RESEARCH ARTICLE | DECEMBER 15 2021 Broadband digital coding metasurface holography Special Collection: Metasurfaces for Photonic Devices Qiang Xiao ; Qian Ma ; Liang Wei Wu; Yue Gou; Jia Wei Wang; Wei Han Li; Rui Zhe Jiang; Xiang Wan; Tie Jun Cui J. Appl. Phys. 130, 235103 (2021) https://doi.org/10.1063/5.0064675 View Online Export Citation Articles You May Be Interested In Light scattering by V4O7 film across the metal–insulator transition J. Appl. Phys. (September 2024) Raman spectra and elastic light scattering dynamics of V3O5 across insulator–metal transition J. Appl. Phys. (January 2021) 15 April 2025 02:04:53 Journal of Applied Physics ARTICLE scitation.org/journal/jap Broadband digital coding metasurface holography Cite as: J. Appl. Phys. 130, 235103 (2021); doi: 10.1063/5.0064675 Submitted: 26 July 2021 · Accepted: 31 October 2021 · Published Online: 15 December 2021 View Online Export Citation CrossMark Qian Ma,1,2,3,a) Liang Wei Wu,1,2,3 Yue Gou,1,2,3 Jia Wei Wang,1,2,3 Wei Han Li,1,2,3 Qiang Xiao,1,2,3 1,2,3 Rui Zhe Jiang, Xiang Wan,1,2,3 and Tie Jun Cui1,2,3,a) AFFILIATIONS 1 Institute of Electromagnetic Space, Southeast University, Nanjing 210096, China 2 State Key Laboratory of Millimeter Wave, Southeast University, Nanjing 210096, China 3 Center of Intelligent Metamaterials, Pazhou Laboratory, Guangzhou 510330, China Note: This paper is part of the Special Topic on Metasurfaces for Photonic Devices. a) Authors to whom correspondence should be addressed: qianma@seu.edu.cn and tjcui@seu.edu.cn ABSTRACT 15 April 2025 02:04:53 Digital coding metasurfaces composed of subwavelength meta-atoms can flexibly control electromagnetic waves to achieve holography, which has great potential in millimeter-wave imaging systems and data storage. In this paper, we propose a 3-bit reflective digital coding metasurface. The incident linearly polarized waves can be transformed into cross-polarized components with distinct phase responses by adjusting the rotational and open angles of the coding elements. The 3-bit phase performance can be retained over a wide bandwidth from 12 to 18 GHz by simultaneously changing the rotational and open angles. Based on the proposed broadband metasurface, broadband holography is successfully demonstrated with the optimization of a modified Gerchberg–Saxton algorithm. As a proof of concept, five schemes with different holograms integrating the letters “S,” “E,” “U,” “X,” and “Z” are simulated from 12 to 18 GHz. Good simulation results validate the performance of the proposed broadband holography, showing a relative bandwidth of 40%. Two prototypes superposing the holograms of letters “U” and “X” are fabricated and measured in a near-field microwave anechoic chamber. The experimental results corroborate well with simulated results, further supporting the demonstration. We believe that the proposed broadband holography based on the digital coding metasurface paves a way to wideband applications for microwave imaging, information processing, and holographic data storage. Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0064675 I. INTRODUCTION As two-dimensional (2D) planar metamaterial, metasurface has distinguished abilities to manipulate electromagnetic (EM) waves by periodically or aperiodically arranging the artificial subwavelength elements in a thin profile. Generalized Snell’s laws1 introducing abrupt phase changes are innovatively put forward, which accelerates the development of metasurfaces, resulting in the properties of EM wave (amplitude, phase, and polarization) to be flexibly manipulated, hence accomplishing exciting physical phenomena, such as perfect lens,2 invisibility cloak3–5 and so on. In 2014, the concept of digital coding metasurfaces was proposed as a new method to manipulate the EM waves by using the digital state manner instead of effective medium paraments from the information perspective,6,43,44 which simplifies the design process and simultaneously abandons manipulation dimensions. Most J. Appl. Phys. 130, 235103 (2021); doi: 10.1063/5.0064675 Published under an exclusive license by AIP Publishing importantly, the digitalization of elements bridges the physical world and the digital world by enabling physical metasurface to interact with digital information,7–9 which is analogous to digital signal processing. Thus, some basic information operations or theories of digital coding metasurfaces have been gradually developed, from convolution,10 addition,11,12 differentiation, and integration13 operation to information entropy,14–16 speeding up the research of multi-functional metasurfaces such as beam steering,17,18 polarization-modulation,19,20 and orbital angular momentum (OAM) generator.21,22 By introducing active components (positive-intrinsic- negative diodes, varactors, and photodiodes) controlled by field-programmable gate arrays (FPGA), the digital programmable metasurfaces are further studied to dynamically manipulate EM waves by encoding the corresponding sequences in real-time, and different EM functions are realized, such as 130, 235103-1 Journal of Applied Physics scitation.org/journal/jap II. PRINCIPLES OF BROADBAND HOLOGRAPHY The principle of broadband holography based on the digital coding metasurface is schematically demonstrated in Fig. 1. The most important core to realize broadband holography is to carefully design a broadband metasurface with good performance, which keeps constant around 45° phase difference of 3-bit digital coding elements. The proposed broadband metasurface can be achieved by combining digital coding elements. For 3-bit digital coding case, the coding elements are encoded as “0,” “1,” “2,” “3,” “4,” “5,” “6,” and “7,” which have the adjacent phase difference of around 45° from 12 to 18 GHz with high reflective amplitude. Note that the incident x-polarized waves are changed to y-polarized waves through the wave-manipulation of the metasurface. Then, by integrating the optimized hologram of the letter “U” in the metasurface, the image of the letter “U” can be reconstructed and clearly observed from 12 to 18 GHz when an x-polarized electromagnetic (EM) wave normally illuminates the proposed metasurface, hence realizing broadband holography. The structure of the proposed coding element is depicted in Figs. 2(a) and 2(b), which are composed of three layers: metal structure layer, metal-ground layer, and substrate layer. Two C-shaped symmetrical rings connected with the metal bar of the width w = 1.5 mm and the length of r = 4.25 mm are printed on the substrate. The substrate F4B has a thickness of h = 3 mm with relative permittivity 2.6 and loss tangent 0.009. The period p of the unit cell is optimized to 10 mm. The proposed coding elements can exhibit broadband performance, similarly performing as the linear polarization conversion resonator as reported in detail in Ref. 44. When an x-polarized wave illuminates the coding element of a rotation angle β = ± 45°, multiple resonate modes supported by the metal bar and the corresponding C-shaped symmetrical rings are excited, resulting in polarization conversion and realizing broadband performance. More importantly, the coding element (β = ± 45°) and its mirror counterpart (β = ∓ 45°) will have the 15 April 2025 02:04:53 nonreciprocity,23,24 space–time-coding metasurfaces,25 and intelligent imaging.26 It is worth mentioning that the programmable metasurfaces are widely explored and become a potential participant in the sixth-generation communication system.27–29 Furthermore, metasurfaces can enable holography and become a potential paradigm of imaging techniques by precisely reconstructing the amplitude and phase information of target objects, emerging the advantage of low profile, small size, and high resolution compared with traditional holography.30 Recently, various emerging forms of metasurface holography were proposed, such as phase-only,31–33 complex amplitude,34,35 helicity-multiplexed,36 and vectorial,37,38 hence facilitating the development of metasurface holography. Furthermore, OAM is introduced to achieve encrypted holography with the advantages of numerous OAM orthogonal modes.39,40 Loading active components in the metasurface, reprogrammable holography can be dynamically realized by updating the metasurface holograms in real-time.41 However, there is little research on broadband microwave metasurface holography, which may further broaden the application boundary of imaging technology. Comparing with the earlier work,32 we further expand the working bandwidth of holography. The imaging efficiencies of the proposed holography are greater than about 50% from 12 to 18 GHz. In addition, inducing of digital coding metasurfaces enables the optimization procedures of holograms to be more flexible, and the information of holograms can be easily edited or processed. 360° phase coverage can be achieved by setting different C-shaped split rings open angles and eight coding elements are selected as 3-bit digital states. To obtain high imaging efficiency, a modified Gerchberg–Saxton (GS) algorithm is applied to generate the final holograms.39,42 We present five proof-of-concept coding metasurfaces with the holograms of letters “S,” “E,” “U,” “X,” and “Z,” successfully verifying broadband holography with a relative bandwidth of 40%. ARTICLE FIG. 1. Schematic diagram of broadband metasurface holography. The proposed metasurface is composed of eight digital coding elements: digital “0,” “1,” “2,” “3,” “4,” “5,” “6,” and “7” with a phase increment of 45°. The letter “U” can be clearly observed from 12 to 18 GHz when x-polarized EM waves illuminate the proposed metasurface. J. Appl. Phys. 130, 235103 (2021); doi: 10.1063/5.0064675 Published under an exclusive license by AIP Publishing 130, 235103-2 Journal of Applied Physics ARTICLE scitation.org/journal/jap same reflective amplitude with a stable phase difference of 180°. To achieve the 3-bit phase characteristic of the coding element, four kinds of coding elements with a phase difference of around 45° can be obtained by tuning the open-angle α of C-shaped symmetrical rings and mirroring these coding elements symmetrically to get the remaining four coding elements. The full-wave simulation of the unit cell is performed in the commercial software, Computer Simulation Technology (CST) Microwave Studio with the unit-cell boundary condition, in which mutual coupling between adjacent elements can be taken into consideration. The entire 360° phase coverage can be achieved by changing the combination of (α, β) under the illumination of an x-polarized wave. It means that 180° phase coverage can be extended to 360° phase coverage by a mirror operation of the rotational angle β in the above coding elements. To simplify the design process, the eight digital coding elements with different combinations of (α, β) = (40°, 45°), (65°, 45°), (90°, 45°), (115°, 45°), (40°, −45°), (65°, −45°), (90°, −45°), and (115°, −45°), in which the adjacent phase difference is around 45° from 12 to 18 GHz, are carefully selected to realize the entire 3-bit phase coverage and hence mimic the digital states: “0,” “1,” “2,” “3,” “4,” “5,” “6,” and “7,” as shown in Fig. 2(d). The reflected amplitude of the cross-polarized J. Appl. Phys. 130, 235103 (2021); doi: 10.1063/5.0064675 Published under an exclusive license by AIP Publishing 15 April 2025 02:04:53 FIG. 2. The geometry structure of broadband coding element and the corresponding reflective performance. (a) and (b) The designed coding element. (c) and (d) The simulated reflective amplitude and phase response of 3-bit coding elements from 10 to 18 GHz. state is greater than −3 dB in Fig. 2(c). The eight digital coding elements have better broadband performance with the phase difference of around 45° from 12 to 18 GHz, which ensures that broadband holography based on the proposed digital coding metasurface can be accomplished over a wide working frequency. III. RESULTS To demonstrate the performance of broadband holography based on digital coding metasurface, five kinds of broadband metasurfaces superposing with the phase-only holograms of letters “S,” “E,” “U,” “X,” and “Z” are, respectively, proposed. Then, the holograms of the above-mentioned letters should be calculated. In practice, the near-field imaging area is usually close to the feed and the metasurface in microwave holography, resulting in that the imaging field distribution will be interfered with by the incident wave. The holograms are optimized by a modified Gerchberg–Saxton (GS) algorithm, which considers the effect of the incident wave to adaptively iterate the corresponding perfectly matched phase distributions. As a proof of concept, the optimized holograms of letters “S,” “E,” “U,” “X,” and “Z” at the initial imaging locations of 320 mm with the center frequency of 15 GHz are calculated by the 130, 235103-3 Journal of Applied Physics ARTICLE scitation.org/journal/jap FIG. 3. The calculated holograms and the corresponding configuration of the designed metasurface. (a)–(e) The optimized holograms by a modified GS algorithm superposing letters “S,” “E,” “U,” “X,” and “Z,” respectively. (f )–(k) The coding elements distributions of five metasurfaces with letters “S,” “E,” “U,” “X,” and “Z,” respectively. reconstructed from 12 to 18 GHz. The concrete relation between the locations, the effective sizes of holographic images, and the working frequency are described as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (x x1 )2 þ (y y1 )2 þ z12 z1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2πf2 (x x2 )2 þ (y y2 )2 þ z22 z2 , ¼ c 2πf1 c (1) where f1 and f2 are two different working frequencies, c is the propagation speed of EM waves in free space, (x, y) are the coordinates of the coding elements, and (xi , yi , zi ) represent the coordinates of 15 April 2025 02:04:53 above modified GS algorithm, as shown in Figs. 3(a)–3(e). According to the above-optimized holograms, five corresponding coding metasurfaces are, respectively, designed as Figs. 3(f )–3( j), each of which consists of 30 × 30 elements with a total size of 300 × 300 mm2 (15 × 15 λ2). The above digital coding metasurfaces are simulated by the time-domain solver of CST Microwave Studio with the openboundary condition. Meanwhile, a standard waveguide working from 12 to 18 GHz is served as an exciting source, normally placed at a distance of 180 mm from the center of the metasurface and thus illuminating five metasurfaces, respectively. The simulated near-field results are depicted in Figs. 4(a)–4(e). As can be observed, letters “S,” “E,” “U,” “X,” and “Z” can be clearly FIG. 4. Full-wave simulated results. (a)–(e) The intensity distributions of letters “S,” “E,” “U,” “X,” and “Z” from 12 to 18 GHz at different locations, respectively. J. Appl. Phys. 130, 235103 (2021); doi: 10.1063/5.0064675 Published under an exclusive license by AIP Publishing 130, 235103-4 Journal of Applied Physics ARTICLE scitation.org/journal/jap FIG. 5. The prototype of metasurface and experimental setup. (a) The metasurface sample corresponding to the hologram of the letter “U.” (b) The near-field experimental setup in microwave anechoic chamber. the holographic images at the working frequency of fi . According to the above relationship, the sizes of the holographic image and the distance between the metasurface and the holographic images vary with the working frequency. After the optimization, the locations of relatively good holographic results are selected as 160, 240, 280, 320, 340, 380, and 460 mm corresponding to 12, 13, 14, 15, 16, 17, and 18 GHz, respectively. It is noticeable that the locations and the effective sizes of holographic images have a gradually enlarging tendency with the increase of working frequencies. As shown in Fig. 4, the relative bandwidth of the proposed broadband holography based on the digital coding metasurface is 40% with high imaging quality. To experimentally verify the performance of broadband holography based on digital coding metasurfaces, two kinds of broadband digital coding metasurfaces, respectively, integrating the holograms of letters “U” and “X” are fabricated and measured, one fabricated sample of which is exhibited in Fig. 5(a). The above broadband metasurfaces with the effective size of 300 × 300 mm2 (15 × 15 λ2) composed of 30 × 30 digital coding elements are fabricated by the printed circuit board (PCB) technology. The samples are measured in the microwave anechoic chamber for the nearfield, and the experimental environment is shown in Fig. 5(b). A standard waveguide working from 12 to 18 GHz serves as a transmitter and the experimental probe connected with the vector network analyzer serves as a receiver. Meanwhile, the polarized state of a standard waveguide is set as vertical polarization and the polarized state of the experimental probe is horizontal. The 15 April 2025 02:04:53 IV. EXPERIMENTAL VERIFICATION standard waveguide is vertically placed on the center of the acrylic board used to support samples, with a distance of 180 mm away from samples. The distance between samples and the experimental probe is set as the different distances varied with the working frequency according to the simulated results in Fig. 4. The scanning plane is parallel to the samples with an effective area of 300 × 300 mm2. First, the probe is moved to a distance of 240 mm from the metasurface and the intensity of the metasurface with the letters “U” and “X” at 13 GHz is, respectively, measured by the probe automatically moving along the x–y plane. Similarly, the probe is successively moved to 280, 320, 340, 380, and 460 mm when the working frequency is set from 14 to 18 GHz. It should be noted that the distance between the metasurface and the probe at 12 GHz is 160 mm, less than 180 mm, and, hence, the intensity of the metasurface at 12 GHz is unable to be measured due to the experiment limitation. All measured results are demonstrated in Fig. 6. The intensity distributions of the letter “U” from 13 to 18 GHz are clearly shown in Fig. 6(a), and Fig. 6(b) displays the intensity distributions of the letter “X” from 13 to 18 GHz. We can observe that the deviation exists between the measured and the simulated results. Limited by the accuracy of the manufacturing technology and the characteristic of the used substrate, the imperfect fabrication and a slight deformation of samples will cause the actual phase deviation. In particular, for higher working frequencies, the imaging effect is slightly different from the simulation because higher tolerance for the accuracy of the fabricated samples is required. Moreover, the performance of the waveguide used as the feed and the imperfect alignment between the waveguide and the metasurface can affect the measured results. The deviation between FIG. 6. The measured results in microwave anechoic chamber. (a) and (b) The intensity distributions of letters “U” and “X” from 13 to 18 GHz at the distance of 240, 280, 320, 340, 380, and 460 mm, respectively. J. Appl. Phys. 130, 235103 (2021); doi: 10.1063/5.0064675 Published under an exclusive license by AIP Publishing 130, 235103-5 Journal of Applied Physics the probe position and the metasurface center can exist in the measured process because the measuring probe is installed manually. To a certain extent, the deviation can be decreased by the advanced fabricated technology and precise position matching. Hence, the relative bandwidth of the proposed broadband holography based on the digital coding metasurface is at least 33.33% with high imaging quality. Moreover, the realized imaging efficiency of the letters “U” and “X” is calculated to be 54.8% and 49.2% at a center frequency of 15 GHz based on the definition of imaging efficiency that the fraction of incident energy that contributes to the transmitted holographic image.45,46 In general, the proposed metasurface accomplishes the function of broadband holography with good performance. V. CONCLUSION ACKNOWLEDGMENTS This work was supported in part from by the National Key Research and Development Program of China under Grant Nos. 2017YFA0700201, 2017YFA0700202, and 2017YFA0700203, the National Natural Science Foundation of China (NNSFC) under Grant Nos. 61871127, 61735010, 61731010, 61890544, 61801117, 61722106, 61701107, 61701108, 61701246, and 61631007, the State Key Laboratory of Millimeter Waves, Southeast University, China (K201924), the Fundamental Research Funds for the Central Universities under Grant No. 2242018R30001, the 111 Project under Grant No. 111-2-05, and the Fund for International Cooperation and Exchange of the National Natural Science Foundation of China (NNSFC) under Grant No. 61761136007. AUTHOR DECLARATIONS Conflict of Interest The authors have no conflicts of interest to disclose. DATA AVAILABILITY The data that support the findings of this study are available from the corresponding authors upon reasonable request. REFERENCES 1 N. Yu, P. 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