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Abstract
Beam scanning based on metasurfaces is widely discussed in recent years owing to its high integration, lightweight, and low cost. However, most of the reported beam scanning metasurfaces operate in either transmission or reflection mode. Here, we propose a full-space beam scanning metasurface based on transmission reflection switchable meta-atom and the quadratic phase distribution. As a validation, a metasurface array with 400 units (20 × 20) was experimentally demonstrated. Beam scanning of ± 35 ° was achieved in both transmission mode and reflection mode. A larger scanning angle (± 45 °) was further verified simulatively with a 900-units (30 × 30) array. The method provides an avenue for expanding the space of electromagnetic wave manipulation and may have great potential in wireless communication and radar detection.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, metasurface, a two-dimensional form of metamaterial, has attracted research interest owing to the ultrathin subwavelength profiles and the flexibility in electromagnetic (EM) wavefront manipulation [1–4]. By properly designing and adjusting unit cells of metasurfaces, the comprehensive functions were achieved [5–8]. More recently, more flexible modulation of the electromagnetic wave was realized with active metasurfaces by integrating PIN diode [9], varactor diode [10], MEMS [11], and so on. Novel beam scanning devices based on active metasurfaces have been extensively studied owing to their high integration and lightweight. As a result, the active metasurfaces have promising applications in wireless communication [12], phased array radar [13], and other fields. For instance, Q. Zeng et al. proposed a dual frequency transmissive metasurface array with beam scanning function by relative rotation [14]. W. Pan et al. created an integrated antenna based on an active frequency selective surface, which realizes transmission beam scanning of ± 30 ° via the five layers cascade metasurface [15]. A beam scanning reflector was proposed by Badreddine Ratni et al. with a simple structure with varactor diodes [16]. However, most beam scanning designs focus on transmission space or reflection space at present.
In order to realize full-space EM wave manipulation, transmission reflection switchable metasurfaces based on polarization switching [17], frequency selective surface [18], and dynamic metasurface [19] have been proposed. Among them, dynamic gratings allow quick and flexible transmission-reflection switching by inserting PIN diodes in the traditional metal one-dimensional or two-dimensional gratings. Independent EM manipulations in the transmission and reflection region were realized by incorporating the dynamic gratings into the metasurfaces design. Li et al. realized the functions of transmission-reflection switching and polarization conversion with the double-layer one-dimensional dynamic and anisotropic structures [20]. Liang et al. realized the transmission reflection switching and four different holograms for different polarization directions and radiation spaces [21]. However, most of the previous works can only realize EM manipulation in a specific direction. The syncretic framework of transmission-reflection switching and beam scanning has not been established until now.
In this paper, we proposed a full-space metasurface integrating the functions of electronically-controlled transmission-reflection switching and beam scanning. The switching function is realized with an orthogonal double-layer dynamic grating by introducing PIN diodes. The geometric phase resonators were designed to simultaneously and quasi-continuously manipulate the transmission and reflection phase. Beam scanning transmission and reflection array were further constructed based on the quadratic phase. A 20 × 20 full-space metasurface array was experimentally demonstrated and beam scanning of ± 35 ° was achieved in both transmission and reflection regions. The transmission-reflection switching and -45° - +45° beam scanning was verified by simulation with a 30 × 30 array.
2. Analysis and design
The proposed full-space beam scanning metasurface is schematically depicted in Fig. 1. Transmission reflection switchable meta-atom (presented in Fig. 2) with a dynamic grating is subtly designed by introducing PIN diodes. As shown in Fig. 1(a), the metasurface operates in transmission mode when the PIN diodes are turned off. While the working mode will be switched into reflection mode by turning on the diodes (shown in Fig. 1(b)). The geometric phase resonators were designed to manipulate the phase of the transmitted cross-polarization EM wave and reflected co-polarized EM wave. The quadratic phase distribution is adopted in constructing the transmission array and reflection array, allowing for large-angle focus and the transformation of the rotational symmetry into translational symmetry [22]. Beam scanning is further achieved by moving the circularly polarized point source on the focal plane.
Fig. 2. (a) Schematic diagram and parameters of the unit cell and the equivalent circuit model of the PIN diode MA4SPS502 in off and on states;(b)-(e) Schematic diagram of the metal layer from the top to the bottom; (c) Top view of and (d) bottom view of the transmission-reflection switching layer, where the direction of the diode is also shown in (c); (b) Top and (e) bottom layer of phase modulation structure;
The phase of transmission and reflection modes introduced by the geometric phase resonators can be analyzed with the Jones matrix. For transmission mode, the transmitted electric field Et of the meta-atoms with different rotating angles under the incident electric field Ein can be expressed as [23]
Equation (1) indicates that the right (left) circularly polarized electromagnetic wave will produce an additional phase related to the rotation angle α of the geometric phase resonators when the incident EM wave is left (right) circular polarization.
For the reflection case, the Jones matrix of the ideal mirror can be expressed as $\left( {\begin{array}{cc} {\textrm{ - }1}&0\\ 0&1 \end{array}} \right)$, where the p-wave will produce a 180° phase delay while the s-wave remains unchanged. Therefore, the reflected circularly polarized electromagnetic wave will firstly produce a polarization conversion. At this time, the left (right) circularly polarized electromagnetic wave is
To realize the beam scanning function, the phase profile of the designed metasurface array adopts the quadratic phase depicted as [24]
where r is the distance from any point on the xy plane to the center of the metasurface, λ is the wavelength and f is the focal length. Previous work has clarified that the relationship between the radiation angle θ and the horizontal shift Δ can be expressed as The designed meta-atom is shown in Fig. 2. The structure is composed of two parts: one is the full polarized transmission-reflection switching dynamic grating, which can be used to realize the switching between transmitting and reflecting of circular polarization incidence wave (Fig. 2(c) and (d)), and the other is the phase modulation structure using geometric phase to realize quasi-continuous phase modulation (Fig. 2(b) and (e)). The main parameters are shown in Table 1.
Table 1. Unit cell parameters and corresponding values in Fig. 2
The switching layer shown in Fig. 2(c) and (d) integrates the PIN diodes (MA4SPS502) on one layer. The braided structure can increase the structural consistency in x-direction and y-direction and the rotational symmetry of the unit cell simultaneously, so that the difference between amplitude and phase of modulated electromagnetic wave in x and y polarizations can be reduced. Besides, higher integration is achieved compared with previous forms of dynamic gratings [20,21]. The equivalent circuit of the PIN diode at on-state and off-state is shown in the inset of Fig. 2(a), where the Rs = 5 Ω, Ls = 0.15 nH, Cs = 0.045 pF. Figure 3 shows the simulated and measured S12 of the transmission-reflection switching layer. It can be seen from the results that high transmittance can be achieved when all PIN diodes are turned off. Nevertheless, the transmittance is less than -10 dB at 7-8 GHz with switching the diodes on. The coincidence S-parameter curves of x-polarization and y-polarization indicate that the design can realize transmission-reflection switching of arbitrary polarization. Therefore, when the incident electromagnetic wave is circular polarization, the transmitted and reflected electromagnetic waves are consistent with the linear polarization results.
Fig. 3. (a) Simulation and (b) measurement of transmission reflection switching layer under different PIN diode states.
Figure 4(a) shows the transmission amplitude (T) and reflection amplitude (R) of the meta-atom for different PIN diodes states, respectively. In order to evaluate the transmission-reflection switching, the isolation degree needs to be considered, which can be defined as:
Fig. 4. (a) Simulated transmittance and reflectance under different PIN diode states; (b) Phase responses at 7.6 GHz of the unit cells with different rotation angle θ. The ideal phase indicates the relationship between the phase response φ and the rotation angle θ of the geometric phase unit cell, which is presented as φ=2θ.
Two metal open rings are designed on the top and bottom layers of the meta-atom. The quasi-continuous phase modulation of cross-polarization transmission (co-polarization reflection) is realized by simultaneously changing the azimuth angle of open rings on the two layers. The transmission phase (trl PIN off) and reflection phase (rll PIN on) exhibit almost the same response at 7.6 GHz (shown in Fig. 4(b)). Compared with resonant metasurfaces, this design can avoid complicated unit cell searching. That is, as long as the parameters with acceptable amplitude are found, 360 ° quasi-continuous phase coverage can be achieved by rotating the open rings.
To verify the above function of the metasurface, we simulate the 20 × 20 array in CST Microwave Studio software firstly. The phases of each meta-atoms on the metasurface are obtained according to Eq. (4). The rotation angles of the meta-atom are further determined by the following formula.
The numerical aperture (NA) of the quadratic phase metasurface determines the maximum scanning angle. When f = D/4, the metasurface can realize the scanning of ± 90 ° theoretically (where D represents the diameter of the metasurface). However, considering the gain, aperture, and cost, the NA of the metasurface designed in this paper is set to 0.707. (The two-dimensional phase distribution and unit structure distribution are shown in Fig. 5) The moving range of the simulated feed Δ is limited to 0 - 60 mm. The corresponding model and circularly polarized feed are established and simulated in CST. Figures 6(a) and 6(b) show the transmission field and reflection field by translating the waveguide feed from 0 to 60 mm at the x-axis (results at 0 mm, 30 mm, and 60 mm).Fig. 5. (a) Optimized 2D phase distribution at 7.6 GHz; (b) Top view of the corresponding layout of the designed metasurface
Fig. 6. Far-field simulation results of 20 × 20 metasurface array at 7.6 GHz in (a) transmission mode and (b) reflect mode, where the angle range is 0 ° - 180 ° in transmission mode and 180 ° - 360 ° in reflection mode, Δ = 0 mm, 30 mm and 60 mm, respectively. The illustrations show the far-field simulation results in percentage, where the intensities of each beam deflection angle are normalized to the maximum, where Δ = 0 mm, 30 mm,60 mm. Far-field simulation results of 30 × 30 metasurface array at 7.6 GHz in (c) transmission mode and (d) reflect mode where Δ = 0 mm, 45 mm, and 90 mm, respectively.
Beam scanning of 0 ° - 35 ° is realized for transmission mode and reflection mode. The isolation degrees of different PIN states under all deflection angles are all above 9.74 dB. The gains are 18.33 dB at 0 ° in transmission mode and 19.73 dB at 180 ° in reflection mode. With the increase of scanning angle, the gain will gradually decrease and the main lope is widened as the movement of the feed causes the deviation of the energy radiation range. The illustrations show the far-field patterns normalizing with the maximum at different angles. Both the transmitted and reflected far-field patterns show obvious differences in the transmission mode and the reflection mode, which indicates that the design can realize the functions of scanning and transmission reflection switching. In the case of the large translation in the reflection mode, the isolation degree decreases slightly, which is due to the coupling between the unit cells that reduces the polarization conversion efficiency of the expected mode when the EM wave is incident at a large angle.
In order to achieve a larger angle scanning, we also simulated a 30 × 30 array with a 120 mm focal length and a 0 - 90 mm movement range. The results are shown in Figs. 6(c) and (d). It can be seen that compared with the 20 × 20 array, the gains are slightly improved and the scanning angle can be increased to 45 °. The feed can be moved further away from the center to achieve a larger scanning angle, but the negative effects of beam width, gain and transmission reflection isolation need to be considered. Note that although only the feed translation along the + x-axis is carried out, the metasurface can certainly realize the full-space beam scanning function due to the symmetry of the phase design of the structure.
3. Fabrication and measurement
The entire metasurface is fabricated using printed circuit board technology to experimentally validate the theoretic and simulated analysis above. Four 0.017 mm-thick metallic patterns are printed on three layers of F4B substrate (2.65 + 0.003i), which are shown in Figs. 7(a) and (d). hgap is controlled by an F4B substrate with a thickness of 0.8 mm (Fig. 7(e)), which is much larger than the height of the package of MA4SPS502. The PIN diodes are soldered by reflow soldering technology. The positive pole and negative pole (ground wire) of the metasurface bias line are all connected in series through wires, thereby realizing the simultaneous turning on and off of all PIN diodes. When the voltage is 0 V and 1.8 V, the PIN diodes are in the off and on states respectively. Subsequent experiments are carried out with a far-field test system in the anechoic chamber and the test details are given in the Appendix.
Fig. 7. (a) Top view of transmission reflection switching layer; (b) Feature of PIN diodes; (c) The measuring setup for performance (shown is the setup of reflecting mode measurement); (d) Top view of the whole sample; (e) View of the F4B substrate used to control hgap.
The transmitted far-field patterns for two working states of PIN diodes are measured with an LCP feed and an RCP receiving horn at first. Then, the horn at the receiving end is replaced with an LCP horn to measure the reflected far-field patterns. Figure 8(a) and (b) show the measured far-field pattern under different PIN working states with the feed located at about z = 120 mm of the metasurface sample and translating in the focal plane.
Fig. 8. (a) Normalized measured radiation patterns of (a) transmission mode and (b) reflection mode where Δ = 0 mm, 30 mm, and 60 mm respectively. (c) and (d) are the normalized results of Fig. 6. (a) and Fig. 6. (b).
It can be seen that the scanning range of 0 - 35° in transmission and reflection mode can be realized at 7.6 GHz, and the side lobes for all titled beams are higher than 10 dB. The isolation degrees of these two modes are all about 10 dB, which is highly consistent with the simulation results. At the same time, the cases shown by the dotted lines are basically submerged in the noise due to the low gains. The slight differences between the simulated and experimental results may be related to the waveguide parameters, processing error, and test environment. The gains and sidelobes can be further improved by optimizing feed [25] and bias voltage [26].
4. Summary
In summary, we proposed a full-space metasurface integrating beam scanning function and transmission-reflection switching. A novel unit cell with dynamic grating integrating PIN diodes and a double-layer open ring is constructed. The switching of the PIN diodes and the feed translation in the focal plane of the quadratic phase array manipulate the transmission-reflection mode and beam angle, respectively. The consistency of S parameters of x-polarization and y-polarization of the transmission-reflection switching layer is verified by simulation and experiment, demonstrating that the switching of full polarization can be achieved. The unit cell realizes quasi-continuous phase modulation by rotating the open rings at the target frequency, and the isolation degree under different diodes states is greater than 10 dB. Two arrays with different numbers of unit cells based on the quadratic phase are designed and simulated, and the 20 × 20 array is processed and verified. The results show that the 20 × 20 array and 30 × 30 array can achieve ± 35 ° and ± 45 ° beam scanning at 7.6 GHz, respectively. The ingenious design method with excellent performance is promising for radar detection and wireless communication.
APPENDIX
Simulation setup: All full-wave simulation results are obtained by CST Microwave Studio. Frequency domain solver is employed to calculate the S-parameters of the unit cell in the range of 7 - 8 GHz. The boundary conditions in the x and y directions are set to the unit cell, while the open (add space) is adopted in the z-direction. The far-field patterns of 20 × 20 and 30 × 30 arrays are calculated by the time domain solver with open (add space) boundary conditions in all directions.
Experiment setup: The far-field experimental setup is shown in Fig. 7. (c). The positive pole and negative pole of the bias line on the metasurface are respectively connected in series to the positive pole and ground of DC voltage power. The PIN diode is turned off or on by controlling the DC voltage source to 0 V and 1.8 V. For the transmission mode, an LCP waveguide work in 7.3 GHz - 7.7 GHz connected with the vector network analyzer (R&S ZVA40) is placed at the focus of the metasurface to transmit electromagnetic waves, the RCP waveguide working as receiving end is placed at another side of the metasurface with a distance over 20 λ, the metasurface is placed on a turntable controlled by computer. The vector network is employed to collect far-field radiation EM with the rotation angle from 0° to 180°. When measuring reflection mode, the waveguide at the receiving end is replaced as the LCP waveguide, and the rotation range of the turntable is adjusted to 180° - 360°.
Funding
Sichuan Science and Technology Program (2020JDJQ0006, 2020YFJ0001); Chinese Academy of Sciences Youth Innovation Promotion Association (2019371); National Key Research and Development Program of China (SQ2021YFA1400121); National Natural Science Foundation of China (61875253, 61975209, U20A20217).
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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