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Abstract
In this paper, a water-based metasurface with adjustable reflection amplitude is proposed. The overall structure uses a transparent substrate as a water-based container, and the upper surface is loaded with a double-ring-shaped resistive film. As the height of the water in the container gradually increases from 0 mm to 0.5 mm, within a broadband range from 0.1 GHz to 30 GHz, the maximum adjustable range of the reflection amplitude is -2 dB to -12 dB. The water-based metasurface switches from a state of strong reflection to a state of absorption. The test results are in good agreement with the simulation results. Because the tunable metasurface is transparent to visible light, it can be used for electromagnetic shielding of windows of airplanes.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid development of electromagnetic metamaterials [1–6], tunable metasurfaces have become a hot research topic, some tunable methods are introduced to flexibly control the electromagnetic properties of metasurfaces. Compared with the traditional metasurface, the tunable metasurface is thinner in spatial dimensions and has the advantages of a low profile. In addition, it can achieve bandwidth expansion and multifunctional switching through electrical, optical, mechanical, and temperature control. It can be divided into optical metasurface, acoustic metasurface, mechanical metasurface, and so on [7–12]. Among them, the optical metasurface is the most common type. It can control the phase, amplitude, resonant frequency, polarization, and other characteristics of electromagnetic waves through sub-wavelength microstructures. The tunable metasurfaces are suitable for many applications such as electromagnetic compatibility equipment, stealth weapons. For example, a plasma-based adjustable active metasurface was designed in [13]. By adjusting the voltage, the resonant frequency of the reflection is continuously adjustable from 9.2 GHz to 10.5 GHz. A broadband tunable metamaterial absorber based on vanadium dioxide was proposed in [14]. The conductivity of VO2 can be adjusted by changing the temperature. The absorption intensity can be achieved from 30% to 100% continuous adjustment in the range of 0.3-0.8 THz. The production of the above structure is complicated, and it is not transparent to visible light.
Water has high dielectric loss and high transparency, so it has been widely used in the design of tunable metamaterials in recent years [15–20]. An elliptical water-based medium container was designed in [21]. By rotating the container, the water takes on different shapes at different angles. The transmission amplitude of the proposal can be adjusted from -8 dB to 0 dB in the 1.1-1.4 GHz microwave band. A water-based continuously tunable bandpass filter is proposed in [22]. By adjusting the amount of distilled water inside these tubes, the equivalent electrical length of the transmission line changes, which can change the resonant frequency from 1.06-1.29 GHz. The performance of the above structure is superior, however, the structure is not transparent to visible light and the tunable bandwidth is relatively narrow.
The tunable metasurface proposed in this paper adopts a simple way to achieve the dynamic adjustment. By adjusting the height of the water in the container, the adjustment of the reflection amplitude can be realized. The water-based metasurface is composed of a frequency selective surface (FSS) [23], a water-medium mixed substrate, and the indium tin oxide (ITO) backplane. When the height of the water is 0 mm, the metasurface shows strong reflection. When the height of water injected into the container increases from 0 mm to 0.5 mm, the proposed water-based metasurface can achieve a continuous change of the reflection amplitude in the 0.1-30 GHz microwave band from -2 dB to -12 dB, and the adjustable center frequency is 15.2 GHz. The proposed water-based tunable metasurface has a simple structure can be applied to electromagnetic shielding of optical windows of planes adapt to different scenarios.
2. Design and simulation
The unit cell of the water-based metasurface is designed as a typical sandwich structure, as shown in Fig. 1. The double-ring-shaped periodic ITO patch is used as FSS, and the polythylene terephthalate (PET) is used as the substrate with a dielectric constant of 3 and loss tangent of 0.002. The top layer and the bottom layer are 300 Ω/sq and 5 Ω/sq ITO films respectively. They are separated by the polymethyl methacrylate (PMMA) container with a dielectric constant of 2.25 and a loss tangent of 0.001. The container is composed of water and air. The optimized parameters of the structure are as follows: r = 5.6 mm, r1 = 3 mm, r2 = 9 mm, w=0.6 mm, h = 2.5 mm, h1 = 0.8 mm, h2 = 0.8 mm, t = 0.175 mm, t1 = 0.125 mm. The size of unit cell is 10 mm × 10 mm, and the total thickness is 3.6 mm. Figure 1(c) is the schematic diagram of the overall structure of the water-based metasurface loaded with FSS. Thin-caliber syringes are connected on both sides to control water injection and drainage of the structure. By using this method, the height of the water in the container can be changed.
Fig. 1. (a) Top view of the metasurface. (b) unit cell and side view of the metasurface. (c) The water-based metasurface of 16×16 units.
The permittivity of water can be described by the Debye formula given below [24,25]:
To explore the function of the water and the FSS, we compare the reflection coefficients of three different models shown in Fig. 3. Model 1 is the structure with FSS but without water. Model 2 is the structure loaded with water-based but without FSS and Model 3 is loaded with both FSS and water-based. Because of the low-resistance ITO backplane, the transmissivity of the entire layer can be neglected. The reflectivity of the structures is of interest. Figure 4(a) presents the reflection amplitudes of these three different structures. From the comparison of the |S11| of Model 1 and Model 3, it can be seen when water is injected into the medium container, the reflectivity of electromagnetic waves decreases obviously. The water in the container result in a strong absorption of microwave. Besides, it can be seen from the comparison between Model 2 and Model 3 that the loading of the resistive frequency selective surface on the upper surface can also decrease reflected wave. Since the FSS on the top of the water-based metasurface optimizes the impedance matching between the structure and the free space, so that more electromagnetic waves can enter the metasurface and be absorbed by the water.
Fig. 3. Three different models: (a) only FSS loaded (b) only water-based (c) FSS loaded and water-based.
In Model 3, when the height of the water in the container changes, the reflected wave amplitude will vary. Figure 4(b) shows the variation of the reflection coefficients of the structure with respect to the height of water. When the container is empty, the reflection is close to -2 dB. As the height of water gradually increases from 0 mm to 0.5 mm, within the frequency range of 0.1-30 GHz, the reflection amplitude can be continuously decreased from -2 dB to -12 dB. By regulating the height of water in the container, the water-based metasurface can switch from a state of strong reflection to a state of absorption. It can be seen in Fig. 4 (b) that when the height of water increases from 0 mm to 0.3 mm, the center frequency of the water-based metasurface shifts slightly to the right. As the water level continues to increase, the center frequency of the water-based metasurface shifts to the left. We analyzed that when the water level inside the container is low, the resonance of the metasurface is mainly determined by the surface FSS and the structure, and the water absorption has little effect on the overall structure. When the height of water reaches a certain height (0.3 mm), the reflection suppression of the water-based metasurface is mainly caused by the absorption of the water. In this case, the height of the water determines the resonant frequency. As the height of the water increases, the equivalent permittivity of the entire structure increases and the resonant frequency decreases.
Fig. 4. (a) Comparison of the reflection amplitude of three different models of simulation. (b) The water increases every 0.1 mm, the simulation result of the reflection amplitude.
In order to explore the mechanism of the water-based tunable metasurface, the surface current distribution and the power loss density of the water-based metasurface under different heights of water was observed. As shown in Fig. 5 (a) and (b), the current is concentrated on the bottom ITO backplane at 0 mm. The metasurface exhibits a strong reflection state at this time. When the height of the water is 0.5mm, the current is mainly concentrated in the water layer, corresponding to the state of absorption. What’s more, there is almost no loss inside the container when the height of water is 0 mm. When the height of water increases to 0.5 mm, the power loss of the water-based metasurface is mainly concentrated in the internal water layer. It shows that the power loss of the structure in the absorbing state is mainly caused by water in the container.
Fig. 5. Distributions of surface current and power loss density of the metasurface at different heights of water: (a) 0 mm. (b) 0.5 mm.
3. Results and Discussion
The sample fabricated of 180 mm × 180 mm (16 × 16 unit cells) is given in Fig. 6 (a) and (b). The pictures show that the water-based metasurface has high transmittance to visible light. The ITO patch on the top layer is etched by high-precision ultra-fast laser technology. Then stick the substrate, backplane and ITO patch on the surface together with an ultra-thin optical adhesive film. The height of water inside the structure is controlled by the syringes on both sides. The whole structure has a low profile of 0.18 λ0, where λ0 is the wavelength of free space at the center frequency.
As shown in Fig. 7(a), the manufactured water-based metasurface sample was measured by vector network analyzer (Agilent E8363B) through the free space method in a microwave anechoic chamber. Two pairs of broadband horn antennas 1-18 GHz and 18-26.5 GHz were used to transmit and receive electromagnetic waves. Fig. 7(b) shows the comparison of measured and simulated results of water-based metasurface samples. The curves show that the water-based metasurface has the function of tunable reflection amplitude. In the 0.1 GHz to 30 GHz microwave band, the |S11| can be continuously adjusted from -12 dB to -2 dB at the center frequency of 15 GHz. The good agreement between simulation and measurement demonstrates that the water-based metasurface can switch between strong reflection and absorption. The deviation between the measurement and the simulation may be caused by the limited structure and manufacturing tolerances.
Fig. 7. (a)Test environment for the sample of water-based metasurface. (b)Measured and simulated |S11| in the normal direction of water-based metasurface.
Finally, Table 1 shows the comparison of performance between the proposed water-based metasurface and the reported water-based structure. Compared with the previous water-based structure, the water-based metasurface proposed in this paper is not only transparent to visible light but also has a broadband tunable function. Besides, the thickness of the whole structure is only 3.6 mm (0.18). The proposal adopts a simple way to achieve the adjustment of reflection amplitude in the range of 0.1-30 GHz by adjusting the height of water. When the height of water is 0 mm, the water-based metasurface realizes the state of strong reflection. When the height of water increases to 0.5 mm, the metasurface switch to the state of absorption.
Table 1. Comparison between the proposal and reported water-based structures
4. Conclusion
In conclusion, this paper proposes a water-based metasurface with continuously tunable reflection amplitude. The height of water is adjusted to achieve continuous adjustment of the reflection amplitude in the range of 0.1-30 GHz. The upper surface of the structure is composed of a frequency selective surface with resistive film. The transparent medium PMMA is used as the container. By adjusting the height of the water in the container, |S11| can be maximally tuned from -12 dB to -2 dB. When the interior of the structure is empty, it corresponds to the state of strong reflection. As the height of the water increases, the state of absorption can be realized. A good agreement has been obtained between the simulated and measured results. The structure is transparent to visible light, so it can be applied to the windows of aircraft to adapt to different environments.
Funding
Technology Program of Shenzhen (JCYJ20180508152233431); National Natural Science Foundation of China (61971340); National Key Research and Development Program of China (2020YFA0709800).
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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