Study on fabrication for broadband perfect absorption in the ghz region based on metamaterials

the structure of resonators, then, the operating frequency range is very narrow. This is the main cause of the lack of MMs with a simple structure, low cost of manufacture that can operate in multiple bands or in a wide frequency range simultaneously. In addition, maintaining multiple absorptions or absorption peaks on a wide range always requires increasing the unit cell dimensions or the physical thickness of symmetrical structures. In particular, constituent materials used in the fabrication of metamaterial absorber (MA) (metal-dielectric) are often inelastic, leading difficulties to cover it on curved surfaces of objects in practice. Therefore, it is still a challenge to uphold studying more advanced features for micro-structure MMs, with multi-band and wideband operation using recent manufacturing. 2. Purposes of the thesis - Building up a theoretical basis, studying MA operating in multi-band and broadband in the frequency region of 2-18 GHz. - Designing and optimizing structural parameters of MA operating in multi-band and broadband based on new models. Researching on MA operating in multi-band and broadband with simple structure and independent to the polarization of incident electromagnetic waves. - Elaborating and applying asymmetry in the geometrical structure of MMs to create multi-band absorption. - Fabricating and investigating electromagnetic characteristics of MA operating in multi-band and broadband in the frequency region of 2-18 GHz. 3 3. Main researching contents of the thesis - The thesis focuses on studying MA operating in the frequency region of 2-18 GHz in 2 main approaches: Asymmetric and isotropic MA with multi-peak; Broadening the absorption range by integrating conductive polymer. - The content of the thesis is based on a consistent combination of theoretical models and simulation software. The experimental process will be conducted to verify some typical simulation results and adaptable to recent manufacturing conditions. Accomplishment: This thesis focuses on solving and improving the problem of electromagnetically MA operating in the frequency of GHz as follows: i) Experimentally tested new models based on the symmetry breaking in the traditional resonant structure in a single unit cell. This is an effective approach to creating dual and multi-peak MPA; ii) Experimentally designed and verified MPA models integrated with low conductivity Polymer materials (partially and completely integrated into the metal resonant structure); iii) Investigated in simulation and experiment, verified the operating stability of multi-peak and wide- range MPA models under the change of incident angle and polarization angle of electromagnetic waves. The thesis is divided into 4 chapters as follows: Chapter 1. Overview Chapter 2. Researching methods Chapter 3. Asymmetric metamaterials absorber with multi-peak Chapter 4. Expanding the electromagnetic wave absorption range by integrating conductive polymer 4 CHAPTER 1: OVERVIEW 1.1. History of metamaterial and applications In terms of electromagnetic structure, metamaterials (MMs) are built up from "meta-atom", which are actually electromagnetic resonant structures much smaller than the operating wavelength. Theoretically predicted and proposed by Veselago in 1968, MMs researching area had been created from a concept of an environment with negative refractive index [negative permeability (µ <0] and permittivity (ε <0) simultaneously on the same frequency range]. This completes the overall picture of the values of the refractive index in all material environments [Figure 1.1]. Especially, in 2008, the phenomenon of perfect absorption of electromagnetic waves in MMs discovered by Landy and his colleagues opened a huge potential in science and technology. Figure 1.1. Classification of materials based on the sign of permittivity and permeability [42]. The first quadrant is conventional material with ε> 0 and µ> 0 simultaneously. The value ε <0 is only observed in the optical frequency area for metal. The case of µ <0 is only observed for some magnetic materials at low frequencies. Especially at the third quadrant, at the same time, two values µ <0 and ε <0 are only achieved in metamaterials. Currently, studies on MMs are not only limited in negative refractive index MMs, but also broaden to many areas and prospects for other modern applications, such as bio-sensors and wireless power transfer (WPT). 1.2. Effective medium theory Since the unit cell dimensions are much smaller than the operating wavelength, we must rely on a theoretical model called Effective Medium Theory (EMT) to design, predict and study the electromagnetic properties of MMs. The two most common EMTs today are known as the effective electromagnetic parameters model proposed by Bruggeman and Maxwell-Garnett. 1.3. Electromagnetic responses of MMs 1.3.1. Electrical resonant structures Firstly, in order to build up and understand the mechanism of electrical interaction occurring within each type of resonant structures, we can start from the principle of effective dielectric control of metamaterials. Another similar way for us is using LC equivalent circuit. The two basic resonators commonly used in traditional MMs are Split-ring resonator (SRR) structure and the cut-wire (CW) structure, as described in Figure 1.7. SRR structure [Figure 1.7 (a)] operates like an oscillating LC circuit and has its own resonant 5 frequency which can be calculated by the formula 𝜔𝐿𝐶 = 1 √𝐿𝐶⁄ . Figure 1.7. Split-ring resonator structure (SRR); (b) Cut-wire structure, and orientation of the external electric field. (c) LC equivalent circuit model and (d) characteristic diagram of the real part (ε ') and the imaginary part (ε' ') of the effective permeability. 1.3.2. Magnetic resonant structure Figure 1.8. (Left) The operating mechanism of SRR to create µeff < 0 and (Right) dispersive diagram of permeability Most of the conventional materials in nature have a positive permeability, only a few materials which have negative permeability. Besides, the magnetic properties of such materials usually exist at low frequencies, and almost extinguished in the GHz frequency region. However, magnetic resonance can also be obtained from non-magnetic materials by stimulating circular currents to create a dipole moment. Based on this principle, in 1999, Pendry proposed the first model to create the magnetic sounding at the GHz frequency region of a periodic sequence of two coaxial SRRs and under the special polarization of incident waves. These results are important prerequisites for designing and controlling the operating frequency of MPAs using electrically resonant and magnetic resonance structures. 1.4. Matching impedance of MMs with free space The impedance of MM can be defined as 𝑍(𝜔) = √𝜇(𝜔)/𝜀(𝜔) = 𝑍𝑟 + 𝑖𝑍𝑖. When condition 𝜇(𝜔) = 𝜀(𝜔) is satisfied, electric and magnetic energy of incident electromagnetic wave would completely propagate through the interface MM and surrounding medium. This phenomenon is the so-called impedance matching between materials and wave-guide medium. In this case, 𝑍(𝜔) = 𝑍0(𝜔) = √𝜇0/𝜀0 ≈ 377 Ω leads to the reflectance at the interface 𝑅(𝜔) = 0, which is also an outstanding property of MMs compared to natural materials. 1.5. Metamaterial perfect absorbers The basic structure of MPAs can be classified into two main types: anisotropic and isotropic. 6 Anisotropic MPAs are commonly designed with three layers: the first layer is composed of periodic metal structures, the middle layer is made of dielectric and a continuous metal layer in the bottom. In order to obtain perfect absorption, two conditions must be satisfied simultaneously: the reflectance and the transmission equal to zero. The electromagnetic waves transmitted to the MPA will not be reflected since the design of MMs satisfies the impedance matching condition in the desired frequency region. Meanwhile, the continuous metal layer prevents all electromagnetic waves from passing through the MPA. Therefore, the absorbance can be calculated based on the formula 𝐴(𝜔) = 1 − |𝑆11(𝜔)| 2. Fig. 1.17. (a) Unit cell of MPA proposed by Landy and (b) Ohmic loss and dielectric loss distributions at a resonant frequency in GHz region. * Energy dissipating mechanism in MPAs The dielectric layer in MPAs, besides providing a space to confine the energy of electromagnetic waves, in some cases it also makes a significant contribution to the absorption mechanism when the dielectric loss is dominant. It is notable that the Ohmic loss also characterizes the energy dissipation of the electromagnetic wave but occurs within the metallic layer, similar to the energy dissipation of the resistor inside the LC resonant circuit. 1.6. Multi-band and broadband Metamaterial perfect absorber Since the extraordinary properties of MPA are all generated by electromagnetic resonances, the operating frequency (perfect absorption) region is commonly very narrow and difficult to adjust. Therefore, it is necessary to increase or expand the operating frequency region of MPAs for practical application. In general, there are some conventional methods for creating a wide-band MPA: resistor integration, using multi-layer structure and combining many resonant structures in the planar unit cell. CHAPTER 2: RESEARCHING METHODS 2.1. Photolithography In order to fabricate MPAs operating in the frequency range of 2 to 18 GHz, we exploit the photolithography method. The initial material is printed circuit boards - PCBs. The PCB printed circuit consisting of a dielectric layer FR-4 with dielectric constant ε equals to approximately 4.3 and the dielectric layer thickness can be varied from t = 0.4 to 1.6 mm covered with a layer of copper (Cu - with a thickness of approximately 0.036 mm) on both sides. The fabricating system of this method has been set up at the Department of Magnetic and Superconducting Materials Physics, Institute of Materials Science, Vietnam Academy of Science and Technology. 7 2.2. Simulation method Based on the finite integration technique (FIT) of Weiland in CST program, we can visually simulate interactions between the electromagnetic field and materials. The obvious advantage of CST is that it could investigate some properties that are difficult to verify and observe experimentally. 2.3. Measurement method In order to experimentally study the absorption properties of metamaterials, reflection and transmission parameters are measured. Measurements in the GHz frequency region are normally performed by the Vector Network Analyzers system in the Anechoic Chamber. 2.4. Calculation method The direct measurements of the effective parameters such as permeability, permittivity, impedance and refractive index of metamaterials are very complicated and difficult. Therefore, the calculation method of Nicolson - Ross - Weir is often used to calculate these parameters in complex expression (refractive index, impedance, dielectric coefficient and permeability) of MMs through scattering data obtained from the simulation process. CHAPTER 3: ASYMMETRIC AND ISOTROPIC MULTI-BAND METAMATERIAL ABSORBER 3.1. Asymmetric effect of coaxial resonant double-ring structure The advantage of this design is the "tightening" of resonant structures to minimize the size of a unit cell. A lattice constant is chosen as a = 10 mm and the structure consists of three layers: metal-dielectric - metal. The dielectric layer (FR-4, thickness td = 0.8mm) has a dielectric constant of 4.3 and a loss tangent of 0.025. Two metallic layers were chosen as Copper (thickness tm = 0.036 mm) with a conductivity of 5.8x107 S/m. To control the resonant frequency in the RD structure and proceed to the fabrication of MPA operating in a broad range of frequency, we created a "gap” - g in the closed resonant ring. Therefore, the new structure becomes a combination of circular disk structure and split-ring resonator (SRD), as shown in Figure 3.1(c). Fig. 3.1. The unit cell of MMs using (a) circular disk structure (RD) (b) circular disk combined with closed ring (RD) and (c) circular disk combined with split-ring resonator (SRD). 8 Fig. 3.5. Dependance of absorption spectra on the width g in SRD structure: (a) simulation and (b) measurement. To further investigate the dependence of the absorption spectrum on the SRD structural parameters, we changed the value of g from 0.2 to 1.4 mm. Obviously, the blue shift was observed in both simulation and experiment, respectively, in Figures 3.5 (a) and 3.5 (b), especially resonant peaks at a lower frequency. The blue shift of the absorption peak is due to the attenuation in value of the effective magnetometer when a narrow gap appears (approximately 1/3 of the effective permeability value in the case of a closed resonant ring). This reduction is proportional to the width of the gap (g). 3.2. Asymmetric effect of closed ring resonance structure Fig. 3.7. (a) Fabricated samples of MMs using closed-ring resonator and (b) unit cell model, (c) Simulation and experiment absorption spectrum. In this section, we propose and investigate another model of asymmetric of MMs with the goal of dual- band absorption in the frequency range of 10 to 15 GHz. Figures 3.7 (a) and 3.7 (b) show the fabrication and design patterns of a unit cell with a closed resonant ring. This structure consists of three layers: metal-dielectric - metal. The dielectric substrate is selected as FR-4 with a dielectric constant and the losses are 4.3 and 0.025 respectively. The thickness of the metal layer (Copper) is tm = 0.036 mm. The other structural parameters are optimized as: R1= 3, R2= 0.7 and a = 10 mm. Figure 6(c) shows the simulated and experiment absorption spectrum, respectively. Obviously, in both cases, the absorption reaches 99% at 13.0 GHz. The mechanism of absorption for this type of structure comes from magnetic resonance, similar to previous studies. 9 Fig. 3.10. (a) Fabricated sample corresponds to (b) an asymmetrical fisheye structure. (c) Simulated and experiment (d) absorption spectrum in case of various distance d. Based on the symmetrical unit cell [Figure 3.7 (b)], we conducted the study of the asymmetric effect in this structure by moving the hole diagonally in the unit cell [adjusted through distance d in Figure 3.10 (b)]. This new structure has a similar shape as the "fisheye" structure. All other structural parameters of the unit cell are kept constant. Results of simulation and experiment of absorption spectral change by distance d are shown in Figures 3.10 (c) and 3.10 (d). Clearly, with d = 1.5 mm, the absorption spectrum exists a single absorption peak at 13.5 GHz (over 99%). As d increases to 2.3 mm, beside the initial absorption peak (unchanged absorbance), the second peak appears around the frequency of 13.5 GHz (approximately 99%). 3.3. Anisotropic dual-band metamaterial absorber In this section, beside breaking asymmetry of anisotropic MPA structure, we propose a new isotropic MPA model in order to dual-band absorption by near-field coupling effect, also known as electromagnetically-induced transparent effect (EIT). The resonant structure consists of three layers: Cu - FR-4 - Cu. Four pairs of metallic dishes are designed to be fixed at the quadratic centres of the unit cell on both the front and back sides of the structure. The simulation results completely are in agreement with the experimental results when two resonant peaks were obtained at 12.88 and 15.56 GHz with absorbances of 90.5% and 90.3%, respectively. 3.4. Multi-band metamaterial absorber with X-shaped structure As the result discussed above, by breaking the symmetry of the coaxial structure or the closed resonant ring, we can easily generate dual and multi-peak absorption using the supercell construction method. However, this method is easy to violate the conditions of effective medium theory when the size of the supercell is approaching to the operating wavelength. In addition, the integration of many structures in one unit cell could make the MPA thicker in order to maintain perfect absorption peaks at multiple frequencies simultaneously. Therefore, to overcome this limitation, we propose an X-shaped MPA structure model. This model is not the only advantage of simplicity in design and manufacture but also expected to increase the 10 number of degrees of freedom of the induced charges when its symmetry is broken, leading to multi-peak absorption. Fig. 3.20. (a) The unit cell structure of an X-shaped MPA oriented in 3D. The front sides of (b) the symmetric and (c) asymmetric structures, where d is the diagonal displacement a metallic wire in the unit cell. Fig. 3.21. Comparison of simulation (blue dashed line) and experiment (solid red line) absorption spectrum of the triple-peak MPA structure in case of (a) d = 0, (b) d = 0.5 and (c) d = 1.0 mm. Simulation and experiment absorption spectrum show that the absorbance in the initial case (d = 0) reaches 24% at 11.4 GHz. When the displacement d = 0.5 mm, we observe three simultaneous absorption peaks appearing at frequencies of 10.8, 11.5 and 14.7 GHz. The absorbances achieved corresponding to the above frequencies are 99%, 99% and 95% respectively. 3.5. Conclusions In this chapter, we have proposed and successfully demonstrated an effective method for generating multi-peak absorption in the GHz frequency region. Firstly, we design and optimize single-peak structures and 11 then, by breaking their symmetry, new emerging absorption peaks appear based on magnetic resonances (for anisotropic MPA model) and near field coupling of resonances (for isotropic MPA models). The above results are verified by the method of calculating LC equivalent circuit model, simulating by CST software and experimental measurement. These results are expected to open up an efficient way of generating broadband absorption for further studies. CHAPTER 4: BROADENING ABSORPTION FREQUENCY RANGE BY INTEGRATING CONDUCTIVE POLYMER 4.1. Broadening the MPA absorption frequency range based on the integration of conductive polymer. Fig. 4.1. (a) Three-dimensional arrangement of the unit cell for wide-band MPA with the polarization of EM wave. (b) Fabricated sample and its magnification for the front and the back layers of 2 × 2 unit cells. (b) Illustrated arrangement for the experimental configuration. With the goal of creating broadband MPA with a simple structure and easy to fabricate, we propose a new MPA model with the "active" role for the third layer (commonly a continuous metallic layer) of an MPA anisotropy. This idea is considered as a new generation of anisotropic MPA, broadband absorption with a planar structure and flexible integration of a variety of materials. In this proposal, by integrating low conductivity materials in combination with a continuous metallic layer, our proposed BMPA is expected to meet the practical requirements of the broad electromagnetic wave. and not dependent on polarity. Generally, most recent BMPAs depend significantly on the polarization angle of the incident wave. To evaluate the advantages of the proposed BMPA, we also investigate the dependence of absorption on the incident angle (θ) for both cases of TE and TM polarization, as shown in Figure 4.4. By changing the incident angle in the case of TE polarization, the simulated absorption spectrum shows that the value off FBW = 51% (at zero angle) is slightly weakened to 32.8% at the incident angle θ = 50° [Fig. 4.4 (a)]. Particularly, Figure 4.4 (b) confirms a good agreement between the result of the measurement absorption spectrum of the fabricated sample and the simulation spectrum. When θ = 5°, the absorbance is greater than 90% from 5.7 to 9.1 GHz (FBW = 46%). At θ = 50 °, the absorbance remains at 90% from 6.07 to 8.41 GHz (FBW = 32.3%). 12 Fig. 4.4 Performance of the wide-band MPA in a wide range of incident angle. (a) Simulated and (b) measured absorption spectra according to the incident angle of EM wave for the TE polarization. (c) Simulated dependence of absorption on the incident angle of EM wave for the TM polarization. 4.2. Broadening the MPA absorption frequency range by completely replacing metallic structure by conductive polymer. Differentially to the model presented above, we propose a new model with a complete replacement of the first metallic pattern with conductive polymer. Figure 4.7 (a) shows a BMPA unit cell consisting of three layers: polymer-dielectric-metal. At the centre, the dielectric layer is carved by four cylinders (radius and height are r and h, respectively). This positions will be completely filled with conductive polymer (σ = 150 S/m). The optimized geometry parameters of BMPA are: a = 6, r = 0.8 and h = 1.8 mm. The dielectric layer FR-4 has thickness t = 2.0 mm and the dielectric constant 4.3. The bottom continuous metal layer is made of Copper (thickness tm = 0.036 mm) with conductivity σc = 5.8x107 S/m. The main idea of this design is the combination of two basic magnetic resonances (created by the interaction between the upper and lower surfaces, and the interaction between adjacent resonant structures) when putting in perfect impedance matching condition at microwave frequency. As observed in Figure 4.7 (b), two nearly perfect absorption peaks (99.99%) appeared at frequencies of 15.3 and 20.1 GHz (red curve). Therefore, the absorption property over the wide frequency range with absorption rates over 90% (the green band) has been achieved from 13.52 to 22.18 GHz. In this range, the FBW is 48.5%. 13 Fig. 4.7. (a) 3D MM structure with the polarization of electromagnetic waves and (b) the corresponding absorption spectrum of proposed BMPA. The blue band represents the absorption frequency range over 90%. 4.3. Conclusions In this chapter, we present the results of integrating and completely replacing metal components in traditional MPA structures by low conductivity polymer. The addition of high loss components in traditional structures to achieve broad absorption also is a potential approach, towards making simple, easy-to-fabricate MM with the elastic feature. These results are an important basis for further studies to integrate MMs in electronic devices that operate stably under manipulation by multifunctional peripheral effects such as temperature, optical, electromagnetic sensors. CONCLUSIONS The thesis "Study and fabrication of broadband superabsorbent materials based on metamaterials" has been carried out at the Academy of Science and Technology and the Institute of Materials Science, Vietnam Academy of Science and Technology. The results related to the thesis have been published and are under review in 06 international and national journals (04 papers published in ISI journals), 02 papers on the proceeding of specialized scientific conferences. The results of the thesis have several considerably contributions as follows: 1. Studied and presented the theoretical basis of MPA to create multi-peak and wide-range absorption models in the frequency range from 2-18 GHz. 2. Proposed and investigated the electromagnetic properties of asymmetrical multi-peak MPAs. For the symmetry breaking of the two coaxial resonances structure: both simulation and experimentation are in agreement with two absorption peaks at 14.0 and 16.2 GHz (over 90%). The proposed MPA can be adjustable based on the narrow gap of the outer resonant ring. This also is the mechanism for creating eight absorption peaks over 80% in the frequency range of 12.5 to 16.2 GHz. Other asymmetric models also demonstrated to 14 be dominant when achieving nearly perfect absorption in the case of dual-band (at 13.05 and 13.5 GHz for fisheye structure) and the case of triple-band (at 9.7, 11.6 and 17.1 GHz for the X-structure.) 3. Proposed and experimentally verified the dual-band and isotropic MPA model operating at frequencies of 12.8 and 15.5 GHz with the absorbance of more than 90% for the first time. The experimental measurement results are consistent with simulation and theoretical calculation. 4. Proposed and demonstrated a new broadband MPA model with the integration of low conductivity Polymer. With the "active" role of the third layer in anisotropic MPA, the absorbance is greater than 90% from 5.7 to 9.1 GHz (FBW = 46%) in both simulation and experiment (under increasing angle conditions up to 400 and for all polar