† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 61461016 and 61661012), the Natural Science Foundation of Guangxi Zhuang Autonomous Region, China (Grant Nos. 2014GXNSFAA118366, 2014GXNSFAA118283, and 2015jjBB7002), and the Innovation Project of Graduate Education of Guilin University of Electronic Technology, China (Grant No. 2016YJCX82).
An ultrathin micro-split Jerusalem-cross metasurface is proposed in this paper, which can efficiently convert the linear polarization of electromagnetic (EM) wave into the circular polarization in ultra-wideband. By symmetrically employing two micro-splits on the horizontal arm (in the x direction) of the Jerusalem-cross structure, the bandwidth of the proposed device is significantly extended. Both simulated and experimental results show that the proposed metasurface is able to convert linearly polarized waves into circularly polarized waves in a frequency range from 12.4 GHz to 21 GHz, with an axis ratio better than 1 dB. The simulated results also show that such a broadband and high-performance are maintained over a wide range of incident angle. The presented polarization converter can be used in a number of areas, such as spectroscopy and wireless communications.
Polarization is one of the fundamental properties of electromagnetic (EM) waves. Many applications and optical devices, such as liquid crystal displays, microwave communications and polarization converters are inherently sensitive to polarization, and hence full control of the polarization states of the EM waves is highly desirable. Conventional polarization-manipulation devices are usually realized by utilizing birefringence behaviors in crystals. In this way, polarization changes are obtained by phase accumulation when the EM waves propagate along the optical components, which leads to thick and bulky configurations in the devices.[1,2] Therefore, it is extremely inconvenient to integrate within ultrathin devices, such as nano-photonic devices and advanced sensors.
Metasurfaces are periodic or quasi-periodic planar arrays of sub-wavelength elements, which can be made on an ultrathin dielectric plate or optical thin film. They have received much attention since they offer reduced losses and lower profiles, and hence are simpler to fabricate than bulk metamaterials.[3,4] When the EM waves irradiate the metasurface, the desired phase discontinuity can be achieved by engineering the geometric parameters of the subwavelength elements such as the shape, size and orientation, yielding anomalous reflections and refractions.[5,6] Owing to such fascinating phenomena, metasurfaces break their dependences on the propagation effect by introducing abrupt changes of optical properties. Hence, many novel physical effects have been presented, such as photonic spin Hall effect,[7] beam focusing,[8] surface plasmon couplers,[9] three-dimensional computer-generated holography image reconstruction, and flat lenses.[10]
Recently, metasurfaces have been employed to manipulate the polarization states of the EM waves. By controlling the amplitudes and phases of the reflected or transmitted waves, many polarization converters have been successfully investigated and demonstrated.[11–20] In reflective linear polarization converters, the electric (symmetric mode) and magnetic (asymmetric mode) plasmonic resonances appear simultaneously.[21] The multi-resonant features can effectively extend the bandwidth. For example, by combining a cut-wire array with a ground metal plane, a broadband terahertz linear polarization converter with an efficiency of up to 80% in a frequency range between 0.8 THz and 1.36 THz has been successfully realized.[22] At microwave frequencies, a double V-shaped metasurface can rotate the linearly polarized wave into the cross-polarized one in an ultra-wideband (from 12.4 GHz to 27.9 GHz) with an over 90% polarization conversion efficiency.[23] As for the linear-to-circular polarization conversion, ultrathin quarter-wave plates based on metasurfaces have also been widely demonstrated.[24–28] However, these polarization devices have the obvious disadvantage of narrow bandwidth, which restricts the practical applications.
Here, we propose an ultra-wideband linear-to-circular polarization converter based on an ultrathin metasurface that is composed of micro-split Jerusalem-cross structures. We show that by employing two narrow slits on the horizontal arms, we can significantly expand the bandwidth of the linear-to-circular polarization converter. Both numerical simulation and experiment results show that the polarization converter can convert the linearly polarized wave into the circularly polarized wave in an ultra-wideband from 12.4 GHz to 21 GHz, and the fractional bandwidth with an axial ratio better than 1 dB reaches 50%. Moreover, its broadband polarization conversion performance is supported in a wide range of incident angles, providing the convenience in practical applications.
The reflection polarization converter is usually made up of a metasurface and a continuous metallic ground spaced by a dielectric plate, in which the metasurface formed by a periodic array of unit cells with an asymmetric structure is considered as an anisotropic homogeneous material with dispersive relative permittivity and permeability. When a plane wave with specified polarization irradiates the polarizer, the reflected EM wave can be decomposed into two perpendicular components which are denoted as
According to the principles described above, we design a micro-split Jerusalem-cross metasurface that can convert linear polarization of the EM wave into circular polarization in an ultra-wide band. The proposed polarization converter is formed by a micro-split Jerusalem-cross metasurface and a metallic ground spaced by an ultrathin dielectric plate as illustrated in Fig.
![]() | Fig. 1. (a) Schematic of linear-to-circular polarization conversion. (b) A unit cell of the proposed polarizer. |
We implement three-dimensional (3D) full wave simulations using commercial software CST Microwave Studio to verify the performance of the ultra-wideband linear-to-circular polarization converter. In simulations, a single unit cell with periodic boundary condition along the x and y direction is employed, and a plane wave with electric field polarized along the u direction (45° relative to the x axis, see Fig.
In order to obtain a further insight into the performance of the proposed circular polarization converter, we introduce the Stokes parameters as follows:[13]
Then, we define the normalized ellipticity as e = S3/S0 to descript the circular polarization ability. When e = 1, the reflected wave is a left-handed circularly-polarized (LHCP) wave; when e = −1, the reflected wave is a right-handed circularly-polarized (RHCP) wave. According to Eq. (
An interesting characteristic of the proposed circular polarization converter is that it can maintain the excellent performance of circular polarization for a large incident angle. Figure
Here, we investigate electromagnetic (EM) response of the proposed metasurface based on an equivalent circuit theory. The proposed metasurface can be modeled as a series LC circuit,[29] in which the capacitor C results from the electric field distribution in the gaps between metallic wires, and the inductor L is related to the current distribution on the metallic wires and decreases with their lengths and widths.[30] To demonstrate it, we observe the surface currents and electric field distributions at four typical frequencies corresponding to 13, 15, 18, and 21 GHz when the y and x polarized waves are normally incident on the metasurface, respectively. The observed results are shown in Figs.
![]() | Fig. 4. Simulated surface current distributions on the cell of metasurface for the y-polarized incident wave at frequencies of (a) 13 GHz, (b) 15 GHz, (c) 18 GHz, and (d) 21 GHz. |
![]() | Fig. 5. Simulated surface electric field distributions on the cell of metasurface for x-polarized wave. (a) 13 GHz, (b) 15 GHz, (c) 18 GHz, and (d) 21 GHz. |
In order to further study the EM response of the proposed metasurface, we first consider the Jerusalem-cross array without ground and dielectric substrate. Then it can be modeled as a two-port network as shown in Fig.
The characteristic of the equivalent two-port network can be analyzed by using the following transfer matrix:
![](cpb_25_12_128102/cpb161268ieqn1.gif)
![](cpb_25_12_128102/cpb161268ieqn2.gif)
![]() | Fig. 6. (a) Equivalent two-port network of the free-standing Jerusalem-cross array. (b) Circuit model of the equivalent cascaded two-port networks. |
If we transform the ABCD matrix of the whole network into S-parameters matrix as
For the y-polarized incident wave, the transmitted wave is mainly related to the inductive effect of the I-shaped arms in the y direction (see Fig.
When a u-polarized wave is incident on the Jerusalem-cross array, it can be decomposed intox- and y-polarized components. Based on the aforementioned theory, the phase of transmitted waves in the x and y direction are respectively related to the capacitance (Ceff) and the inductance (Leff) of the equivalent series LC circuit. Furthermore, the inductive element results in phase delay of y-polarized component, and the capacitance leads to the phase advance of the x-polarized component.[31] Then a phase difference between x and y electric field components can be obtained as
On the other hand, when a metallic ground plane is placed on the other side of the dielectric substrate, the transmitted waves are totally reflected. Hence, an incident wave that illuminates the metasurface undergoes multiple reflections and transmissions between the metasurface and metallic ground, where they interfere with another one to create the final reflected wave.[32] So the dielectric thickness is another critical parameter to affect the phase of the wave transmitting in the dielectric substrate, implying that a dispersion-free broadband polarization converter can be realized by carefully choosing the thickness of dielectric substrate.
In order to study the physical mechanism in more detail, we study the reflection characteristics by using the multiple reflection interference theory and the physical model is shown in Fig.
According to Eq. (
We also compare the performance of the proposed polarization converter with the one consisting of Jerusalem-cross without micro-split (conventional Jerusalem-cross) as illustrated in Fig.
To experimentally validate the proposed polarization converter, the metasurface is fabricated on the F4B substrate by using the conventional printed circuit board (PCB) technique as shown in Fig.
In experiments, we consider only the case of normal incidence. The measured |Ruu| and |Rvu|, and the phase difference Δφvu are represented by dashed lines in Figs.
We present an ultra-wideband and high-performance circular-polarization converter by using ultrathin micro-split Jerusalem-cross metasurface. Numerical simulation and measurement results demonstrate that the proposed design can convert the linearly-polarized EM wave to circularly-polarized wave in a frequency range of 12.4 GHz–21 GHz, with a fractional bandwidth of 50%. Furthermore, the ultra-wideband polarization conversion performance is maintained in a wide range of incident angles. We also investigate the physical mechanism by using the equivalent circuit method. The proposed polarization converter will have many potential applications at the microwave and terahertz frequencies.
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