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Mar 1, 2017 - based ultrathin coding metasurface (less than 0.1 wavelength thick) .... polarizers [5], flat lens [6], holographic imagers [7,8], etc. ... shows broadband reduction of backward scattering in microwave frequencies. ... conductive material, one should carefully optimized the geometric configuration to minimize.

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Coding metasurface for broadband microwave scattering reduction with optical transparency KE CHEN, LI CUI, YIJUN FENG,* JUNMING ZHAO, TIAN JIANG, AND BO ZHU Department of Electronic Engineering, School of Electronic Science and Engineering, Nanjing University, Nanjing, 210093, China. * [email protected]

Abstract: Metasurfaces have promised great possibilities in full control of the electromagnetic wavefront by spatially manipulating the phase characteristics across the interface. Here, we report a scheme to realize broadband backward scattering reduction through diffusion-like microwave reflection by utilizing a flexible indium-tin-oxide (ITO)based ultrathin coding metasurface (less than 0.1 wavelength thick) with high optical transparence. The diffusion-like scattering is caused by the destructive interference of the scattered far-field electromagnetic wave, which is further attributed to the randomly distributed reflection phases on the metasurface composed of pre-designed meta-atoms arranged with a computer-generated pseudorandom coding sequence. Both simulation and measurement on fabricated prototype sample have been carried out to validate its performance, demonstrating a polarization-independent broadband (nearly from 8 GHz to 15 GHz) 10 dB scattering reduction with good oblique performance. The excellent performances can also be preserved to conformal cases when the flexible metasurface is uniformly wrapped around a metallic cylinder. The proposed metasurface may create new opportunities to tailor the exotic microwave scattering features with simultaneously high transmittance in visible frequencies, which could provide crucial benefits in many practical uses, such as window and solar panel applications. © 2017 Optical Society of America OCIS codes: (160.3918) Metamaterials; (050.6624) Subwavelength structures; (310.7005) Transparent conductive coatings; (290.5880) Scattering, rough surfaces; (350.4010) Microwaves.

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#283851 Journal © 2017

https://doi.org/10.1364/OE.25.005571 Received 30 Dec 2016; revised 23 Feb 2017; accepted 24 Feb 2017; published 1 Mar 2017

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1. Introduction The significant advances in the emerging concept of metasurfaces have empowered rapid growths of ultrathin electromagnetic (EM) devices which can arbitrarily control the wavefront by introducing field discontinuities across the interface [1]. The metasurface is generally recognized as a 2D equivalence of metamaterials, which usually consists of flat inhomogeneous array of subwavelength-scaled meta-atoms. The appeal of metasurfaces lies in their maintaining of ultrathin thickness meanwhile creating exotic phenomenon that are otherwise difficult or even impossible with naturally occurring materials, such as anomalous refraction and reflection [1–3]. So far, a number of intriguing EM devices based on metasurface technique have been implemented to demonstrate the unique EM properties, showing a promising prospect for real-world applications, such as invisibility cloaks [4], polarizers [5], flat lens [6], holographic imagers [7,8], etc. Recently, a new concept of digital or coding metamaterial and metasurface has been reported to manipulate the EM wave radiation and scattering by elaborately designing the coding sequences [9,10], which is quite different to the conventional metamaterials typically described with effective medium theory or phase gradients. The newly emerged concept of

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geometric-phase-coded metasurface enables further simplification of the design and optimization procedure of the digital metasurface [11]. In particular, by encoding the metasurface with randomized coding sequences, the diffusion-like scattering can be invoked by the destructive interference of the radiations from each constituent element, resulting in significant reduction of the backward radar cross section (RCS), indicating its potentials in stealth technique and many other useful applications [9–17]. However, this kind of coding metasurfaces are exclusively composed of structured meta-atoms with optically opaque materials (e.g. gold or copper), which may limit their applications on windows or domes for observation and communication. In general, an optically transparent metasurface could be required wherever the optical field continuity for viewing through the metasurface is inevitably necessary. For example, aircraft or satellite windows with reduced RCS to escape from radar detection, room windows to prevent radio leakage and electronic surveillance, aesthetic presentation isolating the unwanted radiation, and observation windows in microwave anechoic chamber or EM shielding room. Furthermore, the metasurface with high optical transparency is also an appealing candidate for integration with solar panels. Interestingly, some works based on optically transparent techniques have shown their potentials in some practical microwave applications, such as frequency selective surface [18,19], metamaterial absorbers [20], and transparent antennas [21,22]. Yet, similar concept has not been introduced to microwave metasurface designs. In this paper, we report the design of a diffusion-like coding metasurface based on highly optically transparent and electrically conductive material (indium tin oxide, ITO) deposited on transparent and flexible back-grounded polyethylene terephtalate (PET) substrate that shows broadband reduction of backward scattering in microwave frequencies. By selecting two particular meta-atoms with out of phase reflection property as the digital bits of “0” and “1”, and extending them according to a pre-design randomized coding sequence, the metasurface can generate a diffusion-like scattering, therefore significantly restrict the specular reflection with at least 10 dB reduction over a broad frequency band from 7.8 GHz to 15 GHz. In addition, the superior performance is insensitive to the incident polarizations, and can be well preserved as the oblique incident angle up to about 50° for both wave polarizations. The use of a plastic substrate has major advantages of lightweight and flexibility, which holds the promise to extend the diffusion-like scattering to conformal cases and therefore may be applied to novel conformal EM shielding and RCS reduction. As an exemplary demonstration, the flexible coding metasurface is conformally wrapped onto a metallic cylinder and achieves broadband performance in suppressing the backward scattering, validating its potential use in window applications as above-mentioned. 2. Design of coding elements The conductive inclusions composing the meta-atom often act as strong EM resonator and responses mainly due to their geometric patterns. In order to design the meta-atom with optically transparency, the conductive materials should simultaneously have high electrical conductivity and high optical transmittance. Fortunately, intense works of transparent conductors in recent decades have reported several realization methods [23], such as transparent conducting oxide [24], metallic nanowire network [25], perforated continuous metallic films [26], and graphene [27]. In this paper, we use the off-the-shelf commercial product of ITO film to design the coding metasurface.

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Fig. 1. (a) Digital elements of “0” (upper-left panel) and “1” (upper-right panel) with optimized geometric parameters of, in millimeters, p = 8, r = 3.5, l = 2.2, t = 2.55. Bottom panel shows the overall view of the coding metasurface with a random distribution of digital elements. (b) Simulated reflection spectra of the digital elements “0” and “1”.

In a 1-bit coding reflective metasurface, two elements with 180° phase difference are utilized to act as the digital byte of “0” and “1”, and further to form certain custom-designed coding sequences to manipulate the EM wave scattering and radiation. The designed digital elements are schematically illustrated in the upper panel of Fig. 1(a), where ITO thin film with a thickness of 185 nm is used to form the top conductive patterns (a square and a circular patch) and the ground plane, while the flexible PET is used as dielectric substrate with a thickness of 2.55 mm, a relative permittivity of 2.65 and loss tangent of about 0.015. The total thickness is less than 0.1λ (λ is the wavelength corresponding to the center frequency of the working band). The conductivity of the ITO film in the microwave region is about 6.76 × 105 S/m, which results in an equivalent surface resistance of 8 Ω/sq. Consider the use of lossy conductive material, one should carefully optimized the geometric configuration to minimize the surface currents flowing on the metal films to reduce the ohmic loss, which may be different to that with complex resonant structures [11–13]. The reflection of current design does not undergo much attenuation as shown in Fig. 1(b), where full wave analysis is performed to curve the EM responses and the reflection amplitude is uniformly kept at nearly unity across the entire frequency band for both two digital elements. Meanwhile, a nearly constant 180° reflection phase difference within a broad frequency band ranging from 8 GHz to 15 GHz can be achieved under the illumination of a plane wave, offering possibility to design a broadband diffusion metasurface for scattering suppression. Since the phase responses of elements are mainly due to the resonant feature of the structure, one should

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properly optimize the geometric parameters of the patches to separate the resonant frequency of the two elements (the resonant frequency of the two elements are 8 GHz and 12 GHz, respectively), thus offering a desired 180° phase difference. Considering a metasurface containing an array of M × N elements and a plane wave illumination, the scattering pattern from the metasurface can be characterized by the superposition of the scattering wave from each constituent element, which can be derived as M

N

E tol =  E m , n (θ , ϕ ) ⋅ e

jϕ m ,n

,

(1)

m =1 n =1

where Em,n is the vector far-field scattered by the element located at position of [m, n], θ and ϕ are the polar and azimuthal angle, respectively. When the initial phase ϕ m , n of the element [m, n] is applied with 0° and 180° randomly, the far-field scattering field will undergo complex interferences in a random manner and thus be reflected to many directions similar to that of light illuminating onto a rugged surface, resulting in low backward scattering. 3. Diffusion-like coding metasurface Considering there are countless possibilities to determine the randomized coding sequences, we just investigate one certain case as a proof of concept to demonstrate the optically transparent diffuse metasurface. We use a configuration of 3 × 3 identical elements as a supercell to better comply with the boundary condition that used in simulation, where periodic boundary hypothesis is used to obtain the optimized geometric parameters. In general, the dimension of the supercell should be around the working wavelength. As for smaller supercell, the diffusion-like scattering can also be excited but with a little narrow bandwidth. Then the digital elements are uniformly extended in a finite sheet of 240 × 280 mm2 according to a computer-generated pseudorandom 1-bit coding sequence, as shown in the bottom panel of Fig. 1(a). The coding metasurface is composed of totally 30 × 35 = 1050 elements. Full wave simulations are performed on the entire coding metasurface to validate the diffusion scattering performance. The three dimensional (3D) far-field radiation patterns under the normal illumination of a plane wave are shown in Fig. 2. It clearly presents that the scattered wave from the metasurface are randomly spread into numerous directions in the whole upper half-space at 9 GHz, 11 GHz and 13 GHz for both x- and y-polarized incidence, indicating its polarization independent, broadband performances. In contrast, the 3D far-field result of a bare same-sized metallic slab only has a dominating specular reflection with ultralow side-lobes, as shown in Fig. 2(d) and (h). The working mechanism can also be extended to oblique incident cases as shown in Fig. 2(i)-(k), where the metasurface works well under the EM waves with different incidence angle (15°, 30°, 45°) at 13 GHz. Similar diffusion-like scattering can also be found under x-polarized incidence or at other frequencies during the working band. For real applications, flexible low scattering metasurface is more favored to use in curved surfaces and objects. Therefore, the proposed metasurface is conformally wrapped around a metallic cylinder with a diameter of 180 mm and a height of 240 mm. The EM wave scattering is calculated when the metallic cylinder is coated with the flexible coding metasurface and illuminated by monochromatic plane wave propagating along –z direction. We can qualitatively see that at different frequencies shown in Fig. 3(a)-(c) the backward scattering is reduced to very low level and re-distributed to enormous directions within a broad bandwidth, which is intrinsically distinct from that of a bare metallic cylinder (Fig. 3(d)) with the scattered waves only radially confined in y-z plane. These results demonstrate that the flexible coding metasurface can significantly suppress the backward scattering in a broad frequency band, revealing its potential uses in conformal applications.

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Fig. 2. The 3D backward scattering patterns under the normal illumination of (a)-(c) xpolarized ((b)-(d) for y-polarized incidence) plane wave at 9 GHz, 11 GHz, and 13 GHz, respectively, as well as the (d) corresponding results ((h) for y-polarized incidence) from a same-sized metallic slab at 13 GHz. The 3D backward scattering patterns of the metasurface under y-polarized incidence with different angle of (i) 15°, (j) 30°, and (k) 45° at 13 GHz, respectively, as well as (j) the corresponding result from a same-sized metallic slab with incident angle of 45° at 13 GHz.

Fig. 3. Calculated 3D backward scattering patterns of (a)-(c) a metasurface-coated cylinder at 9 GHz, 11 GHz and 13 GHz, respectively, as well as (d) bare metallic cylinder at 13 GHz.

4. Experimental results Figure 4(a) shows the photograph of the fabricated prototype. High optical transparency (about 80%) of the sample can be observed. To validate the concept, experimental measurements are carried out in a microwave chamber with a pair of broadband horn antennas serving as the transmitter and receiver. The unity reflection is calibrated to a samesized copper slab. Figure 4(b) shows the measured backward reflections as a function of frequency, which roughly consistent with the full-wave simulated results. A low specular reflection below 0.1 can be obtained from 8 GHz to about 15 GHz for both x- and y-polarized incidences. The reflection amplitude of the metasurface element is uniformly kept at nearly unity across the entire working band, indicating that there is negligible energy dissipation during the EM wave coupling and scattering. Therefore, the low backward reflection of the metasurface is mainly attributed to the diffusion-like scattering. To further confirm the result of low-level backward scattering, we have also measured the E-plane scattering patterns of

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the metasurfaace by experim mentally detectiing the scattereed wave at diffferent reflectioon angles from −65° to 65° with an intterval of 10° while w the transm mitting antennaa is fixed at thee angle of o Fig. 4(b) ob bviously shows that at least 100 dB backwardd scattering redduction in 5°. The inset of all directions can be achieveed across the entire e frequenccy band from 8 GHz to 15 G GHz for xpolarized inccidence (simiilar result is obtained foor y-polarizatiion incidence)), which demonstrate the powerful ability of th he polarizationn-insensitive ccoding metasuurface in b scatteerings. suppressing backward

Fig. 4. 4 (a) Photograph of o the fabricated sample. (b) Simulaated and measuredd reflections of thee flat coding metasurface under the norm mal illumination oof a plane wave.. Inset shows thee ured far-field backward scattering patterns in E-plaane of the flat cooding metasurfacee measu underr the illumination of o x-polarized incid dence.

The robust angular-dep pendent perforrmance is an important criiterion to evaaluate the a The T mirror refflections of thhe coding meetasurface metasurface in practical applications. d obliqu ue incident angles (5° to 55°°, with an incrrement of 10°)) for both measured at different transverse eleectric (TE) and d transverse magnetic m (TM) polarizations aare shown in F Fig. 5(a)(d). It clearly shows that thee ultrathin mettasurface have a good angulaar performancee with the duction bandwiidth nearly uncchanged till thee incidence anngle up to abouut 40° for scattering red all wave polarrizations, whicch further indiccates its potentiial uses in pracctical applicatioons. To test th he conformal case, two fab bricated flexiblle metasurfacee sheets with identical pattern are used to conformally wrap the whole w metallicc cylinder with a diameter of 180 mm. The bare metaallic cylinder is i also tested as the calibratioon of the scatteering. In the exxperiment the incidence is polarized along a x-directio on, and the baackward scatterring is measurred which w employin ng the flexiblle coding meetasurface. Forr a clear shows drasticc reduction when demonstration n, we have measured m the frequency-deependent backkward scatterinng while rotating the cy ylinder arbitrarrily, and the reesults for diffeerent θ ( θ is tthe self-rotatioonal angle of the cylindeer with respectt to x-axis, as illustrated i in F Fig. 3(c)) are pplotted in Fig. 5(e). The excellent redu uction of backw ward scattering g observed from m these resultss can consistenntly cover a broad freq quency band, more or less,, from 8 GHzz - 14 GHz, which experimentally

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demonstrates that the flexible coding metasurface could be readily extended to conformal applications. In general, increasing the cylinder radius could make the phase response distribution become more random, which will benefit the diffusion-like interference. On the contrary, smaller cylinder radius will reduce the degree of disorder of the coding sequence, leading to a less efficient diffusion-like backward scattering.

Fig. 5. Measured far-field scattering reduction for TE-polarized oblique incidence with electrical field along (a) x- (b) y-direction, and for TM-polarized oblique incidence with electrical field along (c) x- (d) y-direction. (e) Measured far-field RCS (radar cross section) reduction of a metasurface-coated cylinder with different self-rotational angles.

5. Conclusion In summary, we have introduced here the concept of ultra-thin optically transparent coding metasurface for backward microwave scattering reduction. The metasurface is constructed by distributing the out-of-phase digital elements made of ITO film on the surface according to the computer-generated randomized coding sequence. We have realized a low backward scattering with at least 10 dB reduction in a broad frequency band when covering the flexible metasurface on flat or curved metallic objects. Experiments have been carried out to validate the simulated predictions, which show stable scattering performance to different incident angles and polarizations. Additional features, for instance, tunability, may be envisaged with tunable optical transparent materials (e.g. graphene) by controlling the external stimulations, which will further extend the impact of this concept [28,29]. In general, we believe that the design methodology is not limited to scattering suppression but can also be applied to other diverse functions, such as anomalous reflection, beam generation, skinny mantle cloak, etc. The proposed design can also be readily scaled to other microwave bands or even terahertz region. With the advantage of flexible implementation and simple design, our proposal may be employed wherever optical field continuity for viewing is necessary, such as in windows, domes and solar panel applications.

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Funding National Nature Science Foundation of China (61671231, 61571218, 61571216, 61301017, 61371034). Acknowledgments This work is partially supported by the Research Innovation Program for College Graduates of Jiangsu Province (KYZZ15_0028), PAPD of Jiangsu Higher Education Institutions, and Jiangsu Key Laboratory of Advanced Techniques for Manipulating Electromagnetic Waves.

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