1. Introduction

Metamaterial has novel functions such as negative refraction [1,2], cloaking [3,4] etc. which are not found in natural materials [5]. A kind of metamaterial with one or a few functional layers, called metasurface, has aroused wide attentions in recent years due to its sub-wavelength thickness and rich properties [6–17]. Dual-functional metasurface with transmission and absorption characteristics has important applications in the communication systems. The conventional band-pass filter or band-pass frequency selective surface (FSS) is transparent to the EM waves with predesigned frequencies. However, the out of band signals will be reflected back [18–20]. These signals will decrease the signal noise ratio (SNR) of the communication systems. Moreover, the reflected waves will lead to the increase of the bistatic radar cross section (RCS) when the band-pass FSS is used as radome [21]. To enhance the SNR of the communication systems or the performance of radome, the out of band signals should be absorbed i.e., the transmission band of the band-pass FSS should be surrounded by absorption bands.

Costa et al. proposed a radome which has absorption band with frequency higher than that of the pass-band. This radome is composed of a resistive FSS and a metallic FSS with band-pass property [22]. In 2015, an absorptive FSS with a transmission band centered at 0.92 GHz and a broad absorption band from 3 to 9 GHz was designed by Chen et al. using the lumped elements [21]. Based on the resistive FSS with lumped elements and band-pass FSS, an absorptive/transmissive radome was proposed to provide a high frequency absorption band and a low frequency transmission band simultaneously [23]. Chen and collaborators proposed a lumped resistor loaded dual-functional metamaterial which has a transmission band centered at 21 GHz and a broad absorption band from 5 to 13 GHz [24]. Apparently, the band-pass FSS with simultaneous double absorption bands (one absorption band has a frequency higher than the pass-band frequency, while the other absorption band has a frequency lower than that of the pass-band) are more practical in real applications [25]. Shang et al. proposed a radome design which has absorption bands locating on the both sides of the transmission band using lumped elements [26]. In 2015, Li and collaborators proposed a dual-functional metamaterial with three layers of metal structures and three layers of different dielectrics with no lumped elements. The maximum transmission of this metamaterial is 41% and the absorption is broadband. This metamaterial has the advantage of incident angle independency but is polarization dependent [27].

In this work, we propose an ultrathin dual-functional metasurface with transmission band surrounded by multiple absorption bands without lumped elements. The performance of the metasurface is independent to the polarization states of the incident wave at normal incidence due to its geometric symmetry. We note that the proposed design is easy to fabricate for the absence of lumped elements, and the measured transmission band and absorption bands of the fabricated sample agree well with numerical results.

2. Design

The unit cell of the proposed metal-dielectric-metal (MDM) structured metasurface is shown in Figs. 1. The top layer of the metasurface is composed of a concentric metallic disk and split rings (see Fig. 1(a)). The bottom layer is composed of a concentric metallic disk which is separated from the top layer with a dielectric layer (the cyan part). Each split ring on the top layer has four arcs. All the splits with the same width of $g$ form a cross structure. As can be seen from Figs. 1, this metasurface with MDM configuration has four-folder rotational symmetry. The structural parameters of the metasurface are shown in Figs. 1(b) and 1(c). The dielectric layer is chosen as lossy FR-4 which has a relative dielectric constant and loss tangent of 4.1 and 0.018, respectively. All the metals on the top and bottom layers are chosen to be copper with a conductivity of $5.8\times {10}^8$$\text{S/m}$.

 

Fig. 1 (a) Schematic of the metasurface unit cell. (b) Top layer and (c) bottom layer of the metasurface unit cell. (d) Top layer and (e) bottom layer of the fabricated metasurface.

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3. Results and discussions

When the incident wave is $x$-polarized, the transmission of the dual-functional metasurface can be expressed as $T_x(\omega )={\left|S_{21}\right|}^2$, while the absorption of the metasurface can be denoted as $A_x(\omega )=1-T_x(\omega )-R_x(\omega )$. $R_x(\omega )={\left|S_{11}\right|}^2$ is the reflection of the metasurface. $S_{21}$ and $S_{11}$ are the scattering parameters. CST Microwave Studio software [28] is used in the simulations to gain the scattering parameters of the dual-functional metasurface. Periodic boundary conditions are utilized in the $x-y$ plane while open boundary condition is used in the $z$ direction.

The structural parameters of the dual-functional metasurface are as follows (in millimeters): $g=0.4$, $r_1=1.1$, $r_2=2$, $r_3=3$, $r_4=3.8$, $r_5=4.8$, $r_6=5.6$, $r_7=6.6$, $r_8=7.5$, $r_9=8.5$, $R=11.5$, $P=23.3$. The thickness of the FR-4 is 1 mm and the thickness of the copper is set as 35 $\text{μm}$. The simulated transmission, reflection and absorption characteristics of the metasurface are shown in Fig. 2 (a). The main transmission band of the metasurface is centered at about 10.48 GHz with a maximum value of 72.5%. Apparently, the thickness of the metasurface is lower than 1/26 of the wavelength of the main pass band. And there are four main absorption bands which are centered at 6.58 GHz, 8.71 GHz, 12.03 GHz and 12.62 GHz. The corresponding absorptions are 96.3%, 92.5%, 87.2% and 62.7%, respectively.

 

Fig. 2 The transmission and absorption characteristics of the metasurface: (a) simulation, (b) measurement for the $x$-polarized incidence.

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To verify the dual-functionality of the metasurface, a sample is fabricated with same parameters as that in simulations. And the sample has a dimension of 58.25$\times$44.27 cm which has 25$\times$19 unit cells (shown in Figs. 1(d) and 1(e)). The PNA-X network analyzer N5242A which is connected to two horn antennas is used in the measurement. The two groups of antennas used in the measurement have frequency ranges of 5-8 GHz and 8-12 GHz, respectively. The transmission of the metasurface is measured in the microwave anechoic chamber. To measure the reflection property of the metasurface, the two horn antennas are placed on the same side of the metasurface.

As shown in Fig. 2(b), the measured transmission, reflection and absorption characteristics of the metasurface are in good agreement with the simulation results. The measured main transmission band of the metasurface is centered at about 10.33 GHz with a maximum value of 63.8%. The transmission of the measured result is lower than the simulation result mainly because the incident angle of the measurement is about 4° considering the size of the horn antennas with frequency range of 8-12 GHz. The fabrication error also leads to the disagreement between the simulation and measurement results. Furthermore, the transmission and absorption of this metasurface show insensitivity to the polarization angle of the incident EM waves at normal incidence for its symmetric structure. All the locations of the transmission bands and absorption bands are almost fixed when the polarization angle increases from 0° to 90° (not shown here).

As demonstrated by Lee et al., resonators with different parameters lead to absorption peaks with different frequencies [29]. The electric and magnetic field distributions at the three main absorption bands and the transmission band are investigated (shown in Figs. 3). As seen, the multiple absorption bands shown in Figs. 2 are aroused from the concentric metallic disk and split rings with different geometric parameters. Apparently, the electric and magnetic fields centered at 6.58 GHz are localized at the outer split rings which have larger dimension. The electric and magnetic fields centered at 12.03 GHz are localized at the inner split rings. At the three absorption bands, the electric field maxima are located at the magnetic field minima.

 

Fig. 3 The $x$ component distributions for both the electric field: (a) 6.58 GHz, (b) 8.71 GHz, (c) 10.48 GHz, (d) 12.03 GHz and magnetic field: (e) 6.58 GHz, (f) 8.71 GHz, (g) 10.48 GHz (h) 12.03 GHz at the main absorption bands and transmission band for the $x$-polarized incidence.

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The surface current distributions at the three main absorption bands and the transmission band are also investigated (shown in Figs. 4). Apparently, the three main absorption bands are attributed to the magnetic resonances from the surface current distributions. As to the absorption bands, the surface currents on the outer split rings are stronger than those on the inner split rings for the lower frequency absorption band while the surface currents on the inner split rings are stronger for the higher frequency absorption band. This result is in good agreement with the electric and magnetic field distributions shown in Figs. 3.

 

Fig. 4 The surface current distributions: (a) 6.58 GHz, (b) 8.71 GHz, (c) 10.48 GHz, (d) 12.03 GHz at the main absorption bands and transmission band for the $x$-polarized incidence.

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From the above analysis, we know that the locations of the absorption bands are determined by the metallic disk and split rings with different dimensions on the top layer of the metasurface. To further understand the transmission characteristics of the metasurface, we analyze the effect of the structural parameter of the metallic disk on the bottom layer. And we find that the transmission band of the metasurface can be tuned by changing the diameter of the metallic disk on the bottom layer while all the other structural parameters remain unchanged (shown in Fig. 5(a)). The transmission band will have a red-shift with the increase of the diameter of the metallic disk on the bottom layer. And the amplitude of the main transmission band will increase when the diameter of the metallic disk on the bottom layer increases from 20 to 23 mm. The locations of the main absorption bands are insensitive to the change of the diameter of the metallic disk on the bottom layer (shown in Fig. 5(b)). The amplitude of the absorption band centered at about 12.03 GHz will increase with the increase of the diameter of the metallic disk on the bottom layer while the amplitudes of the absorption bands with lower frequencies are almost fixed.

 

Fig. 5 (a) Transmission and (b) absorption characteristics of the metasurface when the diameter of the metallic disk on the bottom layer increases from 20 to 23 mm.

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To find the transmission and absorption rules of the metasurface, we investigate the effect of the loss of the dielectric on the characteristics of the metasurface. As is shown in Fig. 6(a), the transmission of the metasurface will decrease with the increase of the loss tangent of the dielectric while all the transmission bands are fixed at about 10.5 GHz. On the contrary, the absorption of the metasurface will increase with the increase of the loss tangent of the dielectric while the positions of the absorption bands are almost fixed. When the loss tangent of the dielectric is zero, the absorption still exist due to the Ohmic loss. Considering the absorption of the metasurface aroused from the Ohmic loss (the black line shown in Fig. 6(b)), the dielectric loss plays a more important role in the absorption of the metasurface than the Ohmic one. Considering the transmission and absorption rules of the metasurface, the loss tangent of the dielectric layer is chosen to be 0.018 to get a high transmission with the absorption properties maintained.

 

Fig. 6 (a) Transmission and (b) absorption of the metasurface as functions of the loss tangent of the dielectric and frequency for the $x$-polarized incidence.

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The performance of the metasurface under oblique incidences is also investigated (shown in Figs. 7). In the TE case, the incident E field is kept parallel to $x$-axis and the direction of the incident H field is deviated from the $y$-axis direction. While in the TM case, the incident H field is kept parallel to $y$-axis and the direction of the incident E field is deviated from the $x$-axis direction. The transmission intensity of the main transmission band will decrease with the increase of the incident angle for both TE and TM incidences. As shown in Figs. 7(a) and 7(b), the main transmission band maintain excellent performances in the angular range of about $\pm$10°. As shown in Figs. 7(c) and 7(d), the absorption bands maintain excellent performances in the angular range of $\pm$25° for both TE and TM incidences. Furthermore, the absorption intensity is insensitive to the increase of the incident angle.

 

Fig. 7 (a) Transmission and (c) absorption of the dual-functional metasurface as functions of the frequency and the incident angle for TE incidence. (b) Transmission and (d) absorption of the dual-functional metasurface as functions of the frequency and the incident angle for TM incidence.

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4. Conclusion

In summary, we propose a dual-functional metasurface with transmissive and absorptive bands simultaneously. The proposed metasurface has a main transmission band in the X band surrounded by several absorption bands in the S, X and Ku bands. Since the absorption bands are adjacent to the transmission band, the SNR of the metasurface is higher than the conventional band-pass filter which is not absorptive. With a total thickness of 1.07 mm, the metasurface is deep sub-wavelength (1/26) compared with the wavelength of its main transmission band. The transmission and absorption characteristics of this metasurface are independent to the polarization states of the incident wave at normal incidence due to its geometric symmetry. Furthermore, the main transmission band maintain excellent performances in the angular range of about $\pm$10° while the absorption bands maintain excellent performances in the angular range of $\pm$25° for both TE and TM incidences. The absorption and transmission bands can be tuned by varying the dimensions of the split rings on the top layer or the metallic disk on the bottom layer of the metasurface, respectively. The metasurface may find important applications in the communication systems.