A multiband circular polarization selective metasurface for microwave applications

In this research article, a multiband circular polarization selective (CPS) metasurface is presented. A reciprocal bi-layered metasurface is designed by introducing the chirality in the structure. The top layer of the proposed metasurface is composed of circular split-ring resonator with a cross shape structure inside it. The same structure is printed on the bottom side of the proposed metasurface by rotating it at an angle of 90° to achieve chirality in the structure. The proposed metasurface is able to add CPS surface capability between 5.18 and 5.23 GHz for y-polarized incident wave. For the frequency band of 5.18–5.23 GHz, the transmission goes up to − 4 dB, while the polarization extinction ratio (PER) reaches up to − 27.4 dB at 5.2 GHz. Similarly, for x-polarized incident wave, three strategic CPS operating bands are achieved within the frequency ranges of 10.64–10.82 GHz, 12.25–12.47 GHz, and 14.42–14.67 GHz. The maximum PER of 47.16 dB has been achieved for the 14.42–14.67 GHz frequency band at 14.53 GHz. Furthermore, the response of the metasurface does not vary against oblique incidences up to 45°. The simple structure, angular stability, multiband and miniaturized size make this metasurface an outstanding applicant for polarization conversion and biomedical applications.

Metamaterials exhibit extraordinary properties and provide ample opportunity to control and manipulate the polarization, amplitude and phase of an electromagnetic wave. Chiral metamaterials, a special subclass of metamaterials, composed of a structure which lack mirror symmetry and its mirror image cannot be superimposed. Chirality is a significant characteristic of materials in the organic world, which exits in natural molecules like proteins, amino acids and carbohydrates 1 . Chiral metamaterial received significant attention since the Pendry et al. 2 introduced the chirality to achieve negative refractive index 3 (NRI). Although, metamaterials have many advantages, howerver, these are replaced by metasurfaces 4 (i.e., 2D analog of metamaterial). The reason being 3D metamaterials face significant fabrication complexities, have bulky size and inherently have a very narrow bandwidth. Owing to the chirality in metasurfaces, a wide variety of remarkable electromagnetic properties can be realized such as giant gyrotropy 5,6 , optical activity (90°-polatization rotation 7-10 ), circular dichroism 11,12 , polarization selectivity [13][14][15] and asymmetric transmission [16][17][18][19][20] .
The asymmetric transmission is realized by Fedotov 21 in 2006 and it has now become a focal point of research to achieve asymmetric linear-to-linear polarization (optical activity) and circular polarization selectivity. Various techniques have been reported on the asymmetric linear-to-linear polarization area through the use of bi-layered 22,23 and multi-layered 24,25 chiral structures. Currently, scientific community has directed attention towards realizing asymmetric linear-to-circular polarization [26][27][28][29][30][31][32] . As it is a challenging task to achieve asymmetric linear-to-circular (CPS) like the ones in 27,28,32 , however, multi-layered structures can be deployed where multi-band or enhanced bandwidth is required.
The multiband circular polarization selectivity was achieved using multi-layered metallic structures on substrate sheet when they only operated at normal incidence 27,28 . The multi-layered structures cause high transmission loss, while reflection coefficient increase due to poor wave impedance matching 32 . Therefore, the research trend is to achieve circular polarization selectivity by using low cost bi-layered chiral metasurfaces. The dual  32 in which CPS operation has been achieved only at the resonance frequency of 14.79 GHz with transmission magnitude of − 4.5 dB, however, the design worked only for normal incidence. From the aforementioned literature, it can be realized that the designs are either low efficient, operate only at the resonance frequencies with narrow bandwidth, or work at normal incidence. In this context, achieving high efficient multiband circular polarization selectivity along with the angular stability can be attractive due to its capability to integrate with various practical applications in polarization manipulation and biomedical devices 33,34 .
In this paper, a novel and efficient bi-layered metasurface is presented with multiband CPS characteristics. The circular polarization selectivity is achieved within the frequency range of 5. 18-5.23 GHz for y-polarization. While for x-polarization, the CPS operation is achieved in three frequency bands, i.e., 10.64-10.82 GHz, 12.  GHz. For C-band (5.18-5.23 GHz), transmission goes upto -4 dB which is the highest magnitude that has been achieved till to date to the best of authors' knowledge. Moreover, the metasurface operates very well for both x-polarizaton (TM) and y-polarization (TE) against oblique incidence up to 45°, offering assistance in polarization conversion applications.

Theoretical analysis
Let us consider that a plane wave is travelling along − z direction whose incident electric field (E i ) and transmitted electric field (E t ) can be stated as 35 : where w and k represent the frequency and wave number, respectively, while the complex amplitudes (E x and E y ) represent the x-and y-components of electric field. The transmission matrix (T-matrix) for linear polarization can be defined as 36 : Here T xx and T xy represent the co-and cross-component of transmission for x-polarized wave, respectively. While T yx and T yy indicates the cross-and co-component of transmission for y-polarized wave, respectively. The transmission component (T.C) can be written as T.C = E t /E i . Moreover, the transmission matrix (T-matrix) for circular polarization can be expressed as 36 : Here + and − represent the right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP), respectively. The T.C cir matrix can be transformed to the Cartesian basis such as 27,32 :

Simulated results
Design of CPS metasurface. The schematic view of the proposed circular polarization selective metasurface along with the top and bottom layers of the unit cell are presented in Fig. 1. The designed metasurface is printed on both sides of the Rogers RT5870 substrate (ɛ r = 2.33, loss tangent = 0.0012) having thickness of 1.57 mm. The 17 µm copper cladding (with a conductivity of 5.80 × 10 7 S/m) is employed to design the metallic split-ring resonator (SRR) and cross-element resonator. The design on the back side of the substrate is rotated by an angle of 90° compared to the front side of the substrate to achieve chirality in the structure. The unit cell is repeated itself across the x-y plane with the same period of 10 mm to generate an array. To optimize the proposed design, its dimensions are varied with the intention to obtain desired results. The optimized parameters of the unit cell are as follows: p1 = 10, p2 = 10, a = 4.5, b = 3.55, d = 0.75, c = 5 and g = 0.75 (all units in mm).
To simulate and analyze the proposed metasurface, a full electromagnetic numerical solver, CST Studio Suite is utilized. To simulate the metasurface, periodic and open boundary conditions along x-and y-directions along with Floquet ports along z-direction are used. The simulated transmission components (i.e., T yy , T xy , T yx , T xx ) for both transverse-electric (TE) and transverse-magnetic (TM) incident waves propagating along the backward (−z) and forward (+z) directions are presented in Fig. 2a  The CPS operating bands are achieved when magnitude ratio and phase difference between co-and crosscomponents lie within (0.85-1.15) and (85°-95°), respectively. In Fig. 3a, the magnitude ratio 37 (T xy /T yy ) remains within (1 ± 0.15) while the phase difference 37 (φ diff = φ xy − φ yy ) between 5.18 and 5.23 GHz remains nearly 90° as indicated in Fig. 3b. Therefore, pure circular polarization selectivity is achieved within 5.18-5.23 GHz when y-polarization is incident. Similarly, when x-polarization is incident on the metasurface, the magnitude ratio (T yx / T xx ) remains nearly equal to 1 and their phase difference (φ diff = φ yx − φ xx ) remains around ± 90° or odd multiples of ± 90° over the frequency ranges of 10.64-10.82 GHz, 12.25-12.47 GHz and 14.42-14.67 GHz. From this, it can be stated that the pure circular polarization selectivity is achieved at all aforementioned frequency bands when TM wave is incident.   Polarization extinction ratio. Polarization extinction ratio (PER) is another criterion to demonstrate the functionality of polarization conversion. It is a parameter that verifies the efficiency of circular polarization selectivity. For y-polarization, PER can be calculated by using Eq. (6) 37 .
To find PER for x-polarization, T +y and T −y need to be replace with T +x and T −x , respectively. For pure CPS operation, the graph of PER should be greater than + 20 dB or less than − 20 dB. In Fig. 5a for y-polarization, PER   Angular stability. The behavior of the CPS metasurface is analyzed at different incidence angles because of the stability requirement for many applications. It can be seen from Fig. 6a that the proposed structure is angularly stable up to an oblique incidence of 45° for y-polarization. Similarly, for x-polarization, the metasurface remains stable up to 45° for the first band (10.64-10.82 GHz) while for the second (12.25-12.47 GHz) and third band (14.42-14.67 GHz), it remains stable up to 15° as presented in Fig. 6b. The response of the designed metasurface is changed at 30° and 45° for the two bands 67 GHz) because of the larger electrical size of the unit cell. Moreover, it is interesting to see that for y-polarization, there is an additional band at resonant frequency of 9.23 GHz which is also performing CPS operation as shown in Fig. 6a.
Surface currents analysis. The physical behaviour behind the circular polarization selectivity can be explained by analysing the surface currents. The circular polarization conversion is the outcome of interlayer (transverse) magnetic dipole to magnetic dipole coupling at the resonances. The electromagnetic waves with in the metallic layers can be shown by eigenmodes of the resonators 28,32 . Figure 7 describes the induced surface currents at multiple frequencies, in which (a), (b) correspond for incident TE wave, while (c)-(h) correspond for incident TM wave. From Fig. 7a,b, it can be manifested that the surface currents on the top and bottom lay-  28 . The current directions on the top and bottom layers of the structure evaluate the type of the transmitted wave (i.e., RHCP or LHCP). Therefore, at 5.21 GHz, the antiparallel currents on the two layers depicts that the transmitted wave is LHCP. Furthermore, it is apparent from Fig. 7c,d that the antiparallel and parallel surface currents exist on the SRR and metallic strips of the bottom layer with respect to the top layer respectively, leading to an LHCP wave at 10.74 GHz for incident TM wave. Similarly, Fig. 7e,f show that the directions of surface currents on the structure represent that LHCP wave is transmitted at 12.35 GHz for TM wave. In addition, from Fig. 7g,h, for incident TM wave at 14.53 GHz, the surface currents on SRR structure and both metallic strips are in same (parallel) direction which show a parallel magnetic dipole coupling 32 along the external electric field. Therefore, it can be stated that the transmitted wave is RHCP at 14.53 GHz.

Experimental results and discussion
In order to validate the simulated results, the proposed CPS metasurface was fabricated on Rogers 5870 substrate. The fabricated sample (having cross-section of 228.6 × 152.4 mm 2 ) consists of 22 × 15 unit cells, as shown in Fig. 8a. The measurements were carried out in an anechoic chamber while the Rogers sheet was placed between two horn antennas as presented in Fig. 8b. These antennas were attached to the Anritsu-MS46122B (vector network analyzer) via coaxial cables. In order to achieve co-polarized transmission (T yy or T xx ), both transmitting and receiving antennas were placed co-polarized, either vertical (for T yy ) or horizontal (for T xx ). To measure crosspolarized transmission, the two antennas were placed orthogonal to each other, i.e., the transmitting antenna was vertically while the receiving antenna was placed horizontally. For the measurements of the transmission components, the free space method was used through the following expression 38 : where w, c and d represent the angular frequency, speed of light and thickness of the sample, respectively. www.nature.com/scientificreports/ between simulated and measured results may be attributed to cable losses, calibration discontinuities, slight misplacement of antennas, and finite size of fabricated sample 39 . Table 1 compares the performance of the proposed circular polarization selective metasurface with some previously reported bi-layered or tri-layered structures in terms of maximum polarization extinction ratio, angular stability, number of bands, number of layers and thickness. Improvement can be clearly seen in terms of PER, angular stability and number of bands but the proposed CPS metasurface has a larger thickness compared to other reported work listed in Table 1.

Conclusion
This paper presents a chiral metasurface which performs multiband asymmetric linear-to-circular polarization. The CPS metasurface design has an array of split ring resonators with a cross-shaped structure inside it. It is shown that the bi-layered metasurface has the ability to achieve circular polarization selectivity at frequency band between 5.18 and 5.23 GHz for a normally incident TE polarized wave and also achieve CPS operation at different range of frequencies of 10.64-10.82 GHz, 12.25-12.47 GHz and 14.42-14.67 GHz for a normally incident TM polarized wave. For 5.18-5.23 GHz frequency band, the transmission goes up to − 4 dB at 5.2 GHz while for the frequency band of 14.42-14.67 GHz, PER goes up to 47.16 dB at 14.53 GHz, which are the maximum values that has been achieved till to date to the best of authors' knowledge. Owing to the miniaturization in unit cell size, polarization selectivity and angular stability up to 45°, the designed metasurface can be suitable for polarization conversion and biomedical applications.

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.  License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.