1. Introduction

Infrared (IR) and radar detection are two common monitoring methods that have been applied in several military and civilian applications. Therefore, stealth in the IR or radar ranges attracts intense research interest, both for military purposes and for preventing the invasion of personal privacy [1–8]. With the rapid development of combined microwave and IR detection systems, it has become increasingly vital to develop compatible stealth technology in both the IR and microwave ranges [9–15]. Numerous composite coating materials such as conductive polymers [11,12], nanomaterials [13,14], and doped oxide semiconductors [9,15] have been proposed and proved to possess bi-stealth properties. For example, hybrid foam demonstrates an effective microwave absorption bandwidth of 5.64 GHz (> 90% absorption), with a peak at 13.04 GHz, and an excellent IR stealth performance due to its optimistic heat insulation function [11]. It is worth noting that the IR and microwave bi-stealth performance of these composite coatings is determined by the intrinsic electromagnetic properties of the materials. Therefore, it is difficult to regulate the complex permittivity and permeability of the materials to simultaneously gain broadband microwave absorption and efficient thermal IR stealth.

The emergence of metamaterials, though, provides a new means to achieve electromagnetic stealth. In 2008, Landy proposed the concept of a perfect metamaterial microwave absorber [16], and subsequently metamaterials have been broadly applied in the fields of electromagnetic absorption and microwave stealth [17–20]. To realize IR and microwave bi-stealth, multilayer metamaterials were proposed and developed, demonstrating more regulation flexibility compared with the composite coatings [21–24]. Recently, due to the additional requirement of optical transparency in some practical scenarios [25–30], transparent conductive materials, such as indium tin oxide (ITO), were utilized in multilayer metamaterial structures to achieve transparent IR and microwave bi-stealth [31–33]. In 2019, Zhang et al. used a two-layer metamaterial constructed from ITO, polyvinyl chloride, and polyethylene terephthalate to achieve high microwave absorption of over 90% from 8 to 18 GHz, low IR emissivity of 0.23, and optical transmittance of 30% [32]. In 2020, Xu et al. also proposed ITO-based metasurfaces to realize microwave absorption of over 90% from 5.4 to 17.6 GHz, IR emissivity of 0.3, and optical transmittance of approximately 75.02% [33]. However, the stability of the functional structures based on these polymers needs to be further improved, especially in a high-temperature environment.

In this study, we propose a multi-band optimization method to achieve IR-microwave compatible stealth, using a particle swarm algorithm to help fabricate the metamaterial directly on the quartz glass [34–39]. Using this method, we can meet the requirements of simultaneous broadband IR and microwave bi-stealth by optimizing the performance of the whole model. We experimentally demonstrate that the designed metamaterial windows (MMWs) possess a high absorption of over 90% from 5.1 to 19.2 GHz, and the IR emissivity of the structure is below 0.15. Additionally, the average optical transmittance can be maintained above 60%. Furthermore, the MMWs are resistant to high temperatures up to 200 °C. Therefore, our MMWs can compete with the best transparent IR and microwave bi-stealth polymer materials reported so far. Our multi-band optimization method provides a new way to design bi-stealth windows that are adaptable to more complicated scenarios in the electromagnetic shielding and stealth fields.

2. Theory and design

The structural diagram of the MMWs is shown in Fig. 1. From the top downwards, the MMWs consist of an IR shielding layer (ISL), a microwave absorption layer (MAL), and a microwave reflection mirror layer (RML). The ISL, which is composed of periodic ITO square arrays on quartz, can transmit microwaves and reflect IR light. For this type of capacitive frequency selective surface (FSS) [40], the penetrable bandwidth and the intensity of the microwaves are significantly determined by the periods and geometrical patterns of the ITO square arrays. The microwaves cannot penetrate the continuous ITO film. However, the FSS, with a periodic ITO structure, can transmit over 90% of microwaves (See Fig. 6(a) in Appendix A). In the IR regime, we investigate the transmittance (T) and reflectivity (R) of ITO film in the range of 3 to 15 µm (See Fig. 6(b) in Appendix A). The permittivity of the ITO in the IR band is described by the Drude model [41]. The surface resistance of the ITO continuous film (ITO-a) used for the ISL is 8 Ω/sq. The emissivity of the ITO continuous film is about 0.1. The emissivity of the ISL is a combination of the emissivities of the quartz and ITO due to the ISL’s film discontinuity. Specifically, the IR emissivity of the ISL can be calculated as [22,31]

 

Fig. 1. (a) The structural diagram of IR-microwave compatible metamaterial windows; the unit structure of the ISL and MAL is illustrated on the right. (b) The photograph of the fabricated MMWs sample with four subunits. The subunit size is 90 × 90 mm² and the fabricated sample size is 180 × 180 mm² for microwave measurement [28]. The insert is a photograph of the fabricated MAL.

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The MAL is constructed using the ITO pattern arrays configured as a microwave resonance absorber. Owing to the pattern’s symmetry, the absorption performance is insensitive to the polarization of the normal incident wave. The RML behaves as a mirror and reflects the transmitted microwaves to the MAL for further absorption. These three different ITO films are deposited on two quartz substrates separated by an air spacer. The surface resistances of the ITO continuous film used for the MAL and RML are 22.5 Ω/sq (ITO-b) and 8 Ω/sq (ITO-c), respectively. Using the rectangular waveguide method, the dielectric constant of the quartz is approximately 3.75(1-j0.01).

To meet the requirements of broadband microwave absorption and low IR emissivity, we adopt the particle swarm optimization (PSO) algorithm to simultaneously optimize the multi-band performance, as shown in Fig. 1(a). The PSO is a population-based stochastic optimization algorithm, which has been broadened applied into constrained nonlinear optimization in photonics design [42–44]. To keep the quality of the device (stable permittivity), the choices of the quartz plate from commercial products are limited with some fixed thickness. But the thickness does influence the performance [45,46]. Besides, the dielectric spacers, which represent capacitance in the equivalent circuit model [47], are related with the resonance band in microwave metamaterial. In this work, the spacer is air, in which the small changes on equivalent capacitance do not have an apparent influence on the performance of the total system. Meanwhile, the w₂ in this paper is in the best scale under the massive and fast fabrication quality. Therefore, the d₁, d₂, d₃, and w₂ are fixed to save optimization time. In the optimization, we only use six structural parameters (L₁, L₂, L₃, w₁, w, g). Figure 2(a) shows the simulation flow chart of the PSO algorithm

 

Fig. 2. (a) Flowchart for structural optimization with the PSO algorithm. (b) During the PSO algorithm (15 particles for 50 iterations), fitness values vary with the number of iterations. Color dots represent the fitness values of the 15 particles in each iteration. The red curve represents the “Global best” which records the minimum value of the “personal best” since the first iteration. (c) The simulated absorptivity spectrum of the MMWs is based on the result of the PSO algorithm.

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To find the best solution for the six parameters, 15 particles representing 15 random solutions are initialized. Then, for the different parameters, the structures’ microwave and IR absorption levels are calculated using the CST Microwave Studio software and ITO emissivity Eq. (1), respectively. Subsequently, a fitness function (FF) is used to judge the solutions. The FF consists of two parts: FF₁ and FF₂. FF₁ is used to judge microwave absorption and is expressed as:

As shown in Fig. 2(b), the PSO algorithm iterates 50 times, and the color dots represent the fitness values of 15 particles in each iteration. “Global best” represents the optimal solution result since the first iteration (red curve). The initial global best fitness value is 72.6. After 28 iterations, the global best fitness value decreases to zero and the optimization reaches convergence. The optimization results confirm the dimensions of the structure in Fig. 1(a): L₁ = 4.2, L₂ = 2.6, L₃ = 2.4, w₁ = 1.1, w₂ = 1.0, w = 0.45, g = 0.05, d₁ = 2.9, d₂ = 2.5, d₃= 1.1 mm. Finally, the simulated absorption spectrum is shown in Fig. 2(c). It reaches over 90% absorption from 5.1 to 19.2 GHz. From the optimization results, the filling coefficient $\eta$ is 0.81, and the related emissivity of our designed ISL is approximately 0.23.

3. Result and discussion

The measurement system of the microwave absorption spectrum is based on an arch measurement system. The sample size is 180 × 180 × 6.5 mm³, and contains four subunits, as shown in Fig. 1(b). It is necessary for the MMWs to maintain high microwave absorption efficiency under different incident angles. As shown in Fig. 3 and Fig. 7 (See Appendix C for more detail), the absorptivity results of the experiment (red curve) and numerical simulation (blue curve) coincide well for the four different incident angles (8°, 15°, 30°, 45°), for both the transverse electric (TE) and transverse magnetic (TM) modes. The absorptivity is defined as A=1-R (Transmission T=0). Owing to the high reflectance of the RML, the microwave transmittance of the MMWs is assumed to be negligible. The experimental results show that the designed MMWs can continue to efficiently (85%) absorb microwaves for TE and TM polarization when the incident angle is 30°. At an incident angle of 8°, near 90% absorption is achieved over a wide microwave band (5.1 to 19.2 GHz) (Figs. 3(a) and 3(c)). When increasing the incident angle, the absorption bandwidth changes little. When the incident angle increases to 45°, the microwave absorptivity of the target band is near 80% and the absorptivity bandwidth is 5.3 to 20.2 GHz. For the TM mode, the absorption efficiency is near 90%, but the absorption bandwidth is 7.2 to 21.3 GHz. The high microwave absorption performance is due to the symmetry of the proposed structures. This causes a significant loss of incident energy in different directions within the resonance cavity.

 

Fig. 3. The simulated (Sim.) and experimental (Exp.) microwave absorption spectrum of our structure under incident angles of 8° and 30° for TE mode (a)-(b) and TM mode (c)-(d).

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In addition, the MMWs also exhibit excellent IR stealth performance. An object’s IR radiation is determined by both surface emissivity and temperature. The IR spectral characteristics of the ITO-a/quartz are measured by Fourier transform IR spectroscopy (FTIR, Bruker Vertex 80v). A 30 × 30 mm² square sample is the same as the original for the tests. The measured transmission and reflection spectra were normalized by a high reflective mirror made with a gold film. Comparing Fig. 4(a) and Fig. 6(b), the experimental and simulation results are in good agreement. Thus, the IR emissivity of the ITO-a/quartz is approximately 0.1 in the wavelength range from 8 to 14 µm. Then, the IR emissivity of MMWs is measured using a thermal camera (PI640, Optris Inc., spectral range: 7.5 to 13 µm), as we reported in a previous study.⁵ As shown in Fig. 4(b), the MMWs, ITO-a/quartz, quartz (without ISL, the upper surface of the MMWs is quartz), and a metal block are placed on a heating plate with a temperature of 70 °C for half an hour. Their IR temperatures are 33.5, 31.0, 63.1, and 27.7 °C, respectively. The corresponding emissivity values are approximately 0.15, 0.1, 0.81, and 0.08, respectively. The measured IR emissivity of the ITO-a/quartz (0.1) is consistent with the result obtained by the FTIR. The measured emissivity of the MMWs (0.15) is lower than the simulation result (0.23), which is possibly due to neglecting the influence of the air space in the simulation. This is verified by measuring the MMWs without air space, giving an emissivity of 0.24 (See Fig. 9 in Appendix D for more detail). The structure of MMWs has two advantages in IR stealth. First, the ITO FSS structure allows the ISL to have a relatively low emissivity. Second, owing to the low thermal conductivity of the air space between the two quartz substrates, the surface temperature of the MMWs would be lower than that of the MMWs without an air space, which further decreases the IR radiation.

 

Fig. 4. (a) Measured IR transmission and reflection spectrum of ITO-a/quartz. (b) Thermal IR images of four samples at the heating plate with a temperature of 70 °C.

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Fig. 5. The measured result of the optical transmittance of the MMWs and every functional layer. The measured transmittance of the MMWs is about 60% from 300 to 800 nm, and the visible light range is represented by the gray shading.

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To analyze the contribution of each component of the MMWs, we numerically simulate these situations in the absence of the ISL, MAL, or RML (See Fig. 8(a) in Appendix C). The microwave absorptivity of the MMWs with or without the ISL is almost unchanged (Fig. 2(c)). In the absence of the MAL, the MMWs lose their ability to absorb microwaves, which shows that the ITO pattern array resonators are predominantly responsible for microwave absorption. When the RML is removed from the structure, the bandwidth of the microwave absorption spectrum narrows significantly and the absorptivity drops to 60%. It is clear that the MAL, air space, and RML form a resonant cavity, where multiple reflections of microwaves form an interference and boost broadband microwave absorption. To explain the mechanism of the broadband absorber, we display the top view of the power loss density distribution at the metamaterials’ three absorptive peaks of 5.9, 11.2, and 17.8 GHz. The power loss density is predominantly distributed on both sides of the ITO pattern at 5.9 GHz, and in the two bevel slits at 11.2 GHz (See Figs. 8(b)–8(d) in Appendix C). At 17.8 GHz, the power loss comes from the coupling between the two absorption modes (5.9 GHz and 11.2 GHz). Overall, therefore, most of the microwaves’ electromagnetic energy is dissipated around the MAL, owing to the ohmic loss of the resistive ITO pattern arrays.

Additionally, the visible performance of the MMWs is systematically investigated. The visible transmittances of the ISL, MAL, and RML are consistently higher than 75% for wavelengths between 300 to 800 nm. The assembled MMWs have a visible transmittance of approximately 60% (Fig. 5). The prepared MMWs are composed of four subunits, each 90 × 90 mm², as shown in Fig. 1(b). The MMWs exhibit high visible transparency and the letters under the MMWs can be clearly observed. The inset of Fig. 1(b) is an optical micrograph of the MAL. In the optimization, we do not consider the visible band since the scale of the parameters is much larger than the wavelength. The MMWs’ high optical performance is predominantly dependent on the high transmittance of the ITO film and quartz, in the visible band.

To analyze the high-temperature resistance performance of the MMWs, the sheet resistance and emissivity of the ITO-a/quartz and ITO-b/quartz layers are measured after annealing in the air (See Table 3 in Appendix E). The sheet resistance of the ITO was measured using the four-probe method, and the voltages and currents were measured using a source meter device (2612 B SourceMeter, Keithley Inc.). The IR emissivity of the ITO-a/quartz is 0.11 at 200 °C for 24 hours. As the temperature is increased to 250 °C, the ITO-a/quartz still maintains a low IR emissivity of approximately 0.11 (1 h) and 0.12 (3 h). The IR emissivity of the ITO-a/quartz does not change significantly at 250 °C. However, the sheet resistance of the ITO-b/quartz changes significantly with temperature: 24.4 (200 °C for 24 h), 29.1 (250 °C for 1 h), and 36.7 Ω/sq (250 °C for 3 h). The increase in the sheet resistance results in the deterioration of the microwave absorption performance of the MMWs (See Fig. 10 in Appendix E). The MMWs annealed at 200 °C for 4 h show little change compared with those without annealing treatment (Fig. 2(c)). For the MMWs annealed at 250 °C for 1 h and 4 h, the absorptivity for 8 to 15 GHz deteriorates to less than 90%. This shows that the ambient temperature can have a significant influence on the microwave absorption performance of the MMWs. However, up to 200 °C, the device can maintain constant performance.

Table 1 summarizes the performance of the related studies reported in recent years. When compared with these other devices, our MMWs exhibit a wider broadband microwave absorption capability and lower IR emissivity. The visible transmission of our multi-band stealth device is over 60%. Additionally, as our MMWs do not use polymers, which is the case with other devices, their high-temperature resistance remains applicable under harsh conditions.

Table 1. Comparison of metamaterial devices.

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4. Conclusions

Thermal IR and microwave compatible stealth windows are designed and fabricated. The PSO algorithm is used to optimize the multi-band performance of the MMWs, which significantly simplifies the optimization of complex systems with multiple constraints. The prepared MMWs exhibit excellent IR and microwave bi-stealth performance with a low emissivity of 0.15, absorptivity over 90% across a wide frequency band of 5.1 to 19.2 GHz, and possess visible transmittance above 60%. In addition, the bi-stealth performance is retained at temperatures up to 200 °C. With these advantages, the MMWs will find wide applications not only in artificial systems for electromagnetic shielding and stealth but also in functional modules, such as microwave nonreciprocal isolators and energy-saving windows.

Appendix A: simulated result of ITO film and ITO FSS of the ISL

We simulate the microwave and IR spectrum of the designed metamaterial structure under a normally incident plane wave source. The polarization direction is set along the x-axis. As shown in Fig. 6(a), the transmittance of ITO film is about zero in the microwave band (yellow curve). However, the ITO FSS was obtained from ITO film and it can keep high transmittance in the target band even when the gap is small. Both the period and gap can affect the microwave transmission performance of FSS, and the appropriate value needs to be obtained through subsequent optimization. At the infrared regime, the thickness of the ITO continuous film (ITO-a) used for ISL is set as 350 nm and the permittivity of ITO is described by the Drude model¹ in the simulation. The epsilon infinity is set as 3.95, the plasma frequency is set as 2π × 2.9 × 10^14 rad/s, and the collision frequency is set as 0.85 × 10^14 1/s. As shown in Fig. 6(b), the reflectivity is about 0.9 from 6 to 15 µm (red curve) and the transmittance is almost zero (black curve). And then we can figure out the absorptivity (blue curve) by A=1-R (Transmission T=0). According to Kirchhoff's law, emissivity is equal to absorptivity. We can know that the IR emissivity of ITO continuous film is about 0.1.

 

Fig. 6. (a) The simulated result of the transmittance characteristic of the ITO FSS as a function of slit gap (g) and period length (p₁). The ITO filling ratio η at each case is also listed. (b) The simulated result of the transmission, absorption, and reflection spectrum of the ITO continuous film (ITO-a/quartz) using the Drude model.

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Appendix B: search method of particle swarm optimization

In the PSO algorithm, the solutions are abstracted into particles with no volume and mass, but only position (x) and velocity (v). These six parameters form a vector X = [L₁, L₂, L₃, w₁, w, g]. The values of the six parameters determine the position of a particle in the 6-dimensional (6-D) solution space. The position of each particle can be represented as a vector x_j= {x_j1, x_j2, …, x_j6}. The particle swarm iteratively searches the 6-D solution space for the optimal position by tracking two “best values”. The personal best (p_b) value is the optimal solution found by the particle itself, and the other is the global best (g_b) value found by the whole group. And then, in each new iteration, the position and velocity of the particle are updated which is based on the latest results of the p_b and g_b. In i_th (i=1,2, …, T=50) iteration, the j_th particle (j=1, 2, …, N=15) update its position x_jk and velocity v_jk according to the following formula:

(7)$$v_{jk}^{i + 1}\textrm{ = }wv_{jk}^i + {c_1}{r_1}^i({p_b}_{jk}^i - x_{jk}^i) + {c_2}{r_2}^i({g_b}_{jk}^i - x_{jk}^i),$$

(8)$$x_{jk}^{i + 1}\textrm{ = }x_{jk}^i + v_{jk}^{i + 1}. $$

where the weight w decreases linearly from 0.8 to 0.4 with iterations.³ r₁ⁱ and r₂ⁱ has a uniform distribution in (0, 1) to ensure the randomness. The c₁ and c₂ are learning factors of individuals and society, where we set c₁=c₂=1.5. The position x and velocity v are initialized according to the following equation:

(9)$$x = {r_3}({x_{\max }} - {x_{\min }}) + {x_{\min }}, $$

(10)$$v = {r_4}({v_{\max }} - {v_{\min }}) + {v_{\min }}. $$

here, r₃ and r₄ are uniform distribution in (0, 1) and the range of values for each parameter are given in the following Table 2.

Table 2. The range of values of each parameter.

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The six parameters in the PSO algorithm program written in MATLAB are updated and then passed back to simulation software. The geometric parameters of the model are updated for a new simulation and the simulation results of commercial software are used by FF to calculate the fitness value. The value of p_b and g_b will be updated according to the calculation results of the new iteration. When the number of iterations reaches the preset value, the optimization ends, and the optimal parameter is output.

Appendix C: simulation and measurement results of microwave characteristics of the sample

Figure 7 shows the simulation and measurement results of the samples at incidence angles of 15° and 45°. When the incident angle increases to 45°, for TE mode, the microwave absorptivity of the target band can be greater than 75% and the absorption bandwidth is 5.3 to 20.2 GHz. For TM mode, the absorption efficiency can still be near 90%, but the absorption bandwidth is 7.2 to 21.3 GHz. It is quite obvious that the experimental results are consistent with the simulation result.

Numerical simulation results of absorptivity were obtained in these cases without ISL (black curve), MAL (red curve), or RML (blue cure), respectively, as shown in Fig. 8. Figures 8(b)–8(d) shows the top view of power loss density distributions at the three absorptive peaks of 5.9, 11.2, and 17.8 GHz for the metamaterial.

 

Fig. 7. The simulation and measurement results of the samples at 15° and 45° incident angles.

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Fig. 8. (a) Simulation results of absorptivity in these cases without ISL, MAL, or RML, respectively. (b)-(d) The top view of power loss density distributions at the three absorptive peaks of 5.9, 11.2, and 17.8 GHz for the metamaterial.

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Appendix D: measurement results of IR and optical characteristics of the sample

As presented in Fig. 9, the MMWs without air space is placed in a heating plate with a temperature of 70 °C for half an hour and the IR temperature is 38.2 °C, which is higher than that with an air space of 33.5 °C. In the absence of air space, the measured emissivity of ISL is about 0.24, which is consistent with the calculated emissivity (0.233).

 

Fig. 9. Thermal IR images of MMWs without air space at the heating plate with a temperature of 70 °C.

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Appendix E: performance of ITO varies with annealing temperature

The IR emissivity of ITO-a/quartz and the sheet resistance of ITO-b/quartz were measured after in-air annealing (heating and cooling) tests under different heating temperatures. The measured results are presented in Table 3. The infrared emissivity of ITO-a/quartz changes little within 250 °C. The calculated value of the emissivity of the ISL is given in parentheses. The variation of sheet resistance value for ITO-b/quartz within 200 °C has little influence on microwave absorption performance as shown in Fig. 10.

Table 3. The variation of two types of ITO/quartz under different heating conditions.

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Fig. 10. The simulated result of microwave absorption performance under different heating conditions.

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Funding

National Key Research and Development Program of China (2018YFA0208401); National Natural Science Foundation of China (62005140); China Postdoctoral Science Foundation (2020M670310).

Disclosures

The authors declare no competing financial 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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