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Abstract
Paper-based metamaterial is one kind of metamaterial based on the paper substrate, recently drawing a lot of attention due to its fascinating features. This paper proposes another fabrication solution to realize paper-based metamaterials by directly drawing instead of inkjet printing. The drawing tools we used included mechanical pencils, conductive ink pens, and a computer-controlled drawing machine. Three types of paper-based metamaterials—polarization converter, absorber, and conformal coding metasurface—have been fabricated by the drawing technique. The performances of these paper-based metamaterials have been validated through both simulations and measurements. The proposed drawing technique offers an economical, convenient, and flexible way to fabricate paper-based metamaterials with the advantages of thinness, lightness, and softness. It would be promising to apply in other regions of microwaves and electronics.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metamaterial (MM) is an artificially engineered material that has exhibited many extraordinary electromagnetic (EM) properties and has drawn significant interest among researchers. Through tuning the constitute parameters of MMs, we can design the EM responses of MMs and obtain some physical characters that do not exist in the natural world [1–4]. After two decades of rapid growth, the applications of MMs have stretched to many areas, such as polarization converters, invisible cloak, metalens, and so on [5–13]. The introduction of MM’s concept into designing novel EM devices achieves advantages in the physical dimension, weight, and flexibility compared with common devices. Recently, paper-based metamaterial—a novel MM composition and fabrication method based on paper substrate instead of dielectric board—has been proposed [14–16]. Due to the properties of paper substrate, the paper-based metamaterial can be extremely thin and light, and it is also convenient to be taken in by bending and folding. Without the chemical process during its fabrication, paper-based metamaterial is also cheaper and more eco-friendly compared with traditional MMs based on printed circuit board (PCB) technology or CMOS technology [17].
The most frequently used technique to fabricate paper-based metamaterials is inkjet printing which uses the inkjet printer to deposit tiny droplets at a specific location of a substrate [18]. This technique can fabricate paper-based metamaterials with high precision, but inkjet-printing facilities are expensive. Drawing MMs directly on paper is a more straightforward way and can be much cheaper than inkjet printing. To make the drawing process automatic and accurate, we adopt a computer-controlled drawing machine, which can control a pen or pencil to draw designed structures on paper.
In this paper, we first introduce the drawing method and its detailed process. Then we demonstrate the performances of three different types of paper-based metamaterials fabricated by the drawing method—a polarization converter, an absorber, and a conformal coding metasurface, as shown in Fig. 1. The agreement between experimental and simulated results of these paper-based metamaterials validates the feasibility and reliability of the proposed drawing method. Meanwhile, the conformal coding metasurface also demonstrated the feasibility of utilizing paper as a substrate to fabricate flexible MMs.
2. Drawing metamaterials on paper
Paper, as a cheap, easily available, and eco-friendly material, has been widely used in our daily life. Besides traditional use, paper has already found applications in many areas like sensing and engineering [19–22]. Compared with many materials we use, paper has the advantages of being light, thin, and soft, which benefits MM applications. The paper we choose here as the substrate of our paper-based metamaterials is the commercially available paper with a thickness of 0.22 mm and relative permittivity of 2.3. This type of paper is suitable for drawing, thin in thickness and light in weight.
MMs usually consist of the dielectric substrate and metallic resonant structures. In order to draw metallic structures on paper substrate, a conductive ink pen is utilized, whose ink has good conductivity and can be deposited on the paper. To obtain the conductivity of the ink, we used this pen to draw some rectangular shapes. Then we measured the geometric dimensions and the electrical conductivity of these shapes. Measured results showed that the conductivity of the conductive ink drawn on paper is about 3 × 106 S/m. For metamaterial absorbers, it is usually necessary to introduce resistors or resistive films to obtain Ohmic loss, especially when the dielectric substrate is thin. Through a series of tests, pencil was chosen to draw resistors and resistive films, because the pencil lead has a certain conductivity and can work well with the metallic structures drawn by conductive ink pen. Traditional pencils are not suitable for drawing resistors because the radius of the pencil lead is too large to draw tiny parts, and the nib will deform in the drawing process. Thus we use mechanical pencils instead to draw paper-based metamaterials. The diameter of the lead in the mechanical pencil is 0.5 mm, which is small enough to draw the resistors we need. The main components of the pencil leads are graphite and clay, and the graphite has certain conductivity. Different types of pencil leads have different graphite-to-clay ratios. Thus they have different conductivities.
Drawing MMs on paper is a delicate process requiring high accuracy in geometry and pressure. Drawing by hand cannot reach the level of precision we need and will take too much time and effort. Instead, we introduce a computer-controlled drawing machine to draw paper-based metamaterials, as shown in Fig. 2(a). The drawing machine has three stepping motors, providing three freedoms of motion for the pen loaded on the drawing machine. Two stepping motors control the pen to move freely in the horizontal plane, while another controls the pen to lift up and down. When the design of the metamaterial is finished, we can control the drawing machine to draw any designed shapes on paper, making the drawing process accurate and automatic. In the actual process of fabrication, a step of drying is also needed. After finishing drawing the conductors, the paper will be dried in an electric-heating drying oven at the temperature of 150 degrees Celsius for one to two hours. This step is to accelerate the drying process of the conductive ink. After the drying process in an electric-heating oven, the conductive ink on paper will be totally dried and the conductivity will reach its maximum.
Drawing resistive films with high accuracy of surface resistivity on paper is a demanding task. Many factors will affect the resistivity of the resistive films drawn by pencils, such as the texture of paper, the ratio of the graphite content in the pencil lead, the density of the drawing path, and the pressure added on the pencil while drawing. As for drawing resistors on paper, the dimension of the resistors is also a factor needed to be considered.
The pressure added on the pencil will affect the density of the pencil traces deposited on paper, thus affecting the resistances of the pencil traces. To obtain the pressure-resistance relationship, we drew some squares using the same mechanical pencil, same paper, and same drawing path but under different pressures. The results are shown in Fig. 2(b). As can be seen, there is an inverse relationship between pressure and resistance. Other factors like the tightness of drawing paths and the speed of drawing are also needed to be considered. There is one more factor to be considered when drawing resistors on paper, which is the length and width of the resistor. According to the basic theory of electronics, the length of the resistor is proportional to the resistance and the width is inversely proportional to the resistance. Therefore, we can adjust the resistance by tuning the length and width of the resistor. However, in practice, we can’t draw resistors of too large size because the design of MMs limits the size of the resistor. We also cannot draw resistors that are too small due to manufacturing error, which is inevitable to some extent in the drawing process.
If we want to improve the durability of the paper-based metamaterials and realize long-term preservation, another process of sealing the paper-based metamaterials with plastic films on both sides of the paper can be adopted. The material of the plastic film is PET with a thickness of 0.065 mm and relative permittivity of 3.5. With the protection of the plastic films, the paper-based metamaterials won’t be spoiled, making them practical and reliable in a broader range of application scenarios.
3. Paper-based polarization converter
Polarization is one of the most significant characteristics of EM waves, and researchers have devoted many efforts to manipulating the polarization state of EM waves [23–27]. In Ref. [23], a wideband linear polarization converter based on a tri-layered MM structure has been proposed, where a Fabry-Perot-like resonance cavity is formed between metallic layers to enhance the polarization conversion efficiency.
Figure 3(a) and 3(b) show the unit-cell of the proposed paper-based polarization converter. It is a linear polarization converter composed of three structural layers. The first and third layers are respectively composed of two metal gratings that are mutually perpendicular. The second layer in the middle consists of one metallic strip orienting at 45°. There is a foam board between each two adjacent layers. In this structure, the top layer is equivalent to an x-polarization filter, where the y-polarized waves pass through the top layer while the x-polarized waves are blocked. Meanwhile, the bottom layer corresponds to a y-polarized filter. The unit-cell has a period of p = 25 mm, and other physical parameters are optimized as h1 = 3 mm, h2 = 3 mm, g1 = 9.25 mm, g2 = 9 mm, w = 6 mm, and l = 26.6 mm. To investigate the transmission characteristics of the proposed unit-cell, a full-wave simulation was performed. From Fig. 3(c), we can see that it can convert the incident wave to a cross-polarized transmitted wave from 3.1 GHz to 7.1 GHz with a conversion efficiency larger than 90%.
Fig. 3. The unit-cell of the paper-based polarization converter. (a) Perspective view; (b) The second layer; (c) The simulated cross-polarized transmission.
The whole structure consists of 12 × 12 unit-cells and its dimension is 300 mm×300 mm, considering the fabrication and experimental conditions. To experimentally verify its performance, a sample was fabricated and tested. The fabrication process includes three steps. Firstly, we let the drawing machine draw the structures on paper with the conductive ink pen. Secondly, we dried the paper in the electric-heating drying oven at the temperature of 150 degrees Celsius for 90 minutes. At last, we assembled the three layers of paper and two boards of foam to obtain the whole structure. Each layer of the polarization converter and the whole structure are shown in Figs. 4(a)–4(d). Measured results of the cross-polarized transmission under normal incidence are shown in Fig. 4(e). It can be seen that the experimental results and the simulation results agree well. The paper-based polarization converter is able to rotate the linear polarization by 90° with a conversion efficiency of over 90% from 3.1 to 6.6 GHz. The performance of the paper-based polarization converter under oblique incidence was also investigated. Measured and simulated performances of the polarization converter for TE polarization under oblique incidence are shown in Fig. 5, which are in good agreement. It can be seen that the polarization converter shows relatively stable performance at an oblique incidence up to 30°.
Fig. 4. The fabricated polarization converter. (a) The whole structure; (b) The first layer; (c) The second layer; (d) The third layer; (e) The simulated and measured results of the cross-polarized transmission under normal incidence.
Fig. 5. The cross-polarized transmission of the paper-based polarization converter under oblique incidence. (a) Simulated results; (b) Experimental results.
4. Paper-based broadband absorber
Metamaterial absorber (MMA) is one of the important branches of MMs, which has a wide range of applications [28–32]. The absorption bandwidth is often a critical performance indicator for the application of the absorbers. In recent years, considerable interest has been attracted in the design of broadband absorbers [33–36]. However, not only are these structures heavy in weight but also require complicated fabrication processes. Hence, we proposed a paper-based broadband absorber which is light in weight and can be easily fabricated.
The proposed paper-based absorber is a broadband absorber composed of two structural layers, which contain two-dimensional periodic arrays of square loops drawn on paper. Figure 6 shows the proposed absorber's unit-cell geometry, consisting of two structural layers, two foam boards, and a metallic backplane made of tinfoil. The top and the bottom structural layers consist of single-square-loops (SSL) and double-square-loops (DSL), respectively. Each square is drawn by conductive ink pen, and at the center of the square’s arm, a resistor is drawn by mechanical pencil. Using the MATLAB optimization program with a genetic algorithm, we optimized the absorber to achieve the maximum -10 dB absorption bandwidth. The final dimensions of the paper-based absorber unit-cell are p = 30.0 mm, d1 = 29.0 mm, d2 = 15.0 mm, d3 = 25.0 mm, w1 = 1.8 mm, w2 = 1.2 mm, w3= 0.6 mm, R1 = 250 Ω, R2 = 300 Ω, R3 = 450 Ω, h1 = 10 mm, and h2 = 10 mm.
Fig. 6. The unit-cell geometry of the proposed absorber. (a) Perspective view; (b) The top layer; (c) The bottom layer.
The whole structure consists of 10 × 10 unit-cells and its dimension is 300 mm×300 mm. To demonstrate its performance experimentally, a sample was fabricated. The fabrication process includes four steps. Firstly, we used the drawing machine to draw the four arms of the squares by the conductive ink pen. Secondly, we dried the paper in the electric-heating drying oven at the temperature of 150 degrees Celsius for 90 minutes. Then, we used the drawing machine to draw the resistors loaded on the squares with mechanical pencils. At last, we assembled the two layers of paper, one layer of tinfoil, and two foam boards together to obtain the whole structure. The fabricated sample is shown in Fig. 7(a) and 7(b).
Fig. 7. (a) The top layer of the paper-based absorber; (b) The bottom layer; (c) Simulated and measured reflection under normal incidence.
Measured and simulated results of the reflectivity under normal incidence are shown in Fig. 7(c). It can be seen from measurements that the fabricated sample achieves 90% absorptivity ranging from 2.1 GHz to 10.5 GHz with a fractional bandwidth of 113% while the total mass of the absorber is only 58.3 g.
5. Paper-based conformal coding metasurface
So far, there are mainly two approaches to obtain radar cross section (RCS) reduction. One approach is to design MMAs [37,38], while the other is based on phase-cancellation mechanism [39–42]. Recently, metasurface combining absorption and phase cancellation mechanisms has been proposed to further broaden the bandwidth of RCS reduction [43–45]. In addition, flexible metasurface is demanded in many microwave applications, such as radar, antenna, aircraft [46–48], etc. The paper-based metamaterial is inherently flexible, so it can be easily wrapped around objects, which provides a new approach to the new field of EM applications and integration.
Based on these, we design a low-scattering paper-based conformal coding metasurface using hybrid mechanisms. The unit-cell of the proposed paper-based coding metasurface, which is composed of two structural layers and a metallic ground plane, is shown in Fig. 8. The top layer consists of the C-shape split resonant ring drawn by conductive ink pen. The reflected phase of the incident wave can be tuned by rotating the orientation of the C-shape split resonant rings. Thus we can take two different unit-cells, which have a 180° reflection phase difference, as ‘0’ and ‘1’ elements, respectively in 1-bit coding. Based on this, EM diffusion can be realized through the random phase distribution design. The other layer consists of the resistive square loops drawn by pencil. It can absorb the incident wave by converting it into Ohmic loss. The phase cancellation and absorption mechanisms respectively play critical roles at two different frequency bands.
Fig. 8. The unit-cell of the proposed paper-based coding metasurface. (a) Perspective view; (b) Bottom layer; (c) Top layer.
Using the MATLAB optimization program with a genetic algorithm, we optimized the unit cell of the paper-based coding metasurface, and the final dimensions are p = 20 mm, p2 = 10 mm, w = 2.3 mm, r = 8.4 mm, θ = 116°, l = 7.8 mm, s = 4.1 mm, h1 = 4 mm, and h2 = 4 mm. The sheet resistance of the square loops is 550 Ω/sq.
To investigate the reflection characteristics of the unit-cell, numerical simulation was performed with a full-wave electromagnetic simulator. Figure 9(a) depicts the simulated reflection magnitudes of the unit-cell under x- and y-polarized normal incidence. To better understand the mechanism of the unit-cell, we can calculate the proportion of the absorbed energy under normal incidence from its co-polarized and cross-polarized components, as shown in Fig. 9(b). It can be seen that as the frequency increases, more energy is absorbed by the resistive square loops. The ‘0’ and ‘1’ elements’ reflection phases of cross-polarization and their difference are given in Fig. 9(c). It is found that these two elements’ reflection phase difference always stays at 180°. Hence the reflected wave can be offset through phase cancellation.
Fig. 9. (a) The reflection coefficients of the unit-cell; (b) The reflection and absorption of the unit-cell; (c) The ‘0’ and ‘1’ elements’ reflection phases of cross-polarization and phase difference.
According to the random coding sequence, we constructed the whole metasurface with the dimension of 800 mm×480 mm, where the proportions of the ‘0’ and ‘1’ elements are both 50%. To analyze the RCS reduction on the curved surface, the proposed metasurface is bent on a metallic cylinder with a height of 480 mm and a radius of 127 mm, as shown in Fig. 10(a). Figure 10(b) shows the simulated RCS of the metasurface and a bare metallic cylinder with the same size under y-polarized normal incidence. Based on this, we can calculate the RCS reduction of the metasurface. It is clearly seen that the metasurface can achieve 10 dB RCS reduction over a frequency band ranging from 8.94 to 11.59 GHz with a fractional bandwidth of 25.8%.
Fig. 10. (a) Configuration of the coding metasurface bent on a metallic cylinder; (b) The simulated RCS of the conformal metasurface and metallic cylinder; (c) The calculated RCS reduction of the conformal metasurface.
To better illustrate the underlying mechanisms of the conformal metasurface, we further investigate the three-dimensional scattering patterns of the metasurface and the metallic cylinder at 9 GHz, 10 GHz, and 11 GHz, as shown in Figs. 11(a)–11(d). The 3D scattering patterns show more visually how the scattered energy of the metallic cylinder and conformal metasurface is distributed respectively in the backward space. In Fig. 11(d), EM waves only reflect into xoz-plane in the case of the bare metallic cylinder. However, in Figs. 11(a)–11(c), the reflected EM waves are redistributed in the whole backward space and dispersed not only along the x-axis but also along the y-axis, which is quite different from the metallic cylinder. Therefore, the backward scattering energy is obviously reduced when employing our conformal metasurface. We also give the two-dimensional scattering patterns in xoz- and yoz-plane at 9 GHz, 10 GHz, and 11 GHz to quantitatively observe the conformal metasurface’s performances, as shown in Figs. 11(e)–11(j). It is seen that in xoz-plane, the scattered energy of the conformal case is much less than that of the metallic cylinder at all angles. This is because the energy is not only absorbed, but also scattered along the y-axis when employing the metasurface, while the scattered energy of the metallic cylinder is mainly concentrated in xoz-plane and remains consistent at all angles. In Figs. 11(h)–11(j), the main lobe of the conformal case is suppressed compared with the metallic cylinder, while the energy of the conformal case is more than that of the metallic cylinder at other angles. These results indicate that the EM waves are diffused mainly along the y-axis.
Fig. 11. The 3D scattering patterns of the conformal metasurface at (a) 9 GHz, (b) 10 GHz, and (c) 11 GHz. The 3D scattering patterns of the metallic cylinder at (d) 10 GHz. The scattering patterns of the metasurface and the metallic cylinder in xoz-plane at (e) 9 GHz, (f) 10 GHz, and (g) 11 GHz. The scattering patterns of the metasurface and the metallic cylinder in yoz-plane at (h) 9 GHz, (i) 10 GHz, and (j) 11 GHz.
The fabrication process of the top layer is similar to the fabrication process of the paper-based polarization converter, which is described in section 3. The fabrication process of the second layer, composed of square loops, is similar to the fabrication process of the paper-based absorber, which is described in section 4, but we only need mechanical pencils here to draw the square loops. After each layer had been drawn, we attached them layer by layer to the metallic cylinder. The fabricated sample is shown in Fig. 12(a).
The RCS property measurement was performed in the anechoic chamber to eliminate the influence of the external environment and two linearly polarized horn antennas were used to transmit and receive the electromagnetic signal, as shown in Fig. 12(b). In the measurement, the calibration using a metallic sphere was performed first, and time gating is utilized to eliminate the effects of static areas, multipath reflections, and other factors in the limited size of the anechoic chamber. Then, we measured the RCS of the conformal metasurface and the metallic cylinder of the same size. At last, we calculated the RCS reduction of the conformal paper-based coding metasurface. The results are shown in Fig. 13. It is noted that 10 dB RCS reduction is achieved from 9.03 GHz to 11.97 GHz with a fractional bandwidth of 28%, which matches well with the simulated one.
Fig. 13. (a) The measured RCS of the metallic cylinder and the conformal metasurface; (b) The simulated and measured results of RCS reduction.
6. Conclusion
In this paper, the technique of drawing MM structures on paper is investigated and the process of fabricating paper-based metamaterials by drawing is demonstrated. The ballpoint pens containing conductive ink are used to draw conductors while the mechanical pencils are used to draw resistors and resistive films. In order to make the drawing process automatic and accurate, a computer-controlled drawing machine is adopted to replace the process of hand-drawing. Three types of paper-based metamaterials—absorber, polarization converter, and conformal coding metasurface—have been designed, fabricated, and measured to test the drawing technique. The agreement between simulated and experimental results of the three paper-based metamaterials demonstrates the feasibility and reliability of the drawing technique. The fabricated conformal coding metasurface also verifies the feasibility of using paper as a flexible metamaterial substrate. Considering the convenience and flexibility of the drawing technique, this work has developed a new method as an alternative to the traditional technique of MM fabrication.
Funding
Fundamental Research Funds for the Central Universities (021014380186); National Key Research and Development Program of China (2017YFA0700201); National Natural Science Foundation of China (61731010, 61801207, 62071215, 91963128); Priority Academic Program Development of Jiangsu Higher Education Institutions; Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Wave.
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.
Supplemental document
See Supplement 1 for supporting content.
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