Simulation of metal via-wall based ultra broadband terahertz full-sized metamaterial absorber

65 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2018-0071 Natural Sciences 2018, Volume 63, Issue 11, pp. 65-70 This paper is available online at SIMULATION OF METAL VIA-WALL BASED ULTRA BROADBAND TERAHERTZ FULL-SIZED METAMATERIAL ABSORBER Tran Manh Cuong 1 , Nguyen Thi Thuy 1 , Do Hoang Tung 2 , Pham Van Hai 1 and Vu Dinh Lam 3 1 Faculty of Physics, Hanoi National University of Education 2 Institute of Physics, Vietnam Academy of Science and Technology 3 Graduate University of Science and Technology, Vietnam Academy of Science and Technology Abstract. We present the calculation results of a full-sized ultra broadband absorber metamaterial structure at THz region from 19-26 THz. After being integrating the metal via- wall and controlling unit cells, this structure provides a broadband absorption and becomes independent on the polarization of the incident wave. This structure can be applied to improve the performance of telecommunications systems such as micro antenna systems, micro-electromagnetic transmitter system. This structure is particularly feasible to integrate into the antenna system due to its size and that it possesses superior electromagnetic properties and is easy to handle. Simulations and calculations show that for the structure after appropriately applied an electromagnetically shielded wall, the absorption bandwidth is increased sharply to 5 THz with the absorptivity is over 95%. This structure can easily integrate with a micro-antenna system and other terahertz imaging or sensor technology. Keywords: Absorber, defect, metamaterial, THz. 1. Introduction Metamaterial is artificially periodic structure which attracted immense research attentions in the recent decade for their exotic dynamic features that are cannot be found in the natural materials. Their properties were first predicted by Veselago [1] and demonstrated experimentally by Smith et al. [2]. Due to the extraordinary properties, metamaterials are promised to provide a numerous outstanding applications such as super-lens, invisible cloaking, and bio sensor, especially in the antenna and the telecommunication fields as well as the perfect absorber [3-5]. Since the first proposal of Landy et al. in 2008 [4], the metamaterial perfect absorber (MPA) which simultaneously excite electric and magnetic resonances to satisfy the impedance matching with the surrounding air has remarkably become one of the most crucial research trends. With the development of science and technology nowadays, especially in the field of electromagnetic telecommunications, great demands on the integration of electronic materials into the system are increasing. Besides, Terahertz is a spectrum that possesses many important technological applications, such as imaging, sensor, recycle energy or micro antenna communication, which all require a perfect absorber to efficiently collect wave energy [3]. Received October 29, 2018. Revised November 15, 2018. Accepted November 22, 2018. Contact Tran Manh Cuong, e-mail address: tmcuong0279@gmail.com Tran Manh Cuong, Nguyen Thi Thuy, Do Hoang Tung, Pham Van Hai and Vu Dinh Lam 66 Therefore, metamaterial terahertz absorber (MTA) has various potential applications and attracts many researchers in the world. The MPA working in THz frequency was first proposed by Tao et al. in 2008 [5]. Since then many structures have been investigated and put forward, nevertheless, the simple designed structure create a narrow perfect absorption band, the ultra- even super- broadband MPA was engineered by complex configuration of structure [6-14]. This paper proposes a full-sized metamaterial configuration which can generate a super broadband absorbance in THz frequency region. Originally, the full-sized structure metamaterial was investigated by varying the number of cell. Interestingly, the absorptivity of this full structure is much better when it is integrated the defects and a metal via wall. In order to obtain the super broadband absorption, two layers via wall were integrated into a full-sized 10  10 unit cell structure. It is found that, the absorptivity of this structure is not only strengthened but also be expanded to 5 THz ultra-broadband absorption. It is worth mentioning that the thickness of substrate is 0.11 λ0 with respect to the center frequency of the operating bandwidth. The presence of the gold via wall plays the principle role in increasing the absorptivity of the full structure. 2. Content 2.1. Numerical design, results and discussions Figure 1. A unit cell with its dimensions A single unit cell of the original structure with the geometric parameters is presented in Figure 1. In the simulation, the incident wave was normal to the structure, the electric and magnetic fields are parallel to the y-axis and the x-axis and corresponding to the electric field E and magnetic field H directions, respectively. The original structure could be designed by applying the laser lithography process with a 1.5 m-thick (td) polyimide dielectric substrate which has 3.5 dielectric constant sandwiched between 0.026 m-thick (ts) gold film in both sides. In the top layer, the 1.25 m-width (t) square ring surround the dish which has the diameter d = 3.5 m. The bottom layer is covered with a full gold film with an electric conductivity σ = 4.56 × 10 7 S/m. The lattice constant of a single unit cell is a = 9 m (Figure 1). Figure 2 exhibits the full structure metamaterial with 10 × 10 unit cells and the total size is 8100 m2. In this work, the absorbance of various full structure metamaterial such as 8 × 8, 10 × 10 are also discussed (the results are in Figure 3), in there, the size and the geometrical parameters of a single unit cell are unchanged. One can see that, without via wall, the structure shows only one principle absorption peak and enduring at 25.5 THz. Figure 2 is also the schematic representing of the 100 unit cells structure with 2 via walls. In fabrication process, the via wall could be realized by filling gold into the holes between two metal layer. The via wall has the depth h = 1.5 m, the same with the thickness of the dielectric layer, and the via radius is rv = 0.6 m. It is noted that at Simulation of metal via-wall based ultra broadband terahertz full-sized metamaterial absorber 67 the position of the via-wall, the defects on the structure are performed by removing unit cells at the same place. For the numerical simulation, we use the commercial CST Microwave Studio [15] based on Finite Integration Technique (FIT) technique. In simulation, a waveport, which is used to simultaneously transmit microwave beam and receive the reflected beam from the sample, is placed upon the structure. The absorption is calculated through the formula A(ω) = 1 − R(ω) = 1 − |S11(ω)|2, where S11(ω) is scattering parameter and R(ω) = |S11(ω)|2 is reflection (transmission is not applied due to the gold layer at the backplate). Figure 2. Optimal full structure with 100 unit cells size Figure 3. Absorption of full metamaterial absorber with different configurations The full structure of 100 unit cells then is integrated 1 and 2 gold via wall. The results show that, for the full structure without via, the principle absorption frequency is centered at 25.5 THz. For the structure with 1 via wall, the absorber broadband has formed and has a relative width of 5 THz with a weak absorption range around 24 THz. To optimize the broadband absorption, the second via wall was introduced to the structure at the position of the third cell layer. The calculated results were shown in the Figure 3. This indicates that the structure with two via wall layers has a super absorbance band of up to 5 THz and the absorption is over 95%. Tran Manh Cuong, Nguyen Thi Thuy, Do Hoang Tung, Pham Van Hai and Vu Dinh Lam 68 Figure 4. Simulation result: Absorptivity curve of the full-sized absorber structure with 2 via wall; comparison with different via radius r1 = 0.6 µm, r2 = 1.0 µm The formation of the ultra-broad absorption band can be explained as: When forming the via wall layer in the structure, this configuration is considered as an electromagnetic blackbody - a type of an electromagnetic resonance cavity, the electromagnetic waves of the absorption frequency range entered and are confined in the structure. This occurs in the form of standing wave electromagnetic resonance in the absorption frequency range. This is related to the Helmholtz effect of the electromagnetic resonance occurring in the cavity structure with a broadband frequency range, and this is related to the size of the resonance cavity in the blackbody, which concerns directly the optimized distance between two layers of via wall of full sized structure [8]. In Figure 4, we show the results of the optimized structure with different radius of the via. One can see that the radius of the gold via is not strongly affect on the absorption band. When the radius varies from 0.6 to 1.0 µm, the absorption is stable. Figure 5. Absorption spectrum at different polarization angle of the incident wave for the structure with 100 UCs and 2 via walls To confirm the polarization insensitivity of the structure, we present the absorbance of the structure with 100 unit cells integrating 2 via walls for different polarization angles in Figure 5. As Simulation of metal via-wall based ultra broadband terahertz full-sized metamaterial absorber 69 predicted, due to the symmetric configuration of the structure, these results show that the absorption is independent of polarization angle of the incoming wave. To understand more the underlying mechanism of energy concentration in the absorption regime, we analyze and observe the electric field and power loss at 19, 23, and 25 THz, which are three main position peaks on the absorption curve (Figure 6), they are chosen for investigating the field in the range of absorption band. One can see that, the electromagnetic energy concentrated mostly on the surface and at the metal via-wall cavity region of the structure. It is noted that the cavity region formed by via-wall acts like an electromagnetic resonance cavity and confines the incoming energy inside. Figure 6. Electric field distribution patterns in the structure at frequencies of (a) 19 THz, (b) 23 THz and (c) 25 THz The distribution of power loss density for the structure at 19, 23, 25 THz are also respectively observed (Figure 7). The power loss is observed in the structure and at the cross section shows that the loss energy concentrates mainly in the dielectric layer and at the wall cavity region at absorption frequency. Figure 7. The distributions of power loss density for the structure at of (a) 19 THz, (b) 23 THz and (c) 25 THz 3. Conclusions We presented a new full-sized metamaterial structure which possessed a unit cells system integrated with the metal via wall working at the THz frequency range. Simulations show that for the optimized structure applying 2 gold via walls, the working bandwidth is increased up to 5 THz with the absorptivity is over 95 %. The electromagnetic energy and power loss in the structure, which are observed at the resonance range, show that the cavity metal wall is the origin of the perfect absorption. This structure can be used to improve the performances of the micro-antennas, high rate telecommunication systems or other terahertz imaging or sensing technology. Acknowledgement. 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