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Abstract
Multifunctional metasurface integrating different functions can significantly save the occupied space, although most of bifunctional metasurfaces reported to date only control the wave in either reflection or transmission regime. In this paper, we propose a scheme that allows one to independently control the reflection and transmission wavefront under orthogonal polarizations. For demonstration, we design a bifunctional metasurface that simultaneously realizes a diffusion reflection and a focusing transmission. The diffusion reflection is realized using a random phase distribution, which was implemented by randomly arranging two basic coding unit cells with the aid of an ergodic algorithm. Meanwhile, the hyperbolic phase distribution was designed to realize the focusing functionality in the transmission regime. To further show the potential applications, a high-gain lens antenna was designed by assembling the proposed metasurface with a proper feed. Both simulation and measurement results have been carried out, and the agreement between the two results demonstrates the validity of the performance as expected. The backward scattering can be reduced more than 5 dB within 6.4-10 GHz compared with the metallic plate. Moreover, the lens antenna has a gain of 20 dB (with around 13 dB enhancement in comparison with the bare feeding antenna) and an efficiency of 32.5%.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Arbitrary manipulation of electromagnetic (EM) wave, an important topic in both scientific and engineering applications, has been boosted since the invention of metamaterials [1,2]. The characteristics of metamaterials can be tailored by changing the arrangement of meta-atoms, leading to a flexible and easy manipulation of EM wave. However, the metamaterials suffer from bulky structure and narrow operational bandwidth. In recent years, the metasurface was proposed to address these drawbacks [3,4]. The metasurface, as a two-dimensional version of the metamaterial, allows one to fully manipulate the propagation of EM wave on the basis of characteristic discontinuities at the surface, including phase, amplitude and polarization [5]. Therefore, the metasurfaces have been widely used in many applications, such as anomalous reflection/refraction [6–8], focus lens [9,10], surface wave generation [11,12], polarization manipulation [13,14], vortex beam generation [15,16], stealth platform [17,18], and so on.
Most of the reported metasurfaces feature a single function only, i.e. manipulating EM waves in the half space [19–22], e.g. focusing in the transmission regime [19], or diffusing in the reflection regime [20]. Some anisotropic bifunctional meta-devices were investigated [23–32]. A bifunctional metasurface was designed by integrating both anomalous reflection and focus in the reflection regime [23]. A reflective metasurface comprises cross-dipole elements was demonstrated to generate vortex wave with dual orbital angular momentum (OAM) modes and dual polarizations [26]. Furthermore, the bifunctional stealth device featuring cloaking and illusion was designed and applied in conformal geometry [31]. Nevertheless, these bifunctional or multifunctional devices are all designed to manipulate EM waves in either reflection or transmission regimes. Although simultaneous manipulation of reflected and transmitted waves was explored in [33,34], they have the same functionality. Therefore, bifunctional metasurfaces with independent control in both transmission and reflection regimes are still not available.
In this paper, we propose a new bifunctional metasurface that integrates different functions in reflection and transmission regimes. The metasurface features diffusion reflection and focusing transmission under orthogonal polarizations. In the reflection mode, we employed the concept of the coding metasurface, utilizing two unit cells with opposite phases as the digital bits “0” and “1”. The diffusion scattering can be realized according to a random coding sequence. In the transmission mode, we designed the transmission phase distribution as a hyperbolic phase profile, so that the transmitted wave can be focused. For practical applications, this metasurface fed by a proper source can perform as a lens antenna according to the reciprocity of EM wave. The simulated and measured results show that the metasurface can significantly reduce radar cross section (RCS) with respect to the metallic plate under x-polarized plane wave incidence, and act as a high-gain lens antenna by assembling with a y-polarized microstrip antenna.
2. Bifunctional metasurface
In practical applications, one expects that the reflected energy can be redirected into various directions, while the transmitted energy can be focused for further processing. Therefore, the full-space bifunctional metasurface integrating diffused reflection and focused transmission is highly demanded and hence considered in this design. Here, the anisotropic metasurface was employed to implement this bifunction under incident waves with orthogonal polarizations. As shown in Fig. 1, one specifies that the x-polarized incident wave is reflected while the y-polarized incident wave is transmitted. In the reflection mode, the random phase profile was designed to disperse the reflected waves into numerous directions, mimicking diffusion phenomenon at visible light spectra. In the transmission mode, the hyperbolic phase profile was designed to focus the transmitted waves at the focal point, acting as a focus lens.
The anisotropic cross-shaped patch was utilized as the basic unit cell, as shown in Figs. 2(a) and 2(b). The unit cell was composed of four metallic layers separated by three identical dielectric substrates. The substrate has a thickness h = 1.5 mm and dielectric constant εr = 2.65. The unit cell in each metallic layer has identical length along y-direction, and the transmission phase can be manipulated with the variation of this length. As for the x-direction, the unit cells in the bottom three layers have the same widths with the unit cell period p, leading to a totally reflected x-polarized wave. The reflection phase is manipulated by changing the width of the top metallic layer. The reflection and transmission coefficient are depicted in Figs. 2(b) and 2(c). One could observe that the reflection phase difference keeps 180° around 9 GHz when w = 8.4 mm and w = 10.73 mm. Meanwhile, the reflection magnitudes of both cases are higher than 0.94, indicating a high-efficiency reflection. In addition, the transmission responses are almost the same for the two cases. And the reflection and transmission magnitudes of cross-polarization components are equal to zero, indicating very low coupling between orthogonal polarizations. To realize high-efficiency focusing, the transmission phase shift need to cover complete 360°with high transmission under y-polarized incidence. Figure 2(d) plots the transmission coefficients against the thickness of substrate. Note that the phase shift range is only 270° in h = 1 mm case, while the other two cases satisfy transmission phase and magnitude requirement. Here, we adopt h = 1.5 mm for a compact design. The transmission phase and magnitude requirement also determines the number of substrate layers. As shown in Fig. 2(e), the phase range can be enlarged with the increment of the layer numbers, leading to three layers structure can satisfy requirement. The phase cover range reaches 360° with the variation of l and the magnitudes are above 0.65 in the case of h = 1.5 mm and three substrate layers.
Fig. 2 (a) The front view and (b) perspective view of unit cell (p = 11 mm, h = 1.5 mm, w0 = 4 mm, l0 = 5 mm). (c) The reflection coefficients and (d) transmission coefficients versus frequency. (e) Comparison of transmission coefficients versus parameter l with different substrate thickness and (f) different layer numbers at 9 GHz.
Based on the little influence between the reflection and transmission under orthogonal incidence, the dual functionalities of the bifunctional metasurface can be independently designed and controlled. The metasurface performs as a diffusion metasurface in the reflection mode, while acting as a focusing lens in the transmission mode. In order to realize a favorable diffusion performance, we utilize the array theory to predict the theoretical scattering pattern and then find the best phase distribution. Since there are only two unit cells with 180° phase difference, we adopt an ergodic algorithm in combination with the one-dimensional (1-D) array theory to obtain the optimum phase distribution. The scattering pattern of the 1-D array composed of five elements can be expressed by superposing the contributions of all the elements, as shown in Eq. (1):
where is the electric far-field pattern of the m-th element; θ is the elevation angle; d denotes the element period (in our case, as mentioned later, one element uses three unit cells in Fig. 2(a) and therefore d = 3p); k0 is the wave number in free space. Assuming that each element has an identical magnitude but different phase, so can be expressed asupon substitution into Eq. (1), it leads towhere is the electric far-field pattern of the basic element; is the phase of the m-th element; AF is the array factor. In our design, can be freely chosen from two values which satisfying a 180° phase difference. We set the reflection phases as 0° and 180° for a convenient design. The element with 0° phase is defined as “0” coding element, while the 180° phase element is defined as “1” coding element. In our design, the metasurface is composed of 5 × 5 coding elements and each coding element uses 3 × 3 unit cells. In order to minimize the peak value of scattered field, all the 25 coding sequences are calculated and compared, leading to the best candidate 11010. Then, the 1-D sequence can be extended to 2-D matrix by using the logistic XOR gate, and the XOR gate is interpreted as: 00→0, 01→1, 10→1, 11→0. The 2-D phase distribution is shown in Fig. 3(a).Fig. 3 The phase distributions for (a) reflection and (b) transmission; (c) top view of the corresponding layout of the bifunctional metasurface.
To focus the transmitted wave, we design a hyperbolic phase profile for the transmission case; the discrete phase profile can be calculated as follows:
where m and n ( = −7, −6, −5, …, 0, …, 5, 6, 7) are the unit cell number along x- and y-direction, respectively. and are phases of the unit cell (m, n) and reference at the center, respectively. F denotes the focal length. Here, we set F = 50 mm at the operational frequency f = 9 GHz. Therefore, the relative phase distribution for transmission case is shown in the Fig. 3(b). It can be clearly seen that the concave phase profile is realized. Based on the designed reflection and transmission phase profile, the bifunctional metasurface can be designed by integrating random phase profile and focusing phase profile along x- and y-direction, respectively. The layout can be implemented based on the proposed anisotropic unit cell, as shown in the Fig. 3(c). It is composed of 15 × 15 unit cells, occupying 165 × 165 mm2. The entire metasurface illuminated by the orthogonal plane waves propagating along -z direction was simulated in the CST Microwave Studio. Under x-polarized plane wave incidence, the incoming wave was totally reflected into various directions, which mimicking the diffusion phenomenon, as shown in the Fig. 4(a). Unlike the specular reflection of the metallic plate, the diffusion scattering leads to both monostatic RCS (MRCS) and bistatic RCS (BRCS) reductions. Note below that the 5 dB bandwidths for both MRCS reduction and BRCS reduction are 6.4-10.1 GHz. Under y-polarized plane wave incidence, the incoming wave transmits through the metasurface and converges in the region around the focal point, as shown in Figs. 4(b) and 4(c). Note from Fig. 4(d) that the peak value of the normalized power density occurs at a distance of 45.2 mm away from the metasurface, which is consistent with the specified focal length. The slight discrepancy can be attributed to the nonuniform transmission magnitude of each unit cell. Overall, the designed metasurface integrates diffusion reflection and focusing transmission together, and each functionality can be independently operated under different polarizations.Fig. 4 Far-field and near-field results under orthogonal polarized wave incidence: (a) 3-D scattering pattern under x-polarized wave incidence; The distributions of (b) electric field and (c) power flow in xoz plane under y-polarized wave incidence; (d) The normalized power flow versus distance.
Due to the incoming plane wave can be converged after transmitting the metasurface, the high-gain lens antenna can be realized if the metasurface was appropriately fed according to the reciprocity of EM wave. Here, we employ a microstrip patch antenna working at 9 GHz as the feed, as shown in the inset of the Fig. 5. The dimensions of the antenna are pa = 16 mm, r = 5.5 mm, a = 3.1 mm. The feeding antenna was placed at the point where the maximum power intensity occurs. The simulated and measured S11 of the lens antenna are depicted in Fig. 5, and the excellent agreement indicates that the lens antenna operating well within 8.5-9.5 GHz. In addition, we employed the far-field and near-field results to verify the high-gain performance of the lens antenna. Note from Fig. 6(a) that a pencil beam is intuitively observed with a peak gain of 20 dB. The high-gain performance is attributed to the fact that the spherical wave front is well transferred into the plane wave front through the metasurface, which can be validated from the near-field result, as shown in Fig. 6(b).
3. Fabrication and measurement
To experimentally validate the theoretic and simulated analysis above, the entire metasurface and feeding antenna prototypes are fabricated using printed circuit board technology. The metasurface sample comprises three identical F4B substrate layers. Each layer is 1.5 mm thick with a relative permittivity of 2.65. The 0.018 mm-thick metallic patterns are printed on both sides of the top and bottom substrate layers. Therefore, the composite structure was assembled by eight dielectric screws via the holes around the edges, as shown in the inset of Fig. 7(a). Furthermore, the microstrip patch antenna is fabricated on a 3 mm-thick F4B substrate layer, and assembled with the metasurface to form a lens antenna, as shown in Fig. 7(b).
The measurement is divided into two parts and carried out in the microwave anechoic chamber. Firstly, the reflection of the metasurface illuminated by x-polarized plane wave is measured. In this part, a pair of the horn antennas connected to the vector network analyzer serving as the transmitter and receiver are placed 3 m far away from the prototype, as shown in Fig. 7(a). Then, the backward reflection of the metasurface can be evaluated by the transmission parameter S21. Moreover, the same-sized metallic plate was also tested as the calibration of the scattering. Figure 8(a) shows the comparison of the simulated and measured reflection reductions. Note that the reflection of the metasurface exhibits 5 dB less than that of the metallic plate between 6.4 and 10 GHz. The reasonable agreement between simulation and measurement demonstrates the feasibility of the theoretic design.
Fig. 8 (a) The simulated and measured results of RCS reduction spectra and (b) the frequency responses of realized gain for lens antenna and feeding antenna.
We further measure the radiation performance of the lens antenna, as shown in Fig. 7(b). The frequency responses of realized gain for both lens antenna and feeding antenna are depicted in Fig. 8(b). It can be clearly observed that the gain has been significantly enhanced for the designed lens antenna, and the peak gain reaches 20 dB which is 13 dB higher than that of the feeding antenna. Figure 9 depicts the measured and simulated 2-D radiation patterns of both lens antenna and bare feeding antenna on two principle planes. Note that a pencil beam is realized as expected. The simulated (measured) half power beam width (HPBW) of the feeding antenna is 81.6° (80.2°) in E-plane and 89.8° (84.6°) in H-plane, which was narrowed to be 12.4° (11°) in E-plane and 12.2°(10.8°) in H-plane after placing the metasurface. Therefore, the peak gain of the feeding antenna has been significantly enhanced by 13 dB by the metasurface. The front-to-back (F/B) ratio is better than 20 dB (18 dB), and side-lobe levels are around 17 dB (17.3 dB) and 16.5 dB (17.8 dB) in E-plane and H-plane, respectively. The aperture efficiency is evaluated by the ratio of realized gain and the utmost directivity, i.e. , where P and Q denote the length and width of the aperture. So the simulated (measured) aperture efficiency is around 32.5% (28.9%). The relative high gain indicate that the proposed lens antenna is applicable to communication systems.
4. Summary
In summary, we designed, simulated and measured a new bifunctional metasurface, which integrated the diffusion reflection and focusing transmission together. The manipulation of reflection and transmission can be independently controlled. On one hand, the diffusion reflection was realized by randomly distributing coding elements with the aid of array theory and ergodic algorithm. On the other hand, the hyperbolic transmission phase profile was designed to achieve the focusing performance. For potential applications, a lens antenna has been implemented by assembling a feeding antenna in the proposed metasurface. The agreement between numerical and experimental results shows that the proposed metasurface suppresses RCS more than 5 dB within 6.4-10 GHz, and enhances the radiation gain of the feeding antenna by 13 dB.
Funding
National Natural Science Foundation China (61372034, 61401191); Natural Science Foundation of Shaanxi Province (2016JM6063); Guangdong Natural Science Funds for Distinguished Young Scholar (2015A030306032), Guangdong Special Support Program (2016TQ03X839), Shenzhen Science and Technology Innovation Committee Funds (KQJSCX20160226193445, JCYJ20150331101823678, KQCX2015033110182368, JCYJ20160301113918121, JSGG20160427105120572), and Shenzhen Development and Reform Commission Funds under Grant [2015] 944.
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