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Abstract
There is a huge challenge to target multispectral compatible designs to satisfy the conflicting parametric requirements according to specific engineering requirements. In this work, a novel design method of multispectral compatible integration based on a lossy capacitive multispectral meta-film (MMF) is proposed. The simple guidelines from the impedance matching conditions of MMF derived from the transmission line model were employed to guide and analyze the broadband microwave absorption behavior. An autonomous optimization platform was constructed to simultaneously realize the customization of low infrared emissivity, as well as the widest microwave absorption bandwidth while ensuring maximum visible transparency. Following the guidance of the design method, a flexible structure with a low infrared emissivity of 0.534, wideband microwave absorption from 8.9 to 16.4 GHz covering X, Ku, and high visible transmission of 70.18% and ultra-thin thickness of 2.3 mm was finally obtained. The experimental results and simulation results were in high agreement, indicating the MMF has great application potential in multispectral stealth on optical windows, further demonstrating the versatility and effectiveness of the design method.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nowadays, with the rapid development of composite detection technologies, the multispectral stealth function integration technology is imperative to counteract the detection of complex electromagnetic (EM) environments [1–4]. Radar and infrared (IR) radiation inspections, as the most common means of detection in the military, have contradictory material requirements. High absorption and low reflection are required in radar stealth, while low absorption or thermal emissivity is demanded in infrared stealth [5–7]. Meanwhile, the stealth structures to meet the visible transparency have become an urgent requirement under special conditions, such as windows on aircraft or cars [8–10]. Conventional stealth materials such as composite nanofibers, conducting polymers, and doped oxide semiconductors have been difficult to meet the gradually increasing demand for multispectral stealth [11–14]. Moreover, they are usually opaque, hindering their application in optically transparent stealth devices [15,16]. Therefore, it is critical to develop materials with variable loss dispersions to satisfy the conflicting parametric requirements for multispectral compatibility.
Metasurfaces are artificial structures that allow flexible manipulation of the propagations of EM, optical, and plasmonic waves [17–21]. Constitutive parameters of metasurfaces can be engineered deliberately to control the waveband response in different spectral regions [22–26]. Commonly, indium tin oxide (ITO) films are processed into functional metasurfaces to achieve multispectral compatibility, thanks to their transparency, flexibility, and customizable sheet resistance [27–29]. In the previous works, most multispectral stealth structures are composed of an infrared shielding layer (IRSL) and radar absorption layer (RAL) [30–37]. The key idea of these designs can be summarized as covering a high radar transmission metasurface with low IR emissivity on the microwave absorber. However, the hybrid IRSL-dielectric-RAL-dielectric-backplane configuration with at least five layers not only leads to a large thickness of the structure but also results in a serious poor visible transparency (typically only about 20%) [35–37]. Such structures make it difficult to ensure adequate visible transparency for practical applications. To break the limitation of visible transparency by multifunctional layers, the core idea is to integrate IR shielding and microwave absorption in a functional layer. Based on the idea, an optically transparent ITO-dielectric-ITO sandwiched structure with high-efficiency absorptivity over 90% in 5.8–8.3 GHz and a low IR emissivity of 0.52 was proposed [38]. But there is no significant performance improvement in this structure compared to the optimal results based on square patches. Qu et al. resorted to machine learning to solve the multispectral compatibility problem of metamaterials [39]. The optimization effect still has much room for improvement due to the lack of theoretical guidance on electromagnetic regulation. Therefore, it is still a huge challenge to target multispectral compatible designs according to specific engineering requirements because of the lack of practical design methods.
In this work, a novel method based on the MMF was proposed that can simultaneously achieve broadband microwave absorption, low IR emissivity, and visible transparency, as illustrated in Fig. 1. The MMF consisting of lossy ITO separated by random blank strips can perfectly balance IR shielding and microwave absorption. The high ITO coverage and capacitive low-pass filtering characteristics facilitate the acquisition of low IR emissivity, while the different sizes of rectangular patches in MMF cells loaded with appropriate losses can excite multiple electromagnetic resonances to obtain wideband microwave absorption. The simple guidelines from the impedance matching conditions of MMF derived from the transmission line model were established to guide and analyze the broadband microwave absorption behavior. An autonomous optimization platform with Python-CST co-simulation based on genetic algorithms (GAs) was constructed, which enables fast and efficient implementation of customizable IR emissivity (IRexp) while obtaining the widest microwave absorption bandwidth. Under the guidance of the method, the IRexp was set to 0.55 as an example. After optimization, the structure with wideband microwave absorption from 8.9-16.4 GHz, a low IR emissivity of 0.534, and a high visible light transmissivity of 70.18% was obtained. In addition, the structure also has excellent characteristics including flexibility, low profile, polarization insensitivity, and good angular stability. It indicates that MMF has a broad application prospect for multispectral camouflage and protection in conformal transparent devices.
Fig. 1. Functional schematic of the proposed structure: wideband microwave absorption, low IR emissivity, visible transparency, flexibility, etc.
2. Design guidelines and realization
The schematic diagram of the structural design is presented in Fig. 2. Here, the sandwich structure is adopted, which means that the composition in sequence from top to bottom is the top functional metasurface, the intermediate dielectric layer and the bottom reflected backplane, as displayed in Fig. 2(a). Compared to previous multispectral stealth structures with multifunctional layers, the number of layers and interfaces of the sandwich structure is streamlined to a minimum, thus ensuring maximum optical transmissivity. To further improve the optical transparency, the Cu micro-mesh with higher visible transparency and better electrical conductivity instead of the commonly used ITO with low sheet resistance was applied as the backplane [40,41]. The micromorphology of the Cu micro-mesh is presented in Fig. 2(c). Meanwhile, to meet the conformal requirements in practical applications, polyvinyl chloride (PVC) was used as the intermediate dielectric layer due to its high optical transparency, ideal flexibility, and good mechanical properties. As shown in Fig. 2(a), the thickness of the intermediate media layer PVC with permittivity of 2.4(1-j0.06) is d. According to the actual conditions, the thickness of PET substrate (ɛ = 3.0(1 − j0.06) attached by the upper MMF and the bottom Cu micro-mesh are 0.175 mm (t1), 0.125 mm (t2), respectively.
Fig. 2. Schematics of (a) the proposed structure (3 × 3 unit cells), and (b) MMF unit cell configuration. (c) Micromorphology of the backplane Cu micro-mesh (scale 200 µm).
Multispectral compatible design is a multi-objective optimization problem. On the one hand, high visible transparency can be ensured through materials selection and structural configuration. On the other hand, unlike previous studies that required the design of multiple functional layers to achieve multispectral stealth, we proposed the MMF illustrated in Fig. 2(b) to simultaneously satisfy both wideband microwave absorption and low IR emissivity, as the design guidelines are described in the following.
2.1 Infrared stealth design of MMF
In the infrared band, the permittivity of ITO can be expressed by the Drude model [42]
where ɛb = 3.9, the plasma frequency ωp/2π = 461 THz, and the collision frequency γ/2π = 28.7 THz. Equation (1) shows that the real part of permittivity of ITO in the infrared band is negative, indicating that ITO has metal-like properties, so it can show low emissivity for infrared stealth. Notably, although continuous ITO films can meet the conditions of IR stealth, they are severely limited in terms of microwave absorption.Hence, the MMF composed of ITO segmented by random blank strips was designed, as displayed in Fig. 2(b). Considering the fact that stable performance under various incidence angles and different polarizations is widely desired in many fields, here we focus on the design of MMF with a four-fold rotational symmetry axis to obtain polarization insensitivity. The geometric parameters are presented in Fig. 2(b), the unit cell period of MMF is P with sheet resistance RITO, and the total number of random blank strips is 4n (n = 3 in the schematic) with width W. With the center of the unit cell as the origin, the positions of random blank strips are L1 to Ln, respectively. Promisingly, the MMF has the ability to perfectly balance IR shielding and microwave absorption. On the one hand, the high ITO duty cycle and capacitive low-pass filtering characteristics are beneficial to obtain low IR emissivity. On the other hand, by introducing appropriate loss, the ITO rectangular patches with different sizes divided by random blank strips can excite multiple electromagnetic resonances to obtain broadband microwave absorption.
Specifically, the IR emissivity of MMF (ɛMMF) can be calculated as [43]
where ɛITO, ɛPET is the emissivity of ITO and its PET substrate, while RITO, SITO are sheet resistance and filling ratio of the ITO parts in MMF, respectively. Apparently, ɛMMF is positively correlated with RITO while negatively correlated with SITO. Therefore, by selecting the appropriate RITO and SITO, it is sufficient to obtain a low IR emissivity value that meets the requirements. Further, the measured IR emissivity spectrum curves of ITO films with different RITO and their PET substrate were displayed in Fig. 3(a) (The measurement process is described in Section IV). The average ɛITO with sheet resistances of 5, 15, 25, 50, and 100 Ω/sq, as well as ɛPET in the infrared band of 8-14 µm are 0.105, 0.166, 0.368, 0.449, 0.719 and 0.877, respectively. It can be seen from Fig. 3(a) that ɛITO can be adjusted by changing its sheet resistance, as well as they are positively correlated. Thus, the curve of ɛITO with RITO is obtained using a polynomial fit, as presented in Fig. 3(b). Further, by bringing the average emissivity values of the materials used and SITO into Eq. (2), the predicted IR emissivity map of MMF can be obtained, as illustrated in Fig. 3(c). The curves shown as the dashed line in Fig. 3(c) are obtained by calibrating a certain IR emissivity value according to the predicted IR emissivity map. And then it is easy to determine the range of RITO and SITO that meet infrared stealth requirements by the calibrated dashed lines.Fig. 3. (a) Measured IR emissivity spectra and (b) average IR emissivity values and the fitted IR emissivity prediction curve of ITO films with different sheet resistances. (c) The predicted IR emissivity map of MMF with the variation of sheet resistance and filling rate of ITO. Inset: the red dashed lines represent the variation curves of sheet resistance and filling rate of ITO for ɛMMF of 0.35, 0.45, 0.55, 0.65, and 0.75, respectively.
2.2 Impedance conditions of MMF for wideband microwave absorption
Transmission line theory is a powerful method to analyze the resonant behavior of microwave absorption structure [44–46]. Here, an equivalent transmission line model is established to give physical insight into the microwave absorption performance of the structure. As a simplification, it can be assumed that the unit cell is terminated by a shortened load due to the zero loss of the Cu micro-mesh backplane, while the effect of PET substrates is negligible because of their ultra-thin thickness. Conforming to the response in an external electromagnetic (EM) field, a generic equivalent transmission line model of the structure is constructed, as shown in Fig. 4(a).
Fig. 4. (a) Unit cell schematic and equivalent transmission line model of the structure. (b) Impedance range of ZMMF at the absorptive band with A > 90% and the ideal ZMMF with respect to θ. (c) The ideal ZMMF with respect to frequency for various thickness (d) and relative permittivity (ɛr) of the intermediate dielectric layer.
The input impedance looking into the top of the intermediate dielectric layer is represented by Zl, which can be treated as a fraction of the transmission line with a certain characteristic impedance Zd. Using transmission line theory, Zl and Zd can be respectively derived as:
where ɛr, d is the relative permittivity and thickness of the intermediate dielectric layer, respectively, β is the propagation constant of incident electromagnetic waves, and Z0 = 377 Ω represents the characteristic impedance of free space. Meanwhile, the impedance of the top patterned ITO metasurface (ZMMF) can also be written asHence, the overall input impedance (Zin) at the top surface of the absorber is given by
When the structure is exposed to incident EM waves, no transmission takes place due to the Cu micro-mesh backplane. Thus, the absorption (A) of the structure is written as a function of the reflection coefficient ($|{\Gamma}|$), as shown in Eq. (7).According to the above formulas, the expressions for Zin and $|{\Gamma}|$ are given by the following equations, respectively:
To make microwave absorbed as much as possible, it is necessary to match the impedance of the structure to the free space. In the case of perfect absorption, i.e., $|{\Gamma}|$ = 0, the ideal impedance of MMF calculated according to Eq. (9) for well matching as depicted in Fig. 4(b), where Re(ZMMF) and Im(ZMMF) denote the real and imaginary parts of ZMMF, respectively. It can be found that the peak value of the ideal ZMMF is equal to Z0 and the distribution of ideal ZMMF with respect to θ has periodicity from Fig. 4(b). According to the equivalent transmission line model in Fig. 4(a), the input impedance Zin is determined by the intermediate dielectric layer and ZMMF. For an in-depth understanding, the effect of various thicknesses (d) and relative permittivity (ɛr) of the intermediate dielectric layer on the ideal ZMMF versus frequency curves are analyzed. The ideal Re(ZMMF) and Im(ZMMF) curves on the frequency range of 2-40 GHz for different values of d when ɛr is 1 and for different values of ɛr when d is 2 mm are obtained by calculation, as shown in Fig. 4(c). It can be seen that as the d or ɛr increases, the peaks of both ideal Re(ZMMF) and Im(ZMMF) tend to redshift, while the distribution period of the ideal ZMMF respect to the frequency is shrinking. Thus, when the intermediate dielectric layer is determined, the broadband absorption is mainly determined by the MMF. Specifically, broadband microwave absorption can be achieved by matching the actual ZMMF with the ideal ZMMF in a frequency band as wide as possible. Generally, absorptivity above 90% can meet the demand of practical applications. Further, the impedance range of the ZMMF with respect to θ for a microwave absorption of not less than 90% based on Eq. (9) is calculated, as shown in Fig. 4(b). Therefore, the calculated results of ZMMF based on the equivalent transmission line model, can be further employed to analyze the intrinsic correlation between the impedance of the MMF and the wideband microwave absorption characteristics, which in turn provides a guideline for the impedance matching conditions for the design of the MMF to obtain broadband microwave absorption.
2.3 Realization of the co-simulation optimization based on GAs
An autonomous optimization platform with Python-CST co-simulation based on GAs was constructed, as illustrated in Fig. 5, which helps to quickly search for the best design that can achieve the expected low infrared emissivity as well as the widest bandwidth microwave absorption to meet the application requirements. As demonstrated in Fig. 5, the GAs runs in the main program in python, and the CST is called repeatedly for modeling, condition setting, and simulation, with the interaction between them done through the VBA interface. The parameters of the new offspring generated by the crossover and mutation are decoded and passed to CST, and then the simulation results of CST are returned to the main program for further fitness evaluation and selection, forming a loop until a predefined termination condition or design goal is reached. To speed up the optimization process, real encoding is used for parameter encoding in optimization [47–49]. Wherein, the parameters to be optimized include the period length (P) and sheet resistance (RITO) of MMF, the width (W), and position parameters (L1 to Ln) of the random blank inserts in MMF, as well as the thickness (d) of PVC.
Fig. 5. The schematic diagram of the MMF design realization process by Python-CST co-simulation based on GAs.
The optimization of low IR emission and wideband microwave absorption is balanced by setting the expected value of IR emissivity (IRexp) to constrain the generation of new offspring and by applying the microwave absorption bandwidth as the fitness evaluation criterion. When the IRexp is given, the IR emissivity curves with RITO and SITO can be fitted according to Fig. 3(c) to derive the range of parameter values that constrain the generation of new offspring, so that the final optimization results satisfy the infrared stealth demands. The whole process realizes the customization of the IR emissivity of MMF. Generally, the fractional bandwidth (FBW) with an absorptivity greater than 90% is defined as the evaluation standard for microwave absorption, which is expressed as follows:
where fH and fL are high and low limits of the bandwidth with the absorptivity above 90%, respectively. Therefore, FBW as the fitness function is employed to evaluate the process of optimization. The bigger value of FBW, the better the microwave absorption of the structure.The infrared stealth performance of multispectral compatible structures in previous works is shown in Table 1. It can be found that the IR emissivity of multispectral compatible structures previously reported is generally distributed between 0.3 and 0.6, indicating that the IR emissivity within this range has practical application value. So, the customized IR emissivity of 0.55 is adopted as an example for optimization in this work. When IRexp = 0.55, as an example, the constraints on the parameters are as follows:
Table 1. Comparison of infrared stealth performance of multispectral compatible structures
Finally, the best-optimized results with IR emissivity of 0.534 and microwave absorption FBW of 59.3% were obtained by co-simulation based on GAs after 100 iterative evolutions, as illustrated by the iteration curve in Fig. 5. Furthermore, the structural performance at different values of n was obtained and compared, as shown in Table 2. Considering the actual conditions of prototype fabrication, the final optimized parameters are obtained as RITO = 53 Ω/sq, P = 14.7 mm, d = 2 mm, n = 3, W = 0.25 mm, L1 = 3.2 mm, L2 = 4.45 mm, L3 = 6.5 mm.
Table 2. Comparison of structural performance at different values of n
3. Analysis and discussion
Full-wave numerical simulations of the structure were carried out by the finite-element frequency-domain method in CST. Floquet ports with normal incident plane transverse electric (TE) waves and transverse magnetic (TM) waves were set in the z-direction for excitation. Periodic boundary conditions (PBCs) along the x and y directions were used to simulate the infinite periodic element. Finally, the frequency-dependent complex S-parameters can be obtained by the frequency domain solver simulation. The absorptivity of the structure under normal incidence can be defined as A(ω) = 1 − R(ω) due to the zero transmittance of the backplane, where R(ω) = |S11|2 is the reflectivity derived from the complex S-parameter. Simulation results show that the proposed structure can obtain broadband microwave absorption with an efficiency above 90% over the frequency range of 8.9 to 16.4 GHz (FBW = 59.3%), as displayed in Fig. 6(a). As a comparison, square patches with the same IR emissivity are optimized to obtain the widest microwave absorption bandwidth covering 8.2 to 11.1 GHz (FBW = 30.1%). Compared to the optimized square patch with the same IR emissivity, the proposed structure has a 97% improvement in the FBW of microwave absorption.
Fig. 6. (a) Comparison of simulated microwave absorption spectra under normal incidence for the proposed structure and the optimized square patch structure. (b) Comparison of ZMMF and ZSquare, as well as impedance range of ZMMF at the absorptive band with A > 90% with respect to frequency.
Further, according to the guidelines from the impedance matching conditions of MMF for wideband microwave absorption discussed in section 2.2, we have calculated the impedance ranges for MMF satisfying an absorptivity greater than 90% in the operating band covering X, Ku of 8-18 GHz, as illustrated in Fig. 6(b). The dispersion curve of the impedance of MMF (ZMMF) is also calculated based on the transmission line model, most of which is in the impedance ranges, indicating that the MMF can achieve a good match with free space over a wide bandwidth. Noteworthy, the Im(ZMMF) is negative in the operating band of microwave absorption, indicating that MMF presents capacitance, verifying that capacitance is beneficial to broadband microwave absorption. Figure 6(b) also presents the impedance of the square patch (ZSquare) calculated based on the same method as the reference. The comparison of the microwave absorption results obtained from the impedance conditions in Fig. 6(b) once again proves the great advantage of MMF in expanding the absorption bandwidth and enhancing the microwave absorption while ensuring the low infrared emissivity. Meanwhile, it can be seen that for both MMF and square patch, the microwave absorption bandwidth judged by the guidelines from the impedance matching conditions of MMF agrees well with the simulated results, further demonstrating that the guidelines are of guiding significance and generality for wideband microwave absorption design of MMF.
For insight into the physical mechanism of the broadband microwave absorption characteristics, electric field E, magnetic field H, and power loss density of MMF, as well as surface-current I distributions on the MMF and backplane under resonant frequencies were studied in Fig. 7. For MMF, the electric field is mainly distributed at the random blank inserts, while the magnetic field is mainly concentrated on the part with ITO attached, as shown in Figs. 7(a)-(d). It can be seen that the electromagnetic field is mainly concentrated at the different sizes of ITO rectangular patches divided by random blank strips or at their edges, which means that the electromagnetic field distribution can be controlled by the different sizes of capacitive patches in the MMF. Moreover, the intensity of electric and magnetic fields is as a whole stronger at 15.6 GHz than that at 10.3 GHz. The power loss density distributions as shown in Figs. 7(e)-(f), are similar to that of the electric field distributions, indicating that the power loss induced by the proposed absorber is attributed to the electric field coupling caused by resistance loss (ohmic dissipation). Under the influence of the incident waves, electrons from MMF move with the external electric field, and the surface current is induced, as shown in Figs. 7(g)-(j). It can be observed that the direction of the current flow on the bottom backplane is opposite to that of the MMF at 10.3 GHz. Such anti-parallel current flows imply the occurrence of magnetic resonances between the MMF and the backplane. For higher frequencies, the working mechanism is quite different. The direction of the current flow between MMF and the backplane is parallel at 15.6 GHz, which indicates the generation of electrical resonance between them. Therefore, by introducing appropriate loss, the ITO rectangular patches with different sizes divided by random blank strips in MMF can excite multiple electromagnetic resonances to obtain broadband microwave absorption.
Fig. 7. Electromagnetic responses of the proposed structure under normally incident TE waves at 10.3 GHz and 15.6 GHz. The distributions of (a),(b) electric field (c),(d) magnetic field (e),(f) power loss density of MMF. Distributions of surface currents on (g),(h) the MMF and (i),(j) the bottom backplane (the color and size of the arrow represent the strength of the surface current, and the direction of the arrow represents the direction of the surface current).
In many practical applications, good angular stability for broadband microwave absorption is essential. Simulated absorptivity spectra under different oblique incidences of irradiation for TE and TM modes are illustrated in Figs. 8(a), (b). Under TE mode irradiation, with the increase of the oblique incidence angle, fL is almost constant and fH gradually undergoes a slow blue shift, while the absorption intensity at the central frequency is slowly weakening. By comparison, it can be observed that under the TM mode, as the oblique incidence angle increases fL is almost constant and fH gradually undergoes a slow redshift, resulting in a gradual narrowing of the absorption bandwidth. Overall, the absorptivity of the structure can be maintained above 85% across the working bandwidth at incident angles within 35° and 50° for TE mode and TM mode, respectively. These results indicate that the structure features a reasonably stable angular performance. Moreover, the structure has the advantage of polarization independence, thanks to its four-fold rotational symmetry, as shown in Fig. 8(c).
Fig. 8. Simulated absorptivity as a function of incidence angle (theta) for (a) TE, and (b) TM polarization. (c) Simulated absorptivity as a function of polarization angle (phi).
4. Experiment and verification
To further validate the performance of our design, a large-scale prototype consisting of 20 × 20 unit cells (294 × 294 mm2 surface area) was fabricated and tested, as displayed in Figs. 9(a), (b). The MMF was fabricated by a laser carving of the commercial ITO film of 53 Ω/sq, as presented in the inset of Fig. 9(c). The backplane Cu micro-mesh with an extremely low sheet resistance of 1 Ω/sq and high transmittance of 86.5% was fabricated by selective electroplating of Cu in roll-to-roll imprinted microgrooves on the PET substrate [50]. Finally, the MMF and backplane were stretched and taped over a 2-millimeter flexible and transparent intermediate PVC layer. An ultra-thin, insulating optically clear adhesive (OCA) was used for interlayer adhesion to maintain high optical transparency of the whole structure. The OCA was mainly used at the edges and center of each layer to make the layers fit tightly and flatly together to form and maintain a holistic state. The high optical transparency, ultra-thinness and insulating characteristics of OCA, as well as the very small area used compared to the layer area of the structure, make its effect on the experimental results so weak as to be negligible. Especially, by using soft PVC, the proposed structure is flexible, as shown in the inset of Fig. 9(b), which can be bent and attached to the curved surface carrier for working.
Fig. 9. Photographs of the fabricated (a) flat and (b) bent structure with great flexibility. (c) Visible transparency measurement of the structure. (d) Measured IR emissivity spectra (Left) and comparison of measured and simulated absorptivity for MMF structure (Right). Inset: photograph of the fabricated MMF.
As seen in Figs. 9(a), (b), the prepared structure shows good transparency, through which the images below can be seen with naked eyes. Further, the average optical transmittance of the the whole sandwich structure in the visible band is approximately 70.18%, as measured by using the optical transmittance tester (LS116), as shown in Fig. 9(c). Figure 9(d) displays the spectral emissivity in the IR band and the spectral absorptivity in the microwave band. The reflectance coefficient (|S11|) in the microwave band was measured using the arch method inside an anechoic chamber. Thus, a comparison of the measured and simulated microwave absorption curves covering X, Ku microwave bands was presented in the right half of Fig. 9(d). It can be seen that the experimental and simulation results are in good agreement. The minor deviation between the simulated and measured microwave absorption curves is primarily due to the fabrication and assembly tolerances, measurement errors, and the variance of permittivity of the substrates. In the infrared band, it is difficult to obtain the simulated emissivity of the structure, due to the large difference between the operating wavelength of EM waves and the parameters of MMF. Therefore, only the experimental discussion results are given here. According to Kirchhoff's law, emissivity is equal to absorptivity under equilibrium conditions. Consequently, the IR emissivity can be obtained by measuring the transmission and reflection spectra of the structure by FTIR Spectrometer (PerkinElmer, Frontier Optica). As a result, the infrared emission curve of MMF was obtained, as displayed in the left half of Fig. 9(d). In the infrared band of 8-14 µm, the measured average IR emissivity of MMF is 0.546, which is very close to the predicted value of 0.534.
To further verify the infrared stealth performance of the MMF, we used an infrared thermal camera (TPH16) to take infrared images infrared images in the range of 8-14 µm of the MMF at high temperatures, as shown in Fig. 10. To ensure uniform heating, a 60 × 60 mm2 glass was placed on the thermal plate as the substrate, and the heating temperature of the thermal plate was set to 80 °C. From Fig. 10(a), it can be seen that the temperatures of the five randomly marked points on the glass were 76.5, 76.4, 76.3, 76.3, and 76 °C, which indicates that the temperature distribution on the glass was fairly uniform. As references, three samples with the same dimensional size (60 × 20 mm2) were prepared, which are continuous ITO of 53 Ω/sq, PET, and MMF, and then placed on the uniformly heated glass in sequence and heated simultaneously. After being stabilized, their temperatures were 48.2, 70.3, and 54.3 °C, respectively, as shown in Fig. 10(b). It can be observed that the emissivity of the proposed MMF is close to that of the continuous ITO film. The surface emissivity of MMF can be calculated by the equation [51]
where Tr is the temperature measured by the IR thermal camera (44.3 °C), Ta is the ambient temperature (22 °C), and T0 is the true temperature (76.3 °C). According to Eq. (13), the IR emissivity of MMF is calculated to approach 0.533, which is close to the measured value of 0.534. It can be seen that the high-temperature MMF shows relatively low IR emission, which is hardly detected by IR detection equipment. The measured result indicates that the structure features high optical transparency, low infrared emission, and broadband microwave absorptions predicted by the simulations.Fig. 10. Thermal infrared image of (a) glass substrate on the thermal plate with a temperature of 80°C and (b) ITO film of 53 Ω/sq, PET and MMF on the glass substrate.
5. Conclusion
Here, we proposed a novel design method based on the MMF to simultaneously achieve wideband microwave absorption, low IR emissivity, and visible transparency. For the visible band, high visible transparency can be maximized by designing the MMF function integration and adopting Cu micro-mesh as backplane. For the infrared and microwave bands, the compatibility of multispectral stealth was achieved through high ITO coverage and the different sizes of capacitive rectangular patches in MMF cells loaded with appropriate losses. Further, customizable IR emissivity and optimal broadband microwave absorption effects were simultaneously realized through GAs-based Python-CST co-simulation. To validate the method, the IRexp was set to 0.55 as an example, the structure with wideband microwave absorption in 8.9-16.4 GHz (FBW = 59.3%), a low IR emissivity of 0.534, and a high visible light transmissivity of 70.18% and a ultra-thin thickness of 2.3 mm was finally obtained. Compared to the optimized square patch with the same IR emissivity, the proposed structure shows a 97% improvement in FBW of microwave absorption. Moreover, the structure also has excellent characteristics of flexibility, polarization insensitivity, and good angular stability. The measurement results agreed well with the simulation results, which demonstrates the versatility and effectiveness of the proposed design method, as well as indicates that the MMF has a broad application prospect in the field of multispectral stealth and compatibility.
Funding
Open Fund of Key Laboratory (JZX7Y201911SY008601); Key Project of National Natural Science Foundation of China (52032004); National Science Fund for Distinguished Young Scholars (51625201).
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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