Open Access
Add to BibSonomy
Abstract
In this paper, an optically transparent coding metasurface structure based on indium tin oxide (ITO) thin films with simultaneously low infrared (IR) emissivity and microwave scattering reduction is proposed. To this end, two ITO coding elements which can reflect 0° and 180° phase responses are firstly designed. Based on these two elements, four coding sequences with different scattering patterns are designed. Three of them can realize anomalous reflections and the fourth can realize random diffusion of normal incident electromagnetic (EM) waves. A prototype of the random diffusion coding metasurface was fabricated and measured. The experimental results show that for normal incident EM waves, at least 10dB backward scattering reduction from 3.8GHz to 6.8GHz can be achieved, and the structure is polarization insensitive. The averaged transmittance of visible light through the coding metasurface reaches up to 72.2%. In addition, due to the high occupation ratio of ITO on the outside of the coding metasurface, a low IR emissivity of about 0.275 is obtained. Good consistency between the experiment and simulation results convincingly verifies the coding metasurface. Due to its multispectral compatibility, the proposed coding metasurface may find potential applications in multi-spectral stealth, camouflage, etc.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the increasing complexity of the battlefield environment, the stealth of a single frequency band has been difficult to meet stealth requirements, the research of multi-spectral compatible stealth has become a hot spot [1–5]. Because infrared stealth and radar stealth have exactly the opposite requirements on the material absorption, radar-infrared bi-stealth has become a hot and difficult point in terms of compatible stealth [6–8]. In view of the requirement of optical transparency for window materials of aircrafts or tanks, the research of optically transparent stealth materials is also put on the agenda. Although it is difficult to achieve both low infrared emissivity and high radar absorption through a single material, many researchers still make a lot of attempts in this area, such as the mesoporous C-SiO2-Fe nanocomposites [9–11] and La1-xCaxMnO3 (0 ≤ x ≤ 0.5) with perovskite-type structure [12]. However, these materials still have the problems of high infrared emissivity or narrow microwave absorption bandwidth, and neither is optically transparent.
Metamaterial is a kind of artificial designed material which is composed of periodic subwavelength metal/dielectric, and can realize the function that the natural material can't [13–15]. For example, through the design of metamaterials, negative refraction [16], perfect lens [17] and polarizers [18] can be realized, which has attracted wide attention. Metasurfaces, as a special two-dimensional (2D) metamaterial [19–21], has thinner thickness and lower insertion loss compared to 3D metamaterials. At present, many researchers have also achieved multispectral compatible stealth by designing metasurfaces. Zhang and Yang proposed a multispectral metasurface with a microwave absorption bandwidth of 8GHz and infrared emissivity lower than 0.2 [22]. Xu gives a radar-IR stealth-compatible structure simultaneously with five strong microwave absorption peaks and low IR emissivity [23]. In addition, for visible transparent metamaterials, Shrestha and Wang demonstrate a visible transparent infrared absorber, which has a greater absorptivity than 80% in the range of 4-16µm [24]. However, none of them achieved the compatible stealth of infrared, radar and visible light at the same time.
Recently, digital coding metasurfaces, as a new concept, has been proposed to manipulate the EM wave radiation and scattering, which is quite different to the conventional metamaterials typically described with effective medium parameters [25–27]. The coding metasurfaces are composed of two types of unit cells, with 0 and π phase responses, which are named “0” and “1” elements, respectively. By coding “0” and “1” elements with controlled sequences (i.e., 1-bit coding), we can manipulate electromagnetic (EM) waves and realize different functionalities, and the concept of coding metamaterials can be extended from 1-bit coding to 2-bit coding or higher. Presently, the research of coding metasurface not only uses copper or aluminum on conventional dielectric substrate to design optical non-transparent coding metasurface, but also begins to use optically transparent materials on the coding metasurface. For example, Jing and Ma designed an optically transparent coding metasurface by using indium tin oxide (ITO) film, which can realize anomalous scattering patterns and diffusion [28]. Based on ITO deposited on flexible polyethylene terephthalate (PET), Chen and Cui designed a diffusion-like coding metasurface that shows broadband reduction of backward scatting in the microwave band [29]. However, neither of these studies is combined with the infrared stealth.
In this paper, with the help of transparent conductive indium-tin-oxide (ITO) thin film, an optical-transparent coding metasurface which can realize microwave scattering reduction and low infrared (IR) emissivity is designed. Two ITO unit cells are designed as digital “0” and “1” elements respectively, whose phase responses are nearly 0 and π from 4GHz to 7 GHz, and the amplitude responses of the two unit cells means that the structure has absorption. In order to show more clearly the ability of the coding metasurface in manipulating EM waves, three coding metasurfaces with certain custom-designed coding sequences are designed firstly. Lately, a random diffusion coding metasurface to reduce the backward radar cross section (RCS) by encoding a random coding sequence is achieved. A prototype of the random diffusion coding metasurface was fabricated and measured. From 3.8 GHz to 6.8 GHz, the reflection of the fabricated random diffusion coding metasurface is lower than −10 dB, that is, the reflectivity is less than 10%, which is in good agreement with the simulation results. What’s more, due to the high filling ratio of the ITO part, the emissivity of the proposed coding metasurfaces is about 0.275 in the IR band of 3-14µm. As the materials are all transparent, the visible light transmittance of the samples is more than 72.2%. The good consistency between the experiment and simulation results convincingly illustrates the important value of the structure in multi-spectral compatible stealth applications.
2. Structure design and analysis
In this article, the coding metasurfaces are designed by using off-the-shelf ITO film commercial products. ITO film technology is developed rapidly with the stealth needs of optically transparent windows for weapons. ITO is transparent in the visible range, and the permittivity in the IR band can be expressed by Drude model [30]
In order to get the 180° phase difference, two unit cells are designed to act as the digital byte of “0” and “1”. As show in Fig. 1(a), the designed digital elements are schematically illustrated, where the ITO thin film with a 6.0Ω/sq sheet resistant is used to form the top conductive patterns (a square patch and a circular patch) and the backplane. The ITO is covered on a glass substrate with a thickness of h. The dielectric constant and loss tangent of glass are 4.6 and 0.015 respectively [28]. In order to better show the structure of the designed digital elements, the size of the square and circular ITO patch in the schematic is smaller than the actual optimized size. The side length of square patch is a and the radius of circular patch is r. An air layer with a thickness of d is used to separate two glass substrates. The responses of the digital elements “0” and “1” was calculated using the Computer Simulation Technology (CST) Microwave Studio software. The optimized structure parameters are p = 6mm, a = 5.9mm, r = 2.8mm, h = 0.5mm, d = 2.5mm. Figures 1(b) and 1(c) shows the simulated phase and amplitude response variation of the “0” and “1” digital elements from 3GHz to 8GHz, respectively. The phase difference ranges from 140° to 220° (180°±40°) in the frequency range of 4 to 7GHz and the phase difference is almost 180° at the frequency of 4.25 GHz and 6.41GHz. The minimum amplitude corresponding to “0” element is 0.5 at 4.7GHz, and the lowest amplitude of “1” element is 0.73 at 7GHz, which means that there is an absorption peak for each of these two elements in different frequency bands, but this is significant for the reduction of RCS.
Fig. 1. (a) Schematic diagram of the designed digital elements “0” and “1” (b) The reflection phase and (c) amplitude of the digital elements “0” and “1”.
When designing the structure of the two unit cells, considering the requirements of infrared stealth, the occupation ratio of ITO should be increased as much as possible, of course, this comes at the expense of bandwidth. Specifically, based on the occupation ratio of materials with different emissivity, the overall emissivity ε of the structure can be calculated as [23]
where ε is the emissivity of the overall structure, the εITO and εs are the emissivity of the ITO and the substrate material for ITO etching, respectively. The fITO is the occupation ratio (ITO area/total area) of the ITO part, and fs=1- fITO, which is the occupation ratio of the bare substrate. The emissivity of ITO with a sheet resistance of 6 Ω/sq is around 0.1, and the emissivity of glass is generally lower than 0.9 [30]. Considering that the number of “0” and “1” elements in the random arrangement is equal, we can calculate the IR emissivity of the overall structure to be 0.24 according to Eq. (2), which basically meets the emissivity requirements of the IR stealth materials. What’s more, since the materials used, ITO and glass substrates, are both optically transparent materials, the coding metasurface is transparent to visible light.In order to show more clearly the ability of the coding metasurface in manipulating EM waves, three coding metasurfaces S1, S2 and S3 with certain custom-designed coding sequences are designed firstly, as show in Figs. 2(a), 2(b) and 2(c). The S4 is the random diffusion coding metasurface. In order to avoid the EM coupling between adjacent coding particles due to the difference in geometric structure, we adopted the super unit cells structure composed of 5 × 5 identical coding particles. The S1, S2, S3 and S4 are composed of 12×12 super unit cells, that is, 60×60 unit particles. The S1 is designed to be “11001100…” along the x direction, S2 is designed to be “010101…” along the x direction, S3 is designed to be “00110011…” both along the x and y directions, which is similar to a chessboard distribution. Finally, S4 is designed to be a random sequence of “101110010010” both along the x and y directions.
Fig. 2. The coding patterns of four coding metasurfaces. (a) S1: 11001100… periodic coding metasurface. (b) S2: 0101…periodic coding metasurface. (c) S3: chessboard coding metasurface. (d) S4: the coding metasurface with a function of RCS reduction
The far-filed scattering patterns of the coding metasurfaces S1, S2, S3 are show in Fig. 3. Figure 3(a) shows the 3D far-filed scattering pattern of the S1 coding metasurface under the normal incident EM wave at 6.41 GHz, and it is obvious that the incident wave is reflected to two symmetrical directions. Through the Fig. 3(b), the 2D far-filed scattering pattern of S1, we can observe that the anomalous reflection angle is about 23° in simulation. What’s more, the angle of reflected wave can be calculated by the Snell’s law [16]
where θ is the angle of reflected wave, λ and Г are the free-space wavelength and periodicity of the coding sequence. According to the Eq. (3), the reflection angle corresponding to the S1 coding metasurface can be calculated as 23.0°, which is in good consistent with the simulation results. Figures 3(c) and 3(d) shows the 3D and 2D far-filed scattering pattern of the S2 coding sequence under the normal incident EM wave at 6.41 GHz. The reflected wave is also transmitted in two symmetrical directions. Compared with the S1 coding sequence, the reflection angle becomes larger. The reflection angle can be obtained around 51.0° through the 2D far-filed scattering pattern. The reflection angle calculated by Eq. (3) is 51.3°, which is also in good agreement with the simulation results. This is also consistent with the conclusion obtained in Reference 28. For the S3 chessboard coding sequence, it can be clearly seen from Fig. 3(e) that the incident wave is reflected to four symmetrical beams with respect to the incident direction, and the reflection angle can be calculated as 33.5° by Eq. (3), which is in good agreement with the reflection angle of 33.0° observed through the 2D far-filed scattering pattern. In short, through the three abnormal reflection coding sequences of S1, S2 and S3, the structure's abilities on manipulating EM waves are verified, and it provides a theoretical basis for the random diffusion coding sequence of S4 to achieve the effect of diffuse reflection.Fig. 3. The 3D and 2D far-filed scattering patterns of the coding metasurfaces under normal incident EM wave at 6.41 GHz. (a) and (b) The S1 coding sequence. (c) and (d) The S2 coding sequence. (e) and (f) The S3 coding sequence.
Finally, for the random diffusion coding metasurface, several different random coding sequences are simulated and the coding sequence S4 of “101110010010” both along the x and y directions is finally selected because of the best simulation effect. Figures 4(a)–4(p) shows the 3D far-filed scattering patterns and E-plane scattering patterns of the random diffusion coding metasurface at 4.3 GHz, 6.0 GHz and 6.7 GHz and a bare metallic slab with the same dimension at 4.3 GHz. Figure 4(a) shows the 3D far-filed scattering pattern of the diffusion coding metasurfaces under normal x-polarized incidence at 4.3 GHz. Compared with the strong back reflection generated by the bare metallic slab at 4.3 GHz as show in Fig. 4(d), through the 3D scattering pattern corresponding to the random diffusion coding metasurface as show in Fig. 4(a), it can be clearly observed that the incident EM waves are reflected uniformly in all directions and the back reflection is significantly reduced. Figures 4(e) and 4(h) respectively show the scattering patterns of the coding metasurface and the bare metallic slab at the E-plane at 4.3 GHz. And the backward RCS value (Theta = 0) of the coding metasurface is 0, while the backward RCS value of the bare metal plate is 16. Hence, a 16 dB backward RCS reduction at 4.3 GHz is achieved for x-polarized incidence through the random diffusion coding metasurface design, which achieves the desired effect. For the y-polarized incidence, the far-filed scattering patterns is almost similar to the x-polarization, indicating that the random diffusion coding metasurface is polarization insensitive. Next, the scattering patterns under normal x- and y-polarized incidence at 6.0 GHz and 6.7 GHz are given in sequence, similar to the 4.3 GHz, and the backward RCS reduction is all above 10 dB. These results mean that the random diffusion coding metasurface can achieve the suppression of back reflection within a certain bandwidth. Furthermore, the 3D far-field scattering patterns in the case of oblique incidence is simulated, and the results are shown in Figs. 4(q)–4(t). As the incident angle increases, the coding metasurface produces specular reflection under the premise of the scattering effect, thereby further reducing the back reflection.
Fig. 4. (a-c) The 3D far-filed scattering patterns and (e-g) E-plane scattering patterns of the random diffusion coding metasurface under x-polarized ((i)-(k) The 3D far-filed and (m)-(o) E-plane scattering patterns for y-polarized) incidence at 4.3 GHz, 6.0 GHz and 6.7 GHz, respectively, as well as the (d) and (h) corresponding results ((l) and (p) for y-polarized incidence) from a bare metallic slab with the same dimension at 4.3 GHz. The 3D far-filed scattering patterns of the random diffusion coding metasurface under x-polarized incidence with different angle θ of (q) 15°, (r) 30° and (s) 45° at 5.4 GHz, as well as (t) the corresponding result from a bare metallic slab with the same dimension with incident angle of 45° at 5.4 GHz.
Considering that both “0” and “1” elements have absorption, in order to more intuitively show the weight of absorption and diffusion in the backward RCS reduction, from the electromagnetic theory, the percentage of reflection, absorption and diffusion energy could be calculated by
Fig. 5. (a) Calculated proportion of absorbed, scattered and reflected EM energy from 3 GHz to 8 GHz. (b) The averaged ratio of the diffusion, absorption and reflection from 4.2 GHz to 7.1 GHz.
3. Experiment and discussion
In order to verify the results obtained by the simulation, the random diffusion coding metasurface with the coding sequences S4 of “101110010010” both along the x and y directions was fabricated and conducted a microwave test. By using the laser etching technique, the structure corresponding to the S4 coding metasurface was etched on the optical-transparent glass substrate as the upper layer. Another ITO-coated-glass sheet was used as the backplane. The dimensions of them are all 360×360mm2, as show in Fig. 6(a). Next, the reflection(S11) of the samples are measured by the Agilent N5224A network analyzer in the microwave anechoic chamber. Two pairs of horn antennas working in 3.75-6GHz and 5.2-8.2GHz are employed as transmitters and receivers. In addition, in order to make the experimental results more convincing, before simulating the reflection of the fabricated sample, the reflection of a bare metallic slab of the same size as the sample is measured to normalize. Figure 6(b) is the simulation and measured reflection results of the S4 coding metasurface under normal x- and y- polarized incidence. The simulated results of the random diffusion coding metasurface are almost the same for the x- and y-polarization, and the reflection is lower than −10dB from 3.8 to 7GHz. The measured results show that the reflection of fabricated random diffusion coding metasurface under normal x-polarized incidence is lower than −10dB from 3.8 to 6.8GHz, that is, the reflectivity is less than 10%, and the y-polarization simulation results have also reached the expected effect. The measured results are in good consistent with the simulated results, indicating that the coding metasurface does indeed have the effect of suppressing the back reflection.
Fig. 6. (a) The photograph of the fabricated random diffusion coding metasurface. (b) Measured and simulated reflection of the random diffusion coding metasurface under normal x- and y- polarized incidence.
As show in Fig. 6(a), through the whole sample, the picture can still be clearly seen, which means that the structure has a high transmittance to visible light. In order to quantify the transmittance, two samples were fabricated separately. One of the samples was neatly arranged with “0” element, and the other sample was neatly arranged with “1” element, and their sizes are all 60×60mm2, that is, 2×2 super unit cells. In addition, samples of backplane with the size of 60×60mm2 were also fabricated. And the transmissivity of these samples in 380-800nm are measured using ultraviolet-visible spectrophotometer respectively. The measurement results are shown in Fig. 7, the average optical transmittances of the samples of “0” element, “1” element and backplane are higher than 85.0%, 85.4% and 84.7% respectively, so the average transmittance of the total structure can be calculated to be higher than 72.2% in 380-800nm.
In order to analyze the infrared stealth ability of the random diffusion coding metasurface, the IR emissivity of the samples is measured by the TSS-5X IR Emissivity meter as shown in Fig. 8. The TSS-5X IR Emissivity meter measures the emissivity indirectly by measuring reflection. Because the transmittance is almost zero, the infrared emissivity can be obtained indirectly by measuring the reflection coefficient of the sample. The emissivities of the 60×60mm2 samples of the ITO with a sheet resistance of 6Ω/sq, glass, “0” element and “1” element was tested several times, and the average emissivities obtained are 0.15, 0.88, 0.17 and 0.38 respectively. Bring ITO and glass emissivity 0.15 and 0.88 obtained from the experimental test into Eq. (2), the theoretical emissivity of “0” and “1” elements are obtained respectively as 0.174 and 0.381, which is in good agreement with the test results. Furthermore, since the number of “0” and “1” elements in the S4 coding metasurface is the same, the emissivity of the “0” and “1” elements can be averaged as the emissivity of the overall structure, and the average emissivity is 0.275, which basically meets the requirements of infrared stealth on the material emissivity.
Fig. 8. The real picture when measuring the IR emissivities of the ITO with a sheet resistance of 6Ω/sq, glass, “0” element and “1” element samples.
Considering the wavelength dependence of the emissivity, which is crucial in many applications, the emissivity of the sample is also tested by the FTIR spectrometer. Since the size of the unit particles (p = 6mm) is close to the diameter of the FTIR spectrometer test light path, this will affect the accuracy of the measurement results, so five different regions of the “0” element and “1” element samples are separately measured, and the test results are shown in Fig. 9. It is obvious that the emissivity of the sample in the mid-infrared atmospheric window (3-5µm) is slightly higher than that of the sample in the far-infrared atmospheric window (8-14µm). The emissivity measured by the FTIR spectrometer is in good consistent with the emissivity obtained by the IR Emissivity meter, which further confirms the ability of the coding metasurface in suppressing IR emissivity.
Fig. 9. (a) The IR emissivity spectra in the infrared band for the fabricated “0” element and (b) “1” element samples.
In order to further show the infrared stealth effect of the structure, the infrared radiation of the ITO film with a sheet resistance of 6 Ω/sq, “0” element, “1” element and glass samples with 60×60 mm2 at different temperatures are photographed by infrared thermal imager. In order to ensure that the samples reach the same temperature, the samples are placed on the heating table at the same time as shown in Fig. 10(a). The model of infrared thermal imager used is G120EX, which works at 8-14µm. The temperature of the metal heating table is measured by thermocouple temperature meter. When the temperature of the metal heating table is 40.6°C, as shown in Fig. 10(b), it can be observed that ITO with a sheet resistance of 6 Ω/sq has the weakest infrared radiation, followed by “0” element and “1” element samples, and glass has the strongest infrared radiation. Figure 10(c) corresponds to the picture when the heating table temperature is 81.4°C. The infrared radiation of “0” element and “1” element samples is significantly weakened compared with the glass. Due to the high occupancy ratio of ITO in the “0” element, the infrared radiation of the “0” element is almost similar to the ITO with a sheet resistance of 6Ω/sq. Since the random diffusion coding metasurface is composed of “0” element and “1” element, it can be considered that the infrared radiation of the coding metasurface is also suppressed, thereby achieving the effect of infrared stealth.
Fig. 10. (a) Visible image of the ITO film with a sheet resistance of 6Ω/sq, glass, “0” element and “1” element samples (b)Infrared thermal image of the samples at 44.1°C and (c) 82.2°C.
4. Conclusion
In summary, an optically transparent coding metasurface using conductive ITO thin film simultaneously with low infrared emissivity and microwave scattering reduction is proposed. Three coding metasurfaces with certain custom-designed coding sequences are designed firstly to verify the ability of the designed coding metasurfaces in manipulating EM waves. Next, a random diffusion coding metasurface to reduce the backward radar cross section (RCS) by encoding a random coding sequence is designed and fabricated. The reflection of the fabricated random diffusion coding metasurface is lower than −10dB from 3.8 to 6.8GHz and the simulation and experimental results are in good consistency. What’s more, the optical transmittances and IR emissivity of the fabricated prototype were measured separately. The measurement result is that the average visible light transmittance is higher than 72.2%, and the IR emissivity is about 0.275. The good performance of experiments and simulations shows the important application value of this random diffusion coding metasurface in the field of multi-spectral stealth, camouflage, etc. What is more noteworthy is that as long as the processing technology allows, the combination of materials used in this article is also suitable for higher frequency microwaves, that is, the terahertz waves.
Funding
National Natural Science Foundation of China (61971435); Shanxi Province Science and Technology Innovation team Foundation of Shanxi Province (2020JQ-471); National Key Research and Development Program of China (2017YFA0700201).
Disclosures
The authors declare no conflicts of interest.
References
1. G. A. Rao and S. P. Mahulikar, “Integrated review of stealth technology and its role in airpower,” Aeronaut. J. 106(1066), 629–642 (2002). [CrossRef]
2. D. Qi, X. Wang, Y. Cheng, R. Gong, and B. Li, “Design and characterization of one-dimensional photonic crystals based on ZnS/Gefor infrared-visible compatible stealth applications,” Opt. Mater. (Amsterdam, Neth.) 62, 52–56 (2016). [CrossRef]
3. J. Kim, K. Han, and J. W. Hahn, “Selective dual-band metamaterial perfect absorber for infrared stealth technology,” Sci. Rep. 7(1), 1–9 (2017). [CrossRef]
4. Y. Pang, Y. Shen, Y. Li, J. Wang, Z. Xu, and S. Qu, “Water-based metamaterial absorbers for optical transparency and broadband microwave absorption,” J. Appl. Phys. 123(15), 155106 (2018). [CrossRef]
5. M. L. Immordino, F. Dosio, and L. Cattel, “Stealth liposomes: Review of the basic science, rationale, and clinical applications, existing and potential,” Nanomedicine (London, U. K.) 1(3), 297–309 (2006). [CrossRef]
6. M. S. Kluskens and E. H. Newman, “Scattering by a Chiral Cylinder of Arbitrary Cross Section,” IEEE Trans. Antennas Propag. 38(9), 1448–1455 (1990). [CrossRef]
7. 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]
8. S. H. Z. Hong, L. W. U. Ijie, T. A. L. Iu, J. I. F. U. H. Uang, W. E. I. J. Iang, and Y. M. A. Ungui, “Transparent transmission-selective radar- infrared bi-stealth structure,” Opt. Express 26(13), 16466–16476 (2018). [CrossRef]
9. T. Wang, J. He, J. Zhou, X. Ding, J. Zhao, S. Wu, and Y. Guo, “Electromagnetic wave absorption and infrared camouflage of ordered mesoporous carbon–alumina nanocomposites,” Microporous Mesoporous Mater. 134(1-3), 58–64 (2010). [CrossRef]
10. J. Zhou, J. He, G. Li, T. Wang, D. Sun, X. Ding, J. Zhao, and S. Wu, “Direct incorporation of magnetic constituents within ordered mesoporous carbon-silica nanocomposites for highly efficient electromagnetic wave absorbers,” J. Phys. Chem. C 114(17), 7611–7617 (2010). [CrossRef]
11. L. Chen, C. Lu, Z. Fang, Y. Lu, Y. Ni, and Z. Xu, “Infrared emissivity and microwave absorption property of Sm0.5Sr0.5CoO3 perovskites decorated with carbon nanotubes,” Mater. Lett. 93, 308–311 (2013). [CrossRef]
12. J. W. Liu, J. J. Wang, and H. T. Gao, “Infrared Emissivities and Microwave Absorption Properties of Perovskite La1-xCaxMnO3 (0 ≤ x ≤ 0.5),” Mater. Sci. Forum 914, 96–101 (2018). [CrossRef]
13. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85(18), 3966–3969 (2000). [CrossRef]
14. D. R. Smith, J. B. Pendry, and M. C. K. Wiltshire, “Metamaterials and negative refractive index,” Science 305(5685), 788–792 (2004). [CrossRef]
15. J. Wang, S. Qu, Z. Xu, H. Ma, S. Xia, Y. Yang, X. Wu, Q. Wang, and C. Chen, “Normal-incidence left-handed metamaterials based on symmetrically connected split-ring resonators,” Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 81(3), 036601 (2010). [CrossRef]
16. N. Yu, P. Genevet, M. A. Kats, F. Aieta, J. P. Tetienne, F. Capasso, and Z. Gaburro, “Light propagation with phase discontinuities: Generalized laws of reflection and refraction,” Science 334(6054), 333–337 (2011). [CrossRef]
17. X. Li, S. Xiao, B. Cai, Q. He, T. J. Cui, and L. Zhou, “Flat metasurfaces to focus electromagnetic waves in reflection geometry,” Opt. Lett. 37(23), 4940 (2012). [CrossRef]
18. N. Yu, F. Aieta, P. Genevet, M. A. Kats, Z. Gaburro, and F. Capasso, “A broadband, background-free quarter-wave plate based on plasmonic metasurfaces,” Nano Lett. 12(12), 6328–6333 (2012). [CrossRef]
19. C. L. Holloway, E. F. Kuester, J. A. Gordon, J. O’Hara, J. Booth, and D. R. Smith, “An overview of the theory and applications of metasurfaces: The two-dimensional equivalents of metamaterials,” IEEE Antennas Propag. Mag. 54(2), 10–35 (2012). [CrossRef]
20. L. Chen, Q. Ma, H. B. Jing, H. Y. Cui, Y. Liu, and T. J. Cui, “Space-Energy Digital-Coding Metasurface Based on an Active Amplifier,” Phys. Rev. Appl. 11(5), 054051 (2019). [CrossRef]
21. Z. Zhang, D. Wen, C. Zhang, M. Chen, W. Wang, S. Chen, and X. Chen, “Multifunctional Light Sword Metasurface Lens,” ACS Photonics 5(5), 1794–1799 (2018). [CrossRef]
22. C. Zhang, J. Yang, W. Yuan, J. Zhao, J. Y. Dai, T. C. Guo, J. Liang, G. Y. Xu, Q. Cheng, and T. J. Cui, “An ultralight and thin metasurface for radar-infrared bi-stealth applications,” J. Phys. D: Appl. Phys. 50(44), 444002 (2017). [CrossRef]
23. C. Xu, B. Wang, Y. Pang, J. Wang, M. Yan, W. Wang, A. Wang, J. Jiang, and S. Qu, “Hybrid Metasurfaces for Infrared-Multiband Radar Stealth-Compatible Materials Applications,” IEEE Access 7, 147586 (2019). [CrossRef]
24. S. Shrestha, Y. Wang, A. C. Overvig, M. Lu, A. Stein, L. D. Negro, and N. Yu, “Indium Tin Oxide Broadband Metasurface Absorber,” ACS Photonics 5(9), 3526–3533 (2018). [CrossRef]
25. T. J. Cui, M. Q. Qi, X. Wan, J. Zhao, and Q. Cheng, “Coding metamaterials, digital metamaterials and programmable metamaterials,” Light: Sci. Appl. 3(10), e218 (2014). [CrossRef]
26. C. Della Giovampaola and N. Engheta, “Digital metamaterials,” Nat. Mater. 13(12), 1115–1121 (2014). [CrossRef]
27. W. Li, T. Qiu, J. Wang, L. Zheng, Y. Jing, Y. Jia, W. He, Y. Han, and S. Qu, “Programmable coding metasurface reflector for reconfigurable multi-beam antenna application,” IEEE Trans. Antennas Propag. Doi: 10.1109/TAP.2020.3010801 (2020).
28. H. B. Jing, Q. Ma, G. D. Bai, L. Bao, J. Luo, and T. J. Cui, “Optically transparent coding metasurfaces based on indium tin oxide films,” J. Appl. Phys. 124(2), 023102 (2018). [CrossRef]
29. K. Chen, L. Cui, Y. Feng, J. Zhao, T. Jiang, and B. Zhu, “Coding metasurface for broadband microwave scattering reduction with optical transparency,” Opt. Express 25(5), 5571 (2017). [CrossRef]
30. C. Zhang, X. Wu, C. Huang, J. Peng, C. Ji, J. Yang, Y. Huang, Y. Guo, and X. Luo, “Flexible and Transparent Microwave–Infrared Bistealth Structure,” Adv. Mater. Technol. 4(8), 1900063 (2019). [CrossRef]
