An Ultrathin Polarization Free Asterick-Shaped Metamaterial Absorber with Quad-Band Characteristics

In this paper, a quad-band absorber is proposed and developed, which is exhibiting ultrathin and polarization insensitive behaviour. It has been designed to be operated in S, C, and Ku bands with absorptions peaks at more than 95%. Te proposed absorber is implemented on a FR4 glass epoxy laminate with an equivalent electrical thickness of 0.0108 λ 0 , where λ 0 is the wavelength corresponding to the lowest frequency of operation. Tis confrms the ultrathin nature of the structure. Te absorption performance of the proposed structure has been characterized under normal and oblique incidences followed by their experimental verifcation. Presented results demonstrate highly polarization-independent behavior of the proposed absorber due to its symmetric geometry. Also, the electromagnetic feld distributions have been studied to acquire better insight of the absorption mechanism corresponding to distinct elements present in the structure. It is characterized in terms of its behavior as metamaterial, which ensures the miniaturization. Te proposed absorber is suitable to be used in applications like radar cross section reduction, stealth technology, radio frequency identifcation, and electromagnetic compatibility.


Introduction
Metamaterial (MTM) is a division of artifcially engineered structures which exhibit simultaneous negative values of permittivity and permeability, therefore enabling researchers to design devices like super lens [1], electromagnetic absorber [2], antenna [3], flter [4], etc. Ever since the investigation by Landy et al. [2] revealed capability of MTM absorbers exhibiting unity absorption, it remains a trending topic among the researchers. MTM absorbers being light weight, compact, and able to realize near unity absorption ofer substantial advantages over conventional absorbers.
MTM absorbers (MMAs) have been designed using split ring resonators (SRR) [5], closed ring resonators (CRR) [6], electric-feld driven LC resonators (ELC) [7], etc. A typical MMA structure consists of metallic pattern printed over grounded lossy substrate ensuring zero transmission of EM waves through the grounded face. MTM absorbers may be capitalized in terms of their design, size, and ability to be used in wide range of applications. MTM absorbers are also capable of exhibiting wide band [6], single-band [8], dualband [9,10], triple-band [11][12][13], and multiband [14,15] absorption performances. Wide band absorbers fnd their applications in stealth technology and anechoic chambers, while multiband absorbers are preferred in applications such as radar cross section reduction, radio frequency identifcation, sensing, imaging, and various other communication systems. Sometimes, the composite structure of the multiband absorbers provides polarization sensitive absorption attributes [16,17]. It means their absorption characteristics may get changed due to change in the orientation of the absorber. Tis is highly undesirable in various practical applications; thus, in the recent years, research focus has been shifted on designing of the multiband polarization insensitive MTM absorber.
Zhai et al. investigated a triple band MTM absorber which comprises a square CRR stacked between two open square rings [11]. Polarization stable behaviour was demonstrated in the paper, and electrical size of the structure was found as 0.119 λ 0 with respect to the lowermost frequency of 3.29 GHz. Later, a triband polarization insensitive absorber using circular fractal structure was proposed by Jiang et al. [12]. Tis fractal structure consists of circular ring resonators embedded inside an outer circular ring etched over FR4 substrate of 2 mm which negotiate with the electrical size and thickness of the structure. In addition, it ofers irregular absorption at the third resonance mode. Furthermore, an MTM absorber using a dual layer approach was suggested, in which each layer of proposed design comprises three starshaped concentric rings stacked together [13]. It was concluded that the application of dual layer instead of single layer improved absorptivity signifcantly, but it severely afects the thickness of an MTM absorber along with notable increment in electrical length. Chaurasiya et al. proposed a structure using a circular ring and a mended cross-like structure with curved circular patches placed inside it [14]. Structure being four-fold symmetric is polarizationinsensitive, but the unit cell size of the structure is quite large which limits its applications in some practical scenarios. Furthermore, another quad-band MTM absorber consisting of four-solid squares embedded inside an outer square ring is presented for C and X-band applications [15]. Tis absorber ofers polarization insensitivity and wide angle stable absorption along with being ultrathin (0.0015λ 0 ). Moreover, a low-profle (0.024 λ 0 ) penta-band trigeminalshaped MTM absorber is proposed and investigated in [16], which shows polarization insensitivity at lower frequency and polarization sensitivity at higher frequency in microwave range. Recently, Lakshmi et al. reported a conformal triple band microwave metamaterial absorber, but the rate of absorption is very low at second band [17]. Also, absorption performance is degrading in relation to polarization and incident angle variation due to its complex design. After an intensive literature survey, it is concluded that there are further possibilities of improvement in peak absorptivity along with the miniaturized unit cell size and thickness of the absorber structure. Some of the recent works based on heptaband THz MMAs for biomedical applications and sensing applications are also discussed in [18,19].
Tis paper presents a polarization-insensitive and ultrathin quad-band metamaterial absorber for S, C, and Ku band applications. Te geometry of the top surface of the investigated structure consists of segmented eight-arm asterick-shaped resonator enclosed by a square ring with trimmed edges. Te aforementioned structure is further embedded inside an outer square ring resonator. Te proposed structure exhibits absorption characteristics at 3.1 GHz (S-band), 5.42 GHz (C-band), 16 distinct four bands. Further, surface current and electric feld distributions were studied to elaborate the functioning of the proposed structure. Te metamaterial property of the suggested MTM absorber is confrmed through efective permeability, permittivity, and dispersion plots. Absorption performance is also studied for transverse electric (TE) and transverse magnetic (TM) polarization for investigating wide incident angle behaviour. Te paper is composed as follows: Section 2 elaborates the geometric layout, evolution steps, polarization, and incidence angle stability analysis of the proposed absorber. Electric and magnetic felds are explored to defne the absorption phenomenon at diferent bands which is included in Section 3. Te metamaterial behaviour of the proposed unit cell is also explained in this section. Experimental results validating simulated ones and performance comparison of the presented work with specifc multiband MTM absorbers are portrayed in Section 3. Section 5 summarizes the features and fndings of the suggested work along with futuristic possibilities.

Geometry and Progressive Studies of the Suggested Absorber
2.1. Schematic of the Proposed Absorber. Figure 1 displays the schematic layout of the proposed quad-band absorber unit cell with specifed design parameters. Te structure is constructed on two-sided copper coated (0.035 mm thick) commercial FR4 laminate of permittivity (ε r ) 4.4, dielectric loss tangent (tan δ) 0.02, and thickness 1 mm. Te top metallic layer of the intended structure has an inner part as a segmented eight-arm asterick-shaped resonator which is enclosed inside a square ring with trimmed edges at the corners. Furthermore, this ring is encompassed by another square ring which is at the periphery of the unit cell. Te absorption rate A(ω) of the absorber structure is calculated by equation (1) [2].
where |S 11 (ω)| 2 and |S 21 (ω)| 2 refer to the refected and transmitted powers, respectively. Te back side of the designed structure is completely coated with copper; hence, all the incident EM waves will be refected back and the transmission through absorber structure will be zero, i.e., |S 21 (ω)| 2 = 0. Terefore, equation (1) can be rewritten as equation.
Te physical dimensions of the unit cell are confgured in such a way that the input impedance of the MTM unit cell (Z in ) becomes equal to the free space impedance (Z 0 ) at operating frequency to ensure minimum refection.

Evolution
Steps of the Proposed Absorber. To know the operation of the absorber, the complete step by step design procedure of the intended absorber is discussed in this section. To illustrate this, proposed structure has been analyzed with respect to four distinct confgurations that are referred to as Structure-A, Structure-B, Structure-C, and Structure-D as represented in Figure 2. Structure-A implies a square ring, and further on simulation, it gives two absorption bands at 2.93 GHz and 20.78 GHz with peak absorbances of 99.28% and 66.02%, respectively, as seen in Figure 3(a). Moreover, it also ofers a spurious band at 15.56 GHz with very less absorptivity of 28.54%. Furthermore, Structure-B shows an inner square ring which has trimmed edges at the corners. Figure 3(a) suggests that it individually ofers an absorption band at 4.85 GHz with a signifcant absorption level of 80.43% and a spurious band at 16.77 GHz with 36.49% absorptivity. An absorption peak is found at 17.13 GHz with absorptivity of 91.49% due to Structure-C, which consists of a segmented 8-arm asterickshaped resonator. Te systematic amalgamation of aforementioned confgurations is denoted by Structure-D (proposed structure), and its absorption response is displayed in Figure 3(b). It can be observed that all the bands ofered by the individual resonators are intact after combining them with signifcant improvement in absorptivity. However, this led to right shift of some bands. Te proposed structure was excited with the help of Floquet ports, and master-slave boundaries are used as periodic boundaries. Te whole simulation of this work was carried out in Ansys Electronics Desktop 2023 R2 Version.
Te probable reason for this shift could be the capacitive coupling between the elements. Also, the band due to Structure-C along with spurious bands ofered by Structure-A and Structure-B has been merged together, resulting in wide absorption band. Terefore, the suggested absorber exhibits quad-band absorption functionality with peak absorptivity of 99.37%, 99.04%, 98.61%, 95.12%, and 95.57%

Studies of Absorptivity for Diferent Polarization and
Incident Angles. Typically, the performance of microwave absorbers should be intact irrespective of their orientation position and direction of impinging EM wave. In other words, their absorbance characteristics should remain the same with respect to their rotation in azimuthal (xy) plane and change in the incident angles. As suggested in the various literature studies, polarization insensitiveness can be attained using a fourfold symmetric structure. To illustrate the polarization stable characteristics, the proposed absorber is analyzed under diferent polarization angles (ϕ) varying from 0°to 90°in the xy plane. Figure 4 suggests that the absorptivity level is maintained throughout all the four working bands when polarization angle is varied from 0°to 90°. Terefore, it can be stated that the structure is polarization-insensitive, and hence, it can be placed in any orientation in xy plane.
Next, the absorption behaviour is analyzed with respect to the variation in incident angle under TE and TM polarization as shown in Figures 5(a) and 5(b), respectively. Te results assure that the adequate absorptivity is maintained while varying the incident angle from 0°to 45°; however, two additional peaks suddenly emerge near 10 GHz and 12 GHz after incident angle of 45°under both TE and TM polarization cases. Te reason for emergence of these spurious bands could be interaction of the incident waves with the diferent parts of the resonating elements at higher angles. Hence, it is afrmed that the proposed structure indicates appreciable absorption characteristics for a wide range of incidence angles.

Parametric Study and Absorption
Mechanism of the Proposed Absorber

Parametric Analysis Study.
It is very important to examine the resonance behaviour of the designed absorber with the help of parametric analyses. As it is already discussed in the previous section that the frst band is originated due to outer square ring only; therefore, in this section, we have considered the design variables that are responsible for exciting bands other than the frst band. We altered the two parameters, namely, the width of inner square ring having trimmed edges (e) and the gap between the fragments of segmented asterick-shaped resonators (h) and studied its efect on the absorptivity performance of the entire structure. All the simulation work carried out in this paper is as follows.

Variation in Width (e) of Ring with Trimmed Edges.
In this analysis, e has been varied from 0.6 mm to 1.4 mm as shown in Figure 6. When e is increased, the fourth band gets shifted towards lower frequency due to increase in coupling capacitance between outer and inner rings. In addition, its efect is less pronounced on frst two bands. However, the third band is shifting randomly due to the combined efect of change in the gaps among the segmented asterick-shaped resonator and inner and outer rings. A good absorption rate and bandwidth are achieved at e � 1 mm.

Variation in the Gap of Segmented Star Shape (h).
Parameter h is also an important design parameter as it is used to tune the higher absorption bands. To verify the same, h is varied from 0.2 to 1.1 mm. Note that frst two bands and fourth band are contributed by outer and inner rings only; hence, no efect of this variation is observed on frst two bands, while there is a negligible efect on the fourth band. As h increases, third absorption band is decreased gradually as this band arises due to the fragmented structure. A good absorption rate and bandwidth are obtained at h � 0.2 mm.

Electric Field and Surface Current Distribution Study.
Electric feld and surface current distribution analyses are utmost useful to get better insight of absorption mechanism. Literature studies suggest that high electric feld concentration is related to the electric excitation and the magnetic resonance can be represented by circulatory fow of surface current. Figure 7 displays the electric feld distribution at all the fve absorption peak frequencies. After analyzing the electric feld distributions in Figures 7(a) and 7(e), it is found that

International Journal of Antennas and Propagation
there is strong electric feld concentration present in the outer ring resonator at 3.10 GHz and 20.29 GHz. It means that the frst and last absorption bands are solely due to outer square ring. It is seen from Figure 7(b) that, at 5.42 GHz, a notable amount of electric feld is populated in horizontal limbs of the inner trimmed ring resonator and outer square ring. It is stated in sub-Section 2.2 that the second band is originated due to square ring with trimmed edges, but it got shifted to the higher frequency due to capacitive coupling between outer and intermediate rings. So, outer and inner rings play a pivotal role in introduction of absorption band at 5.42 GHz. Te study of feld distribution at 16.65 GHz and 17.56 GHz as depicted in Figures 7(c) and 7(d), respectively, reveals that the segmented asterick-shaped resonator plays a vital role in origination of third band as its inner edges experience the highest electric feld intensity. A little amount of electric feld can also be noticed on the edges of the inner trimmed ring resonator and some parts of the outer ring. Figure 8 depicts the surface current distribution at all the frequencies where the proposed structure exhibits maximum absorption. A careful study of surface current distribution demonstrates that it is following the same pattern as electric feld. In addition, the surface current on front and rear faces is in antiparallel direction at all the frequencies, which gives rise to magnetic excitation. Te above study corroborates the statements made in sub-Section 2.2 about the contribution of various parts in absorption and provides better understanding of the absorption process of the proposed structure.

Analysis of the Proposed Unit Cell for MTM
Characteristics. Recent research reports recommend that employing MTM is one of the best techniques to achieve miniaturization of microwave structures. Terefore, the proposed unit cell has been characterized to examine the metamaterial behaviour. To do this, frstly, two-port analysis was performed on the proposed unit cell applying masterslave boundary conditions and Floquet port excitation. Subsequently, permittivity and permeability characteristics have been retrieved with the help of the MATLAB script [20] using the S parameter extraction method of the unit cell described in [21]. Figures 9(a) and 9(b) show the curve of the real and imaginary parts of the permittivity and permeability, respectively.
Te values of permittivity and permeability at peak absorption frequencies obtained from Figure 9 are noted in Table 1 for better grasp. It can be noticed that the real part of permittivity and permeability at 17.56 GHz are negative, thus confrming the metamaterial property of the proposed absorber. Also, at other frequencies except for 16.65 GHz, the real part of permeability is negative and permittivity is close to negative. Tese retrieved parameters are quite close to each other which satisfy the condition of maximum absorption. Te values being very close implies that the input impedance of the medium matches perfectly with the free space impedance. Tis leads to minimal refections and maximum absorptivity.
Left-handed property of the metamaterials can be explained with the help of the dispersion diagram. To demonstrate this characteristic, dispersion profle of the proposed structure is plotted in Figure 10. Any dispersion curve can be classifed into two regions, right-handed (RH) region and left-handed (LH) region. LH region indicates the area where the plot has a negative slope while the RH region has a positive slope. Tis curve has been obtained with the help of equation (3) [21]: where d represents the periodicity of the unit cells. Figure 10 demonstrates that the frst band lies in the LH region, whereas the other three bands lie in the composite right/lefthanded (CRLH) region. Hence, the proposed structure can be regarded as the MTM-based structure and thus achieves miniaturization.

Experimental Results and Validation
Te proposed absorber is implemented on a 1 mm thick and 1Feet 2 FR4 laminate to validate the absorbance behaviour as shown in Figure 11. Te experiments were conducted in a free space environment using the method outlined in [21,22]. Te measurement was carried out using Anritsu S820E (1 MHz-30 GHz) handheld vector network analyzer (VNA) and two standard horn antennas covering the entire range of the operation. VNA was calibrated over the range of 2 GHz-22 GHz before performing the measurements. Initially, refection from a copper sheet of the identical size was recorded to take the losses due to free space into consideration. Later on, the absorber prototype was placed at exactly the same position to obtain the refections from it. Te diference between the later and former provides the refection coefcients of the proposed absorber. Finally, absorptivity can be easily calculated with the help of the relation expressed in equation (2). Figure 12

International Journal of Antennas and Propagation
After this, polarization free behaviour of the proposed absorber is confrmed experimentally. To do this, the fabricated prototype is rotated in the xy plane while keeping the position of the horn antennas fxed. First, a copper sheet of identical size was placed at diferent angles from 0°to 90°in the step size of 15°, and corresponding refections received at the horn antenna were recorded. Te same procedure is then followed for the prototype, and the refections at the receiver horn antenna were stored. Te diference between both the cases provides the refection coefcients for varying azimuthal angles (ϕ). Equivalent absorptivity is then calculated and is plotted in Figure 13(a). It can be deduced that the proposed absorber is functioning well without any deviation in the absorptivity level. It is a very important characteristic which enables users to position the absorber orientation whichever way it is needed in the xy plane. Figure 13(        International Journal of Antennas and Propagation displays the experimental setup for carrying out the absorptivity measurements under normal and oblique incidences in free space. Next, the proposed absorber was also characterized under oblique incidence variations, and an arrangement was established provisionally. Under this setup, an arc was drawn on the foor to adjust the positions of the horn antennas (as per Snell's law of refection) at diferent angles from 0°to 45°in the step size of 15°while keeping the absorber sample under test fxed. Figures 14(a) and 14(b) illustrate the results obtained from the oblique incidence measurements under TE and TM polarization cases, respectively. Spurious bands near 10 GHz and 12 GHz are emerged at 30°and 45°as noted in the simulated results. All bands are intact when EM waves are incident obliquely on the surface of the absorber. Tus, it can be said that the proposed absorber is suitable for wide incident angle absorption.
Te proposed work is compared particularly with the recently reported metamaterial-based ultrathin multiband absorbers, and summary is presented in Table 2. In the table,   λ 0 represents the free-space wavelength which corresponds to the lowest absorption frequency (LAF). It illustrates that the proposed absorber is miniaturized in nature as compared to all other MTM-based absorbers. Also, it ofers polarization-insensitive behaviour with four numbers of bands. Moreover, the thickness of the intended absorber is smaller than that of the compared works; thus, it can be said that the suggested absorber can be employed for radar cross section reduction, stealth technology, radio frequency identifcation, and electromagnetic compatibility applications in the S, C, and Ku bands.

Conclusion
A novel, miniaturized, ultrathin, and polarization insensitive metamaterial inspired quad-band absorber has been studied and realized in this paper. Polarization free and wide incidence angle stable properties of the suggested absorber have been confrmed with the help of absorption performance under normal and oblique incidences, respectively, which was also validated through subsequent measurements. Te study of electric feld and surface current distribution authenticates the inferences drawn during parametric investigations. Moreover, the metamaterial property of the proposed structure was also elaborated using permittivity, permeability, and dispersion plots. Compared to other reported MTM-based microwave absorbers, the proposed structure ofers much better electrical thickness of 0.0103 λ 0 except for the absorber reported in [17]. Te proposed work suggests further opportunities in enhancing the miniaturization and ultrathin nature of the microwave absorbers exhibiting multiple bands.

Data Availability
Te data used to support the fndings of this study are included within the article.

Disclosure
A preprint of this work is available in [24].