Defect in the structure generates multi-band perfect metamaterial absorber in THz regime

156 JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2017-0044 Mathematical and Physical Sci. 2017, Vol. 62, Iss. 8, pp. 156-161 This paper is available online at DEFECT IN THE STRUCTURE GENERATES MULTI-BAND PERFECT METAMATERIAL ABSORBER IN THz REGIME Tran Manh Cuong Faculty of Physics, Hanoi National University of Education Abstract. In this paper, we present a multi-band perfect metamaterial absorber in the THz band applying defects to the absorber structure. Open boundary condition with an excitation port is used for the simulation of the full structure. A controlled defect was then introduced into the structure to study multi-band absorption. Multi-perfect absorption peaks were observed at 19.8 and 23.2 THz for the structure with a defect of 4 unit cells. This is a simpler method to generate a THz multi-frequency absorber compared with ones in the previous works. This is the first study considering the influence of micrometer structural defect on the absorption frequency in metamaterial absorber. Therefore, the findings of this study are valuable for research which focuses on high frequency communication applications, identifications, imaging or sensor technologies. Keywords: Absorber, defect, metamaterial, reflection coefficient, THz. 1. Introduction The emergence of metamaterials has generated a number of research in the filed of material science. The study of an artificial metal-dielectric periodic configuration which possesses special electromagnetic characteristics does not exist in conventional natural materials and its particular applications has been attracting increasing interest for many decades [1, 2]. With the development of technology, the era of electronic and electromagnetic devices has just sprang. Most of the unimaginable applications could be applied more in the near future. The metamaterial is a good sample which can certainly help this process accelerate. The study of perfect metamaterial absorbers (MPAs) - metamaterials which can completely absorb electromagnetic wave (in all regions ranging from MHz frequency band to infrared and visible region and beyond, in which we study the THz region) has caught the attention of different research groups worldwide [2]. Metamaterial absorber has many potential applications in various fields such as sensors, thermal imaging, radar shields, satellite communications, high-performance antennas, radiation or enhance the energy absorption of solar cells, etc [3-5]. However, many challenges must be dealt with, to improve the characteristics of metamaterial absorbers as they create multi-band or broadband [7-12]. In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. This frequency range plays a very importance role in the identification of chemical substances, in the communication of aircraft to satellite, or satellite to satellite, terahertz waves can also produce images with very high resolution, and spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry. Received July 13, 2017. Accepted August 9, 2017. Contact: Tran Manh Cuong, e-mail: tmcuong@hnue.edu.vn Defect in the structure generates multi-band perfect metamaterial absorber in THz regime 157 This paper presents a new method by exploiting defects under control in the metamaterial structure. Multi- absorption peaks were generated at around 18, 19 and 23 THz for the structure containing the defect of 4 or 16 unit cells. Being under control, these defects can help create new absorption peaks leading to expansion of the absorption band of the structure. As far as we know, this method is considered as the first method being used so far. Compared to other current methods, this method is simpler, more effective and feasible, and more practical. The calculations are performed using Finite Integration Technique (FIT) with CST Microwave studio package. 2. Result and discussion In our study, the defect under controls will be investigated in order to study the physical mechanism of new absorption peaks. In one of our recent studies in the GHz frequency range, we used the defect in the structure to create multi-band absorber metamaterial structure and this structure works well in the range of 23 GHz [6]. This type of defects has the advantage in controlling the desired frequency band, as well as expanding the operating frequency range of the absorber structure. For the study, we choose the structure with 100 unit cells because this dimension is optimal for the sake of simplicity in our investigation. This configuration was started in GHz range [6]. In this paper, the dimension of the structure is optimized for working in the THz frequency range. Figure 1 shows a unit cell of proposed metamaterial absorber. Three layers are included for this configuration: the top layer consists of one gold rectangular shape (conductivity  = 4.561 × 107 Sm -1 ) and one gold circle inside. The bottom layer includes a gold background plane, which was separated by 1.5 µm FR4 dielectric layer (The electric permittivity and loss tangent of the middle layer are εr = 4.5 and tan δ = 0.025, respectively). The optimal parameters are applied as follows: a = 9 m, b = 6.5 m, c = 3.5 m. Figure 1. Dimension of a fundamental unit cell Figure 2. The structures containing defects Tran Manh Cuong 158 Figure 3. The reflection spectrum of all simulation cases Figure 4. Electric vector distribution for the 4 unit cells defect case The study structure was evaluated when a number of unit cells were removed from the center of the top layer while the symmetry of the structure was kept unchanged. The defects with 1, 4 unit cells, 9 unit cells and 4 × 4 (16 unit cells) is simulated (Figure 2). We do not studythe larger defects due to their correlation size and the absorption structure. The simulation result of reflection coefficients of the structures with different defects are shown in Figure 3. We found that the defects of 4 and 16 unit cells strongly affect the absorption frequency. When removing 4 unit cells, the two absorption frequencies are 19.8 THz and 23.2 THz (fundamental absorption frequency). In the case of 16 unit cells defect (4 × 4 cells), three peaks were observed (18 THz, 20.8 THz, and 23.1 THz). Structures with 1 and 9 unit cell defects have only one absorption frequency. Therefore, we concentrated on analyzing the absorption peaks of the structure with the defect of 4 and 16 unit cells. To understand the physical mechanisms of absorption structures, we observed the electromagnetic energy focusing on the surface of the 4-unit cell defect structure. The field distribution of the structure was observed by simulation and is shown in Figure 4. We observed that at 19.8 THz, the distribution of electric field was mainly found at the defect position. The second absorption band appears at 23.2 THz. The electromagnetic density at 19.8 THz is distributed more densely than the density distribution at 23.2 THz, therefore the reflectivity Defect in the structure generates multi-band perfect metamaterial absorber in THz regime 159 at 19.8 THz is less dense than the reflectivity at the frequency 23.2 THz. Furthermore, at this frequency, we can see that the electric field and the magnetic field are trapped in the defect region, therefore, the defective region is considered as a rectangular resonant cavity. A standing wave mode is formed at the resonance frequency which helps confine the energy inside the resonant cavity. It is worth pointing that, for our structure, it is easy to create a multi-band resonance. This feature is different from other common absorption structures which usually consist of individual unit cells. The distribution of loss density energy in the structure at 19.8 THz and 23.2 THz also indicates that most of the electric field density concentrates inside the cavity. For a defect structure of 4 × 4 unit cells, the electric field distribution density is observed and shown in Figure 5 as follows: Figure 5. Electric vector distribution for the 16 unit cells defect case For this structure from Figure 5 with 4 × 4 unit-cell of the defect, it is noted that the size of the resonant cavity is larger. Three new absorption bands were observed at 18, 19.8 and 23.1 THz. The distribution of electric field vector for this structure at the absorption frequencies is illustrated in Figure 5. The physical mechanism of this structure is considered to be similar to the 2 × 2 unit cell defect case. In order to have a better understanding, we examined the absorber structure with the change in position of the defect. This time, the defect was moved to the edge of the structure as illustrated in Figure 6. The dimension of the defect was maintained as precedent. For the sake of simplicity, the defects with 1 and 2 × 2 unit cells were studied in this investigation. Figure 6. The structure containing defects at the egde position. Tran Manh Cuong 160 Figure 7. The reflection spectrum of all cases from Fig. 6 Figure 7 shows the reflection coefficients of the structures with the different position of defect. The calculation reveals that each structure with the same defect size but located in different positions has absorption peaks at the same frequency band. In other words, the absorption frequency of the structure does not depend remarkably on the position of the defect. In case of one unit cell defect in the middle and the edge of the structure there is an absorption peak at 23.2 THz. With a defect of 4 unit cells (2 × 2 cells) at the center and edge of the structure, there are two absorptions at 19.8 THz and 23.2 THz. Following similar simulation results of defect structures at the center, we consider electromagnetic energy focusing on the surface of the structure in order to understand the physical mechanisms of absorption of these structures,. What can be found is that the electric field density is concentrated mainly in the defective regions in both structures. Therefore, the absorption peaks at these frequencies occur. Moreover, defects in the absorption structure act as a resonant cavity, which confines the electromagnetic energy within it and produce a high absorption peak for the absorption structure at 19.8 and 23.2 THz. 3. Conclusion In this study, we simulated the absorption properties of the perfect metamaterial absorber by optimizing the size and position of the unit cell defect in the structure at the THz frequency regime. The results show that, by applying the convenient controlled defect electromagnetic cavity, multi- perfect absorption peaks can be generated at around 18, 20 and 23 THz for the structure containing the defect of 4 or 16 unit cells. Moreover, the simulation shows that the absorption is independence from the position of the defect in the structure. This research provides a better understanding of the interaction between electromagnetic waves and absorption structures in order to achieve the perfect absorption of electromagnetic waves in the THz frequency bands. By using this method, one can effectively control the frequency of absorption and development of multi- or broadband absorption metamaterials at THz region. Acknowledgmen: This research was funded by the Vietnam National Foundation for Science and Technology Development (Grant No. 103.99-2017.26). Defect in the structure generates multi-band perfect metamaterial absorber in THz regime 161 REFERENCES [1] Veselago, V. G, 1968. The electrodynamics of substances with simultaneously negative values of ε and μ. Soviet Physics Uspekhi, Vol. 10, No. 4, pp. 509-514. [2] Landy, N.I., Sajuyigbe, S., Mock, J.J., et al., 2008. Perfect metamaterial absorber, Phys. Rev. Lett., 100, (20), p. 207402. [3] H. Tao, E. A. Kadlec, A. C. Strikwerda, K. Fan, W. J. Padilla, R. D. Averitt, E. 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