INTRODUCTION
Metamaterials are synthetic materials tailoring electromagnetic properties that are challenging to obtain in nature. Metamaterials have attracted broad interest and have led to many potential electromagnetic applications, from microwaves to optical regimes. These applications reflect the unique characteristics of metamaterials, such as the ability to tailor electromagnetic properties 1. Metamaterials are widely used in optics, especially as researchers seek to increase the image resolution of lenses by adjusting negative refractive index values 2,3. Besides being used in perfect lenses, metamaterials are also widely applied as biosensor metamaterials 4,24, perfect absorbers 5,6, intelligent surfaces 7, and high refractive index materials 8,9. The use of metamaterials is also highly developed in the field of aeronautics, especially in radar and the camouflaging of aircraft. With the addition of metamaterials to the fuselage, aircraft can be rendered undetectable by enemy radar 10,11. The manufacture of metamaterials is carried out according to user needs by designing specific unit cells to deliver desired functions 12,13.
Several metamaterial studies use circular unit cells 12,14,15 as the design of the unit cells and paper as the dielectric material 12,16,17. Paper is a promising material for use in electronic platform devices because it is inexpensive and environmentally friendly. Yudistira et al. 12 designed and simulated a metamaterial based on circular unit cells with paper as the dielectric to obtain a negative refractive index. The simulation achieved a high figure of merit (FoM) negative refractive index using a circular design on paper. However, they have not created a sample due to the fabrication challenges on paper.
The fabrication method is a significant issue for creating metamaterials operating at the optical and microwave spectrums. The print circuit board (PCB) method is often applied to microwave metamaterial fabrication 18,19,20; however, this method is seldom used on paper. This study proposes a hand-painting method to fabricate a microwave metamaterial with a negative refractive index. Hand painting is a simple method for fabricating microwave metamaterials and is viable for use on paper, including glossy paper as used in this study. Metamaterials with a negative refractive index are widely applied to perfect lens applications. Simulation results are validated using experimental results.
MATERIAL DESIGN, SIMULATION, AND FABRICATION PROCESS
A negative refractive index was obtained on a circular design in a previous study 12,21. Loop surface current existing in the surface of the circular shape produces the magnetization phenomenon, the presence of which is indicated by the permeability of the sample. The huge electric field existing in the gap between the circular shape is responsible for the polarization phenomenon, with its permittivity describing the polarization of the sample. The metamaterial design is illustrated in Figure 1a. The circular diameter (D), length of unit cell (L), unit cell gap size (2G), paper thickness (T1), and silver thickness (T2) are 40 mm, 50 mm, 10 mm, 1 mm, and 0.1mm, respectively.
Numerical simulation work employed the CST Studio Suite 2019 software, with the circular conductor model as silver and the substrate model as paper. The x-y boundary condition is described as the unit cell, and the z boundary condition is described as open (add space). The properties of silver and paper are 13,000 S/m for conductivity (conductive material), and 2.31 for relative permittivity (dielectric material), respectively. The 3 – 6 GHz frequencies are used in the simulation spectrum.
Metamaterial sample fabrication is performed using a 3D printing mold and a paintbrush by the inexpensive and straightforward hand-painting method. The first fabrication process is making a mold using a 3D printing machine. This mold is then placed onto glossy paper. A paintbrush is then used to apply the silver ink. The 3D printing cast can help the painting process achieve a perfect sample shape. When the painting process is complete, the mold is removed from the glossy paper. The fabricated metamaterial by hand painting is presented in figure 1b.
The microwave measurement system presented in figure 1c is set up to measure the fabricated metamaterial by hand painting’s performance. The vector network analyzer Measall KC901V is used on the microwave measurement set up. The generated spectrum of vector network analyzer Measall KC901V can be tailored from 9 kHz to 6.8 GHz. Two Deepace R101C antennas are connected to a vector network analyzer as transmitter and receiver antennas. The range frequency of the Deepace R101C antenna is 730 MHz – 6.5 GHz. Therefore, the frequency spectrum used in this experiment is similar to the simulation range of 3–6 GHz. The microwave propagates through the metamaterial. Therefore, the presented scattering parameter (S-parameter) on the vector network analyzer depends on the metamaterial’s properties. The S-parameter from the experiment is used to validate the S-parameter from the simulation.
THE METAMATERIAL PROPERTIES PREDICTING
The reviewed S-parameters on this work are reflection (S11) and transmission (S12). The relationships between reflection (S11) and transmission (S12) and impedance (z) and refractive index (n) are described by Equations 1 and 2 12,22.
where k0
Calculating refractive index is similar to calculating impedance by inverting Equations 1 and 2:
A passive medium is usually applied for the metamaterial. The real impedance and the imaginary refractive index are greater than zero to describe a passive medium. The electrical properties of metamaterial are sensitive to the perturbation of the S-parameter. Therefore, the refractive index can be predicted from another equation not involved in specifying the root sign Equations 4 and 5. Equation 7 can predict the refractive index (n):
Equation 8 and 9 are used for the estimation of permeability and permittivity.
RESULTS AND DISCUSSION
Figure 2a presents the simulation’s transmission result (black line) and reflection result (red dashed line). The peak of low transmission is 0.02286 at 5.44 GHz, and the peak of high reflection is 0.97494 at 5.38 GHz.
Discrepancies between the simulation and experiment results exist as shown in Figure 2b. The lowest peak frequency is 0.02 at 5.44 GHz for the simulation (black line) and 0.21 at 5.17 GHz for the experiment (red dashed line). The simulation shows a tendency to follow a downward pattern before rising, while the experiment shows rises and falls but still has one lowest peak frequency. The perturbation of transmission experiment results is due to existing noise during the measuring of the sample. The lowest peak frequency of the simulation is at 5.44 GHz, while the lowest peak frequency of the experiment is at 5.17 GHz. The lowest peak frequency of the simulation is close to the lowest peak frequency of the experiment. Several factors, such as the silver ink’s non-uniform thickness and the imperfect circular shape of the fabricated samples, can affect the disparity between simulation data and experiment data. Overall, the experiment results have a similar trend to the simulation results.
The metamaterial impedance and refractive indexes are retrieved from Equations 4 and 7, respectively. The retrieving of metamaterial properties, such as impedance and refractive index, requires the scattering parameter results, transmission, and reflection, as in Figure 2a. Figures 3a and 3b present the impedance and refractive indexes of the metamaterial, respectively. The real impedance and imaginary refractive index have positive values at 3–6 GHz that fulfill the passive medium criteria. Refractive index with negative values is yielded at 3–3.6 GHz. High transmission performance is delivered at the spectrum of the negative refractive index. The permittivity and permeability properties are involved in the proving of refractive index with negative value performance.
Permittivity and permeability are estimated using Equations 8 and 9, respectively. Figures 4a and 4b present permittivity and permeability, respectively. The permittivity describes the polarization in the unit cell, which is mainly yielded in the tiny gap of the unit cell. Meanwhile, permeability represents the magnetization of the unit cell, produced due to its loop current. The fundamental idea of a negative refractive index condition is that the controlling unit cells simultaneously yield the negative real permittivity and the negative real permeability. However, Figure 4 presents the positive real part of the permittivity and the permeability of the negative refractive index at 3–3.6 GHz. Tung et al. 23 have introduced another basic concept for negative refractive index metamaterial conditions by stating that Equation 10 must be fulfilled to achieve negative refractive index in a metamaterial:
Figure 5 presents the calculation of
\epsilon
CONCLUSION
This study successfully fabricated a negative refractive index
metamaterial based on circular shape with paper as dielectric by using
the hand-painting method. The limitations of the fabrication method
produce the disparity between experiment and simulation results. The
hand-painting method has limitations, such as the non-uniform
conductive silver line thickness and the imperfect circular shape of
the fabrication sample. Overall, the experiment results show a similar
trend to the simulation results. The calculation of
\epsilon