Multi-Band Polarization Insensitive Ultra-Thin THz Metamaterial Absorber for Imaging and EMI Shielding Applications

In this article, a multi-band polarization-insensitive ultrathin (0.036⋋) single layer metamaterial absorber is designed for THz imaging and EMI shielding. A unique oval-shaped structure is compact with three circular ring-shaped resonators is proposed with a unit cell dimension of 36×36×19.6μm3. The absorbance of the proposed multiband MMA is above 90% for three bands i.e. 98.57%, 90% and 99.85% at 5.58, 7.98-8.84, 11.45THz frequency respectively. Return loss is nearly the same for the changing incident and polarization angle. Therefore, this metamaterial absorber with a wide range of polarization insensitivity is found and it is also suitable for various applications such as quantum RADAR Imaging, energy harvesting, and optoelectronic devices. INDEX TERMS MMA (Metamaterial Absorber), Triple-band, Polarization-Insensitive, EMC (Electromagnetic Compatibility), Terahertz (THz), EMI (Electromagnetic Interference)


I. INTRODUCTION
n recent progresses, metamaterials have entered in various practical applications such as sensing, cloaking, stealth or absorber, and communication [1][2][3][4][5]. Among these applications absorbers have been found feasible and applicable in a microwave, terahertz as well as the infrared band for applications like terahertz and infrared imaging, perfect absorbers, wave manipulation, energy harvesting, chemical detection and, radioactive cooling. Moreover, it is also used for single-band, dual-band, multiband and wideband applications. Metamaterials in terahertz range are popular due to bandwidth scalability, high security, and strong penetration [6-7, 17, 18]. Metamaterial absorbers are generally arranged as a threelayered structure comprising of a ground plane as a bottom layer, a dielectric as middle, and a resonant periodic structure as a top layer. In comparison with like Salisbury and Jaumann metamaterial absorbers are an alternative that are used in fifthgeneration fighter plane stealth to improve the isolation of MIMO antenna, imaging, sensing, Radar cross-section and electromagnetic interference reduction [8][9]. Moreover, multiband absorbers that are polarization as well as incident wave insensitive have been designed and validated with a thickness lower than the conventional absorbers having absorptivity above 90% for S, and X-band applications [10][11]. In this paper, an ultra-thin microwave absorber is designed that works at three frequency bands i.e. band-1(5.42-5.74 THz), band-2 (7.98-8.84THz), and band-3(11.36-11.54THz) with high absorptivity in X-band. The absorption for the three intended bands is above 90% for TE & TM polarization. The threelayered structure is simple to design and fabricate using commercially available copper-clad laminate. Section-II shows the design of the proposed MMA structure. Section-III depicts the simulated results of the proposed MMA in terms of return loss, absorbance, reflectivity, current density, RCS, and electric field distribution. Conclusion of the proposed designing work is summarized in section-IV.

II. DESIGN OF MMA STRUCTURE
The proposed metamaterial absorber comprises two oval geometries organized as a flower-shaped pattern having three ring-shaped slots cut at the center and a metallic ground plane separated by a dielectric layer, as shown in Figure 1. The MMA is designed on an FR-4 substrate with 19.6μm height (h) and copper plated (σ = 4.2×10 7 S/m) of 3μm thickness (t). FR4 is commercially available and an alternative to the expensive dielectrics with satisfactory response for the THz range of interest [37][38]. The ground made up of a copper conductor acts as conducting mirror so that there is no transmission of the incident wave and thus converting the incident wave into heat that minimizes the reflected wave which produces high absorbance. The radius and width of all the rings are optimized and the geometrical details of the structure are mentioned in Table-1 36 19.6 3

FIGURE 2
Dimensions of a triple-band absorber Absorption is measured by reflection coefficient (S11) and transmission coefficient (S21) using the simple relation given in (1) [21]: The ground plane is a metallic sheet therefore no transmission takes place, so the transmission coefficient (S21) becomes zero. So, (1) is reduced to (2) [21]: Equation (4) comes from (2) and (3) [21], Where, S11coand S11cross are the reflection coefficient of copolarized and cross-polarized EM waves as shown in Figure  5(c). The S-parameters are calculated using the simulations done on CST-Microwave Studio for a frequency sweep from 5THz to 12THz. The absorber impedance should be comparable to the free space impedance to achieve perfect absorption. The normalized impedance can be calculated using (5): The total impedance of the metasurface absorber Zin and reflection coefficient is given as: Equations (10) and (11) indicate the constraints upon relating s (6)- (9) in agreement to match with the free space impedance, i.e. Zin = Z0. : Put the value of and from (10) in (11) and change "cot" form to "tan" form gives (12) that can be further solved by (13): : √ K -LMN; Putting value of 92πfL − 1 2πfC H < from (11) to (13) gives (14): The value of R, L, and C can be estimated from (15) and (16) as: For maximum absorption condition, the impedance of the absorber should be equal to the characteristic impedance of free space as per maximum power transfer theorem i.e. Zin = Z0 = 377Ω in [5][6][7][8][9].

III. RESULTS & DISCUSSION
The proposed triple-band MMA is simulated with unit cell boundary condition on the x-y axis and incident wave on the zaxis to generate a return loss curve as shown in Figure 5 (a). Absorptivity at the intended band 5.58THz, 8.71THz, and 11.46THz is above 90% and for band-1(5.42-5.74 THz), band-2 (7.98-8.84THz), and band-3(11.36-11.54THz) having above 90% as shown in Figure 5 (b). The absorption of the proposed MMA is calculated by (2) [9]. The absorber structure is horizontally and vertically polarized waves to obtain co and cross-reflection coefficients by assigning the Floquet ports on the top of the unit cell as indicated in Fig 5(c). Figure 5(d) shows that the phase change due to resonance, indicating the phase variation of ±180° at 5.58THz, 8.71THz, and 11.46THz.  Figure 6 indicates the results of absorbers that are required to recognize the exotic behavior of the material in terms of effective relative permeability/permittivity and refractive index, due to geometrical alterations, not reflected in natural materials. Relative permeability is having a negative peak value, at the resonant frequencies. Figure 6  where, the parameters ω, μc, t, τ, h, e and kB are radian frequency, chemical potential, temperature, scattering rate, Plank's constant, electronic charge and Boltzmann's constant. Figure 7 depicts the performance of the absorber under extreme conditions of orientation and wave incidence. In Figure 7 (a) polarization angle "ϕ" is varied from 0° to 90° with an incident wave at normal i.e. "θ = 0°". It is observed that the absorber designed is insensitive to the polarization angle. The incidence angle is varied from 0° to 90° with "ϕ = 0°", it is found that the absorber is also insensitive to incidence angle, thus reducing RCS at three frequency bands as shown in Figure 7(b). RCS reduction is used in stealth for fifthgeneration aircraft to minimize the reflected power for detecting the stealth aircraft. (a) Simulated reflection coefficient "S11" of the absorber for various (a) polarization angles "ϕ'' at normal incidence (b) incidence angles "θ" at "ϕ = 0. "(c)TE polarization, (d) TM polarization The polarization angle ϕ, is varied from 0° to 90°keeping incident angle θ = 0°, it can be observed from Figure 7(a)there is no variation in absorptivity, which indicates that the absorber is polarization insensitive. Also, it is clear from Figure 7(b) that if the incident angle θ is varied from 0° to 90°, the structure holds good reliability. Therefore, the absorber is insensitive to the angle of incidence of the approaching electromagnetic wave . Fig 7 (c) and (d) indicate the TE and TM polarization respectively. show that for lower frequencies the outer structure is responsible for absorption, and at higher frequencies, the electric field is concentrated more on the inner rings. The proposed MMA provides good absorbance and low profile compared with the referred articles as shown in Table-II.

IV. CONCLUSION
A single layer ultra-thin multi-band THz MMA with polarization-insensitive has been presented with high absorbance at three distinct frequencies 5.58, 8.71, and 11.45THz. The proposed structure has high absorbance with ultra-thin 0.013λ thickness with existing literature used in Table-II. So this MMA is used in many defense applications like stealth Fifth-generation stealth aircraft for RCS reduction, MIMO antenna isolation improvement, EMI shielding, energy harvesting, EMC reduction, THz imaging [18], optoelectronic devices, and sensing in the intended band.