In this paper, broadband microwave absorbers utilizing water-based metamaterial structure elements have been proposed and investigated. We employ water into the metamaterial structure unit-cell of the absorber as primary resonant elements such as the water-droplet, or water-tube structure. By investigating the resonant modes and the coupling between the water elements and the surrounding dielectrics, it is found the inherent multi-resonance of the proposed metamaterial structures could result in a broadband microwave absorption. For water-droplets design, 90% microwave absorption has been achieved from 7.5 GHz to 15 GHz, while for water-tube design, a much broader bandwidth from 5 GHz to 15 GHz is obtained for nearly 90% microwave absorption. The broadband absorption performance has been verified by both full wave simulation and experimental measurement. We believe the proposed broadband water-based absorber may find some applications in microwave stealth and electromagnetic compatibility technology.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Microwave absorbers have been widely used in civil and military applications for several decades, such as in radar cross section (RCS) reduction, antenna beam forming or electromagnetic interference (EMI) reduction [1–3]. As the fast development of modern electromagnetic (EM) devices, to satisfy the compact and tight requirement for integration application recent researches have mainly focused on achieving lighter and thinner absorbers with broader working bandwidth. However, traditional EM wave absorbing materials, such as the ferro-magnetic materials used in the microwave frequency, mostly work on their high loss characteristics, which inherently have difficulties in reducing volume and weight [4-5]. Recently metamaterials have attracted plenty of interests to develop various novel EM devices due to their extraordinary properties [6–12]. Therein the emergence and fast development of EM absorbers utilizing metamaterials have led to a powerful impact on the traditional absorption material [13–27], which have gain the advantages of MMs such as electrically thin thickness [14, 19, 24], moderate weight and designable absorption band [22, 25]. Those advantages have great application potentials in EM sensors, detectors, energy harvesting and stealth technology [28, 29].
Since the first proposal of perfect metamaterial absorber by N. Landy, most of the researchers have focused on designing novel sub-wavelength metallic resonant structures, regarding that the inherent electric and magnetic resonances would bring wave impedance matching to that of free space, meanwhile the parasitic loss would damp the incident energy . However, such resonances are always highly dispersive, which inherently limits the absorption bandwidth, and prevents its uses in practical broadband applications. Subsequently many attempts have been proposed to improve the absorption bandwidth, such as developing multi-resonant unit-cells, utilizing vertical multilayer structures or introducing ohmic loss components into the resonators [30–32]. However, these efforts will usually increase the complexity and the cost of the device fabrication.
More recently all-dielectric metamaterials have been proposed to gain the advantages of broader bandwidth compared with metamaterials utilizing metallic resonant structures in case of their moderate dispersion property [33–35]. Among those chosen dielectrics, water, regarded as a ubiquitous existing dielectric material in the world, exhibits some specific properties for developing metamaterial devices. The real part of water’s dielectric permittivity rapidly decreases as frequency increases while the loss tangent significantly increases in the microwave region . Meanwhile, water is in liquid form at room temperature, which enable the possibility of constructing a flexible layout.
As the simplest and typical existing form of water in the environment, water droplets have also been investigated and considered as a candidate to construct tunable metamaterial structures including EM wave absorbers [36, 37]. By control the volume or temperature of the water-based meta-particles, their EM properties can be tuned [38–40]. Utilizing water as the primary resonant element to design EM absorbers would greatly decrease the commercial cost. However, so far the absorption bandwidths of the previous reported water-based EM absorbers are still not satisfactory. Aiming to obtain broader bandwidth meanwhile with easy fabrication technologies, we herein propose the EM absorber layouts based on water resonators and investigate their microwave absorption performance.
In this paper, following the metamaterial idea, firstly a broadband absorber based on periodic dielectric holes filled with water droplets is proposed. The proposed absorber mainly operates due to the unique dielectric permittivity of water. The absorption performance can be tuned by manipulating the volume of the water droplets. By properly control the volume of the water-droplets, the proposed metamaterial absorber achieves an absorption efficiency above 90% in the broadband frequency range from 7.5 to 15 GHz, which is better than previous reported designs. Secondly, to simplify the fabrication procedure, we propose another metamaterial structure absorber design by directly injecting water into plastic tubes, therefore resulting a water-tube absorber. Through some optimization approaches, the absorption performances have been well investigated, which achieve 90% absorption efficiency ranging from 5 to 15 GHz. We believe the proposed designs could find various applications in electromagnetic compatibility technology.
2. Broadband absorber based on water-droplet metamaterial structure
Owing to the liquid form of water at room temperature, the shape of water droplet introduced in the unit-cell of the proposed metamaterial structure will be determined by the surrounding material. Meanwhile the resonant modes of the unit-cell are dominated by the shape of the water droplet. Therefore, in the unit-cell design, determining the shape of the water-filled area is the essential issue. The broadband absorption property of the proposed water absorbers comes from the multiple resonances in the unit-cells and the coupling among them. We start from choosing a specific shape (sphere, cylinder) of the water-droplet as the dominant resonant element. After several iterations of optimization, we find that the truncated cones shape exhibits better multiple resonances behavior, which leads to a broader absorption bandwidth. While considering the fabrication difficulty, we propose to employ a circular truncated-cone-shaped water droplet. Through optimization, we determine the detailed geometric parameters of the unit-cell. The schematic view of the unit-cell for the proposed water metamaterial structure absorber is shown in Figs. 1(a) and (b). As we can see from the Fig. 1(a), a hole of specific shape is drilled in the dielectric substrate (FR4) backed by metallic ground. The dielectric substrate has a relative permittivity of 4.3 and loss tangent of 0.025, with a thickness h of 3 mm, which we use in the EM simulations. The hole is designed to have a shape of inverted circular truncated cone. Its bottom radius and top radius are 1 mm and 4 mm, respectively. The background copper thin film has a thickness of 0.017 mm, and other dimension parameters of the structure are as follows: w1 = 12 mm, w2 = 2 mm. When water droplet is injected into the unit-cell, it will firstly fill the hole. If the volume of the water droplet is a little larger than that of the truncated-cone-shaped hole, due to surface tension, it will naturally form a spherical cap in the top of the water droplet, as shown in Fig. 1(b). A photo of the fabricated sample is shown in Fig. 1(c). Holes of circular truncated cone shape are drilled in the substrate layer through milling machine. Water droplets are then injected into the holes by using a hand held pipettor which is generally used for the accurate and precise sampling and dispensing of liquid volumes. It can be seen from the figure, as we predicted, water spherical caps are formed over the top of whole sample. The whole dimension of the sample has reached 240 mm × 240 mm, including 200 unit-cells in total.
To examine the EM wave absorption performance, we then simulated the absorption spectrum utilizing commercial full-wave EM simulation software. For the specific water material, a Debye model has been introduced in the software to properly represent the dielectric permittivity of water under ambient temperature. To verify, we also experimentally measure the permittivity of distilled water using Keysight 85070E dielectric probe kit. The comparison between the ideal permittivity model in the software and that of the experimental measurement are shown in Fig. 2, which indicates good agreement between them. As labeled in the schematic, for a y-polarized normal incident wave, the absorption spectrum is simulated at the frequency range from 6 to 18 GHz. The absorptivity is defined as A(ω) = 1−T(ω) −R(ω), where T(ω) and R(ω) represent the transmittivity and reflectivity, respectively. Because of the metallic back, the transmittivity equals to zero across the entire frequency range. Thus the absorptivity is reduced to A(ω) = 1−R(ω). The simulated absorption spectra of the designed structure loading or without loading water droplets are given in Fig. 3(a). It can be seen from Fig. 3 that after injecting water droplets into the holes in the substrate, a quite broad absorption bandwidth is achieved in microwave range. The absorption coefficient stays over 90% ranging from 6.5 GHz to 14.6 GHz.
The absorption band is mainly composed of three absorption peaks, as we labeled in the absorption curve. To better understand the working mechanism of the proposed absorber, we have investigated the current density distributions in the unit-cells at each absorption peak. The water droplet with high permittivity acts as a dielectric resonator. From the distribution plots in Fig. 3(b), we can obtain that through the interaction between the incident EM wave and the water droplets, the EM energy mainly concentrates in the water area, vortex-like current loops are induced and formed in the water droplet indicating different resonant modes. As a result, the current loops are localized in the water droplet which is surrounded by the FR4 layer. Because of the large loss tangent of water, most of the incident EM energy will be dissipated in the vortex in the structure. As labeled in the figure, peak A resulted from the fundamental mode of the induced vortex-like current loop, the induced current flows downward and upward around the center of water droplet. While Peak B and Peak C may correspond to the higher resonant modes as two and three current vortex are formed respectively inside the water droplet as shown in Fig. 3(b). Therefore the broadband EM wave absorption associated with this water-based metamaterial structure is resulted from the multi-resonant modes of the water droplet.
To verify the predicted performance, we have carried out the experiment test on the prototype sample. The measurement is conducted in a microwave anechoic room, where two broadband horn antennas as the source and the receiver are fixed on an arch frame connecting to an Agilent vector network analyzer N5247A to measure the microwave reflection from the sample placed below. The measured frequency band is from 6 GHz to 18 GHz. The comparison between the simulated and experimental absorption spectra is shown in Fig. 4(a). The results clearly indicate a broadband microwave absorption. The measured absorption band slightly shifts to higher frequency compare with the simulation with the merge of Peak A and B. As we can see the simulated current density distribution of Peak A in Fig. 3, one of the strongest parts is located in the bottom side of the truncated-cone-shaped hole, where the water-droplet directly attach with the metallic back. However, in the fabrication, to avoid piercing the metallic back, the milling machine will leave more or less dielectric material on the metallic back, which will influence the resonant mode of Peak A and B. Besides, the deviation of the permittivity value of the substrate from the ideal value used in the simulation as well as the variations in the size and shape of the drilled holes also bring in absorption frequency shift. Nonetheless the fabricated sample still obtains a quite broadband absorption bandwidth ranging from 7.5GHz to 15GHz with over 90% absorption rate.
We also investigate the influence on the absorption property caused by the amount of the water by both simulation and experimental measurement. The results are shown in the Figs. 4(b)-(c). In the simulations, different volumes of water droplets are injected into the unit-cells. The different volumes of the water droplets can be represented by applying different heights of the water level in the holes, which is described by the parameter hs labeled in Fig. 1(a). It can be observed that the absorption spectrum entirely shifts to lower frequency domain as the volume of the water droplets is gradually increased. While in the measurement, we control the volume of the water droplets by using the hand held pipettor which can cover a volume range from 10 to 100 μl. We prepare three samples by controlling the volume of the water droplets of 50 μl, 66 μl and 90 μl. Then we measure the absorption spectrum of each sample respectively. According to the experimental results shown in Fig. 4(b), the lower frequency edge of the absorption band also moves to lower frequency as the volume of water droplets is increased. This phenomenon of absorption-band shifting can be explained as that the volume of the water droplets is increased, the dielectric resonator and hence the induced currents loop in the droplet becomes enlarged. Thus the resonant mode will couple to lower frequency EM wave. Although the change of the water droplet volume will bring in absorption-band shift, the broadband absorption bandwidth remains. We can also conclude that, if the volume of the water-droplet injected in the sample is not uniform and varies in a limited range (which may be the case in the test), the whole absorber will maintain a broadband absorption bandwidth with slight absorption-band shift.
To investigate whether the water-droplet structure has good absorptivity for oblique EM wave incidences, we study the influences on the absorption performances for oblique incidences by full-wave simulations. Absorption spectra of the water-droplet structure for different incident angles are shown in Fig. 5. From the figure, we find that the absorption performance of the water-droplet structure does not change for small TE oblique incident angles (up to 30 degree). Even when the incident angle increases to 50 degrees, it still remains over 80% absorptivity from 7.5 to 15 GHz. For TM oblique incidences, absorption peak A and B move slightly to higher frequency with very high absorptivity as the incident angle increases. The influences on peak C are much obvious, it moves to much higher frequency resulting in a better high frequency performance. Even when the incident angle increases to 60 degrees, the absorber could still achieves over 80% absorptivity from 7.5 to 14.5 GHz. Therefore the water-droplet structure can be regarded as a wide angle absorber.
3. Broadband absorber based on water-tube metamaterial structure
As we discussed above the shape of the water droplet dominates the resonant mode and the absorption performance. The water droplet injecting procedure is a time-consuming process and water-droplet shape cannot be easily maintained especially when moving. To simplify the fabrication procedure, we introduce a more robust design by utilizing water-tube layout to construct water-based broadband absorber.
We first investigate the absorption performance of the simplest model. Assuming the unit-cell is constructed with one PVC plastic tube filled with water placed over a metallic back sheet, as schematically shown in Fig. 6(a). The design parameters as labeled in the figure are as follows: d = 6mm W = 8mm t = 0.5mm. The simulation result of microwave absorption is demonstrated in Fig. 6(b). Obviously this structure is polarization dependent. For the incident polarization that is orthogonal to the water-tube, it can achieve an over 90% absorption frequency band from 4.5 to 8 GHz, which is mainly composed of two absorption peaks, as are labeled A and B in the figure. The simulated current density distributions at the two resonant peaks are demonstrated in Fig. 6(c), which representing two basic resonant modes of the proposed water-tube structure. From the figure, we can see that similar as the water droplet design, the induced current loops mainly locate inside the water-area. Peak A is related to the fundamental mode with single vortex-like current loop, while Peak B is caused by the second higher mode with double vortex-like current loops. For the polarization parallel to the water-tube, the EM response is not strong enough to induce well absorption effect in this frequency range.
To investigate whether the water-tube structure has good absorptivity for oblique EM wave incidences under the orthogonal incident polarization, we also study the influences on the absorption performances for oblique incidences by full-wave simulations. Absorption spectra of the simple water-tube structure for different incident angles are shown in Fig. 7. From the figure, we find that for TE oblique incidences when the incident angle increases, absorption peak A moves to lower frequency while peak B moves to higher frequency. Therefore for small oblique incidence it even gains slightly broader bandwidth. When the incident angle increases to 30 degrees, it remains over 80% absorptivity with broader bandwidth. When the incident angle increase to 60 degrees, the absorption spectrum will split into two resonant peaks. For TM oblique incidences, absorption peak A and B move slightly to higher frequency with very high absorptivity when the incident angle increases. Even when the incident angle increase to 50 degrees, it remains nearly the same 90% absorptivity bandwidth with that of the normal incidence. It implies the simple water-tube structure also has good absorptivity for oblique incidences.
For the orthogonal incident polarization, we also investigate the influence on the absorption performance of water-tubes with different diameters. As shown for the simulation results in Fig. 8(b), when the diameter is varying from 2.5 mm to 6.4 mm, the whole absorption band will shift gradually to lower frequency. It can be explained that the change in the diameter brings in the change of the size of the induced current loops inside the water-tube, resulting in the whole resonant frequency shift. Therefore, inspired by the absorption band broadening solution by introducing random sized resonant elements to realize multi-resonance, we propose to use water-tubes of different diameters to construct a broadband MM absorber. A random sequence generation function is chosen to generate different one dimensional (1D) random arrays of differently sized water-tubes. The generated water tube arrays contain random sequence of three water-tubes with diameters ranging from 4 mm to 6 mm. The thickness of the PVC layer of the water-tube is less than 0.5mm, leading to a total thickness of the absorber less than 7 mm. After optimization, two random arrays have been chosen and their results are shown in Fig. 8(c). The use of differently sized water-tubes brings in multiple absorption resonances for the whole absorptive structure, leading to a broadening of the total absorption bandwidth. The simulated results in Fig. 8(c) demonstrate a broad absorption bandwidth coinciding with our prediction.
We then carry out the experimental verification of the broadband polarization dependent water-tube absorber. Aiming to obtain broad absorption bandwidth, we fabricate the prototype samples with differently sized water-tube arrays and test their performance. The schematic of the whole absorber with a photo of the fabricated sample are shown in Fig. 8(a), where water-tubes of different diameter are fixed onto one metallic plate parallel to each other. In the practical realization, the fixation of the water-tubes may introduce some parameter variation which leads to extra water-tube diameter variation into the absorber layout. As the bending and twisting of the water-tubes make some part of the water-tube become thinner. Therefore, as shown in Fig. 8(b), the thinner diameter will lead to absorption at higher frequency band. Thus, the fabricated absorber exhibits a broader absorption bandwidth comparing with the simulation result, especially at higher frequency band as shown in Fig. 8(c). The measurement indicates that the water-tube absorber can achieve 90% absorption rate in a quite broad bandwidth ranging from 5 GHz to 13 GHz.
At last, we expand the water-tube design into a polarization insensitive layout. For convenient, we twist one single water-tube to form a rectangular spiral shape to form the absorber. The quasi rotation symmetry of the spiral structure leads to a polarization insensitive layout as schematically shown in Fig. 9(a). The whole absorber can be regarded as a quasi-periodic layout of the basic water-tube unit-cell. Therefore, water-tube unit-cell is still the dominate sub-wavelength resonant element. By choosing 6 mm as the diameter of the water-tube, the simulated absorption spectra for different incident polarizations in Fig. 9(b) demonstrate the polarization insensitive property of the proposed structure. The absorption performances present little difference for the two orthogonal polarizations, and demonstrate a broadband property.
To verify the design, we fabricated a prototype sample and tested its performance as shown in Fig. 9(b). As we indicated above, the introduced extra parameter variation during the fabrication and fixation of the water-tubes may gain much broader bandwidth than that of the simulation, resulting in an extra broadband absorption effect, especially at the higher frequency band. The results in Fig. 9(b) shows that a broadband microwave absorption with nearly 90% absorption rate can be achieved from 5 GHz to 15 GHz. The measured absorption peaks coincide with the simulation results roughly well. Besides the broadband property, the water-tube design is a totally flexible layout which can be easily applied to curved metallic surfaces, which implies that the proposed water-based absorber may have good application potential.
In summary, we propose and investigate the absorption performances of metamaterial structure absorbers based on resonant water elements. Utilizing water-droplet and water-tube designs, two broadband metamaterial absorbers are proposed and realized in microwave frequency regime with broad absorption bandwidth. For water-droplet design, 90% absorption rate from 7.5 GHz to 15 GHz has been achieved. While for water-tube design, nearly 90% absorption rate from 5 GHz to 15 GHz has been achieved. The broadband absorption performances of the proposed absorbers have been verified through both simulations and experiments. Both designs can be easily fabricated in simple laboratory conditions especially for the water-tube design. We believe the proposed designs will provide a cheap and convenient solution to realize broadband microwave absorbers.
National Natural Science Foundation of China (NSFC) (61571218, 61671231, 61571216).
This work is partially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Wave.
References and links
1. K. C. Pitman, M. W. Lindley, D. Simkin, and J. F. Cooper, “Radar absorbers: better by design,” in IEE Proceedings F (Radar and Signal Processing) (IET, 1991), Vol. 138, pp. 223–228.
2. B. A. Munk, Frequency Selective Surfaces: Theory and Design (Wiley Online Library, 2000), Vol. 29.
3. S. M. Abbas, A. K. Dixit, R. Chatterjee, and T. C. Goel, “Complex permittivity, complex permeability and microwave absorption properties of ferrite–polymer composites,” J. Magn. Magn. Mater. 309(1), 20–24 (2007). [CrossRef]
4. J. L. Wallace, “Broadband magnetic microwave absorbers: Fundamental limitations,” IEEE Trans. Magn. 29(6), 4209–4214 (1993). [CrossRef]
5. M. B. Amin and J. R. James, “Techniques for utilization of hexagonal ferrites in radar absorbers. Part 1: Broadband planar coatings,” Radio Electron. Eng. 51(5), 209–218 (1981). [CrossRef]
6. R. S. Kshetrimayum, “A brief intro to metamaterials,” IEEE Potentials 23(5), 44–46 (2005). [CrossRef]
7. S. Zouhdi, A. Sihvola, and A. P. Vinogradov, Metamaterials and Plasmonics: Fundamentals, Modelling, Applications (Springer Science & Business Media, 2008).
8. G. Oliveri, D. H. Werner, and A. Massa, “Reconfigurable electromagnetics through metamaterials—a review,” Proc. IEEE 103(7), 1034–1056 (2015). [CrossRef]
9. C. Huang, Y. Feng, J. Zhao, Z. Wang, and T. Jiang, “Asymmetric electromagnetic wave transmission of linear polarization via polarization conversion through chiral metamaterial structures,” Phys. Rev. B 85(19), 195131 (2012). [CrossRef]
10. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies,” Science 314(5801), 977–980 (2006). [CrossRef] [PubMed]
14. Y. Pang, J. Wang, H. Ma, M. Feng, Y. Li, Z. Xu, S. Xia, and S. Qu, “Spatial k-dispersion engineering of spoof surface plasmon polaritons for customized absorption,” Sci. Rep. 6(1), 29429 (2016). [CrossRef] [PubMed]
15. X. Yin, C. Long, J. Li, H. Zhu, L. Chen, J. Guan, and X. Li, “Ultra-wideband microwave absorber by connecting multiple absorption bands of two different-sized hyperbolic metamaterial waveguide arrays,” Sci. Rep. 5(1), 15367 (2015). [CrossRef] [PubMed]
16. Y. Zhang, Y. Feng, B. Zhu, J. Zhao, and T. Jiang, “Graphene based tunable metamaterial absorber and polarization modulation in terahertz frequency,” Opt. Express 22(19), 22743–22752 (2014). [CrossRef] [PubMed]
18. Y. Cui, K. H. Fung, J. Xu, H. Ma, Y. Jin, S. He, and N. X. Fang, “Ultrabroadband Light Absorption by a Sawtooth Anisotropic Metamaterial Slab,” Nano Lett. 12(3), 1443–1447 (2012). [CrossRef] [PubMed]
19. X. Shen, Y. Yang, Y. Zang, J. Gu, J. Han, W. Zhang, and T. Jun Cui, “Triple-band terahertz metamaterial absorber: Design, experiment, and physical interpretation,” Appl. Phys. Lett. 101(15), 154102 (2012). [CrossRef]
21. C. M. Watts, X. Liu, and W. J. Padilla, “Metamaterial electromagnetic wave absorbers,” Adv. Mater. 24(23), OP98–OP120 (2012). [PubMed]
23. B. Zhu, Z. Wang, C. Huang, Y. Feng, J. Zhao, and T. Jiang, “Polarization insensitive metamaterial absorber with wide incident angle,” Prog. Electromagnetics Res. 101, 231–239 (2010). [CrossRef]
25. W. Padilla and X. Liu, “Perfect electromagnetic absorbers from microwave to optical,” SPIE Newsroom 14, 03137 (2010).
26. J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, “High performance optical absorber based on a plasmonic metamaterial,” Appl. Phys. Lett. 96(25), 251104 (2010). [CrossRef]
27. O. Luukkonen, F. Costa, C. R. Simovski, A. Monorchio, and S. A. Tretyakov, “A thin electromagnetic absorber for wide incidence angles and both polarizations,” IEEE Trans. Antenn. Propag. 57(10), 3119–3125 (2009). [CrossRef]
29. J. Hao, L. Zhou, and M. Qiu, “Nearly total absorption of light and heat generation by plasmonic metamaterials,” Phys. Rev. B 83(16), 165107 (2011). [CrossRef]
30. H. Xiong, J.-S. Hong, C.-M. Luo, and L.-L. Zhong, “An ultrathin and broadband metamaterial absorber using multi-layer structures,” J. Appl. Phys. 114(6), 064109 (2013). [CrossRef]
31. F. Ding, Y. Cui, X. Ge, Y. Jin, and S. He, “Ultra-broadband microwave metamaterial absorber,” Appl. Phys. Lett. 100(10), 103506 (2012). [CrossRef]
32. S. Gu, J. P. Barrett, T. H. Hand, B.-I. Popa, and S. A. Cummer, “A broadband low-reflection metamaterial absorber,” J. Appl. Phys. 108(6), 064913 (2010). [CrossRef]
33. P. Moitra, Y. Yang, Z. Anderson, I. I. Kravchenko, D. P. Briggs, and J. Valentine, “Realization of an all-dielectric zero-index optical metamaterial,” Nat. Photonics 7(10), 791–795 (2013). [CrossRef]
34. A. Ahmadi and H. Mosallaei, “Physical configuration and performance modeling of all-dielectric metamaterials,” Phys. Rev. B 77(4), 045104 (2008). [CrossRef]
36. A. Andryieuski, S. M. Kuznetsova, S. V. Zhukovsky, Y. S. Kivshar, and A. V. Lavrinenko, “Water: promising opportunities for tunable all-dielectric electromagnetic metamaterials,” Sci. Rep. 5(1), 13535 (2015). [CrossRef] [PubMed]
37. Y. J. Yoo, S. Ju, S. Y. Park, Y. Ju Kim, J. Bong, T. Lim, K. W. Kim, J. Y. Rhee, and Y. Lee, “Metamaterial absorber for electromagnetic waves in periodic water droplets,” Sci. Rep. 5(1), 14018 (2015). [CrossRef] [PubMed]
38. Q. Song, W. Zhang, P. C. Wu, W. Zhu, Z. X. Shen, P. H. J. Chong, Q. X. Liang, Z. C. Yang, Y. L. Hao, H. Cai, H. F. Zhou, Y. Gu, G. Q. Lo, D. P. Tsai, T. Bourouina, Y. Leprince-Wang, and A. Q. Liu, “Water-resonator-based metasurface: an ultrabroadband and near-unity absorption,” Adv. Opt. Mater. 5(8), 1601103 (2017). [CrossRef]
39. Y. Pang, J. Wang, Q. Cheng, S. Xia, X. Y. Zhou, Z. Xu, T. J. Cui, and S. Qu, “Thermally tunable water-substrate broadband metamaterial absorbers,” Appl. Phys. Lett. 110(10), 104103 (2017). [CrossRef]
40. X. Huang, H. Yang, Z. Shen, J. Chen, H. Lin, and Z. Yu, “Water-injected all-dielectric ultra-wideband and prominent oblique incidence metamaterial absorber in microwave regime,” J. Phys. D Appl. Phys. 50(38), 385304 (2017). [CrossRef]