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Abstract
We present experimentally a double-arrow metasurface for high-efficiently manipulating the polarization states of electromagnetic waves in the dual-band. The metasurface is capable of converting a linearly polarized (LP) incident wave into a circularly polarized (CP) wave or its cross-polarized LP wave at different frequencies. It is numerically shown that in the two bands from 14.08 to 15.71 GHz and from 17.63 to 19.55 GHz the metasurface can convert the LP wave into CP wave, of which the axis ratio is lower than 3 dB. Meanwhile, the proposed metasurface also can convert the LP wave into its cross-polarized LP wave at 13.39 GHz and 20.29 GHz. To validate the theoretical analysis and simulated results, a prototype is fabricated and measured. The experimental results are reasonably consistent with the theoretical and simulated results, which demonstrates that such a metasurface can successfully achieve dual-band and dual-mode polarization conversion.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Polarization, as one of the most important characteristics of electromagnetic waves, plays an irreplaceable role in many practical applications [1]. Traditionally, birefringent materials, such as liquid crystals [2,3], are considered as ways to control polarization rotation. However, these materials suffer from bucky sizes and relatively large insertion losses. It is highly demanded to find novel ways to manipulate polarization high-efficiently and conveniently. Metasurfaces [4–8], a kind of planar periodic artificial structures with extraordinary electromagnetic characteristics, have received significant attention in electromagnetics recent years. On account of the main advantages compared with bulky metamaterials, such as low profile, low loss, and thin thickness, metasurfaces show more flexibility in tailoring and controlling electromagnetic wave [9–11]. Various research interests have been concentrated on metasurfaces, giving birth to many applications, such as polarization conversion [12–15], high gain antennas [16–18], wavefront manipution [19,20], holograms [21–23], and perfect absorption [24–27].
In the past few years, studies have shown great potential of metasurfaces for the manipulation of polarization of electromagnetic waves. For example, Zhu et al. [12] designed a reflective metasurface that can achieve linear to right/left circular polarization conversion from 2.4 GHz to 2.6 GHz. Jiang et al. [28] numerically proposed a high-efficiency reflective metasurface operating from 0.6 THz to 1.41 THz realizing linearly to left/right circular polarization conversion simultaneously. Chen et al. [29] reported a metasurface for linear to linear polarization conversion in an ultra-wideband regime. Multi-layer configurations [30,31] were also proposed for improving the bandwidth of linear to circular polarization conversion in wide band, at the expense of complexity in fabrication and relatively high thickness. Furthermore, based on the principle of polarization conversion, many works alter propagation phase and geometric phase together to realize multi-function devices [32,33], which provide a promising route in wireless communication systems. Even though significant achievements have been made in this emerging field, it is still highly desired to find a metasurface with simple configuration yet could realize multi-mode polarization conversion in multiple bands.
In this paper, we present a single-layer reflective metasurface made of double-arrow resonators. Such a metasurface can convert the incident linearly polarized (LP) wave into cross-polarized LP wave near the two resonance frequencies with the polarization conversion almost 100% and into circularly polarized (CP) wave in dual-band with the axis ratio lower than 3 dB. Moreover, when excited by $x$- or $y$- LP wave, the reflected beam could be converted into left-handed circularly polarized (LCP) wave or right-handed circular polarized (RCP) wave, respectively. To validate the theoretical and simulated results, a prototype with $20 \times 20$ unit cells is fabricated and measured. The experimental results are in reasonably good agreement with the theoretical and simulated ones over the whole frequency band. The proposed metasurface with simpler structure and ultra-thin thickness can realize two functions in two bands, which could find great potential in polarization manipulations such as designing CP antennas and have great prospects in wireless communications.
2. Theoretical analysis and unit cell design
Assuming a linearly polarized planar wave propagates from $+z$ direction to the metasurface with polarization along the $y$ direction,
the reflective wave can be written asBased on the theoretical analysis, the unit cell of the metasurface is designed with sandwiched structure consists of a metallic double-arrow resonator, a dielectric substrate (Rogers 4350B, $\varepsilon _{r}=3.48$, $\tan \delta =0.0037$), and a metallic ground, as shown in Fig. 1. The parameters of the top metallic layer are as follow: $p=10$ mm, $a=3.75$ mm, $g=1.25$ mm, $w=0.5$ mm, $t=9.6$ mm. The thicknesses of dielectric layer is 1.524 mm (0.08$\lambda$), which is much less than the wavelength, so it can be considered as ultra-thin metasurface.
Fig. 1. (a) The top view of the unit cell. (b) Scheme of the unit cell under the illumination of a linearly polarized wave.
The commercial software CST Microwave Studio is used to simulate the performance of the metasurface. Figure 2 shows the reflective coefficients of the metasurface under $y$-polarized incident wave and $x$-polarized incident wave. It could be seen from Figs. 2(a) and 2(c) that the magnitudes of the reflective coefficients fro the two cases are all approximately equal in the two bands from 14.08 GHz to 15.71 GHz and from 17.63 GHz to 19.55 GHz, which implies high-efficient polarization conversion. In addition, it can be found in Fig. 2(b) that the phase difference keeps $-90^{\circ }$ from 14.08 GHz to 19.55 GHz, which demonstrates that the reflective wave is an RCP wave. On the contrary, it is shown in Fig. 2(d) that the phase difference is $90^{\circ }$ in the same broad band, which leads to an LCP wave. Furthermore, the cross-polarized magnitude is near -40 dB while the co-polarized magnitude is 0 dB at 13.39 GHz and 20.29 GHz, respectively. Consequently, the proposed design could convert the incident LP wave into its cross-polarized LP wave at these two frequencies. Hence, the proposed design could perfectly realize the two types of polarization conversion simultaneously according to the simulated results.
Fig. 2. Simulated reflective magnitudes and phases of $y$-polarized incident wave[(a) and (b)] and $x$-polarized incident wave [(c) and (d)].
3. Simulation results analysis
For circularly polarized wave, its conversion performance can be depicted by axis ratio (AR). In general, it demonstrates that good circularly polarized wave is obtained when the AR is lower than 3 dB. In Fig. 3(a), AR is found to be less than 3 dB in dual bands, which shows good linear-to-circular conversion performance. And for linearly polarized wave, its performance is depicted by polarization conversion ratio (PCR), which is defined as
In Fig. 3(b), it is clearly that the PCR is almost 100% 13.39 GHz and 20.29 GHz, which indicates that the metasurface realizes good linear polarization conversion. Moreover, at these two frequencies, the AR values are near 30 dB. According to the definition of AR, at these two resonance points, the reflective waves are nearly pure LP waves. As a result, the metasurface would high-efficiently convert the incident LP wave into CP wave in the dual-band and into its cross-polarized directions near these two frequencies.To understand the physics mechanism of the polarization converter, Figs. 4, 5 and 6 give the current distributions on the unit cell at 14.7 GHz, 13.39 GHz and 20.29 GHz under the $y$-polarized and $x$-polarized incident waves. In Fig. 4, it can be found that the current distributions of different sides of the unit cell are orthogonal at 14.7 GHz, which results in a CP reflective wave. Furthermore, we can find that the net currents on the top and bottom layers are completely opposite at the two resonance frequencies, implying that these currents form a current loop which could in turn induce magnetic fields. Then, we can decompose the induced magnetic field along $x$- and $y$- axis, $\vec {H}_x$ and $\vec {H}_y$. Since $\vec {H}_y$ is parallel to the incident electric field, that is vertical to the incident magnetic field, it could realize a $90^\circ$ polarization conversion. While $\vec {H}_x$ is vertical to the incident electric field, they would keep in the same polarization. Similarly, in Fig. 6, it shows the current distributions of the unit cell under $x$-polarized incident wave at two different frequencies. Same as the former analysis, it forms current loops and induces magnetic field due to the opposite directions of two layers’ currents. We can also find that after decomposing, one component can realize cross-polarization conversion while the other component well keeps its polarization.
Fig. 4. Distributions of the surface current on the two metallic parts of the metasurface unit cell under $y$-polarized incident wave ((a) and (b)) and $x$-polarized incident wave ((c) and (d)) at 14.7 GHz.
Fig. 5. Distributions of the surface current on the two metallic parts of the metasurface unit cell and mechanism schematic under $y$-polarized incident wave at 13.39 GHz ((a), (b) and (c)) and 20.29 GHz ((d), (e) and (f)).
Fig. 6. Distributions of the surface current on the two metallic parts of the metasurface unit cell and mechanism schematic under $x$-polarized incident wave at 13.39 GHz ((a), (b) and (c)) and 20.29 GHz ((d), (e) and (f)).
4. Measured results
In order to experimentally verify the theoretical analysis and simulation results of the proposed design, a metasurface sample with $20\times 20$ unit cells is fabricated using printed circuit board technique. The experimental setup and fabricated sample are shown in Figs. 7(a) and 7(b). The two antennas, one as a transmitter and the other as a receiver, are linked to a vector network analyzer to obtain reflective coefficients of the metasurface.
The comparison between the simulated and experimental magnitudes and phases of the reflective coefficients of the metasurface under different polarized incident wave are shown in Fig. 8. Due to limitation of the broadband antennas in our lab, we can only measure the results from 12 GHz to 18 GHz, which covers over 60% of the total operating frequency band as shown by the simulated results in Fig. 2. From Figs. 8(a) and 8(c), one can see that the whole trend of the measured magnitudes consistent reasonably well with the simulated results except a little frequency shift. Similarly, from Figs. 8(b) and 8(d), we can find that the measured results of the reflective phases are in agreement with simulated results. Obviously, the phase difference maintains nearly $90^{\circ }$ and $270^{\circ }$, respectively. As a consequence, the incident linearly polarized wave could be converted into LCP wave or RCP wave, respectively. The discrepancies between the theoretical and experimental spectra are due to the influence of the experimental surroundings and measurement errors. Overall, the measured results show that the metasurface could indeed convert the incident LP wave into CP wave in dual bands.
Fig. 8. Simulated and experimental results of the magnitudes and phases of the reflection coefficients of the metasurface under $y$-polarized incident wave ((a) and (b)) and $x$-polarized incident wave ((c) and (d)).
5. Conclusion
In summary, we have presented a reflective metasurface for realizing linear-to-circular and linear-to-linear polarization conversion in two bands. Numerical results demonstrated that the reflective coefficients of the metasurface are well satisfied the ideal conditions in dual bands. The analysis of the AR and current distributions at the three key frequencies show that the proposed design could perfectly achieve different polarization conversion. Furthermore, the experimental sample was fabricated to validate the designed functionalities. The experimental results of reflective coefficients are in reasonably good agreement with the simulation results. The proposed dual-band and dual-mode metasurface can flexibly control the polarization states of electromagnetic waves and may have potential prospects in many applications.
Funding
National Natural Science Foundation of China (51777168, 61701303); Natural Science Foundation of Shanghai (17ZR1414300).
Disclosures
The authors declare no conflicts of interest.
References
1. L. H. Nicholls, F. J. Rodríguez-Fortu no, M. E. Nasir, R. M. Córdova-Castro, N. Olivier, G. A. Wurtz, and A. V. Zayats, “Ultrafast synthesis and switching of light polarization in nonlinear anisotropic metamaterials,” Nat. Photonics 11(10), 628–633 (2017). [CrossRef]
2. B. Vasić, D. C. Zografopoulos, G. Isić, R. Beccherelli, and R. Gajić, “Electrically tunable terahertz polarization converter based on overcoupled metal-isolator-metal metamaterials infiltrated with liquid crystals,” Nanotechnology 28(12), 124002 (2017). [CrossRef]
3. P. Chen, W. Ji, B.-Y. Wei, W. Hu, V. Chigrinov, and Y.-Q. Lu, “Generation of arbitrary vector beams with liquid crystal polarization converters and vector-photoaligned q-plates,” Appl. Phys. Lett. 107(24), 241102 (2015). [CrossRef]
4. L. Li, T. J. Cui, W. Ji, S. Liu, J. Ding, X. Wan, Y. B. Li, M. Jiang, C.-W. Qiu, and S. Zhang, “Electromagnetic reprogrammable coding-metasurface holograms,” Nat. Commun. 8(1), 197 (2017). [CrossRef]
5. N. Yu and F. Capasso, “Flat optics with designer metasurfaces,” Nat. Mater. 13(2), 139–150 (2014). [CrossRef]
6. M. R. Akram, G. Ding, K. Chen, Y. Feng, and W. Zhu, “Ultrathin single layer metasurfaces with ultra-wideband operation for both transmission and reflection,” Adv. Mater. 32(12), 1907308 (2020). [CrossRef]
7. J. Zhang, X. Wei, I. D. Rukhlenko, H.-T. Chen, and W. Zhu, “Electrically tunable metasurface with independent frequency and amplitude modulations,” ACS Photonics 7(1), 265–271 (2020). [CrossRef]
8. B. Liu, W. Zhu, S. D. Gunapala, M. I. Stockman, and M. Premaratne, “Open resonator electric spaser,” ACS Nano 11(12), 12573–12582 (2017). [CrossRef]
9. D. Lin, P. Fan, E. Hasman, and M. L. Brongersma, “Dielectric gradient metasurface optical elements,” Science 345(6194), 298–302 (2014). [CrossRef]
10. A. A. High, R. C. Devlin, A. Dibos, M. Polking, D. S. Wild, J. Perczel, N. P. de Leon, M. D. Lukin, and H. Park, “Visible-frequency hyperbolic metasurface,” Nature 522(7555), 192–196 (2015). [CrossRef]
11. P. R. West, J. L. Stewart, A. V. Kildishev, V. M. Shalaev, V. V. Shkunov, F. Strohkendl, Y. A. Zakharenkov, R. K. Dodds, and R. Byren, “All-dielectric subwavelength metasurface focusing lens,” Opt. Express 22(21), 26212–26221 (2014). [CrossRef]
12. H. Zhu, S. Cheung, K. L. Chung, and T. I. Yuk, “Linear-to-circular polarization conversion using metasurface,” IEEE Trans. Antennas Propag. 61(9), 4615–4623 (2013). [CrossRef]
13. Y. Yang, W. Wang, P. Moitra, I. I. Kravchenko, D. P. Briggs, and J. Valentine, “Dielectric meta-reflectarray for broadband linear polarization conversion and optical vortex generation,” Nano Lett. 14(3), 1394–1399 (2014). [CrossRef]
14. W. Liu, S. Chen, Z. Li, H. Cheng, P. Yu, J. Li, and J. Tian, “Realization of broadband cross-polarization conversion in transmission mode in the terahertz region using a single-layer metasurface,” Opt. Lett. 40(13), 3185–3188 (2015). [CrossRef]
15. X. Gao, W. L. Yang, H. F. Ma, Q. Cheng, X. H. Yu, and T. J. Cui, “A reconfigurable broadband polarization converter based on an active metasurface,” IEEE Trans. Antennas Propag. 66(11), 6086–6095 (2018). [CrossRef]
16. Y. Pan, P. Hu, X. Zhang, and S. Zheng, “A low-profile high-gain and wideband filtering antenna with metasurface,” IEEE Trans. Antennas Propag. 64(5), 2010–2016 (2016). [CrossRef]
17. L. Zhang, X. Wan, S. Liu, J. Y. Yin, Q. Zhang, H. T. Wu, and T. J. Cui, “Realization of low scattering for a high-gain fabry–perot antenna using coding metasurface,” IEEE Trans. Antennas Propag. 65(7), 3374–3383 (2017). [CrossRef]
18. H. Li, G. Wang, H.-X. Xu, T. Cai, and J. Liang, “X-band phase-gradient metasurface for high-gain lens antenna application,” IEEE Trans. Antennas Propag. 63(11), 5144–5149 (2015). [CrossRef]
19. L. Shao, W. Zhu, M. Y. Leonov, and I. D. Rukhlenko, “Dielectric 2-bit coding metasurface for electromagnetic wave manipulation,” J. Appl. Phys. 125(20), 203101 (2019). [CrossRef]
20. M. R. Akram, M. Q. Mehmood, X. Bai, R. Jin, M. Premaratne, and W. Zhu, “High efficiency ultrathin transmissive metasurfaces,” Adv. Opt. Mater. 7(11), 1801628 (2019). [CrossRef]
21. G. Zheng, H. Mühlenbernd, M. Kenney, G. Li, T. Zentgraf, and S. Zhang, “Metasurface holograms reaching 80% efficiency,” Nat. Nanotechnol. 10(4), 308–312 (2015). [CrossRef]
22. X. Ni, A. V. Kildishev, and V. M. Shalaev, “Metasurface holograms for visible light,” Nat. Commun. 4(1), 2807 (2013). [CrossRef]
23. W. Ye, F. Zeuner, X. Li, B. Reineke, S. He, C.-W. Qiu, J. Liu, Y. Wang, S. Zhang, and T. Zentgraf, “Spin and wavelength multiplexed nonlinear metasurface holography,” Nat. Commun. 7(1), 11930 (2016). [CrossRef]
24. Y. Yao, R. Shankar, M. A. Kats, Y. Song, J. Kong, M. Loncar, and F. Capasso, “Electrically tunable metasurface perfect absorbers for ultrathin mid-infrared optical modulators,” Nano Lett. 14(11), 6526–6532 (2014). [CrossRef]
25. L. Cong, S. Tan, R. Yahiaoui, F. Yan, W. Zhang, and R. Singh, “Experimental demonstration of ultrasensitive sensing with terahertz metamaterial absorbers: A comparison with the metasurfaces,” Appl. Phys. Lett. 106(3), 031107 (2015). [CrossRef]
26. J. Xie, W. Zhu, I. D. Rukhlenko, F. Xiao, C. He, J. Geng, X. Liang, R. Jin, and M. Premaratne, “Water metamaterial for ultra-broadband and wide-angle absorption,” Opt. Express 26(4), 5052–5059 (2018). [CrossRef]
27. W. Zhu, F. Xiao, I. D. Rukhlenko, J. Geng, X. Liang, M. Premaratne, and R. Jin, “Wideband visible-light absorption in an ultrathin silicon nanostructure,” Opt. Express 25(5), 5781–5786 (2017). [CrossRef]
28. Y. Jiang, L. Wang, J. Wang, C. N. Akwuruoha, and W. Cao, “Ultra-wideband high-efficiency reflective linear-to-circular polarization converter based on metasurface at terahertz frequencies,” Opt. Express 25(22), 27616–27623 (2017). [CrossRef]
29. H. Chen, J. Wang, H. Ma, S. Qu, Z. Xu, A. Zhang, M. Yan, and Y. Li, “Ultra-wideband polarization conversion metasurfaces based on multiple plasmon resonances,” J. Appl. Phys. 115(15), 154504 (2014). [CrossRef]
30. L. Peng, X.-F. Li, X. Jiang, and S.-M. Li, “A novel thz half-wave polarization converter for cross-polarization conversions of both linear and circular polarizations and polarization conversion ratio regulating by graphene,” J. Lightwave Technol. 36(19), 4250–4258 (2018). [CrossRef]
31. S. M. A. M. H. Abadi and N. Behdad, “Wideband linear-to-circular polarization converters based on miniaturized-element frequency selective surfaces,” IEEE Trans. Antennas Propag. 64(2), 525–534 (2016). [CrossRef]
32. Y. Yuan, K. Zhang, X. Ding, B. Ratni, S. N. Burokur, and Q. Wu, “Complementary transmissive ultra-thin meta-deflectors for broadband polarization-independent refractions in the microwave region,” Photonics Res. 7(1), 80–88 (2019). [CrossRef]
33. K. Zhang, Y. Yuan, X. Ding, B. Ratni, S. N. Burokur, and Q. Wu, “High-efficiency metalenses with switchable functionalities in microwave region,” ACS Appl. Mater. Interfaces 11(31), 28423–28430 (2019). [CrossRef]
