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Abstract
This paper presents a novel approach for achieving a multifunctional metasurface capable of multiband compatible stealth. The metasurface is designed with a single-layer metallic structure that integrates functions of radar cross-section (RCS) reduction, laser stealth, and infrared shielding simultaneously. The reduction of RCS is achieved by developing two sub-cells that employ the interference cancellation principle, leading to a 10 dB decrease in RCS across a broad frequency range of 13-21 GHz. The laser stealth capability is attained by implementing a chessboard phase distribution in the array, also based on the interference cancellation principle, efficiently cancelling the specular reflection at the laser wavelength of 1.06 µm. The significant difference in wavelength between microwaves and lasers ensures that their operational characteristics do not interfere with each other. Additionally, the metasurface exhibits an infrared shielding property with an extremely low emissivity (less than 0.03) in the infrared atmosphere window of 3-5 µm and 8-14 µm, enabling the infrared stealth capability. The proposed metasurface demonstrates exceptional performance and has an extremely thin single-layer structure, indicating that it has a promising potential for future applications in multiband compatible stealth.
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1. Introduction
In recent years, there has been a growing interest among researchers in the development of multifunctional compatible camouflage techniques [1–5]. This interest has been driven by the rapid development in multispectral detection technology, which has led to the exploration of various camouflage methods such as infrared stealth, radar stealth, and laser camouflage. One particular focus in this area has been on the compatible stealth design based on metasurfaces, which are two-dimensional planar substitutes for conventional three-dimensional metamaterials [6]. Metasurfaces have unique properties in controlling electromagnetic waves and have considerable benefits for practical applications due to their simple structure and ease of fabrication [7–10]. Consequently, metasurfaces have been extensively investigated for their potential in achieving multifunctional compatible camouflage.
The challenge of achieving multiband compatible camouflage lies in the conflicting requirements of different stealth technologies. For instance, infrared camouflage requires low absorptivity and high reflectance characteristics in order to achieve invisibility in the infrared spectrum. This requirement aligns with Kirchhoff’s law, which is the basic theory for infrared stealth [11]. On the other hand, laser stealth and microwave stealth require high absorptivity and low reflectance to minimize the intensity of reflected waves, which is exactly the opposite of the requirements for infrared stealth. These conflicting requirements are due to their different detection mechanisms employed by each method. Microwave and laser detection are typically active detection methods based on radar, while infrared detection is usually passive based on radiometers. Intuitively, it seems impossible to find materials that satisfy these conflicting parametric requirements simultaneously. Despite its difficulty, several designs based on metasurface have been proposed in an effort to overcome this conflict and achieve multiband compatible camouflage using metasurfaces. One common strategy is to use an infrared shielding layer on the top layer, which is commonly made of a large-area material with low emissivity and small gaps. This configuration allows microwaves to pass through it to the underlying radar absorber layer, which achieves microwave stealth by incorporating extra absorbing material structures [12,13]. Meanwhile, absorbers are also commonly used for laser and microwave stealth purposes [14,15].
Recent studies have primarily focused on bifunctional compatible stealth designs, including microwave-infrared stealth and infrared-laser stealth [16–19]. The capability of compatible stealth can be obtained through the use of chemical stealth materials or the design of physical structures. The chemical method is to manufacture thin films by combining different chemical materials, which can exhibit high absorptivity in the microwave band and low emissivity in the infrared band [20–26]. In 2020, Gu et al. introduced a multifunctional bulk hybrid foam that demonstrated a 10 dB reduction in radar cross-section from 12.36 to 18 GHz. This foam also exhibited excellent heat insulation and thermal stealth properties [21]. Similarly, in 2022, Wu et al. proposed a nano-micro CuS@rGO lightweight aerogels that achieved a 10 dB RCS reduction from 10 to 18 GHz and also lowered surface temperature [22]. The advantage of the chemical method is its ability to be applied to structures of various shapes. However, a challenge arises when attempting to combine laser and infrared stealth using chemical methods, as they require opposite characteristics in close frequency bands. This contradiction hinders the development of multifunctional compatible stealth technologies. Furthermore, the physical structures can integrate functions of laser stealth and infrared shielding, and there have also been several studies on the compatible stealth techniques [27–30]. For instance, in 2017, researchers developed a dual-band wide-angle metamaterial absorber that could absorb laser wavelength of 10.6 µm and 1.06 µm, while also having low emissivity in the infrared band. The average emissivity in the 3-10 µm range was measured at 0.15 [31].
Currently, there is a significant importance in the integration of laser, infrared, and microwave stealth capabilities into a unified structure. A study in 2020 proposed a hierarchical metamaterial for laser-infrared-microwave compatible camouflage [32]. This metamaterial was able to reduce the reflected energy of lasers at 1.06 µm and had an emissivity lower than 5% in the band of 3–14 µm. Also, the incident microwave in the 7-12.7 GHz band can pass through the metasurface and then be absorbed by the bottom absorber. The bottom absorber is a multilayer structure, consisting of resistive sheets embedded in two FR4 spacers. Moreover, in 2022, Feng et al. introduced a large area hierarchical metasurface for laser-infrared-microwave stealth [33]. This design achieved an emissivity lower than 0.2, an ultrawide microwave absorption band in 2.7-26 GHz range, and low laser reflectance at 1.06 µm and 10.6 µm. This design also used multi-layer resistive films to serve as the microwave absorber. These studies indicate that using complex absorber structures to achieve microwave and laser stealth can lead to a highly complex overall structure, thereby increasing processing difficulty and cost. Therefore, improving the control capability of metasurface and integrating the functions of laser-infrared-microwave compatible stealth into a single-layer metasurface is expected to reduce the complexity of the overall structure.
In this paper, a multifunctional metasurface for laser-infrared-microwave compatible stealth is proposed using a single-layer all-metallic structure. The metasurface is designed to reduce radar cross section by 10 dB within the 13-21 GHz frequency range for microwave stealth. For laser stealth, the metasurface operates at a wavelength of 1.06 µm, resulting in a 30 dB reduction in specular scattering. Both the microwave stealth and laser stealth rely on the phase control capability of the designed metasurface, and their operation characteristics do not conflict with each other because of the significant difference in wavelengths. Additionally, the all-metallic metasurface demonstrates the ability to achieve infrared stealth by emitting ultralow levels of infrared radiation, with an emissivity lower than 0.03 in the 3-14 µm range. Therefore, the metasurface we proposed has the advantages of multifunctionality and good properties, with simple structure of single layer. This new design concept present new opportunities for the development and application of metasurfaces that are compatible with laser, infrared, and microwave camouflage stealth.
2. Design and results
To achieve multifunctional stealth capabilities that is compatible with laser, infrared, and microwave frequencies, the metasurface must exhibit low reflectance in the laser band, low emissivity in the infrared band, and low reflectance in the microwave band. This research aims to progressively attain these characteristics, beginning with the fundamental function and advancing towards more sophisticated capabilities, ultimately enabling the metasurface to effectively operate stealth capabilities across multiple frequency bands. The schematic of the proposed multifunctional metasurface for compatible camouflage is depicted in Fig. 1. It follows a typical reflective metasurface configuration, consisting of a metallic surface on the top layer, a substrate and an air layer in the middle, and a metallic layer on the bottom serving as the ground plane. On the top layer, Gold (Au) is chosen as the metal material due to its exceptionally low emissivity in the infrared band and its resistance to oxidation, making it inherently suitable for infrared camouflage. Through the design and arrangement of the unit cells in an array, the metasurface is capable of providing multifunctional camouflage that is compatible with microwave and laser stealth. Consequently, an integrated design of three stealth functions can be realized.
Fig. 1. Schematic of the multifunctional metasurface for laser-infrared-microwave compatible camouflage
The design process consists of three primary stages. Initially, a metasurface is designed using two sub-cells with a 180°±37° phase difference to achieve stealth capability in the microwave frequency range, employing the interference cancellation principle. This design results in a 10 dB reduction in radar cross-section within the 13-21 GHz frequency band. Subsequently, the capability for laser stealth is integrated into the previously developed microwave metasurface. Given the significant difference in wavelength between microwaves and lasers, it is feasible to incorporate metasurfaces for both microwave and laser frequency bands, with their respective operational characteristics not mutually affecting each other. Therefore, the artificial micro/nano structures of the laser metasurface can be designed and fabricated on the upper surface of the microwave metasurface. In the final stage, the infrared shielding function with low emissivity characteristic in the infrared band is also achieved, and the performance of infrared stealth is analyzed. In the following, a detailed introduction to the design principles and results of each part will be provided.
2.1 Design for microwave stealth
In microwave, the primary objective of the metasurface is to maintain the stealth capability by minimizing its radar cross-section. Using the exceptional capability of metasurfaces in manipulating electromagnetic waves, we have designed a metasurface to achieve microwave stealth as an initial step. The configuration of the designed metasurface is illustrated in Fig. 2(a), featuring a chessboard-like arrangement composed of two sub-cells, denoted as subcell I and subcell II. Subcell I has a fully covered metallic patch on the top layer, while subcell II does not. For both subcell I and subcell II, they have same dielectric layer, air layer and ground layer. The material of the substrate is F4B, with a relative permittivity of 2.2 and a loss tangent of 0.0009. The purpose of the air layer between the substrate and ground is to adjust the working band of RCS reduction.
Fig. 2. (a) Schematic of the multifunctional metasurface for microwave stealth, (b) Phase difference of two sub-cells, (c) Far field scattering patterns of the metasurface at 17 GHz, (d) RCS reduction of the metasurface compared to PEC plate.
The basic theory for reducing radar cross-section is based on the concept of interference cancellation. The RCS reduction effect depends on the phase difference between the two unit cells, which can be calculated as [34–37]
where φ1 and φ2 are the reflected phases of the two unit cells. According to this theory, it is necessary to satisfy the reflection phase difference of 180°±37° between two unit cells so that the 10 dB RCS reduction can be achieved. This necessitates that the unit cells in the metasurface are designed to possess a nearly identical reflection amplitude close to 1, and a reflection phase difference of 180°. In this design, the phase difference is determined by the presence or absence of top layer patches. The thickness of the F4B (h1) is 0.2 mm, while the thickness of the air (h2) is 4.0 mm, and the period of the sub-cell (l) is 50.5 mm. Given that both Au and PEC exhibit a reflectance of 100%, the focus is solely on attaining a phase difference of 180°. To evaluate the phase response of designed unit cells, numerical simulations are performed using the commercial software CST Microwave Studio with periodic boundary conditions and plane wave excitation. The simulated results are shown in Fig. 2(b). It is noted that the phase difference maintains 180°±37° in a wide frequency band of 13-20 GHz. Consequently, these two subcells satisfy the criteria for employing the interference cancellation principle.To evaluate the performance of microwave stealth, a metasurface array is constructed using the designed two subcells. The metasurface array is 202 × 202 mm in size and consists of 4 × 4 subcells. These subcells are arranged in a chessboard pattern, as depicted in Fig. 2(a). Full-wave simulations were performed using the Time domain solver of CST. A plane wave with x-polarization propagating along the negative z-axis was used as the excitation, and field monitors were employed to obtain the radar cross-section results. A perfect electric conductor (PEC) plate of the same dimensions was also simulated for comparative purposes. The three-dimensional scattering pattern in the far field at a frequency of 17 GHz is illustrated in Fig. 2(c), showing the reflected wave pattern divided into four diagonal directions in the 45° and 135° plane. As shown in Fig. 2(d), a 10 dB reduction in radar cross-section is observed within the frequency range of 13-21 GHz, corresponding to a bandwidth of 47.0%. Especially, the peak RCS reduction exceeded 40 dB at 17.5 GHz, indicating a highly effective reduction in radar cross section. Furthermore, an analysis of a metasurface without the substrate was conducted to simulate the ideal wave propagation in air, resulting in an extended bandwidth of 50%. The simulation results align with expectations and are marginally lower than the ideal scenario.
Additionally, the relationship between structural parameters and the microwave stealth performances is investigated. Based on the interference cancellation principle, the RCS reduction is related to the phase difference of two subcell, which is determined by the height of air layer (h2). From the simulation results, our designed structure realizes max RCS reduction at 17.5 GHz. Therefore, the phase difference of two unit cells and RCS reduction at 17.5 GHz are simulated as h2 varies, as illustrated in Fig. 3. It is evident that the phase difference remains within the range of 180°±37° as h2 increases from 3.5 mm to 4.5 mm, resulting in a consistent RCS reduction of better than -10 dB. Overall, the simulation results confirm that our designed structure aligns with theoretical expectations.
In the previous discussion, it is noted that there is no metallic patch on the top layer for subcell II. In the subsequent investigation, the top layer of subcell II will be substituted with a frequency-selective surface (FSS), as illustrated in Fig. 4(a). This modification aims to facilitate the incorporation of laser stealth capabilities and enhance infrared stealth performance by expanding the coverage area of metallic materials. It is well-known that periodic patches with gaps are transparent for microwaves of specific bands based on the standing wave resonance theory [38], and microwaves can pass through them as if there are no obstacles. Consequently, the modified subcell II exhibits identical electromagnetic responses to the original one, and the metasurface comprising new subcells can still generate a chessboard-like phase arrangement, as illustrated in Fig. 2(a).
Fig. 4. (a) The structure of FSS, (b) Simulated reflection magnitudes and phases of the subcell II with and without FSS.
As mentioned previously, the periodic patches with gaps can be understood as an FSS. This FSS exhibits ideal reflection characteristics only at the resonant frequency, allowing microwaves with frequencies lower than the resonant frequency to pass through. The resonant frequency can be calculated using the following equation [38],
where n = $\sqrt{\varepsilon_{r}}\,=\,\sqrt{2.2}$ is the refractive index of F4B, $w$ = d = 100 µm is the width of the patch and c = 3 × 108 m/s is the speed of light in vacuum. The gap width between the patches is 1 µm, so p = 101 µm. It is obvious that as the patch size decreases, the resonant frequency increases proportionally. Through calculations, it has been ascertained that the resonant frequency f equals 1 THz for this design. This indicates that low frequency microwaves ranging from 10-30 GHz can efficiently transmit the FSS structure.In order to evaluate the influence of FSS on microwave transmission, a comparative analysis was conducted using subcell II with and without FSS. The numerical simulation was performed using the frequency domain solver in CST Microwave Studio. The simulation results are depicted in Fig. 4(b), showing that the reflection magnitudes of subcells with and without FSS are both equal to 1, and the reflection phase with FSS is marginally offset. This indicates that the interaction between FSS and the dielectric layer will have a minor influence on the reflection phase, which can be rectified by adjusting the thickness of the air layer. Here, by modifying the thickness of the air layer from the original 4 mm to 3.6 mm, the desired phase response can be obtained. Consequently, the modified subcell II, utilizing FSS as the top surface, exhibits identical electromagnetic responses to the original subcell without FSS, and the far-field scattering pattern of whole metasurface remains unchanged, as depicted in Fig. 2(c). The metasurface, composed of subcell I and modified subcell II, has successfully achieved the function of microwave steal and has created conditions for integrating laser stealth and infrared steal functions.
2.2 Design for laser stealth
Next, the function of laser stealth is developed based on the previously designed microwave stealth metasurface. Given the significant difference in wavelength between microwaves and lasers, it is feasible to incorporate metasurfaces for both microwave and laser frequency bands, with their respective operational characteristics not mutually affecting each other.
Similar to radar detection, laser detection is also an active detection method. Consequently, achieving laser stealth involves diminishing the reflection signal of the target to the laser, thereby reducing the probability of the target being detected. The primary objective for laser stealth is to decrease the surface reflectivity of the target and minimize the laser radar scattering cross-section (LRCS) of the target. According to the generalized Snell’s law [39], abnormal reflection will occur by introducing abrupt phase change of metasurface. Utilizing the interference cancellation principle, as illustrated in Fig. 5(a), the electromagnetic wave is reflected at the interface, while the incident wave reaches the all-metal structure. Upon reaching the surface, the reflected wave acquires a propagation phase. As the plate is composed of a perfect electric conductor, the reflection coefficient is -1. Consequently, different reflection phase β1 and β2 can be generated by varying thickness of different units. By appropriately adjusting the phase difference, the overall reflected electromagnetic wave, which is the superposition of these multiple reflections, can be effectively eliminated. As a result, units with the identical amplitudes and 180° phase difference can direct the reflected wave towards nonthreatening angles. Based on this principle, a laser stealth structure is proposed as shown in Fig. 5(b), which is an all-metal metasurface array with chessboard-like configuration. The working wavelength of the structure is designed at 1.06 µm.
Fig. 5. (a) Schematic of phase canceling for laser wave, (b) The all-metal metasurface array with chessboard-like configuration.
The proposed metasurface consists of two unit cells, as shown in Fig. 6(a). To simplify the structure, the only difference between the two unit cells is their thickness. By introducing a thickness difference equivalent to one-fourth of the incident wavelength, a phase difference of 180° in the reflected wave can be achieved, which meets the requirements of the interference cancellation principle. The geometrical parameters of the structure, including d1 = 1 µm, p1 = 2 µm, h3= 0.265 µm, and h4= 0.1 µm, were used in the simulations of the electromagnetic responses of the two unit cells. CST Microwave Studio was employed for these simulations, with periodic boundary conditions applied in both x and y directions, and an open boundary in the z direction. As shown in Fig. 6(b), a phase difference of 180° is observed at the designed central wavelength of 1.06 µm, which is depicted as the red dashed line, and this phase difference remains close to 180° across a wide range of wavelengths. The reflectance of the two unit cells is consistently above 90% and approaches 100% due to the total reflection characteristics of gold. Therefore, these simulation results are consistent with theoretical calculation and demonstrate the suitability of the proposed structure for a chessboard arrangement.
Fig. 6. (a) The two unit cells of the metasurface, (b) Reflection phase and phase difference of two unit cells.
Additionally, the relationship between structural parameters and the laser stealth performance is investigated. According to the interference cancellation principle, the thickness of the unit cell (h3) is a significant parameter which determines the laser stealth performance directly. Thus, we simulated the phase difference and LRCS reduction as h3 varies, as depicted in Fig. 7. It is observed that the phase difference exhibits a linear decrease as h3 increases from 0.25 µm to 0.28 µm, with the reduction in LRCS reaching its peak of 30 dB at h3 = 0.265 µm, where the phase difference is approximately 180°. These results demonstrate that the designed metasurface structure achieves the laser stealth capability with theoretical expectations.
Using the designed unit cells, a metasurface array is constructed with a phase distribution resembling a chessboard pattern. For this kind of chessboard-like phase arrangement, the direction of the reflected field (θ, φ) can be calculated by the following equations [34,40]
where dx = dy = d1 = 1 µm are the length of the unit cells, and λ=1.06 µm is the wavelength of incident light. According to Eq. (1), φ=±45° can be obtained, indicating that the energy of the reflected field is mainly concentrated at angles of 45° and 135°. Through calculation, the elevation angles are determined to be θ=±48.5°. Thus, the specular reflected wave can be effectively minimized.Full-wave simulation was performed to validate the effectiveness of the designed metasurface for manipulating laser beams. The 20 × 20 array (as shown in Fig. 5(b)) is simulated using the time domain solver in CST. A plane wave with x-polarization was used as the excitation, propagating along the negative z-axis. The scattering pattern of the metasurface at the φ=45° plane is shown in Fig. 8(a). The results indicate that the reflection elevation angle θ=±48.5°, which is in accordance with the theoretical calculations. The simulated monostatic scattering pattern is shown in Fig. 8(b). It is noted that the specular reflectance is dramatically reduced. The scattering pattern of the designed metasuface has a reduction of 30 dB at the wavelength of 1.06 µm compared to the reference Au plate, which verifies that the designed metasurface has the capability of laser scatting reduction.
Fig. 8. (a) Scattering pattern of the metasurface in ϕ = 45° plane at 1.06 µm. (b) The comparison of monostatic scattering patterns of the metasurface and Au plate at 1.06 µm.
So far, the designed metasurface has able to achieve both microwave and laser stealth functionalities. Next, efforts will focus on achieving infrared stealth, effectively combining all three stealth functions into a unified metasurface structure.
2.3 Design for infrared stealth
In the infrared band, the effect of thermal infrared stealth is determined by infrared emissivity. The emissivity of an object is related to the properties of the material. According to the Kirchhoff’s law, to nontransparent material, the relationship between the infrared emissivity (E), absorptivity (A), and reflectivity (R) can be expressed as
Therefore, the metasurface’s emissivity in infrared band can be obtained through the absorptivity results. Low emissivity refers to a low level of absorption and a high level of reflectivity.
As mentioned previously, in order to meet the requirement for high microwave transmittance of the metasurface, it is necessary to introduce gaps between the all-metal arrays. The existence of gaps may potentially influence the effectiveness of infrared stealth characteristic. In this case, the infrared emissivity can be calculated as [41,42]
where $\varepsilon$ is the emissivity of the metasurface. $\varepsilon_{d}$=0.955 is the emissivity of FR-4 in infrared wavelengths 3 to 14 µm, and is the emissivity of the gold. $\varepsilon_{m}$ is the filling ratio of the metal material. The configuration of the infrared shielding structure is shown in Fig. 9(a), where there are extremely thin gaps between the patches. The period of these gaps is p = 101 µm, and the size of metal part is d = 100 µm. Thus, the filling ratio ${f}_{m}$ can be calculated as 98.03%.Fig. 9. (a) Diagram of the metasurface for infrared stealth, displaying a portion of subcell II. (b) Simulated emissivity of the metasurface in 3-14 µm.
To characterize the emissivity accurately, full-wave simulations are performed using frequency domain solver. A plane wave with x-polarization was used as the excitation, propagating along the negative z-axis, with periodic boundary conditions applied in both x and y directions, and an open boundary in the z direction. Benefited by all metal design of the array, the emissivity of the metasurface is ultralow and the average emissivity is 0.016 in the wavelength ranges of 3-14 µm, as shown in Fig. 9(b). In the infrared atmosphere window of 3-5 µm and 8-14 µm, the average emissivity is 0.015, 0.02, respectively, indicating a good infrared stealth capability.
Hence, through theoretical analysis and numerical simulations, we have demonstrated the laser-infrared-microwave compatible camouflage functionalities of the designed metasurface. For the manufacturing of this type of metasurface, high-precision micro/nano fabrication techniques are required to obtain the desired microstructures. The metasurfaces in Ref. [3,33,43–45] are similar to our design, which can offer guidance for the future fabrication of this metasurface.
2.4 Comparison
In order to better understand the performance of our designed metasurface, we have compiled a comparison of infrared emissivity, RCS reduction bandwidth and laser specular reflectance performances of some typical works in Table 1. The traditional bifunctional compatible stealth designs mainly focused on infrared emissivity and then added microwave or laser stealth capabilities [1,3,12,17], whereas our work integrates all three shielding functions together. Besides, compared to [15,27,32,33], our design has achieved a lower emissivity in infrared band. In [33], the 10 dB RCS reduction bandwidth is exceptionally wide due to the use of a multilayer absorber, whereas our design achieves a bandwidth of 13-21 GHz based on a single-layer structure. It should be emphasized that our approach simplifies design due to the use of interference cancellation principle. Therefore, our proposed metasurface has the advantages of multiple functionality, simple structure, and high performance, providing a novel method for designing multispectral compatible camouflage metasurfaces.
Table 1. Comparison of our work and previous researches
3. Conclusion
In this paper, a multifunctional metasurface for laser-infrared-microwave compatible stealth is proposed based on a simple single-layer structure. The metasurface integrates with RCS reduction, laser stealth and infrared shielding function together. The reduction of RCS is achieved by developing two sub-cells that employ the interference cancellation principle, leading to a 10 dB decrease in RCS across a broad frequency range of 13-21 GHz. The laser stealth function can dramatically reduce specular reflectance and the LRCS has a reduction of 30 dB at the laser wavelength of 1.06 µm compared to the reference gold (Au) plate. In addition, the infrared shielding function has ultralow emissivity lower than 0.03 in the 3-14 µm range. This novel metasurface, along with its design methodology, offers valuable insights for the development and application of camouflage metasurfaces that are compatible with laser, infrared, and microwave frequencies.
Funding
National Natural Science Foundation of China (62201118); Fundamental Research Funds for the Central Universities (DUT20RC(3)048); Project of State Key Laboratory of Millimeter Waves (K202405).
Acknowledgments
The authors would like to thank the School of Information and Communication Engineering, Dalian University of Technology, and State Key Laboratory of Millimeter Waves for their support.
Disclosures
The authors declare no conflict 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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