With the advancement of modern science and technology, detection methods have become increasingly diversified, rendering single-frequency band stealth materials inadequate for contemporary detection environments. Currently, radar and infrared detections are the predominant means of surveillance. Radar cross-section (RCS) reduction serves as a crucial method and evaluation metric for radar stealth [1,2]. The main approach to achieving infrared camouflage is to reduce the infrared discrimination between the target and background [3,4]. In complex infrared backgrounds, relying on a single infrared emissivity proves insufficient for effective camouflage. Customized infrared camouflage relying on the infrared emissivity gradient is more advantageous. Metasurfaces, as two-dimensional structures composed of sub-wavelength elements arranged according to specific patterns, exhibit unique characteristics in electromagnetic wave manipulation [5–9]. Customized control over electromagnetic waves can be achieved through meticulous design of metasurface elements [10–12]. Furthermore, these element structures typically consist of metallic and dielectric materials. The low processing costs and ease of maintenance associated with these materials are emerging as pivotal directions in the research of novel stealth technologies [3,4,13–16]. To adapt to the complex environment where low infrared emissivity and high infrared emissivity coexist, a radar stealth-infrared camouflage compatibility metasurface requires meta-atoms with customized infrared emissivity. Various studies have explored different metasurfaces to achieve effective camouflage across multiple spectra [16–18]. For instance, a study by Zhu et al. introduced a device for multispectral camouflage consisting of a ZnS/Ge multilayer and a Cu-ITO-Cu metasurface [19]. This approach aimed at wavelength-selective emission for camouflage in different spectra. Additionally, transparent metamaterials can be utilized for multispectral camouflage, as demonstrated by Lee et al. [20], achieving high transmissivity in the visible regime. Furthermore, the development of dynamically tunable metasurfaces has been proposed for multispectral camouflage and radiative cooling by Zhou et al. [21]. This metasurface incorporates materials such as gold, antimony selenide, and aluminum to enable adaptability across various spectra. Moreover, multifunctional-hierarchical flexibility metasurfaces have been proposed for multispectral compatible camouflage of microwave, infrared, and visible spectra by Kang et al [22]. These metasurfaces offer versatility and effectiveness in achieving camouflage across multiple spectra. Additionally, Luo et al. proposed a large-area low-cost multiscale-hierarchical metasurface, which has been developed for multispectral compatible camouflage of dual-band lasers, infrared, and microwave [23]. Xue et al. proposed one layer of periodic conducting patches with a high occupation ratio that is put on the top of the metasurface and can reduce the infrared emissivity effectively; the patches can also be used as a frequency-selective surface to couple with the microwave layer [24], which simplifies the structure. The camouflage at high temperatures is also studied through high-temperature metal materials [15,25]. Limited by resonant interference, the above research still separates the devices in different spectra into multi-layers. The multi-functional layers can realize excellent microwave absorption and extremely low infrared emissivity, which partly ignores the depth, weight, structural complexity, and processing cost of the metasurface. In addition, for the high-infrared-radiation environment or hybrid infrared radiation backgrounds where low infrared radiation and high infrared radiation coexist, such as forests, bushes, and densely populated construction areas, the traditional low infrared emissivity camouflage methods will be more obviously observed. So a digital camouflage with various infrared radiations is required. To solve the above problems, a non-invasive inset-integrated meta-atom is proposed, which can achieve a single-layer metasurface simultaneously with coded microwave reflectivity and digitalized infrared emissivity [26]. However, when interfered with by the Lorentz resonance, the infrared emissivity range is limited to maintaining the microwave performance.

To improve the compatibility performance of integrated metasurfaces, in this paper, a single-functional layer metasurface simultaneously with microwave scattering reduction and customized infrared emissivity is designed first. In the metasurface, the split square ring serving as a microwave device can realize polarization conversion at 3.5–5.5 GHz, which reduces the co-polarized scattering field. For the cross-polarized scattering, the metasurface is arranged according to a coding metasurface [27], in which the scattering beams are reshaped and spread to the sides. The metal patches in the middle of the metasurface serve as an infrared device, which can regulate the infrared emissivity by changing the sizes. For customizing infrared camouflage, the metasurface requires meta-atoms with customized infrared emissivity. Generally, the low infrared emissivity requires the meta-atom with a high occupation ratio, in which a large area of a metal patch is used to fill the surface. However, the high occupation ratio will interfere with the scattering reduction function due to the Lorentz resonance from the metal patch. To address the problem, the metasurface is optimized based on decoupling Lorentz resonance. By dividing the patch into four parts, the Lorentz resonance in the scattering reduction band is suppressed and shifted to high frequency. After optimization, the range of the infrared emissivity is expanded from 0.60–0.80 to 0.51–0.80, and the microwave performance is not interfered with. The sample is fabricated and experimented in microwave and infrared bands, and the measured results are consistent with the simulated results. The work offers theoretical support for the integration and enhancement of the multispectral metasurface. The work can be used for stealth in the S-band and C-band, which is essential for low detectability over long distances. In infrared camouflage, the designability of infrared emissivity provides a higher degree of customized camouflage according to different infrared backgrounds.

Figure 1(a) gives the overall design framework. The metasurface is composed of three layers. As Fig. 1(b) shows, the bottom is the metal copper, with the height $h_1=0.018\mathrm{mm}$. The second layer is the dielectric layer, for which the material is polytetrafluoroethylene (called F4B as well), with dielectric constant $\epsilon =2.65$, loss tangent $\tan \delta =0.001$, and $h_2=6.5\mathrm{mm}$. The top layer is the functional layer, which is made of copper as well. First, the microwave device is designed, which is called a polarized rotator. The shape of the rotator is a split open ring. The period of the meta-atom $p=10\mathrm{mm}$, the opening seam width $l=1\mathrm{mm}$, the width of the ring $d=0.5\mathrm{mm}$, the outer side length of the ring $b=8\mathrm{mm}$, and the thickness $h_3=0.018\mathrm{mm}$.

The initial rotator is simulated in CST to analyze the polarization conversion performance. The polarization conversion rate (PCR) is used to measure the efficiency of the polarization conversion, which is the dimensionless data in the range of zero to one. Take the $x$-polarized incident wave for example; the PCR represents the $y$-polarized reflected wave rate of the reflected wave. The PCR is defined as $$\mathrm{PCR}=\frac{r_{xy}^2}{r_{xx}^2+r_{xy}^2}=\frac{r_{yx}^2}{r_{yy}^2+r_{yx}^2},$$(1)where $r_{xx}=r_{yy}$ represents co-polarization and $r_{xy}=r_{yx}$ represents cross-polarization. The co-polarized reflectivity, cross-polarized reflectivity, and PCR are given in Fig. 1(c) after simulation. Through observing the data curves, there are three resonant peaks in the 3.5–7.0 GHz range. Under the assistance of three resonant peaks, more than 90% polarization rotation efficiency is achieved in the whole frequency band.

Then the mechanism of the polarization conversion is analyzed. The three resonant frequency points can be classified into electric resonance and magnetic resonance. The resonance between incident EM waves and the metasurface generates the phase gradient in different directions of the reflected waves, which is the main factor in polarization conversion.

The $y$-polarization wave is chosen as the incident wave, and the $u$- and $v$-coordinate system is introduced. The $u$-axis is along a 45° direction concerning the $y$-axis and perpendicular to the $v$-axis, as shown in Fig. 1(d). Then we consider that the EM wave is polarized along the $y$-axis. The electric field can be decomposed into two orthogonal components (directions $\overrightarrow{u}$ and $\overrightarrow{v}$). Hence, the electric field of the incident EM wave can be expressed as $$\overrightarrow{E}=\overrightarrow{u}{E}_{iu}{e}^{i{\phi }_u}+\overrightarrow{v}{E}_{iv}{e}^{i{\phi }_v},$$(2)where $\overrightarrow{u}$ and $\overrightarrow{v}$ are the meta-atom vectors in the $u$- and $v$-axes, respectively. When the EM wave is incident into the surface, the electrons motivated by the waves will move along the copper pattern. The equivalent current direction can be classified into two types. For the first type, as shown in Fig. 1(d) panel (i), the equivalent direction is the $v$-axis, and in the $u$-direction, the surface can be regarded as a perfect electrical conductor (PEC), causing a 180° phase change of the $u$-polarization wave. In the $v$-direction, the reflective phase is composed of three parts: the phase caused by magnetic resonance between the rotator and metal plate, the transmission phase in the dielectric layer, and the initial reflective phase. When the reflective phase in the $v$-direction is the same as the incident phase, the field synthesized by $E_{ru}$ and $E_{rv}$ will be changed to the $x$-direction. Therefore, the direction of reflective polarization is rotated by 90°. For the second type, as shown in Fig. 1(d) panel (ii), the equivalent direction is the $u$-axis; the surface can be regarded as a perfect electrical conductor (PEC), causing the $v$-polarization wave to have a 180° phase change. In the $u$-direction, the reflective phase is composed of three parts: the phase caused by magnetic resonance between the rotator and metal plate, the transmission phase in the dielectric layer, and the initial reflective phase. For the multiple plasmon resonance, there are mainly two types. The first one is the electric resonance, and the second one is the magnetic resonance. The electric resonance is produced by the oscillations of the free electrons in the metal copper. Motivated by the reflected EM waves, the free electrons will move in one specific direction, which results in the reflected phase mutation in that direction. When the direction of the surface current on the top is counter-parallel to the current on the bottom, the magnetic resonance is formed, which can result in the reflected phase mutation as well. When the reflective phase in the $u$-direction is the same as the incident phase, the field synthesized by $E_{ru}$ and $E_{rv}$ will be changed to the $x$-direction.

For an infrared (IR) emissivity gradient, the key is to design and regulate the surface emissivity of materials. According to Kirchhoff’s law, in thermal equilibrium, the emissivity and absorptivity of a material are numerically equivalent at a given temperature and wavelength. Emissivity is an intrinsic property of the material itself. Most common metals exhibit high reflectivity with low absorptivity, making them suitable for use in coating-type infrared low-emissivity materials. Some precious metals such as gold (Au), platinum (Pt), and silver (Ag) have demonstrated excellent low-emissivity properties. Additionally, certain common metals can also achieve low emissivity while being more cost-effective, including aluminum (Al), copper (Cu), and zinc (Zn). Empirical formulas are generally employed to determine macroscopic infrared emissivity [28,29]: $$\epsilon ={\epsilon }_m·f_m+{\epsilon }_d·(1-f_m),$$(3)where $\epsilon$ is the symbol of emissivity, ${\epsilon }_m$ represents the metal emissivity, ${\epsilon }_d$ represents the dielectric emissivity, and $f_m$ represents the occupation ratio. Commonly the copper emissivity is 0.1 while the F4B emissivity is 0.9 [28,29].

Based on the above principle, the infrared design idea is shown in Fig. 2(a). The metal patches as the infrared device are inserted in the middle of the metasurface. The sizes of the patches are represented by parameter $x$. Due to the low infrared emissivity of copper, the infrared emissivity can be modulated by changing the sizes of patches. As the occupation ratio increases, the infrared emissivity will decrease. In that case, the infrared emissivity characteristics of metasurfaces can be customized and designed according to the environmental requirements.

The microwave device and the infrared device are on the same layer with the same height. It is essential to discuss whether the microwave rotator is interfered with when the occupation ratio changes. The co-polarized reflectance, cross-polarized reflectance, and PCR at different occupation ratios are simulated and the results are shown in Figs. 2(b)–2(d), respectively. From the results, when the size of the patch is more than 5 mm (occupation ratio: 0.375), the polarization conversion bandwidth is reduced. If the microwave performance is maintained, the range of the occupation ratio is limited, which weakens the infrared performance. To avoid interference between the rotators and patches, the key is to decouple.

Resonance is a universal phenomenon in nature. In the EM field, resonance occurs when the external excitation is consistent with the natural frequency of the system. Lorentz resonance is a classical model describing the forced vibration of charged particles (such as electrons) in an electromagnetic field and is particularly important in explaining optical dispersion and polarized conversion between media and metals. This model successfully reveals the basic mechanism of the interaction between electromagnetic waves and matter by analogy with the simple harmonic vibration model of the spring oscillator. Resonance occurs when the input frequency of the electromagnetic wave is equal to the eigenfrequency [30].

To find the resonant frequencies of the rotator and patches, the reflection coefficients are simulated successively; the results are shown in Figs. 3(a) and 3(b). As the size of the patches increases, the resonant frequency moves in the low-frequency direction, which approaches the resonant frequencies of the rotators. The electromagnetic response of different devices will inspire new resonance and change the initial electric field, resulting in changes in the reflective phase. That is the reason why the polarization conversion bandwidth is reduced when the size of the patch is more than 5 mm.

To reduce the influence of large-sized patches on polarization conversion, the resonance between the rotator and metal patch should be decoupled at 3.5–7 GHz. The surface current of the initial rotator (without patch) and rotator (with patch size 5 mm) at 7 GHz is simulated and the results are shown in Figs. 3(c) and 3(d). Compared to the initial rotator, the current on the patch is anticlockwise, which is opposite to the rotator. According to the right-hand rule, the patch will reduce the magnetic resonance in the incidence direction of electromagnetic waves, which will stimulate more current in the rotator, and the initial polarization rotation mode is broken.

If the patch is divided into four small parts with slits, the current in the middle will be reduced and the magnetic resonance in the opposite direction is weakened obviously. According to the simulated result in Fig. 3(e), the current is suppressed to the initial level, which verifies that the decoupling effect appears. The width of the slit is only 0.1 mm, which will not influence the original infrared emissivity.

The co-polarized reflectance, cross-polarized reflectance, and PCR of the meta-atoms improved by decoupling Lorentz resonance at different occupation ratios are simulated again. The results are shown in Figs. 4(a)–4(c), respectively. From the results, the interference from the infrared patches to the rotator is reduced. The polarization conversion performance can be maintained until the size of the patch is more than 6.0 mm, which means the lowest infrared emissivity can be reduced from 0.60 to 0.51.

Improved by Lorentz resonance suppression, the patches with slits are used in the meta-atoms to replace the initial whole patches. To embody the infrared gradient design, the four types of meta-atoms are chosen according to the sizes of the patches. The chosen sizes are 0 mm, 2 mm, 4 mm, and 6 mm, and the infrared emissivity is 0.80, 0.76, 0.67, and 0.51, respectively. The metasurface is arranged by the above meta-atoms and forms spatial gradients of infrared emissivity.

In 2014, Ding and Monticone introduced the P-B phase theory [31,32], which can be used in broadband phase modulation. The phase of electromagnetic waves could vary with the rotation of metallic patterns on the surface of an element, establishing a corresponding relationship whereby a metal’s rotation induces a phase difference. If the rotator is rotated 90°, the phase of the cross-polarization reflective wave will realize a change of 180°, and the amplitude remains unchanged. To validate this theory, comparisons are made between cross-polarized reflective amplitudes and phases post-rotation versus those pre-rotation, as illustrated in Fig. 5(a). The findings depicted align with theoretical expectations. In the same year, Cui et al. proposed design principles for coding metasurfaces [27], offering innovative strategies for controlling electromagnetic wavefronts. According to the coding metasurface, the initializing meta-atoms are called code “0”, while the rotated meta-atoms are called code “1”. The alternatively arranged phase coding sequence “00001111……” is introduced to build the coding metasurface; Fig. 5(b) shows the diagrammatic sketch of the metasurface code 0 and code 1. The metasurface can scatter the vertical incident electromagnetic waves into two beams, and the reflective waves have deviated, causing a great reduction of the radar cross section (RCS) in the vertical incidence [32].

To verify the functional effect of the metasurface under the alternative arrangement, a $200\mathrm{mm}\times 200\mathrm{mm}$ metasurface is constructed, which consists of $20\times 20$ structural units and is divided into four regions according to the infrared emissivity gradient. The CST is utilized to perform the far-field simulation with the setup of the three monitoring points at 3.6 GHz, 4.6 GHz, and 5.6 GHz. To emphasize the effect of the metasurface, we simulate the same size as the ordinary copper metal plate. The simulation results of the two surfaces are shown in Fig. 5(d). The backward RCS energy of the metasurface is reduced compared with the ordinary copper plate, and the main energy is divided into two beams and scattered in other directions. The energy that still exists in the backward direction is because of the side-valve effect and the influence of the infrared devices. To further compare the backscattering RCS reduction effect of the metasurface, the RCS in the vertical direction is compared with the copper plate of the same size, as shown in Fig. 5(e). The metasurface has a significant backscattering RCS reduction effect, and the backscattering RCS reduction value at 3.5–7 GHz is 5 dB on average. Some frequency bands even reach a 10 dB reduction. Notably, RCS reduction is just one example of microwave performance. Through coding theory, the metasurface can be encoded to anomalous reflections, multi-order vortex waves, and so on through different requirements.

A prototype of the metasurface is fabricated through conventional printed circuit board (PCB) techniques. The microwave experiment measuring system is illustrated in Figs. 6(a) and 6(b). The measurements were carried out in a microwave anechoic chamber based on a network analyzer (Agilent E8363B) with two pairs of broadband antenna horns whose frequency bands are 2–6 GHz and 6–12 GHz. The center of the transmitting antenna and receiving antenna is placed in line with the normal metasurface. In this case, the vertical incidence of the antenna was first aligned to the metal backplane, and the RCS data was normalized for calibration to eliminate the effects of coupling between the antennas. The detailed photograph of it is illustrated in Fig. 6(b), which is the same as the metasurface in the CST. The size of this metasurface is $200\mathrm{mm}\times 200\mathrm{mm}$ including 400 meta-atoms. Then, the antenna was aligned to the sample surface to measure the co-polarization reflection coefficient, and the RCS reduction data of the metasurface was directly obtained, which is shown in Fig. 6(d). The measured and simulated results are similar, with slight difference in frequency. The difference between simulation and experimental curves is due to the changes in the permittivity of F4B occurring during the processing, which can lead to the frequency shift in the RCS reduction band. The simulation and experimental curves are consistent in trend, showing more than 50% RCS reduction at 3.5–5.5 GHz, which fully demonstrates the microwave performance of the metasurface.

Subsequently, the infrared properties of the samples were systematically evaluated. Initially, a TSS-5X infrared emissivity tester was employed to measure the infrared emissivity across various regions of the sample. A metal probe was positioned vertically on the surface, and readings were recorded once stability was achieved. The specific experimental setup and results are illustrated in Figs. 7(a)–7(d). It is observed that the infrared emissivity of the regions with different occupation ratios is measured at 0.51, 0.70, 0.82, and 0.87, respectively. These values align closely with theoretical predictions and exhibit pronounced gradient characteristics. To obtain a more precise assessment of the sample surface’s infrared emissivity, an infrared spectrometer was utilized to analyze its emissivity within the wavelength range of 3–14 μm. The results obtained are presented in Fig. 7(e). The meta-atoms with different occupation ratios yield a stable gradient feature in terms of infrared emissivity on the sample surface, further verifying the designability of the infrared signature of the meta-atom. The difference between the measured values and theoretical values is caused by the position deviation of the detected light spot. Moreover, the errors can also come from the inherent roughness and wear on the sample surface. Finally, comprehensive infrared imaging of the sample was conducted using an infrared imager. These imaging results were subsequently compared against those from a standard copper plate as shown in Fig. 7(f). In contrast to single-point measurements from copper plates’ thermal emission properties, significant gradient changes were observed across different regions of fabricated samples. Four kinds of infrared emissivity form a hierarchical infrared pattern, which can match different infrared backgrounds formed by different materials. By adjusting the proportions of different infrared emissive regions, the metasurface can camouflage in different environments with low infrared radiation or high infrared radiation. To prove the temperature robustness, the sample was placed on the heating plate. At 60°C and 90°C, the infrared images at different heating times are observed and recorded in Fig. 7(g).

In the infrared images from the infrared imager, according to the Stefan-Boltzmann law, the radiated power of the sample can be expressed as $$M=\epsilon \sigma T^4,$$(4)where $\epsilon$ represents the infrared emissivity of the sample, $\sigma$ represents the Stefan constant, and $T$ represents the surface temperature. In infrared images, the surface temperature is inferred under the $\epsilon$ of 1 (default value for infrared imager) and the $M$ (perceptual intensity of infrared imager). So, they can build an equation: $$M={\epsilon }_{\text{Actual}}\sigma T_{\text{Actual}}^4={\epsilon }_0\sigma T_{\text{Inferred}}^4.$$(5)

The $T_{\text{Actual}}$ are provided in Figs. 7(a)–7(e). The relationship between the ${\epsilon }_{\text{Actual}}$ and apparent temperature $T_{\text{Inferred}}$ can be calculated through $$\frac{{\epsilon }_{\text{Actual}}}{{\epsilon }_0}={\left(\frac{T_{\text{Inferred}}}{T_{\text{Actual}}}\right)}^4.$$(6)

Equation (6) can reflect the positive correlation between temperature and emissivity. In the case of heat source heating, different emissivity regions have different effects on radiation heat dissipation, resulting in slight differences at apparent temperature.

The microwave and infrared characteristics of the sample are tested, which effectively verifies the accuracy of the design theory, and proves that the metasurface has compatible properties of adjustable infrared space and scattering reduction.

Table 1 presents a comprehensive comparison of various characteristics between our proposed metasurface and previously reported advanced camouflage devices. The results demonstrate that the metasurface proposed in this work exhibits a simpler structure (single functional layer compatible with microwave and infrared camouflage) and a broader infrared emissivity gradient.Table 1.

Comparison of the Proposed Metasurface with Previously Reported Studies

In the microwave experiment, the experimental systematic errors mainly include antenna noise interference, fabrication precision, the finite number of meta-atom cells, and the loss of transmission lines.

In the infrared experiment, the experimental errors were mainly reflected in the infrared emissivity of different regions. In the process of average emissivity measuring through the TSS-5X infrared emissivity tester, the metal probe can cover the whole meta-atom, and the results are closest to the theoretical values. However, the roughness of the sample surface will make the test result higher than the theoretical value. In the infrared spectrum experiment, the test aperture is smaller than the meta-atom, which will not cover the whole meta-atom. The results will be influenced by the occupation ratio in the aperture area. In the infrared imaging experiment, because of the convection and heat dissipation, the actual temperature on the platform is less than the setting temperature, which makes the calculated emissivity from the perceived temperature lower than the actual emissivity. In addition, the perceived errors can result in fluctuation of infrared emissivity.

In this paper, a single functional layer metasurface simultaneously with microwave scattering reduction and customized infrared emissivity is proposed. The metasurface integrates the microwave device and infrared device into the same layer. To reduce the interference of different devices and improve multispectral compatibility, the meta-atoms of the metasurface are promoted by decoupling Lorentz resonance. The initial infrared patches are divided into four parts with slits, which can shift the resonant frequency of the patches to high frequency. The Lorentz resonance in the polarization conversion band is suppressed, and the polarization conversion performance of meta-atoms can be maintained with the high occupation ratio. After the optimization, the infrared emissivity range is expanded from 0.60–0.80 to 0.51–0.80. To verify the design method, the sample was fabricated and subjected to experiments in microwave and infrared bands, and the measured results were found to follow the simulated results. In future studies, more mechanisms can be combined to optimize the rotator to extend the microwave functional bandwidth and infrared radiation range.