A Polarization-Dependent Frequency-Selective Metamaterial Absorber with Multiple Absorption Peaks

Abstract

A polarization-dependent, frequency-selective metamaterial (MM) absorber based on a single-layer patterned resonant structure intended for F frequency band is proposed. The design, fabrication, and measurement for the proposed absorber are presented. The absorber’s absorption properties at resonant frequencies have unique characteristics of a single-band, dual-band, or triple-band absorption for different polarization of the incident wave. The calculated surface current distributions and power loss distribution provide further understanding of physical mechanism of resonance absorption. Moreover, a high absorption for a wide range of TE-polarized oblique incidence was achieved. Hence, the MM structure realized on a highly flexible polyimide film, makingthe absorber suitable for conformal geometry applications. The proposed absorber has great potential in the development of polarization detectors and polarizers.

1. Introduction

Electromagnetic metamaterials, which represent a class of artificial structures whose electric and magnetic response can be controlled freely, have attracted a significant amount of attention [1,2,3]. Due to their unique benefits, metamaterials have provided many important effects, such as invisible cloaking [4], negative refractive index [5], and super-lens [6].

Since the metamaterial absorber (MA) was presented in 2008 by Landy et al. [7], metamaterial has attracted many researchers because of its advantages, such as size minimization and suitable thickness, for applications using electromagnetic wave absorbers [8,9,10]. In recent years, various structures, such as split rings [11], closed rings [12], and fishnet structures [13], have been proposed for MAs. A certain progress has been made in broadband MAs developing [14,15]. Meanwhile, some studies on tunable absorbers [16,17] and multiband absorbers [18,19,20] have been performed. Currently, research is focused on absorbers intended for application in polarization imaging and polarization detecting. Therefore, the polarization-dependent frequency-selective absorbers are highly demanded. For most single patterned resonant structures, the number of absorption peaks was fixed or the absorbers were polarization-independent [21,22]. Hokmabadi et al. proposed a stereometamaterial perfect absorber, wherein absorption frequency is related to polarization angles [23]. Hu et al. presented a polarization-dependent MA with different resonant frequencies in two orthogonal directions [24]. However, these structures have only one polarization-dependent absorption peak, which limits their application in spectroscopic imaging and detecting, wherein multiple absorption peaks are required [25,26].

In this paper, the focus is on the F frequency band (F-band), and a simple polarization- dependent frequency-selective metamaterial absorber is proposed. The proposed structure has three separate absorption peaks with high absorption for TE-polarized and TM-polarized normal incident waves. Furthermore, the absorption peaks can be dynamically controlled by adjusting of a polarization angle of the incident wave. Compared to the previously reported MAs, the MA proposed in this paper has several advantages. Firstly, compared to complex resonance structures with multiple absorption peaks, such as all-in-one packed super-unit cell [27,28] and stacked layers [29], the proposed absorber has only one patterned resonant structure, which meets current demands on miniaturization and simplification. Secondly, the multiple absorption peaks can be dynamically tuned by changing of polarization angles of the incident wave. Lastly, the proposed MA structure is realized on a highly flexible polyimide film without a rigid substrate, so the absorber can be easily used in the non-planar and conformal-geometry applications.

2. Structure and Design

The proposed MA structure and the unit cell structure of the copper pattern are presented in Figure 1. Unlike previously reported results, where multiple different-sized metal patterns were used to obtain multi-band absorption, here, the simple etching of a C-shaped slot into a circular patch, provides a triple-band absorption. The geometrical dimensions of the copper pattern shown in Figure 1b are a = 820 μm, d = 60 μm, r₁ = 112.5 μm, r₂ = 161 μm, and r₃ = 367.5 μm. The copper pattern with a thickness of 0.5 μm and an electric conductivity of (5.8×10⁷) S/m was printed on the top layer of a thick dielectric substrate with a thickness of 50 μm that was made of polyimide film with a relative permittivity of 3.6 and a dielectric loss tangent of 0.02. Moreover, a thick copper plate that acts as a ground plane was printed on the bottom of the dielectric substrate.

The absorptivity can be calculated by

A = 1 − |S₁₁|² − |S₂₁|²

(1)

where A, S₁₁, and S₂₁ are the absorptivity, the reflection coefficient, and the transmission coefficient, respectively. In this case, the transmission coefficient S₂₁ is zero because the thickness of the metal ground plane is much larger than its skin depth, so the absorptivity is determined by A = 1 − |S₁₁|².

3. Simulated Results and Discussion

The simulations were conducted in CST Microwave Studio 2014 (Darmstadt, Germany), using the finite-element frequency-domain method, and the unit cell boundary conditions were utilized in both x and y directions. Meanwhile, the Floquet port condition was employed in the z-direction. Since the proposed absorber has similar absorption characteristics for TE-polarized and TM-polarized normal incident waves, only the electromagnetic responses for the TE-polarized incident wave, whose electric field is parallel to the x-axis, were calculated for the normal incidence angle. Three distinct absorption peaks, which are at 92.4 GHz, 102.0 GHz, and 130.1 GHz with absorptivities of 88.1%, 98.2%, and 94.6%, respectively, are presented in Figure 2. In addition, three resonance peaks were labeled as mode f₁, mode f₂, and mode f₃ starting from low to high frequencies. From Figure 2, the separation of the first two adjacent resonant peaks of the MA is not high enough, which may hinder its applications in the multiband detection. This can be improved by reconstructing the geometries of the proposed structure.

In order to show the intrinsic mechanism of the proposed metamaterial absorber, the surface currents distributed on the metal pattern and the metal ground plane that correspond to three absorption peaks were calculated and the obtained results are presented in Figure 3.

As shown in Figure 3a, for mode f₁, the surface currents on the metal pattern diverge from inner circular patch to two arms of the closed ring, and then converge to the lower left corner of the ring, which means that mode f₁ represents a typical dipole resonance. Meanwhile, the surface currents on the metal ground plane, shown in Figure 3b, are reversed, and circulating current loops are formed because the current on two metal layers are anti-parallel, so the magnetic response is excited.

Similarly, for mode f₂, the electric response is excited by surface currents on the metal pattern that flow along two arms of the closed ring, forming a dipole resonance (Figure 3c). However, compare with mode f₁, the surface currents on the closed ring in mode f₂ diverge from the lower right corner and converge to the upper left corner. According to results presented in Figure 3d, the magnetic response of mode f₂ is also excited by circulating currents between two metal layers. In the case of mode f₃, the surface currents are mainly concentrated on the connecting bar between a circular patch and a closed ring (Figure 3e), and mode f₃ represents a dipole resonance that mainly depends on the connecting bar dimensions.

In order to demonstrate the absorption mechanism, the power loss density in the dielectric layer and the ohmic losses at resonator surface for mode f₁ − f₃ were determined, and the obtained results are illustrated in Figure 4 and Figure 5, respectively. Dielectric losses of the absorber are mainly concentrated in areas with a strong electric field, while the ohmic losses at resonator surface are determined by surface currents. In addition, it has been observed that the power losses in the dielectric layer are more than dissipated on the resonator surface, and the polyimide film has a very important role in the absorption of the incident waves.

4. MA Absorption Dependence on the Wave Incident Angle and Polarization

The absorption characteristics of the proposed structure for oblique incident wave with TE polarization were simulated, and the obtained results are shown in Figure 6a. During the calculation, E-field was in the x-direction, and H-field and wave propagation directions varied simultaneously for angle θ.As shown in Figure 6a, when the oblique incident angle increases from 0 to 60°, the absorption peaks at the three resonance frequencies are about 90% or greater, which is caused by a resonator structure, wherein for TE polarization a strong electric resonance can be excited with a relatively small field strength.

For the TM-polarized wave, whose H-field is in the x-direction, the absorption characteristics for the oblique incident wave were also simulated, and the obtained results are shown in Figure 6b. However, as can be seen in Figure 6b, when the incident angle increases, the absorption characteristic for TM-polarized oblique incidence deteriorates rapidly at all resonance modes.

Nonetheless, we were focused on absorption characteristics for different polarization angles for the normal incident wave. For TE and TM polarization, E-field changes for angle φ with respect to the x-and y-direction, respectively. The simulated responses for TE and TM waves for different polarization angles are presented in Figure 7a,b, respectively. As shown in Figure 7a, for resonant modes f₁ and f₃, the absorption is high when φ is in the range 0–45°. However, the resonance of mode f₂ decreases gradually when φ changes from 0 to 45°. According to results presented in Figure 7a, it can be concluded that the resonance of mode f₂ vanishes at φ = 45° because the surface current on the resonator surface in mode f₂ (Figure 3c) mainly flows perpendicularly to the direction of the electric field vector. Therefore, the electric response cannot be excited.

Similarly, when TM polarization angle is changed, the absorption peaks for mode f₂ are above 80%, while the absorptivities of mode f₁ and f₃ decrease gradually when φ changes from 0 to 45°. Therefore, the proposed structure provides both single-band and multi-band absorption for different polarization angles.

5. Experimental Results

The MA was fabricated using standard photolithography. Firstly, the copper with a thickness of 0.5 μm was deposited on the bottom surface of the polyimide film. Then, the photolithography was used to define the metallic patterned resonator at the top film surface. Lastly, the copper was evaporated, and a metal lift-off process was used to complete the pattern transfer. The fabricated sample composed by 18×18 unit-cells with a dimension of 1.5 cm×1.5 cm is displayed in Figure 8.

The free space measurement method was adopted in the experiment. A pair of horn antennas were connected to the Agilent N5224A vector network analyzer (Agilent Technologies Inc, Santa Clara, CA, USA) via VNA extenders VDI VNAX600 (Virginia Diodes Inc, Charlottesville, VA, USA) that were used for frequency range 90–140 GHz. In this case, one antenna acted as a transmitting antenna and other as a receiving antenna. In order to avoid the reflections from the environment, the calibration was performed as follows. The reflections from an identical copper plate, which acts as an ideal reflector, were firstly measured. Then, the actual reflections from the sample were calculated by comparison of the reflection from the structure and the reflection from the copper plate. It should be noted that, for the given sample dimensions, a minimum far-field distance of 40 cm must be met. Moreover, the measurement of responses for different incident angles has not been carried out due to the large scattering of this finite sample at wide incident angles [30].

The measured MA absorption for various polarization angles (φ) for both TE-polarized and TM-polarized normal incident waves is presented in Figure 9. For TE-polarization, a dual-band absorption is achieved for a polarization angle of 45°. However, only one absorption peak with absorptivity of above 70% is achieved for polarization angle of 45° in the case of TM-polarized incident wave, which is in agreement with the simulated results.

6. Conclusions

A simple design of a polarization-dependent frequency-selective metamaterial absorber based on a circular patch with a C-shaped slot is presented. In contrast to previous studies on compression of multi resonators, in the proposed design, a multi-band absorption is obtained using a simple resonant structure. Furthermore, changing the polarization angle of the incident wave, a triple-band, a dual-band, and a single-band absorptions can be achieved, which is required in the polarization-dependent detection. In order to demonstrate the absorption mechanism, the electric field distribution, the surface current distribution, and the power-loss distributions were obtained. The proposed design was verified both by simulations and experiments, which were performed using the free space measurement method. A good agreement between simulated and measured results was achieved, which proves a high absorption performance of the proposed MA.