To be published in Journal of the Optical Society of America B: Title: Authors: Accepted: Posted Doc. ID:
Dual-band 2-bit coding metasurface for multifunctional control of both spatial waves and su Shahid Iqbal,Shuo Liu,GUODONG BAI,Muhammad Furqan,Hamza Madni,Tie Jun Cui 11 December 18 12 December 18 345529
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Dual-band 2-bit coding metasurface for multifunctional control of both spatial waves and surface waves S HAHID I QBAL1, 2, S HUO LIU1,2, GUO DONG BAI 1,2, MUHAMMAD FURQAN3, HAMZA AHMAD MADNI 1,2,4, AND TIE J UN CUI 1,2* 1State
Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210096, China Innovation Center of Wireless Communication Technology, Southeast University, Nanjing 210096, China 3National mobile Communication Research Laboratory, Southeast University, Nanjing 210000, China 4Department of Computer Engineering, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan, 64200, Pakistan. *Corresponding author:
[email protected] 2Synergetic
Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX
We present a reflection-type 2-bit digital coding metasurface to achieve dual-band functionalities in two different operating bands, independently. We aim to design various coding sequences to achieve the pre-desired functionalities, which no longer require time-consuming optimization of the distribution of reflection phases. Two main contributions are provided in this work. Firstly, we show that by changing the operational frequency band between the lower (X-band) and higher (Ku-band) frequency bands, we can realize the beam switching between the opposite half planes. Secondly, the proposed dual-band 2-bit coding metasurface is further extended for conversion from the spatial waves to surface waves in one frequency band, and beam deflection/focusing in another band. Samples are fabricated to experimentally validate their frequency-dependent performance, which show high efficiency and good agreement with simulations results. We remark that the proposed concept can be readily extended to terahertz and optical regimes, which may find potential applications in multifunctional meta-devices with multispectral features.
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OCIS codes: (160.3918) Metamaterials; (230.0230) Optical devices; (160.4670) Optical materials © 2018 Optical Society of America
http://dx.doi.org/10.1364/AO.99.099999 1. INTRODUCTION Metasurfaces (MSs) [1-2] are highly encouraged as the ideal methodology of two-dimensional manmade materials having the superb beauty of controlling the amplitude, phase, and polarization of the incident electromagnetic (EM) waves. The MSs are designed by subwavelength scatterers that offer a new way for constructing ultrathin planar devices with the features of easy fabrication, and low profile [3-10]. In addition, significant work has been done on MSs with multibands [11-17], high-efficiency [18-23], and polarization conversion [24-26] in microwave to optical frequency regimes. Meanwhile, the demands of multi-functional but single MS-based devices are rapidly increasing for high integrated systems that will significantly reduce the size and cost. To fulfil such expectations, some bi-functional MSs have been designed for either transmission-type or reflection-type to achieve functionalities of deflection and focusing under various polarizations [27]; and anomalous reflection or transmission under the circularly-polarized incidence [28]. Although bifunctional MSs with high efficiencies have been studied in many references [29-31], but it is worth noted that all the existing bi-functional MSs are associated with single frequency band in either reflection or transmission mode [32-40]. On the other hand, reflectiontype MSs have become promising candidates for optical manipulation due to their broadband performance and high-efficiency [36,37]. Thus, a need is still existed to design
multi-functional and multi-spectral MS for reflection-type mode. Recently, the concept of coding MS [41] has been reported to manipulate the EM waves by elaborately designing the coding sequences, which is quite different to the traditional MSs and easier for practical implementations. Since then, based on this coding MS concept, various functionalities [42-49] have been achieved by simply implementing the corresponding coding sequences such as beam steering [41,43-45], beam shaping [41,49], beam focusing [44-45], and random EM-wave scattering [48]. In addition, a new measure to achieve conversion from spatially propagating wave (PW) to surface wave (SW) [49] has been introduced for different polarized incident waves. Similar to traditional MSs, these coding MS designs are also applicable only for either single operating frequency or in a narrow band. To overcome these issues, 2-bit anisotropic coding MS [45] and 1-bit bi-functional coding MS [43] have been investigated to perform bi-functional operations for the two orthogonal polarizations and beam shaping, respectively. In the meanwhile, bi-functional gap plasmon MS [46] has been proposed to realize PW-SW conversion and beam steering for incident orthogonal polarization in the optical regime. Now the question raised that whether we can control both spatial waves and surface waves by using dual-band 2-bit coding MS in reflection-type mode, when the aperture remains unchanged. To address this question, in this work, 1
we proposed and experimentally demonstrated a recipe to construct a bi-layered reflection-type coding MS that has tendency to perform various distinct operations in two different frequency bands. For the proposed concept, sixteen quantized reflection phases are obtained for dual-band 2-bit coding MS that obey the binary rule of 2(m.n). In this regard, ‘m’ is the number of operating bands and ‘n’ is the number of quantized phases and this work is accomplished by using metallic patches of different size. The beauty of the proposed concept is that the same proposed design is used to achieve different functionalities in each working frequency band. For simplicity of discussion, here, we demonstrated two working band as illustrated in the schematic diagram of Fig. 1. In this scenario, Fig. 1a shows the frequency dependent beam switching to opposite half-planes in which the beam deflection occurs to left side in lower (X) band while in higher (Ku) band the beam deflection occurs to right side. Besides the beam modulation for reflected spatial waves, PW-SW conversion and anomalous reflection are realized using the same coding MS in lower and higher bands, respectively, for the specific coding sequence as shown in Fig. 1b. Both simulation and experimental work have been performed and the obtained results validate that different bi-functionalities can be simultaneously obtained in X-band and Ku-band from the same designed coding MS.
dielectric spacers F4B, which has ε = 2.65 and tangent loss δ = 0.001. Therefore, the geometry of the coding particle is described by p = 7mm as the lattice constant, h = 1mm as the thickness of each dielectric substrate, and t = 0.018mm as the thickness of each copper metallic patch and ground layer. Similarly, w1 and w2 show the side lengths along x-axis of the inner and top metallic patches respectively, while l1 and l2 describes the side lengths along y-axis of inner and top patches, respectively. For dual-band 2-bit coding MS, 16 coding particles are obtained by optimizing the dimensions, l1, w1, l2 and w2 of the two patches as described in Fig. 2b. These 16 coding particles are demonstrated in Eq. (1). In the following, commercially available software CST, MWS is employed to demonstrate the effectiveness of the proposed unit cell under normal incidence of x-polarized plane waves with periodic boundary condition and Floquet port excitations. Optimized geometrical parameters for all sixteen coding particles are obtained by applying parametric sweeps for the values of the side lengths of both patches in the unit cell. The required reflection phases for 16 coding particles are obtained in two different bands (lower band 9.2 - 10 GHz and higher band 14.4 - 15.1 GHz) under the normally incident x-polarized wave as shown in Fig. 2d and 2e. In this scenario, for each 4 identical reflection phases in the lower band there are 4 reflection phases in the higher band. Whereas, two
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Fig. 1. Schematic illustration of 2-bit dual-band multifunctional coding MS that embed different dual functionalities into a single design. (a) Schematic diagram of the proposed design is used to achieve the optical properties of beam switching to opposite half-planes in both lower (fl) and higher (fh) bands. (b) PW-SW conversion and beam deflection/focusing for fl and fh, respectively.
_________________________________________________ 2. DESIGNING METHODOLOGY A. Unit-cell design and simulation To start with, the unit cell of the proposed coding MS is composed of two rectangular (and/or square) metallic patch resonators and a ground metallic layer to avoid transmission that guarantee complete reflection, as shown in Fig. 2a. The two patches and ground metallic layer are separated by two
Fig. 2. Demonstration of unit cell and all 16 meta-atoms. (a) Unit cell transparent view. (b) Top view of all 2-bit dual-band coding meta particles. Whereas, each binary number along the horizontal axis shows the phase state in the lower band while, the binary numbers along the vertical axis show the phase state in the higher band. (c) Phase response of first four meta-atoms in lower and higher band where, the shaded region with red color shows the lower band and blue color represents the higher band. (d) Phase response of all sixteen meta-atoms in lower band. (e) Phase response of all sixteen meta-atoms in higher band. (f) Amplitude response of all sixteen meta-atoms in lower and higher band. _____________________________________________________________ 2
consecutive phases are 90o ± 10o out of phases from each other to mimic 2-bit dual-band coding particle (Fig. 2c). We remark that for every four consecutive coding particles (elements in the same row of the matrix M2-bit) of dual-band coding MS, the phase in the lower band should have the same state, while in the higher band it should have a phase coverage of 270o, which is independent of other reflection phases. In this configuration, a phase difference among rows elements in the higher band does not affect the independent nature of reflection phases and hence will not affect the proposed predesigned functionalities. Meanwhile, Fig. 2f represents the amplitude response of all sixteen meta-atoms in lower and higher band. B. Full-wave simulation and design Based on the selection and spatial distribution of coding particles, the proposed coding MS enable us to achieve different functionalities in two different operating bands, independently. Full wave Simulations are performed by using CST, MWS with open add space boundary condition under both normally and obliquely incident x-polarized plane waves. The concept of super unit cell is employed for various coding sequences to avoid the undesired coupling between different coding particles that have different patch sizes and placed adjacently. Moreover, the other advantage of super unit cell is to have freedom for the period of coding sequence to manipulate the reflected beams in different direction or to convert the spatial waves into surface waves.
bands. For the current design, the coding sequence has the binary pattern as: 00/11, 00/11, 00/11, 01/10, 01/10, 01/10, 10/01, 10/01, 10/01, 11/00, 11/00, 11/00, 00/11, 00/11….. The simulation results for this proposed structure can be seen in Fig. 3 that perfectly switch the beam to opposite half-planes. Whereas, Figs. 3a and 3b represents 3-D and 2-D normalized scattering patterns in lower band, respectively. Similarly, the simulation results for both 3-D and 2-D normalized scattering patterns in the higher band can be seen in Figs. 3c and 3d, respectively. To evaluate the efficiency of the dual band coding MS for the manipulation of spatially propagating waves, we define the efficiency as the ratio of the intensity of the radiation beam between the sample and a perfect electric conductor (PEC) having the same dimension. Although we have shown only the normalized results of scattered fields for beam deflection to opposite half planes that have unity amplitude (0 dB), it can be determined that the overall efficiency is not less than 80% in both operating bands. Moreover, the deflection angle for the proposed design in two operating bands can calculated by Eq. (2), that has very good agreement with the simulation results as shown in Fig. 3b and 3d. - 1 l ù (2) qd = Sin é ê Gû ú ë
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3. RESULTS AND DISCUSSIONS A. Beam controlling of reflected propagating waves
whereas λ is the wavelength of the incident wave and Γ is the period of the coding sequence. Moreover, the scattered field can be flexibly steered to different directions in the same half plane, which have the same off-diagonal elements but with different periods of the coding sequences as predicted by Eq. (2).
Although, extensive research has been carried out to realize beam steering and anomalous reflection into various directions using MS [41-44,50,51] while, these meta-devices are applicable only for single band. Here, we demonstrate frequency dependent beam deflection by constructing 2-bit dual-band coding MS with various coding sequences and by proper selection of the coding particles from the matrix M2-bit.
M
2- bit
æ 00 / 00 ç ç ç 01 / 00 ç =ç ç ç 10 / 00 ç ç ç11 / 00 ç è
00 / 01 01 / 01 10 / 01 11 / 01
00 / 10 01 / 10 10 / 10 11 / 10
00 / 11ö ÷ ÷ ÷ 01 / 11÷ ÷ ÷ ÷ 10 / 11÷ ÷ ÷ ÷ 11 / 11÷ ø
(1)
In Eq. (1), the off-diagonal particles like 00/11, 01/10, 10/01, 11/00, show opposite direction of the discrete phase gradient in lower and higher bands. Whereas, the binary states before the slash demonstrates lower band while after the slash is for higher band. Therefore, the existence of opposite phase gradients in the two different working bands gives freedom to switch the reflected fields to opposite half-planes by sharing the same aperture. To verify the beam switching property, the proposed concept of super unit cell is adopted, and the coding MS consist of the off-diagonal elements of the Eq. (1). Therefore, the full structure is composed of 48×48 coding particles, which has period of 12 unit cells, and x-polarization is adopted for both lower and higher
Fig. 3. Beam switching to opposite half-planes by off-diagonal elements. (a) 3-D far field scattering pattern in lower band. (b) Normalized 2-D far field scattering pattern in lower band. (c) 3-D far field scattering pattern in higher band. (d) Normalized 2-D far field scattering pattern in higher band. The structure size is 48×48 meta-atoms with a period of 12 particles. The coding sequence has the binary pattern as 00/11, 00/11, 00/11, 01/10, 01/10, 01/10, 10/01, 10/01, 10/01, 11/00, 11/00, 11/00, 00/11, 00/11…... _____________________________________________________________
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B. PW-SW conversion One of the important properties of coding MS is the conversion of PW-SW and controlling the flow of SW [44,49]. However, previously designed structures can only operate in a single band that can either perform PW-SW conversion or spatial wave manipulation for a specific coding sequence. Here, our proposed design has the capability to embed these two functions (PW-SW conversion and spatial wave manipulation) into a single meta-device controlled by the frequency band. The working principle of propagating-to-surface wave conversion in lower band and anomalous reflection in the higher band can be explained in the following. It is already established that for PW-SW conversion, the period of super unit cell of a gradient MS should have a high phase coverage (ideally 2π radian) and should be smaller or equal to the length of the operating wavelength, i.e., l inc G£ 1 . The proposed design provides a reasonable distance between the two operating frequencies of lower (X-band) and higher (Ku-band) frequency band. Furthermore, the specific coding elements for the proposed structure are available in Eq. (1) with the periods of 4 unit-cells. Most importantly, the period size of the designed structure is less than the operating wavelength but still has high phase coverage up to 300o that fulfills the conditions required for PW-SW conversion in lower band. Moreover, in higher band the same structure has a period length larger than the operating wavelength.
named by MS2, MS3 and MS4 containing either diagonal elements or off-diagonal coding elements only. The coding sequence of MS2 is 00/00, 01/01, 10/10, 11/11, 00/00, 01/01…., with a period of 4 particles. Similarly, the coding pattern for MS3 is 00/11, 01/10, 10/01, 11/00, 00/11, 01/10…., also having a period of 4 particles. Also, for MS4 the coding sequence is 11/11, 10/10, 01/01, 00/00, 11/11, 10/10…., which are the same diagonal elements as in MS2 but distributed in opposite direction having same period of MS2. The overall size of each of these designed structures is 24×24 unit cells with an area of 168×168 mm2. To clearly observe the propagation of converted surface waves, the dielectric sheets F4B with thickness of t2 = 4mm are placed next to each side of the design. Simulations are carried out in CST using waveguide port as an excitation source, which is set to illuminate the designed structure normally with x-polarized plane waves. Simulation results provided in Figs. 4a-4c show that the incident waves have been converted to surface waves efficiently by all designs in lower band with the center frequency of 9.8 GHz. The conversion efficiency ‘η’ for PW-SW of the designed MS converter can be defined as h = ps po , where Po is the incident power and Ps is the
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power flow carried by surface waves. The efficiency reaches η ≈ 65 % for the designed structure. The major reasons for a relatively lower efficiency are: firstly, the momentum mismatch between the dielectric substrate and the PW-SW converter. Secondly, the smaller size of super unit cell where all the elements are different and not repeats itself in the same super unit cell, due to which some undesired coupling occurs which degrade the efficiency of PW-SW conversion. Moreover, it is worthy to mention that the propagation’s direction of converted surface waves can be controlled by the gradient of the coding sequence. For example, in case of MS2 and MS3 that have an increasing phase gradient in the +y direction, the converted surface waves propagate to the -y direction since most of the energy reflects opposite to phase gradient. In contrast to MS2 and MS3, converted surface waves propagate to the +y direction for MS4 (Fig. 4c). 4. FABRICATION AND MEASUREMENT
Fig. 4. Simulation results for propagating to surface wave conversion in terms of near-field (Ex). (a) PW-SW converter (MS2) with coding sequence 00/00, 00/01, 10/10, 11/11, 00/00, 01/01…... (b) PW-SW converter (MS3) with coding sequence 11/00, 01/10, 10/01, 00/11, 11/00, 01/10…... (c) PW-SW converter (MS4) with coding sequence 11/11, 10/10, 01/01, 00/00, 11/11, 10/10…... In all cases, the period of the coding sequences is 4 unit cells and the overall size is 24×24 unit cells. _____________________________________________________________
To realize PW-SW conversion from the proposed 2-bit dual-band coding particles, we designed three full structures
Two MS samples labeled as MS1 and MS2 are fabricated using the standard PCB technology to verify the functionalities of beam switching, beam bending, and PW-SW conversion. The coding particles encoded in sample MS1 are derived from Eq. (1) with a period of 12 unit cells (Fig. 5a), which represent off-diagonal elements such as 00/11, 00/11, 00/11, 01/10, 01/10, 01/10, 10/01, 10/01, 10/01, 11/00, 11/00, 11/00, 00/11, 00/11, 00/11, 01/10, 01/10, 01/10.… Similarly, for fabricated sample MS2, the coding pattern is 00/00, 01/01, 10/10, 11/11, 00/00, 01/01…., with a period of 4 unit cells as shown in Fig. 5b. The overall fabricated size of the sample MS1 for beam switching property is 350×350mm2 composed of 48×48 meta-atoms. Similarly, the overall size is 168×168mm2 with 24×24 unit cells for the sample MS2 that is fabricated for beam bending and PW-SW conversion. The substrate used 4
here for both samples is F4B, which has the same material parameters. Three separate experiments are performed to verify the simulation results of the proposed MS such as: beam switching [(Fig. 5a)], beam deflection [(Fig. 5b)], and PW-SW conversion [(Fig. 6)]. For beam switching and beam bending, we measured the far field patterns in a microwave anechoic chamber by employing two different horn antennas as excitation sources in lower and higher operating band.
higher band for sample MS2, which has a close agreement with the simulation results of Fig. 5e. Furthermore, experimental results for PW-SW conversion in lower band are presented in Fig. 6 that are derived from the designed MS2. In Fig. 6a, the coding sequence of MS2 is similar to that of Fig. 5b. Meanwhile, horn antenna is used as a feeding source placed at 8cm above the sample MS2 that can be seen in Fig. 6b. In this case, the dielectric substrate F4B of area of 250×70mm2 and thickness of 4mm is placed next to the designed sample to receive the surface waves generated on the sample.
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Fig. 6. Experimental results for PW-SW conversion in lower band. (a) Top view of fabricated sample MS2 with attached dielectric sheet (black colored) (b) Near field measurement setup. (c) PW-SW conversion near field simulation results. (d) Normalized near field experimental results of PW-SW conversion. The absence of surface waves propagating in the +y direction is due to the fact that our experimental setup can only scan the dielectric board at one side of the MS converter. Therefore, we have measured the surface waves propagating towards -y direction only because maximum amount of energy propagates in the -y direction for the design in experiment. _____________________________________________________________
Fig. 5. Experimental setup and results for beam switching to opposite half-planes in both lower (X) band and higher (Ku) bands and beam deflection in higher band. (a) Experimental setup for beam switching to opposite half-planes with metasurface sample MS1 top view. (b) Experimental setup for beam deflection with metasurface sample MS2. (c) Comparison of measured and simulation results in lower band using metasurface MS1. (d) Comparison of measured and simulation results in higher band using metasurface MS1. (e) Simulation results in higher band using metasurface MS2. (f) Comparison of measured and simulation results in higher band for sample MS2. _____________________________________________________________
In this regard, a receiving antenna (not shown in figures) is used to receive the reflected fields for both X and Ku bands. The measured far field scattering patterns for frequency dependent beam switching operation are shown in Figs. 5c and 5d, which have very close agreements with the simulation results especially at the desired deflection angles as predicted by Eq. (2). Similarly, Fig. 5f shows the measured far field scattering pattern of the deflected beam in
A microwave probe connected to an automatic 2-D translation stage was carefully adjusted at 1mm above the dielectric substrate to scan the Electric field (Ex) on the substrate. In the experiment, the microwave probe was able to automatically scan a rectangular area of 90×70mm2. The measured near field is plotted in Fig. 6d that has good agreement with the simulation results of Fig. 6c, that is the plus point to validate the proposed concept. One may notice in Fig. 6 that the wavelength of the simulation and measured results are different, which is due to the compression of Fig. 6c (simulation results) so that the figures should have the same size and symmetry. Since the area of the dielectric board on which the simulation results are plotted is 144×228 mm2, while the actual measured area is 70×90 mm2 in experiments, which should explain the discrepancy between the simulation and experimental results.
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5. CONCLUSION In summary, we designed a frequency dependent 2-bit coding MS to embed different dual functionalities into a single design. This dual-band coding MS can independently manipulate the reflected beam for different functionalities at each band based on the spatial distribution of coding particles. The functionality of beam switching to opposite half-planes is achieved. Besides this, the designed coding MS can convert the propagating waves into surface waves in one band and can manipulate the propagating reflected beams in the other band. Two different designs MS1 and MS2 are fabricated to experimentally verify the beam switching operation, PW-SW conversion and beam deflection, respectively. The measurement results of the fabricated coding MS are highly consistent with the simulation results. We believe that the proposed design can open doors for multispectral multifunctional operations such as frequency dependent multi-directional MS antennas,
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multicolor holography for color display and frequency beam splitter for the spatial separation of broadband signals modulated with various frequencies in the future works. Moreover, the simple nature of the proposed design will motivate to scale down the size of meta-atom to achieve various functionalities in terahertz and optical regime.
Funding. This work was supported in part by the National Key Research and Development Program of China (2017YFA0700201, 2017YFA0700202, 2017YFA0700203), in part by the National Natural Science Foundation of China (61631007, 61571117, 61501112, 61501117, 61522106, 61731010, 61735010, 61722106, 61701107, and 61701108), and in part by the 111 Project (111-2-05). H. A. Madni acknowledges the support of the Postdoctoral Science Foundation of China at Southeast University, Nanjing, China, under Postdoctoral number 201557. Acknowledgment. S. Iqbal designed the devices and carried out the simulations and experiments. S. Liu, G. D. Bai, M. Furqan, H. A. Madni and T. J. Cui analyzed the data and interpreted the results. S. Iqbal, H. A. Madni, and S. Liu drafted the manuscript with the input from the others. T. J. Cui supervised the project. REFERENCES 1. 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 Antenn. Propag. M. 54, 10-35 (2012). 2. N. Yu, and F. Capasso, “Flat optics with designer metasurfaces,” Nat. Mater. 13, 139-150 (2014). 3. A. Pors, O. Albrektsen, I. P. Radko, and S. I. Bozhevolnyi, “Gap plasmon-based metasurfaces for total control of reflected light,” Sci. Rep. 3, 2155 (2013). 4. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, “Metamaterial Electromagnetic Cloak at Microwave Frequencies,” Science 314, 977–980 (2006). 5. A. Alù, A. Salandrino, and N. Engheta, “Negative effective permeability and left-handed materials at optical frequencies,” Opt. Express 14, 1557–1567 (2006).
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