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Abstract
In this paper, an optically transparent and wideband absorber/reflector with switchable states and tunable frequency spectrum is presented. The proposed structure consists of a Polydimethylsiloxane (PDMS) layer with microchannel structures and an Indium Tin Oxide (ITO) layer as the metal panel. The switching function is implemented by controlling the injection and discharge of pure water, and the switchable frequency band of the absorbing and reflecting states ranges from 7.9 to 34.4 GHz with a fractional bandwidth of 125.2%. The tunable properties are achieved by changing the concentration of the injected saline water. In addition, the distributions of the electric field, the magnetic field and the power loss density are used to further understand the physical mechanism of the structure. Moreover, it also performs well under different polarizations and incident angles. For validation, a transparent and wideband absorber/reflector is fabricated and tested, and the simulated and measured results are consistent with each other.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metamaterial absorbers have been generally carried forward because of their wide applications in various areas in the past few decades, for example, RCS reduction, resduction electromagnetic stealth and imagining system [1,2]. The Salisbury screen can realize a narrowband absorbing performance by placing a resistive layer at a quarter wavelength distance from the conductive plane [3]. The Jaumann absorber, by cascading several resistive sheets, achieved broadband absorption ability, but the structural thickness was increased [4]. Compared with the conventional absorbers [5], [6], electromagnetic absorbers based on frequency selective surfaces (FSSs) can reduce the thickness and improve the absorption performance. Besides, the lumped components such as PIN diode, varactor diode, Schottky diode, and fluidic control can be used in FSS structure for controllable proposes [7–9].
The metamaterial absorbers usually have limitations in absorption band. Nowadays, the design of water-based all-dielectric metamaterials have become a hot-spot [10–12], which can generate electric or magnetic resonances by appropriately adjusting the shape and direction of the dielectric resonators. Switchable all-dielectric metamaterials are easy to be realized because of the fluidity of water [13], [14]. As one of the most plentiful materials in nature, water has been extensively investigated in the field of electromagnetics because of its merits of easy availability, low cost and environmental protection.
During the past years, the research on water-based wideband metamaterial absorbers has become popular. Periodic water droplets are used to design a metamaterial absorber for the first time, which covers 8 - 18 GHz [15]. In [16] and [17], multiband coherent perfect absorption and ultra-wideband absorption were presented, and grid-shaped water-based metamaterials were also used. In [18], water-resonator-based metasurface achieves tunable absorption from 20 to 42 GHz by changing the height of the water shape. In addition, a water-based metamaterial is proposed to achieve broadband absorption and optical transparency, where indium tin oxide (ITO) and polymethyl methacrylate (PMMA) materials with good transparency are employed [19]. A novel electromagnetic absorption metasurface based on cylindrical water resonator is proposed in [20]. More recently, a microchannel in polylactic acid (PLA) is fabricated by 3D printing technology, and the absorptivity of the absorber above 90% ranges from 9.3 to 49.0 GHz [21]. A water-based wideband microwave switchable absorber/reflector is proposed in [22], where a switching function between the absorbing state (10.52 - 20.04 GHz) and the reflecting state (5 - 26.5 GHz) is realized through the injection and discharge of pure water.
In this article, a flexible, wideband, and optically transparent stealth structure is designed. As far as we know, it is the first multifunctional design that the advantages of wideband switchable radar absorbing/reflecting states, tunable frequency spectrum, and optical transparency. Compared with traditional absorber/reflector structures, where active devices are added for switching and tuning, this structure adopts microfluidic technology to realize the switching function between absorption and reflection. At “with water” state, wideband absorption properties are easily achieved by injecting pure water into the microchannel of the structure. This design allows draining pure water quickly, the entire band possesses an excellent reflective performance at “without water” state. Besides, a tunable frequency spectrum can be attained by adjusting the concentration of saline water. In Table 1, the proposed structure is compared with other reported water-based metamaterials, and the proposed design has reconfigurable features, wideband, transparency, polarization insensitivity and angular stability. Besides, flexible materials (e.g. ITO, PET, PDMS and water) are employed in the entire designing process so that the proposed structure can be applied in certain applications that require flexibility. In addition, the copper sheet is replaced by an ITO layer as the reflective background, which achieves good optical transmittance.
Table 1. Comparison with other water-based metamaterial designs
The remainder of this paper is organized as follows. Section 2 describes the details of the proposed structure, including the simulation results, parametric analyses and distributions of electric and magnetic field. In Section 3, in order to validate the simulation results, a prototype of the design is fabricated and tested. Finally, a conclusion is drawn in Section 4.
2. Metamaterial absorber/reflector
2.1 Design and performance
Figure 1(a) shows the unit cell of the metamaterial-inspired absorber/reflector. The basic element is based on the Swastika-Shaped structure in [21], which consists of modified cross-shaped channels with four water injection ports. The micro-fluidic channels are formed by bonding two transparent PDMS layers. A transparent ITO layer, whose square resistance value is 6 Ω/sq, is attached to the PDMS layer as the metal background. Figure 1(b) shows the side view of the structure, and the heights of the three layers from top to down are h1, h2, and h3. The length and the width of water layer are l and w, and the period of the unit cell is P. The optimal geometric parameters in the simulation are P = 15 mm, L = 11 mm, w = 1.8 mm, h1 = 2 mm, h2 = 0.7mm and h3 = 1.2 mm.
In the numerical simulation, the permittivity and the loss tangent of the PDMS material are 2.72 and 0.0027, respectively. The Debye model is used to attain the permittivities of both pure water and saline water, which is described as a function of the resonant frequency ω, the temperature T and the salinity S [23], [24]:
Fig. 2. Real and imaginary parts of pure water and saline water calculated by using the Debye model (0 - 35 GHz).
As can be observed from Fig. 3(a), when the modified cross-shaped channels are filled with pure water, the proposed structure acts as a wideband absorber, and the absorbing bandwidths of TE and TM polarizations are larger than 24.6 GHz (7.9 - 34.4 GHz) under normal incidence. Instead, when the pure water is withdrawn from the channel, the design works as a reflector in the entire absorbing frequency band for different polarizations. Therefore, by switching the working states of the proposed structure between the states of “with water” and “without water”, the reconfigurable performance of absorption and reflection can be attained. Figure 3(b) shows the effect of saline water concentration on the absorption characteristics. As the concentration of saline water increases, the absorption band spectrum further decreases. The circumstance of oblique incidence for pure water is also analyzed and presented in Fig. 4, and the results show that the wideband absorber/reflector performance of the proposed structure holds steady at an incident angle up to 30° at different polarization.
Fig. 3. Simulation results under TE and TM modes. (a) Pure water. (b) Saline water with different concentrations.
2.1 Parameter analysis
In Fig. 5, a parametric analysis is conducted to analyze the influence of the pure water layer on the structure performance. In Fig. 5(a), the absorption peak shifts to the lower frequency and the reflecting performance is basically unaffected as the width of the microchannel increases from 1.6 mm to 2.0 mm. The effect of the water layer thickness is the same as that of the microchannel as shown in Fig. 5(b). The microchannel width of 1.8 mm and the water layer height of 0.7 mm are finally selected to attain good absorbing and reflecting properties.
Fig. 5. Reflection characteristics of the pure water for different parameters under TE mode. (a) Width of the microchannel w. (b) Thickness of the microchannel h2.
2.3 Distributions of the electric field and the magnetic field
To analyze the working principle of the proposed structure at the absorbing state about pure water, the distributions of the electric field (E-field) and the magnetic field (H-field) at different resonance frequencies are simulated and the results are given in Fig. 6(a) and Fig. 6(b), respectively. At 8.9 GHz, the E-field mainly concentrates on the top PDMS layer, and a loop H-field around the middle water layer can be obviously found. The phenomenon indicates the existence of strong electric and magnetic resonances at this frequency. The E-field mainly distributes above and below the water layer at 20.6 GHz, and the H-field distribution is similar with a relative weaker intensity. Due to the joint effect of these resonances, wideband absorbing properties can be achieved. Meanwhile, the distributions of the power loss density at the two resonant frequencies are shown in Fig. 6(c), and it can be observed that the power loss can be largely attributed to the existence of the water layer.
Fig. 6. Distributions of the electric field, the magnetic field and the power loss density under “with water” mode. (a) Electric field distributions at 8.9 GHz (up) and 20.6 GHz (down). (b) Magnetic field distributions at 8.9 GHz (up) and 20.6 GHz (down). (c) Power loss density at 8.9 GHz (left) and 20.6 GHz (right).
3. Fabrication and measurement
To verify the simulation results of the wideband metamaterial absorber/reflector, a prototype consisting of 10 × 10 elements is fabricated as shown in Fig. 7. The bottom PDMS layer carrying the micro-fluidic channel is fabricated by using the soft lithography technique, and it is then bonded with the upper PDMS to form a complete structure [25]. The ITO layer and the PDMS layer are carefully attached together to ensure that there is no air bubble between the two layers. An amount of 17.739 mL pure water is required to fill the whole structure, a syringe is used for the injection and discharge of the pure water. Besides, a micro pump can be further employed to provide a faster and more stable control at a constant speed.
3.1 Measurement of the electromagnetic property
The reflection coefficients of the sample are measured by the free-space method in a microwave anechoic chamber under different states as shown in Fig. 8(a). In the actual test, the Agilent N5245A Vector Network Analyzer (VNA) is connected with two wideband horn antennas placed on the same side of the sample, one serving as the transmitter and the other as the receiver. Due to the wide absorption/reflection bandwidth of the structure, two sets of antennas (1 - 18 GHz and 18 - 26.5 GHz) are used. Two horn antennas are placed at about 2 m from the sample to ensure that the test can meet the far-field requirement. The measurement process is divided into following steps: first, the reflection coefficients are obtained with the sample in the foam frame; then, the reflection coefficients in the first step are normalized to the measured results of the copper sheet with the same size.
Fig. 8. Measurement setup. (a) Electromagnetic characteristics test. (b) Optical transparency property test.
Figure 9 shows the simulation and experimental results under two different states. For different polarizations, the proposed structure has a −10 dB absorbing bandwidth in the band of 6.5 - 26.5 GHz at “with water” state and a −3 dB reflecting bandwidth of 4 - 26.5 GHz at “without water” state. The measurement results agree well with the simulation results. And the main reasons of the reflectance coefficient variation (frequency shift and bandwidth reduction) can be contributed to the thickness and the material characteristics of PDMS. Besides, other influence factors, such as the room temperature of pure water and the bending of the structure surface, should also be considered as the possible uncertainties. The oblique incidence circumstance is also studied, and the results of the proposed structure prove to be stable under different incident angles up to 30°. Besides, the results of saline water with the concentration of 0.9% are also given in Fig. 10, and the test results are in consistent with the simulation results.
Fig. 9. Measurement results of the pure water under oblique incidence. (a) 4 - 18 GHz under TE mode. (b) 4 - 18 GHz under TM mode. (c) 18 - 26.5 GHz under TE mode. (d) 18 - 26.5 GHz under TM mode.
3.2 Measurement of the optical transparency
In recent years, transparent and flexible materials like ITO and PDMS with good optical transparency are applied in metamaterial designs [26,27]. Figure 8(b) shows the optical transparency that is measured by the transmittance tester (LH-221). Due to the size of the sample and the measurement error, multiple measurements are conducted to obtain the average value in the optical transmittance test. Through calculations, the averaged optical transmittance of the visible light in the “with water” and “without water” is 64.7% and 63.3%, respectively, which indicates that the proposed structure is optically transparent in visible wavelengths (380 - 780 nm). Due to the entire structure is filled with pure water, the surface inside the PDMS microfluidic structure becomes smoother, which reduces the diffuse reflection and improves the optical transparent performance.
4. Conclusion
An optically transparent and wideband absorber/reflector is proposed in this paper. The structure possesses switchable absorbing/reflecting states and a tunable frequency spectrum in the operating frequency range of 6.5 - 26.5 GHz, having the advantages of optical transparency, polarization independence and angle stability. Besides, the field distributions of the design are analyzed to explain its wideband absorption. Finally, the switching and the tuning characteristics as well as the optical transparent properties of the proposed structure are verified. Due to its reconfigurable performance and optical transparency, the proposed design can be used in the windows of the military and civil equipment.
Funding
Key laboratory of radar imaging and microwave photonics (Nanjing University of Aeronautics and Astronautics) (NJ20210002); National Natural Science Foundation of China (61871219).
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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