In this paper, a transparent metamaterial absorber (MA) loaded with water substrate is presented, which can simultaneously achieve enhanced broadband microwave absorption and tunable infrared radiation. As a proof, the indium tin oxide (ITO) films are first introduced here as a frequency selective surface (FSS) on the top layer and reflective backplane on the ground layer. Next the distilled water combined with the polymethyl methacrylate (PMMA) substrate is employed as a hybrid substrate in the middle. Simulation and experimental measurements show that the transparent water-substrate MA can achieve broadband microwave absorption with efficiency over 90% in the frequency band of 6.4-23.7GHz, and the proposed hybrid substrate has almost no influence on its original transmittance. Moreover, owing to the available water circulation system, the infrared radiation of the proposed MA is also demonstrated to be controlled by the temperature of the injected water. Based on its multifunction and high performance, it is expected that the proposed design may find potential applications, such as glass window of stealth equipment, electromagnetic compatible buildings/facilities, etc.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In the past few years, the electromagnetic absorbing materials have always been investigated and innovated due to their extensive application, such as radar stealth , electromagnetic compatibility [2,3], energy harvesting [4–6], thermal emitter [7,8], imaging system [9–11], and so on. With the rapid development of metamaterials, the subwavelength structure was also introduced to electromagnetic wave absorption, and the according metamaterial absorber(MA) was demonstrated to obtain more diversified absorption performance. In fact, the perfect MA was firstly proposed as a new artificial medium which achieved near unity absorption at a certain frequency . The corresponding metal-dielectric-metal construction can excite and adjust the electric and magnetic resonances simultaneously and then contributed the desired impedance matching. Inspired from the three-layered construction, a series of single-, dual-, and multi-band absorbers were flexibly gained [13–18]. Meanwhile, more and more attention was paid to the achievement of broadband absorption due to the requirements in practical application. On the one hand, a direct way which attempted to overlap the multiple absorption peaks in spectrum was proposed here to achieve desired broadband absorption. The multiple resonators with deliberately regulated period length were assembled together on the same plane [19,20] or stacked together in the multilayer construction [21–23]. On the other hand, with the help of outer elements, the MAs can be further improved to acquire desired broadband absorption, such as loading lumped resistors to the metallic resonators [24,25], replacing the dielectric substrate with magnetic materials [26–28] or using all dielectric materials as resonators [29–31].
In recent years, transparent broadband MA attracted our attention due to its great application in the window of stealth equipment and electromagnetic compatible construction. Based on this, a lot of attempts were proposed to achieve effective integration of broadband microwave absorption and high optical transparency [32–35]. Guo et al. developed an optically transparent and flexible MA based on aluminum-wire-grid resonator and PMMA spacer . The as-developed MA achieved broadband absorption with efficiency over 90% in 5.8-12.2GHz and averaged optical transmittance of about 62%. Then, Cui et al. carried out a simple implement of transparent broadband MA by replacing aluminum-wire-grid resonator with ITO FSS . Moreover, Guan et al. developed ITO as standing-up resonators arranging on reflective backplane periodically to achieve broadband microwave absorption as well as high optical transparency, which was demonstrated to be superior to the former multi-layered construction . However, the former researches in the literature concentrated on the optimization of subwavelength structure, and less attention was paid to the transparent substrate. In fact, distilled water has a frequency-dispersive permittivity and high transmittance characteristic, which can be seen as a good candidate for further enhancement of broadband microwave absorption in transparent MA.
On illumination, a comprehensive scheme of transparent water-substrate MA is proposed in this paper, which can simultaneously achieve enhanced broadband microwave absorption and tunable infrared radiation. In implement of the proposed MA, the ITO films are firstly introduced here as FSS on the top layer and reflective backplane on the ground layer. Then, the distilled water combined with PMMA substrate is employed here as a hybrid substrate in the middle. Due to the frequency dispersive permittivity of distilled water, the as-proposed MA can achieve broadband microwave absorption with the efficiency over 90% in the frequency band of 6.4-23.7GHz. Then, the proposed hybrid substrate is also demonstrated to have almost no influence on its original transmittance. In addition, owing to the available water circulation system, the infrared radiation of the proposed MA can be easily controlled by the temperature of injected water. Based on its multifunction and high performance, it is expected that the proposed attempt will be popularized in the window of stealth equipment and electromagnetic compatible construction.
2. Broadband microwave absorption
2.1 Designs and simulations
The schematic of the transparent water-substrate MA is depicted in Fig. 1(a). The as-proposed MA is constructed based on the combination of ITO FSS, PMMA substrate, water substrate and ITO backplane. Due to the fluid characteristic, the distilled water needs to be packaged into a dielectric container for the practical application. Thus, the spacer is designed as a hybrid substrate consists of dielectric substrate and water substrate. The thicknesses of dielectric substrate and water substrate are dp and dw, respectively. On the top layer, the ITO FSS consists of square-shaped and cross-shaped units are printed on an ultrathin polyethylene terephthalate(PET) substrate, which is expected to achieve desired impedance matching during a broad frequency band. The thickness of the PET substrate is df. The length and the width of the cross-shaped unit are l and w. The length of square-shaped is a. The two kinds of unit are assembled together with the dimension of p. On the ground layer, the integrated ITO film is also used here as reflective backplane. The sheet resistances of the top-layered ITO FSS and bottom-layered ITO backplane are R1 and R2, respectively. Meanwhile, the two ducts are set between the inside and out space in the water-substrate MA for the circulation of distilled water, which is also shown in Fig. 1(b). In the design process, the complex permittivity of distilled water is provided by theoretical model under the room temperature(23°C) and a standard atmospheric pressure. The permittivity of the PMMA is 2.25(1-j0.01), while the permittivity of the PET is 2.8(1-j0.03). The simulation is carried out by the CST MWS software. The absorptive efficiency of the water-substrate MA under normal incidence can be defined as A(ω) = 1-R(ω)-T(ω) = 1-|S21|2-|S11|2, where A(ω), |S11|2 and |S21|2 are the absorbance, reflectivity and transmissivity, respectively. Due to the minimized sheet resistance of ITO backplane with R2 = 8.0Ω/sq, the averaged transmissivity for the normal incidence is about 0.0013, which is approximated as a specular reflection. When giving the other parameters as follows: df = 0.175mm, dp = 2.5mm, dw = 1.0mm, l = 4.7mm, w = 1.5mm, a = 7.6mm, p = 12.5mm, and R1 = 85.0Ω/sq, the proposed transparent water-substrate MA can achieve broadband microwave absorption with the efficiency more than 90% in the frequency band of 6.4-23.7GHz, as shown in Fig. 2(a). Then, the simulated power loss of the constitutive component in the proposed MA is also calculated in Fig. 2(b). From the calculated results, it is obvious that the ITO FSS on the top layer contributes the major power loss in the water-substrate MA, while the ITO backplane takes effect only for the low frequency band of 4.0-10.0GHz. Moreover, the water substrate used here also contributes the partial power loss for the frequency above 6.0GHz. Thus, the three components work together contributing the desired broadband microwave absorption. Figure 3 also shows the simulated absorption spectra of the water-substrate MA under different oblique TE and TM incidences. With the increased angle of TE incidence from 0 to 50°, the proposed MA can still achieve broadband absorption with the efficiency over 80% during the operating frequencies of 6.4-23.7GHz. By comparison, for the oblique incidence of TM wave within 70°, the proposed MA always exhibits broadband absorption with the efficiency over 90%.
2.2 Results and discussions
To illustrate the enhanced broadband microwave absorption, the contrast between the proposed transparent water-substrate MA and the other models are discussed here. Figure 4(a) shows Model 1 of the transparent water-substrate MA which is constituted by ITO FSS, hybrid substrate and ITO backplane. Herein, the hybrid substrate consists of water substrate and PMMA substrate. Figure 4(b) shows model 2 consists of ITO FSS and hybrid substrate. Figure 4(c) shows model 3 of original PMMA-substrate MA which is constituted by ITO FSS, PMMA substrate and ITO backplane. In order to get a clear contrast, the hybrid substrate in model 1 and model 2 should have same thickness with the PMMA substrate in model 3, and the other structural parameters of the three models should be consistent. Figure 4(d) shows the simulated absorption spectra of the three models, the black line, red line and blue line reflect the model 1, model 2 and model 3, respectively. Comparing the absorption spectra between model 1 and model 2, it is obvious that their broadband absorption spectra are consistent during the high frequency band of 12.8-23.7GHz, but the first absorption peaks at lower frequency is not achieved in model 2 when removing the reflective backplane. The following comparison of model 1 and model 3 demonstrate that the combination of upper ITO FSS and ground ITO backplane directly inspires the first absorption at lower frequency. Due to hybrid substrate in model 1 has a larger equivalent permittivity than the PMMA substrate in model 3, the first absorption peak frequency of model 1 is necessarily lower than model 3. Thus, it can be concluded that the hybrid substrate used here as spacer in transparent MA cannot only provide an controllable equivalent permittivity but also make full use of frequency dispersive permittivity to further enhance its broadband microwave absorption. Meanwhile, the simulated absorption spectra of the proposed MA by varying the thickness dp and dw in hybrid substrate are also discussed in Figs. 4(e) and 4(f). With the increased thickness dp in PMMA substrate, the lower-frequency absorption peak will be almost unchanged, while the other two absorption peaks will gradually move to higher frequency. Similarly, with the increased thickness dw in water substrate, the first absorption peak will gradually move to low frequency while the two absorption peaks at high frequency will be almost unchanged. Thus, in the proposed MA, the ITO FSS and the water substrate spaced by the PMMA substrate can be seen as absorbing structure which achieves two absorption peaks at high frequencies, while the ITO FSS and the ITO backplane spaced by the hybrid substrate can also be seen as another absorbing structure which achieves lower-frequency absorption. The three absorption peaks overlapped together contributing the enhanced broadband microwave absorption.
In order to further explore the physical principle of the proposed transparent water-substrate MA, the surface current distributions at the absorption peak frequencies of 7.3, 18.0, and 23.0GHz are given in Fig. 5. Due to the upper ITO FSS is closed to the backplane, near field coupling will be induced which make it possible to generate anti-parallel currents between the top and ground layers. From the cross-sectional view of the surface current distributions in Figs. 5(a) and 5(b), the excited surface current on the upper ITO FSS is indeed anti-parallel to the ground backplane. For the higher frequency of 23.0GHz, the space is not thin enough to its wavelength, and the excited surface current almost concentrates on the upper surface in Fig. 5(c). Meanwhile, the current density is also changed with different absorption peak frequencies. From the top view, Fig. 5(a) shows that the excited current evenly distributed on the upper surface of ITO FSS at the first absorption peak frequency of 7.3GHz. Figure 5(b) shows that the excited current is enhanced in the center of the square-shaped unit at the second absorption peak frequency of 18.0GHz. Figure 5(c) also shows that the excited current is enhanced in the center of square-shaped and cross-shaped units simultaneously at the third absorption peak frequency of 23.0GHz. In general, the enhanced surface current on the upper surface combining with the ohmic loss from the ITO FSS contribute to the highly effective absorption performance, following the relation Ploss = I2·R, where I is the induced current and R is the effective resistance.
On top of the highly effective absorption at the three resonant frequencies, the bandwidth of each absorption peak can also be seen as another important factor for the achievement of broadband absorption in the proposed MA. As shown in Fig. 6(a), when increasing the sheet resistance of ITO FSS R1 from 5 to 85Ω/sq, the Q factor of the three absorption peaks will be gradually decreasing leading to the enhancement of absorption bandwidth based on its definition Q = ω0 × PT/PL = f0/Δf, where ω0 and f0 are the resonant frequencies. PT and PL are stored and dissipated energy. Δf is the absorption bandwidth. The three absorption peaks with enhanced bandwidth overlap together contributing the continuous broadband absorption performance during the frequency band from 6.4 to 23.7GHz. Thus, the sheet resistance R1 of ITO FSS is demonstrated to be an important parameter for the achievement of broadband microwave absorption.
To give an experimental demonstration, a sample of transparent water-substrate MA is fabricated. As the insert shown in Fig. 6(b), the dimension of the fabricated sample is 230 × 230mm2, which consists of 17 × 17 unit cells. In the fabrication process, one ITO film backed with ultrathin PET substrate was etched into the designed FSS structure by the lithography technology. Another ITO film backed with glass substrate was used here as backplane. Then, the PMMA substrate was constructed as an open container by the laser beam cutting technology so as to fill the distilled water. In the next assembling process, the three components were glued together, and the two ducts were set here between the inside and out space. After injecting the distilled water, the sample of transparent water-substrate MA was finally obtained. The experimental measurement was carried out in microwave anechoic chamber. The system is based on an Agilent E8363B network analyzer with four pairs of broadband antenna horns respectively working in the frequency bands of 4-8, 8-12, 12-18 and 18-26GHz. In the measurement process, the distilled water used here is always under the temperature of 23°C and a standard atmospheric pressure during the measurement process. At last, the measured and simulated absorption spectra are illustrated in Fig. 6. As expected, the good agreement between the simulated and measured results validates that transparent MA incorporated with water substrate can further enhance its broadband microwave absorption performance.
3. Optical transmittance
ITO film printed on transparent substrate has always been the best choice for transparent conductive film. Thus, the combination of ITO FSS and ITO backplane spaced by transparent substrate has been demonstrated to be an effective approach to transparent MA. In pursuit of broadband microwave absorption, the multi-layered construction of ITO FSS is proposed. However, due to the interference effect, the increased layer of ITO FSS inevitably leads to the decrease of optical transparency . To get an effective integration of broadband microwave absorption and high transmittance characteristic, water substrate is introduced into transparent MA. The aforementioned discussion has demonstrated that the absorption bandwidth can be effectively broadened in the proposed MA. In this part, our attention concentrates on it transmittance characteristic. Figure 7(a) shows the photographs of near and distant views through the fabricated sample. Both of them are clear enough to our naked eye. Then, to get more accurate experimental demonstration, the optical transmittance spectrum is measured by the spectrometer. For comparison, the fabricated samples of the water-substrate MA and PMMA-substrate MA corresponding to the model 1 in Fig. 4(a) and model 3 in Fig. 4(c) are simultaneously fabricated here. Due to the periodic pattern of ITO FSS on the top layer, the measured averaged optical transmittance of each sample needs to be calculated by the weighted average method of the area. As shown in Fig. 7(b), the red line reflects the averaged optical transmittance spectrum of the water-substrate MA while the black line reflects the PMMA-substrate MA. After loading with the water substrate, the optical transmittance is slightly reduced during the wavelength range of 400-550nm while kept changed during the wavelength range of 550-800nm. Calculated results show that the averaged optical transmittances of the water-substrate MA is 71.1% in a wavelength range of 400-800nm, about 1.7% lower than the PMMA-substrate MA. The slight difference is hard to be noticed by our naked eye. Thus, it can be concluded that the water substrate introduced to the transparent MA can enhance broadband microwave absorption while has almost no influence on its original transmittance.
4. Tunable infrared radiation
The capacity to dynamically tune the radiation property of a material in infrared frequency is becoming an exciting prospect due to the extended range of possible applications [36–38]. In fact, the infrared radiation of a material is more conveniently controlled via its chemical composition or kinetic temperature, following the relation M = ε(T)·σ·T^4, where ε(T) is infrared emissivity, T is the temperature. In the proposed MA, the water substrate compared to other constitutive materials has a greater specific heat capacity. The corresponding infrared radiation of the proposed MA is almost controlled by the temperature of the water substrate and less influenced by the environmental temperature. To give a simple experimental demonstration, the fabricated samples of water-substrate MA and PMMA-substrate MA were placed in the oven simultaneously under the environmental temperature of 45°C, as shown in Fig. 8(a). Then, a thermal infrared camera was employed here to record the infrared radiation imaging of the two samples after 1 minute, 2 minutes, and 3 minutes. As shown in Fig. 8(b), when the two fabricated samples were placed in the oven for 1 minute, both of them exhibited low infrared radiation. After 3 minutes, the infrared radiation of the PMMA-substrate MA was enhanced while water-substrate MA still had a low radiation. After 5 minutes, with the increased temperature of water, the water-substrate MA also exhibited enhanced infrared radiation, but its infrared radiation was still lower than the adjacent PMMA-substrate MA. Based on the measured results, it is obvious that the temperature of water substrate is demonstrated to have a great influence on the practical infrared radiation. Meanwhile, owing to the available circulation of the injected water, the infrared radiation of the proposed transparent water-substrate MA can be easily controlled by the temperature of injected water.
In conclusion, we have demonstrated that distilled water can be used as a substrate in transparent MA for further enhancement of broadband microwave absorption as well as tunable infrared radiation, as confirmed by both of the calculated results and the experimental measurements. Compared to the multi-layered construction in the literature, the proposed water-substrate MA gets an effective integration of broadband microwave absorption and high transmittance. Simulation and experimental measurement show that the proposed MA can achieve broadband microwave absorption with efficiency more than 90% in the frequency band of 6.4-23.7GHz, and the water substrate introduced here has almost no influence on its original transmittance. Meanwhile, Owing to the available water circulation system, the infrared radiation of the proposed MA is also controlled by the temperature of injected water. Based on its multifunction and high performance, it is expected that the as-presented MA obtained in our work will have a great application prospect, such as glass window of stealth equipment, electromagnetic compatible buildings/facilities, and ETC systems.
National Natural Science Foundation of China (Grant No. 61471388 and No. 61671467), the China Postdoctoral Science Foundation (Grant No. 2015M572561), the Foundation for the Author of National Excellent Doctoral Dissertation of the People’s Republic of China (Grant No. 201242), and the Shanxi Province Scientific and Technology Innovation Team Foundation of China (Grant No. 2014KCT-05).
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