Open Access
Add to BibSonomy
Abstract
In this paper, we propose an interesting thermally tunable broadband metamaterial absorber based on ionic liquids at the microwave band, which has distinct modulation characteristics in different frequency bands. Numerical simulation results show that the absorption decreases with the increase of temperature in the low-frequency band from 2-10GHz, which decreases to 60% at 100 °C. Meanwhile, the absorption increases with the increase in temperature in the high-frequency band from 25GHz to 48GHz. In addition, the absorber still has good broadband absorption without the metal substrate, and the absorption reaches more than 80% in the frequency band of 13.96-34.10GHz. As an all-dielectric metamaterial absorber, its absorption increases with the increase in temperature, which reaches more than 90% in the range of 20.44-50GHz at 100 °C. At last, the designed metamaterial absorbers have been fabricated based on ionic liquids, and experimental results are presented to demonstrate the validity of the proposed structure. Furthermore, the simple design and wide frequency tuning range of the absorbers can promise a great potential application in sensors, detection, and frequency-selective thermal emitters.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
IN 2008, it immediately became a research hotspot after the first perfect metamaterials absorber (MMA) was proposed by Landy [1]. With the rapid development of MMAs, which have always been widely used such as electromagnetic imaging [2,3], polarizers [4–6], stealth [7–9], and radio frequency [10,11]. Many MMAs have been proposed and demonstrated in the microwave, terahertz, infrared, and visible light to ultraviolet frequencies [12–16]. Unfortunately, the absorption bandwidth and strength of these reported MMAs are usually narrow and fixed, which greatly limited their practical applications. Therefore, to fully extend the functionality of MMAs, it needs to dynamically control the resonant frequency.
In the microwave frequency band, the tunable MMAs are mainly achieved by the following methods. Firstly, introduce lumped elements in an array of metal resonant elements. Fang J and his peers combined active metamaterials and passive absorbing honeycombs by adjusting the bias voltage of PIN diodes to achieve active tuning of radar absorption [17]. Wu T et al. proposed a band splicing strategy with a varactor annular split ring (CSR), which realized the controllability of the two frequency bands in the on and off states of the pin [18], which greatly expanded the tunable bandwidth. Secondly, the performance of the absorber can be dynamically controlled by manipulating the graphene Fermi level with an external electric field. Zhang J et al. proposed a graphene sandwich-structured metasurface, the tunable absorption with the dual, single, and broadband are realized by applying a bias voltage to graphene layers [19]. Using the controllability of the bias voltage to the impedance, the dynamic modulation of the absorbing frequency, bandwidth, and amplitude is achieved [20]. Thirdly, the use of liquid crystal to realize the regulation of electromagnetic waves. In 2017, Deng G et al. established a liquid crystal tunable MMA in the microwave F-band and realized a 6.4% shift in the effect of resonant frequency by using the influence of voltage on the orientation of the liquid crystal [21]. Lv J F achieved a shift of 7.8% of the center frequency by adjusting the bias voltage from 0 to 1 [22]. However, these tunable MMAs suffer from some drawbacks, such as the frequency tuning range being small, the structure being complex and difficult to construct, and the external control device being complex. Therefore, it is very necessary to design a new type of tunable MMA with a wide frequency tuning range and simple structure.
Based on the frequency dispersive permittivity of water at microwave frequencies, Pang Y et al. realized the broadband and thermally tunable absorption performance [23]. Kong X's team also proposed a novel water-based reconfigurable selective grating to achieve thermal tuning above the transmission band, which exhibits the characteristics of absorbing and reflecting electromagnetic waves under water-filled and non-water-filled conditions, respectively [24]. Simultaneously, Kong X et al. proposed a liquid reconfigurable stealth window composed of a metamaterial absorber, which not only has a high absorption rate but also realizes the mutual switching of the transmission band and the absorption band under the differential dosage of water and alcohol [25]. As a liquid, although the above-mentioned water has the characteristics of ultra-broadband absorption, low cost, and environmental friendliness, it also has the disadvantages of poor stability, easy condensation and volatilization, and easy pollution. It is found that ionic liquids, as new soft media, not only have broadband absorption properties but also dielectric properties that are sensitive to temperature changes.
Based on the sensitivity of dielectric properties to temperature, an ionic liquids-based broadband absorber is designed to achieve dynamic regulation in different frequency bands by changing the temperature. In the low-frequency band, the absorption rate decreases with the increase in temperature; the absorption rate increases with the increase of the temperature in the high-frequency bands, which shows the opposite trend in the low-frequency band. When the metal substrate is removed, the all-dielectric metamaterial absorber can still maintain over 80% absorption at 13.96∼34.1GHz, and the absorption rate increases with the increased temperature. At 100 °C, the absorption rate remains above 90% in the frequency band of 20.44-50GHz. Compared with reported tunable MMAs, the MMAs demonstrated to have the advantages of large frequency tuning range, frequency selective control function, and simple unit structure design.
2. Design and simulation
2.1 Thermally tunable absorber
The ionic liquids-based MMA is a sandwich structure consisting of three layers of resin and a bottom metal plate, and the ionic liquid ([EMIM][N(CN)2]) is then injected into the cube dielectric as shown in Fig. 1. Besides, the dielectric cover situated upon ionic liquids can improve impedance matching and enhance absorption effectively. The side length and height of the ionic liquids are defined as q and d, respectively. The thicknesses of the top dielectric cover, wall width, and dielectric substrate are defined as t2, k, and t1, respectively. The bottom of the resin is a layer of metallic copper with a thickness of 0.1mm, which acts as a reflector. The x and $y$ direction is set as the boundary condition of the unit cell, the z direction is set as the open boundary, and the incident electromagnetic wave is irradiated from the positive direction of the $z$-axis into the absorber. The dielectric constant of resin is $\varepsilon = 3 \cdot (1 \pm 0.02)$, the conductivity of metal copper is $5.8 \times {10^7}S/m$, and the simulation frequency band is set to 0-50GHz.
In order to improve the absorbing performance, the finite difference time domain method was used to discuss and optimize the structural parameters of the model. In the structure, the influence of the unit side length p, the ionic liquids side length q, and the ionic liquids depth d on the absorption is mainly considered.
When the side length of the absorber unit increases from 19mm to 21mm, the wave absorbing effect is shown in Fig. 2(a). With the increase of unit side length, the absorption rate of the absorption peak increases in the range of 2-10GHz and moves to high frequency gradually. It can be seen from the figure that the absorption rate remains above 90% when p = 20mm is used as the intermediate variable. At 35-50GHz, the absorption peak of the absorption rate shows a gradually decreasing trend, and the absorption rate drops by 80% at 21mm. Figure 2(b) shows changes in the absorbing performance of ionic liquids under different side lengths from 17.2mm to 18.8mm, which is opposite to the change of the parameter p. In the low-frequency band (2-10GHz), the absorbing effect becomes worse with the increase of q, and in the middle-high-frequency band, the absorption rate increases with the increase of q. When the thickness d of ionic liquids increases from 4mm to 6mm at a unit interval of 0.5mm, the absorption peaks at low-frequency and high-frequency shift to low frequency. The difference is that the absorption rate shows a downward trend at low frequencies, and the absorption rate is opposite at high frequencies, as shown in Fig. 2(c). In general, the resonance frequency and absorption performance are closely related to p, q, and d. Considering the experimental conditions and design requirements, p = 20mm, q = 18mm, and d = 4mm is finally selected.
Fig. 2. (a) Influence of parameter p on absorbing performance (b) Influence of parameter q on absorbing performance (c) Influence of parameter d on absorbing performance.
Previous studies have confirmed that temperature has a great influence on many physical and chemical properties of ionic liquids [26], in which temperature is closely related to dielectric properties. We measured the dielectric properties of ionic liquids [EMIm][N(CN)2] based on an open-ended coaxial probe system by using an Agilent Network Analyzer N5247A. As shown in Fig. 3, both the real and imaginary parts of ionic liquids [EMIm][N(CN)2] increase with increasing temperature. The imaginary part ${\varepsilon ^{^{\prime\prime}}}(\omega )$ shows the increased amplitude in the low-frequency band is significantly higher than that in the high-frequency band. The values depend on conductance and polarization, the conductivity loss is found to be dominant for dielectric loss in the low-frequency band, and the polarization loss plays a major role in high-frequency bands. This is because the increase in temperature can reduce the viscosity of anion, while the increase in temperature can reduce the viscosity, and enhance the mobility of free ions.
Fig. 3. Curves of the dielectric properties of ionic liquids as a function of temperature (a) real part (b) imaginary part.
Based on the variation of dielectric properties of ionic liquids with temperature, a thermally tunable absorber is obtained as shown in Fig. 4. Obviously, it can be obtained that the absorption peak and bandwidth can be changed correspondingly according to the change in the environment temperature. Moreover, the MMA has distinct modulation characteristics in the two frequency bands of 2-10GHz and 35-48GHz. Figure 5 shows the changes in the absorption peaks in the above two frequency bands more clearly. As shown in Fig. 5(a), with the increase in temperature, the absorbing performance will decrease greatly and the absorption peak shifts to low frequency. When the temperature is 100 °C, the absorbing rate at 5.55GHz decreases by 40%. Oppositely, the absorption rate at the frequency of 42.10GHz of the absorber increases from 80% to 1 as the temperature rises, as shown in Fig. 5(b).
Fig. 4. (a) Absorption performance of the MMA at different temperatures (b) Absorption intensity at different temperatures.
Next, the incidence angle and polarization angle of the absorber are simulated and analyzed. The influence on the absorption performance of TE and TM polarization is very small due to the high symmetry of the model, and the incident wave is TM polarization in this paper. As shown in Fig. 6, when the absorber increases from 0° to 30°, the change in absorption rate is small. With the increase of the incident angle, the absorption rate of the absorber was significantly reduced. When the angle gradually increases, the absorption rate of the full frequency band will decrease, and when the incidence angle is 60°, the absorption rate is about 70% in the frequency band of 7.2-44.1GHz, as shown in Fig. 6(a). However, the absorption performance is basically unchanged when the polarization angle is increased from 0° to 90° with a step width of 30° in Fig. 6(b), which is because of their high symmetry in structure.
Fig. 6. (a) The effect of different incident angles on the absorbing performance in TM mode (b) The effect of different polarization angles on the absorbing performance in TM mode.
The absorbance of the absorber is defined as $A(\omega ) = 1 - R(\omega ) - T(\omega )$, where $R(\omega )$, $A(\omega )$ and $T(\omega )$ are denoted as absorptivity, reflectance, and transmittance, respectively. As shown in Fig. 7, the transmission of the absorber is approximately 0 and can be ignored here. The MMAs have four absorption peaks in the microwave frequency band of 0-50GHz, and the corresponding frequencies are 5.55GHz, 21.35GHz, 34.10GHz, and 48.35GHz.
Fig. 7. Curves of reflection, transmission, and absorption of the absorber (Schematic diagram of energy loss in the $xoy$ plane).
Firstly, the two absorption peaks (21.35GHz, and 34.10GHz) exhibiting the broadband absorbing effect were analyzed. As shown in Fig. 8, the magnetic field is mainly concentrated in the ionic liquids at 21.35 GHz, forming a closed annular region, which makes the electric dipoles in it generate strong electric resonance, and then a larger energy loss occurs in the field. Moreover, it can be seen from Fig. 8(c) that in this frequency band, the energy loss is mainly concentrated in the upper half of the ionic liquids, and the absorption rate reaches 99%. Compared with the former, the magnetic field forms two electric dipoles in opposite directions inside the ionic liquids at 34.10 GHz. The superposition and cancellation of the two electric dipoles have a very strong electrical resonance, and the surface of the ionic liquids produces a “chessboard"-like energy loss.
Fig. 8. (a) Electric field, (b) magnetic field, (c) x-z energy loss density distribution at each frequency (d) x-y energy density distribution.
At 5.55GHz, the magnetic field is mainly distributed in the ionic liquids, and a closed electron flow is formed, resulting in a strong electric dipole. Different from 21.35GHz, the energy loss is mainly concentrated in the middle and upper part of the ionic liquids in the form of strips. The frequency corresponding to the last absorption peak is 48.35 GHz, which achieves an absorption rate of 99.5%. It is not difficult to find that four closed magnetic field vector rings are formed in the ionic liquids and the surrounding dielectric cavity, and the adjacent magnetic field vector rings have opposite directions, which still show the electric resonance effect of the electric dipole after superposition. By comparing the electric field, magnetic field, and energy loss distribution area, it is found that in the energy loss diagram, the electromagnetic waves are mainly consumed in the form of strips with alternating “red, green, and blue” colors.
In order to further analyze the thermally tunable characteristics of the absorber, the energy loss at different temperatures was observed, as shown in Fig. 9. At 5.55 GHz, the energy loss is mainly concentrated in the upper-middle region of the ionic liquids. The color of the red strips representing consumption gradually disappears, the energy loss gradually decreases, and the performance of the absorber gradually becomes weaker as the temperature increases. It shows that with the increase in temperature, the imaginary part of the complex permittivity of the ionic liquids increases gradually, and the impedance matching gets worse. At 42.10GHz, the energy loss gradually increases as the temperature increases, while the absorbing performance also becomes stronger. Meanwhile, it can be found that the energy loss is concentrated on the upper side of the ionic liquids and the inner side of the dielectric cover mainly.
2.2 All-dielectric temperature tunable absorber
The unit sample of the all-dielectric thermally tunable absorber is obtained by removing the metal substrate based on the thermally tunable absorber, the 3D schematic diagram and side cross-sectional view are shown in Fig. 10. It is not difficult to find that the all-dielectric absorber without the metal substrate also exhibits a good absorption effect of more than 80% in the frequency range of 15-35 GHz. Similar to the absorption performance exhibited by the thermally tunable absorber at 15-48 GHz, the absorption rate of this absorber increases with the increase in temperature, as shown in Fig. 11. At 100 °C, the absorption rate reaches more than 90% in the range of 20.44-50GHz.
Fig. 11. (a) Curves of absorbing wave at different temperatures (b) Plot of absorbing wave intensity at different temperatures.
Because this type of absorber is only composed of a resin cavity and ionic liquids without any metal components, which cannot ignore the influence of transmittance for the absorption, as shown in Fig. 12. It can be seen from the figure that the frequency range of 12.25-35.39GHz, the reflection loss is always below -10dB and the transmission loss is greater than -8dB.
The energy loss at different temperatures and different planes was observed to further analyze the temperature tunable characteristics of the absorber, taking the energy loss diagram of 27.50GHz as an example in Fig. 13. At 27.50 GHz, the energy loss is mainly concentrated in the upper-middle region of the ionic liquids. The energy loss gradually increases as the temperature increases, and the performance of the absorber gradually becomes stronger. It shows that the dielectric properties of ionic liquids increase gradually with the increase in temperature, and increase the impedance matching.
3. Experiment results and discussion
Two kinds of thermally tunable MMAs were made and tested to prove the broadband and thermally-adjustable absorption performances. As shown in Fig. 14(a), We fabricated the proposed absorber containing 9 × 9 units via 3D printing. Then injected the ionic liquids into the square groove of 3D printing and covered the groove bracket. Finally, the copper foil tape is attached to the bottom to be used as a metal back plate. Figure 14(b) is the operating environment of the experiment, in which the absorbing model is set in the middle of the sample measuring platform, and two horn antennas are installed on the test interface in three frequency bands (3-20GHZ, 20-26GHZ, 26-40GHZ) respectively, and connected with the Agilent Vector Network Analyzer (Agilent N5247A) through RF cables. At the early stage of the experiment, a metal plate of the same size as the model needs to be used for normalization calibration. The model to be tested heated to a specific temperature is immediately placed on the sample test table, and its reflection parameters are obtained and recorded. Simulation results and experimental results when the temperature varies from 20 °C to 100 °C in Fig. 15(a). It can be found that the absorption rate decreases with the increase in temperature at 3-10 GHz; The absorption rate decreases from 1 to 0.6 at 5.5GHz, and the experimental results are consistent with the simulation results. In the range of 17-40GHz, the absorption rate increases with the increase in temperature, which remains above 90%, with a good absorption effect. It also was verified the variation of the all-dielectric absorber with temperature through experiments simultaneously. It can be seen from Fig. 15(b) that the absorption rate reaches more than 90% at 22.44-50GHz and 100 °C as the temperature increases, which is similar to the simulation results. Compared with the simulation, the cut-off frequency of the absorbing curve has a slight blue shift, accompanied by the appearance of burrs. The reasons for the error are as follows: The specific heat capacity of solid and liquid is different, which leads to the slight difference in temperature between the medium and ionic liquids after heating; The ionic liquids in the groove are not evenly distributed.; The ionic liquids are mixed with trace impurities(such as water), which affect the dielectric properties ; The surface of the model is not uniform enough. In addition, the test between 40-50GHz cannot be realized due to the limitation of experimental conditions, but from the trend of the curve, we can predict that there will be absorption effects matching the simulation results in the range to be tested.
Fig. 15. Comparison of simulation and experiment(a) absorber with metal substrate and (b) all-dielectric.
4. Conclusions
In summary, a thermally tunable broadband MMA based on ionic liquids at microwave bands are designed, simulated, and measured. There are distinct modulation characteristics in different frequency bands, the absorption decreases with the increase of temperature in the low-frequency band from 2-10GHz, but the absorption increases with the increase in temperature in the high-frequency band from 25GHz to 48GHz. In addition, an all-dielectric MMAs is obtained when the underlying metal is removed, which remains sensitive to temperature modulation. Meanwhile, The absorption rate increases with the increase of temperature, and the absorption rate reaches above 90% in the range of 20.44-50 GHz at 100 °C. Compared with the reported tunable MMAs, the absorber has the advantages of a wide frequency tuning range and a simple structure.
Funding
National Natural Science Foundation of China (62061025).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (62061025).
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
References
1. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef]
2. N. I. Landy, C. M. Bingham, T. Tyler, N. Jokerst, D. R. Smith, and W. J. Padilla, “Design, theory, and measurement of a polarization-insensitive absorber for terahertz imaging,” Phys. Rev. B 79(12), 125104 (2009). [CrossRef]
3. J. Shabanpour, S. Beyraghi, and H. Oraizi, “Reconfigurable honeycomb metamaterial absorber having incident angular stability,” Sci. Rep. 10(1), 14920–8 (2020). [CrossRef]
4. F. Ahmed, T. Hassan, and N. Shoaib, “Comments on “an ultrawideband ultrathin metamaterial absorber based on circular split rings”,” Antennas Wirel. Propag. Lett. 19(3), 512–514 (2020). [CrossRef]
5. H. F. Zhang, H. Zhang, Y. Yao, J. Yang, and J. X. Liu, “A band enhanced plasma metamaterial absorber based on triangular ring-shaped resonators,” IEEE Photonics J. 10(6), 1–9 (2018). [CrossRef]
6. S. Guo, C. Hu, and H. Zhang, “Unidirectional ultrabroadband and wide-angle absorption in graphene-embedded photonic crystals with the cascading structure comprising the Octonacci sequence,” J. Opt. Soc. Am. B 37(9), 2678–2687 (2020). [CrossRef]
7. R. Yahiaoui, J. P. Guillet, F. de Miollis, and P. Mounaix, “Ultra-flexible multiband terahertz metamaterial absorber for conformal geometry applications,” Opt. Lett. 38(23), 4988–4990 (2013). [CrossRef]
8. T. Beeharry, R. Yahiaoui, K. Selemani, and H. H. Ouslimani, “A dual layer broadband radar absorber to minimize electromagnetic interference in radomes,” Sci. Rep. 8(1), 382 (2018). [CrossRef]
9. H. Zhang, H. F. Zhang, G. B. Liu, and H. M. Li, “Ultra-broadband multilayer absorber with the lumped resistors and solid-state plasma,” Results Phys. 12, 917–924 (2019). [CrossRef]
10. H. F. Zhang, H. Zhang, J. Yang, and H. M. Li, “Unidirectional absorption in a three-dimensional tunable absorber under oblique incidence,” Plasmonics 14(4), 985–991 (2019). [CrossRef]
11. H. Kurihara, Y. Hirai, K. Takizawa, T. Iwata, and O. Hashimoto, “An improvement of communication environment for ETC system by using transparent EM wave absorber,” IEICE Trans. Electron E88-C(12), 2350–2357 (2005). [CrossRef]
12. J. Tak and J. Choi, “A Wearable Metamaterial Microwave Absorber,” Antennas Wirel. Propag. Lett. 16, 784–787 (2017). [CrossRef]
13. A. Mohanty, O. P. Acharya, B. Appasani, S. K. Mohapatra, and M. S. Khan, “Design of a Novel Terahertz Metamaterial Absorber for Sensing Applications,” IEEE Sensors J. 21(20), 22688–22694 (2021). [CrossRef]
14. N. Lee, T. Kim, J. S. Lim, I. Chang, and H. H. Cho, “Metamaterialselective emitter for maximizing infrared camouflage performance with energy dissipation,” ACS Appl. Mater. Interfaces 11(23), 21250–21257 (2019). [CrossRef]
15. C. Enkrich, M. Wegener, S. Linden, S. Burger, L. Zschiedrich, F. Schmidt, and C. M. Soukoulis, “Magnetic metamaterials at telecommunication and visible frequencies,” Phys. Rev. Lett. 95(20), 203901 (2005). [CrossRef]
16. J. Tang and S. He, “A novel structure for double negative NIMs towards UV spectrum with high FOM,” Opt. Express 18(24), 25256–25263 (2010). [CrossRef]
17. J. Fang, J. Huang, Y. Gou, and Y. Shang, “Research on broadband tunable metamaterial absorber based on PIN diode,” Optik (Munich, Ger.) 200, 163171 (2020). [CrossRef]
18. T. Wu, W. Li, S. Chen, and J. Guan, “Wideband frequency tunable metamaterial absorber by splicing multiple tuning ranges,” Results Phys. 20, 103753 (2021). [CrossRef]
19. J. Zhang, Z. Li, L. Shao, and W. Zhu, “Dynamical absorption manipulation in a graphene-based optically transparent and flexible metasurface,” Carbon 176, 374–382 (2021). [CrossRef]
20. C. Huang, C. Ji, B. Zhao, J. Peng, L. Yuan, and X. Luo, “Multifunctional and tunable radar absorber based on graphene-integrated active metasurface,” Adv. Mater. Technol. 6(4), 2001050 (2021). [CrossRef]
21. G. Deng, T. Xia, S. Jing, J. Yang, H. Lu, and Z. Yin, “A tunable metamaterial absorber based on liquid crystal intended for F frequency band,” Antennas Wirel. Propag. Lett. 16, 2062–2065 (2017). [CrossRef]
22. J. F. Lv, C. Ding, F. Y. Meng, J. Q. Han, T. Jin, and Q. Wu, “A tunable metamaterial absorber based on liquid crystal with the compact unit cell and the wideband absorption,” Liq. Cryst. 48(10), 1438–1447 (2021). [CrossRef]
23. Y. Pang, J. Wang, Q. Cheng, S. Xia, X. Y. Zhou, Z. Xu, and S. Qu, “Thermally tunable water-substrate broadband metamaterial absorbers,” Appl. Phys. Lett. 110(10), 104103 (2017). [CrossRef]
24. X. Yan, X. Kong, Q. Wang, L. Xing, F. Xue, Y. Xu, and X. Liu, “Water-based reconfigurable frequency selective rasorber with thermally tunable absorption band,” IEEE Trans. Antennas Propag. 68(8), 6162–6171 (2020). [CrossRef]
25. X. Kong, W. Lin, X. Wang, L. Xing, S. Jiang, L. Kong, and M. Liu, “Liquid reconfigurable stealth window constructed by a metamaterial absorber,” J. Opt. Soc. Am. B 38(11), 3277–3284 (2021). [CrossRef]
26. F. Yang, J. Gong, E. Yang, Y. Guan, X. He, S. Liu, and Y. Deng, “Microwave-absorbing properties of room-temperature ionic liquids,” J. Phys. D: Appl. Phys. 52(15), 155302 (2019). [CrossRef]
