Flexible optically transparent ultra-wideband millimeter-wave metasurface with infrared stealth and polarization insensitivity

Abstract

Traditional microwave absorbers with limitations of narrow band absorption, single functionality, and difficulty in conforming to complex surfaces are no longer sufficient to meet the demands of modern equipment for multispectral stealth. This paper proposes a flexible optically transparent metasurface (FOTMS) with simultaneously ultra-broadband millimeter-wave absorption and infrared (IR) stealth. Tri-layer indium tin oxide (ITO) patterned layers were designed and etched onto flexible transparent polyethylene terephthalate (PET) substrate, achieving radar-IR bi-stealth, optical transparency and a flexible configuration. Both simulated and experimental results indicate that the metasurface features absorptivity more than 90% within 15–38.7 GHz, while exhibits 10 dB radar cross section (RCS) reduction for curved configurations. Additionally, the metasurface displays polarization insensitivity and stability at different incidences and polarization angles. Meanwhile, the entire structure maintains a high optical transmittance of 85.09%, and the average infrared emissivity of metasurface is 0.47 for 3–14 μm. This work provides an effective strategy for optical windows and multispectral stealth.

Graphical abstract

Keywords

infrared stealth / metasurface / amplitude modulation / flexible / optical transparency / polarization insensitivity

1 Introduction

The rapid advancement of radar detection technology and the increasingly complex electromagnetic environment pose severe challenges to the survivability of military targets in modern warfare. Electromagnetic (EM) stealth technology, therefore, remains a significant importance due to its crucial role. Its objective is to reduce the radar cross section (RCS) of a target [1−4]. Therefore, coating targets with absorption materials can significantly enhance their stealth performance in complex environments. However, the traditional radar absorbing structure, such as the Salisbury screen [5], Dallenbach absorbers [6] and magnetic absorbers , typically have limitations including narrow band absorption and significant thickness.

Unlike traditional absorbers that rely on the inherent properties of materials to achieve loss, the absorption of metamaterials stems from their unit cell design and chemical composition [7−9]. Landy et al. [10] proposed a perfect metamaterial absorber, achieving near perfect absorption performance, which became a significant milestone in the development of metamaterial absorbers. However, the extremely narrow absorption bandwidth, strong polarization sensitivity, and poor angular stability severely limit its practical application. Since then, numerous metasurface, which are the two-dimensional equivalent of metamaterials, have been designed and examined [11, 12]. Optical metasurfaces overcome the traditional limitation of bulky devices. With subwavelength-scale thicknesses, they can manipulate the properties of light, enabling functions such as ultra-thin lenses, beam deflection and holographic imaging [13, 14]. However, most of the existing optical metasurfaces are fabricated on rigid and planar substrates, which limits their potential toward practical applications requiring conformal coverage. Consequently, the research of flexible metasurfaces has emerged [15−18]. Owing to the flexible transparent ITO-coated-PET films, Hayat et al. [19] introduced a broadband metasurface absorber in microwave frequency that possesses remarkable optical transparency and flexibility. A low-profile 1-bit diffusion metasurface absorber integrating dual mechanisms has been developed, which demonstrates high optical transmittance and mechanical flexibility [20]. Having addressed the fundamental issues of conformal and optical transparency, the research has mainly focused on enhancing the performance of metasurface. For expanding the absorption bandwidth, the multilayer resonant structures have been proposed in Refs. [21−25]. A cross-square ring staggered rectangular metamaterial absorber with a multilayer composite structure was designed, which realized broadband absorption ensured jamming of passive microwave remote sensing [26]. Furthermore, the broadband absorption can be further realized by combining resistive film and magnetic loss layer. An absorber effectively achieves absorption from 2 to 18 GHz with its thickness of 7 mm [27]. In EM environments, the polarization direction of radar waves is often unknown and random [28−31]. Therefore, it is a crucial characteristic for metasurface absorbers with polarization insensitivity. Soghi et al. [32] introduced a single-layer optically transparent broadband microwave absorber. The design was featured by a fractal octagonal ring resonator, exhibiting broadband, wide-angle, and polarization-insensitive microwave absorption. Based on eight trapezoidal symmetrical structures, and blended three types of materials: vanadium dioxide (VO₂), silicon dioxide (SiO₂), and Dirac semimetal (DSM), successfully achieved four distinct absorption modes with excellent polarization insensitivity [33].

With the development of multi-spectrum composite detection, the traditional stealth technology in a specific spectral band is difficult to protect the target, which led to a surge of radar-IR bi-stealth [34, 35]. The current approach to achieving radar-IR bi-stealth involves cover the microwave absorber with low-IR-reflectivity material. Common coverings include frequency selective surface [36, 37] and high-metal-filling checkerboard-shaped metasurfaces [38]. In Ref. [39], an interesting method has been proposed to realize both tunable absorption and IR emissivity by incorporating a water layer. Yet, there are limitations in terms of robustness and integration. Subsequently, to integrate the multispectral-compatible camouflage and in-band signal communication, Ge et al. [40] proposed an optically transparent metasurface with multifunctional capable of microwave absorptivity and low IR emissivity, based on the synergistic effects of periodic ITO square patches, ITO narrow rings, and metal-meshed FSS structures. As development, the metasurface-based absorbers not only achieve RCS reduction by absorbing EM energy, but also offer advantages of thin thickness, function integration, and flexible manipulation of multispectral waves [41−45]. Yet, few of them can simultaneously integrate all these properties into a metasurface.

Inspired by the research mentioned above, this paper proposed a flexible metasurface that integrates infrared stealth, ultra-wideband absorption in millimeter-wave, optical transparency, flexibility, and polarization insensitivity. Figure 1 illustrates a potential application scenario of the metasurface, which can be integrated into an aircraft windshield and with multispectral stealth. The infrared stealth layer (IRSL) comprises a periodic array of ITO square patches etched onto flexible PET, which is subsequently coated over the absorber surface to achieve both low infrared emissivity and efficient millimeter-wave absorption. The center-symmetric micro-structure ensures perfect polarization insensitivity, while synergistic resonance between patterned layers enables an ultra-wideband absorption in 15−38.7 GHz. FOTMS maintains excellent optical transparency with a transmittance of 85.09%. Furthermore, the underlying mechanism behind ultra-wideband microwave absorption were explored in depth through equivalent circuit model theory and field distributions. Simulated and measured results validate the effectiveness of FOTMS.

2 Model and theory of the metasurface

2.1 Model design

Figure 1 demonstrates the schematic of the proposed flexible optically transparent metasurface with high millimeter-wave absorption and low infrared emissivity. Figure 1(b) illustrates a unit cell of the micro-structure for FOTMS, which consists of an IRSL, three PET dielectric substrates, two ITO films, and Polyvinyl Chloride (PVC) dielectric substrate. The designed IRSL features high microwave transmissivity and low infrared emissivity. Here, ITO square with a sheet resistance of 6 Ω/sq patches etched on a PET substrate with a thickness of 0.125 mm were selected for the IRSL. The PET dielectric substrate has excellent flexibility and optical transparency, featuring a dielectric constant of 2.4 and a loss tangent of 0.06. The ITO1 film is composed of a closely connected resonant ring and a concave equilateral dodecagonal structure. The opening gap and inner diameter of the resonant ring are w₂ and l₁, and the side length of the concave equilateral dodecagon is l₂. The structure of the ITO2 film is a square with side length l, which completely covering the PET. The thicknesses of ITO1 and ITO2 films are 150 nm and 200 nm, respectively, and their sheet resistances are 140 Ω/sq and 15 Ω/sq, respectively. Additionally, PVC dielectric substrate with a dielectric constant of 2.4 and a loss tangent of 0.06 is employed to support entire micro-structure.

Due to the central symmetry of ITO1 and ITO2 film, the metasurface is insensitive to the polarization direction of the incident wave. So, it shows almost the same absorption for x- and y-polarization. The specific structural parameters of FOTMS are detailed in Table 1.

2.2 Operation principle

Absorbers are typically composed of multiple layers of materials. By adjusting the appropriate materials and geometric micro-structures, they can reduce the reflection of incident EM waves and enhance their absorption. In this work, when the waves are incident to the surface of FOTMS from free space, the incident EM energy adheres to the law of conservation of energy. The total incident power is decomposed into three components: a part is reflected due to impedance mismatch, another portion is absorbed after resonance with the pattern layer, as well as loss within the dielectric substrate and materials, whereas the remaining EM waves transmit through the underlying PET dielectric substrate. This energy conversion process can be expressed by the following equations:

(1)R(\omega )+A(\omega )+T(\omega )=1,

(2)R(\omega )={\left|S_{11}\right|}^2,

(3)T(\omega )={\left|S_{21}\right|}^2,

where A(ω), S₁₁ and S₂₁ are the absorptivity, reflection coefficient and transmission coefficient, respectively.

To further illustrate the working mechanism of the proposed FOTMS, we utilize the equivalent circuit model (ECM) to reveal the analytical insight of the operation principle [46, 47]. The corresponding equivalent circuit diagram is presented in Fig. 2(a), and the detailed parameter extraction process is described in Supplementary Note 1. Since the ITO2 layer is a continuous low-sheet-resistance conductive film, its electromagnetic characteristics are close to a perfect electric conductor, so the S₂₁ approaches 0. Further, the absorptivity of this metasurface can be expressed as

(4)A(\omega )=1-{\left|S_{11}(\omega )\right|}^2.

In order to ensure that the S₁₁ of the circuit model matches the results derived from the full-wave simulation software CST. The unit cell boundary conditions are set in x−y plane, while the open boundary condition is chosen in z direction. Using software Advanced Design System (ADS) to verify the ECM and determine the final circuit component values. The optimized values are listed in Supplementary Table 1. The reflection coefficients simulated by ECM and CST are compared in Fig. 2 under x-polarization and y-polarization. It can be observed that the S₁₁ of FOTMS remains below −10 dB within the frequency range of 15−38.7 GHz, and the results obtained by the full-wave simulation and ADS are in excellent consistency, which verifies the validity of ECM. In addition, the micro-structure shows almost the same absorption for x-polarization and y-polarization, which proved its polarization insensitivity characteristic. Therefore, the subsequent analysis will focus solely on the case of x-polarized incidence.

The purpose of infrared stealth is to avoid detection by IR detectors, with its physical essence lying in controlling the target’s energy of thermal radiation. Besides, the effective control requires a quantitative analysis of thermal radiation principles. According to the law of thermodynamics, any object at a temperature above absolute zero emits EM radiation, typically in the infrared part of the spectrum, with its total energy of thermal radiation can be calculated by the Stefan−Boltzmann law,

(5)E=e\sigma T^4,

where e is the surface infrared emissivity, σ represents the Stefan−Boltzmann constant, and T is the surface temperature. As the temperature of absorber rises during operation and is difficult to monitor. It is a realistic way to reduce the emissivity for the realization of infrared stealth. Based on the composition of IRSL layer, the infrared emissivity e can be approximated as [41]

(6)e=e_{\mathrm{I}\mathrm{T}\mathrm{O}}{f}_{\mathrm{I}\mathrm{T}\mathrm{O}}+e_{\mathrm{P}\mathrm{E}\mathrm{T}}{f}_{\mathrm{P}\mathrm{E}\mathrm{T}},

(7)f_{\mathrm{I}\mathrm{T}\mathrm{O}}+f_{\mathrm{P}\mathrm{E}\mathrm{T}}=1,

where f_ITO and f_PET represent the filling rate of ITO film and PET dielectric substrate, respectively. Here, ITO exhibits a low IR emissivity of 0.09, in contrast to PET, which has a high emissivity of 0.9. In addition, the regulatory mechanism of e needs to be revealed by Kirchhoff’s law,

(8)e=A(\omega )=1-{\left|S_{11}(\omega )\right|}^2.

This formula provides clear guidance for our design. Thus, to achieve low infrared emissivity, a functional layer with high reflectivity in the infrared band must be constructed on the outer surface of the absorber.

3 Design and analysis of metasurface

3.1 Design and optimization of metasurface

In this work, the FOTMS has been designed with the aimed to achieve both ultra-broadband absorption and low infrared emissivity. The design ideas and optimization process for the FOTMS will be discussed in detail hereinafter.

To implement the ultra-broadband absorption, the structure of the ITO1 film was initially designed and subsequently optimized. Polygonal symmetrical structures typically demonstrate ultra-broadband absorption and exhibit insensitive to polarization.

Based on this, the micro-structure began with a concave equilateral dodecagonal pattern, and the subsequent evolution process is illustrated in Fig. 3(a). At a specific frequency f₁, intense localized resonance is excited, forming narrowband absorptive peak. To expand the bandwidth, a resonant ring is further introduced around the concave equilateral dodecagonal. Thereby, the metasurface in stage-2 successfully excited two independent resonant modes corresponding to frequencies f₂ and f₃. Owing to the weak coupling between the two resonant modes, it can be observed that the absorption rate significantly decreases between the two absorption peaks. To achieve a smooth transition and consequently realize ultra-broadband absorption, the resonant ring was connected to four sides of the concave equilateral dodecagon. This critical geometric connection signifies the completion of the structural evolution process. Next, we present the full-wave EM simulation and analysis of three stages to validate the effectiveness of the design concept. As shown in Fig. 3(b), the metasurface in stage-1 exhibits absorption with the rate above 90% within 28.41−38.43 GHz. However, the coverage is limited to the lower frequency segment of the Ka band and the higher frequency segment of the K band. As expected, the absorption peak of stage-2 shifts to 22.17 GHz and it exhibits a new absorption peak at 41.46 GHz. Thus, 90% absorptivity is achieved in the frequency of both 17.64−31.56 GHz and 40.47–42.27 GHz, which means that a continuous, highly efficient absorptive frequency band could not be formed. After adding the rectangular connector block, absorption rates above 90% can be achieved in the frequency band ranging from 17.5 to 41.5 GHz, providing coverage across the entire K and Ka band, as well as parts of the Ku and V band. The parameters of metasurface from stage-1 to stage-3 have all been adjusted and the detailed parameter values are presented in Supplementary Table S2. To clearly demonstrate the advantages of the proposed design, we further compare the absolute and fractional bandwidths of the three stages in Supplementary Table S3. To this end, Fig. 3(c) shows the input impedance curve of different stages. According to the equivalent circuit theory, optimal absorption is achieved when the real part approaches 377Ω and the imaginary part approaches 0. Compared to the earlier stages, we note that in the 90% absorption band of 17.5 to 41.5 GHz, the real and imaginary parts of the impedance in stage-3 are closest to 1 and 0, respectively.

Through the design and optimization of structure for ITO1 film, the metasurface absorber has achieved ultra-wideband absorption. On this basis, the IRSL will be incorporate into the metasurface absorber to achieve IR stealth. The IRSL consists of ITO square with a sheet resistance of 6 Ω/sq patches etched on a PET substrate. Based on Eq. (13), the IRSL has the lowest IR emissivity when f_ITO = 1. However, as shown in Fig. 4(a), such an ITO film that almost completely covers PET substrate will lead to significant millimeter-wave reflection and reduce the transmission of millimeter-wave. Therefore, as shown in Fig. 4(b), it is divided into N square patches with a period of w₄, and the distance between adjacent squares is d. In order to illuminate the impact of N on the absorption performance, a parametric study was performed using full-wave simulation software CST. Figure 4(c) illustrates the absorption corresponding to distinct N values under x-polarized wave excitation. We observe that, the peak absorption frequency increases as N increases, yet remains below the original absorption frequency. Due to the smaller period of the ITO square patches relative to the bottom absorber unit, the accumulation of charge generates a capacitance in the gap between adjacent patches, thus reducing the resonant frequency of the absorption structure. Therefore, to achieve high microwave absorption in the high frequency band, it is necessary to reduce the number of patches, which means a small filling ratio of ITO on the IRSL. The parameters of IRSL can, therefore, be adjusted to meet the specific requirements for microwave absorption and infrared stealth during the subsequent construction process.

For an intuitive comparison of IR characteristics, four different samples (N = 1, 25, 100, and 400) with identical size of 35 × 35 mm² were prepared and placed on the 82 °C heating plate, as shown in Fig. 4(d). All samples were examined using an IR thermal camera. Figure 4(e) indicates that as the N value increases, the surface radiation temperature of the sample rises accordingly, reflecting an increase in IR emissivity. After comprehensive evaluation of their impact on millimeter-wave absorption and IR emissivity, the optimal value of N is determined to be 400.

After the optimization of the IRSL, the final model in the building process, namely FOTMS, has been designed. As shown in Fig. 4(f), the proposed FOTMA achieves over 90% absorptivity in the frequency range of 15–38.7 GHz, with its absorption peak shifts to the low frequency direction. This peak shifts to the low frequency direction. This frequency shift results from optimal impedance matching within the corresponding band, indicating that the IRSL layer finely tunes the EM response. While absorption visually represents the final outcome of energy dissipation, the underlying reasons for achieving efficient absorption at ultra-broadband require further analysis.

3.2 Analysis of the metasurface

The electric field distributions in IRSL, ITO1 layer and ITO2 layer at the three representative frequencies of 15 GHz, 20 GHz, and 38.7 GHz are shown in Fig. 5(a). We note that the electric energy in the IRSL layer is primarily concentrated in the gaps of ITO square patches, which leads to enhanced absorption. Besides, due to the increased frequency, the electric field is enhanced at different positions of the ITO patches. In the ITO1 layer, the electric field energy is mainly concentrated at the edges of the concave equilateral dodecagonal and the upper and lower arms of the conductive ITO rings, which spatially corresponds to the IRSL. This indicates that the electric field originates from electromagnetic coupling with the IRSL. In the ITO1 layer, the electric field energy density is extremely low. This phenomenon can be explained from both the perspective of material properties and functional design. First, the ITO2 layer is fabricated as a continuous, low-sheet-resistance conductive film (15 Ω/sq in this paper) that behaves close to a perfect electric conductor (PEC). According to the boundary conditions of electromagnetic fields, the tangential electric field at the surface of PEC must be zero. Additionally, the primary function of the bottom ITO2 layer is not to absorb electromagnetic waves but to act as a reflective layer. It reflects electromagnetic energy not fully dissipated by the upper layers back into the structure, allowing this energy to pass through the loss layers again. Surface current analyses of each lossy layer at resonance frequency were performed to visually represent the loss mechanism.Close contact between the IRSL and the ITO1 layer will induce near-field coupling, thereby easily generating antiparallel currents between them. As shown in Fig. 5(b), the surface current is parallel to the direction of the incident electric field under x-polarized wave excitation. Meanwhile, the direction of the surface currents in IRSL and ITO2 are antiparallel to the ITO1 layer, generating two closed current loops that results in magnetic resonance and energy damage. Also, the ohmic loss of the ITO film can be calculated by Joule’s law, Ploss = I²R, the term I represents the excited current, and R is the square resistance value of the ITO film. This synergistic interaction between the magnetic resonance and ohmic losses, leads to remarkable energy dissipation, consequently, the FOTMS developed in this work demonstrates ultra-broadband absorption performance.

The above analysis of the absorption mechanism and performance exhibited by FOTMS was conducted under normal incidence. However, in practical applications, electromagnetic waves often incident at arbitrary angles. Therefore, evaluating the stability of FOTMS under oblique incidence is crucial for assessing its practical engineering value. The incidence direction of electromagnetic waves is determined by the zenith angle θ and the azimuth angle φ. Here, θ represents the deviation of the EM waves from the z-axis, while φ defines the angle between the projection of the incident wave onto the XOY plane. Altering the azimuth angle φ effectively changes the polarization direction of the incident wave. As shown in Fig. 6, the FOTMS exhibits significant absorption over the incident angle θ from −60° to 60°. Furthermore, owing to the pronounced symmetrical structure of FOTMS, the absorption almost does not change with the φ, thus, demonstrates great polarization angle insensitivity.

With the continuous advancement of radar technology, traditional stealth techniques have become inadequate for demands of increasingly diverse and complex EM environment. Consequently, the analysis of RCS reduction has become particularly crucial.

Owing to the flexibility of the designed metasurface, it can be appropriately curved with a curvature angle of α. Figure 7 illustrates the simulated monostatic RCS of the FOTMS and PEC with the same area, subject to normal incidence and oblique incidence with 30° of different polarized waves. For the planar structure, Fig. 7(a) shows that across the entire absorptivity band under x- and y-polarization, a significant reduction in RCS is achieved, with the maximum reduction exceeding 35 dB at a frequency of 20 GHz. As shown in Fig. 7(b), the peak value of RCS reduction for x-polarization and y-polarization reaches 27 dB and 26 dB, respectively, at corresponding frequencies of 23 GHz and 21.5 GHz, which confirms that the FOTMS maintains excellent performance even at an incident angle of 30°. Figures 7 (c) and (d) show the scattering pattern within the XOZ and YOZ planes of the FOTMS and a PEC with the same area, subject to normal incidence and oblique incidence with 30° of different polarized waves, respectively. It can be observed that the FOTMS demonstrates significant RCS reduction under normal incidence of both x-polarized and y-polarized waves. When the incident wave arrives at an angle of 30°, the FOTMS can not only effectively reduces the scattered energy in the specular direction but also realizes the reduction of the corresponding backscatter. Compared with the PEC, the RCS reduction performance of the FOTMS is better across the scattering angle ranges from 0° to 360°. Although the reduction in certain angles is slightly lower than the PEC, the proposed metasurface consistently maintains a stable RCS reduction below 10 dB. In the curved state (α = 90° and α = 180°), when it is the normally incident EM waves along the z-axis, Figs. 7(e) and (f) shows that a great RCS reduction could be realized within the frequency band under x-polarization. Meanwhile, for y-polarized wave, the proposed metasurface absorber consistently maintains a stable RCS reduction within specific frequency bands. In the case of oblique incidence with 30°, the monostatic RCS of FOTMS exhibits a significant reduction compared to PEC, as shown in Figs. 7(g) and (h). However, its reduction is slightly less than that observed for normal incidence, and the frequency corresponding to the maximum reduction also shifts. This occurs because the oblique incidence of EM waves affects the impedance matching. Even so, it can maintain a RCS reduction of nearly 10 dB for the defined frequency range. Overall, our design demonstrates satisfactory performance across various conditions, significantly reducing RCS and making it suitable for stealth applications.

4 Experimental results and analysis

4.1 Fabrication of sample

To confirm the actual performance of the proposed absorber, a sample with dimensions of 250 mm × 250 mm was fabricated. In the preparation of IRSL and ITO layers, a uniform ITO film was first deposited on a flexible PET substrate using magnetron sputtering technology, as shown in Fig. 8(a). traditional sputtering technology is carried out under high vacuum conditions, where incident ions (Ar⁺) bombard the target under the action of electric field, causing neutral atoms or molecules on the target surface to gain sufficient kinetic energy to leave the target surface and deposit on the substrate surface to form a thin film. However, electrons are affected by electric and magnetic fields, causing drift, which results in low sputtering efficiency. To improve sputtering efficiency, strong magnets are installed below the target, with the center and periphery serving as N and S poles respectively. Due to the Lorentz force, electrons are confined around the target and continuously move in circular motion, generating more Ar⁺ bombardment on the target and significantly improving sputtering efficiency [26]. Subsequently, specific patterns are achieved throughlaser etching. Additionally, two support structures were fabricated using 3D printing technology to support sample with a central angle of 90° and 180°, respectively. Following that, the four layers of FOTMS, which includes the IRSL, two ITO films and a flexible PVC substrate, were fixed in the conformal framework. Figures 8(c) and (d) depict the sample in the state of planar and bending, providing evidence of its excellent flexibility.

4.2 Experiment and discussion

For verification, the optical transmittance of the sample was first measured using a LS116 light transmittance meter. As illustrated in Fig. 9(a), the FOTMS maintains excellent optical transparency with a transmittance of 87.37%. Based on the single-point transmittance test, to further assess the overall transparency of the entire surface, the sample is evenly divided into 10 × 10, a total of 100 square sub-regions. The transmittance of each region is measured sequentially, and the corresponding heat map is shown in Fig. 9(b). The average transmittance of all areas was calculated to be 85.09%. Additionally, in order to test the absorption performance of the FOTMS, it was located at a microwave anechoic chamber, and its reflection coefficient was measured using free-space technique. As shown in Fig. 9(c), the transmitting and receiving antennas connected with the vector network analyzer through transmission lines are positioned on the same side of the sample, with absorptive materials placed between them for isolation. The measured results and their comparison to the simulated results are illustrated in Figs. 9(d) and (e). It can be seen that the absorption of the proposed FOTMS remains above 90% in the frequency ranges of 15.3−38.4 GHz and 15.6−38.1 GHz under x-polarization and y-polarization, respectively. To be noted, there is an error between the test result and the simulation result, which is related to the loss of dielectric, and the insertion loss of devices in the high-frequency test environment. Despite slight variations, the experimental results are consistent with the simulated results. This provides evidence that the FOTMS exhibits ultra-broadband absorption and high absorption efficiency.

To demonstrate the stealth characteristics of the proposed FOTMS, its millimeter-wave stealth performance was first evaluated by testing RCS reduction, as depicted in Fig. 10(a). The sample is placed on the platform under the horn antennas surrounded by absorbing materials, where its RCS reduction is obtained by comparison to a copper-clad plate (CCL) with the same area. Additionally, the angle of incident electromagnetic wave can be altered by adjusting the robotic arm. As shown in Figs. 10(b) and (c), FOTMS achieves great RCS reduction underblending configuration under normal incidence with both x-polarization and y-polarization. Furthermore, FOTMS maintains a RCS reduction of 10 dB within specific frequency bands even at an oblique incidence of 30°, as shown in Figs. 10(d) and (e). While the maximum reduction exceeding 35 dB and 25 dB can be realized with a curvature angle of 90° and 180°, respectively. Furthermore, it can be seen that the experimental results match well with the simulated results, which verify the stealth performance under millimeter-wave.

The following experiments focused on the IR stealth performance of FOTMS. The infrared emissivity result was measured by the TSS-5X IR emissivity meter first. Fixed the sample horizontally on the platform and used an infrared detector to obtain the infrared emissivity. The specific experimental setup and result are illustrated in Fig. 10(f). Then, as depicted in Fig. 10(g), the reflection spectra were measured using a Fourier transform infrared spectrometer (Thermo Scientific Nicolet IS50). Then the IR emissivity can be calculated based on Eq. (6). Figure 10(h) shows that the average IR emissivity of the proposed IRSL structure is 0.47 in 3–14 μm band, significantly lower than that of PET. To further examined the IR stealth performance of the proposed metasurface, the thermal IR images of a 35 mm × 35 mm IRSL sample was measured with the help of an IR camera, which works in the range of 8−14 μm. Simultaneously, a PET without top IRSL has been used as a reference sample in Fig. 10(i). Both samples were heated on a thermal plate for 5 min. As expected, Fig. 10(j) illustrates that the radiative temperatures of the sample were much lower than the PET without top IRSL. Therefore, the proposed metasurface can effectively realize IR stealth.

Traditional fractional bandwidth (FBW) reflects bandwidth performance but fails to account for the physical dimensions of absorber unit structures. It isone of the core objectives to achieve the widest possible absorption bandwidth within minimal achievable unit volume for application, especially in space-constrained electronic devices. We apply an evaluation metric proven effective in prior research, known as the cost-benefit absorption bandwidth (BW_CEA) [50]. It can be written in the form

(9){BW}_{\mathrm{C}\mathrm{E}\mathrm{A}}=BW/R_v.

The quantity R_v = V/{\lambda }_0^3 describes the relative volume of a unit cell. Where the term V is the total volume of a unit cell, and {\lambda }_0 represents the wavelength at the center frequency of the band over which the absorptivity exceeds 90%.

Figure 11 presents a comparative analysis of the key performance metrics between the proposed metasurface and previously published works. It clearly demonstrates that our design simultaneously realizes all four functionalities of low IR emissivity, optical transparency, millimeter wave absorption, conformal, and performs the most cost-efficient bandwidth. A detailed comparison data is illustrated in Supplementary Table S4.

5 Conclusion

In conclusion, we have designed, fabricated and measured a FOTMS with infrared stealth. The absorber demonstrates exceptional performance, achieving over 90% absorption across the 15 to 38.7 GHz frequency range, encompassing the K, Ka, Ku, and V bands. Its symmetrical structure ensures polarization insensitivity and maintains high absorption at oblique incidence angles up to 60°. Additionally, the entire structure maintains a high optical transmittance of 85.09%, which is critical for practical applications. By comparing the RCS reduction of a PEC, the stable absorption and millimeter-wave stealth performance of the FOTMS at different incident angles was verified. Furthermore, IR stealth is achieved by patching ITO squares on the IRSL, with the infrared emissivity of 0.47 in 3−14 μm IR band. The experimental measurements of both millimeter-wave absorption and IR stealth characteristic showed excellent agreement with the simulated results, demonstrating its potential in the field of multispectral stealth application.

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