Tài liệu Tt luan an tieng anh trang

MINISTRY OF EDUCATION VIETNAM ACADEMY AND TRAINING OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ……..….***………… PHAM THI TRANG INVESTIGATION ON CONTROLLING THE RESONANCE FREQUENCY AND THE RESONANCE AMPLITUDE OF METAMATERIALS Major: Electronic Materials Code : 62.44.01.23 SUMMARY OF DOCTORAL THESIS IN MATERIAL SCIENCE Ha noi – 2017 The thesis has been completed at: Institute of Materials Science Vietnam Academy of Science and Technology. Science supervisors: 1. Assoc. Prof., Dr. Vu Dinh Lam 2. Assoc. Prof., Dr. Le Van Hong Reviewer 1: Assoc. Prof., Dr. Trinh Xuan Hoang Reviewer 2: Assoc. Prof., Dr. Nguyen Minh Thuy Reviewer 3: Assoc. Prof., Dr.Sc. Pham Van Hoi The thesis was defended at National level Council of Thesis Assessment held at Graduate University of Science and Technology - Vietnam Academy of Science and Technology at on , 2017. Thesis can be further referred at: - The Library of Graduate University of Science and Technology - National Library of Vietnam INTRODUCTION In this day and age, the scientific and technological revolutions of researching new materials are proceeding remarkably worldwide. Researching and seeking new materials which are better, cheaper replacing traditional ones have become a vital need. Moreover, this also creates materials owning new properties, compared with traditional materials in realistic application. Metamaterials are the materials which have artificial structure, formed by arranging and managing the order of unit cells. Shapes and sizes of unit cells play the role as “atoms” in traditional materials. Properties of metamaterials can be changed with shape, composition, and order of unit cells. There are many methods of researching into metamaterials, MPA is one of the most considerable. This material was put forward and proved firstly in 2008 by Landy and his partners. Landy proved metamaterials can entirely absorb electromagnetic wave energy and does not reflect. Metamaterials electromagnetic absorber have many advantages, they are thin, easy to be made, cheap, easily controlled, especially controlled by peripheral influences. Numerous MPAs are proposed and researched with the exploitation of structural artificiality, in order to search for the simple structure, possessing large working frequency band to apply in reality, such as medical, science devices, energy battery, invisible cloaking [4]. Furthermore, materials are really potential in other aspects like frequency filter, resonance, antenna and biological sensor. Basing on interesting and well – applied properties of metamaterials, it is worth every scientist of concern all around the world. Metamaterials, in general, operate with the bedrock is existence of magnetic and electromagnetic resonance when interact with magnetic and electromagnetic elements of electromagnetic wave radiating. However, resonant properties usually occur in the range of narrow frequency band and depend on polarization of electromagnetic wave. Therefore, before putting metamaterials into reak applications, it is necessary to solve a number of problems: seeking materials which have simple structure, is easy to be manufactured deploy in applications and not up to electromagnetic wave polarization; large working frequency band materials and electromagnetic wave isotropic receiving materials are of broad interest [5]. In the aspect of a researching group at Institute of Materials Science, fellow Do Thanh Viet successfully defended his PhD thesis about electromagnetic entirely absorbing materials in 2015. His thesis has concentrated on researching to maximize the capability of structure of structure and widening working 1 frequency band of electromagnetic absorbing materials, based on cut wire pair structure and ring structure in the range of GHz frequency. Nevertheless, structures researched by PhD Do Thanh Viet only absorb electromagnetic wave in one direction, not in reverse one. Meanwhile, fellow Nguyen Thi Hien successfully defended her PhD thesis in the early of 2016 about materials which refractive index is negative. This thesis focused on capability of widening working frequency band by using Hybridization plasmon effect. In this effect, she researched interaction (exterior) among structural layers’s impacts on properties of materials. In this thesis, fellows focused on two chief contents below: i) ii) Widening working frequency band of metamaterials by using internal interaction in structure. Particularly, researching the impact of dielectric layer, the impact of structural asymmetrical on widening of negative permeability range of materials. Searching for metamaterials which have isotropic absorbing structure, do not depend on electromagnetic wave polarization. For these reasons, the thesis aims to design and manufacture metamaterials which: i) ii) Have simple structure, wide working frequency band. Highly symmetrical structure, absorb isotropic electromagnetic wave. Objects of research are metamaterials absorbing electromagnetic wave. Scientific and realistic significance of the thesis: Thesis is a basic work of research. Researches solve problems like widening working frequency band of metamaterials with simple, easy to be made structure. Besides, thesis has successfully found out metamaterials which have electromagnetic isotropic absorbing structure, do not depend on polarization of electromagnetic wave working in GHz frequency range. Similarly important is that this is the precursor of following researches at high frequency range, further is completely controlling technology of manufacturing metamaterials which operate in infrared and visible frequency range, with many applications in reality. The thesis is divided into 4 chapters: Chapter 1: Overview of metamaterials. Chapter 2: Method of research. 2 Chapter 3: Impacts of parameter structure on negative permeability range’s width of metamaterials. Chapter 4: Electromagnetic isotropic absorbing metamaterials. Layout of thesis: Consist of 117 pages, comprising: Beginning, 4 chapters of content with 86 pictures, conclusions, continued ways of research, list of reference documents, works published and appendix. Main results of thesis have been published on 6 newspapers and magazines at home and abroad as well as conference proceedings. CHAPTER 1: OVERVIEW OF METAMATERIALS 1.1. Metamaterials 1.1.2. Classification of metamaterials Two principal quantities determining electromagnetic wave transmittance in materials is permittivity ɛ and permeability µ. Classification of metamaterials may base on negative or positive value of these two quantities (Picture 1.2). Figure 1.2: Parameter space for the real parts of permittivity and permeability According to Picture 1.2 graph, metamaterials can be classified as 4 main types: - Negative dielectric materials (Electric materials): ɛ < 0. Negative permeability materials (Magnetic materials): µ < 0. Negative refractive index materials (Left – handed materials): n < 0. Electromagnetic entirely absorbing materials. 3 1.2. Metamaterial perfect absorber (MPA) 1.2.2 Absorption mechanism of metamaterials MPA structure consist of 3 layers: (i) top layer is a metal circulating structure, (ii) the center is dielectric structure and (iii) bottom s metal panel. Recently, there are two main ways popularly used to explain the roles of MPA structure’s layer. 1.2.2.1. Absorption mechanism based on impedance matching Electromagnetic waves to the interface surface can be reflected, transmitted, absorbed, scattered or can be excited by surface plasmonics. The scientists have demonstrated that for MPA, at the frequency of absorption occurs, scattering and wave phenomena in surface are negligible [4]. Therefore, we can calculate the absorptivity as follows: A=1–R–T In which: T is the transmission, R is the reflection, A is the Absorption. In case if the electromagnetic wave is perpendicular to the sample we have the reflection: 2 z  z0  n R  r z  z0 r  n 2 With Z =  /  is the impedance of MPA and Z0 =  0 /  0 is the impedance of the air. In case MPA has the third layer is covered by full metallic, T will be equal to 0, then the absorption is equal to: 2 z  z0  n  1 r A=1–R=1– z  z0 r  n 2 So the idea to make an MPA is through the impedance matching of the material with the air environment to cancel the refraction, while the copper film at the back layer is thick enough to block all transmission. The cyclic flat structures by metal in the front and the backside metal plate are separated by the dielectric, create magnetic resonance. When the electromagnetic wave energy is dissipated on the metal and the dielectric layer of the material, a small fraction of the energy is used and maintained to produce magnetic resonance, which is responsible for the charge. This is the cause of metamaterial perfect absorber. 4 1.2.2.2. Absorption mechanism based on destructive interference Wave interference theory negates the role of magnetic resonance. In this case, the two metal layers in the absorbing material act as wave reflecting surfaces. We will selection the dielectric material with reasonable thickness and dielectric constant such that the waves are reflected from the metal circular layer and the overlap of multiple reflections between the two opposite metal layers leads to destructive reflection completely. In addition, the back metal plate acts as an electromagnetic interference block that leads to zero transmittance. That is the cause of the largest absorptivity. Figure 1.15: Light transmission model to MPA [42]. 1.2.3. Some methods of manufacturing transform materials The first MPA structure proposed by Landy et. al. in 2008. It was a complicated structure which based on the resonance ring. Therefore, this structure had a lot of difficulties in fabrication and measurement. Since then, the process of finding an optimal MPA structure continues to be strong in all frequencies ranging from GHz, THz to optical frequency [45-50] typically focusing on a number of structures such as ring structure, dish structure ... by the geometrical symmetry of them. Materials absorb at a certain frequency, so they will only be applicable in limited circumstances. For most applications, the material should be designed with broad band absorption characteristics, independent from wave polarization. The independent from wave polarization can be achieved by the symmetry structures of the top metal elements. To achieve wide band absorption, several methods have been proposed such as placing more than one resonant structure on a super unit cell. Basically, there are two methods for designing transform materials with a wide frequency range: (i) structures are placed on the same plane as the super unit cell and (ii) structures are placed on the perpendicular axes to the sample 5 plane. In the case of different sized resonant structures arranged on the same plane to create the super unit cell, there will be more absorption peak at different frequencies. When these peaks are close together, a wide absorption band is obtained. In the case of cells placed on an axis perpendicular to the sample plane, the sample creates a pyramidal structure, each of which forms a resonance peak. These resonance peaks are close together, stacked together to form a wide absorption band [26-27]. 1.4. The conclusion of chapter 1 Chapter 1 presents an overview of transform materials: the concept of transform materials, the classification of transform materials and some applications of transform materials. In particular, the thesis focuses on how MPAs operate in different frequency bands with the applications of this material. In addition, the thesis details the absorption mechanism of the modified material to explain the structural role of the MPA, as well as the design and fabrication methods for materials with different frequency bands. CHAPTER 2: RESEARCH METHODS. 2.1. Approach and method of studying material change The interactive studies of transform materials with electromagnetic waves used in the thesis are based on a combination of three methods which are physical modeling method based on the LC equivalent circuit, the simulation method and empirical methods. 2.3. Physical modeling method based on the LC equivalent circuit So far, the interaction of transform materials with electromagnetic waves has often been explained based on the LC equivalent circuit, proposed by Zhou et. al. [59]. From this model we can calculate approximately the absorption frequency, or the frequency at which magnetic resonance phenomena occurs according to structural parameters. 2.4. The simulation method The thesis use CST program to study the electromagnetic field interaction with materials, because CST software that has been featured in prestigious magazines. Furthermore, the Institute of Materials Science bought this software. One of the important steps in using the CST software is to set input parameters, including: materials (available from the bank of available materials or the inclusion of new materials which are not available in the simulation 6 program), shape, size and structural parameters of the base cell, boundary conditions, material surroundings. The obtained output parameters include: complex scattering parameters such as transmission coefficient S21, reflection coefficient S11, whereby the reflectance R (ω) and the transmittance T (ω) can be obtained from T(ω) = |S21|2 and R(ω) = |S11|2 and the phases of the electromagnetic waves passing through the transform material structure allow us to calculate the absorptivity of material A = 1 - (T + R). 2.5. Experiment methods Figure 2.13: The process of metamaterial fabrication The material is selected for operation in the frequency range of 12 - 18 GHz so the thesis proceeds to make the material by lithography method. Modeling process: The initial material used for the production of the sample was a conventional printed circuit board (SME, Korea) consisting of a FR -4 dielectric (Cu coated both sides). 2.6. Conclusions of Chapter 2 Chapter 2 presents the research methods used in the thesis such as simulation methods, experimental methods and LC models for studying the interaction between electromagnetic waves and MM. CST commercial software based on FIT were used. Theoretical models are calculated based on model of Zhou's group. Lithography technology is used to fabricate the sample sand a vector network analyzer is used for the measurements of materials. Chen's algorithm also is used to calculate other effective parameters of material (  ,  , n, z ). 7 CHAPTER 3: EFFECTS OF STRUCTURAL PARAMETERS TO THE WIDTH OF NEGATIVE PERMEABILITY RIGIME OF METAMATERIALS. In this thesis, I investigate the expansion of the negative permeability regime of MMs by CWP structure, which is a traditional and fundamental structure of MMs with the advantages of simple structure, easy to manufacture and apply to reality. At first, we increased the width of negative permeability regime by changing dielectric thickness. Noticeably, we demonstrate the general function determined magnectic resonance frequency, the expansion of negative permeability regime based on internal interaction. With the desire to find another way to expand the regime of negative permeability, we investigate the effects of symmetry and asymmetry of CWP structure in direction of electric field, magnetic field and both directions. 3.2 The role of dielectric thickness on the expansion of negative permeability regime 3.2.1 Theoretical model Magnetic resonance frequency of CWP structure was calculated by Zhou et al [59] based on equivalent LC circuit in equation (2.6) (Chapter 2). According to this equation, magnetic resonance frequency is only depended on the length of CW and permittivity ɛ of the dielectric spacer. However, some experimental results shows that the thickness of the dielectric spacer has strongly impact on magnetic resonance frequency [60, 61, 79]. Therefore, we propose a different model to calculate inductance Lm and capacitance Cm in LC circuit model by considering 2 CWs interaction. Then, inductance Lm and capacitance Cm are defined by: Lm  0lw 2  w  ts  2  td  2ts  , Cm   0c1lw td (3.1) Where l is the length, w is the width and ts is the thickness of CW, td is the thickness of dielectric spacer, c1 is coefficient of 0,2 ≤ 𝑐1 ≤ 0,3. Magnetic resonance in this case is defined as: f  c  l 2 c1 (1  2ts / td ) / (1  ts ) w (3.2) Equation (3.2) shows that magnetic resonance frequency is depended not only on the length of CW and permittivity, but also on the width of CW and the thickness of dielectric substrate 𝑡 𝑑 . Noticeably, when the thickness of CW ts is much smaller than the width (ts << w), ts is tiny in comparison to the thickness of 8 dielectric spacer (ts << td), then equation (3.2) revert to (2.6) of Zhou. So, inductive interaction is not considered in Zhou’s equation. After that, in order to study the role of the dielectric spacer’s thickness on the expansion of negative permeability regime. The relative expansion of negative permeability region is calculated as: f 1  1 f0 1 F (3.12) Where F is structural coefficient [80]: F 2 0 Nl 2td Lm 2 td l  w  ts   ax a y az w td  2ts 2 (3.4) The results of (3.4) and (3.12) shows that when the thickness of dielectric spacer increases, the region of negative permeability would be expanded. 3.2.2. Simulated and experimental results Figure 3.4 shows the simulation and experiment results of transmission of the structure with different td. (a) (b) Figure 3.4: The simulated (a) and experimental (b) results of transmission working with td Table 3.1: Comparison among simulation, experiment and calculation methods about magnetic resonance frequency working with td 𝑓 𝑚 (GHz) 𝑓 𝑚 (GHz) 𝑓 𝑚 (GHz) 0.4 0.8 Theory 14.02 14.18 Simulation 13.97 14.17 Measure 14.07 14.43 1 14.33 14.27 14.51 𝑡𝑑 (mm) 9 From the Table, we see that the method given is appropriate. In particular, simulation results and experiments show that the magnetic resonance region is expanded more effectively when increasing td. For a better observation, Figure 3.5 shows the results of the transmission vs the thickness of the dielectric . The results show that the width of the transmission is very narrow, the intensity is weak when the dielectric thickness is small. However, when this thickness increases to 1 mm, the width of the transmission spectrum is broaden with greater intensity. Figure 3.5: Influence of dielectric thickness on transmission spectrum. Base on Chen's retrieval algorithm [57], Figure 3.6 (a) shows the dependence of magnetic permeability on the dielectric thickness. The results shows a significant improvement of the permeability region as the thickness increased. Specifically: when the dielectric thickness increased from 0.2 to 1.0 mm corresponding to the osmotic magnitude extending from 3.6% (0.55 GHz at center frequency 13.79 GHz) to 17% (2.42 GHz at center frequency 14.26 GHz). We then calculate the region width from dielectric thickness to dielectric based on Equation 3.13 of the theoretical model. The results obtained from the above two methods are presented in Table 3.2. Table 3.2: Influence of td on the expansion of negative magnetic region 0.2 0.4 0.6 0.8 ∆f/f Theory 3.7% 6.1% 8.8% 11.8% ∆f/f Simulation 3.6% 5.7% 8.5% 14.5% 1 15.1% 17.0% td (mm) 10 The results reconfirm a great agreement between the simulation and theoretical calculations. Therefore, the thickness of the dielectric layer plays an important role in widening the working frequency range of our metamaterial. When the thickness of the dielectric layer increases, the absorption bandwidth also increases. 3.3. Effect of asymmetry property on negative permeable region extensibility of CWP structure 3.3.1. Equivalent LC circuit model Figure 3.7(a) shows asymmetric structure of CWP following the electric direction through increasing the length of one bar CW and fix the size of remaining one. Figure 3.7(b) presents circuit model LC which is equivalent to asymmetric CWP structure. The structure asymmetry leads to changing charge distribution on two head of a pair CW, creating distinct capacitance values with a small error C (because of the change of dielectric thickness td which connects between two opposite charge heads). These capacitance values generate veryclosing magnetic resonances. By this combination, negative permeable region is expanded. (a) (b) Figure 3.7: (a) CWP structure transform from symmetry to asymmetry (electric field direction E), (b) equivalent circuit of asymmetric CWP structure. Following (2.2) and (2.6), we define expression of relative extensibility of negative permeable region in asymmetric structure in comparison with that of symmetric structure. t f  1 d 1 f td (3.13) Where ∆C/C is capacitance-changing magnitude to initial capacitance value ratio. 11 This ratio is determined based on parameters of asymmetric CWP structure as follows: C  C 1 t 1 d td 1 (3.14) 2  l  t d    1 1 . td  td  Where: Figure 3.8 describes the growth of (3.15)  l   f td l , versus , when l  0 or   0  . It f td td  td  is clear that when l increases, the thickness of dielectric layer connecting two electrolysis increases, resulting in the increase of relative extensibility of magnetic resonance region. Particularly, when l  2td , the negative permeable extensibility can be reached 50%. 1.8 td/td 1.6 f/f 1.4 td/td, f/f 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 l/td Figure 3.8: The dependence of l f td , on . td f td Without loss of generality, we next investigate collapsion of symmetry property of CWP structure following to magnetic direction of electromagnetic wave by increasing the width w of one bar and fixing remaining CW bar. Figure 3.9 expresses symmetric CWP structure transformed to asymmetric CWP structure following w direction (width direction of bar). When increasing the size of w which is similar to increasing the length l, however, electric field formed between two bars CW has perpendicular direction with that of projected electromagnetic waves. Therefore, electric field strengh corresponding to additional area c1l w is negligible, capacitance formed by structure changes little from C to C  C ' ( C ' : small value). Hence, negative permeable region is extended but that is negligible. 12 Figure 3.9: CWP structure transformed from symmetry to asymmetry following magnetic direction 3.3.2. Effect of structural asymmetry under electric direction In this section, we perform simulation and experiment to CWP asymmetric material with parameters l = 0 mm, 0.5 mm and 1.0 mm. Network constants of base cell ax = 3.5 mm, ay = 9mm. Dielectric layer FR-4 is used with dielectric constant of   4.3 and thickness of td = 0.4 mm. Metal substance in structure is Cu with conductivity of 5.96.107 Sm-1. The length of bar CW in initial symmetric structure is l = 5.5 mm, width of w = 1 mm and thickness of ts = 0.036 mm. Figure 3.10 shows results of spread spectrum based on simulation and experiment of asymmetric structure CWP with ∆l set at 0, 0.5 and 1.0 mm and ∆l/td set at 0, 1.25, 2.5, respectively. Figure 3.10: Spread spectrum for simulation and experiment for CWP structure when breaking structural asymmetry with l = 0, 0.5, 1 mm. It is clear that the spread spectrum is almost similar for two methods. In order to observe more clearly about magnetic resonance extensibility, it is better to simulate with parameter l varying from 0 to 1.2 mm. We can see that (figure 3.11) when l increasing, magnetic resonance region is extended. In order to re-verify magnetic resonance extensibility by Chen, real part of permeability magnitude of material is computed based on simulation figure 13 expressed as in figure 3.12. The simulation results shows that, negative permeability magnitude is extended gradually as ∆l increases. Particularly, the increasing of ∆l from 0 to 1.25 td corresponds to the negative permeability magnitude extended from 12.8% (bandwidth of 1.8 GHz at resonant frequency of 14 GHz) to 23.1% (bandwidth of 3.08 GHz at resonant frequency of 13.38 GHz). At l  2td , the negative permeability value is increased to 24.8% (bandwidth of 3.31GHz at resonant frequency of 13.376 GHz), at l  2.5td the negative permeability magnitude is increased to 27.8% (3.7 GHz at central frequency of 13.3 GHz). Figure 3.11: Spread spectrum of asymmetric CWP structure when ∆l changes in range of 0 – 1.2 mm. Figure 3.12: Permeability magnitude when ∆l is 0, 0.125td, 1.5td, 2td, 2.5td. In order to explain more clearly about causes of magnetic resonance extensibility, we perform to simulate distribution of electromagnetic lines corresponding to following values: ∆l = 0, 1.5td, 2.5td. 14 (a) (b) (c) Figure 3.13: Distribution of electric field at magnetic resonance frequency on CWP structure with: (a) ∆l = 0, (b) ∆l = 1.5td, (c) ∆l = 2.5td. It is clear that the distribution of magnetic lines at ∆l = 0, ∆l = 1.5td is almost similar, this demonstrates that there has been charge transfer to bar head CW 2. Nevertheless, at ∆l = 2.5td electricity distribution region between bar head CW1 and extra length of bar CW2 is partially uniform and non-uniform electric field, this demonstrates that the electrolysis does not transfer completely to head point of CW2. In other words, the electrolysis only have limited distribution. That is why negative permeable region initially increases linearly and reaches to saturation point of about 25 – 28 %. 3.3.3. Effect of structural asymmetry under magnetic direction Figure 3.15 shows spread spectrum results through simulation and experiment of asymmetric CWP structure following magnetic direction when ∆w increases. Figure 3.15: Spread spectrum of asymmetric CWP structure when w rises, (a) Simulation, (b) Experiment. From above results, we can see that when the width w of CW2 increases, magnetic resonance region is extended negligibly. This is explained clearly in theoretical model in 3.3.1. By destroying symmetry of structure CWP, it is successful in designing one structure which negative permeable region is widen. Analytical results have shown that negative permeable area is extended much more effectively as 15 destroying the structural symmetry under electric direction than magnetic direction. 3.3.4. The affection of the asymmetrical structure according to two directions This thesis continue to study the asymmetrical CWP structure with the increase of CW2 bar in both length and width. The magnetic resonance region had been enlarged as expected. However, this structure has a transmittance which strongly depends on the polar angle that leads to a disadvantages in practical work. For the desire that find out a structure which can expand the negative permeability and does not has an impact on the polarization of the electromagnetic wave, we chose the double rings structure and started to simulate and experiment the transmission spectrum as well as what we have done with CWP structure. The result indicates that the asymmetrical double rings structure also has the same characteristics with the asymmetrical CWP structure. Although, the transmission spectrum of this structure does not depend on the polarization of the electromagnetic wave, which is the dominance of the double rings structure. 3.5. The summary of chapter 3 In summary, chapter 3 has researched the effect of the polarization of electromagnetic wave on the absorption properties of materials. The results show that meta-material which has CWP structure strongly depend on the polarization of electromagnetic wave meanwhile the double ring metamaterial is totally independent from the polarization. Besides, the this thesis has worked on the effect of the thickness of the dielectric layer on the resonance frequency and the width of the negative permeability frequency region of CWP structure material by using the internal interaction model (between metal bar in one unit cell). The final result indicates that the thicker dielectric layer is, the bigger resonance frequency and the width of negative permeability frequency region are. The result of simulation is compared to the result of calculation from LC circuit model and verify by experiments. This is an important result which contribute to research about absorbing electromagnetic wave metamaterial that has wide active region or negative refractive index material. Especially, this thesis has used the theoretical model utilizing LC equivalent circuit and simulation method. The effect of asymmetry of the unit cell (CWP and rings structures) on expanding the negative permeability 16 frequency region has researched. The result shows that the asymmetry along the length of CW bar heavily affects the expending working frequency band from 12.8 to 27.8% when ∆l changes from 0 to 2.5td. Meanwhile, the asymmetry along the width has a slight effect on the extensibility of working frequency band. The model has been built conformably for metamaterial not only at GHz frequency region but also the THz region. The experiment result is quite coincide to the simulation result and theoretical calculation. CHAPTER 4: ISOTROPIC METAMATERIAL ELECTROMAGNETIC WAVE ABSORBER Structure of perfect absorption material usually concludes the dielectric layer in the middle, the resonance structure layer in the front face and metal plate in the back. However, for the common designs, metamaterial can absorb the electromagnetic wave from only 1 certain direction and cannot absorb from the opposite direction. In the effort of finding isotropic perfect absorption material, chapter 4 will propose a symmetric structure based on the transformation of CWP structure as well as square pair structure, diamond pair structure and ring pair structure. With this idea, the electromagnetic wave perfect absorber is obtained by combining the loss of electric resonance and magnetic resonance on the material. 4.1. Electromagnetic wave isotropic absorption metamaterial The research of traditional metamaterial absorbers began from CW structure. But this structure provides the absorbance which depends on the polarization of electromagnetic wave. With the desire of overcoming that disadvantage, we turn to research MPA with square structure and rings structure. Those structures receive electromagnetic wave in only 1 certain direction and cannot receive it in opposite direction. That leads to a limit in practical application. In the next part of this thesis, we focus on finding isotropic metamaterial absorber. 4.2. Isotropic metamaterial electromagnetic wave absorber To obtain isotropic metamaterial electromagnetic wave absorber, the thesis begin at CWP structure-metamaterial. This is a traditional structure known as 2 types of common resonance: magnetic resonance and electric resonance. The resonance happened at low frequency is magnetic resonance and is electric resonance at high frequency. The idea here is creating a perfect absorber based on the combination of the loss at electric and magnetic resonance. 17 4.2.1. Metamaterial absorber based on CWP structure Figure 4.11: A single unit cell of CWP structure with parameters Figure 4.12: The simulation results: (a) transmission and (b) absorption spectra of CWP structure The unit cell of CWP structure-metamaterial is presented on figure 4.11. Transmission spectrum and absorption spectrum are presented in figure 4.12. The results show that there are two absorption peaks at two different frequencies (12.0 and 13.8 GHz). Those two peaks are corresponding with magnetic and electric resonance frequencies with the absorbance 60% and 40% alternately. This result suggests us that if shift these two peaks closer to each other, especially overlapping, we can obtain the perfect metamaterial absorber. However, controlling the structure parameters is facing with many difficulties because of their resonance properties [59]. Thus, this thesis focuses on finding material which has more symmetric structure and easier in controlling the resonances. 4.2.2 Square pair structure metamaterial Starting from CWP structure metamaterial, by increasing the width (w) as equal to the length (l), other parameters are maintained, we obtain square pair structure metamaterial. Figure 4.14 presents absorption spectrum of square structure material with two absorption peaks at two different frequency (f 1=11.2 GHz and f2=14.7 GHz). The first absorption peak is 52% caused by magnetic 18