S. DEEPA NIVETHIKA: A SIMPLE STRUCTURED MULTIBAND TERAHERTZ METAMATERIAL ABSORBER ... 225–230 A SIMPLE STRUCTURED MULTIBAND TERAHERTZ METAMATERIAL ABSORBER WITH A HIGH Q FACTOR ENOSTAVNO STRUKTURIRANI VE^ PASOVNI TERAHER^NI METAMATERIAL ZA ABSORBERJE Z VISOKIM Q FAKTORJEM S. Deepa Nivethika Vellore Institute of Technology University, Chennai – 600127 Prejem rokopisa – received: 2023-11-07; sprejem za objavo – accepted for publication: 2024-02-27 doi:10.17222/mit.2023.1040 A terahertz Eight-/Nine-/Twelve-/Fourteen-/Sixteen-Band Metamaterial Absorber (MMA) for sensing applications is built and simulated. The substrate is sandwiched between the bottom ground plane and the top patch structure of this primitive MMA. The top patch is made up of two concentric circular ring resonators. This structure generates a multiple number of multi bands without utilising stacked layers, multiple resonators, or overlapping in a single unit cell by altering the radius of the top patch structure within the shorter frequency range of 0.8 THz to 1.2 THz. The polarisation and angle insensitivity properties are inves- tigated by shifting the angle values from 0 to 90 degrees. To learn about the inside mechanism of the planned structure, the mag- netic field distribution, electric field distribution and surface current distribution plots are explained. For the sixteen-band MMA, the Q-Factor and full width half maximum are also determined. This proposed MMA will be used in biosensing applica- tions, sensors and wireless communications. Keywords: metamaterial, absorber, multiband, terahertz, Q-factor Povzetek: avtor tega ~lanka je zgradil in simuliral teraher~ni osem, devet, dvanajst, {tirinajst in {estnajst pasovni absorberski metamaterial (MMA; angl: Metamaterial Absorber) za senzorje. Avtor je substrat kot sendvi~ namestil med spodnjo osnovno plo{~o in vrhnjo plastjo te primitivne MMA strukture. Vrhnja plast je bila izdelana iz dveh koncentri~nih resonatorskih Cu obro~ev. Ta struktura generira ve~kratno {tevilo ve~pasovnosti, ne da bi zato uporabili zlog plasti, ve~vrstne resonatorje ali prekrivanje v enojni celici s spreminjanjem polmera zgornje strukture znotraj kraj{ega frekven~nega obmo~ja od 0,8 THz do 1,2 THz. Avtor je ugotavljal, s spreminjanjem kota med 0 in 90 stopinjami, polarizacijo in lastnosti kotne neob~utljivosti. Spoznal in razlo`il je notranji mehanizem ve~plastne planarne strukture, porazdelitev magnetnega polja, porazdelitev elektri~nega polja in potek povr{inskih tokov. Za {estnajst pasovni MMA je dolo~il Q-Faktor in polno {irino polovi~nega maksimuma. Ta predlagani MMA bodo uporabili za biosenzorske aplikacije ter senzorje za brez`i~ne komunikacije. Klju~ne besede: metamaterial, absorber, ve~pasovnost, terahertz (THz), Q-faktor 1 INTRODUCTION The last section of the electromagnetic field that needs to be examined is the terahertz wave (THz), which ranges from 0.1 to 10 THz. THz ray absorbing in atomic and biomolecular platforms is driven by the stimulating of intrinsic and inter-molecular movement, which in- clude weakly linked molecules with bonds composed of hydrogen along with weak relations that involve van der Waals forces, which is important for a compound and bi- ological molecules detection due to its location within infrared rays and microwave frequencies. 1,2 Some com- pounds, such as illegal substances, 3 antibacterial medica- tions, 4 herbicides, 5 peptides, 6 organisms that have been genetically modified, 7 melamine, 8 which and other dan- gerous remains, 9 have been found as a result of absorp- tion by identification through pounding powder samples into the pellets. Though, this kind of approach is best suited for investigating the possibility of THz detection, rather than analysing tiny amounts. Metamaterials (MMs), which are the periodic engi- neered electromagnetic media- with a dimension level lower than a wavelength of the stimuli from the outside that exhibit features that are absent in nature. 10 They were originally developed to achieve intriguing features like negative refractive index and invisibility. 11 The growth of MM study in the last 20 years has focused on multiple topics that were highlighted in certain reviews, among them all-dielectric MMs, adaptable MMs, versa- tile MMs, meta components, graphite MMs, adjustable MMs, and the meta surfaces. THz metamaterials, ini- tially presented by Prof. Xiang Zhang’s team, 12 have sparked widespread attention and are currently a hot topic in THz research and development, involving MM modulators, MM polarizers, THz wave generators, THz wave absorbers, and compression imaging. 13 It is significant to note that, the purpose of this ap- proach is not to improve a single aspect of the MMA, but rather to retain high absorbance at incident angles of both TM and TE polarisation in an identified range of THz frequencies. In this paper we offer an ultra-thin polarisation and incidence-angle-insensitive sixteen-band MMA. In the following sections, the relevant resonance Materiali in tehnologije / Materials and technology 58 (2024) 2, 225–230 225 UDK 66.081.2:681.586.4 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(2)225(2024) *Corresponding author's e-mail: deepanivethika.s@vit.ac.in (S. Deepa Nivethika) frequencies and absorption rate are discussed in graphs and tabulation forms. For the sixteen-band MMA, a higher Q factor can be offered. Metallic symmetrical cir- cular ring resonators with compact and ultra-thin fea- tures can be offered in a unique shape. For both the TE as well as TM modes, the proposed metamaterial ab- sorber exhibits virtually perfect absorption over a wide angle of incidence, up to 90°. In spite of its superior per- formance, this ultra-thin absorber is suitable for a wide range of applications, including spacecraft radar systems, concealed imaging, and electromagnetic disturbance re- duction. Surface currents and electromagnetic field dis- tributions are also examined in this paper to better under- stand the resonance mechanism. This research demonstrates that the suggested absorber can retain a high level of absorbance even at large incidence and polarisation angles. 2 STRUCTURE AND DESIGN The proposed structure design, simulation and analy- sis is made using CST (computer simulation technology) Microwave Studio Software. Different developing stages of the MMA are shown in Figures 1a to 1c and the 3-d view is shown in 1d. This polarization-insensitive struc- ture consists of three layers. The polyimide substrate is placed in between the two copper layers. The dielectric constant value ( r ) of the polyimide is 3.5 and the electric tangent (tan ) is 0.0027. The thickness of the substrate is 0.125 mm and the thickness of the ground plane is 0.001 mm. The conductivity of the copper is = 5.8 × 107 S/m. 14 Table 1: Parameters and dimensions of the MMA Parameters Dimensions (mm) Length and Width of the substrate and ground plane (L×W) (0.25×0.25) R1, R2, R3, R4 0.06, 0.14, 0.16, 0.24 Thickness of the substrate 0.125 mm Thickness of the ground plane 0.001 mm The top patch consists of two concentric circular res- onator structure, which is a very simple and easy struc- ture. R1, R2, R3, R4 represented the radius value of the inner and outer circular resonators. And the length and width of the ground plane and substrate layer is the same. The corresponding dimensions are shown in Ta- ble 1 and marked in Figure 1d. 2.1 Boundary Conditions and Absorption Rate of MMA The unit-cell periodic boundary rule is used in both the x and y directions, besides the z free boundary situa- tion, as shown in Figure 2. The simulation system is used to calculate the variables for angular frequency ( ), absorbance (abs), reflections (ref), and transmittance (tra). The formula Abs ( )=1–Ref( )-Tra ( ) is used to determine the absorption. The spectrum of the trans- mission confirms the null reception due to the existence of a thick, conductive, ground copper layer that is higher than its skin depth ( ). As a consequence, the absorbance is determined only by the reflected component. Because the designed structure produces improved reflections in the suitable bands, absorption is improved. The absorp- tion coefficient will be one if the impedance is appropri- ately matched with open space. If both the inner and outer rings are present it means the proposed structure resonated at sixteen different fre- quencies, i.e., 0.80 THz, 0.83 THz, 0.876 THz, 0.895 THz, 0.92 THz, 0.93 THz, 0.957 THz, 0.962 THz, 0.988 THz, 0.997 THz, 1.02 THz, 1.04 THz, 1.09 THz, 1.112 THz, 1.137 THz and 1.154 THz, with absorption rates of 93 %, 86 %, 99.6 %, 86.5 %, 94 %, 93 %, 98 %, 96 %, 97 %, 95 %, 98 %, 96 %, 91 %, 91.2 %, 80 % and 99 %, respectively, which is shown in Figure 3c. Also, chang- S. DEEPA NIVETHIKA: A SIMPLE STRUCTURED MULTIBAND TERAHERTZ METAMATERIAL ABSORBER ... 226 Materiali in tehnologije / Materials and technology 58 (2024) 2, 225–230 Figure 1: Different developing stages of the MMA: a) inner circle only, b) outer circle only, c) both inner and outer circle- polyimide in yellow, copper in green, d) perspective view of the MMA (3-layers) Figure 2 Unit-cell boundary along with axis ing the top-patch parameter values gives the Eight-/ Nine-/Twelve-/Fourteen bands which are explained in the upcoming sections (i.e., the parameter study). 3 RESULTS AND DISCUSSION 3.1 Absorption Rate Characteristics The different developing stages of the MMA are shown in Figures 1a to 1c and the corresponding absorp- tion-rate curves are shown in Figures 3a to 3c.I nt h e first structure 1a only the inner circle is present at the time the structure is resonated at different frequencies, i.e., 0.86 THz, 0.87 THz, 0.91 THz, 0.95 THz, 0.97 THz, 1.01 THz, 1.05 THz, 1.052 THz, 1.06 THz and 1.17 THz, with absorption rate of 99 %, 90 %, 99 %, 98 %, 90 %, 71 %, 92 %, 91 %, 92 %, and 85 % respec- tively which is shown in Figure 1b only the outer circle is present that time the structure is resonated at six dif- ferent frequencies 0.86 THz, 0.867 THz, 0.945 THz, 0.98 THz, 1.07 THz, and 1.1 THz with absorption rate of 98 %, 90 %, 88 %, 87 %, 98 %, and 99 %, respectively, which is shown in Figure 3b. 3.2 Polarization ( ) and Angular ( ) Stability The projected absorption of the suggested MMA is shown in Figures 4a and 4b, the oblique angles ( )of S. DEEPA NIVETHIKA: A SIMPLE STRUCTURED MULTIBAND TERAHERTZ METAMATERIAL ABSORBER ... Materiali in tehnologije / Materials and technology 58 (2024) 2, 225–230 227 Figure 4: Absorbance characteristics of the proposed structure: a) different incident angles ( ), b) different polarization angles ( ), in figure, T-theta, P-Pi Figure 3: a) Inner circle only presents 10 resonance bands, b) outer circle only presents 6 resonance bands, c) both inner and outer patch present 16 resonance bands for both TE and TM mode the incidence and polarisation angle ( ) can be changed from 0° to 90° in the spectral range of interest (0.8 THz to 1.2 THz). When the polarisation-angle and incidence-angle variables were altered, the structure’s parameters such as the absorption and frequency of resonance remained con- stant. Consequently, the recommended MMA exhibits an excellent polarization-independent absorption capability. This is mostly due to the unit-cell uniformity of the pro- posed MMA. Because of the polarization-insensitive and angle-insensitive characteristics this structure finds ap- plications in spacecraft satellite communication and po- larization imaging. 3.3 Electric Field, Magnetic Field and Surface Current Distribution for Sixteen Frequencies The physical mechanism of the MMA is analysed from the electric field, the magnetic field and the surface current distribution plots, which are shown in Figure 5a [a-p], 5b [a-p], and 5c [a-p]. The absorber resonated at sixteen bands which are 0.80 THz, 0.83 THz, 0.876 THz, 0.895 THz, 0.92 THz, 0.93 THz, 0.957 THz, 0.962 THz, 0.988 THz, 0.997 THz, 1.02 THz, 1.04 THz, 1.09 THz, 1.112 THz, 1.137 THz and 1.154 THz. The intensity cal- culator, which is located alongside the distribution plots, explains the related sixteen bands’field distributions. We will distinguish which places in the picture have more field distributions and which places have fewer field dis- tributions based on that image. Figure 5a shows (c, f, g, i, n, o, p) with a high electric field distribution at the di- electric’s surface and across the patch resonator struc- ture, and (a, b, d, e, h, j, k, l, m) with an electric field dis- tribution at select points on the dielectric and patch resonator. Figure 5b depicts the magnetic field disper- sion. The largest magnetic distribution is shown in Fig- ure 5b (e, h, j, k, l, n, o, p) near the surface of the dielec- S. DEEPA NIVETHIKA: A SIMPLE STRUCTURED MULTIBAND TERAHERTZ METAMATERIAL ABSORBER ... 228 Materiali in tehnologije / Materials and technology 58 (2024) 2, 225–230 Figure 5: a) Electric field distributions, b) magnetic field distributions, c) surface current distributions: a) 0.80 THz, b) 0.83 THz, c) 0.876 THz, d) 0.895 THz, e) 0.92 THz, f) 0.93 THz, g) 0.957 THz, h) 0.962 THz, i) 0.988 THz, j) 0.997 THz, k) 1.02 THz, l) 1.04 THz, m) 1.09 THz, n) 1.112 THz, o) 1.137 THz and p) 1.154 THz tric and across the patch resonator, whereas a little less magnetic distribution is shown in Figure 5b (a, b, c, d, f, g, i, m). Figure 5c depicts the surface-current distribu- tion plot. The distribution of the current is greatest at the dielectric surface and the top-patch resonator structure in all the plots from a to p. 3.4 Study of design parameters The parameter analysis assists us in determining the best absorption-rate and resonance-frequency values. Changing the parameter values results in a change in the number of bands and resonant frequencies. The goal of this effort is to achieve multiband absorption between 0.8 THz and 1.2 THz. As a result, we varied the parame- ters and recorded the outcomes. The graphs in Figure 6a and 6b are created based on the different types of outputs with respect to the radius (R3 and R1) from the top-patch structure. First, we adjusted the outer circle R3 value from 0.12 mm to 0.20 mm in 0.2 mm steps, as shown in Figure 5a. At that time, we had the most bands (16), with R3 values of 0.12, 0.16, and 0.20 mm. Furthermore, we obtained fourteen bands with R3 values of 0.14 and 0.18 mm. We choose 0.16 mm as the optimal R3 value from these five distinct outputs because the absorption rate is strong at sixteen various frequencies, as shown in Table 3. Second, we adjusted the outer circle R1 value from 0.02 mm to 0.06 mm in 0.2 mm steps, as shown in Figure 6b. At that time, we had the most bands (six- teen), and the R1 value was 0.06 mm. Furthermore, we obtained eight, nine, and twelve bands with R1 values of (0.02, 0.04, and 0.05) mm, respectively. As a result of the investigation, we obtained the maximum multiband at sixteen different frequencies with the highest absorp- tion rate. When the outer circle radius was 0.24 mm, 0.16 mm, the resonant frequencies in the THz frequency ranges in 0.80, 0.83, 0.876, 0.895, 0.92, 0.93, 0.957, 0.962, 0.988, 0.997, 1.02, 1.04, 1.09, 1.112, 1.137, 1.154 with an ab- sorptivity of 93 %, 86 %, 99.6 %, 86.5 %, 94 %, 93 %, 98 %, 96 %, 97 %, 95 %, 98 %, 96 %, 91 %, 91.2 %, 80 %, 99 % and hence 16 peaks were achieved. When the outer circle radius was 0.14,0.06 mm, the resonant frequencies in the THz ranges in 0.80, 0.83, 0.876, 0.895, 0.92, 0.93, 0.957, 0.962, 0.988, 0.997, 1.02, 1.04, 1.09, 1.112, 1.137, 1.154 with the absorptivity of 93 %, 86 %, 99.6 %, 86.5 %, 94 %, 93 %, 98 %, 96 %, 97 %, 95 %, 98 %, 96 %, 91 %, 91.2 %, 80 %, 99 %, so 16 peaks were achieved. When the dielectric constant and the permittivity are independently manipulated, the electric and magnetic fields could possibly be absorbed. With the help of the manipulation techniques in the reflectivity, permittivity and permeability, the impedance matching with respect to the free space can be achieved for a highly capable ab- sorber. 3.5 Quality factor (Q-factor) The Q value (defined as the frequency of the reso- nance point divided by its Full-Width Half Maximum S. DEEPA NIVETHIKA: A SIMPLE STRUCTURED MULTIBAND TERAHERTZ METAMATERIAL ABSORBER ... Materiali in tehnologije / Materials and technology 58 (2024) 2, 225–230 229 Figure 6: Absorption-curve as a function of: a) radius R3 and b) radius R1 Table 2: FWHM and Q for sixteen resonant frequencies Band No. Original Frequency (THz) Absorption (%) FWHM Q-Factor 1 0.80 93 0.00365 219.17 2 0.83 86 0.00471 176.22 3 0.876 99.6 0.00138 634.78 4 0.895 86.5 0.00474 188.81 5 0.92 94 0.00266 345.86 6 0.93 93 0.00255 364.70 7 0.957 98 0.00449 213.14 8 0.962 96 0.00126 763.49 9 0.988 97 0.00279 354.12 10 0.997 95 0.00243 410.28 11 1.02 98 0.0032 318.75 12 1.04 96 0.00293 354.94 13 1.09 91 0.00273 399.26 14 1.112 91.2 0.0047 236.59 15 1.137 80 0.00212 536.32 16 1.154 99 0.00225 512.88 (FWHM)) is a common parameter for deciding which frequencies to use the resonating mode on. It might be able to inform us absolutely whether the frequency selec- tion can be used for sensing. As the Q value increases, so does the detecting capabilities. 20 Table 2 shows the Q value for each frequency range, as well as a clarification of the Q value. All the frequency modes have a Q value greater than 170. When compared to the other 15 modes, the eighth mode has an extremely high Q-factor value. The sixteen-band absorber design has considerable po- tential in sensor-related specialty due to its high Q-factor. 4 CONCLUSION We have created and simulated a terahertz Eight-/ Nine-/Twelve-/Fourteen-/Sixteen-Band Metamaterial Absorber (MMA) for sensing applications. By varying the radius of the top-patch structure within the narrow frequency range of 0.8 to 1.2 THz, this structure gener- ated a large number of multi bands without the use of stacked layers, numerous resonators, or overlapping in a single unit cell. By varying the angle values from 0 to 90 degrees, the polarisation and angle insensitivity proper- ties were examined. The electric field distribution, mag- netic field distribution, and surface-current distribution plots are explained to understand the internal mechanism of the proposed structure. For the sixteen-band MMA, the Q-Factor and full width half maximum are also deter- mined. The highest value of the Q-Factor obtained from this work is 763.49. As a result, the suggested MMA will very certainly be used in biosensing and refractive-index sensing applications. This work is tabulated and com- pared to prior works. 5 REFERENCES 1 B. Ferguson, X. C. Zhang, Materials for terahertz science and tech- nology, Nature Mater, 1 (2002), 26–33 2 M. Walther, B. Fischer, A. Ortner, A. Bitzer, A. Thoman, H. Helm, Chemical sensing and imaging with pulsed terahertz radiation, Ana- lytical and bioanalytical chemistry, (2010), 397 3 K. Kawase, O. Yuichi, W. Yuuki, I. Hiroyuki, Non-destructive terahertz imaging of illicit drugs using spectral fingerprints, Optics express, 11, (2003) 20, 2549–2554 4 L. Yang, G. 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DEEPA NIVETHIKA: A SIMPLE STRUCTURED MULTIBAND TERAHERTZ METAMATERIAL ABSORBER ... 230 Materiali in tehnologije / Materials and technology 58 (2024) 2, 225–230