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Abstract
Considering that typically more than two pin diodes or other tunable elements are required in the unit cell of polarization-insensitive reconfigurable metasurfaces (RMs), this paper proposes a new approach to design a polarization-insensitive RM unit using only one VO2 chip. A polarization-insensitive phase-modulated metasurface (PMM) using single VO2 chip is presented. The surface layer is composed of an outer ring and an inner cross, with a VO2 chip loaded at the connection of the cross. As the VO2 chip can be connected with the metal patch on all sides, only one VO2 chip is used in this polarization-insensitive design. By thermally controlling VO2 chips switch between low-resistance and high-resistance states, the PMM achieves a 1-bit phase shift within 180° ± 37° from 7.85 to 15 GHz. A prototype is fabricated and measured, and the measured results have verified the correction of the design and analysis of the designed PMM.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid development of radio technology, there is an urgent need for electromagnetic signal control and processing. Numerous electromagnetic technologies such as radar cross-section (RCS) reduction [1], frequency selection [2], and antenna detection technology [3,4] have rapidly developed have emerged and developed. Significantly, the reconfigurable metasurface (RM) has become a research hotspot for reconfiguring electromagnetic (EM) wave propagation due to its unique characteristics. To adapt to the complex and changeable environment, RMs have been widely applied to realize frequency selection [5,6], absorption/transmission [7,8], RCS modification [9], and polarization conversion [10] of EM waves.
Many active components have been integrated into RMs, including PIN diodes [11,12], varactors [13,14], and MEMS switches [15,16]. PIN diodes and varactors have the advantages of low cost and miniaturization, but they perform poorly in the millimeter-wave band due to the influence of parasitic parameters. MEMS switches can work up to the millimeter-wave band, featuring low-loss and high isolation. However, they are relatively high cost and need a high driving voltage. In recent years, vanadium dioxide (VO2) [17–19] as a phase change material has become a research hotspot in RMs. It belongs to a metal-insulator transition (MIT) material, behaving like a dielectric at room temperature and becoming conductive when heated up above the transition temperature. During the transition from insulator to metal, the electrical resistivity of VO2 drops rapidly by 3-5 orders of magnitude [20], which makes VO2 an attractive material for switches. In the field of RMs, VO2 has been used widely in terahertz [21–23] and optical bands [24,25], while relative research in microwave frequency bands is gradually enriching. In [26], a thermal-switchable metamaterial absorber based on the phase-change material of VO2 has been provided. In [27], the broadband phase-modulated metasurface (PMM) based on VO2 patches has verified the application of VO2 in microwave PMMs for the first time.
Due to the massive use of the above tunable elements, the cost of RMs has become a problem that must be taken into consideration. Especially, polarization-insensitive RMs are costlier as more components need to be used to modulate both X- and y-polarized EM waves. At least two tunable elements are required in a single unit cell of polarization-insensitive metasurfaces [28,29]. Four PIN diodes have been used in the unit cell of a dual-polarized programmable metasurface [30]. M. Wang et al. has used four varactor diodes to achieve a dual polarization multifunctional reconfigurable metasurface [31]. A novel 1-bit RRA element capable of working in dual-linear or dual-circular polarization modes depending on two pin diodes has been presented [32]. Moreover, as the price of PIN diodes increases with frequency, the issue of high cost for polarization-insensitive RMs is particularly prominent at high operating frequencies, which limits their widespread use. Therefore, reducing the cost of polarization-insensitive RMs is an urgent and worth researching issue.
In this study, a design of a polarization-insensitive PMM using a single thermally-stimulated VO2 chip is provided. This design allows for reducing the cost of polarization-insensitive PMMs by reducing the number of components used in a single unit cell. As VO2 undergoes tremendous conductivity change during the insulator-metal transition, the designed PMM can achieve a reconfigurable reflection phase. The active components used in the PMM, VO2 chips, are fabricated by cutting VO2 thin film deposited on sapphire substrates into small chips. They are extremely low-cost, and customizable in dimensions, further reducing the cost of the designed polarization-insensitive PMM. Besides, VO2 chips are activated thermally, and no additional feeding lines are introduced, which avoids complex drive circuit design and the impact of feeding lines on PMM. Full-wave simulation results show that the designed PMM achieves a phase difference within 180° ± 37° from 7.85 to 15 GHz, with a fractional bandwidth of 62.6%.
2. Description of the metasurface
An ideal 1-bit PMM should have two states, and the phase difference between them should be kept at 180°. Therefore, the surface layer usually contains active components to achieve reflection phase modulation. To acquire a broadband PMM, a 1-bit metasurface based on a single VO2 chip has been designed in this section. A 3-D geometrical topology of the PMM is depicted in Fig. 1. The surface layer is composed of an outer ring and an inner cross, with a VO2 chip loaded at the connection of the cross. It can be observed that the side length of the VO2 chip is longer than the width of the cross. The surface layer is supported by a 3 mm thick dielectric substrate with the relative permittivity of εr = 2.65. A metal ground is attached to the other side of the dielectric substrate to make all incident waves reflect without passing through.
Fig. 1. 3-D geometrical topology of the proposed PMM (P = 9 mm, r = 3.15 mm, d = 0.25 mm, w = 1 mm, g = 1.5 mm, and h = 3 mm).
As VO2 chips exhibit high resistance at room temperature and low resistance at high temperature, the structure can be represented in two states of 1-bit PMM. For analysis convenience, we regard the PMM with VO2 chips in low-resistance and high resistance as state “0” and state “1”, respectively. The equivalent circuit model of the PMM and surface structure of two states is shown in Fig. 2. The outside ring and inside cross are represented by two inductances, L1 and L2; C1 represents the coupling capacitance between unit cells. The VO2 chip can be equivalent to a resistance Ron when it is in the low-resistance state. When the VO2 chip is in the high-resistance state, the circuit is disconnected, which leads to a gap capacitance Coff. Both the free space and dielectric spacer can be regarded as transmission lines. Y0 and YC denote the characteristic admittance of free space and dielectric, respectively. h is the thickness of the dielectric layer.
Fig. 2. Equivalent circuit model of (a) the PMM, (b) surface structure of state “0” and (c) surface structure of state “1”.
For a conventional reflective metasurface, the equivalent admittance of the surface layer Ysurf is a complex admittance due to its lossy properties and can be expressed as
The equivalent susceptance of the dielectric layer can be expressed as
where δ = βh, β represents the propagation constant of the dielectric layer. According to the transmission line theory, the reflection coefficient can be obtained as follows [33]Therefore, the reflection phase of the metasurface can be calculated by
PMMs show different reflection coefficients in the two states. Suppose the reflection coefficients of the two states are Γ1 and Γ2, respectively; the phase difference between them can be expressed by
Connect the reflection coefficient and input admittance with a Smith admittance chart [34], as shown in Fig. 3. Gray circles and arcs represent the conductance and susceptance of normalized input admittance, respectively. The reflection phase φ can be known from the angle between the line from the admittance point to the origin and the positive X-axis. The phase difference is the angle between the lines of two admittance points and the origin. Since the reflection phases of the two states have different rates of change, keeping the 180° phase difference is realizable only at a single frequency and impossible over a wide band. Thus, the 180 ± 37° phase difference is usually adopted as the criterion in many published works to achieve the 10 dB RCS reduction [35]. Figure 3 shows the input admittance condition that the PMM should satisfy to achieve a 180° ± 37° phase difference between the two states at 10 GHz. For example, when the input admittance of state “0” is known as shown, marked as “+” in the chart, the PMM will get a phase difference of 180° ± 37° if the input admittance of state “1” is in the blue sector range. Particularly, if the input admittance falls within the red dotted line, the phase difference is equal to 180°.
In addition, it can be observed that the phase difference between the two states of the structure can reach 180° only when the input conductance of the two states is opposite. If one state is capacitive, the other state must be inductive. The inductive circuit can be achieved by the RL circuit, while the capacitive circuit can be achieved by the LC circuit. Since the dielectric layer with the metal ground can be equivalent to an inductor, lumped elements that can switch between R and C should be loaded into the surface of the PMM. According to the above equivalent circuit analysis, the structure based on a single VO2 chip can be represented in two states of 1-bit PMM.
3. Description of the metasurface
To achieve a wideband phase modulation, the PMM needs to meet the admittance condition over a wide band, which demands that the reflection phase of the two states should change slowly with frequency and have a similar rate of change. To make the reflection phase slowly vary with frequency, the surface admittance of the PMM needs to change with frequency slowly. The weaker the Q value, the weaker the circuit resonance strength. A larger parallel inductance will reduce the Q value, and can help slow the rate of surface admittance changes with frequency [7]. Hence, the width of the outer ring is relatively narrow for a large parallel inductance.
As the value of g directly affects the value of the gap capacitance Coff introduced, the determination of g is crucial for the phase difference between the two states. The reflection phase is mainly affected by the imaginary part of surface admittance; thus, we will only take the imaginary part of the surface admittance into consideration. Figure 4 shows the normalized surface admittance curves of two states varying with frequency when the value of g is 1.2 mm, 1.5 mm, and 1.8 mm, respectively. In the meantime, the surface admittance condition of state “1” that obtains a phase difference of 180° ± 37° with state “0” is indicated in turquoise. When the admittance of state “1” satisfies the admittance condition, the phase difference between the two states can be kept within 180° ± 37°. We can see from Fig. 4 that as g increases, gap capacitance Coff decreases, and the resonance frequency of state “1” increases. When g is 1.5 mm, the surface layer of state “1” resonates at the same frequency with the 180° admittance curve, which means a 180° phase difference between the two states is realized at this frequency. In this case, the bandwidth of phase difference within 180° ± 37° reaches the widest, about from 7.85 to 15 GHz. Therefore, the value of g was determined to be 1.5 mm.
Fig. 4. Normalized surface admittance of two states at (a) g = 1.2 mm, (b) g = 1.5 mm, and (c) g = 1.8 mm.
Full-wave simulation results are obtained using CST Microwave Studio, as shown in Fig. 5. It can be noticed that the reflection phases of the two states change slowly with frequency and have a similar rate of change over a wide band. Especially, the reflection phase of state “0” changes very gently over the entire simulation frequency range. Due to the large distance between resonance points, its phase change does not exceed π. By comparison, the phase change of state “1” is slightly more dramatic, from 9 to 14.68 GHz. If the resonance frequency of 14.68 GHz can be moved higher, the bandwidth may be wider. Around 9 GHz, state “1” shows a phase inversion with a reflection phase of 180°, while state “0” generates a parallel resonance with a reflection phase of 0°, achieving a phase difference of 180°. The phase modulation band with a phase difference of 180° ± 37° is marked in blue (7.85-15 GHz), which is in good agreement with the above admittance analysis.
To further analyze the working mechanism of the unit cell, we next investigate the surface current distribution of the two states under vertically incident TE-polarized wave around 9 GHz, as shown in Fig. 6. For state “0”, the vertical arm of the inner cross is well electrically connected at the location of the VO2 chip. The current on the outer ring and the inner cross both flows downward. For state “1”, the vertical arm is broken at the location of the VO2 chip. The current flowing through the outer ring and inner cross of state “1” forms multiple loops. The surface current is antiparallel to the current on the metallic ground in both states, which generates magnetic resonances within the structure.
Fig. 6. Current distribution of (a)state “0” and (b)state “1” under vertically incident TE-polarized wave around 9 GHz.
Due to the slight differences in working principles between VO2 chips and other traditional components, the effect of its main parameter, the square resistance Ron, on the performance of the PMM needs further analysis. Figure 7 shows the reflection amplitude of state “0” and phase difference vary with Ron. It can be observed that the reflection loss of state “0” increases with the increase of square resistance Ron. The lower the square resistance Ron, the lower the reflection loss. However, due to the limitation of the current preparation process, the square resistance of VO2 chips cannot be very low. When VO2 chips are in the high resistance state, the square resistance is large enough to be equal to disconnection, so the reflection loss is small, less than 0.4 dB. While the reflection amplitude of state “0” changes obviously with Ron, the simulated results show that the phase difference hardly changes with Ron.
Fig. 7. Effect of Ron on the performance of the PMM: (a) reflection amplitude and (b) reflection phase difference.
By combining theoretical derivation and Smith chart analysis, we have studied why the phase difference does not change with the square resistance Ron of VO2 chips. When Ron is relatively low, the conductance G of surface admittance will be small and much smaller than the susceptance B. According to Eq. (6), the effect of Ron on the reflection phase of state “0” can be ignored. We mark the simulated surface admittance of state “0” with different Ron on the Smith admittance chart at 10 GHz, as shown in Fig. 8. Ron varies from 0 to 1000 Ω at intervals of 50 Ω. When Ron is zero, which means G is also zero, and the admittance point falls on the outer circle. As the Ron increases, the admittance point begins to move inward. From the chart, we can know that the simulation results are according to the results in Fig. 7. When the Ron is less than 100 Ω, the reflection phase changes very slightly, which verifies what we said above that the reflection phase of PMM is mainly affected by the imaginary part of surface admittance. Therefore, the phase difference between the two states hardly changes with Ron.
4. Performance of the metasurface
4.1 Numerical analysis
As the unit cell is designed entirely symmetrical to the coordinate axes, the designed PMM is insensitive to the polarization of the incident wave, effective in both horizontal polarization and vertical polarization waves. In general, multiple two-port components, such as PIN diodes and MEMS switches, need to be used in a single polarization-insensitive unit cell. However, in this polarization-insensitive design, only one VO2 chip is used in each unit cell. This is due to the fact that VO2 chips can be cut to any size desired, and square chips are used in this work, which has good symmetry. Therefore, using VO2 chips as an active component can greatly reduce the number of components used during the design of polarization-insensitive PMMs.
The simulated reflection phase difference of the PMM at both TE- and TM-polarized oblique incidences are shown in Fig. 9. Overall, the performance of the PMM performance is relatively stable at both TE mode and TM mode. With the increase of the oblique incidence angle, the fluctuation of the reflection phase difference becomes a little more severe. The phase modulation bandwidth under oblique incidence becomes a little wider than that under vertical incidence instead. At TE mode, for oblique incidence up to 30°, the phase difference changes slightly so dramatically that it is out of the range of 180° ± 37°. Under TM polarization, the phase difference curves show some difference from those under TE polarization. The bandwidth reaches its widest when the oblique incidence angle is 30°. Besides, curves show a discontinuity at about 12.4 GHz in the case of oblique incidence. The reason is that state “1” generates single-point resonance wave absorption at this frequency, which causes the discontinuity of the reflection phase of state “1”.
Fig. 9. Full-wave simulation results of VO2-based PMM: (a) TE mode and (b) TM mode under different oblique incidence angles.
With the help of COMSOL Multiphysics 5.6, we conduct thermal simulations on the designed PMM. The bottom temperature of PMM is set as a constant temperature of 100°C at the beginning to simulate the temperature provided by the heating plate, and returns to room temperature 250°C after 2 minutes. The convection power density between the structure and the air is set to 5 w/m2. Figure 10 shows that the temperature of VO2 chips changes with time under the set thermal excitation. It took 19 seconds for the temperature of VO2 chips to rise from room temperature to 680°C. Then, the thermal equilibrium is reached, and the temperature of the VO2 chips is stable at 930°C. The time required for heat dissipation from the highest temperature to the room temperature is relatively long, about 65 s. Compared with the switching speed of current PIN diodes, the phase transition speed of VO2 chips under this thermal excitation mode is slower.
4.2 Measurement validation
A 210 nm VO2 thin film was deposited on a 500 um sapphire substrate by magnetron sputtering. During the deposition, the substrate temperature was maintained at 4250°C with a sputtering power of 200W. The accelerated ion bombards the vanadium target, bombards the atoms of the vanadium target, and obtains the deposited material, which is deposited on the substrate and condensed into a VO2 film. After deposition, the film was annealed at 5250°C in a vacuum furnace for 3 hours. The VO2 film in the metal state was characterized by a grazing angle X-ray diffraction (XRD) measurement achieved with a Philips X’Pert Pro MPD diffractometer, using Cu-Ka radiation with a wavelength of 1.5406 Å. As shown in Fig. 11, among them, there is the strongest peak at the position of 2θ = 27.9°, corresponding to the crystal plane (011) of VO2. The measured XRD peaks are in accordance with the VO2 standard XRD spectrum (JCPDF Card 43-1051), which indicates a high purity characteristic of the prepared VO2 film.
The square resistance of deposited VO2 film can be evaluated by a multifunctional digital four-probe tester. As shown in Fig. 12, the square resistance of the VO2 film is measured by FPT while heated by a heating plate. At the room temperature of 250°C, the square resistance of the VO2 film is about 0.3 MΩ/□. With the increase of temperature, the square resistance of VO2 film starts to decrease slowly. When the temperature exceeds 700°C, the square resistance suddenly drops sharply and remains stable at 57 Ω/□ finally. The square resistance ratio of fabricated VO2 film between the metal state and insulating state exceeds three orders of magnitude. When the temperature gradually cools from 950°C to room temperature, the temperature point at which the square resistance rises suddenly becomes about 600°C, with a thermal hysteresis interval of 100°C.
To further verify the correctness of the theoretical analysis and simulation results, the proposed VO2-based PMM is fabricated and experimentally characterized. Figure 13(a) and (b) show the photograph of the VO2 chips and a partial view of the PMM prototype. According to the results of the above simulation analysis, the VO2 film is cut into chips by laser cutting technology with a dimension of 1.5 × 1.5mm2. The processed PMM is composed of 2 × 12 unit cells with a whole size of 18 × 108 mm2. VO2 chips are pasted in the center of the surface layer with silver paste. Then, put the sample in the oven and bake it for 30 minutes at 80°C to solidify the silver paste. Figure 13(c) and (d) show the measurement setting, and the performance of the VO2-based PMM is measured in the parallel plate waveguide (PPW). The PMM is placed in the center of the flat plate, with a heating plate bonded to its back. The two ports of the PPW are connected to the vector network analyzer. As the designed PMM is a reflective metasurface, only S11 needs to be tested. When the VO2-based PMM is in state “1”, the heating plate doesn’t work, and the S11 of state “1” is measured at room temperature. When the VO2-based PMM is in state “0”, the heating plate heats the sample to 1000°C to realize the phase change of VO2 chips, with a rated power of 9 W. A metal plate of the same size can be considered to achieve total reflection, and the reflection loss of the PMM is obtained by comparing it with the reflection amplitude of the metal plate. Due to the error of the test system, when the reflection amplitude of the PMM is higher than that of the metal plate, the reflection amplitude of the PMM is regarded as 0 dB.
Fig. 13. (a) Sample of VO2 chips, (b) Partial view of PMM, (c) Measurement environment, and (d) Heating plate.
Figure 14 presents the simulated and measured reflection amplitude and phase difference of the VO2-based PMM. The test frequency range is about 7-16 GHz. The measured phase modulated band of phase difference basically within 180° ± 37° is about from 7 GHz to 14.67 GHz, which was generally consistent with the simulation results. The difference between the measured and simulation results is mainly due to the diffusion of the silver paste during the process of pasting VO2 chips onto the surface layer, which leads to the change of the gap capacitance Coff. At the same time, the measured reflection loss of two states is greater than that of simulation results. This difference comes from the fact that the deposited VO2 film is uneven, with a minimum square resistance of 57 Ω/□, due to the limitations of the current processing. In addition, the VO2 film has no protection and may be oxidized during heating, resulting in an increase in resistivity. Although the measured reflection loss of PMM is slightly large around 11 GHz, the reflection loss of both states is large, and the difference between them is small, which is good for phase cancellation and RCS reduction. Besides, the error brought by the test system also contributes to the difference between the simulated and measured results.
4.3 Discuss
We compared our design against several similar designs taken from the literature in terms of the active component, phase-modulated band, fractional bandwidth, and polarization sensitivity (Table 1). Most of the previous studies indicate that only one active component is needed for designing a single-polarized unit cell, while more than two active components are required in a dual-polarized unit cell. In our work, only a single VO2 chip is used in the unit cell of the polarization-insensitive PMM. To some extent, the application of VO2 chips reduces the number of components used in polarization-insensitive metasurfaces. Metasurfaces are often composed of a large number of unit cells, and reducing the number of active components used in a single unit cell will greatly reduce the cost of metasurfaces, which is of great research significance. Moreover, VO2 chips are inherently low-loss, and do not increase in price with increasing frequency. When polarization-insensitive PMMs operate at high frequencies, the use of VO2 chips not only improves RF performance, but more importantly, reduces costs to a greater extent.
Table 1. Performance Comparison
Meanwhile, the PMM also has some drawbacks. Since the PMM is based on thermal effects, the temperature of VO2 chips changes slowly, resulting in a limited speed of phase switching. Besides, the size of VO2 chips is large, and the square resistance is a little high, causing a high reflection loss. Further studies on electrically excited VO2-based phase-change switches should be carried out, and the problems of slow speed and large loss will be solved.
5. Conclusion
In summary, a polarization-insensitive PMM using a single VO2 chip is proposed and fabricated in this paper. The cleverness of this design lies in the fact that the single VO2 chip is connected with metal on its four sides, which enables the design of a polarization-insensitive unit cell using only one VO2 chip. However, this is not possible with two-port components, such as diodes and MEMS switches, as multiple two-port components need to be used in a polarization-insensitive design. In addition to reducing the cost, the VO2 chips can avoid the influence of parasitic parameters so that the metasurfaces can work at the high-frequency range. Moreover, as the VO2 chip is directly fabricated by laser cutting the VO2 film, it also has the advantages of simple processing and customizable size.
Funding
Provincial Natural Science Foundation of Hunan (No. 2022JJ30665); Science and Technology Innovation Program of Hunan Province (2020RC2046,2020RC2048); National Natural Science Foundation of China (61901492,61901493).
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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