Open Access
Add to BibSonomy
Abstract
Metamaterials offer exciting opportunities for developing multispectral stealth due to their unique electromagnetic properties. However, currently transparent radar-infrared-visible compatible stealth metamaterials typically involve complex hierarchical designs, leading to thickness and transparency limitations. Here, we propose an integrated metamaterial for multispectral stealth with high transparency. Our design features an ITO/dielectric/ITO sandwich structure, with the upper-layer ITO acting as a resonator for broadband microwave absorption while maintaining a high filling ratio to suppress infrared (IR) radiation. Experimental results demonstrate excellent performance, with over 90% microwave absorption in 8–18 GHz, an IR emissivity of approximately 0.36 in 3–14 µm, an average optical transmittance of 74.1% in 380–800 nm, and a thickness of only 2.4 mm. With its multispectral compatibility, the proposed metamaterial has potential applications in stealth and camouflage fields.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Inspired by the camouflage of animals in nature, the technology of camouflage and stealth has been widely employed in military applications. The rapid advancement of multispectral detection technology has made it imperative to develop materials that can achieve stealth in multiple frequency bands, including radar, infrared (IR), and visible [1,2]. Radar and IR detection are commonly utilized detection techniques in various applications [3,4]. Radar detection is an active method that involves transmitting microwave signals and receiving echoes to detect targets. Achieving radar stealth typically involves utilizing absorbing materials with unit absorptivity to absorb incoming radar waves. IR detection is a passive method that relies on detectors to identify the IR waves radiated by targets. In contrast to radar stealth materials that require unit absorptivity, achieving IR stealth entails utilizing stealth materials with zero emissivity (absorptivity). Radar and IR compatible stealth materials should satisfy both unit and zero absorptivity in the different spectral regimes, which makes it challenging [5–7].
Traditional radar-IR compatible stealth materials are mainly achieved by mixing metal powders with radar absorbing materials [8,9] or coating IR low-emissivity coatings on radar absorbing materials [10]. However, they generally suffer from narrow radar absorption bandwidth or high IR emissivity issues. Metamaterials, with their exotic electromagnetic properties, undoubtedly provide new opportunities for the development of multispectral stealth materials and have bee demonstrated to offer promising solutions to this challenge. Researchers design multispectral stealth metamaterials for radar and IR by integrating a metal capacitive frequency selective surface that exhibit high reflectivity to IR waves and high transparency to microwaves on a radar absorber [5,11]. In addition, for further compatibility with visible stealth, the researchers used transparent materials to design radar-IR compatible stealth materials to achieve radar-IR-visible compatible stealth design, which can also be used in some special scenarios, such as aircraft cockpits [12,13]. Nevertheless, incorporating multiple functional layers in the metamaterial design leads to challenges such as increased layer count, substantial thickness, intricate fabrication processes, and limited transparency [14,15]. Despite our recent work has aimed to design transparent radar-IR-visible compatible stealth metamaterials through an integrated design approach, it remains challenging to achieve both broadband radar absorption and low IR emissivity simultaneously [16–18].
In this paper, we propose a novel highly transparent multifunctional integrated metamaterial for radar-IR-visible compatible stealth. By carefully designing the structure, the limitations of traditional hierarchical designs are overcome, achieving broadband radar absorption and low IR radiation using only a single layer of ITO frequency selective surface (FSS). Specifically, by arranging different sized ITO structures on the FSS, we achieve resonance excitation at different frequencies, resulting in radar absorption exceeding 90% in 8–18 GHz (i.e., radar X- and Ku-bands). Additionally, the high filling ratio of ITO on the FSS effectively suppresses IR emissivity in 3–14 µm, reducing it to approximately 0.36. The fabricated prototype demonstrates a high average optical transmittance of 74.1% in 380–800 nm. The experimental and simulation results exhibit a strong agreement, confirming the potential application value of the proposed metamaterial in the field of multispectral camouflage and stealth.
2. Structure design and analysis
ITO is an N-type oxide semiconductor material that exhibits optical transparency. It is composed of a mixture of indium oxide and tin oxide. Due to its transparency, ITO has found extensive applications in liquid crystal displays, photovoltaic devices, and architectural glass. The permittivity of ITO in the IR band can be expressed by Drude mode [12,19,20]
Figure 1 illustrates the schematic diagrams and geometrical parameters of the proposed metamaterials. In the initial design phase, we aimed for the radar stealth research of the metamaterial to cover the 8–18 GHz frequency range. This frequency range, commonly referred to as the X and Ku bands, is widely employed in military radar systems, encompassing airborne fire control radar systems, missile guidance systems, and synthetic aperture radar for imaging radar. Therefore, it is crucial to conduct research on radar stealth materials within this frequency band. To achieve visible light transmission, optically transparent materials are utilized in the entire structure. We first designed the structure of the ITO resonant absorber using a common split-ring resonator structure. We optimized the split-ring structure through parameter sweep to ensure its absorption frequency range covers 8–18 GHz as much as possible. Subsequently, based on the capacitive frequency selective surface with low-pass and high-impedance characteristics, we added small square patches to the bare glass substrate not covered by the ITO resonant absorber. These patches further increased the filling ratio of the low-emissivity ITO without significantly affecting the microwave absorption performance, thereby suppressing the infrared radiation of the metamaterial structure. Finally, we performed a comprehensive optimization of the entire metamaterial structure through parameter sweep to ensure its microwave absorption frequency range covers 8–18 GHz.
Fig. 1. The schematic diagrams and geometrical parameters of the proposed metamaterials: (a) three-dimensional view and (b) top view of the unit.
Specifically, both the upper and lower layers consist of ITO glass with a sheet resistance of 8 Ω/sq, and the ITO is facing outward. The upper layer ITO features an FSS structure as depicted in Fig. 1(b), while the lower layer acts as a reflective backplane. The glass substrate has a thickness of h = 0.3 mm, a dielectric constant of 4.6, and a loss tangent of 0.015. For ease of description, we refer to the outermost ring with a gap as the “split ring”, the square ring with the largest side length in the split ring as “ring 1”, the square ring with the second side length as “ring 2”, and the central square patch as the “center patch”. An air layer with a thickness of d = 1.8 mm separates the upper and lower layers. The entire metamaterial structure has a thickness of approximately 2.4 mm. The optimized geometric parameters are as follows: p = 12 mm, a = 6.2 mm, b = 1.2 mm, m = 1.6 mm, n = 0.4 mm, s = 1.1 mm, t = 0.3 mm, w1 = 0.15 mm, w2 = 0.175 mm, and w3 = 0.1 mm. Generally, the emissivity of a structure composed of materials with different emissivity can be calculated using the equation:
where ɛ represents the emissivity of the overall structure, while ${\varepsilon _{ITO}}$ and ${\varepsilon _s}$ represent the emissivity of the ITO film and the substrate material, respectively. The ${f_{ITO}}$ denotes the occupation ratio (ITO area/total area) of the ITO part, and ${f_s}$ is the occupation ratio of the bare substrate, that is, ${f_\textrm{s}} = 1 - {f_{\textrm{ITO}}}$. The ITO film with a sheet resistance of 8 Ω/sq has an emissivity of approximately 0.15, while glass has an emissivity of around 0.85. With an occupation ratio of ITO in the upper structure at 68.8%, the resulting IR emissivity of the proposed metamaterial can be determined to be below 0.37.The numerical simulations were conducted using the commercial software CST Microwave Studio. Figure 2(a) illustrates the simulated absorption, reflection and transmission of the proposed metamaterials from 8 to 18 GHz under normal incidence. The blue curve represents the transmission, indicating near-zero transmission across the entire frequency band. Analysis of the curve reveals that the reflection is less than 10%, and the absorption is higher than 90% in 7.8–19.0 GHz, achieving broadband high-efficiency microwave absorption. Furthermore, the absorption for different polarization angles (φ = 0°, 20°, 45°, 70°, and 90°) was simulated under normal incidence, as depicted in Fig. 2(b). The absorption is higher than 90% in 7.8–19.0 GHz when φ = 0° and 90°; it is also higher than 90% in 8.0–18.8 GHz when φ = 20°, 45°, and 70°, indicating that the structure is polarization independent. Since electromagnetic waves are rarely vertically incident in real conditions, the absorption characteristics of TM and TE modes at different incidence angles (θ = 0°, 15°, 30°, and 45°) were simulated, as shown in Figs. 2(c-d). For TM polarization, the absorption bandwidth remains relatively stable for incidence angles below 30°. However, at θ = 45°, the absorption only exceeds 90% in 15.3–20.0 GHz. In comparison, the proposed metamaterial exhibits poorer angular stability for TE polarization. At an incidence angle of 30°, the absorption surpasses 90% solely in 8.0–10.5 GHz and 12.0–15.2 GHz.
Fig. 2. Simulated results of microwave response: (a) simulated absorption, reflection and transmission of the proposed metamaterials under normal incidence, (b) simulated absorption under normal incidence with different polarization angles, (c) simulated absorption of the proposed metamaterials under TM and (d) TE waves with different incident angles in the x-z plane.
In order to suppress the IR radiation of the proposed metamaterials without affecting the microwave absorption properties, a peripheral ring of square patches was introduced around the perimeter of the central ring structure. The absorption characteristics were further simulated with and without the peripheral square patches, as depicted in Fig. 3. The results indicate that the addition of the peripheral square patch causes the absorption band shift to the low frequency, and reduces the absorption effect of the high frequency (18.0–24.0 GHz). However, the overall microwave absorption bandwidth remains largely unaffected.
Fig. 3. Simulated absorption of the proposed metamaterials under normal incidence with or without peripheral square patch.
To further elucidate the absorption mechanism of the proposed metamaterials, the distributions of surface current, electric field component Ez and power loss were monitored at the resonance frequencies of 8.8 GHz and 18.2 GHz. At the low frequency of 8.8 GHz, as illustrated in Fig. 4(a), the obvious surface current takes place on “split ring” and “ring 1” and there has an enhancement of electric field on the “split ring”, “ring 1” and outside of “ring 2”, which means that a strong electrical resonance is generated. In addition, the power loss is also basically concentrated on the corresponding position. Similarly, at the high frequency of 18.2 GHz, as illustrated in Fig. 4(b), strong electrical resonance is excited on the inner side of “ring 2” and on the “center patch”, with corresponding concentration of surface current and power loss. In summary, electrical resonance is excited at different frequencies through superimposing structures of different sizes on the upper-layer ITO, and the equivalent impedance of the structure is matched with free space impedance over a broadband range, thereby achieving broadband microwave absorption.
Fig. 4. Absorption mechanism analysis: (a) the surface current, electric field component Ez and power loss distribution of the proposed metamaterials at the resonance frequencies of 8.8 GHz and (b) 18.2 GHz.
3. Experimental results
To further validate the proposed metamaterials, a prototype with dimensions of 350 × 350 mm2 was fabricated and measured. The fabrication process of the ITO structures involved utilizing a high-precision laser etcher (SC-K750), which utilized laser etching technology to selectively remove excess portions of the ITO thin film layer while preserving the designed pattern, thus achieving the fabrication of the proposed metamaterial specimen. Figure 5(a) presents a photograph of the upper layer of the prototype in an outdoor environment. In Fig. 5(b), a two-layer structural photograph of the prototype is presented, demonstrating the clear visibility of different colored paper placed beneath it, thereby indicating effective visible light transmission of the fabricated prototype. As depicted in Fig. 5(c), etched structures can be observed on the upper ITO layer.
Fig. 5. Prototype fabrication: (a) and (b) photographs of the fabricated prototype, (c) etched structures on the upper ITO layer.
The optical transmittance of the fabricated prototype was measured using an ultraviolet-visible spectrophotometer to assess its performance. Figure 6 illustrates the measurements conducted on the upper ITO glass layer, the lower ITO glass layer, and the entire prototype. Notably, the transmittance curves of the upper and lower layers show a substantial degree of overlap within the visible light spectrum ranging from 380 to 800 nm. The average transmittance of the complete prototype surpassed 74%, suggesting the excellent transparency of the proposed metamaterials.
Fig. 6. Measured optical transmittances of the upper layer, the lower layer and the total fabricated prototype.
The microwave absorption performance of the fabricated prototype was assessed using the Agilent N5224A network analyzer in a microwave anechoic chamber. Two pairs of broadband horn antennas working in 6–18 GHz and 18–22 GHz were utilized as transmitter and receiver, respectively. To ensure greater accuracy in the experimental results, a copper plate with identical dimensions to the fabricated prototype was initially employed for normalization purposes during the measurement process. By fixing the direction of the antenna horn and rotating the fabricated prototype, the reflection spectra under differently polarized wave incidences were measured. The measured absorption results of TM and TE waves under normal incidence are presented in Figs. 7(a-b), respectively, and compared with the simulated absorption. The measured results are relatively consistent with the simulations, but discrepancies between the measured and simulated results exist due to fabrication imperfections of the samples and differences in the environmental and material electromagnetic parameters used in measurements and simulations. Nevertheless, the measured results are sufficient to confirm the broadband absorption performance of the proposed metamaterial.
Fig. 7. Measured and simulated absorption of the proposed metamaterial under normal incidence of (a) TM and (b) TE waves.
The emissivity of the fabricated prototype was measured using a TSS-5X IR emissivity meter (spectral response range: 2–22 µm) and an FTIR spectrometer, as it is a crucial criterion for describing the IR stealth capabilities of materials. Figure 8(a) illustrates a photograph of the measurement of IR emissivity on the fabricated prototype using the IR emissivity meter. The IR emissivity meter was employed to measure the IR emissivity of the fabricated prototype, the ITO film with a sheet resistance of 8 Ω/sq, and glass, resulting in emissivity of 0.36, 0.13, and 0.87, respectively. By substituting the measured emissivity values of the ITO film and glass into formula 2, the IR emissivity of the proposed metamaterials can be calculated to be approximately 0.361, which closely matches the measured emissivity of 0.36 obtained from the IR emissivity meter. Subsequently, the IR emissivity of the fabricated prototype was measured using an FTIR spectrometer within the wavelength range of 3–14 µm. Considering that the period of the proposed metamaterial unit cell (p = 12 mm) exceeds the diameter of the FTIR spectrometer’s test light path, resulting in potential variations in emissivity across different measuring regions, five distinct regions were selected for measurement. The measurement results are depicted in Fig. 8(b). The obtained emissivity of approximately 0.36 exhibits good consistency with the emissivity obtained by the IR emissivity meter, further validating the IR stealth performance of the proposed metamaterials.
Fig. 8. Measurement of IR emissivity: (a) photograph of the measurement of IR emissivity on the fabricated prototype using the IR emissivity meter, (b) the IR emissivity of the fabricated prototype in 3–14 µm measured by FTIR spectrometer.
In order to visually demonstrate the IR stealth performance of the proposed metamaterial, an IR thermal camera was utilized to capture IR thermal images of the fabricated prototype at various temperatures. Figure 9(a) presents the IR thermal image of the prototype at room temperature, revealing effective suppression of IR radiation from the human body in the region covered by the prototype. Additionally, IR thermal images were taken of the ITO film with a sheet resistance of 8 Ω/sq, the fabricated prototype, and glass at different temperatures. As depicted in Fig. 9(b), all three samples were simultaneously placed on the heating platform for comparative analysis. The heating platform is uniformly heated to reach a consistent temperature. The temperature of the heating platform is measured using a thermocouple thermometer. IR thermal images of the prototype were captured using an IR thermal imager when the heating platform reached stable temperatures of 111.8°C, 215.8°C, and 310.3°C, as shown in Figs. 9(c-e). Here, only the thermal IR image captured when the heating platform reached a temperature of 310.3°C is analyzed. The IR radiation of the ITO film with a sheet resistance of 8 Ω/sq is observed to be the weakest. Compared with glass, the fabricated prototype exhibits a significant suppression of IR radiation, greatly reducing the likelihood of detection by IR detection equipment. It should be noted that the heating platform used here is a metal heating platform with inherently low infrared emissivity, resulting in the coldest apparent temperature in the captured infrared thermal image. In summary, the experimental results have successfully demonstrated the expected outcomes, achieving the intended purpose of IR stealth.
Fig. 9. The IR stealth performance: (a) IR thermal image of the prototype at room temperature, (b) photographs of ITO with a sheet resistance of 8 Ω/sq, fabricated prototype and glass, (c), (d) and (e) IR thermal image of ITO with a sheet resistance of 8 Ω/sq, fabricated prototype and glass at high temperature.
4. Conclusion
In summary, we demonstrate a metamaterial based on the ITO/dielectric/ITO sandwich structure for multispectral stealth of IR and radar. The upper-layer ITO acts as a resonator for broadband microwave absorption while having a high filling ratio to achieve low IR radiation. Compared to traditional hierarchical design that integrates metal capacitive FSS on radar absorbers, the proposed metamaterial effectively controls the number of layers, resulting in an increased visible light transmittance of 74.1%. Furthermore, by properly tuning the structural parameters and sheet resistance of ITO, the proposed metamaterial exhibits relatively good IR and radar compatibility stealth performance, with IR emissivity of approximately 0.36 in 3–14 µm and microwave absorption exceeding 90% in 8–18 GHz. However, challenges and limitations may arise when operating beyond the radar stealth frequency range designed by us. To address this, one can consider measures such as utilizing multi-layer stacking structures, composite material design, further optimizing structural parameters, and developing new materials to further expand the microwave absorption bandwidth. In our future studies, we will investigate the development of flexible and highly transparent metamaterials to maintain multispectral stealth performance for arbitrary surfaces [21–23]. In addition, we will further enhance its multispectral adaptive camouflage performance by developing and utilizing active materials such as phase change materials [24,25], light-sensitive materials [26,27] and electrically tunable materials [28–32]. Furthermore, the principles underlying the proposed metamaterial can also provide insights for electromagnetic wave control in the fields of camouflage, stealth, and beyond.
Funding
National Natural Science Foundation of China (52073303); Natural Science Foundation of Hunan Province (2021JJ10049).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. R. Hu, W. Xi, Y. Liu, et al., “Thermal camouflaging metamaterials,” Mater. Today 45, 120–141 (2021). [CrossRef]
2. Y. Wu, S. Tan, Y. Zhao, et al., “Broadband multispectral compatible absorbers for radar, infrared and visible stealth application,” Prog. Mater Sci. 135, 101088 (2023). [CrossRef]
3. W. Xi, Y.-J. Lee, S. Yu, et al., “Ultrahigh-efficient material informatics inverse design of thermal metamaterials for visible-infrared-compatible camouflage,” Nat. Commun. 14(1), 4694 (2023). [CrossRef]
4. H. Zhu, Q. Li, C. Tao, et al., “Multispectral camouflage for infrared, visible, lasers and microwave with radiative cooling,” Nat. Commun. 12(1), 1805 (2021). [CrossRef]
5. T. Kim, J.-Y. Bae, N. Lee, et al., “Hierarchical Metamaterials for Multispectral Camouflage of Infrared and Microwaves,” Adv. Funct. Mater. 29(10), 1807319 (2019). [CrossRef]
6. N. Lee, J.-S. Lim, I. Chang, et al., “Flexible Assembled Metamaterials for Infrared and Microwave Camouflage,” Adv. Opt. Mater. 10(11), 2200448 (2022). [CrossRef]
7. X. Feng, M. Pu, F. Zhang, et al., “Large-Area Low-Cost Multiscale-Hierarchical Metasurfaces for Multispectral Compatible Camouflage of Dual-Band Lasers, Infrared and Microwave,” Adv. Funct. Mater. 32(36), 2205547 (2022). [CrossRef]
8. T. H. Ting, K. H. Wu, J. S. Hsu, et al., “Microwave absorption and infrared stealth characteristics of bamboo charcoal/silver composites prepared by chemical reduction method,” J. Chin. Chem. Soc. 55(4), 724–731 (2008). [CrossRef]
9. T. Wang, J. He, J. Zhou, et al., “Electromagnetic wave absorption and infrared camouflage of ordered mesoporous carbon–alumina nanocomposites,” Microporous Mesoporous Mater. 134(1-3), 58–64 (2010). [CrossRef]
10. L. Liu, R. Gong, Y. Cheng, et al., “Emittance of a radar absorber coated with an infrared layer in the 3∼5µm window,” Opt. Express 13(25), 10382–10391 (2005). [CrossRef]
11. H. Tian, H.-T. Liu, and H.-F. Cheng, “A thin radar-infrared stealth-compatible structure: Design, fabrication, and characterization,” Chin. Phys. B 23(2), 025201 (2014). [CrossRef]
12. C. Zhang, X. Wu, C. Huang, et al., “Flexible and Transparent Microwave–Infrared Bistealth Structure,” Adv. Mater. Technol. 4, 1900063 (2019). [CrossRef]
13. C. Xu, B. Wang, M. Yan, et al., “An optical-transparent metamaterial for high-efficiency microwave absorption and low infrared emission,” J. Phys. D: Appl. Phys. 53(13), 135109 (2020). [CrossRef]
14. Y. Cui, J. Wang, H. Sun, et al., “Visible Transparent Wideband Microwave Meta-Absorber with Designable Digital Infrared Camouflage,” Adv. Opt. Mater. 12(4), 2301712 (2024). [CrossRef]
15. Z. Meng, C. Tian, C. Xu, et al., “Multi-spectral functional metasurface simultaneously with visible transparency, low infrared emissivity and wideband microwave absorption,” Infrared Phys. Technol. 110, 103469 (2020). [CrossRef]
16. Z. Meng, C. Tian, C. Xu, et al., “Optically transparent coding metasurface with simultaneously low infrared emissivity and microwave scattering reduction,” Opt. Express 28(19), 27774–27784 (2020). [CrossRef]
17. C. Xu, B. Wang, M. Yan, et al., “An optically transparent sandwich structure for radar-infrared bi-stealth,” Infrared Phys. Technol. 105, 103108 (2020). [CrossRef]
18. R. Zhu, J. Wang, C. Xu, et al., “Integrated design of single-layer multispectral metasurface with broadband microwave polarization rotation and low infrared emissivity,” Infrared Phys. Technol. 111, 103546 (2020). [CrossRef]
19. C. G. Granqvist and A. Hultåker, “Transparent and conducting ITO films: new developments and applications,” Thin Solid Films 411(1), 1–5 (2002). [CrossRef]
20. J. K. Pradhan, S. Anantha Ramakrishna, B. Rajeswaran, et al., “High contrast switchability of VO2 based metamaterial absorbers with ITO ground plane,” Opt. Express 25(8), 9116–9121 (2017). [CrossRef]
21. C. Wen, B. Zhao, Y. Liu, et al., “Flexible MXene-Based Composite Films for Multi-Spectra Defense in Radar, Infrared and Visible Light Bands,” Adv. Funct. Mater. 33(20), 2214223 (2023). [CrossRef]
22. J. Zhang, Z. Li, L. Shao, et al., “Dynamical absorption manipulation in a graphene-based optically transparent and flexible metasurface,” Carbon 176, 374–382 (2021). [CrossRef]
23. J. Nam, I. Chang, J.-S. Lim, et al., “Flexible Metasurface for Microwave-Infrared Compatible Camouflage via Particle Swarm Optimization Algorithm,” Small 19(46), 2302848 (2023). [CrossRef]
24. W. Li, M. Xu, H.-X. Xu, et al., “Metamaterial Absorbers: From Tunable Surface to Structural Transformation,” Adv. Mater. 34(38), 2202509 (2022). [CrossRef]
25. J. Lee, H. Sul, Y. Jung, et al., “Thermally Controlled, Active Imperceptible Artificial Skin in Visible-to-Infrared Range,” Adv. Funct. Mater. 30(36), 2003328 (2020). [CrossRef]
26. A. M. Shaltout, V. M. Shalaev, and M. L. Brongersma, “Spatiotemporal light control with active metasurfaces,” Science 364(6441), eaat3100 (2019). [CrossRef]
27. T. Cui, B. F. Bai, and H. B. Sun, “Tunable Metasurfaces Based on Active Materials,” Adv. Funct. Mater. 29, 14 (2019). [CrossRef]
28. M. S. Ergoktas, G. Bakan, E. Kovalska, et al., “Multispectral graphene-based electro-optical surfaces with reversible tunability from visible to microwave wavelengths,” Nat. Photonics 15(7), 493–498 (2021). [CrossRef]
29. O. Balci, E. O. Polat, N. Kakenov, et al., “Graphene-enabled electrically switchable radar-absorbing surfaces,” Nat. Commun. 6(1), 6628 (2015). [CrossRef]
30. Y. Jia, D. Liu, D. Chen, et al., “Transparent dynamic infrared emissivity regulators,” Nat. Commun. 14(1), 5087 (2023). [CrossRef]
31. M. Li, D. Liu, H. Cheng, et al., “Manipulating metals for adaptive thermal camouflage,” Sci. Adv. 6(22), eaba3494 (2020). [CrossRef]
32. H. L. Wang, H. F. Ma, M. Chen, et al., “A Reconfigurable Multifunctional Metasurface for Full-Space Control of Electromagnetic Waves,” Adv. Funct. Mater. 31(25), 2100275 (2021). [CrossRef]
