infrared atmospheric window. When the incident angle is changed within the range of 45 degrees, the average absorption rate is still as high as 90%. The experimental results of microwave and infrared are consistent with the theoretical calculation and simulation. The measurement results of optical transparency and flexibility also correspond to the predicted results.
Fig. 1. Schematic diagram of achieving microwave absorption, low infrared emission and optical transparency.
2. Design and methods
In order to achieve infrared radar compatible stealth, IRSL and RAL are designed to overcome the contradiction between infrared stealth and microwave stealth. The design goal of IRSL is to transmit microwaves and reflect infrared back to the incident space. The RAL is responsible for absorbing the microwaves that pass through the IRSL. By designing the area and shape of the ITO patched on the PET, the efficient transmission of microwaves and the perfect reflection of infrared can be achieved concurrently. The electromagnetic characteristics of ITO in the infrared band can be described by Drude mode l [31]:
Among them, the values of ${\varepsilon _b}$, ${\omega _p}$, ${\omega _c}$ are 3.9, 461THz, 28.7THz. The metal-like properties of ITO can produce the phenomenon of low infrared emission similar to metal. Another advantage is that the transparency of ITO determines that the metamaterial structure designed with it can be used in scenarios that require optical transparency. Surface structure composed of materials with different emissivity, its emissivity can be calculated by the following formula [31]:
Among them, $\varepsilon$ is the emissivity of the material, $f$ is the occupation ratio of the material. Therefore, millimeter-scale etching of the low-resistance ITO board can allow microwaves to pass through and maintain the characteristic of low infrared emission. The structure is periodically arranged after plating ITO on PET with a side length of 0.5. According to formula (2), the emissivity of the overall structure is about 0.3 (The emissivity of ITO and PET are respectively 0.1, 0.9). Infrared reflection occurs at a few microns on the surface of materials, thus the low emission of the entire structure can be achieved by controlling the infrared emissivity of IRSL. The structure designed in this paper is shown in Fig. 2. The thickness of IRSL is dIRpet, and the thickness of RAL is dpet, as illustrated in Fig. 2(a). Figure 2(b)(c) shows the front view of IRSL and RAL respectively, in which the structural parameters of ITO are clearly marked on it. Through optimization calculation, the best parameters obtained are dIRpet = 1.5 mm, dpet=2 mm, p=8 mm, e=0.45 mm, c=0.5 mm, r1=0.5 mm, r2=3.3 mm, r3=3.5 mm, r4=4 mm.
Fig. 2. (a) Perspective view of the absorbing structure. (b) Front view of IRSL. (c) Front view of RAL.
Figure 3(a) shows the absorption, transmission and reflection diagrams of the structure obtained by periodically repeating the above unit structure in the horizontal and vertical directions. It can be seen that the absorption rate in the range of 8.7-32 GHz is above 90%, and the absorption bandwidth is 23.3 GHz. The structure has the same absorption of different polarized waves, as displayed in Fig. 3(b). Figure 3(c)-(d) is the absorption rate at different angles in TE and TM polarization modes. The absorption effect becomes inferior and the bandwidth is narrowed as the angle of incidence increases in the case of TE polarization incidence; the absorption effect does not change significantly with the increase of incident angle, but the bandwidth is widened when TM polarized wave is incident. This enlightens that the absorption effect of this structure on TM polarized waves is better than that of TE polarized waves. The above-mentioned proves the periodic structure designed has polarization insensitivity and large angle incidence stability.
Fig. 3. (a) Absorption, reflection and transmission diagrams at normal incidence. (b) The absorption pattern of TE and TM polarized incidence. The absorption graphs of (c) TE and (d) TM polarization at 0°, 15°, 30°, and 45° incident angles.
To understand how the two functional layers of IRSL and RAL take effect to achieve broadband and efficient microwave absorption, we modeled and simulated in the software. The resulting curve is shown in Fig. 4. For IRSL, the transmission rate in the 5-35 GHz frequency band is over 80%, which proves IRSL has reached the predetermined requirements. Without IRSL, the absorption effect of RAL alone is not satisfactory. Compared with the absorptance curve in Fig. 3(a), it can be seen that the IRSL designed in this structure not only has the effect of reducing the infrared emissivity, but also produces a stronger absorption effect under the combined action of the RAL. When the FSS is removed, the absorption bandwidth of this structure does not narrow, but the absorption effect becomes worse, as shown in Fig. 4(c). Figure 4(d) displays the performance after removing the reflective backplane of the structure. It can be seen that there is a significant decline. The designed structure stacks the original different functions together, so that the performance of each functional layer has been improved on the basis of keeping the original ability unchanged.
Fig. 4. Absorption graph of (a) IRSL, (b) RAL, and (c)structure without the top layer ITO simulated in the software. (d) Absorption reflection and transmission when there is no bottom reflective backplane.
In order to further explore how the structure's absorbing principle is achieved, we monitored the electric field, magnetic field and surface current at 11.72 GHz and 27.44 GHz, expecting to give a reasonable explanation by observing their distribution. Figure 5 displays the electric field, magnetic field and surface current distribution at the two representative wave-absorbing frequency points. Figure 5(a)(b) shows the electric field distribution of IRSL and RAL at 11.72 GHz; (c)(d) is at 27.44 GHz. Strong electrical resonance occurred in both IRSL and RAL layers. The strong resonance occurs at the etched part of the ITO FSS, which is the interface where ITO and PET contact. Figure 5 (e) and (h) are the magnetic field distribution diagrams at 11.72 GHz and 27.44 GHz, respectively. When the frequency point changes, the position of the magnetic field in the upper and lower layers of PET will change. It proves that PET of the whole structure can consume microwave in the form of magnetic resonance. Figure 5(f)(g) and (i)(j) are the surface current distribution diagrams at two frequency points. As the phase changes, the current peak value will alternate between the upper and bottom layers of RAL at 11.72 GHz; when the frequency is 27.44 GHz, the current peak value will alternate between the upper surface of IRSL and RAL. Furthermore, the ITO coated upper surface of IRSL and RAL and the reflective backplane will generate induced current, which is converted into heat energy in the form of Ohm's law (Ploss=I2R) and consumed. After the above analysis, it can be concluded that every part of the designed structure is helpful for wave absorption. The enhanced current and the ohmic loss on the ITO surface, the dielectric loss of PET are all consuming the incident microwave, which creates an ultra-wideband absorption phenomenon.
Fig. 5. Perspective view of (a) IRSL and (b) RAL electric field at 11.72 GHz. (c)(d) Perspective view of electric field at 27.44 GHz. The magnetic field distribution diagram at (e) 11.72 GHz and (h) 27.44 GHz. Surface current distribution graphs at (f)(g) 11.72 GHz and (i)(j) 27.44 GHz.
3. Experimental demonstration
In order to confirm whether the experimental result is in accordance with the simulation, we processed the sample and tested its absorbing performance. Figure 6(a) is a schematic diagram of the sample being tested in a microwave anechoic chamber. Due to the limitation of processing technology, IRSL and RAL are pasted by optically clear adhesive (OCA), which will have a certain impact on the microwave absorption effect and frequency band. Test result is shown in Fig. 6(b), from which we can see that the experimental data is basically consistent with the theoretical prediction.
Fig. 6. (a) Schematic diagram of the experiment in the microwave anechoic chamber. (b) Comparison chart of simulation and test results in the software.
The average emissivity of the sample in the infrared atmosphere window is gauged by the TSS-5X IR Emissivity meter. As shown in Fig. 7(a), it is approximately 0.3. Furthermore, the spectral response of the infrared atmospheric window is precisely represented by the FTIR spectrometer, as shown in Fig. 7(b). The results measured by the two apparatus are consistent with the theoretical calculation.
Fig. 7. (a) Physical image when measuring emissivity with emissivity meter. (b) Spectra measured by infrared spectrometer.
As expected, the fabricated sample is transparent. Due to the influence of ITO during the etching process, the transmittance of the whole structure will decrease to a certain extent, which has a great relationship with the processing technology. Figure 8(a) shows the transparency of the sample under naked eyes, and Fig. 8(b) exactly expresses the visible spectrum transmittance of the sample. It is worth mentioning that the metamaterial designed is flexible, which means it can be bent to adapt to more application scenarios.
4. Conclusion
In this paper, theoretical calculations and simulation are carried out for the objective of infrared radar compatible transparent materials. The structure designed in this paper can achieve higher than 90% absorption in the 8.7-32 GHz. When the incident angle changes within ±45°, the absorption effect is still considerable. The mechanism of absorbing wave is analyzed. In addition, the infrared emissivity is only 0.3. It is also transparent in the optical band. Flexible properties greatly broaden its scopes of application. The measured results of microwave absorption are consistent with the software simulation and theoretical calculation. The work of this paper can be used for reference to broaden the application scenarios of transparent materials with infrared radar compatible stealth characteristics.
Funding
Natural Science Foundation of Shanxi Province (2019JZ-40, 2020JQ-471); National Natural Science Foundation of China (12004437, 21471159).
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, “Ultra-wideband flexible transparent metamaterial with wide-angle microwave absorption and low infrared emissivity”.
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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