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Abstract
This work proposed and demonstrated a bi-functional metamaterial to implement the multispectral camouflage in infrared and microwave bands. Aiming at integrating broadband, wide-angle and low infrared emissivity into one structure, the bi-functional structure is made up of three metasurface layers with different functions. Specifically, a metasurface superstrate based on hexagonal metallic patch was deployed to achieve a low infrared emissivity and a high transmittance of microwave simultaneously. In the framework of equivalent circuit model, the bi-functional structure was designed and optimized. A dielectric transition layer was introduced into the structure to obtain better microwave absorption performances. A sample of such structure was prepared based on optimized geometric parameters and tested. The simulated and measured results indicate that the novel hexagonal patch metasurface superstrate significantly reduces infrared emissivity and the measured emissivity of the structure is about 0.144 in 8-14µm infrared band. Meanwhile, the multilayered structure has a broadband absorption band from 2.32 GHz to 24.8 GHz with 7 mm thickness and is equipped with good angular stability under oblique incidence. In general, the method and specific design proposed in this work will benefit utilizing metasurface to implement bi-functional microwave and infrared camouflage materials with outstanding performances, which are promising for extensive applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid development of electronics and information technology, multispectral complementary detection technologies, which combine detectors based on different working principles and electromagnetic (EM) wave bands, are fast-growing over past decades [1,2]. Deploying multispectral camouflage materials is an efficient strategy against multispectral detection technologies [3,4]. Therefore, in recent years, extensive and increasing researches have focused on such materials, which can manipulate EM wave over two bands, for example, microwave and infrared bands [5,6], microwave and visible light bands [7,8], microwave, infrared and visible light bands [9–13], and so on [14].
Since detectors working in microwave and infrared bands are very common and powerful at present, multispectral materials for simultaneous microwave and infrared camouflage have attracted the most attention [15,16]. According to the Stefan–Boltzmann law, one of effective strategies to reduce the probability of detection by infrared detector is deploying low infrared emissivity (IE) materials [17]. As known, in the infrared band, high reflectivity means low emissivity for non-transparent materials due to the Kirchhoff's law of thermal radiation [17] . Therefore, metallic materials with high conductivity exhibit low IE and are often used to prepare infrared camouflage materials since they can reflect the incident infrared EM wave [18]. On the other hand, microwave camouflage materials usually require high absorptivity to reduce the echo signal [2]. However, the high conductivity of metallic materials will evoke strong reflection to microwave. Apparently, there is a conflict between requirements on properties of materials for infrared and microwave camouflage, which is difficult to be overcome by nature materials due to their intrinsic dispersion features in the microwave and infrared bands [14]. Metamaterials, as an emerging new type of artificial materials based on designed sub-wavelength elements, allow frequency selective EM responses at different bands, for example, the band-pass response at a certain band. Therefore, metamaterials can be transparent for microwave and reflect EM wave in the infrared band, which can provide a promising approach to overcome this conflict [12,19,20].
During the years, researchers have already proposed numerous metamaterials, which can integrate broadband microwave absorption [5,6,9–11,13] and low infrared emissivity [21–23] together. For example, Zhong et al proposed a microwave-infrared bi-camouflage metamaterial structure with high absorptivity from 1.5 GHz to 9 GHz and the IE of 8-14$\mathrm{\mu} \textrm{m}$ is about 0.52,which is a little bit high [11]. Aiming to achieve a low infrared emissivity, Tan et al designed a multilayered metamaterial structure with the IE of 8-14$\mathrm{\mu} \textrm{m}$ is 0.245, achieving absorptivity greater than 0.9 ranging from 5.7 GHz to 16.5 GHz [9]. Very recently, to obtain better comprehensive performances of microwave absorption and infrared emissivity, Zhang et al designed a flexible metamaterial based on a patterned graphene film. Such metamaterial can achieve the absorptivity lower than 0.9 from 1.96 GHz to 20.32 GHz and the IE of 8-14$\mathrm{\mu} \textrm{m}$ is about 0.35 with the thickness as 16.5 mm [20]. Despite those significant progress regarding to microwave and infrared bi-camouflage metamaterials, there still exist some issues. Meanwhile, the structure also faces challenges from multi-based detection, as the EM wave may incident from different angles [24,25], the work mentioned above can maintain stable absorption performance when the EM wave incident angle up to 45$^\circ $ at most. What’s more, Zhong and Tans’ works depend on full-wave simulation while Zhang introduced the equivalent circuit model (ECM) into the design, improving the efficiency. Several related work has adopted intelligent algorithm into the design and gain good result [19,23,26]. Therefore, an efficiently, fast and accuracy design method, a low IE and wide-angle broadband microwave absorption integrated bi-functional structure is still desired.
In this work, we proposed and demonstrated a bi-functional camouflage metamaterial with a low IE and broadband wide-angle microwave absorption based on a multilayered metasurface (MS) structure. The metamaterial was analyzed and optimized in the framework of ECM. Firstly, A MS based on metallic hexagonal patches is deployed on the top of whole structure, which has a very low IE since the filling ration of metallic patches is 83%. Such MS achieves a high transmittance in microwave band after optimizing its geometrical parameters. Then, a resistive gear patterns MS was designed to achieve broadband absorption. Additionally, we adopt magnetic material to expand the absorption band, especially in low frequency. In order to achieve better microwave absorption performance, we introduced a dielectric transition layer to improve the impedance matching of the whole camouflage structure. Finally, the sample of whole bi-functional camouflage structure was fabricated and tested. The measured results indicate the IE is about 0.144 at 8-14$\mathrm{\;\ \mu m}$ band, the microwave absorptivity is more than 0.9 from 2.32 GHz to 24.8 GHz while the thickness is only 7 mm (0.054${\lambda _L}$). Moreover, the microwave absorptivity can be maintained more than 0.8 for the incident angle of 40$^\circ $ under transverse electric (TE) and 60$^\circ $ under transverse magnetic (TM) polarizations. The simulated and measured results exhibit good agreement, validating the outstanding performances of proposed bi-functional structure.
2. Design and analyze
Figure 1 shows the schematic of the infrared and microwave bi-functional structure and its working mechanism. The whole structure is made up of an infrared shielding layer (IRSL), a transition layer (TL), a microwave absorber (MA) including a resistive microwave absorption layer (RMAL), a dielectric spacer and a magnetic microwave absorbing layer (MMAL), a metallic backboard from top to bottom. The top IRSL satisfies the requirements that both the low IE and high transmittance of the microwave. With the help of TL, the microwave transmitted into the MA and improved the impedance matching between the IRSL and the MA. Consequently, the microwave would be absorbed effectively, thus, the infrared and microwave bi-functional effect is achieved.
2.1 Design and analyze of IRSL
As mentioned above, the infrared camouflage material achieves low emissivity by reflecting EM wave in the infrared band. Meanwhile, it should be transparent for EM wave in the microwave band so that they can get into the microwave absorption structure and then be dissipated in such structure. Obviously, the infrared camouflage material needs a low-pass response to the incident EM wave in the frequency domain. As known, the metasurface based on the periodic array of sub-wavelength metallic patches can achieve a low-pass response [27–29]. Here, we deployed the hexagonal metallic patch to design the IRSL, which can exhibit high microwave transmittance while also maintains a low IE.
Figure 2(a) plots the unit cell of the IRSL, which is a hexagonal copper patch embedded on the F4B (${\varepsilon _{F4B}}$= 2.2, $\mathrm{tan\delta }$ = 0.003) substrate layer. The radius of the hexagon is r and the gap between each hexagon is g, ${h_1}$ represents the thickness of the F4B which is 0.25 mm. The IE of the proposed IRSL can be calculated by the following equation [19],
Fig. 2. (a) Unit cell of the IRSL, (b) relationship between radius of hexagonal patch and IE, (c) transmittance under different values of $\textrm{r}$.
Furthermore, we established the ECM of hexagonal copper patch, which is illustrated by Fig. 3(a). As shown, the hexagonal copper patch can be characterized by a shunted capacitor just like the square patch. However, comparing to the square patch, the hexagonal patch introduces a transition from a square to a triangular lattice as Fig. 3(b) shows. The major difference is that triangular lattice is denser than the square lattice and has a smaller equivalent period ${p_{\textrm{eff} - \textrm{hex}}}$. The distribution function of a lattice can be expressed using a function that is periodic with the lattice period, allowing for the replacement of the original period with an equivalent one. The distribution functions of square and triangular lattices can be expanded using two-dimensional Fourier transforms [31]:
According to the ECM of square patch introduced in [32], we proposed formulas for the equivalent capacitances of the hexagonal patch:
Figure 3(c) plots both the calculated and simulated transmittance results. As shown, they agree to each other very well, which verifies the validity of the ECM. Moreover, it can be found that the transmittance under TE and TM polarization is almost unanimous, which means the IRSL is polarization insensitive.
Fig. 3. (a) ECM of IRSL, (b) schematic of square and triangular lattices, (c) calculated and simulated transmittance. (d) normalized electric field distributions of the IRSL at different frequencies.
The normalized electric field distributions on the IRSL at different frequencies are presented in Fig. 3(d). As shown, at 3 GHz, the electric field mainly concentrates in the gap between each hexagonal patch and can transmit forward through the gap. The electric field concentrating in the gap becomes weak at 16 GHz. Therefore, amplitude of the transmitted EM wave through the gap will decrease, which causes a reduced transmittance as shown in Fig. 3(c). Furthermore, when the frequency increases to 60 GHz, the electric field distribution on the IRSL becomes to be uniform. Hence, most of the incident EM wave will be reflected by the IRSL and the transmittance will be very small, since the filling ratio of hexagonal metallic patch is large. Overall, the field distribution results confirm the low-pass feature of the IRSL based on array of hexagonal metallic patches, which is mentioned above.
2.2 Design and analyze of MA
Figure 4(a) shows the schematic diagram of the proposed MA, which consists of a RMAL and a MMAL from top to bottom. They are separated by the ethylene-vinyl acetate (EVA) foam with a dielectric constant of 1.07 [34]. In this work, the paraments are as follow: $p = 10\textrm{mm}$, ${h_2} = 5.25\textrm{mm}$, ${h_3} = 0.75\textrm{mm}$, ${r_1} = 8\textrm{mm}$, ${r_2} = 4.8\textrm{mm}$, $w = 1.5\textrm{mm}$, $s = 0.8\textrm{mm}$, $g = 1.2\textrm{mm}$, $Rs = \; 68\Omega /sq$, respectively. Figure 4(b) plots the EM parameters of the MMAL ranging from 1 GHz to 26.5 GHz.
Fig. 4. (a) Unit cell of the MA, (b) EM parameters of the MMAL, (c) ECM of the MA, (d) simulated and calculated absorptivity of the MA.
For the convenience of the codesign with the IRSL, we also construct the ECM of the MA, which is plotted in Fig. 4(c). As shown, the RMAL can be equivalent to a series resonance circuit with an impendence ${Z_{RMAL}}$ as follow,
where $R,\; {L_2},\; {C_2}$ represent the equivalent resistance, inductance, capacitance of the RMAL, respectively. The ABCD transmission matrix of the MA can be calculated as follows,Then, the reflection coefficient $\textrm{R}$ can be calculated as follow,
When the microwave incidents into the MA, it would be reflected, transmitted or absorbed. Considering the presence of a metal backplate, the transmittance can be neglected. Therefore, the absorptivity of the MA can be expressed as follows [35],
Figure 4(d) plots the simulated absorptivity spectrum of the MA. As shown, such MA has a good absorption performance ranging from 2.8 GHz to 20.2 GHz. Since the gear pattern do not have accuracy equivalent circuit analytic formula, the equivalent circuit paraments was fitted from the simulated absorptivity spectrum by commercial software Advanced Design System (ADS), which are as follows, $\textrm{R\; }$= 150ohm, ${\textrm{L}_2}$= 1.209nH, ${\textrm{C}_2}$= 0.095pF, respectively. Comparing the simulated and fitting results shown in Fig. 4(d), it can be found that the ECM we established can describe the EM response of the proposed MA well.
For further analysis of the working mechanisms, we investigated the power loss density distribution on the MA at different frequencies, which are shown in Fig. 5. It can be found that the power loss is mainly concentrated on the MMAL at 3 GHz, but mainly concentrated on the RMAL at 16 GHz. At 8 GHz, both the MMAL and RMAL have significant contributions to the power loss of incident microwave. Apparently, the MMAL is responsible for low frequency wave absorption while the resistive gear patterns MS is responsible for high frequency wave absorption. Both the MMAL and RMAL are helpful for the middle frequency wave absorption.
Fig. 5. Power loss density distribution of proposed MA structure at ${f_1} = 3GHz$, ${f_2} = 8GHz,{f_3} = 16GHz.$
2.3 Design and analyze of the TL
The proposed IRSL and MA was combined together to construct the bi-functional metamaterial, which is shown in Fig. 6(a). In order to achieve infrared and microwave bi-functional simultaneously, we proposed a TL between the IRSL and the MA, the TL is an EVA foam with the certain thickness ${h_4}$. According to the ECMs of IRSL and MA discussed earlier, we established the ECM of the composite structure as shown in Fig. 6(b). Then we can calculate the ABCD transmission matrix of the whole structure as follows:
Fig. 6. (a) Schematic, (b) ECM, (c) calculated absorptivity and (d) normalized input impedance of the bi-functional metamaterial absorber.
The absorptivity of the whole bi-functional metamaterial can also be calculated from the transmission matrix presented in Eq. (21) based on Eq. (18–20). Figure 6(c) plots calculated absorptivity spectrums of the bi-functional metamaterial for ${\textrm{h}_4}$ ranging from 0 mm to 1 mm. As shown, absorptivity of the bi-functional structure is smaller than 0.9 from 4.8 GHz to 8.4 GHz for ${h_4} = 0\textrm{mm}$, which means the IRSL is stacked on the MA directly. We also found that when ${h_4} = 0.25\textrm{mm}$, the absorptivity is better but still very close to 0.9 around 6 GHz. Meanwhile, the efficient absorption (Absorptivity ≥ 0.9) starts from 2.2 GHz for ${h_4} = 1\textrm{mm}$, but the absorptivity at high frequency deteriorates and the whole efficient absorption bandwidth is narrower than other different heights. Therefore, the thickness of the TL ${h_4}$ was chosen to be $0.75mm$, which can balance the requirements of thickness and absorption bandwidth as Fig. 6(c) shows. Figure 6(d) plots the normalized impedances of the original MA, bi-functional metamaterial with and without the TL. As shown, compared with the original MA, the structure directly covered by the IRSL without a TL doesn’t achieve better impedance matching results for frequency band under 18 GHz. In fact, the input impedance of bi-functional metamaterial without a TL deviates from the wave impedance of free space greater than the original MA from 4.8 GHz to 18 GHz, which is consistent with the worse absorptivity performance presented in Fig. 6(c). On the other hand, when we introduce a TL with appropriate thickness (0.75 mm in this work), the bi-functional metamaterial can achieve better impedance matching results than the original MA, that its normalized resistance is closer to 1 and normalized reactance is closer to 0 at most frequencies, which also corresponds to its better absorptivity performance presented in Fig. 6(c). Apparently, the TL plays an important role for the bi-functional metamaterial to achieve satisfying absorption performances and should be elaborately designed.
3. Experimental verification
3.1 Verification of the infrared camouflage performance
To reveal the contribution of the IRSL for low IE of whole metamaterial, we adopted printed circuit board (PCB) techniques to fabricate an IRSL, the designed hexagon copper was platted on the 0.25 mm F4B medium as Fig. 7(a) and (b) show.
Fig. 7. Photographs of (a) IRSL, (b) IRSL under microscope, measured result for the (c) IRSL’s emissivity, (d) F4B’s emissivity ranging from 8-14µm.
The IE measurement is carried out by the IR2 Infrared Emissometer with the accuracy smaller than ±0.02 [6]. According to the Kirchhoff's Law of Thermal Radiation, under condition of thermodynamic equilibrium [17], the IE could be obtained indirectly by measuring the reflection coefficient of the sample. When the modulated infrared wave incidents upon the sample, the wave reflected from the sample would be trapped by the receiver and then the IE can be measured precisely. As shown in Fig. 7(c), at 250$^\circ \textrm{C}$, the corresponding infrared spectral band is 8-14$\mathrm{\mu} \textrm{m}$ and the measured IE of the IRSL is 0.144 within a reasonable margin of error [9]. Therefore, the IRSL reaches the low IE requirement. Moreover, the measured IE of the F4B is 0.942 shown in Fig. 7(d), which is similar to IE used in the Eq. (1).
3.2 Verification of the microwave camouflage performance
To verify the microwave camouflage performances, the prototype sample of proposed MA was fabricated. In the MA sample, the RMAL with resistive gear patterns was fabricated on PI films through the silkscreen printing process. The planar size of RMAL is 300mm × 300 mm. At the bottom of the MA, there is a MMAL (GXR-60G75, produced by Guanxu New Materials Company, China). The RMAL, EVA medium and the MMAL are bonded together with binder to construct the MA sample. Furthermore, the IRSL sample was adhered to the initial MA sample with an EVA lamina as the TL to construct the sample of proposed bi-functional metamaterial. The reflectivity spectrums of them were measured on an arch system shown in Fig. 8(a), which consists of an Agilent E8363A vector network analyzer, two pairs of broadband double-ridged horn antenna (1 GHz to 18 GHz), standard K-band horn antenna (18 GHz to 26.5 GHz) and a pair of power-driven antenna hangers for oblique incidence measurement. The radius of arch is 2.6 m, which satisfies the NRL standard [36].
Fig. 8. (a) Photographs of the initial MA sample and measurement setup, (b) measured and simulated measured absorptivity of the MA under normal incident. (c) photographs of the bi-functional metamaterial sample, (d) measured and simulated absorptivity of the bi-functional metamaterial under normal incident.
Figure 8(b) shows the simulated and measured absorptivity of the initial MA sample under normal incident. As shown, the simulated and measured results have a good agreement and indicate that absorption bandwidth of the initial MA is from 2.83 GHz to 19.8 GHz. Figure 8(d) plots the absorptivity of the bi-functional metamaterial under normal incidence. It can be found that the measured results agree well with the simulated ones. The measured absorption bandwidth is from 2.32 GHz to 24.8 GHz, which confirms that the TL can improve the microwave absorption performance of the composited bi-functional metamaterial. Moreover, the sample of bi-functional metamaterial can be bent as shown in the insert photo of Fig. 8(d) since the components of such structure are all, which has profits for conformal applications.
In addition, we also investigated absorption performances of the MA and the bi-functional metamaterial under oblique incidence. Figure 9(a) and (b) plot the measured absorptivity heat maps of the MA versus frequencies and incident angles of the incident EM wave for TE and TM polarizations, respectively. As shown, the measured absorptivity of the initial MA is more than 0.8 ranging from 2.9 GHz to 17.6 GHz under the incident angle $\mathrm{\theta }$ up to ${40^\textrm{o}}$ for TE polarization, and 2.6 GHz to 18.3 GHz under the incident angle $\mathrm{\theta }$ up to ${60^\textrm{o}}$ for TM polarization. Furthermore, Fig. 9(c) and (d) present the results of the whole bi-functional metamaterial for TE and TM polarizations, respectively. It can be found that the measured absorptivity of the bi-functional metamaterial is more than 0.8 ranging from almost 2.5 GHz to 20 GHz under the incident angle $\mathrm{\theta }$ up to ${40^\textrm{o}}$ for TE polarization, and 4.8 GHz to 25 GHz under the incident angle $\mathrm{\theta }$ up to ${60^\textrm{o}}$ for TM polarization. Apparently, the bi-functional metamaterial with the IRSL superstrate based on the metallic hexagonal patch is equipped with better angular stability for oblique incidence than the initial MA.
Fig. 9. Heat maps of absorptivity at different incident angles of the initial MA under (a)TE, (b) TM polarization Heat maps of the bi-functional metamaterial under (c)TE, (d) TM polarization.
4. Discussion
In this work, we proposed a hexagonal patch MS as the superstrate of proposed bi-functional metamaterial to achieve comprehensive requirements of low IE, high microwave transmittance and angular stability under oblique incidence.
As known, figure of merit (FoM) is used to evaluate absorption, bandwidth, thickness and other performance of absorber comprehensively, which is defined as follows [33],
where $\Delta f$ is continuous bandwidth defined by absorptivity above 0.9, ${f_0}$ and ${f_L}$ denote the center working frequency and lower operating frequency, respectively. The FoM of the bi-functional structure is 30.62, showing perfect performance at many aspects. Compared with the state of the art shown in Table 1, our bi-functional structure shows comprehensive advantages of low IE, broadband microwave absorption and wide-angle incident stability.
Table 1. Comparison of Bi-functional Structure with the state of the art
As known, there is a physical limitation of the thickness for the passive non-magnetism absorber, which was figured out by Rozanov in Ref [38]. Rozanov limitation is a vital indicator to evaluate the relationship between the thickness and absorption performance. The Rozanov limitation ${h_{roz}}$ is calculated as follows,
In most related works presented in Table 1, square patch topology was deployed to design the IRSL. To comprehensive analysis, we also numerically investigated the influence of an IRSL based on square patch pattern to microwave absorption performances of the bi-functional metamaterial. In simulation, the gap between each patch and length of each patch were set as 0.1 mm and 0.9 mm, respectively. As Fig. 10(a) shows, unlike the proposed IRSL, the IRSL utilizing square patch pattern will degrade the impedance matching of the MA without the TL (h4 = 0 mm). Therefore, we also introduced a TL into the structure. Figure 10(a) plots the calculated absorptivity curves for different thickness of TL. Comparing those curves, it can be found that the thickness h4 = 1 mm has an optimal performance for proposed design. As a comparison, Fig. 10(b) plots the absorptivity curves of the MA, the proposed bi-functional metamaterial and the simulated result of the MA with a square patch based IRSL superstrate under normal incident. As shown, with appropriate thickness of the TL, both the IRSL utilizing hexagonal and square patch can match the MA very well since their absorption FBW are almost same. However, the TL for proposed hexagonal patch based IRSL has smaller thickness.
Fig. 10. Absorptivity of (a) the MA and bi-functional metamaterials with square patch based IRSL for different thickness of TL. (b) the MA and bi-functional metamaterials with hexagonal and square patch based IRSL for optimal thickness of TL.
Angular stability under oblique incidence is also an important performance of bi-functional camouflage structure. As we mentioned above, the IRSL and its TL can improve the impedance matching performance of the whole bi-functional metamaterial, including the angular stability under oblique incidence. It has already been verified by the measured results presented in Fig. 9(a)-(d). To investigate this phenomenon further, we calculated absorptivity of the MA, the bi-functional metamaterials with different IRSLs versus the incident angle of EM wave. Figure 10(a)-(d) plot the results for different frequencies. As shown, at 3 GHz and 8 GHz, the original MA have relatively stable responses versus incident angle since the period of MA unit cell is much smaller than the working wavelength of EM wave. Meanwhile, the synergy of the resistive MS and the MMA can achieve better angular stability partly since they have different absorption mechanisms [42]. The calculated results also indicate that, at those two frequencies, the hexagonal patch based IRSL mainly improves angular stability of the bi-functional metamaterial for the TE mode. On the contrary, the square patch based IRSL only improves the angular stability slightly at 3 GHz and even worsen the angular stability at 8 GHz. At 16 GHz, absorptivity of the original MA for TE and TM modes have clear departure when the incident angle is large. Both the IRSLs with hexagonal and square patch can eliminate this departure efficaciously and improve angular stability of the bi-functional metamaterial for both TE and TM modes. In addition, the proposed IRSL based on the triangular array of hexagonal patches has better angular stability than that based on the tetragonal array of square patches. This phenomenon has already been figured out by Munk and Chen et al. that the triangular lattice can achieve better angular stability under oblique incidence in their previous studies on the influences of lattice types to the angular stability of FSS [27,41]. The scenario at 24 GHz is quite different with those at low frequencies. As shown in Fig. 11(d), the IRSLs can improve absorption performances for both normal and oblique incidence. However, the departure between the absorptivity of TE and TM modes for the bi-functional metamaterials covered by the IRSL under oblique incidence at 24 GHz is visibly larger than the results at 3, 8 and 16 GHz. The phenomenon occurs at 24 GHz mainly because the period of IRSL unit cell is close to the wavelength of EM wave at such frequency and the grating-lobe effect under oblique incidence is more significant in this scenario [27].
Fig. 11. Comparisons between the absorptivity of the MA and bi-functional metamaterials with hexagonal, square patch based IRSLs versus incident angle at different frequencies: (a) 3 GHz, (b) 8 GHz, (c) 16 GHz, (d) 24 GHz.
5. Conclusion
In this paper, we proposed a multilayered bi-functional structure with a MS superstrate based on the hexagonal patch to achieve a low IE and broadband wide-angle microwave absorption. The IRSL superstrate based on the hexagonal patch not only has a low IE, but also achieve a high transmittance of 0.85 for microwave below 26.5 GHz. The composite MA has an optimized resistive gear patterns MS and a MM layer. A dielectric TL is deployed between the IRSL and MA to achieve better impedance matching of whole camouflage structure. The ECM is utilized to design and optimize the structure. The sample of such bi-functional structure was prepared and tested. Both the simulated and measured results indicate the bi-functional camouflage structure can obtain greater than 0.9 absorptivity from 2.32 GHz to 24.8 GHz with 7 mm thickness and a low IE about 0.144 in the infrared band of 8–14$\mathrm{\;\ \mu m}$. Furthermore, the absorptivity can be maintained around 0.8 for the incident angle up to 40° under TE and 60° under TM polarizations. The proposed bi-functional structure would have a broad application prospect in infrared and radar wave bi-functional camouflage in various aspects.
Funding
National Natural Science Foundation of China (62371222); Open Fund of Key Laboratory of Materials Preparation and Protection for Harsh Environment, Ministry of Industry and Information Technology of the People's Republic of China (56XCA22042); Fundamental Research Funds for the Central Universities Electromagnetic Wave Characteristic Information Regulation Technology Research (0210-14380193); Priority Academic Program Development of Jiangsu Higher Education Institutions; Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Wave.
Acknowledgments
All authors thanks to Mr. Kong and Mr. Ge from Yangzhou Sparkle Co. Ltd for their support in the infrared testing.
Disclosures
The authors declare no conflicts of interest.
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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