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Abstract
A novel reconfigurable metasurface for multiple functions is designed and investigated. The lattice of the metasurface is proposed to realize integrated absorption and polarization rotation and named as polarization conversion absorber (PCA). Then, lattices are arranged together to realize a certain function. Due to the independence of each lattice, the metasurface can present different performance based on different arrangement principle. Magnitude, polarization and phase modulation can be presented by manually arranging metasurface with 6 × 6 lattices. Both simulations and measurements prove that the proposed method provides a simple, flexible and effective strategy for multifunctional metasurface design.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetic (EM) waves can be characterized by fundamental properties, such as amplitude, phase and polarization state. Metasurface, a two-dimensional metamaterials, opens up more possibilities in manipulating above EM characteristics in propagation [1–3].
In recent years, metasurface has attracted tremendous attention due to its ability of tailoring the characteristics of EM waves. For instance, the perfect metamaterial absorber was firstly proposed by Landy in 2008, which could modulate the amplitude of reflected waves via nearly 100% absorption [4]. Then absorbers with low or wide operating bandwidth are developed [5–15]. Polarization conversion surfaces could convert the polarization state of transmitted or reflected waves due to the asymmetric structure on the metasurface [16–26]. With the combination of coding principles and the generalized Snell’s law [27,28], the propagation direction of EM waves could be changed by phase distribution control on the metasureface. Lens [29,30], beam splitter [31,32], and phase gradient metasurface [33,34] are various applications of such principles. And when such principles apply to the scattering problem, anomalous reflection and diffusion can be obtained using the proper array arrangements [35,36]. However, most of the aforementioned metasurfaces only focus on one certain function, which limits their applications in modern systems with multiple demands.
Some pioneer metasurfaces are proposed to meet diversified needs of the modern system [37–42]. In [37], a beam steering array antenna was achieved via tunable phase control based on the coding concept. A programmable metasurface with dynamic polarization, scattering and focusing control was presented in [38], which could realize the polarization and phase modulation simultaneously. As presented in [39], a metasurface for magnitude and phase modulation is proposed, which could achieve absorption and scattering diffusion under the combination of the polarization conversion unit cell and the absorption unit cell. However, these metasurfaces only control one or two characteristics of EM waves. To the author’s best knowledge, it is rarely reported that the metasurface can achieve magnitude, polarization and phase modulation simultaneously.
In this paper, the metasurface for magnitude, polarization and phase modulation is proposed. Firstly, the lattice with integrated magnitude and polarization modulation is proposed and named as polarization conversion absorber (PCA). The proposed lattice could realize absorption at low frequency and polarization conversion at high frequency simultaneously. Then, lattices can be arranged manually for different functions based on different arrangement principles. For demonstrated purposes, magnitude, polarization and phase modulation could be achieved by the reconfigurable metasurface with 6 × 6 lattices. Finally, the result shows explicitly that the proposed metasurface can be applied in a variety of ways, such as absorption, polarization conversion and scattering diffusion.
2. Design of the reconfigurable metasurface
2.1 Multifunctional metasurface design
To realize the multifunction metasurface, a polarization conversion surface (PCS) is designed firstly, of which the outside is a square loop. Then slots are made in the outside square loop. Finally, four lumped resistors are inserted into the square loop and the absorptive ability is integrated into PCS. Key points in this method are listed as follows: 1. insert resistors have little influence on the polarization rotation efficiency of PCS; 2. the band of absorption is different from the band of polarization conversion. The design method is illustrated in the Fig. 1.
The structure and details of the PCA are shown in Fig. 2. The PCA is composed of three layers. The top layer is composed of a cross arrow along the diagonal direction and a square loop loaded with four-sided lumped resistors. The middle layer is a 4mm dielectric substrate, the relative dielectric constant and loss tangent of which are 2.2 and 0.001, respectively. The bottom layer is a full metallic plate, whose conductivity and thickness are 5.8 × 107 S/m and 0.0036 mm, respectively. Four-sided 70 Ohm resistors loaded on the square loop are utilized to achieve maximum absorption, while the cross arrow along the diagonal direction is chosen to form the asymmetric structure for wideband polarization rotation. The details of the PCA are: P = 10mm, a = 9mm, w = 0.5mm, g = 1mm, lt1 = 7.37mm, lt2 = 4.1mm, lt3 = 1mm, wt1 = 1.1mm. To achieve a reconfigurable metasurface, 6 × 6 lattices are needed, and each lattice contains 4 × 4 unit cells.
2.2 Uniform layout for magnitude and polarization modulation
As shown in Fig. 3(a), a uniform layout for absorption and polarization conversion integration is composed of lattices following the same orientation. The absorption and polarization conversion of EM waves in uniform layout result from the lumped resistor loss and asymmetric reflection over certain frequency range. The matrix can be extended to describe the relationship between incident and reflected electric fields as:
where R and I with subscript lin represent reflected and incident EM waves with specific linear base. According to Eq. (1), if the amplitude of incident electric field Iy is zero, accompanying with the minimization of the co-polarization reflection rxx and cross-polarization reflection ryx, the remarkable absorbing property will be achieved. As for the polarization rotation, if the co-polarization reflection rxx is minimized and cross-polarization reflection ryx is maximized, the polarization rotation will be realized.
Fig. 3 Performance of the uniform layout. (a) Top view. (b) The reflected amplitude. (c) The reflected phase. (d) Absorption rate. (e) Polarization conversion ratio (PCR). (f) The azimuth angle ψ and ellipticity angle κ. (g) Axial ratio.
Properties of the uniform layout are analyzed by HFSS, using the periodic boundary condition and floquet ports. It can be seen from Fig. 3(b) that the simulated bandwidth of rxx less than −10 dB is 4.25 GHz to 5 GHz, 11.5 GHz to 16.25 GHz and 20.6 GHz to 20.95 GHz. Meanwhile, the ryx more than −10 dB ranges from 10.4 GHz to 21.6 GHz. And the reflected phased is also shown in Fig. 3(c).
Due to the full metallic plate on the bottom layer, the frequency dependent transmitted power is zero, and then the absorption rate of the PCA can be simplified by
in which Ax(f) is the absorption rate, Rx2(f) and Ry2(f) are co-polarization reflected power and cross-polarization reflected power, respectively. In order to maximize the absorption rate of the PCA, the impedance should match with that of the free space to minimize the reflected power Rx2(f) and the conversed power Ry2(f). Figure 3(d) shows that the polarization-insensitive absorption band of the uniform layout is from 4.25 GHz to 5 GHz in good accord with the rxx less than −10dB, where the absorption rate is more than 90%. Moreover, the polarization rotation efficiency of the uniform layout is reviewed and shown in Figs. 3(e)-3(g). Owing to the anisotropy of the uniform layout, the reflected wave generally consists of both co- and cross-polarized components. For an x-polarization incidence, the co- and cross-polarized components are defined as [26]What’s more, the polarization conversion ratio (PCR) is written asTo understand the polarization state of the reflective waves, the azimuth angle ψ and ellipticity angle κ are introduced and expressed as follows: Then, the axis ratio is calculated as:As depicted in Fig. 3(e), the PCR more than 90% are from 11.7 GHz to 16.15 GHz and from 20.65 GHz to 20.95 GHz. And PCR are 0.999, 0.977 and 0.977 at 12.2 GHz, 15.2GHz and 20.8GHz, respectively. Compared with the PCS, the lumped resistors have little influence on the polarization conversion performance of the PCA. As illustrated in Fig. 3(f), the azimuth angle is about 90 degree from 11.7 GHz to 16.15 GHz and 270 degree from 20.65 GHz to 20.95 GHz, resulting in linear-to-linear polarization rotation. What’s more, the ellipticity angle κ is approximately 45 degree ranging from 17.7 GHz to 20 GHz, achieving linear-to-circular polarization rotation. The abrupt angle change in Fig. 3(f) can be attributed to the combination of the approximate similarity of numerical values and the 90 degree phase difference between rxx and ryx. The axial ratio performance is also depicted in Fig. 3(g). Hence, the uniform layout could realize absorption and polarization rotation simultaneously.
2.3 Coding layout for magnitude and phase modulation
To obtain a diffusion pattern, two lattices with phase difference are required and then optimally arranged by algorithms. Fortunately, there is a 180° phase difference between the proposed lattices and that with rotation angle of 90° in the polarization rotation bandwidth. The proposed lattices and that with rotation angle of 90° are named lattice ‘0’ and ‘1’ respectively, and illustrated in Fig. 4(a). In the case of normal incidence, antenna array theory offers an efficient way to fast predict the scattered field of the metasurface. To achieve a layout with better diffusion performance, the aim is to minimize the maximum of the scattering field. The function can be expressed as:
where Am,n represents the pattern function of the latticem,n, φm,n is the phase addition after the reflection from latticem,n, d is the distance between the structural centers of two adjacent lattices, θ is the elevation angle, and φ is the azimuth angle. As illustrated in Fig. 4(b), the co- and cross-polarized reflected magnitudes are almost same in the operating bandwidth. Hence, the lattice ‘0’ and ‘1’ are assumed to share the same pattern function. Moreover, the 180° phase difference exists in cross-polarized reflected waves, as presented in Fig. 4(d). In this layout, additional phases of “0” and “1” lattices are set as 0° and 180°, respectively.Fig. 4 Performance of different lattices. (a) Top view. (b) Reflected magnitude. (c) Reflected phase. (d) Phase difference.
According to [35], a coding layout for diffusion at 14GHz could be obtained through the Simulated Annealing Algorithm (SAA). The design flow of the coding layout is summarized in Fig. 5(a). To avoid the time-cost full wave simulation, Matlab is involved in the rapid calculation and optimization of the layout and the scattering field. Moreover, the best arrangement is depicted in Fig. 5(c). Figures 5(b) and 5(c) illustrate the 3-D calculated scattering pattern of metallic plate and the coding layout under the normal incidence, respectively. Comparing with metallic plate, part of the backscatter energy has been diffused into all direction, resulting in backward RCS reduction respectively.
Fig. 5 Design of coding layout. (a) Design flow. (b) Calculated metallic plate. (c) Calculated coding layout.
Figure 6(a) shows the monostatic RCS versus frequency of both objects. Note that a reflected suppression is obtained in a wide bandwidth ranging from 3GHz to 23GHz, and four reduction peaks are at 4.3GHz, 11.4GHz, 15.1GHz, and 21.1GHz. The bistatic RCS reduction in the specular direction under different incident angle has been given in Fig. 6(b). Note that though the RCS reduction bandwidth achieved by the phase cancellation is shifted downwards due to the alteration of phase response at high frequency range, a wideband RCS reduction is realized for x-polarized until the incident angle up to 30°. To better show the stealth performance under normal wave incidence, three-dimensional (3-D) RCS patterns of the coding layout at reduction peaks are plotted in comparison with the metallic plate, as shown in Figs. 6(c)-6(j). As expected, the incident energy was absorbed at 4.3GHz, leading to low backward reflection. Opposing to the strong mirror reflection by the metallic plate, the backward reflected waves are diverted into various directions at 11.1GHz, 15.0GHz and 21.1GHz. The above results sufficiently prove that array theory combining with optimization algorithms offer an efficient way for this purpose.
Fig. 6 Scattering performance under x-polarized incidence. (a) Monostatic RCS under normal incidence. (b) Bistatic RCS reduction in the specular direction under different incident angles. (c) Metallic plate at 4.3GHz. (d) Metallic plate at 11.1GHz. (e) Metallic plate at 15.0GHz. (f) Metallic plate at 21.1GHz. (g) Coding layout at 4.3GHz. (h) Coding layout at 11.1GHz. (i) Coding layout at 15.0GHz. (j) Coding layout at 21.1GHz.
3. Fabrication and measurement
To validate the mentioned properties above, 36 lattices with same orientation have been fabricated and tested, as shown in Fig. 7(a). Each lattice is manufactured by printed circuit board processing technology. For a solid structure, lattices are fixed in a rigid foam frame, whose dielectric constant of the frame closes to 1. Since all lattices are copied individuals, they can be assembled for different layouts by simply changing the arrangement orientation of lattices. Figures 7(b) and 7(c) are completed states of the reconfigurable metasurface for different functions.
Fig. 7 Fabricated reconfigurable metasurface and the measurement setup. (a) The partially completed state. (b, c) Completed states: (b) uniform layout, (c) coding layout. (d) The basic measurement setup.
The basic measurement setup in an anechoic chamber is illustrated in Fig. 7(d). Two horn antennas are connected to the Agilent Vector Network Analyzer (N5230C) as a transmitter and receiver respectively. The horn antennas are capable to receive both polarization reflected waves by rotating the receiving horn antenna 0 degree and 90 degree, respectively. Then, the scattering performance is evaluated by the transmission coefficients obtained by N5230C. Gate-reflect-line calibration in time-domain analysis kit of N5230C is employed to further eliminate the noise in the environment
The measured results along with their simulated ones are demonstrated in Fig. 8 when the x-polarized incidence propagated. Due to the limitation of the experimental condition, the measured frequency range is only from 3.0 GHz to 18.0 GHz. The measured results coincide with the simulated ones, except for the discrepancy in the absorption band, which can be attributed to the imperfect soldering of embedded resistors. In addition, the difference between real and assumed characteristic values of the dielectric substrate and misalignment of distinction are other reasons for the distinction between simulated and measured RCS reduction spectrums. Nevertheless, the comprehensive capabilities of reconfigurable metasurface to manipulating magnitude, polarization and phase of reflected waves have been confirmed from the measured results.
Fig. 8 Measured and simulated results. (a) The reflected amplitude of the uniform layout. (b) The reflection reduction magnitude of the irregular layout compared to full metal board.
4. Conclusion
A new concept for multifunctional metasurface design has been proposed in this article. The lattice has been designed to show great ability of absorption and polarization conversion. Then, the reconfigurable metasurface can be carried out by arranging these lattices in various layouts. The uniform layout and coding layout are investigated to show the comprehensive capabilities of the metasurface for EM wave manipulation. The proposed reconfigurable metasurface offers a cost-efficient approach to multiple-function achievements, which has great potential values to be applied in target stealth. Moreover, the concept can be transplant to other frequency ranges as well.
Funding
National Natural Science Foundation of China (NSFC) (61671464, 61471389, 61801508, 61701523); Postdoctoral Innovative Talents Support Program of China (BX20180375).
Acknowledgments
Yulong Zhou and Xiangyu Cao contributed equally to this work. We also thank the reviewers for their valuable comments.
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