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Abstract
Multi-functional metamaterial absorbers have attracted considerable attention for applications in the microwave frequency regime. In this paper, we report the design, fabrication, and characterization of frequency-selective absorbers, which exhibit substantial absorption property within a pre-defined frequency band, while at the same time behaving as a highly transparent screen in another targeted frequency band. The proposed designs consist of a symmetrically patterned indium tin oxide film acting as an absorbing layer, two dielectric substrates, and a cross-slot metal sheet frequency selective surface playing the role of a transmitting layer. In order to validate the functionalities of the designed absorbers, equivalent circuit models, full-wave numerical simulations and measurements are presented. The measured results, in good agreement with the numerical ones, show that the proposed designs realize 80% broadband absorption over the desired frequency range and possess a transparent window in a higher or lower frequency band for a wide range of incidence angles up to 60°. These performances suggest that the proposed designs are promising candidates for multi-functional scattering control and communication applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past decade, metamaterial absorbers (MAs) have attracted substantial research interests owing to their unique advantages of ultrathin thickness [1] and near unity absorption [2]. With the emergence of absorbers [3], MAs have been extensively investigated for practical applications, including electromagnetic (EM) shielding [4,5], EM compatibility solutions [6], and radar cross-section (RCS) reduction [7]. To achieve near-perfect absorption, the structures of MAs generally consist of electric resonators and a mirror reflector separated by a dielectric substrate [8,9], which leading to a strong reflection outside the absorbing frequency band(s). As a result, the reflected beams of MAs produce a large RCS, which can still be detected by bistatic radar systems. For applications in the communication domain, several novel absorber designs have been reported to achieve some particular characteristics, such as MAs simultaneously possessing transmission windows and an absorption frequency band, also named as frequency-selective absorbers (FSAs).
Recently, a variety of FSAs have been realized [10–17], which are based on the combination of circuit analog absorbers and frequency selective surfaces (FSSs). In general, according to the relative position of the absorption and transmission frequency bands, we can classify FSAs into the following categories: I) FSAs with an absorption band below the transmission band, II) FSAs with an absorption band above the transmission band, and III) FSAs showing transmission performance within the absorbing band. Among these designs, common band-pass LC resonators are introduced to obtain high transmission efficiency. To realize the pre-defined absorption function, FSAs are usually designed by loading lumped elements in the metallic resonator [10–14,16,17]. However, the shortcomings of lumped components include difficulty in fabrication and assembling, and vulnerable to damage. Besides, in the transmission band, the lumped components will introduce undesired insertion losses. The method of cascading multiple metallic/dielectric layers is proposed to avoid the use of lumped elements [15], but the multi-layer structure has the disadvantage of possessing a large volume and is more sensitive to fabrication inaccuracy. It is worth pointing out that most of FSAs use low permittivity material such as foam as spacer which limits their practical applications [10–14,16]. For example, the insufficient supporting force of the substrates may cause deformation. Therefore, it is of significant importance to propose a judiciously designed compact FSA structures satisfying for sturdy structure, low profile, and excellent transmission property.
In this paper, acting in a customizable manner, we investigate two compact designs of FSAs based on ITO films. Via different designs of the metasurface elements, both FSAs exhibit high absorption in the same frequency band, but open transmission windows in two different frequency bands. Numerical results show that the first design achieves an absorption of higher than 80% covering the X-band and a transparent band in the S-band under vertical polarization. Meanwhile, the second design exhibits an absorption greater than 80% covering the X- and Ku-bands and a highly transmissive window in the Ka-band. As a proof-of-concept example, a prototype of the first design is fabricated and tested. The experimental results show a good agreement with simulated ones. The approach reported in this research can further be extended to higher frequencies for terahertz and optical domains by considering proper optical materials and technological fabrication constraints.
2. Concept and design
By combining the working principles of MA and FSS, a class of efficient FSAs is proposed in this section. Thanks to judicious designs of the patterns of the ITO film on the top layer and the metallic patch on the bottom layer of the metasurface elements, the proposed FSAs achieve absorption and transmission characteristics within two respective specified frequency ranges. The operating principle of the proposed FSAs is illustrated in Fig. 1. The incident wave (marked in purple) operating in the X-band and X/Ku bands, respectively, is absorbed by the FSA and the incident wave (marked in red) operating in the S-band and Ka-band, respectively, is transmitted through the FSA.
An ITO film is used as the absorbing layer with a sheet resistance of 100 $\Omega$ /sq. As shown in Fig. 2(a), an ITO film as absorbing layer placed on the top of substrate I and a metallic patch as transmitting layer etched on the bottom of substrate II are integrated as an elementary element of the two proposed FSAs. The dielectric materials of substrate I is Rogers 5880 with a relative permittivity of $\varepsilon_r$ = 2.94 and tan $\delta$ = 0.002 is utilized for substrate II.
Fig. 2. Geometry of the proposed integrated absorption–transmission structures. (a) Three-dimensional illustration of FSAs. (b) (c) Top view of two proposed FSAs. (d) (e) Bottom view of two proposed FSAs. (The optimized geometrical dimensions of FSAs are: $p_1$ = 13 $mm$, $p_2$ = 6.5 $mm$, $h_1$ = 1.575 $mm$, $h_2$ = 1.464 $mm$, $a$ = 4.6 $mm$, $b$ = 1.1 $mm$, $c$ = 4.54 $mm$, $d$ = 1.06 $mm$, $l_1$ = 12.6 $mm$, $l_2$= 6.8 $mm$, $g_1$ = 0.2 $mm$, $g_2$ = 2.1 $mm$, $w_1$ = 3.4 $mm$, and $w_2$ = 0.76 $mm$).
The patterns of the absorbing layer employed in this work are depicted in Figs. 2(b) and (c). For the first design (structure I), to achieve the characteristics of the band-pass FSS in S-band, the transmitting layer of the unit cell is formed by a modified Jerusalem Cross (JC) slot etched in a square conducting ground plane, as illustrated in the left column of Figs. 2(d) and (e). For the second design (structure II), a classical crossed (CC) slot is employed to guarantee a transmissive behavior in the Ka-band. All elementary elements of the proposed FSAs are arranged periodically with periods $p_1$ and $p_2$ for structure I and structure II, respectively.
To analyze the generality of our designs, a general transmission line model [Fig. 3(d)] can be built based on Fig. 3(a), where h$_1$, $h_1$ represent the thicknesses and the equivalent characteristic impedance of the dielectric substrates, and $Z_d$ are the impedances of the absorbing layer and transmitting layer, respectively. Regarding this model as a two-port network, an ABCD matrix can be expressed as follows [18]:
Fig. 3. Equivalent circuit of the proposed FSAs and their simulation comparisons in HFSS and AWR. (a) and (d) General physical and circuit model of FSAs respectively. (b) and (e) The equivalent circuits of structure I and structure II. (c) and (f) Comparison of calculated and simulated scattering parameters of structure I and structure II, respectively. (The simulated curves are obtained from HFSS software using transverse electric (TE) polarized incidence).
Next, the reflection coefficient $\textit {S}_{11}$ and the transmission coefficient $\textit {S}_{21}$ of this network can be calculated by the relation between the ABCD matrix and S matrix [19]:
When the EM waves impinge on the surface of the medium from port P-1, the absorption is expressed as:
Consequently, to design the preset functionalities of structure I, it is reasonable to set $\left | {{S_{11}}} \right |\textrm { = }0$ and $\left | {{S_{21}}} \right |\textrm { = }0$ in the X-band while setting $\left | {{S_{11}}} \right |\textrm { = }0$ and $\left | {{S_{21}}} \right |\textrm { = }1$ in the S-band. Meanwhile, to achieve the performance of structure II, it is preferable to design $\left | {{S_{11}}} \right |\textrm { = }0$ and $\left | {{S_{21}}} \right |\textrm { = }0$ in X-band and Ku-band, while setting $\left | {{S_{11}}} \right |\textrm { = }0$ and $\left | {{S_{21}}} \right |\textrm { = }1$ in the Ka-band. Note that Z$_u$, Z$_d$, and $Z_1$ are impact factors of the values of S$_{11}$ and S$_{21}$.
To further analyze the operating principle of the proposed FSAs, their equivalent circuits are investigated as shown in Figs. 3(b) and (e). The ITO film of the FSAs can be modeled as an RLC series circuit [20]. The ideal transmitting layer can be represented by a parallel LC circuit when the FSS is realized by a CC slot for structure II [21], and by a series capacitance $C_{15}$ when the JC slot is adopted in structure I. It should be mentioned that we added the resistance to $Z_d$ to characterize the influence of the intrinsic impedance of the absorbing layer on the transmitting sheet. In addition, two thin dielectric layers with thicknesses $h_1$ and $h_2$ are exploited as lossy transmission lines in this design.
Figure 3(b) shows the equivalent circuit of structure I, in which the impedance of the absorbing layer (Z$_{u1}$) is composed of series-parallel R$_{11}$, L$_{11}$, C$_{11}$, R$_{12}$, L$_{12}$, C$_{12}$, and C$_{13}$ with a resonance frequency in the X-band. And the impedance of the transmissive layer (Z$_{d1}$) consists of series-parallel R$_{13}$, L$_{1}$, C$_{14}$, and C$_{15}$ with a resonance frequency in the S-band. According to the equivalent circuit of structure II [Fig. 3(e)], we observe that the impedance of the absorbing layer (Z$_{u2}$) is composed of tandem R$_{21}$, L$_{21}$, and C$_{21}$ which has a resonance crossing the X- and Ku-bands. And the impedance of the transmissive layer (Z$_{d2}$) consists of series-parallel R$_{22}$, L$_{22}$, and C$_{22}$ with a resonance frequency in the Ka-band.
With the aid of the above analyses, the impedance of the absorbing and transmissive layers of the two FSAs can be written as:
Furthermore, the equivalent circuit models have been analyzed using AWR Microwave Office software to obtain the reflection and transmission coefficients of the proposed FSAs, as plotted in Figs. 3(c) and (f). Additionally, the corresponding optimized circuit parameters of the two structures are listed in Table 1 and Table 2, respectively. In light of the theoretical analyses, it is straightforward to observe that the reflection and transmission coefficients of FSAs can be tuned by changing the value of inductances and capacitances of the FSA structures in their respective equivalent circuit, i.e., by controlling the geometrical parameters of the absorbing and transmissive layers. Hence, by optimizing the geometrical parameters, the physical dimensions of the FSAs can be determined [see Figs. 2(b)–2(e) ].
Table 1. Variable Value of Structure I
Table 2. Variable Value of Structure II
3. Numerical validation
The two proposed unit cells are simulated under normal plane wave incidence using ANSYS HFSS software. Figures 4(a) and (b) present the scattering parameters of the two proposed FSAs for both transverse electric (TE) and transverse magnetic (TM) polarized incident waves. It is worth noting that the gray areas in Figs. 4(a) and (b) mark the transmission bands (insertion losses less than 2 dB) of the FSAs.
Fig. 4. Simulation results for different incident waves with different polarizations of structure I and II, respectively. (a) (b) Scattering parameters . (c) (d) Absorption curves. (e) (i) Absorption and (f) (j) transmission efficiencies for TE polarization. (g) (k) Absorption and (h) (l) transmission efficiencies for TM polarization.
Taking the TE-mode incidence as an example and considering the effect of cross-polarization, Eq. (4) is written as
Figures 4(e)-(h) shows the transmission and absorption responses of structure I under oblique wave incidence with different polarizations. For TE polarization, the transmission and absorption with varying frequency and incidence angle are depicted in Figs. 4(e) and (f). Structure I possesses a transmission window in the S-band within a field-of-view up to 65$^{\circ }$ oblique incidence, and an X-band absorption band up to 60$^{\circ }$ incidence. Under TM polarization, the absorption [Fig. 4(g)] of the FSA is maintained in the X-band up to 50$^{\circ }$ incidence angle. Moreover, Fig. 4(h) shows that the transmission window exists in the S-band for over 80$^{\circ }$ incidence angle. Similarly, the performances of structure II are illustrated in Figs. 4(i)-(l). Figures 4(i) and (j) show the transmission and absorption under the TE polarization. It exhibits high absorption in the X- and Ku-bands for incidence angles smaller than 66$^{\circ }$, as shown in Fig. 4(i). From Fig. 4(j), it is evident that structure II presents transmission characteristics in the Ka-band when the TE-mode incidence angle is below 25$^{\circ }$. Moreover, structure II presents high absorption in the X- and Ku-bands for incidence angle up to 70$^{\circ }$ and displays a transmission window in the Ka-band within a 10$^{\circ }$ incident angle under TM polarized wave illumination.
To summarize, the proposed FSAs show good performances of absorption along with a transmission window for both TE and TM polarizations over a wide field-of-view. In addition, the functionalities of the transmission window and absorption of structure I are less sensitive to incidence angle for both TE and TM polarizations. Structure II shows that the transmission property is more stable under TE polarized wave, whereas it retains the absorption property over a wider range of incident angle under TM polarized wave.
In oreder to investigate the contribution of each layer in the implementation of the absorption-transmission property, the surface current distributions in the absorption and transmission bands for the TE polarized incident wave are shown in Fig. 5 for structures I and II, respectively, where the direction of the induced current is marked by arrows. Figures 5(a) and (b) displays the surface current distribution at the transmission peak occurring around 2.56 GHz. As can be seen, the surface current is more concentrated on the slot structure of the transmissive layer to ensure excellent transmission performance. Meanwhile, due to the semiconductor nature of ITO, weak current flow is also generated on the absorbing layer. By contrast, Figs. 5(c) and (d), which illustrate the surface current distributions at absorption peak occurring around 10 GHz, reveal that the absorbing layer reacts more to the incident wave, resulting in stronger surface currents. Besides, the current along the magnetic field direction is stronger than that in the electric field direction. On the other hand, at the transmission peaks of structure II, the current is distributed in the electric field direction of the CC slot for the transmissive layer, which agrees with the theoretical analysis. Besides, Figs. 5(g) and (h) manifest that the surface current at the absorption frequency is concentrated on the absorbing layer.
Fig. 5. Surface current distributions of structure I and structure II for normal incidence at different frequencies of (a) and (b) 2.56 GHz, (c) and (d) 10 GHz, (e) and (f) 26.94 GHz, and (g) and (h) 13 GHz.
The above analysis unambiguously demonstrates the contribution of the ITO film and FSS layer of the proposed FSAs in the realization of absorption and transmission characteristics. In other words, the absorbing layers of the FSAs are relatively dominant in the implementation of the absorption, and the transmission performance is mainly produced by the resonance of the transmissive band-pass FSS in the S- and Ka-band for the two designed FSAs. It should be noted that the impedance of ITO film is resistive, which results in small insertion losses of the structure in the transmission frequency band.
4. Experimental verification
To experimentally validate the proposed methodology, a proof-of-concept prototype of structure I is physically implemented. A photograph of the prototype fabricated utilizing printed circuit board (PCB) and laser etching techniques is presented in Figs. 6(a) and (b). 2 $\times$ 2 square-ring patches are etched periodically on the upper surface of substrate I through laser etching. The transmissive layer on the bottom of substrate II is fabricated through PCB technique. Meanwhile, the thickness of substrate I and substrate II is, respectively, 1.575 mm and 1.464 mm, as applied in numerical simulations. The fabricated FSA (structure I) is composed of 23 $\times$ 15 unit cells and covering a surface area of 299 mm $\times$ 195 mm.
Fig. 6. Fabricated sample and experimental measurement of the FSAs in structure I. (a) Top view and (b) bottom view of the fabricated sample. (c) Reflection and (d) transmission measurement setups. Comparison of the simulated and measured scattering parameters (e) and absorption (f) of the proposed FSA (structure I) under the TE mode wave with different incident angles.
The absorption-transmission functionality of the sample is experimentally measured in a microwave anechoic chamber. The experimental setups exploited for reflection and transmission measurements are shown in Figs. 6(c) and (d), respectively. Two linearly dual-polarized horn antennas operating from 2 GHz to 18 GHz are connected to the Agilent vector network analyzer (VNA) through coaxial cables. The FSA prototype is surrounded by microwave absorbing materials to avoid diffraction at the edges. The horn antennas are placed at 2 m away from the sample for both transmission and reflection measurements. In the first step, two horn antennas are calibrated in the free space transmission (in the absence of the FSA prototype). In the second step, the calibration of reflection is completed with a sheet of copper acting as a reference reflecting mirror. Then, the transmission of the prototype is measured utilizing incident quasi-plane waves emitted by a horn antenna. The reflection coefficient is measured by placing the emitting and receiving horn antennas on the same side of the prototype and inclined with an angle of about 5$^{\circ }$ with respect to the normal on the FSA sample [22].
The simulated and measured results with different incidence angles are presented in Figs. 6(e) and (f), from which a qualitative agreement between simulated and measured results can be observed. In Fig. 6(e), the measured and simulated transmission coefficients both indicate that the proposed FSA has a transmission window with insertion losses lower than 2 dB from 2 GHz to 3.12 GHz in the S-band under normal incidence. Besides, the FSA behaves as a transparent transmission windows in the S-band within 60$^{\circ }$ incidence. The absorption performance extracted from the experimental reflection and transmission data [Fig. 6(f)] explicitly demonstrates that the proposed FSA exhibits 80% absorption from 9.18 GHz to 11.06 GHz in the X-band for incidence nagle up to 60$^{\circ }$. As the incidence angle is further increased, the absorption bandwidth gradually decreases and shrinks towards the central frequency (around 10 GHz).
To highlight the advantages of our proposed FSAs, a comparison between the proposed absorbers and those reported in literature is shown in Table 3. It can be clearly observed that the proposed structures feature better absorption performances and reflectivity reduction. Besides, the absorption-transmission-integrated behavior remains stable up to 65$^{\circ }$ incidence angle, which can be advantageous for stealth functionality in radome applications.
Table 3. Comparison with Existing Works
5. Conclusion
In this paper, a class of bi-functional FSAs based on circuit-level analysis and numerical simulations is designed. The proposed FSAs realize the characteristics of a broadband absorber in a targeted frequency band and a highly efficient band-pass filter in a higher or lower frequency band. Numerical simulations show that the proposed FSAs exhibit a broad absorption bandwidth and a valid EM transmission window. In addition, the simulation results also demonstrate the polarization sensitivity and incident angle sensitivity of suggested FSAs. To analyze the absorption and transmission mechanisms of the designed structures, the surface current distributions in the absorption and transmission bands are investigated and discussed. Furthermore, a proof-of-concept prototype is fabricated by PCB and laser etching techniques, and measured in an anechoic chamber. Experimental results were in accordance with the simulated ones. The performances observed suggest that such class of FSA structures can be exploited for multi-functional scattering control and communication applications.
Funding
Natural Science Foundation of Shaanxi Province (2020JM-078); National Natural Science Foundation of China (61801109, 61801366); Xidian University special fund for industry-university-research cooperation (XWYCXY-012020016-HT); High Level Innovation and Entrepreneurial Research Team Program in Jiangsu.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (NSFC) under Grant 61801366, the National Natural Science Foundation of China (NSFC) under Grant 61801109, the Natural Science Foundation of Shaanxi Province under Grant 2020JM-078, and the High Level Innovation and Entrepreneurial Research Team Program in Jiangsu.
Disclosures
The authors declare no conflicts of interest.
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