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Abstract
In this paper, an optically transparent ultra-wideband electromagnetic absorber (EMA) is designed using indium tin oxide (ITO) film. To achieve a transparent property, the traditional dielectric substrate and copper are replaced by polyethylene terephthalate (PET) and ITO film. The ultra-wideband absorbing is achieved through the combination of multilayer periodic ITO structures etched on flexible PET layers. The simulation results show that the design can absorb EM waves in the frequency range of 3.72-42.42 GHz (relative bandwidth 165.75%), while its overall thickness is only 0.669λ0 (λ0 is the wavelength at the center frequency of EMA). The design also has the property of polarization insensitive and angular stability. Finally, the absorber is fabricated and measured to prove the effectiveness.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Electromagnetic absorber (EMA) is an important device widely used in electromagnetic compatibility (EMC), electromagnetic stealth, information security and other fields [1]. With the rapid development of science and technology, various electronic components produce a large amount of electromagnetic radiation in different frequencies, which puts forward higher requirements on the parameters of EMAs such as performance, size, and multi-function.
The first prototype of EMA was derived from the Salisbury Screen proposed by Winfield Salisbury in 1952 and suggested to reduce the radar cross section (RCS) of military equipment [2]. It consists of three parts: a top metal film, a dielectric plate with a quarter thickness of the working wavelength and a bottom metal reflector. Later, many improvement work were proposed to achieve the purpose of broadband, multifunction and complex environment adaptation. Among these works, the Jaumann absorber [3] is one of the most representative one aiming at increasing the bandwidth of the Salisbury screen by overlapping multiple Salisbury screens. In 2009, A. K. Zadeh et. al. proposed a circuit analog (CA) design method for fast and efficient design of wideband absorbers [4]. Based on these work, in the recent years, researchers are interested in improving the property of EMAs for specific applications, some of these work such as: in 2017, Sim D U et al, proposed an EMA with an absorption band covering 0.6-3.27 GHz [5]; in 2018, Y. Han et al. proposed two new types of dual-polarization frequency selective absorbers, which are based on a two-layer resonance structure under multi-mode and has a relative bandwidth greater than 100% [6]; in 2019, X. Yuan et al. proposed and implemented a reactive diffusion system mode broadband EMA based on a resistive metasurface characterized by Rosler’s chaos [7].
During the design of traditional EMAs, it is difficult to balance the bandwidth, thickness, absorbing performance and angular stability of the absorber, and researchers usually ignore the optical characteristics [8]. However, the application potential brings new requirements for transparent EMA, such as in windows or transparent films [9,10]. In a word, recent research on transparent EMAs is aimed at achieving “thin thickness, light weight, high absorption rate, wide bandwidth and transparency” [11–14]. Based on these analyses, in this work we further improved the EMA techniques by achieving more bandwidth, less thickness and high absorption.
In this paper, a transparent EMA based on ITO film is designed to achieve ultra-wideband from C to Ka band. The absorber adopts ITO film and PET instead of the traditional resistive film and dielectric plate to realize optical transparency for the overall structure. The EMA is firstly analyzed using the equivalent circuit model (ECM) and the surface electric distribution for its absorption mechanism and then simulated to achieve the required absorbing performance. After that, it was fabricated into a 350 × 350 mm sample consisting of 60 × 60 units. Both simulation and measurement results show that the transparent EMA proposed in this work has an absorption rate of over 90% in 3.72-42.42 GHz. This absorption bandwidth is much wider than most of the recent similar work and covers most commonly used microwave frequency bands of C-, X-, ku-, k-, ka-, thus can be better used for broadband electromagnetic stealth. The design is also proved to be angular stable, that is to say, when the incident angle is increased from 0° to 45°, an 85% absorption rate for TE and TM polarized incident waves can still be guaranteed.
2. Design and analysis
2.1 Structure design
In the design of transparent absorbing structure, ITO film is used to form lossy surface structures and PET is used as the supporting substrate. The proposed absorbing unit is a four-layer structure with three air layers as shown in Fig. 1. Each layer structure is composed of a designed resistive ITO film structure and supported by a PET layer. The upper three layers consist of two square loop patches of different sizes, as shown in Fig. 1(c) (the left subfigure shows the structure of the top two layers and the middle one shows the structure of the third layer). These structures are all made of ITO films with a square resistance of 100$\mathrm{\Omega }/\textrm{sq}$. The bottom layer uses an ITO film with a square resistance of 6$\mathrm{\Omega }/\textrm{sq}$ to replace the traditional reflective metal ground.
Fig. 1. Structure of a unit for the proposed absorber (a) 3D view (b) Side view (c) Top view of each layer, from left to right: top layer, second layer, third layer.
As illustrated in Fig. 1, the periodicity of the unit cell is p. The sheet resistance of the bottom and other ITO film layers are 6$\mathrm{\Omega }/\textrm{sq}$ and 100$\mathrm{\Omega }/\textrm{sq}$. The relative dielectric constant of the PET with a thickness of 0.175 mm is 3 and the loss tangent is 0.006. The proposed transparent EMA has a low profile of 0.669λ0 (λ0 is the wavelength at the center frequency of EMA). The structure is inherently polarization-insensitive due to its symmetric property. Besides, other parameters are listed in Table 1.
Table 1. Structural Geometric Parameters (unit: mm)
2.2 Equivalent circuit analysis
According to the transmission line theory, the PET layer can be regarded as a section of transmission line with a characteristic impedance of ZPET. To further understand the absorption mechanism, the ECM of the designed unit is shown in Fig. 2. Because the squared resistance of the bottom ITO film is 6$\mathrm{\Omega }/\textrm{sq}$, it can be considered as an ideal conductor and described using a wire in the ECM. The patches on each layer of the three-layer structure are equivalent to inductors Li. For the capacitance effects, there are two contributions: 1 from the gap between the two square loops in a unit and 2 from the gap between two units. To simplify analysis, we combine these two capacitors as a Ci. The resistive ITO structure can be equivalent to resistance R to dissipate incident wave, as explained in [15–18].
Considering factors such as thickness, absorbing bandwidth, absorbing performance and application prospect, we achieve broadband absorption effect by combining three-layer ITO multi-resonant structures. Using the equivalences described above, the ECM of the EMA is established as shown in Fig. 2. In the ECM, the total input impedance Zin of the EMA can be calculated via the following formula:
In addition, the absorption rate of the absorber can be calculated by [19]:
where T is the transmission rate, R is the reflection rate, S21 is the transmission coefficient, and S11 is the reflection coefficient. When electromagnetic wave incidents into the EMA, because the bottom ITO has a very small square resistance, most of the energy will be reflected by the bottom ITO film and dissipated by the absorber, thus the formula can be simplified as:Using the help of the Advanced Design System (ADS) software, the values of the circuit components can be obtained for broadband performance as shown in Table 2.
Table 2. Design parameters
In addition, Fig. 3 compares the absorption rate obtained by full wave simulation and the ECM. It can be seen that the two absorption rate curves are almost coincident, which proves the effectiveness of the ECM.
2.3 Surface current analysis
In order to better understand the absorption mechanism, Fig. 4 shows the surface current distribution on each layer at two frequencies of 4.7 GHz and 13.7 GHz, where the absorption rate reaches its maximum. It can be seen from Fig. 4(a) that at 4.7 GHz, the current distribution on each layer is strong, in which the current is distributed upwards on the bottom layer and downwards on the other three layers (Here we default upward to the north direction). Therefore, a magnetic resonance is formed between the bottom layer and each square patch layer. Also because the current on the other layers is in the same direction and forms a closed loop, it forms an electric resonance. Similarly, in Fig. 4(b), the current distributions of the top and bottom layers are downwards, while the current distributions of the other two layers are upwards. At this time, an electric resonance and two magnetic resonances are generated inside the EMA. In summary, when the current distribution on the surface is large, the ohmic loss on the resistive film will also increase thus convert more electromagnetic energy into heat.
Fig. 4. Simulated current distribution on the surface of each layer at: (a) 4.7 GHz, (b) 13.7 GHz. From left to right: top layer, second layer, third layer and bottom layer.
2.4 Angular simulation
Figure 5 shows the simulated absorption rate of the EMA under TE and TM polarizations. It can be seen that the absorption rate is more than 90% in 3.72-42.42 GHz (relative bandwidth 165.75%) in both polarizations, covering C-, X-, ku-, k-, ka- bands. This proves the EMA to be ultra-wideband and polarization stable. In addition, the absorption rate of the EMA under different incident angles are also presented and showed to be stable within the incident angle of 45°.
Fig. 5. Simulated absorption rate under different incident angles. (a) TE polarization (b) TM polarization
3. Experimental verification
To verify the ultra-wideband absorption characteristics of the proposed EMA, a sample is fabricated as shown in Fig. 6(a), in which the magnetron sputtering technique is used to etch ITO on the transparent substrate PET. The sample consists of 60 × 60 units with a total size of 350mm × 350mm. The optical transmittance of the sample is measured using the light transmittance instrument LH-221, as shown in Fig. 6(b). By taking 10 points at equal intervals on the four sides of the sample and measuring their light transmittance, finally the optical transmittance is obtained (by taking the average of the measuring values) to be 43.5%.
Fig. 6. Optically transparent samples and transmittance measurement. (a) Transparent ultra-wideband EMA sample (b) Measuring optical transmittance.
The reflection/transmission coefficient of the sample is measured in an anechoic chamber, as described in Fig. 7. Because the EMA has ultra-wideband characteristics, the measurement is done by separating the bandwidth into different segments of 3-18 GHz, 18-26.5 GHz and 26.5-40 GHz (this band is not measured due to lack of high frequency measurement equipment).
Fig. 7. Measurement configuration of the reflection/transmission coefficient in an anechoic chamber. Left: Reflection measurement. Right: Transmittance measurement.
As illustrated in Fig. 8, we have measured both the transmittance (S21) and reflectance (S11) parameters under two conditions: 1 a metal plate; 2 the designed absorber. Then using:
Fig. 8. Measured Absorption rate under (a) TE polarization and (b) TM polarization. Left two subfigures: 3-18 GHz. Right two subfigures: 18-26.5 GHz.
During angular measurement, due to the lack of an arched measurement system, to increase the accuracy of measurement, the main lobe of the antenna should point at the center of the absorber during each measure. Therefore, during the measurement of each angle, we have carefully changed the angle between the transceiver antenna and the absorber and adjust the position to make sure the phase center of the antenna pointing at the center of the absorber, then redo the measure as described for vertical incident case. Using this way, we tried our best to make sure the measured results are accurate.
As previously discussed, only the first two frequency bands are measured and the results are shown in Fig. 8. As shown in Fig. 8(a) and (b), the measured absorption rates agree well with the simulation results under different incident angles in both the TE and TM polarizations. To prove the advantage of our design, Table 3 compares proposed EMA with similar designs proposed in recent years. It can be seen that this work has better performance in balancing bandwidth and thickness, at the same time the characteristics of transparency and angular stability are not compromised.
Table 3. Performance Comparison with published EMAsa
4. Conclusion
This paper proposes an ultra-wideband EMA composed of optically transparent resistive ITO films and PET layers. Simulation shows that the 90% absorbing band covers 3.72-42.42 GHz (relative bandwidth 165.75%) and keeps stable in the angular range of 45° under both TE and TM polarizations. Due to measurement conditions, only 3.72-26.5 GHz are measured and the results agrees well with the simulations. This work presents our effort on pushing the EMA techniques towards ultra-wideband and light transparency for both potential civil and military applications.
Funding
National Natural Science Foundation of China (NO. 61871219); Nanjing University of Aeronautics and Astronautics (xcxjh20210401, xcxjh20210410); Jiangsu Provincial Key Laboratory of Electrochemical Energy Storage Technologies, Nanjing University of Aeronautics and Astronautics (NJ20210002).
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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