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Abstract
The past few years have witnessed the great success of artificial metamaterials with effective medium parameters to control electromagnetic waves. Herein, we present a scheme to achieve broadband microwave low specular reflection with uniform backward scattering by using a coding metasurface, which is composed of a rational layout of subwavelength coding elements, via an optimization method. We propose coding elements with high transparency based on ultrathin doped silver, which are capable of generating large phase differences (∼180°) over a wide frequency range by designing geometric structures. The electromagnetic diffusion of the coding metasurface originates from the destructive interference of the reflected waves in various directions. Numerical simulations and experimental results demonstrate that low reflection is achieved from 12 to 18 GHz with a high angular insensitivity of up to ±40° for both transverse electric and transverse magnetic polarizations. Furthermore, the excellent visible transparency of the encoding metasurface is promising for various microwave and optical applications such as electronic surveillance, electromagnetic interference shielding, and radar cross-section reduction.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Microwave radiation, which comprises electromagnetic waves ranging from 300 MHz to 300 GHz, plays an extremely important role in mobile/satellite communications, radar detection, and remote sensing [1,2]. Recently, the arrival of fifth-generation networks (450 MHz–52 GHz) and Internet of Things technologies have made electromagnetic interference shielding a critical requirement for electronic devices used in the industrial, medical, and aerospace fields [3–5]. Moreover, the increasing radiation interference intensity presents a new challenge for the stability and security of electronic systems.
Endowing shielding materials with visual transparency is an urgent need yet an important challenge in the development of functional devices and components for optical windows in practical applications [6,7]. Considering this situation, numerous studies have focused on metal-based structures and materials, such as metal meshes, metal nanowires, and metal oxides, to realize transparent shielding solutions [8–14]. Excellent barriers against electromagnetic waves as well as high optical transmittance can be simultaneously achieved using such materials; however, these methods suffer from strong specular reflection to incident microwaves, which cannot fundamentally eliminate electromagnetic pollution, thereby concentrating the backward scattering energy.
Recent advances in metamaterials/metasurfaces composed of artificial subwavelength units have provided opportunities to manipulate the amplitude, phase, and polarization of electromagnetic waves, leading to a series of remarkable functionalities such as invisibility cloaking, superlensing, and perfect absorption [15–20]. Furthermore, the advent of digital/coding metamaterials, rather than conventional metamaterials described by effective medium parameters, has demonstrated the potential to control the wavefront of electromagnetic waves as desired owing to the tunable combination of the constituent units [21–29]. However, most of these attempts are based on optically opaque elements and suffer from low visibility. To date, although transparent indium tin oxide and graphene films have been proven to be feasible for fabricating metamaterial devices, they either have an inherently brittle mechanical character or cannot be easily scaled up for large-scale applications [30–33]. Therefore, a method that combines high visible transparency and good flexibility is urgently needed.
In this study, we proposed a coding metasurface based on ultrathin doped silver films for the first time; the metasurface features high transparency, broadband low specular reflections at wide incident angles, and strong shielding capability. In addition, a genetic algorithm-based scattering optimization model was developed with the goal of minimizing the variance of backward scattering energy in all directions. Through a rational design of the coding elements and an appropriate arrangement of the coding sequences, minimum specular reflection and uniformly distributed backward scattering were achieved. Both simulation and experimental results reveal that more than 85% backward scattering reduction was achieved from 12 to 18 GHz with satisfactory angular tolerance of up to ±40° for both transverse electric (TE) and transverse magnetic (TM) polarizations. Moreover, the average relative visible transmittance of the proposed coding metasurface was ∼95.9%, indicating high transparency compared to that of other reported coding metamaterials/metasurfaces. The strategy described in this study can open a new path to the extensive use of coding metamaterials for various optical applications that are difficult to realize with conventional techniques and is compatible with flexible platforms.
2. Methods, experiment, and results
2.1 Design of coding metasurface
As illustrated in Fig. 1(a), the proposed microwave transparent coding metasurface for ultralow reflections consists of two types of subwavelength elements, through which the reflection can be nearly uniformly distributed with a small intensity in various directions. Subwavelength elements A and B are both composed of ring structures with different dimensions, as shown in Fig. 1(a), which are located on the ground metal layer separated by polymethyl methacrylate. In general, the ground metal layer in the elements completely eliminates the transmittance in the optical spectrum. In this work, we employed ultrathin silver to construct a ground layer to realize the visible transparency of the entire structure.
Fig. 1. a) Schematic of the transparent coding metasurface and the geometric structure of coding element. Simulated reflection phase responses b) and reflection amplitude c) of the optimized elements A and B.
Elements A and B were designed to provide the similar high amplitude but a large reflection phase difference in a wide frequency band. Numerical simulation using commercial CST Microwave Studio was performed to optimize the structural parameters of the coding elements. In the simulations, the periodic boundary conditions in the x and y directions and open boundary condition in the z direction were selected to ensure a better accuracy of the calculated results. The reflection phases of the coding elements can be adjusted by varying parameters such as the outer and inner radii, that is, r1 and r2 of the rings, length of unit cells d, and dielectric thickness t. To obtain better reflection phase responses over a wider frequency range, the structural parameters are changed in a reasonable range, as shown in Fig. S1 (Supplement 1). Figure 1(b) demonstrates the simulated reflection spectra of both elements after parameter optimizations, where the dimensions of optimized elements A and B are r1 = 1.1 mm, r2 = 0.6 mm, r1´ = 2.2 mm, and r2´ = 1.4 mm at d = 5 mm and t = 2 mm.
It is clear that a smooth and nearly linear phase variation with the increasing frequencies is obtained, with sufficient phase difference from 10.5 to 18.5 GHz covering the entire Ku band by the two elements. Notably, the phase difference of the coding elements in the shadow region of Fig. 1(b) is in the range of [145°, 215°], which is determined as the effective phase interval by the criteria of far-field reflection amplitude [22,23], and is not limited to 180°. In addition, as illustrated in Fig. 1(c), high reflection amplitude (∼1) without any absorption dips of elements A and B is obtained in the broadband frequency range, which ensures subsequent modulation of microwave reflection intensity for coding metasurface.
A coding metasurface can be used to control electromagnetic waves by appropriately distributing elements A and B to form a specific coding sequence. Considering the coding metasurface with M × N elements, the far-field scattering pattern can be calculated from the superposition of scattering waves by treating each element as a radiation source. The electric field at the arbitrary spatial position $(\theta ,\varphi )$ in a spherical coordinate can be expressed as follows:
In this work, we aimed to reduce specular reflection and obtain uniform backward scattering patterns, where the reflected microwave energy is redistributed as omnidirectionally as possible, leading to a diffusion-like reflection. To realize reflection suppression in the maximum bandwidth, the genetic algorithm (GA) optimization is used for designing the coding sequences [34]. A schematic of the GA for choosing coding metasurface is shown in Fig. 2. For this purpose, we propose that the objective function of the scattering regulation in the GA optimization model is set to solve the minimum scattering energy variance in each direction of the half space. The smaller the variance, the more uniform the energy allocated in each direction; consequently, a diffusion-like reflection with small intensities in each direction can be anticipated. Finally, the optimal solution is the reflection phase layout matrix of all elements. Thereafter, according to the calculated corresponding reflection phase, the optimal coding sequence of elements can be obtained.
To better comply with period boundary conditions and reduce coupling between different elements, we employed a subset made of 3 × 3 identical elements as a whole unit representing 0° or 180° phase responses in the simulation [27,29]. The overall coding metasurface contains 39 × 39 elements. We continued to use the GA for multiple optimizations to determine the final arrangement of the coding elements. In the optimization process, we discovered that the fitness value decreased with an increase in the number of iterations and finally converged to the same value at different initial random matrices as illustrated in the inset of Fig. 2, which indicates that the minimum variance is obtained via the GA. Thereafter, we obtained the optimal coding sequence from the optimization method, as shown in Fig. 2.
To verify the correctness and validity of the coding metasurface, a full-wave simulation based on CST was conducted to calculate the reflection and far-field scattering patterns. Figures 3(a)–(f) illustrate the scattering patterns of the optimized coding metasurface and metallic plate under normal illumination at 10.5, 13.5, and 18.5 GHz respectively. In contrast to the large main lobe concentrated in the specular direction produced by the metallic plate, numerous reflection beams in various directions with low intensities are generated by the coding metasurface.
Fig. 3. Simulated far-field scattering patterns under normal incidence for (a–c) the optimized coding metasurface and (d–f) metallic plate at 10.5, 13.5, and 18.5 GHz, respectively; Simulated RCS of the optimized coding metasurface and metallic plate in the (g) xoz and (h) yoz planes at 13.5 GHz.
The overall scattering patterns of the coding metasurface are disordered at three frequencies, and specular reflections are no longer dominant. The backward energy distribution of the coding metasurface is similar to a diffusion-like reflection, whereas there is a single concentrated reflected beam for the metallic plate. For a quantitative comparison, Fig. 3(g)–(h) show the simulated far-field radar cross-section (RCS) at 13.5 GHz in the xoz and yoz planes, and it is clear that a nearly 20-dB RCS reduction is achieved in the specular direction with no significant side lobes covering all directions compared with the metallic plate, manifesting the significant reflection suppression effect of the coding metasurface in the entire half space.
Next, to investigate the angular dependence of the optimized coding metasurface, we examined the specular reflection at different incident angles (0–40°) for both TE and TM polarization waves. Under the illumination of normal incidence, as shown in Fig. 4, the simulated result presents a specular reflection coefficient below 0.1 in a wide frequency band, indicating excellent reflection suppression properties. In fact, because of the reflection phase discontinuity between neighboring coding elements, reflected beams of the coding metasurface cannot constructively interference as a metallic plate, thus greatly suppressing the reflection energy and redirecting them in other directions. Furthermore, ultra-low reflections (<0.1) from 10.5 to 18.5 GHz are achieved over a large angle of incidence for both polarizations, which can be expected from the effective phase interval of coding elements in Fig. 1(b). The bandwidth continues to shrink with the increase in the incident angle; when this incident angle is less than 40°, the reflection coefficient is still not greater than 0.2 in a wide frequency range. Notably, the reflection coefficient is normalized to the specular reflection from the metallic plate at normal or oblique incidences, which is used to show the specular reflection reduction, and the sum of the reflection coefficients at all scattering angles represents the actual reflection performance of the coding metasurface. In addition, the coding metasurface is insensitive to the different polarizations, and the reflection remains unchanged for TE and TM waves, which is attributed to the symmetry of the coding element and their random arrangement. The far-field scattering patterns of the coding metasurface at different incident angles are depicted in Fig. S2 (Supplement 1), all of which exhibit uniformly distributed backward reflections.
Fig. 4. Simulated normalized reflection spectra of the optimized coding metasurface under oblique incidences of a) TE and b) TM polarizations.
2.2 Experimental details
Ultrathin doped Ag films were deposited from pure Ag and Cu targets by the co-sputtering method, as shown in Fig. 5(a). Thereafter, we employed standard photolithography combined with laser direct writing and reactive ion etching processes to fabricate coding metasurface samples based on the as-prepared ultrathin Ag films; the process flow is shown in Fig. 5(b). Photographs of the fabricated ultrathin Ag film and coding metasurface sample are shown in Fig. 5(c), all exhibiting excellent visible transparency, through which the logos can be clearly observed.
Fig. 5. a) Schematic of co-sputtering process. b) Process flows of coding metasurface based on ultrathin Ag film. c) Photographs of the prepared ultrathin Ag film and coding metasurface with and without photoresist. d) SEM and AFM images of ultrathin Ag film. e) Photographs of the fabricated coding metasurface in the bending state.
The composition of ultrathin Ag films can be adjusted by varying the source power of the targets. In our experiments, the calibrated sputtering rates of Ag and Cu were 1.109 and 0.019 nm/s, respectively, resulting in ∼2% Cu in the ultrathin doped Ag films. Previous research has proved that the introduction of a small amount of additive metals, such as aluminum, copper, nickel, and chromium, into Ag films during the deposition process can significantly improve the optoelectronic performance of Ag films, particularly at ultrathin thicknesses (<15 nm) [35,36]. Here, 8-nm ultrathin Ag films on polyethylene terephthalate (PET) substrate were co-sputtered by controlling the deposition rates and time, and scanning electronic microscopy and atomic force microscopy were used to characterize the surface morphologies of the as-grown films, all of which exhibited smooth and continuous surface features, as illustrated in Fig. 5(d). The measured sheet resistance of the 8-nm ultrathin doped Ag is ∼12.6 Ω/sq compared to that of ∼4.5 kΩ/sq for the pure Ag film with a thickness of ∼9 nm, verifying the good electrical performance. To further enhance the optical transmittance of ultrathin Ag films, 40-nm indium tin oxide overcoats and undercoats function as optical antireflection layers by suppressing the strong reflection from air/Ag interfaces, which can simultaneously improve the electrical conductivity of ultrathin Ag films. The measured average visible transmittance of ultrathin Ag with antireflection layers is ∼96.5%, in contrast to that of ∼75% for the pure 8-nm Ag film, as plotted in Figure S3, Supplement 1, exhibiting clearly improved visibility due to antireflection coatings and ensuring the excellent optical performance of the coding metasurface.
Considering the actual processing capacity of the facilities, the entire coding metasurface sample (18 cm × 18 cm) was divided into four equal-sized pieces, which were accurately bonded together according to the original arrangement of the coding sequence by optical clear adhesion (100 μm, Sanling Co.). Figure 5(c-2) shows the fabricated coding metasurface sample (1/4 sizes), through which the patterned ultrathin Ag structures and the background picture can be clearly observed. Notably, the photoresist on the coding elements is not removed from the surface so that the ring structures can be clearly observed, as ultrathin Ag itself is highly transparent. The average relative visible transmittance of the coding metasurface based on the experimental result of ultrathin Ag was ∼95.9%, which was obtained by calculating the obscuration ratio of coding elements from the coding sequence. Furthermore, the coding metasurface sample on the PET substrate shows favorable flexibility, as shown in the inset of Fig. 5(e), which can be bent to a large angle without affecting the overall performance. As ultrathin Ag film has outstanding mechanical behaviors compared to most of the transparent metal oxide films, the coding metasurface here inherits its good ductility and bendability, which holds great promise in the next-generation flexible electronics. For real applications, the proposed transparent flexible coding metasurface is more compatible to complex and conformal curved surfaces, such as windows, domes, solar cell panels and can be further extended to wearable electronics as it can be wrapped around the desired object with great microwave scattering reduction performance.
2.3 Experimental analysis
To experimentally verify the low-reflection properties of microwaves, the standard bow method was used to test the far-field scattering performance of the fabricated coding metamaterial sample. As shown in Fig. 6(a), two linearly polarized horn antennas connected to the vector network analyzer serving as the transmitter and receiver were installed on the arc frame and could be moved within a range of ±80°. Microwave-absorbing materials (blue pointed cone) are laid around the sample holder to reduce unnecessary reflection. In addition, a metallic plate of the same size was measured in the experiment for comparison. The maps of simulated and measured far-field scattering results under normal incidence for TE polarization are depicted in Figs. 6(b)–(e). Limited by sample size, the coding metasurface was measured in the range of 12–18 GHz. At normal incidence, the measured reflection coefficient of the optimized coding metasurface was ∼0.15, exhibiting a significant reduction capability of microwave reflection. The experimental results are slightly higher than those from simulation owing to the reduced electrical conductivity of ultrathin Ag after the etching process and microwave leakage of the spliced samples. In addition, from these figures at specific frequencies, it is clear that a reduction of over 20 dB compared to the metallic plate in other scattering directions is achieved, and the measured overall trends agree well with those from simulation, thereby avoiding a significant increase of scattered waves in other directions.
Fig. 6. a) Microwave far-field reflection measurement setup. b) Measured normalized reflection of the optimized coding metasurface at normal incidence. c–f) Simulated and measured far-field RCS reduction for the coding metasurface and metallic plate in the x-z plane at 12, 14, 16, and 18 GHz respectively.
Considering the reflection properties at oblique angles, we present the specular reflection spectra from 10 to 40° for TE and TM waves, as shown in Fig. 7. The averaged normalized specular reflection coefficients from 12 to 18 GHz at different incident angles are all less than 0.15, which indicates that the specular reflection is significantly suppressed in the presence of the coding metasurface. These measurements indicate that the optimized coding metasurface can achieve low reflection and uniform backward scattering over a broad frequency range within a wide range of incident angles. Moreover, the measured shielding effectiveness of the coding metasurface is shown in Fig. S4, Supplement 1, exhibiting a strong shielding capability (∼28 dB) owing to the back ultrathin Ag layer, which implies that only ∼0.0015 of the incident energy can be transmitted.
Fig. 7. Measured normalized reflection spectra of the coding metasurface at different incident angles. a) 10°, b) 20°, c) 30°, and d) 40°.
3. Conclusions
In conclusion, a coding metasurface based on ultrathin doped Ag was proposed with low reflection, for the first time. We established a GA-based scattering optimization model with the goal of minimizing the variance in the backward scattering energy. Both numerical simulation and experimental results demonstrate the desired broadband and wide-angle diffusion of microwaves with high optical transmittance, which is attributed to the employment of ultrathin Ag and rational layout of coding elements. The measured results exhibit a relative visible transmittance of ∼95.9%, specular reflection of ∼15% with uniform backward scattering, and a strong shielding effectiveness of ∼28 dB was obtained. The proposed coding metasurface provides new opportunities for controlling the scattering of microwaves for optoelectronic devices in optical applications.
Funding
Natural Science Foundation of Heilongjiang Province (LH2020F016); China Postdoctoral Science Foundation (2019M661272); National Natural Science Foundation of China (61975046, 62005065).
Acknowledgments
We would like to express our sincere gratitude to CST Ltd., Germany, for providing a free package of CST MWS software to the CST Training Center (Northeast China Region) at our institute.
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.
Supplemental document
See Supplement 1 for supporting content.
References
1. H. Chen, W. Ma, Z. Huang, Y. Zhang, Y. Huang, and Y. Chen, “Graphene-based materials toward microwave and terahertz absorbing stealth technologies,” Adv. Opt. Mater. 7(8), 1801318 (2019). [CrossRef]
2. S. Zhong, W. Jiang, P. Xu, T. Liu, J. Huang, and Y. Ma, “A radar-infrared bi-stealth structure based on metasurfaces,” Appl. Phys. Lett. 110(6), 063502 (2017). [CrossRef]
3. M. Han, C. E. Shuck, R. Rakhmanov, D. Parchment, B. Anasori, C. M. Koo, G. Friedman, and Y. Gogotsi, “Beyond Ti3C2Tx: MXenes for Electromagnetic Interference Shielding,” ACS Nano 14(4), 5008–5016 (2020). [CrossRef]
4. F. Shahzad, M. Alhabeb, C. B. Hatter, B. Anasori, S. M. Hong, C. M. Koo, and Y. Gogotsi, “Electromagnetic interference shielding with 2D transition metal carbides (MXenes),” Science 353(6304), 1137–1140 (2016). [CrossRef]
5. O. Balci, E. O. Polat, N. Kakenov, and C. Kocabas, “Graphene-enabled electrically switchable radar-absorbing surfaces,” Nat Commun 6(1), 1–10 (2015). [CrossRef]
6. X. Zhu, J. Xu, F. Qin, Z. Yan, A. Guo, and C. Kan, “Highly efficient and stable transparent electromagnetic interference shielding films based on silver nanowires,” Nanoscale 12(27), 14589–14597 (2020). [CrossRef]
7. L. C. Jia, D. X. Yan, X. Liu, R. Ma, H. Y. Wu, and Z. M. Li, “Highly efficient and reliable transparent electromagnetic interference shielding film,” ACS Appl. Mater. Interfaces 10(14), 11941–11949 (2018). [CrossRef]
8. H. Wang, Z. Lu, Y. Liu, J. Tan, L. Ma, and S. Lin, “Double-layer interlaced nested multi-ring array metallic mesh for high-performance transparent electromagnetic interference shielding,” Opt. Lett. 42(8), 1620–1623 (2017). [CrossRef]
9. Z. Y. Jiang, W. Huang, L. S. Chen, and Y. H. Liu, “Ultrathin, lightweight, and freestanding metallic mesh for transparent electromagnetic interference shielding,” Opt. Express 27(17), 24194–24206 (2019). [CrossRef]
10. Y. Han, J. Lin, Y. Liu, H. Fu, Y. Ma, P. Jin, and J. Tan, “Crackle template based metallic mesh with highly homogeneous light transmission for high-performance transparent EMI shielding,” Sci. Rep. 6(1), 1–11 (2016). [CrossRef]
11. N. Zhang, Z. Wang, R. Song, Q. Wang, H. Chen, B. Zhang, H. Lv, Z. Wu, and D. He, “Flexible and transparent graphene/silver-nanowires composite film for high electromagnetic interference shielding effectiveness,” Sci. Bull. 64(8), 540–546 (2019). [CrossRef]
12. C. Liang, K. Ruan, Y. Zhang, and J. Gu, “Multifunctional flexible electromagnetic interference shielding silver nanowires/cellulose films with excellent thermal management and joule heating performances,” ACS Appl. Mater. Interfaces 12(15), 18023–18031 (2020). [CrossRef]
13. . C.Yuan, J. Huang, Y. Dong, X. Huang, Y. Lu, J. Li, T. Tian, W. Liu, and W. Song, “Record-High Transparent Electromagnetic Interference Shielding Achieved by Simultaneous Microwave Fabry–Pérot Interference and Optical Antireflection,” ACS Appl. Mater. Interfaces 12(23), 26041–26049 (2020). [CrossRef]
14. N. Erdogan, F. Erden, A. T. Astarlioglu, M. Ozdemir, S. Ozbay, G. Aygun, and L. Ozyuzer, “ITO/Au/ITO multilayer thin films on transparent polycarbonate with enhanced EMI shielding properties,” Current Applied Physics 20(4), 489–497 (2020). [CrossRef]
15. N. I. Zheludev and Y. S. Kivshar, “From metamaterials to metadevices,” Nat. Mater. 11(11), 917–924 (2012). [CrossRef]
16. Q. Ma, G. D. Bai, H. B. Jing, C. Yang, L. Li, and T. J. Cui, “Smart metasurface with self-adaptively reprogrammable functions,” Light Sci Appl 8(1), 1–12 (2019). [CrossRef]
17. W. Cai, U. K. Chettiar, A. V. Kildishev, and V. M. Shalaev, “Optical cloaking with metamaterials,” Nat. Photonics 1(4), 224–227 (2007). [CrossRef]
18. H. Li, L. Fu, K. Frenner, and W. Osten, “Cascaded plasmonic superlens for far-field imaging with magnification at visible wavelength,” Opt. Express 26(8), 10888–10897 (2018). [CrossRef]
19. Y. Li, Z. Liu, H. Zhang, P. Tang, B. Wu, and G. Liu, “Ultra-broadband perfect absorber utilizing refractory materials in metal-insulator composite multilayer stacks,” Opt. Express 27(8), 11809–11818 (2019). [CrossRef]
20. L. Li, T. J. Cui, W. Ji, S. Liu, J. Ding, X. Wan, Y. B. Li, M. Jiang, C.W. Qiu, and S. Zhang, “Electromagnetic reprogrammable coding-metasurface holograms,” Nat. Comm. 8(1), 1–7 (2017). [CrossRef]
21. 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]
22. T. J. Cui, S. Liu, and L. L. Li, “Information entropy of coding metasurface,” Light Sci Appl 5(11), e16172 (2016). [CrossRef]
23. S. Liu, T. J. Cui, Q. Xu, D. Bao, L. Du, X. Wan, W. X. Tang, C. Ouyang, X. Y. Zhou, H. Yuan, H. F. Ma, W. X. Jiang, J. Han, W. Zhang, and Q. Cheng, “Anisotropic coding metamaterials and their powerful manipulation of differently polarized terahertz waves,” Light Sci Appl 5(5), e16076 (2016). [CrossRef]
24. L. Zhang, X. Q. Chen, S. Liu, Q. Zhang, J. Zhao, J. Y. Dai, G. D. Bai, X. Wan, Q. Cheng, G. Castaldi, V. Galdi, and T. J. Cui, “Space-time-coding digital metasurfaces,” Nat. Comm. 9(1), 1–11 (2018). [CrossRef]
25. R. Y. Wu, C. B. Shi, S. Liu, W. Wu, and T. J. Cui, “Addition theorem for digital coding metamaterials,” Adv. Opt. Mater. 6(5), 1701236 (2018). [CrossRef]
26. L. H. Gao, Q. Cheng, J. Yang, S. J. Ma, J. Zhao, S. Liu, H. Chen, Q. He, W. Jiang, H. Ma, Q. Wen, L. Liang, B. Jin, W. Liu, L. Zhou, J. Yao, P. Wu, and T. Cui, “Broadband diffusion of terahertz waves by multi-bit coding metasurfaces,” Light Sci Appl 4(9), e324 (2015). [CrossRef]
27. L. Liang, M. Qi, J. Yang, X. Shen, J. Zhai, W. Xu, B. Jin, W. Liu, Y. Feng, C. Zhang, H. Lu, H. Chen, L. Kang, W. Xu, J. Chen, T. J. Cui, P. Wu, and S. Liu, “Anomalous terahertz reflection and scattering by flexible and conformal coding metamaterials,” Adv. Opt. Mater. 3(10), 1374–1380 (2015). [CrossRef]
28. L. Ali, Q. Li, T. A. Khan, J. Yi, and X. Chen, “Wideband RCS reduction using coding diffusion metasurface,” Materials 12(17), 2708 (2019). [CrossRef]
29. D. S. Dong, J. Yang, Q. Cheng, J. Zhao, L. H. Gao, S. J. Ma, S. Liu, H. Bin Chen, Q. He, W. W. Liu, Z. Fang, L. Zhou, and T. J. Cui, “Terahertz broadband low-reflection metasurface by controlling phase distributions,” Adv. Opt. Mater. 3(10), 1405–1410 (2015). [CrossRef]
30. 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), 5571–5579 (2017). [CrossRef]
31. 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]
32. Z. Meng, C. Tian, C. Xu, J. Wang, X. Li, S. Huang, Q. Fan, and S. Qu, “Optically transparent coding metasurface with simultaneously low infrared emissivity and microwave scattering reduction,” Opt. Express 28(19), 27774–27784 (2020). [CrossRef]
33. H. Chen, W. B. Lu, Z. G. Liu, and M. Y. Geng, “Microwave programmable graphene metasurface,” ACS Photonics 7(6), 1425–1435 (2020). [CrossRef]
34. J. Zhang, G. Wang, T. Wang, and F. Li, “Genetic Algorithms to Automate the Design of Metasurfaces for Absorption Bandwidth Broadening,” ACS Appl. Mater. Interfaces 13(6), 7792–7800 (2021). [CrossRef]
35. C. Zhang, Q. Huang, Q. Cui, C. Ji, Z. Zhang, X. Chen, T. George, S. Zhao, and L. J. Guo, “High-performance large-scale flexible optoelectronics using ultrathin silver films with tunable properties,” ACS Appl. Mater. Interfaces 11(30), 27216–27225 (2019). [CrossRef]
36. D. Gu, C. Zhang, Y. K. Wu, and L. J. Guo, “Ultrasmooth and thermally stable silver-based thin films with subnanometer roughness by aluminum doping,” ACS Nano 8(10), 10343–10351 (2014). [CrossRef]
