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Abstract
In this paper, we propose a reconfigurable metasurface based on liquid metal for flexible beam-steering. The Gallium alloy with low melting temperature (about 30°C) is employed for easy structure reconfiguration. By designing specific dimension of cavity, we make the liquid metal in it easily form into desired sizes, to generate distinct phase responses. Two metasurface elements with four phase responses are designed, simulated and measured. Based on the above elements, various scattering fields can be realized within our design. We present four schemes to achieve single- and dual-beam fields with different beam-deflecting angles. The measured results show great agreement with the simulations, validating our design. In addition, every two columns of the metasurface are grouped into a single composite form, which promises a customizable combination for metasurface pattern. Compared to the previous reconfigurable works for metasurfaces, the method via liquid metal possess lower cost, easier fabrication, and will further enrich the manipulation method for metasurfaces.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterials are artificial three-dimensional materials with intriguing electromagnetic (EM) characteristics [1], which are usually unattainable in nature. The impressive design freedom in metamaterial structure has enabled many unique applications to be implemented, such as negative index [2], super lens [3], and cloaks [4]. Such outstanding properties are still maintained on metasurfaces. Metasurfaces are the 2D version of metamaterials [5], but requires lower profile and less complexity. The phase and amplitude of spatial EM wave can be effectively manipulated using the reflective or transmissive structure of metasurfaces [6]. Due to the outstanding specialties of metasurfaces, various applications based on them have been realized, such as communication [7], hologram [8] and vortex beams [9].
In many applications, the reconfigurable capability is very demanding for multi-functionality. Various reconfigurable methods such as PIN diode [10,11], mechanically-deformation [6,12], are developed. Some other tunable mechanism like saline water [13], liquid crystal [14] and active components [15,16] are reported. These works usually rely on the fixed metal structure, and the functionality is relatively inalterable when the structure is determined. However, this drawback can be addressed by using liquid metal in the metamaterial structure. Liquid metal is a good electrical conductor with soft and deformable shape [17]. The common liquid metals are Hg, Ga and its alloy, like Eutectic Gallium-Indium (EGaIn). Compared with Hg, whose toxicity obviously limits its applications, Ga has much lower toxicity. Consequently, Ga and its alloy have been utilized in various fields from biotherapy [18] to plasmonic devices [19,20]. One of the interesting applications for liquid metal is tunable absorber [21–25], which is usually fabricated in polydimethylsiloxane (PDMS) to alter resonance structure. The liquid metal enclosed in the PDMS channels can be refilled into designed shape [21–25], to tune the EM responses as wish. Some stretchable materials embedded with liquid metal have been proposed to realize absorber [26] and cloaking [27], because of good fluidity of EGaIn. In addition, utilizing good mobility of liquid metal, reconfigurable antenna [28] and filter [29] based on Ga alloy have been presented. The electric structure formed by EGaIn is changed by air pressure in PDMS, to further alter the EM responses of antennas or filters.
Previous researches on reconfigurable design using Ga or EGaIn usually realize single function like tuning frequency point or absorptivity. The micro-channel fabrication in PDMS also requires complicated process [25]. In this work, we demonstrate a method for reconfigurable metasurface based on liquid metal (EGaIn), to achieve flexible beam manipulation. The hollowed structure in the acrylic substrate allows EGaIn to redistribute into desired dimension, so as to generate specific phase responses. The composite design for the metasurface array enables flexible pattern groups, to realize beam-deflection, single- and dual-beam scattering fields. The measured and simulated data show good consistency. We believe this work will further extend the design freedom for liquid metal metasurfaces. In addition, the low cost and easy fabrication of our design may have potential applications in sensing and detection fields.
2. Principle and design
As showed in Fig. 1, the designed metasurface is composed by two kinds of unit cells, which have different cavity for liquid metal to fill. Due to its room-temperature melting point, EGaIn in substrate cavity can be refilled into designed structure with impact of heat stimulus and gravitational forces. Under the illumination of plane wave (linear polarization), the liquid metal in different sizes reflects distinct phase responses because of their different electric length. Since EM theory works in both microwave and optical domain, our ideas for liquid metal metasurfaces are also feasible for higher frequency band, even in optical domain. Four phase responses with 90° interval are designed into two kinds of elements, which are encoded as cell A and cell B. The units in the same columns share the same shape dimension, while the units in different columns can be redistributed into different sizes. Each two columns are grouped into a composite column form, which allows us a free combination for different phase patterns. As a result, four phase states can be arranged into diverse metasurface patterns, to generate single- or multi-beam scattering fields with different angles. To exhibit its beam-steering performance, we present four schemes to respectively realize single-beam deflection and dual-beam deflection with different scattering directions.
Fig. 1 The schematic of the liquid metal metasurface for flexible beam-steering. The liquid metal in specific structures generates distinct phase responses. With appropriate heat stimulus, the structure of liquid metal metasurface can be easily changed, to realize various beam-steering.
To realize the aforementioned functions, two simple unit cells (cell A and cell B) are designed, as Figs. 2(a) and 2(b) depicted. The structures from the top to bottom are acrylic substrate with the height H2 = 2 mm, FR4 (with a dielectric constant of 4.4 and loss tangent of 0.009) substrate with the height H1 = 1 mm and copper ground. At the interface between the top acrylic layer and the FR4 substrate, there is a cavity with metal liquid inside. The period size A of the square element is 15 mm. Figures 2(c) and 2(d) display the top- and side-view of the cells A and B respectively. The elements should be excited by a plane wave, with the polarization along y-axis, as we marked in Fig. 2(c). The detailed dimensions of cavity in cells A and B are clearly depicted and the shape of the cavity is like a polygon structure. As we all know, the electric length along the polarization direction (along y-axis in Fig. 2) mainly determined the reflected-phase response for a simple patch element. Consequently, we design four dimensions in two kinds of unit cells (B and F in cell A, G and L in cell B) to achieve four distinct phase responses, which are encoded into State 1-4. Two rectangular cavities are connected with a trapezoid channel. Almost the same volume (about 20 mm3) of EGaIn is injected into each element to fill one of the rectangular cavities. The thickness of the cavity T is 1 mm. The detailed sizes of cell A are designed as following: B = 3 mm, C = 7.1 mm, D = 13 mm, E = 3.2 mm, F = 6.9 mm, while the dimensions of cell B are provided as below: G = 7.55 mm, I = 3 mm, J = 9 mm, K = 3 mm, L = 8.5 mm. Please note that a little excrescent volume is designed into the element structure, to prevent the error of EGaIn injection volume, which may result in the error in final electric length. The injection volume of EGaIn is designed to be 20 mm3, so the precise dimensions of the four rectangular cavities are listed as following: 3*6.67 mm (State 1), 6.9*2.9 mm (State 2), 7.55*2.65 mm (State 3), 8.5*2.35 mm (State 4).
Fig. 2 The structure illustration for the designed element, and its EM responses. (a) and (b) The structures of the cell A and cell B. (c) and (d) The top- and side-view of the cell A and cell B. (e) and (f) The reflected phase and amplitude responses of the proposed elements, with four different states.
The element simulations are performed in the commercial software, CST Microwave Studio. In Fig. 2(e), the reflected phase responses of the metasurface elements with four different states are listed. We can observe that different states with distinct phase responses in C band. At 7.5 GHz, the phase responses of four states are 144.6°, 59.3°, −32.2°, −124.4° respectively. The phase difference between adjacent two states is almost 90°, promising the good phase manipulation performance. For simplicity, we encode them as digits “0”, “1”, “2”, “3” respectively in the following illustration. When the working frequency switches from 6 GHz to 9 GHz, the reflected amplitude responses of four different states are almost similar, as showed in Fig. 2(f), where four states all remain at about 0 dB, suggesting good performance. To clearly list the above information, we summary the element states and the related phase responses in one table, as listed in Table 1. The gray blocks in the element indicate the structure filled by liquid metal.
Table 1. The phase responses of four states for cell A and B
3. Results
To clearly exhibit the performance of the proposed metasurface, we design four schemes with distinct patterns, as given in Fig. 3. The whole metasurface contains 14*14 elements, with a size of 210*210 mm. Since the metasurface is fabricated into the composite columns, the phase pattern only varies along y-axis. Figure 3(a) exhibits scheme A with coding sequence of “01230123” that can generate a single-beam field scattering at 45°. The coding sequence of scheme B is “00112233” to achieve single-beam at about 20°. Both schemes C and D can realize the dual-beam forming. The coding sequences are designed to be “00220022” in scheme C and “00002222” in scheme D. The scheme C can generate dual-beam to point at ± 45°, while the scheme D can reflect two beams at about ± 20°.
Fig. 3 The coding patterns of the designed four schemes. (a) and (b) The schemes A and B to realize single-beam fields, with distinct beam deflection angles. (c) and (d) The schemes C and D to realize dual-beam fields, with distinct beam deflection angles.
We conduct full-wave simulations in the commercial software, CST Microwave Studio, for all four schemes. Figure 4 shows the calculated 3D far-field results for four schemes with different coding sequences at 7.5 GHz. To clearly distinguish the difference of scattering fields among these four schemes, the radiation power in Fig. 4 is all normalized. In Figs. 4(a) and 4(b), the field data is normalized to 10.8dB. We can notice that the energy of scattering fields generated by schemes A and B are nearly the same, but the deflecting angles of single-beam are quite different. The gains of the scattering beams in schemes A and B are all above 0.95. And the single-beam deflection angles are 45° (for scheme A) and 20° (for scheme B). Figures 4(c) and 4(d) provide the simulated far-field results of schemes C and D, in which the energy is normalized to 9.5dB. Both radiation patterns generate two beams but their deflecting angles are different. According to the generalized Snell’ law [5], larger pattern period produces a smaller deflection angle. The beam deflecting angle is about ± 45° for scheme C and ± 20° for scheme D. In these two cases, most of the energy is concentrated in the two main beams and the normalized gains vary from 0.75 to 0.85. Therefore, these simulated results of four schemes verify our design.
Fig. 4 The three-dimensional far-field results in simulations. (a) and (b) The simulated results for single-beam steering (scheme A and B). (c) and (d) The simulated results for dual-beam steering (scheme C and D).
To clearly elucidate the beam-steering performance, the 2D far-field results at 7.5 GHz are provided in Fig. 5, corresponding to the four designed schemes. Figure 5(a) shows the simulated results for schemes A and B, which achieve single-beam with different deflecting angles. The single-beam deflection angles of schemes A and B are about 45° and 20° respectively. In these two schemes, there is a slight difference in the reflected gains when deflecting angle changes and the reflecting gains of schemes A and B are about 10 dB. The results of schemes C and D are listed in Fig. 5(b), two reflected beams can be observed at ± 45° in scheme C and ± 20° in scheme D. Under the illumination of plane wave, both schemes generate symmetrical beam patterns. By comparison, the gain of dual-beam is a little smaller than the gain of single-beam. As labeled in Fig. 5(b), the gain of dual-beam scattering field is about 7 dB. Thus, the above observation further suggests the good agreement between simulations and our design targets.
Fig. 5 The two-dimensional far-field results in simulations. (a) The simulated results for single-beam steering (schemes A and B), as well as a PEC with the same dimension for comparison. (b) The simulated results for dual-beam steering (schemes C and D).
In metasurface fabrication, as showed in Fig. 6, the detailed process is presented. Figure 6(a) provides the detailed fabrication structure of metasurface sample. The whole metasurface is composed of FR4 substrate and acrylic cavity like a sandwich structure, in which EGaIn is injected into the cavity within a needle injection. The FR4 substrate and acrylic board are adhered by Ultraviolet Rays (UV) glue. UV glue is a kind of compound glue that will solidify rapidly under ultraviolet light. In the realistic fabrication process, we firstly adhere the middle acrylic board and FR4 substrate; then inject EGaIn into central cavity; finally close the element with top acrylic board. The EGaIn shape transformation process is depicted in Fig. 6(b). We first employ a thermostatic electric heater to melt EGaIn; then the metasurface is set upright to make the liquid metal refill the structure, with the impact of the gravitational force; finally, we cool and solidify the EGaIn. However, when we inject EGaIn into acrylic structure and before we thoroughly bond the whole structure, the EGaIn is exposed to the air and its surface is oxidized during this time. Its surface will be oxidized into a thin “skin”, which will prevent EGaIn to redistribute freely [30]. To dissolve this “skin”, we added a small amount of low concentration hydrochloric acid (HCl of wt10%) in the cavity structure. It is noteworthy that the room temperature in our experiment is 26°C. Therefore, we need to heat up the metal and melt it faster, so as to realize structure reconfiguration. Besides, we recommend to operate the liquid metal below its melting point, so that the shape of the liquid metal is solid, which is more convenient in experiment installation.
Fig. 6 The fabrication illustration and the photograph of the fabricated metasurface sample. (a) The fabrication structure of the liquid metal metasurface. (b) The reconfigurable process for liquid metal. (c) The composite form for final fabrication sample. (d) The photograph of the fabricated metasurface. (e) The schematic illustration of the far-field measurememnt.
Figure 6(c) shows the composite components (including FR4 substrate, top and middle acrylic boards) of fabricated sample. As we mentioned above, each two columns are fabricated into a composite form, for more flexible pattern combinations. Therefore, there are two methods to manipulate the phase distribution on the metasurface: one is liquid metal reshaping, the other is arranging these composite pieces to obtain the desired pattern. By combining the above two methods, we can realize various phase patterns. Take the schemes B and D for example, when we need to transform the phase pattern in scheme B into scheme D, we only need to heat up three composite pieces (state 3 into state 2, state 1 into state 0) in the metasurface and refill the liquid metal. The final fabrication sample is given in Fig. 6(d), and a clear element photograph is listed in the lower right corner subgraph. The far-field measurements are performed in a standard chamber room. We provide a schematic illustration in Fig. 6(e). the metaurface and its feed source are fixed on a rotatable table, and a receiving horn is placed at 10 meters away from the rotatable table to measure the far-field results in a two-dimensional plane.
Figure 7 shows the measured 2D far-filed results (in solid line) that agree well with the simulations (in dotted line). We can clearly observe the single-beam schemes A and B in Fig. 7(a), as well as the dual-beam schemes C and D in Fig. 7(b), which are corresponding to Figs. 5(a) and 5(b). The deflecting angles of the single-beam schemes in experiments are about 45° (scheme A) and 20° (scheme B), while the deflecting angles of the dual-beam schemes are ± 45° (scheme C) and ± 20° (scheme D). The measured energy distributions are in reasonable agreement with the simulated results (as listed in dotted line). But there are still slight deviations in energy and deflecting angle. The minor error between the simulated and the measured results is mainly owing to the following reasons: (1) the manual operation in measurement process, such as the position deviation in the installation on the measurement platform; (2) the manual error in fabrication process, like manual injection of EGaIn, which may lead to the slight difference in EGaIn amount; (3) the machine error in fabrication process, like the manufacture discrepancy in laser cutting for acrylic cavity.
Fig. 7 The two-dimensional far-field results in measurements. (a) The measured results (in solid line) for single-beam steering (schemes A and B), as well as the simulated results for comparison (in dotted line). (b) The measured results (in solid line) for dual-beam steering (schemes C and D), as well as the simulated results for comparison (in dotted line).
4. Conclusion
In summary, we demonstrate a reconfigurable metasurface based on the liquid metal (EGaIn), to realize flexible beam-steering in scattering fields. By exploiting the good liquidity and conductivity of liquid metal, EGaIn can be easily transformed into different structure sizes, to generate distinct phase responses. We design two reconfigurable unit cells equipped with four phase responses (almost 0°, 90°, 180°, 270°). And the presented metasurface is fabricated into composite forms for flexible pattern combinations. Four pattern schemes including single- and dual-beam forming, are designed to demonstrate beam-steering performance. Both single-beam and dual-beam scattering fields with different deflecting angles exhibit great consistency between simulations and measurements.
In contrast to the traditional reconfigurable methods, such as PIN diode or varactor, the present metasurface yields lower cost and system complexity, as well as good performance on scattering-field manipulation. Besides, our design ideas are possible to be realized in optical domain, within the high-accuracy platform like microfluidics channel [31]. For linear-polarization plane wave, the presented design is also feasible in THz frequency band. We envision that by applying specific microfluidics channel, the liquid metal can be injected into it, to be filled into desired structure. And the liquid metal inside microfluidics structure can be controlled by air pump, to achieve various functionalities. We believe this work may further enrich the modulation methods for metasurface, and facilitate some potential applications in smart sensing and detections.
Funding
National Natural Science Foundation of China (11404207); SHIEP Foundation (K2014-054, Z2015-086); The Local Colleges and Universities Capacity Building Program of the Shanghai Science and Technology Committee, China (15110500900).
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