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Abstract
Increasing attention on microwave ultra-broadband metamaterial absorbers has been paid due to their promising applications. While most microwave ultra-broadband metamaterial absorbers developed so far are based on metallic resonant structures, dispersive dielectric water-based metamaterial opens a simpler and more versatile route for the construction of polarization- and angle- insensitive ultra-broadband absorption. Here, we review the recent progress of water-based metamaterial absorbers by providing an illustration of the mechanisms to realize ultra-broadband, tunable and multi-functional absorption. We also address the further development direction and some potential novel applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, substantial progress in microwave absorption has been achieved for various novel potential applications, such as radar stealth, radiation prevention, and energy collection [1–5]. With the development of metamaterials [6–8], metamaterial absorbers can substantially dissipate microwave energy by manipulating effective permittivity and permeability parameters simultaneously based on the impedance-matching principle [9]. Due to the limitation between quality factor and resonant intensity, only high absorption at a single frequency can be achieved in such designs [10]. In order to break this application limitation, coupled integration methods are proposed to construct dual-band and multi-band metamaterial absorbers by combining metamaterial absorber units with coupled adjacent resonance peaks vertically or horizontally [11–13]. The dual-band and multi-band metamaterial absorbers have more than one range of absorption frequency, but the continuous broad frequency band cannot be covered in these designs [14]. Although tunable metamaterial manipulation methods, such as diodes [15], graphene [16], liquid crystal [17], electric field [18], phase change material [19], and thermal control [20], are successfully used to fabricate frequency tunable metamaterial absorbers for continuous broad frequency band absorption, the simpler ultra-broadband metamaterial absorbers are highly sought after for better novel applications.
In order to achieve ultra-broadband absorption, the strategy of employing a self-similar structure to realize coupled resonance peaks is proposed in [21]. Usually, the microwave ultra-broadband metamaterial absorbers are integrated coupled self-similar metallic resonance structures with adjacent resonance peaks. High absorption can be achieved in ultra-broad frequency band by vertical and horizontal integration methods. Resonance modes of absorber correspond to frequency point within absorption band, while the resonance at lower or higher frequency exists in the parts with larger or smaller dimensions, respectively. Due to the coupled resonance modes, the absorber can achieve the impedance-matching condition in the ultra-broad band with flat dispersion of constitutive parameters [22]. However, the metallic ultra-broadband flat dispersion structures are fabricated with complex integration and optimization process. It is important to find a simpler and more practical design method to reach the theoretical minimal thickness with high absorption in low-frequency regime [23].
Most of the microwave ultra-broadband metamaterial absorbers developed so far are based on metallic resonant structures, while the others are dielectric resonant structures. Recently, the promising periodic dielectric water-droplet WMMA is proposed [24], effectively utilizing the constitutive dispersive permittivity of water. The promising water-based metamaterial with plenty of advantages, such as biocompatibility, easy availability, versatile tunability, and optical transparency, can be applied in radar stealth, energy harvesting, and radiation prevention. In this paper, we focus on the potential of microwave ultra-broadband absorption based on water-based metamaterials and analyze the mechanisms of dielectric resonance, spoof surface polarization, and diffraction gratings in WMMA. Moreover, we exhibit the recent progress in WMMAs in thermal tunability, ionic tunability, shape tunability, and multiple functions. Finally, the future development prospects and novel potential applications of microwave WMMA are discussed.
2. Dispersive relation for better absorption performance
The dielectric water exists as a liquid from 0 to 100 °C at atmospheric pressure [25]. The charge center shift of liquid water molecules in the polarized state process can be described by Debye equations. At the room temperature $T = 20\circ{C}$, the angular frequency $\omega $ and permittivity $\varepsilon = {\varepsilon _{real}} + j{\varepsilon _{image}} = {\varepsilon _{real}}({1 - jtan\delta } )$ are shown in Eq. (1–4). The symbols ${\varepsilon _\infty }({\omega ,T} )$, ${\varepsilon _s}({\omega ,T} )$ and $\tau ({\omega ,T} )$ are optical permittivity, static permittivity and rotational relaxation time, respectively. (More details are available in [26])
As shown in Fig. 1, the permittivity and loss tangent curves show that water has extremely high loss within a certain microwave frequency band, due to the energy dissipation of the molecule polarization process.
Considering a sub-wavelength metamaterial absorber with thickness d as a uniform equivalent layer, the normalized impedance ${z_r}$, and refractive index ${n_r}$ can be extracted by scattering parameters [27].
Fig. 1. The permittivity and lost tangent of water in microwave frequency regime at room temperature. Authors’ figure.
Since the relative permittivity, relative permeability, and absorptivity are obtained by ${\varepsilon _r} = \frac{{{n_r}}}{{{z_r}}}$, $\; {\mu _r} = {n_r}{z_r}$, and $A = 1 - {|{{S_{11}}} |^2} - {|{{S_{21}}} |^2}$, high absorption means minimizing $|{{S_{11}}} |$ and $|{{S_{21}}} |$. Moreover, the impedance-matching condition ${z_r} = 1$ or ${n_r} = \sqrt {{\varepsilon _r}{\mu _r}} = {\varepsilon _r} = {\mu _r}$ for perfect absorption should be satisfied:
The first equation in (7) is the impedance-matching condition, for minimizing the reflection from the surface of the metamaterial absorber. The second one means the dissipation inside should be large enough to attenuate the energy of incidence. Since the wave vector is proportional to the frequency, a larger thickness is required to achieve the same absorption at a lower frequency. Theoretically, the thickness d of the absorber should not be less than the limitation calculated by the reflection coefficient spectrum $R(\lambda )$ [28]:
To minimize the practical thickness as close as possible to the theoretical limitation, the absorber should possess flat ultra-broadband dispersion constitutive parameters. According to the Kramer-Kronig relation [8],
3. Coupled resonance mechanism of broadband absorption
3.1 Dielectric resonance mode of the single structure
From the viewpoint of scattering theory, all scattering objects can be equivalent to effective electric or/and magnetic polarizability densities [30]. The electric and magnetic resonance modes are both supported in dielectric materials [31], even the toroidal mode [32]. For non-dispersive dielectric structures, the supported low-coupled low-order and high-order resonant modes cannot achieve ultra-broadband absorption [33], however, the coupled adjacent resonances can extend the absorption frequency band [34]. Since the real part of the permittivity of water is inversely proportional to the squared incident wavelength, the coupled adjacent resonant modes can be excited in a certain frequency band simultaneously for the ultra-broadband impedance-matching condition. In addition, the extremely high loss tangent in the microwave frequency band indicates high absorption [35]. For a single resonant structure, the magnetic resonances are stronger than electric resonances, so the excited multiple low-order and high-order vortex current can achieve ultra-broadband high absorption [36].
Single types of resonant structures, such as spherical [26], water-droplet [24], and rectangular [35–37] designs, have relatively narrow absorption frequency bandwidth for poor excited resonant modes and low coupling strength. After optimization, a variety of adjacent strongly coupled resonant modes can be excited to achieve ultra-broadband absorption [38–46]. As shown in Fig. 2(b), the single water-droplet structure can support coupled strong resonance peaks A and C to realize ultra-broadband absorption. Figures. 2(c-d) show low-order and high-order vortex currents corresponding to different resonant loops of magnetic resonances. Since the length of the current loop is inversely proportional to the resonant frequency, higher frequency means shorter vortex current loops [38]. Limited by the designed structure, the magnetic resonances of a single water-droplet structure have low coupling strength, which leads to several isolated absorption peaks. As shown in Fig. 2(e), the optimized water-droplet structure with a bottom cone can support more resonance modes, therefore the high-coupled adjacent peaks can achieve ultra-broadband absorption [46].
Fig. 2. The coupled dielectric resonance modes of WMMAs. (a) The 3D schematic diagram of droplet-shaped WMMAs. (b) The simulation and experiment results of absorption relations. The density distribution of induced current at resonance (c) peak A and (d) peak B. (e) The 3D schematic diagram and (f) absorption curves of optimized droplet-shaped WMMAs. (g-i) The vortex current distribution of optimized droplet-shaped WMMAs at resonance peaks A, B, and C. (a-d) Reproduced from [24] under CC BY 4.0. (e-i) Reproduced from [46] with permission. Copyright 2018, Optica Publishing.
3.2 Dielectric resonance mode of horizontal and vertical integrated structure
According to the absorption superposition method [34], the horizontal-integrated resonant structures can achieve a broader absorption frequency band by coupling the adjacent ones [46,47]. As shown in Fig. 3(a), since the single water-tube structure support two adjacent resonant modes A and B, the total absorption frequency band is relatively narrow. In contrast, as shown in Fig. 3(b), because the excited resonant peaks are highly connected to geometry parameters, the rectangular spiral-shaped structure with extended absorption frequency band can be achieved by integrating water-tubes with different diameters [5].
Fig. 3. The Horizontal and Vertical Integrated Structure water-based material absorbers. The 3D schematic diagram of (a) tube-shaped water-based absorber and (b) horizontal integrated tube-shaped water-based absorber. (c) The absorption of tube-shaped water-based absorber for y-polarized and x-polarized normal incidence. (d) The vortex current loop density distributions for at resonance peak A and B. (e) The experimental and simulated absorptivity of the horizontal integrated tube-shaped water-based absorber. (f) The vertically integrated design processing diagrams of the channeled water-based absorber. (g) The absorptivity curves of Model (I-VI). (h) The schematic diagram of fish-bone-shaped metal and layered water absorber. (i) The simulated and measured absorption spectrum of fish-bone water-based absorber. (j) The schematic diagram of glass caped moth-eye water-based absorber. (k) The absorption performance of moth-eye water-based absorber. (a-e) Reproduced from [46] with permission. Copyright 2018, Optica Publishing. (f-g) Reproduced from [50] with permission. Copyright 2021, Optica Publishing. (h-i) Reproduced from [51] under CC BY 4.0. (i) Reproduced from [40] with permission. Copyright 2021, Optica Publishing.
The horizontal-integrated method not only leads to a large period of the absorber unit cell but is also difficult to excite the diffraction grating resonance mode for high absorption in higher frequency [48]. In order to achieve a smaller absorber unit cell, structures with adjacent resonance frequency bands can be superimposed vertically [49]. As shown in Fig. 3(f-g), the channel water-based absorber is composed of three-layered water-based structures in Model_IV. The small cross structure in Model_III improves the absorption in the middle frequency band (30–50 GHz), and the square ring structure with a long resonant loop in Model_IV ameliorates the absorption in low-frequency band (10–20 GHz). The vertical-coupled large cross structure, small cross structure, and square ring structure can achieve ultra-broadband high absorption [50]. As shown in Figs. 3(h-i), the water-based substrate coupled with a tapered metal structure can also support multiple resonance modes to achieve ultra-broadband absorption. With the increased resonant frequency, the long resonant loop at the bottom of the structural changes to the shorter resonant loop in the top structure [51]. As illustrated in Figs. 3(j-k), due to the variable aspects ratio, the water-based tapered moth-eye structure can also support multiple resonance modes to achieve ultra-broadband absorption [40].
Most vertically superimposed water-based absorbers have a large thickness after forwarding design and local optimization [51] and utilize the tapered structure for ultra-broadband absorption [52]. According to Rozanov limit [28], both the larger degrees of freedom in optimization degree and more deliberately designed optimization algorithm are required to reduce the thickness close to the theoretical limit [23]. Under the same design complexity, the initial absorbing frequency of water-based structures with dispersion characteristics is lower than metallic structures [40,51].
3.3 Diffraction gratings and surface plasmonic polarizations modes
Through the horizontal or vertical broadband integration method, we can extend the absorption frequency band, and improve the low-frequency absorption. Due to the constitutive high loss tangent, the high-frequency absorption of WMMA with a two-dimensional grating arrangement mainly depends on the exciting two-dimensional diffraction grating mode [48]. The water-based absorbing structure with periodic length p and refractive index n can support zero-order diffraction grating mode and first-order diffraction grating mode (higher-order modes are negligible), while the zero-order diffraction can excite the dielectric resonance mode. When incident wavelength $\lambda $ satisfies Eq. (10),
the first-order diffraction grating mode can be excited, the diffraction angle $\theta $ of which can be described by Eq. (11) asDue to the large value of loss tangent of water in high frequency, the high density at the top of the water-based absorbing structure can be achieved by first-order diffraction grating mode. When the absorber has a metal substrate to prevent backscattering, the horizontal wave vector component of first-order diffraction grating mode can excite the confined surface plasmonic polarization mode in the dielectric-metal surface. The relation between the horizontal component of the incident wave vector ${k_x}$ and SPP mode wave vector ${k_{SPP}}$ satisfies Eq. (12) [53],
However, the intensity of the excited SPP resonant mode is closely related to the frequency. In the high frequency, the diffraction component of incidence is absorbed in the top of the water-based absorbing structure, so the excited SPP resonant mode is weak. When the frequency decreases, the diffraction component excites strong SPP resonant mode in the dielectric-metal confined surface.
As shown in Fig. 4(b-e), four high absorption resonance peaks are supported in the typical two-dimensional grating WMMA in Fig. 4(f). Since the metal substrate prevents transmission at low frequency, the high absorption at frequency ${f_1}$ and ${f_2}$ are mainly due to the dielectric resonance and metal substrate interaction. When the frequency increases, the high absorption at ${f_3}$ depends on the SPP resonant mode excited by the diffraction grating mode, while extremely high absorption in the top at ${f_4}$ is achieved by the dominant diffraction grating mode [54]. Considering the ground-free case, the first-order diffraction grating mode can only exist at the higher frequency of 20 GHz. Moreover, the magnetic and electric component distributions in Figs. 4(h-j) demonstrate that the SPP resonant mode can be effectively excited by diffraction grating mode [55].
Fig. 4. The diffraction gratings and surface plasmonic polarization modes of WMMA. (a) The 3D schematic diagram of cylindrical WMMA. (b-e) The energy loss density distributions of cylindrical WMMA at resonance peaks ${f_1}$, ${f_2}$, ${f_3}$, and ${f_4}$. (f) The absorptivity comparison of the proposed absorber, water layer with metal plate and water resonator without metal plate, as well as the energy loss distribution of ground-free water-based absorber with diffraction grating mode. (g) The 3D diagram of sphere-cap WMMA. (h) The magnetic component ${H_x}$ and (i) electric component ${E_\textrm{z}}$ of sphere-cap water-based absorber for diffraction grating mode. (j) The absorption spectrum of sphere-cap WMMA. (a-f) Reproduced from [54] with permission. Copyright 2018, Optica Publishing. (g-j) Reproduced from [55] with permission. Copyright 2017, John Wiley & Sons Publishing.
Only WMMA with periodic two-dimensional gap distribution can support the exciting diffraction grating mode. Therefore, the WMMA with no obvious periodic gap distribution is difficult to excite the two-dimensional diffraction mode for high-frequency absorption [29,39,43,44]. According to Rozanov limit, the correlation between the thickness limit and high-frequency absorption is weak, so it is easy to achieve a thinner absorber in high-frequency band [28]. By introducing a two-dimensional grating distribution structure in WMMAs, the high-frequency absorption is effectively improved by utilizing the diffraction grating mode and corresponding SPP resonant mode [38,54,55]. In addition, the high absorption of ground-free WMMA in high frequency can also be achieved by introducing the two-dimensional grating design [40,41].
4. Polarization- and angle- insensitivity
All of the WMMAs with multiple symmetry are characterized by both polarization insensitivity and high absorption rate to the incident wave [38–40,42–44]. A typical example is the optimized fish-bone WMMA with quadruple rotational symmetry, which can achieve perfect polarization-insensitive absorption as shown in Fig. 5(a).
Fig. 5. The polarization- and angle- insensitive absorption of WMMA. (a) The 3D schematic diagram of the optimized fish-bone water-based absorber. (b) The simulation result of absorption with different polarization angles. The absorptivity of channeled WMMA with oblique (c) TE incident wave and (d) TM incident wave. (a-b) Reproduced from [51] under CC BY 4.0. (c-d) Reproduced from [50] with permission. Copyright 2021, Optica Publishing.
For the oblique incidence condition, the excited resonance intensity of the water-based dielectric structure is relatively sensitive to the horizontal component of the magnetic field [5,10,45]. For larger incident angles, the resonance intensity decreases obviously under oblique TE incidence due to the decreased horizontal component, while the resonance intensity decreases slightly under oblique TM incidence due to the unchanged parallel magnetic field. As shown in Fig. 5(c-d), the WMMAs can still obtain high absorption under an incident angle as large as 60°, which can be applied in wide-angle absorption [50].
5. Tunable absorption
5.1 Thermal tunability
As shown in Eq. (13–15), the Debye model parameters of water, including optical permittivity ${\varepsilon _\infty }(T )$, static permittivity ${\varepsilon _0}(T )$, and relaxation time $\tau (T )$, can be described as a function of temperature [25]. Since the polarity of liquid water decreases due to hydrogen bond fracture after heating, the real and imaginary parts of permittivity, as well as the loss tangent angle $tan\delta $ within 0–40 GHz, decrease with higher temperature as exhibited in Fig. 6(b). For dielectric resonance in WMMA, the lower $tan\delta $ means lower absorption intensity, while the variable real part of permittivity achieves the offset of the resonance peak.
Fig. 6. The thermal tunability and stability of WMMAs. (a) The permittivity of water as a function of frequency for the temperature 0–100 °C. (The solid lines correspond to the real part, dashed lines to the imaginary part). (b) The loss tangent $tan\delta $ works as a function of frequency for the temperature range of 0–100 °C. (c) The 3D schematic diagram of thermally tunable metamaterial absorber. (d) The absorption of thermally tunable metamaterial absorber for different temperatures varying from 10°C to 80°C. (e) The 3D schematic diagram of thermal stable metamaterial absorber. (f) The absorption relations of thermal stable metamaterial absorber for different temperatures varying from 0°C to 100°C. (a) Reproduced from [26] under CC BY 4.0. (b) Authors’ figure. (c-d) Reproduced from [56] with permission. Copyright 2016, AIP Publishing. (e-f) Reproduced from [54] with permission. Copyright 2018, Optica Publishing.
The proposed absorber composed of metal-pattern and water-layer is illustrated in Fig. 6(c), where the water layer works as the dielectric lossy layer. Moreover, the permittivity $\varepsilon $ of the water layer can be represented as the lumped element capacitance C in a resonant circuit with a parallel connection. Therefore, it is effective to adjust the amplitude and phase of resonance by the thermal control of the water layer [56]. The variation proportion of the imaginary part in Fig. 6(d) is larger than the real part under increasing temperature, achieving a large change of loss tangent. Therefore, the absorption intensity at the coupling of resonance peaks decreases significantly due to weaker resonance peaks, showing promising thermal tunable absorption applications [51].
Besides, the absorption with thermal insensitivity also has novel applications. As shown in Fig. 6(b), the loss tangent of water is sensitive to temperature within 0–10 GHz, so it is difficult to realize a thermal stable absorption design [37]. Most of the thermal stable WMMAs developed so far only works in the higher frequency band ($> $ 10 GHz) [38,41,54]. Coupling with the thermal-insensitive SPP mode and diffraction grating mode, the thermal stable absorption of WMMA can be realized.
When there is a metal substrate, the WMMA in Fig. 6(e) can also support SPP mode. Compared with absorbers composed of metal-pattern and water layer, the WMMA has high thermal stability due to the coupled strong diffraction grating and SPP mode [54]. In addition, the high thermal stable absorption can also be achieved by coupling the diffraction grating mode and dielectric mode in a ground-free structure [41]. Further works on how to achieve thermal stable absorption at lower frequency are still challenging.
5.2 Ionic tunability
According to Debye relations Eq. (1–4), the loss tangent of water at low frequency is small, so the WMMA cannot achieve high absorption due to the low energy loss of the resonance loop. The loss tangent of water at low frequency can be effectively improved by adding ions to form an ionic solution for higher low-frequency energy loss. Taking the common ion NaCl as an example, the modified Debye model of saline is [57]:
Compared with the unmodified Debye model, $i\frac{{{\sigma _{NaCl}}({T,N} )}}{{\omega \varepsilon _0^\ast }}$ is added to the imaginary part of permittivity. As shown in Fig. 7(b), saline has a larger imaginary part of permittivity in the low-frequency band than water, and thus the metamaterial absorber based on saline has higher low-frequency absorption under the same structure. As shown in Fig. 7(e), when the normalized salinity S is 0.25, the absorption of the circular truncated cone in Fig. 7(c) at low frequency is greatly improved. Figure. 7(d) shows that the strong dielectric resonance loss at low frequency solves the common problem of low absorption below 3 GHz in WMMAs [58].
Fig. 7. The ionic tunable absorption of WMMAs. (a) The real part and (b) imaginary part of permittivity $\varepsilon $ as a function of frequency for different normalized salinity. (c) The 3D schematic diagram of saline tunable metamaterial absorber. (d) The loss density distributions of saline tunable metamaterial absorber from left to right are operating at 1.73, 30.6, and 50.2 GHz, respectively. (e) The absorption of saline tunable metamaterial absorber for different salinity. (f) The schematic ionic tunable absorber with programmable design. (a-e) Reproduced from [58] with permission. Copyright 2020, Optica Publishing. (f) Reproduced from [60] with permission. Copyright 2019, AIP Publishing.
When the frequency increases, the permittivity of water has a low correlation with salinity, so the saline-based metamaterial absorber can be regarded as the pure water-based structure. Due to the existence of multiple resonance modes in higher frequency bands, such as dielectric resonance mode, SPP mode, and diffraction grating mode, the ions have less effect on absorption in the higher frequency range.
As shown in Fig. 7(e), the tunable absorption can be achieved by adjusting the salinity in low-frequency band. Inspired by thermal tunable WMMA, to reduce the influence of SPP mode and diffraction grating mode for high-frequency sensitive tunable absorption, the ionic-WMMA composed of the water layer and metal structure is proposed. Further works of ionic tunable absorption are expected on the multi-manipulation with an electric method such as programmable tunable control [59]. The programmable beam control absorption structure shown in Fig. 7(f) can adjust the amplitude and phase of different spectrum reflection peaks corresponding to the on and off states of the diode [60]. The multiple tunable absorption applications need to be further studied.
5.3 Shape tunability
Compared with absorbers of solid state, the WMMAs have unique tunability to control the absorption due to the liquid state. The developed shape-control design methods are divided into two categories: one is to use water pipes or microfluidic systems to control the water content in the structure [55,61,62], and the other is to reserve part of the space in WMMA to control the shape by gravity [26,63].
As shown in Fig. 8(a), the shape of WMMA is tuned by controlling the internal water pressure in the PDMS deformable container with a microfluidic system. When water pressure is high, the increased volume brings a longer dielectric resonance loop with both longer path and dielectric resonance peak with lower frequency. While the diffraction grating mode and SPP resonance mode keep unchanged, so the lower coupling between adjacent resonant peaks leads to lower absorption [55]. As shown in Fig. 8(b), for the metal-water-based structure, the position of the resonance peak can be effectively adjusted by controlling the thickness of the water layer. The increased thickness of the water layer leads to redshift and blueshift of low-frequency and high-frequency resonance peaks, respectively, due to the decreased thickness of the air layer above [61].
Fig. 8. The shape-tunable WMMAs. (a) The 3D schematic diagram of the micro-channel shape-controlled water-based absorber. (b) The 3D schematic diagram of the layer-tunable thick-controlled water-based absorber. (c) The 3D schematic diagram of the gravity-controlled rotational tunable water-based absorber. (d) The schematic diagram of the gravity-controlled switchable water-based absorber. (The blue and white structures are water and container, respectively.) (e) The foldable mechanical-controlled shaped-tunable WMMA. (a) Reproduced from [55] with permission. Copyright 2017, John Wiley & Sons. (b) Reproduced from [61] with permission. Copyright 2021, RSC Publishing. (c) Reproduced from [63] with permission. Copyright 2016, AIP Publishing. (d-e) Authors’ figures.
The second gravity-controlled tunable route depends on the angle between the structure and the gravity direction. However, when the gravity-tunable plane of the overall structure is perpendicular to the ground, the tunability will be lost. Moreover, the fine-tuning control cannot be achieved by gravity. As shown in Fig. 8(c), half of the cavity filled with water can achieve different shapes and different transmission properties by rotating the overall structure [63]. Figure. 8(d) shows a possible tunable process of absorbing switch. An hourglass-like water-based structure can reach two states of high and low absorption through the principle of connector [26].
By utilizing the external fluidic systems and gravity, the shape of the water-based absorbing structure varies. However, fluidic control changes the water content, while gravity regulation does not. For the tunable method with unchanged water content, inspired by origami mechanical force control [64], the water-based material can be bound through an elastic container, and the mechanical deformation of the structure in Fig. 8(e) can be realized by using external mechanical force to adjust the absorption.
6. Multi-functional absorption
6.1 Optical transparent function
Optical transparent absorbers with high ultra-broadband microwave absorption have application value in camouflage, energy shielding, and photovoltaic system. The dispersion characteristics of water in near-infrared and visible frequencies can be described by the Sellmeier equation (extinction coefficient is ignored) [65]:
As shown in Fig. 9(a), the refractive index of water in the near-infrared and visible frequency bands is close to 1, so the water is optically transparent with a high loss tangent in microwave frequency. Therefore, optically transparent broadband microwave water-based absorber can be realized by combining optical transparent materials with water [40,61,62,66,67].
Fig. 9. The optical transparent function of WMMAs. (a) The refractive index n is calculated by the Sellmeier equation in the wavelength range from 0.2 to 2 $\mu m$. (b) The 3D schematic diagram of typical optical transparent WMMA. (c) The experimental demonstration of near and distant views for optical transparent WMMA. (d) The comparison between PMMA-substrate metamaterial absorber and water-substrate metamaterial absorber. (a-d) Reproduced from [62] with permission. Copyright 2018, Optica Publishing.
The dielectric container of WMMA can be fabricated with PMMA, glass, PET, and other optical transparent dielectric material [68,69], while the metallic substrate and pattern layer can be replaced by printable optical transparent metal ITO [70]. As exhibited in Fig. 9(c), near and distant views of WMMA in Fig. 9(b) shows that the absorber has promising performance in optical transmission experimentally. As shown in Fig. 9(d), the comparison with PMMA-based metamaterial absorber proves that the WMMA is optically transparent with 60% optical transmissivity [61,62]. To realize the complex design with optical transparent property, we can choose transparent resin as the material of the container for 3D printing.
6.2 Flexible structure
Because PMMA, resin, and other deformable dielectric materials are not suitable for flexible structure, the WMMAs in Fig. 10(a) can be fabricated by a soft dielectric material and a thin metal layer, such as PDMS and copper [55]. As shown in Fig. 10(c), the reflection spectrum of different directions of curved WMMA shows a high omnidirectional absorption. Intuitively, when a curved surface is illuminated under the normal plane wave, the reflection wave scatters into different directions. As illustrated in Fig. 10(b), the bare metal shows a relatively high omnidirectional reflection spectrum. This comparison indicates that the water-based structure contributes a lot to omnidirectional absorption.
Fig. 10. The flexible WMMA. (a) The schematic diagram and of flexible WMMA. The reflectance spectrum at different reflective positions for (b) bare metal curved surface and (c) curved water-based absorber when R = 200 mm. (a-c) Reproduced from [55] with permission. Copyright 2017, John Wiley & Sons Publishing.
Due to the limitation relation between liquid water and dielectric container, the geometry of water-based structure varies with the curved flexible base structure, so the connected water layer structure may have uneven overall distribution. Therefore, the base structure deformation should be considered in the design of flexible water-based structures, and only the specific water-based absorber is suitable for flexible design [29].
6.3 Tunable infrared radiation
The infrared radiation of the material follows the relation $M = \varepsilon (T )\sigma {T^4}$, where $\varepsilon (T )$ is infrared emissivity, and T is the temperature. The dynamically tunable radiation property of materials in infrared frequency has introduced a wide range of novel applications [71,72]. The most convenient route for controlling radiation is changing the chemical composition or kinetic temperature. Since the water has a large specific heat capacity, the corresponding infrared radiation of the WMMAs can be effectively tuned by the injected water [61,62].
As shown in Fig. 11(b-d), the infrared radiation of the WMMA is lower than that of the PMMA-based absorber, thus can be controlled by injected water [66]. However, the absorption in low-frequency band will change simultaneously with the variable structure of the water layer. Further works on WMMA with tunable infrared radiation and stable microwave absorption are expected.
Fig. 11. The infrared-radiation tunable WMMA. (a) The schematic diagram of water-based absorber with control channels. (b-d) The IR radiation patterns of water-substrate absorber (left side) and PMMA-substrate absorber (right side) with initial heated temperature 45°C at room temperature after 1, 3, and 5 min. (a-d) Reproduced from [62] with permission. Copyright 2018, Optica Publishing.
Table 1. The Comparison between Selected Ultra-broadband Microwave Water-based Absorbers
7. Outlook
As a dispersive material for the microwave metamaterial absorbers, water-based materials show several natural advantages, notably in terms of the high loss tangent in the microwave frequency band, and the flat dispersion relation in ultra-broadband absorption like metallic structures [22]. Although most WMMAs in Table 1 show a promising absorbing ability, the inverse design method lacks theoretical instruction [75]. Further works are expected at goal-oriented inverse design with data-driven route for WMMAs [76], so the WMMAs with thickness approaching theoretical limit can be achieved. Although water has a low loss tangent in low frequency (1–5 GHz) for absorption, this can be improved by either adding ionic in the water for larger conductivity or replacing the water with ionic liquid [77]. Due to the large constitutive loss tangent in high-frequency band, the WMMA can easily achieve large absorption in high-frequency band. Towards the challenging theoretical relative bandwidth limit (200%) and high absorption at extremely low frequency band (L band or even lower), we can achieve this goal by utilizing the optimized water-based materials for higher absorption in low frequency band, such as saline. Further explorations are expected in the tunable and multi-functional WMMAs. The thermal tunable, electrically tunable and shape tunable WMMA can be fabricated by introducing phase change material [78] and electric tunable material [17]. The mildly changed material parameter of water combined with the abruptly changed one of phase change material provides versatile tunable functions. Based on the tunable design, the WMMAs with multiple functions, such as wavefront control [79], polarization conversion combined with absorption [80,81], and so on, can be realized by combing the water and metallic structure, which is achieved by controlling the water content with reversible and switchable functions. Further, the dynamic absorption spectrum controlled by water content can be applied in the stealth technology to make the target scattering in line with the environment for stealth technology, and the applications for the optical window can also be obtained by water. Moreover, the easy-to-get and cheap water and complex container fabricated by 3D printing technology show promising potential in practical application. At last, further broadband functions and other applications can be explored by the constitutive property of water-based materials [82–88].
Funding
National Natural Science Foundation of China (92166107, 51872154).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available within the article.
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