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Abstract
Polarization, as an important property of light, has been widely discussed in modern detecting and radar systems. A polarization converter that can be used to achieve dynamic control is regarded as an excellent alternative for implementing the integrated functionalities of communication and stealth. In this work, we propose a paradigm of meta-converter for dynamic polarization states shifting from linear-to-linear (LTL) to linear-to-circular (LTC) polarization. The strategy is achieved by loading voltage-controlled PIN diodes on the double-arrows metallic meta-resonators. The operation modes can be switched by changing the bias voltage. When the PIN diodes are turned on, the polarization meta-converter (PMC) will reflect and convert a linearly polarized electromagnetic (EM) wave into a circularly polarized one in 5.6–15.5 GHz with an axial ratio (AR) below 3dB. When the PIN diodes are turned off, the PMC will reflect and convert a linearly polarized EM wave into the orthogonal counterpart in 7.6–15.5 GHz with a polarization conversion ratio (PCR) over 88%. Simulations and experimental results show a good agreement, which manifests the feasibility of our proposed meta-converter. Moreover, the proposed PMC has great potential for polarization-dependent communication and stealth systems.
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1. Introduction
With the rapid development of wireless communication systems, vast quantities of attention have been devoted to controlling the polarization of broadband EM waves, such as radars, satellite communications, and remote sensing. Recently, metamaterials have shown great advantages in manipulating the polarization of EM waves and provide new schemes for manipulating EM waves. As a 2-D version of metamaterials, metasurfaces have the advantages of lower lossy, simpler manufacturing, and lighter weight. However, most metasurfaces only focus on the single function of polarization conversion or fixed-band absorption [1–4], and they cannot meet the increasing requirements for dynamic or reconfigurable features. At present, active electronic devices (such as PIN diode, varactor, and MEMS switch) and materials with phase change properties are two strategies to achieve reconfigurable characteristics. The former have the advantages of good electrical control performance. In addition, they possess relatively simple and flexible structures [5,6]. The latter use tunable materials such as graphene [7,8], plasma [9,10], liquid crystal [11,12], and vanadium-dioxide [13–15] to adjust the conductivity. The sensitivity of the optical properties to external stimuli makes it possible to dynamically control the metamaterial.
Polarization is essential to the properties of EM waves. Consequently, the combination of active or reconfigurable metasurfaces with polarization has become a hot research topic. A large number of investigations are focused on passive polarization conversion metasurfaces with transmission and reflection types, including linear-to-linear (LL) [16–18], linear-to-circular (LC) [19–21], and circular-to-circular (CC) [22,23]. However, such converters typically implement a single type of conversion. With the addition of active devices with reconfigurable properties, metasurfaces that can realize the function of dynamic regulation have been proposed. By using PIN diodes, the patch antenna based on metasurfaces can perform a variety of reconfigurable functions at the same feed port, such as switching from right-hand circular polarization (RHCP) to left-hand circular polarization (LHCP) modes [24] and from circular polarization (CP) to linear polarization (LP) [25]. In addition, some reconfigurable metasurfaces can change the state of waves between transmission and reflection [26–28]. Despite the successful realization of reconfigurable polarization conversion metasurface in the above literature, the operating frequency bands are not wide enough. Moreover, there lacks the research on polarization converters that can dynamically shift polarization states and combine LP and CP conversion. Therefore, further exploration of reconfigurable polarization converters is still needed.
Here, we propose a polarization meta-converter (PMC) that can realize dynamic broadband polarization states shifting. By changing the biasing condition of PIN diodes, the PMC adjusts the amplitude and phase of EM waves and transforms the state of polarization of the reflected wave. When the PIN diode is turned on, the PMC reflects and converts a linearly polarized EM wave into a circularly polarized one; When the PIN diode is turned off, the PMC reflects and converts a linearly polarized EM wave into its orthogonal counterpart. The proposed metasurface exhibits advantages of high conversion efficiency for linear polarization (LP) and circular polarization (CP), and miniaturized microstructure.
2. Meta-atom design and simulation
Figure. 1 indicates the conceptual illustration of the polarization meta-converter. The proposed meta-converter can be used to perform two different functionalities at the same frequency band. Hence, the design of meta-atom is the key to realize these functionalities. The meta-atom of the proposed PMC is illustrated in detail in Fig. 2. It consists of three layers in total. From top to bottom, they are F4B dielectric substrate, metal backplate, and FR4 dielectric substrate. The first dielectric substrate is F4B with dielectric constant εr= 2.65, and loss tangent tan δ= 0.001. A double arrow-shaped metal patch is etched on the F4B dielectric substrate. There is a gap with width w = 0.4 mm in the direction of the arrow, and a PIN diode is integrated into this gap and connected to the patch on both sides. In order to reduce the negative influence caused by the feeder lines, we integrate two inductors (type of LQW18AN27NGOOD) on the substrate. The metal ground is between the F4B dielectric substrate and the FR4 dielectric substrate. The last dielectric substrate is FR4, and the DC feeding line is printed on the back. Metal vias with radius r = 0.3 mm connect the upper and lower patches, through which the DC feeding can provide the bias voltage to the PIN diode. The other geometric parameters are as follows: a = 13 mm, b = 1.1 mm, c = 0.7 mm, g = 0.5 mm, s = 0.7 mm, l1 = 1.7 mm, l2 = 0.7 mm, h1 = 4 mm, h2 = 0.1 mm.
Fig. 1. (a) The schematic model in the ON-state of PIN diodes, (b) The schematic model in the OFF-state of PIN diodes.
Fig. 2. The metasurface meta-atom: (a) 3D schematic of the meta-atom structure, (b) Detail display of the metal double arrow and bias circuit.
To realize the reconfigurable characteristic, we use PIN diode with the type of MADP-000907-14020P, which has the advantages of small capacitance, short transit time, and high sensitivity. The equivalent circuit of the PIN diode is shown in Fig. 3. When the diode is in the ON-state, it behaves like a resistor in series with an inductor. When the diode is in the OFF-state, it behaves as a capacitor in series with an inductor (Ron = 7.8 Ω, Coff = 0.026 pF, LS = 30 pH). To simultaneously control the states of diodes of the entire circuit, the PIN diodes are in series with the others.
CST Microwave Studio is used for the analysis and optimization of the meta-atom of the PMC. To explore the performance of the proposed polarization conversion, ryy and rxy express the co- and cross-polarized reflection amplitudes, respectively. Here, the subscripts x- and y- indicate the polarization direction of the EM wave. When the diode is in different states, the simulated results of co- and cross-polarized reflection coefficients are demonstrated in Fig. 4. Obviously, when the diode is turned on, the simulated co-polarized reflection amplitude is approximately equal to the cross-polarized reflection amplitude, and the phase difference between the simulated co-polarized reflection and the cross-polarized reflection is about 90° in a frequency band of 5.6–15.5 GHz. Reflected waves are circularly polarized waves. However, when the diode is turned off, the simulated co-polarized reflection amplitude is low. In contrast, the cross-polarized reflection amplitude is high in a frequency band of 7.6–15.5 GHz. The highest amplitude of cross-polarized reflection reaches 0.99 at 8.3 GHz. It is clearly observed that the reflected waves are mainly vertically polarized.
Fig. 4. (a) The simulated reflective coefficients ryy and rxy and their phases φyy and φxy when the diode is turned on. (b) The simulated reflective coefficients ryy and rxy when the diode is turned off. (c) The calculated AR for LTC polarization conversion. (d) The calculated PCR for LTL polarization conversion.
The polarization conversion ratio (PCR) can be shown in the following equation, which represents the efficiency of LTL polarization conversion.
The axial ratio (AR) and phase difference between the simulated co-polarized reflection and the cross-polarized reflection waves are essential to analyzing the circular polarization conversion. AR can be shown in the following equation.
3. Analysis and discussion
To discuss the circular polarization conversion efficiency of the proposed PMC, we suppose a y-polarized EM wave incident from the + z direction to the PMC. Hence, the electric field of the incident EM wave can be expressed as ${\vec{E}_i} = {\vec{E}_y} = \hat{y}{E_0}{e^{i{k_z}z}}$. The reflected EM wave can be expressed as
The coefficients of the reflected EM wave can be expressed as ${R_{yy}} = {r_{yy}}{e^{i{\varphi _y}}}$ and ${R_{xy}} = {r_{xy}}{e^{i{\varphi _x}}}$, where ryy and rxy are the amplitudes of reflection coefficients Ryy and Rxy; φyy and φxy are the phases of reflection waves. Then we use RLCP-y and RRCP-y to denote the LTC polarization conversion reflection coefficients. Assuming that rLCP-y, rRCP-y and φLCP-y, φRCP-y represent the corresponding amplitudes and phases, respectively. The electric field of the reflected LCP wave and the RCP wave can be expressed as
Notice that ${\vec{E}_r} = {\vec{E}_{LCP}} + {\vec{E}_{RCP}}$ . Combining formula 4, 5, and 6, we can derive the following equation,
It can be found that to achieve efficient conversion from linear polarization to circular polarization, the amplitudes of the co- and cross-polarized reflection coefficients should be equal and as high as possible. At the same time, the phase difference between co-polarized and cross-polarized reflection coefficients is nπ/2 (n is an integer). To study polarization states of reflected waves, four Stokes parameters are introduced as follows, $I = {|{{r_{yy}}} |^2} + {|{{r_{xy}}} |^2}$, $Q = {|{{r_{xy}}} |^2} - {|{{r_{yy}}} |^2}$, $U = 2|{{r_{yy}}} ||{{r_{xy}}} |\cos \varDelta \varphi$, and $V = 2|{{r_{yy}}} ||{{r_{xy}}} |\sin \varDelta \varphi$. As illustrated in Fig. 5(a), the polarization azimuth angle α and the ellipticity angle β describe the direction of the principal axis of the ellipse and the shape of the ellipse. And they can be obtained by
Fig. 5. (a) The calculated azimuth angle α and the ellipticity angle β and a sketch of the polarization azimuth angle α and the ellipticity angle β. (b) The polarization ellipses of the reflected wave at different frequencies (4, 6, 8, 10, 12, and 14 GHz). (c) A pair of two perpendicular directions u- and v-. (d) When the diode is in the ON-state, the co-polarized reflection amplitudes and phases under u- and v- polarized waves incidence.
When the diode is turned on, the calculated β exceeds 33° in a frequency band of 5.5 - 15.5GHz and even approaches 45° at 10GHz. Therefore, reflected waves can be considered as close to circularly polarized waves. Figure 5(b) demonstrates that the polarization ellipses are all viewed as circles at different frequencies (f = 6, 8, 10, 12, and 14GHz). Furthermore, the physical mechanism of the polarization conversion is investigated by analyzing surface current distributions. We define the perpendicular directions, u- and v- along the direction of the arrow, as shown in Fig. 5(c). Then we obtain the simulation results under the incident of u- and v-polarized EM waves. Figure 6 shows the surface current distributions on the double arrow-shaped metal and the metal backplate at three resonant frequencies of 6.1 GHz, 10 GHz, and 14.1 GHz. Under the u-polarized EM waves, strong surface currents are induced on the obliquely placed double arrow-shaped metal, which are anti-parallel at 6.1 GHz or parallel at 10 GHz and 14.1 GHz to the surface currents on the metal backplate. Hence, strong resonances are generated in the u-direction. As demonstrated in Fig. 5(d), the reflection amplitude ruu exceeds 0.9 in a frequency band of 5.8–15.2 GHz. When the v-polarized EM waves are incident on the metasurface, the induced surface currents are weak on the double arrow-shaped metal and metal backplate. Hence, the resonance is negligible. Moreover, the reflection amplitude rvv is close to 1 and there is a variation in the reflected phase φuu and φvv, with a phase difference of approximately π/2. An electric field is combined by two orthogonal electric fields (u- and v-polarized incident wave), as shown in the following equation:
Fig. 6. The surface current distributions on metallic parts (the double arrow-shaped metal and the metal backplate) when the diode is in the ON-state. The black arrow is the main direction of the surface current: (a-c) Under u-polarized waves incidence at 6.1 GHz, 10 GHz, and 14.1 GHz, respectively. (d-f) Under v-polarized waves incidence at 6.1 GHz, 10 GHz, and 14.1 GHz, respectively.
When Δφ = π/2 and the modulus satisfy ruu= rvv, u- and v-polarized reflected waves will be combined into a circularly polarized wave.
When the diode is turned off, as shown in Fig. 7(a), the co-polarized reflection amplitudes ruu and rvv are almost equal, and the reflection phases difference approach π. According to Eq. (11), when ruu= rvv and Δφ = π, u- and v-polarized reflected waves will be combined into fields along with the x-direction. And the incident polarization is rotated by 90°. Moreover, we study the physical mechanism of the polarization conversion through surface current distributions. Figure 7 shows the surface current distributions on the meta-atom and the metal ground under u- and v-polarized waves incidence at two resonant frequencies of 8.3GHz and 14GHz. The symmetric and antisymmetric couplings of current generate electric and magnetic resonance, respectively. As shown in Fig. 7(b), under the incidence of u-polarized waves, the magnetic resonance is generated by anti-parallel surface currents at 8.3GHz. Furthermore, the electric resonance is generated by parallel surface currents at 14GHz, as shown in Fig. 7(c). Under v-polarized wave incidence, the induced current distributions on the double arrow-shaped metal and metal backplate are weak, as shown in Figs. 7(d) and 7(e).
Fig. 7. When the diode is in the OFF-state, (a) the co-polarized reflection amplitudes and phases under u- and v-polarized waves incidence. And the surface current distributions on the metallic parts (the double arrow-shaped metal and the metal backplate): (b) and (c) Under u-polarized waves incidence at 8.3 GHz and 14 GHz, respectively. (d) and (e) Under v-polarized waves incidence at 8.3 GHz and 14 GHz, respectively.
4. Fabrication and measurement
To verify the feasibility of the reconfigurable PMC, a prototype consisting of 20*20 meta-atoms with the size of 260*260 mm2 has been manufactured and measured, as shown in Fig. 8(a). In the measurement setup, we use two sets of 2-18GHz wideband horn antennas. One horn antenna works as the transmitting antenna (signal source) and the other horn antenna works as the receiving antenna. They are connected to two ports of a vector network analyzer. The measurement is carried out in a microwave anechoic chamber to reduce the influence of noises. The experimental setup is illustrated in Fig. 8(c), in which the two antennas are symmetrically placed close to each other and far enough from the prototype. The transmitting and receiving antenna are at the same height to ensure that the EM wave emitted by the transmitting antenna can be effectively reflected by the sample and received by the receiving antenna. At the same time, a DC power supply is used to supply a voltage for the embedded PIN diode through the feeding line printed on the back, as shown in Fig. 8(b).
Fig. 8. (a) and (b) Front and back of the fabricated PMC sample, respectively. (c) The experimental setups of measurement in the microwave anechoic chamber.
First, the bias voltage is changed to 27V for the LTC polarization conversion. The results are shown in Fig. 9(a) and (c). The measured reflection amplitude ryy and rxy are approximately equal and the phase difference between co-polarized and the cross-polarized reflection is about 90°. The calculated AR is less than 3dB in the frequency bands of 6.2–12.4 GHz and 12.8–15.5 GHz, the PMC reflects and converts a LP wave into a CP one. Without the DC power supply, the results are illustrated in Fig. 9(b) and (d). The measured co-polarized reflection amplitude ryy is low. However, the cross-polarized reflection amplitude rxy is high. Furthermore, the calculated PCR is over 88% in a frequency band of 7.8–15.0 GHz, the PMC reflects and converts a linearly polarized EM wave into its orthogonal one. There will inevitably be a small angle here that will introduce oblique incidence when the transmitter and the receiver are located side by side. And it may cause inaccuracy in the experimental results [29]. Besides, there are tolerances in the fabrication and measurement processes. These cause the simulation and experimental results are not exactly the same. Nevertheless, the experimental results are generally in line with the simulation results.
Fig. 9. The measured reflective coefficients ryy and rxy and their phases difference when the diode is turned on. (b) The measured reflective coefficients ryy and rxy when the diode is turned off. (c) The simulated and measured AR for circular polarization conversion. (d) The simulated and measured PCR for linear polarization conversion.
5. Conclusion
In summary, we have proposed a polarization meta-converter for reflective polarization states shifting with broadband characteristic based on PIN diodes. To verify the efficiency of the proposed PMC, we have fabricated a sample polarization converter. Simulation and measurement results have shown that the PMC reflects and converts a linearly polarized EM wave into a circularly polarized one or its orthogonal one by controlling the bias voltage. Furthermore, numerical simulations indicate that the polarization conversion ratio is over 88% in the frequency range of 7.6–15.5 GHz, and the axis ratio for the reflected wave is lower than 3 dB in the frequency range of 5.6–15.5 GHz. The proposed PMC may find potential applications in wide-band reconfigurable antennas, polarization-controlled devices, stealth surfaces, and so on.
Funding
National Natural Science Foundation of China (61971437); China Postdoctoral Science Foundation (2019M651644).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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