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Abstract
Metasurfaces have been verified as an ideal way to control electromagnetic waves within an optically thin interface. In this paper, a design method of a tunable metasurface integrated with vanadium dioxide (VO2) is proposed to realize independent control of geometric and propagation phase modulation. The reversible conversion of VO2 between insulator phase and metal phase can be realized by controlling the ambient temperature, which enables the metasurface to be switched quickly between split-ring and double-ring structures. The phase characteristics of 2-bit coding units and the electromagnetic scattering characteristics of arrays composed of different arrangements are analyzed in detail, which confirms the independence of geometric and propagation phase modulation in the tunable metasurface. The experimental results demonstrate that the fabricated regular array and random array samples have different broadband low reflection frequency bands before and after the phase transition of VO2, and the 10 dB reflectivity reduction bands can be switched quickly between C/X and Ku bands, which are in good agreement with the numerical simulation. This method realizes the switching function of metasurface modulation mode by controlling the ambient temperature, which provides a flexible and feasible idea for the design and fabrication of stealth metasurfaces.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Stealth technology is an important part of modern electromagnetic technology research, which has important application value in communication, military and detection fields. For radar stealth technology, the radar cross section (RCS) of the target is usually used to evaluate its ability to evade detection [1]. The methods to achieve stealth function mainly include absorbing the incident electromagnetic wave or making it reflect abnormally, so that the detector cannot receive the electromagnetic scattering signal of the target. For the methods of using traditional materials to absorb electromagnetic wave, they are mainly divided into electrical loss materials and magnetic loss materials. However, electrical loss materials usually have the disadvantages of narrow working band width and insufficient loss capacity, while magnetic loss materials have the disadvantages of heavy structure and high density [2]. As artificial periodic composite structures, metamaterials can achieve functions that natural materials do not have through structural design, such as invisibility cloaks [3,4], negative refraction [5,6], perfect absorbers [7,8] and flat lens [9]. Metamaterials are widely used in the field of radar stealth because they can flexibly modulate electromagnetic wave in specific frequency bands, as well as their thin thickness, light weight and flexible design methods [10]. RCS reduction can be effectively realized by designing appropriate metamaterial structure and arrangement.
For low reflectance metamaterials, they are mainly designed through material loss and electromagnetic resonance [11,12]. In recent years, the theory of abnormal scattering of incident waves through phase gradient metasurface has attracted widespread attention [13,14]. By modulating the phase of the metasurface unit to generate a stable phase difference, directional scattering of electromagnetic wave energy can be realized. With the proposal of digital metasurfaces and coding metasurfaces [14–19], the operating bandwidth of scattering type metasurfaces has been effectively expanded, which provides a new way for the digital development of electromagnetic wave modulation. Meanwhile, in order to increase the flexibility of metasurfaces in complex electromagnetic environments, tunable metasurfaces have been developed rapidly [20–22]. At present, the main modulation methods are electrical modulation methods using PIN tubes, diodes or graphene [23–25] and light intensity modulation methods using photosensitive films or photosensitive semiconductors [26,27]. These methods are flexible in tuning and have a high degree of design freedom, but there are still challenges in the fabrication accuracy of large-scale samples and the modulation stability of electromagnetic wave. In addition, some researchers have used phase change materials such as VO2 and germanium antimony tellurium (GST) to achieve the tunable function of metasurfaces [22,28–34], but due to the limitations of sample fabrication, most of the studies are in the terahertz range.
In this paper, a novel microwave low reflectance tunable metasurface based on phase change material VO2 is reported. Through the design and fabrication of the composite structure unit array, VO2 is used as a temperature-controlled conductivity switch. Using the difference in conductivity of VO2 before and after phase transition, the composite structure has the function of geometric and propagation phase modulation respectively, so as to achieve RCS reduction in different frequency bands. Through analysis and verification, it is found that the experimental and simulation results are in good agreements with each other, which verifies the feasibility of constructing composite structures based on VO2 and the independence of geometric and propagation phase modulation. This work provides a new idea for the research of tunable low reflectance metasurfaces.
2. Design of tunable phase modulation unit cells
2.1 Geometric and propagation phase modulation of unit cells
VO2 is a kind of metal oxide with reversible phase transition property. When the temperature exceeds 340 K, VO2 shows an excellent insulator-to-metal phase transition behavior. The change of VO2 microstructure before and after phase transition will lead to a 103 times abrupt change in electrical conductivity. According to this property, researchers applied VO2 to the fields of intelligent temperature-controlled thin films and electromagnetic modulation [35–38]. In this paper, VO2 is applied as the metal pattern layer for the design and fabrication of metasurface in the radar frequency range. In this process, we combined the relevant research results [39,40] and adopted the high-temperature waveguide method to test the electromagnetic performance before and after the phase transition of VO2 to ensure the correctness of the electromagnetic simulation. In order to improve the deposition quality of VO2 thin film, silicon dioxide (SiO2) is selected as the substrate. SiO2 has relatively stable chemical properties, with high temperature resistance and low dielectric constant, which can ensure the tunable function of metasurface.
The sandwich unit cells are designed as shown in Fig. 1. The periodicity of unit cells is p = 10 mm and they are divided longitudinally into three layers. The top layer consists of a composite structural pattern of VO2 and copper films with a thickness of 200 nm. The middle dielectric layer is SiO2 dielectric substrate with a thickness of 3 mm, which has a relative permittivity of ε = 3.75 and loss tangent of tanδ = 0.0004. The bottom layer is a metal reflective layer with a thickness of 2 mm. In order to realize the function of dynamic modulation, the split ring of unit cell is made of copper, which corresponds to the yellow part in Fig. 1(a). At the same time, phase change material VO2 is used to fill the notch of the outer split ring and the entire inner ring, corresponding to the red part in Fig. 1(a). Since the conductivity of VO2 has an abrupt change before and after the phase transition, this design can change the symmetry of the equivalent conductive unit structure by controlling the temperature. The local coordinate axis of the unit is defined as the UV coordinate axis and its rotation angle relative to the XY axis is θ. The optimized structural parameters of unit cells are as follows: for the outer ring, the radius is r1 = 4 mm, the width is s1 = 1 mm, the notch width is w = 0.5 mm and the rotation angle θ is a variable. For the inner ring, the radius r2 is a variable and the ring width is s2 = 1 mm. Figure 1(b) shows the shape of the equivalent conductive pattern of unit cell at different temperatures. The working principle is that at room temperature of 25 °C, VO2 presents insulator phase and the corresponding region has no electromagnetic resonance characteristic. At this time, the conductive pattern is equivalent to the split ring structure and the geometric phase modulation can be realized through rotating the split ring. When the temperature is raised to 75 °C (the temperature set here is slightly higher than the transition temperature threshold to ensure the phase transition of VO2 occurs), VO2 presents metal phase. At this time, since the corresponding regions of VO2 are connected with the metal region, the equivalent conductive pattern becomes the double rings structure and the propagation phase modulation can be realized through changing the size of inner ring.
Fig. 1. Schematic diagram of geometric / propagation phase modulation. (a) Schematic diagram of unit cell structure. (b) Schematic diagram of equivalent conductivity pattern of unit cell at 25 °C and 75 °C. (c) Schematic diagram of geometric phase modulation of unit cell. (d) Schematic diagram of propagation phase modulation of unit cell.
The modulation principles of geometric phase and propagation phase are further analyzed to study the electromagnetic characteristics of unit cells before and after the phase transition of VO2. When the electromagnetic wave passes through the metasurface, the reflected wave can be expressed as:
Subsequently, the rotation angle are set to θ = ±45°, and two optimized matrices can be obtained as $\left( {\begin{array}{cc} 0&{ - 1}\\ { - 1}&0 \end{array}} \right)$ and $\left( {\begin{array}{cc} 0&1\\ 1&0 \end{array}} \right)$. It can be seen that the two types of unit cells realize linear polarization conversion of electromagnetic wave, while the cross polarization components between the two types of unit cells have the same amplitude and the phase difference is 180°. Based on the geometric phase modulation, the design of unit cell structures with opposite phase of reflected wave can be realized at room temperature of 25 °C.
The propagation phase modulation of unit cell at 75 °C is shown in Fig. 1(d). Since the unit cells possess structural symmetry along both x-axis and y-axis, the cross polarization component of Jones matrix is 0, while the co-polarization components are equal, so that the polarization conversion of electromagnetic wave cannot be realized. The linear Jones matrix of unit cells is represented as:
2.2 Phase characteristics analysis of coding unit cells
The 2-bit coding unit cells are designed as shown in Fig. 2(a). The unit cells are designed and analyzed by full-wave electromagnetic simulation with CST Microwave Studio. The periodic boundary conditions are applied to simulate the infinite periodic cells. The “00” and “01” unit cells (“10” and “11” unit cells) are obtained by changing the size of inner ring, which have the propagation phase difference of nearly 180° in a certain frequency band. The “00” and “10” unit cells (“01” and “11” unit cells) are obtained by rotating the outer ring structure, which have the geometric phase difference of nearly 180° in another frequency band. Therefore, the function of 180° phase difference in different frequency bands can be realized through structural design Keeping the other structural parameters unchanged, the rotation angle θ and the inner ring radius r2 of the composite unit structure are optimized. The optimized results are as follows: “00” unit cell has a structural size of “r2 = 1.5 mm, θ=-45°”, “01” unit cell has a structural size of “r2 = 2.1 mm, θ=-45°”,"10” unit cell has a structural size of “r2 = 1.5 mm, θ=+45°” and “11” unit cell has a structural size of “r2 = 2.1 mm, θ=+45°”.
Fig. 2. Schematic diagram of 2-bit coding unit cells design. (a) Schematic diagram of unit cells. (b) Reflection amplitudes and phases of “00” and “10” unit cells (“01” and “11” unit cells) based on geometric phase modulation. (c) Reflection amplitudes and phases of “00” and “01” unit cells (“10” and “11” unit cells) based on propagation phase modulation.
For the reflective chessboard structure metasurface, when the reflection phase difference of adjacent units meets 180° ± 37°, the monostatic RCS reduction of more than 10 dB can be achieved for the vertically incident electromagnetic wave [42]. Figure 2(b) shows the reflection amplitudes and phases of “00” and “10” unit cells (“01” and “11” unit cells). It can be seen that the amplitudes reach the minimum value of 0.99 at the frequency point of 6.7 GHz, and the amplitude fluctuations in the whole frequency band are very small, which can be approximated as 1. In the frequency band from 6.8 GHz to 10.6 GHz, the phase difference satisfies the condition 143°≤ | Δ φ| ≤ 217°, which proves that the phases of reflected waves are opposite between unit cells in this frequency band. Figure 2(c) shows the reflection amplitudes and phases of “00” and “01” unit cells (“10” and “11” unit cells). It can be seen that the amplitudes reach the minimum value of 0.98 at 14.2 GHz, which are close to 1 in the whole band. In the frequency band from 14.4 GHz to 17.2 GHz, the phase difference satisfies the condition 143 °≤ | Δ φ| ≤ 217 °, which proves that the phases of reflected waves are opposite between unit cells in this frequency band. Therefore, the characteristics of geometric phase and propagation phase can be modulated by switching the temperature.
3. Design and analysis of tunable array
3.1 Design of regular array
Based on the design results of 2-bit unit cells, the regular cell array M1 with tunable operating band is proposed, as shown in Fig. 3(a). The numbers in M1 array correspond to those in Fig. 2(a). The green numbers represent the geometric phase modulation part of the unit cells and the blue numbers represent the propagation phase modulation part of the unit cells. The frequency points 8.7 GHz and 15.8 GHz are selected to study the scattering characteristics of the M1 array before and after phase transition of VO2, which are the middle frequency points of the frequency bands with 180° phase difference between the unit cells at 25°C and 75°C, respectively. At 25°C, the M1 array shows a regular arrangement of geometric phase modulation unit cells along x-axis direction. The phase differences between the unit cells at 8.7 GHz and 15.8 GHz are 197.6° and 91.6°, respectively. At 75 °C, the M1 array shows a regular arrangement of propagation phase modulation unit cells along y-axis direction. The phase differences between the unit cells at 8.7 GHz and 15.8 GHz are 0.3° and 203.5°, respectively. The combination of 3 × 3 unit cells is set as a supercell to reduce the in-plane coupling and satisfy the periodic boundary conditions of simulation. The combined supercells are arranged into M1 array, which is composed of 6 × 6 supercells horizontally and longitudinally, with an array area of 180 × 180 mm2.
Fig. 3. Tunable M1 array: (a) Unit cells arrangement of the M1 array. (b, c) Far-field scattering patterns at 25 °C for 8.7 GHz and 15.8 GHz, respectively. (d, e) Far-field scattering patterns at 75 °C for 8.7 GHz and 15.8 GHz, respectively.
The far-field scattering characteristics of M1 array at different temperatures and frequencies are simulated with CST Microwave Studio The plane wave incident perpendicular to the array is set as the excitation source to meet the far-field conditions. Figure 3(b∼e) show the one-dimensional far-field scattering plots, and the inserts show the three-dimensional far-field scattering patterns. Phi = 0° means RCS data along the xz plane are selected and Phi = 90° means RCS data along the yz plane are selected. It can be seen that at 25 °C, M1 array shows obvious electromagnetic scattering characteristic at 8.7 GHz. The energy in the main lobe direction is suppressed and scattered to other directions, which matches the phase cancellation characteristic between the unit cells at this frequency point. In addition, M1 array shows a small amplitude of electromagnetic scattering at 15.8 GHz, but the energy in the main lobe direction is still large, and only a small part of electromagnetic wave are scattered to other directions. This is mainly due to the small phase difference between unit cells at this frequency point, which leads to the weak electromagnetic scattering characteristic. At 75 °C, M1 array shows full-wave reflection characteristic in vertical direction at 8.7 GHz, which is because there is no phase difference between unit cells at this frequency point. At 15.8 GHz, the electromagnetic scattering characteristic is exhibited, which shows the energy in the main lobe direction is obviously suppressed. This phenomenon matches the phase cancellation characteristic between unit cells at this frequency point.
According to the generalized Snell’s law [43], when the electromagnetic wave incident on the metasurface, the incidence angle θi, reflection angle θr and phase gradient at the interface of metasurface dΦ/dl should be satisfied:
According to Eq. (6), the anomalous reflection angles for normal incidence are calculated and compared with the simulation results, as shown in Table 1.
Table 1. Comparison between simulation and calculation of electromagnetic wave scattering angle
In Table 1, sim. represents the simulation results and cal. represents the calculation results. It can be seen that the calculation results are basically consistent with the simulation results. Obviously, based on the joint modulation of geometric and propagation phase, M1 array shows different electromagnetic scattering characteristics at different temperatures, which realizes the dynamic modulation of operating frequency band.
The reflection characteristics of M1 array at 25 °C and 75 °C are further simulated. Figure 4(a) shows the reflectivity of M1 array at 25 °C, which is less than -10 dB in the frequency band of 6.7∼10.5 GHz. This is basically consistent with the frequency band of 6.8∼10.6 GHz with geometric phase cancellation function as shown in Fig. 2(b). Figure 4(b) shows the reflectivity of M1 array at 75 °C, which is less than -10 dB in the frequency band of 14.4-17.2 GHz. This frequency band is identical to the frequency band of 14.4-17.2 GHz with propagation phase cancellation function as shown in Fig. 2(c). The simulation results fully verify above inference. The designed M1 array achieves low reflection characteristics of different frequency bands at different temperature points.
3.2 Design of random array
Furthermore, the unit cells in the M1 array are arranged in a disordered manner to study the electromagnetic scattering characteristics of the array under the random arrangement of unit cells. In order to enable the electromagnetic wave to be scattered uniformly, all the unit cells in the horizontal or vertical rows of the array are arranged randomly, instead of the combined form of supercells. In addition, in order to ensure the credibility of the conclusion, the arrays are still composed of unit cells with the same number and size as M1, with a size of 180 mm in both horizontal and vertical directions.
At 25 °C, the electromagnetic scattering characteristics of the array depend on the arrangement of geometric phase modulation units, that is the arrangement of green numbers in Fig. 3(a). Therefore, it is possible to achieve uniform electromagnetic scattering in the xoz plane by randomly arranging them in the horizontal direction. The horizontal elements are set to [0, 0, 1, 1, 0, 1, 1, 0, 0,1, 0, 0, 0, 1, 1, 0, 1, 1, 1, 1], while the blue numbers corresponding to the propagation phase modulation units remain unchanged. At 75 °C, the electromagnetic scattering characteristics of the array depend on the arrangement of propagation phase modulation units, that is the arrangement of blue numbers in Fig. 3(a). Similarly, uniform electromagnetic scattering can be achieved in the yoz plane by randomly arranging the blue numbers in the vertical direction. The vertical elements are set to [0, 0, 1, 1, 0, 1, 1, 0, 0,1, 0, 0, 0, 1, 1, 0, 1, 1]T, while the green numbers corresponding to the geometric phase modulation units remain unchanged. The random arrays are designed by this method. Figure 5(a) illustrates the structure of M2 array which achieves uniform electromagnetic scattering in the xoz plane, and Fig. 5(b) illustrates the structure of M3 array which achieves uniform electromagnetic scattering in the yoz plane, where “0” and “1” respectively represent unit cells with opposite phases.
Fig. 5. Tunable random arrays design results. (a) Schematic diagram of M2 array structure. (b) Schematic diagram of M3 array structure. (c, d) Three-dimensional far-field scattering patterns at 8.7 GHz and 15.8 GHz for M2 and M3 arrays at 25 °C. (e, f) Three-dimensional far-field scattering patterns at 8.7 GHz and 15.8 GHz for M2 and M3 arrays at 75 °C.
Three-dimensional far-field scattering patterns of random array at different temperatures and frequencies are simulated, as shown in Fig. 5(c)∼(f). Figure 5(c) shows the far-field scattering pattern of M2 array at 25 °C. It can be seen that M2 array has uniform scattering characteristic of electromagnetic wave at 8.7 GHz, which is attributed to the uniform distribution of elements and the phase difference between unit cells close to 180°. At 15.8 GHz, M2 array shows partial scattering characteristic of electromagnetic wave, but the reflected wave in vertical direction is still strong. This is because the small phase difference between unit cells at this frequency point, which leads to the limited modulation of electromagnetic wave using the elements uniform arrangement method. The far-field scattering pattern of M2 array at 75 °C is shown in Fig. 5(e). Since the arrangement of propagation phase modulation elements are not changed, it can be equivalent to the regular array M1, showing the electromagnetic reflection characteristic in the vertical direction at 8.7 GHz and electromagnetic scattering characteristic at 15.8 GHz, which are consistent with the illustrations in Fig. 3(d, e). The far-field scattering pattern of M3 array at 25 °C is shown in Fig. 5(d). Since the arrangement of geometric phase modulation elements are not changed, it can be equivalent to the regular array M1. It shows electromagnetic scattering characteristic at 8.7 GHz and only a small part of electromagnetic waves are scattered at 15.8 GHz, which are consistent with the illustrations in Fig. 3(b, c). Figure 5(f) shows the far-field scattering pattern of M3 array at 75 °C. It shows the electromagnetic reflection characteristic in the vertical direction at 8.7 GHz, which is due to the phase difference between unit cells is 0 at this frequency point. At 15.8 GHz, it shows relatively uniform electromagnetic scattering characteristic, which is due to the uniform arrangement of elements and the phase difference between unit cells close to 180°. In general, M2 array and M3 array can achieve uniform electromagnetic wave scattering under different conditions, and the independence of geometric phase modulation and propagation phase modulation in the array is verified.
Then, the reflection characteristics of M2 and M3 arrays at 25 °C and 75 °C are discussed. The simulation result of M2 array at 25 °C is shown in Fig. 6(a). It can be seen that the reflectivity is less than -10 dB in 6.9∼10.2 GHz frequency band, which is basically consistent with the operating bandwidth of M1 array at this temperature. However, its performance is significantly weakened. This is because M2 array is composed of single unit cells, which are easy to generate strong in-plane coupling in the absence of supercells. At the same time, the reflectivity of M2 array is less than -10 dB in 14.4∼17.2 GHz frequency band at 75 °C, as shown in Fig. 6(c). This frequency band is completely coincident with the reflection frequency band of M1 array at this temperature, because the arrangement of propagation phase modulation unit cells at this temperature is the same as M1 array. For M3 array, the reflectivity is less than -10 dB in 6.7∼10.5 GHz at 25 °C, as shown in Fig. 6(b). This frequency band is completely coincident with the reflection frequency band of M1 array, because the arrangement of geometric phase modulation unit cells at this temperature is the same as M1 array. Figure 6(d) shows the simulation result of M3 array at 75 °C. It can be seen that its reflectivity is less than -10 dB in 14.5∼17.2 GHz band. Compared with M1 array, its performance is obviously weakened, which is also caused by in-plane coupling. The above conclusions are basically consistent with Fig. 4, which further indicates the independence of geometric phase modulation and propagation phase modulation.
Fig. 6. Reflectivity curve plots of random arrays at different temperatures. (a) Reflectivity curve of M2 array at 25 °C. (b) Reflectivity curve of M3 array at 25 °C. (c) Reflectivity curve of M2 array at 75 °C. (d) Reflectivity curve of M3 array at 75 °C.
4. Fabrication and measurement
4.1 Sample fabrication
In order to verify the reliability of the simulation results, M1 and M2 arrays were fabricated in a cleanroom environment by magnetron sputtering technology and laser selective etching technology. Due to the limitation of the coating technology, both M1 and M2 were decomposed into 9 pieces of 50 × 50 mm2, 6 pieces of 50 × 30 mm2 and 1 piece of 30 × 30 mm2 samples.
The fabrication process is shown in Fig. 7. First, a 200 nm thick copper film layer have been obtained on the cleaned SiO2 substrate using DC magnetron sputtering method. Then the laser scanning system was used for selective etching, as shown in Fig. 8, and the etching processing parameters are shown in Table 2. The amorphous VO2 film layer with a thickness of 300 nm have been obtained using RF magnetron sputtering from a VO2 target in an argon atmosphere, and then patterned composite unit structure was formed by laser selective etching. Finally, the sample was annealed in vacuum and held for 2.5 h at 530 °C, so that the insulator phase VO2 film could be transformed into metal phase VO2 film with conductivity abrupt change characteristic.
Table 2. List of laser etching parameters
The final sample structure is that VO2 film is attached to the copper film: VO2 film has poor conductivity at room temperature and its thickness is relatively thin, which does not affect the geometric phase modulation of metasurface. When the ambient temperature reaches the phase transition threshold, the conductivity of VO2 film increases significantly, since the lower layer of the metasurface was originally attached with the copper film, so the overall conductivity of the structure is almost unchanged, which does not affect the propagation phase modulation of the metasurface. The area of the fabricated metasurface arrays M1 and M2 is 180 × 180 mm2, as shown in Fig. 9(a).
Fig. 9. (a) The photography of far field test and the fabricated sample. (b) Measured XRD results of VO2 film after annealing. (c, d) Simulated and measured reflectivity results of M1 array at 25 °C and 75 °C respectively. (e, f) Simulated and measured reflectivity results of M2 array at 25 °C and 75 °C respectively.
4.2 Experimental results
X-ray diffractometer was used to test the phase of VO2 film before and after annealing, as shown in Fig. 9(b). There was no obvious diffraction peak before annealing, which indicates the film was amorphous. After annealing, the crystallization state was significantly improved, and the diffraction peak in the XRD image coincided well with the monoclinic VO2(M) standard card (JCPDS, No.72-0514). The conductivity of VO2 film attached to the surface of SiO2 was measured at 25 °C and 75 °C by using the dual electric four probe resistivity tester. The results showed that the VO2 film presented insulator phase before phase transition, and the conductivity was 8.7 × 104S/m after phase transition, which confirmed that VO2 film has obvious conductivity after phase transition. The reflectivity of the sample was tested in the microwave anechoic chamber, as shown in Fig. 9(a). The heating device, thin-layer metal plate and metasurface sample were placed on the system support in order to test the reflectivity values for electromagnetic waves with the frequency band of 2∼18 GHz at 25°C and 75°C respectively. In this method, the heating platform was placed under the metal plate, which can not affect the electromagnetic echo. At the same time, we used the infrared thermometer to monitor the temperature of metasurface in real time to ensure the sample can be heated sufficiently. When the heating stage was set at 85 °C and after more than five minutes of heat conduction, the surface of sample can be stabilized at 75 °C. The test results are shown in Fig. 9(c)–(f), which show that the reflectivity of M1 array is less than -10 dB in 6.8∼11.2 GHz frequency band at room temperature of 25 °C. When M1 array was heated to 75 °C by heating device, its reflectivity is less than -10 dB in 14.7∼17.4 GHz frequency band. Similarly, at room temperature and 75 °C, the reflectivity of M2 array was less than -10 dB in 7.3∼10.9 GHz and 15.2∼17.4 GHz frequency band respectively.
Compared with the simulated and measured results of M1 and M2 at room temperature, the results show that the reflectivity plots move slightly to the high frequency band, which is due to the inevitable slight ablation of the unit cells in the process of laser patterning etching. However, the results are generally consistent, which further verifies that the insulator phase VO2 film does not affect the electromagnetic resonance characteristics of the composite metasurface unit cells. At the same time, there are differences between the simulated and measured results after temperature rises, which shows that the measured operating frequency bands are narrowed and another extreme point appears near 6.8 GHz. These are mainly because the conductivity of the metal phase VO2 film is different from copper film after the temperature rises, and the connection between VO2 and copper becomes worse due to the stress during the annealing process, which makes the connection area of composite unit cannot be treated as an integrated structure. Therefore, the actual electromagnetic characteristics are not exactly the same as the simulation settings, which will lead to the phase difference between unit cells are different from the simulation results. In general, the experimental results still show the low reflection characteristics of different frequency bands through temperature switching.
5. Conclusion
In this paper, we have integrated phase change material VO2 film into metasurface. Through the phase transition of VO2, the composite unit cells can switch between the split-ring and double-ring structures, so as to realize the integration of two independent modulation modes based on geometric and propagation phase. Based on this principle, the 2-bit unit cells were designed to realize the phase cancellation function of the array in different frequency bands at different temperatures. Then the regular arrangement array and random arrangement array were designed, and the results show that electromagnetic waves have different electromagnetic scattering characteristics in different frequency bands before and after phase transition of VO2. The test results show that the prepared sample has broadband low reflection characteristics of different frequency bands before and after temperature switching, the experimental and simulation results are in good agreements with each other, which confirms the independence of geometry and propagation phase modulation. Compared with the traditional electronic modulation methods of lumped elements, the integrated phase change material VO2 film does not need to consider the complex fabrication processes such as wiring and welding. Therefore, it has advantages in fabrication and modulation, which provides a new idea for the design and fabrication of tunable radar stealth metasurfaces.
Funding
National Natural Science Foundation of China (52175405); Natural Science Foundation of Hubei Province (2020CFB423).
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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