1. Introduction

The asymmetry in electromagnetic (EM) wave transmission plays an important role in building a series of EM devices, which involve asymmetry or one-way propagation effects in their physical mechanisms, such like antenna radome, EM wave isolators and circulators [1]. These devices are of fundamental importance both in microwave and photonic communication systems. However, most of these designs require non-reciprocity associated with bulky ferrite materials and work under large static magnetic field biasing [2], making them difficult to be applied to integrated microwave and nano-photonic components.

Recently, metamaterial has attracted much attention due to its extraordinary electromagnetic properties not existing in natural materials, such as the negative refraction [3, 4], super lensing that breaks the refraction limit [5, 6], invisibility cloak [7, 8], etc. As an important branch, many metamaterial structures have been proposed to manipulate the polarization of EM waves achieving conversion or separation for different polarizations through either transmission or reflection [9–12]. Especially, the chiral metamaterial structures have been investigated extensively to realize asymmetric EM wave transmission, such as the asymmetric transmission of circular polarized wave through single layer planar chiral plate [13–15], or the asymmetric transmission for linear polarized wave through composite layers of chiral structures [16–18]. Such asymmetric transmission phenomenon does not require magnetic biasing and is irrelevant to the non-reciprocity of the Faraday Effect in magneto-optical media [2]. If we investigate the transmission properties of linearly polarized EM wave propagating through a metamaterial structure by transmission matrix representation, the asymmetric transmission originates from the off-diagonal terms of the effective transmission matrix getting swapped upon the reversal of the direction of propagation [19].

Another important branch of metamaterial application is the perfect EM wave absorber based on metamaterial structure [20–23]. The perfect EM wave absorption is usually achieved through adjusting the dimensions of electric and magnetic resonators inside the metamaterial unit cell so as to match the effective wave impedance to that of the free space, as well as to dissipate the incoming wave with proper resonant loss. Such metamaterial absorber is highly dependent on the resonant structure and therefore, its design can be scalable to different frequency bands [22, 23] and can be easily made for particular wave polarization or to control the polarization [24–27]. Most of the metamaterial absorber designs involve a metallic sheet at the back-side of the structure which leads to EM wave absorption as the wave incident to the front-side but total reflection when wave incident to the back-side. This makes the absorber opaque to EM wave incidence from both sides. It would be particularly interesting and practically useful to achieve one-way transparency (or one-way absorption) for linearly polarized EM wave propagation, and only a few theoretical explorations have been reported [28–30]. Thus, it is still lack of satisfactory design and validation of one-way absorber that enables EM wave absorption along one direction, while transparency along the opposite direction operating on both orthogonal linearly polarized EM waves.

In this paper, we propose an electrically thin, one-way EM wave absorber for linear polarizations based on composite metamaterial working in the microwave regime. The energy of certain linearly polarized EM wave can be fully absorbed along one propagation direction, but will be almost totally transmitted through the structure along the opposite direction. Notably, for the linearly polarized wave orthogonal to the previous case, the absorber exhibits a totally reversed propagation property. The performance of such one-way absorber has been verified through both EM simulation of the optimal design and experimental measurement on a fabricated prototype.

2. Concept and practical design

We aim to realize a one-way absorber for selective linear polarizations based on metamaterial structure. For possessing the polarization selective absorption property, a polarization selective absorber structure could be used in the design. But to achieve one-way linear polarization absorption which involves the asymmetric transmission, it is not easy to realize through single slab structure. To satisfy the requirement, we try to utilize the composite metamaterial concept to combine different functionalities of different metamaterial structures together. Such composite may leads to special properties which do not belong to any individual metamaterial structure. The proposed design consists of two metamaterial slabs, one possessing polarization rotation property for linearly polarized wave, the other with polarization dependent high absorption. One detailed unit-cell of the proposed composite metamaterial is schematically shown in Fig. 1. The left part is a slab which consists of two metallic layers and a dielectric interlayer, and the right part is a multilayer slab structure with two dielectric layers sandwiched between three metallic layers.

 

Fig. 1 Detailed composition of a unit cell and its working mechanism of the proposed polarization selective one-way absorber.

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The left slab works as a 90° polarization rotator which achieves polarization conversion between orthogonal linear polarizations. Meanwhile the right part is employed as a bi-directional polarization selective EM wave absorber, which only absorbs y-polarized EM wave (vertical polarization) and behaves transparent to x-polarized wave (horizontal polarization). When an x-polarized EM wave propagates along the positive z-axis direction (from left to right) into the composite structure, it is firstly converted to y-polarized EM wave by the polarization rotator and then absorbed by the latter absorber. While when the incident x-polarized wave propagates along negative z-axis direction, it almost transmits through the bi-directional absorber without much attenuation and is then converted into y-polarized wave through the second slab and emits out. For the orthogonal case, when a y-polarized wave transmits through the designed system along the positive z-axis direction, it becomes x-polarized wave due to polarization rotator’s effect and then keeps the polarization property till transmitting through the bi-directional absorber without much energy loss. When the y-polarized wave comes from the opposite direction, it is absorbed immediately by the bi-directional absorber with almost no energy transmission through the composite structure. The working mechanism analyzed above is also schematically illustrated in Fig. 1.

To enable the 90° polarization rotation functionality for the left slab, a bi-layered chiral metamaterial composed of enantiomeric patterns similarly to that proposed in [10] is employed, which could achieve high efficient cross-polarization conversion with independence on the incident polarization azimuth. The inset of Fig. 2(a) shows the schematic diagram of one unit cell of such bi-layered chiral metamaterial. The unit cell of the polarization rotator has a chiral structure which includes four identical cut-wires forming a square on the upper surface and an enantiomeric pattern on the lower surface. Such structure possesses a fourfold rotational symmetry and therefore guarantees the insensitivity to the polarization azimuth of the incident wave. The chirality in the propagation direction ensures that high efficient cross-polarization conversion is possible and giant optical activity is gained by the transverse magnetic dipole coupling among the metallic wire pairs of the enantiomeric patterns.

 

Fig. 2 The photography of the fabricated prototype sample with schematic views of unit cells of (a) the polarization rotator, and (b) the polarization dependent bi-directional absorber. (c) The measurement setup.

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The right slab works as a bi-directional polarization selective absorber which absorbs one particular linearly polarized wave and exhibits almost transparent to the cross-polarized wave. To enable such property, a metamaterial resonant structure similar to the design in [25] has been chosen as the prototype of our design. As shown in Fig. 1, the specifically designed bi-directional polarization dependent absorber (right slab) consists of three metallic layers. The upper one and the lower one have the same shape in geometric dimension and position in the x-y plane, which achieves the symmetry along propagation direction. Such structure can be regarded as cascading of three Class 1 layers in [19], which results in a bi-directional symmetry transmission property for linearly polarized EM wave. The inset of Fig. 2(b) illustrates the geometry of one unit cell of the bi-directional absorber viewed from one side. The unit cell includes four metallic split-cut-wires on both upper and lower surface, and four metallic strips in the middle metallic layer. When an x-polarized wave impinges normally on the slab, it will transmit through the slab without much attenuation. When the incident wave is y-polarized, it will encounter high absorption. Comparing with the layout in reference [25], we increase the amount of cut-wire-pairs in the structure of one unit cell to minimize the transmission loss for x-polarized wave, meanwhile retaining the absorption rate for y-polarized wave. We also applied different values to l₁ and l₂, as well as w₁ and w₂, for the optimization of both the transmission and absorption rate.

3. Full wave simulation and experimental measurement

To validate the concept of the one-way absorber proposal, we design a prototype working in the microwave frequency. The bi-directional absorber is arranged with metallic cut-wires parallel to y-axis. We choose copper as the metallic layer and FR4 board as the dielectric layer. The copper layer has standard thickness of 17 μm, while the FR4 board has a thickness of 0.2 mm, a measured permittivity of about 4.2 and a loss tangent of 0.031. The transverse dimension of a unit cell is 16.8 × 16.8 mm². Through full wave EM simulation, the optimum result is obtained with the geometrical dimension of w₁ = 1.1 mm, w₂ = 2.1 mm, l₁ = 15.9 mm, l₂ = 16.8 mm, d = 0.9 mm. The performance of such absorber is testified through both simulation and experiment means. The whole sample is composed of 11 × 14 unit cells as shown in Fig. 2(b). In the experiment, both transmission and reflection measurement are carried out through our free space measurement system consisting of a pair of point focus lens antenna and a vector network analyzer, as demonstrated in Fig. 2(c). The simulated and measured results are presented in Fig. 3(a). For an incident y-polarized wave propagating along z-axis, the absorber achieves a peak absorption rate of 85.8% in simulation and 85.6% in measurement at 9.84 GHz. At the same frequency, for an incident x-polarized wave, it becomes highly transparent with an transmission of 85.5% in simulation and 85.9% in measurement. The simulation and measurement results agree with each other quite well. The propagation properties along the positive and negative direction are identical through both simulation and experimental observations.

 

Fig. 3 (a) Simulation and measurement results of the bi-directional polarization absorber: absorption for y-polarized incidence and transmission for x-polarized incidence. (b) Comparison of the absorption of the composite structure with different spacer thicknesses D for x-polarized incident wave propagating along the positive z-axis. The absorber only curve is for y-polarized incident wave.

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For the polarization rotator, a 0.762 mm thick Taconic TLY-5 is chosen as the substrate with a dielectric constant of 2.2 + 0.0009i. The copper layers still have a standard thickness of 17 μm on both surfaces. To have coincident working frequency with the bi-directional absorber and gain a high polarization conversion rate, the length L, the width w and the air gap g in Fig. 2(a) are optimized to be 11.8 mm, 2.8 mm and 0.3 mm, respectively. The fabricated rotator is also shown in Fig. 2(a).

To design a composite metamaterial with different functional slabs, an important issue is to ensure that the constituted slabs will not greatly affect the functionality of each other. Near field coupling between composite metamaterial layers may lead to the splitting of the decoupled resonant state [31], which depends strongly on the distance between the composite layers. Thus the separation distance between the two composite layers should be properly chosen to control the coupling strength. An air spacer is inserted between the polarization rotator and the bi-directional absorber to form the proposed polarization selective one-way absorber. To investigate the influence on the absorption property caused by the compositing, we explore the propagation absorption for different spacer thicknesses and compares with that of the single absorber. The simulated results are shown in Fig. 3(b). As is indicated in the figure, small spacer thickness introduces strong coupling between the two slabs. When the thickness decrease below 3 mm, an obvious peak split on the absorption performance can be observed, similar to that analyzed in [31]. When the thickness enlarges over 4 mm, the absorption peak grows even higher with slightly shifting to lower frequencies. Thus, if the spacer thickness is properly chosen, the absorption performance of the composite structure remains almost the same as the single absorber, which means the concept of combining different functionalities together into one composite structure is feasible. At last, considering the energy transmission property for orthogonal polarization, the thickness of the air spacer is optimized as 4 mm. In the fabrication, the two slabs of the composite are fixed with plastic screw pins to ensure the thickness of the air spacer.

Next we explore the asymmetric propagation of the one-way absorber. For convenience, we define “+” (positive) as the propagation direction along the positive z-axis and “−” (negative) as the opposite direction for further discussion. We simulate and measure the propagation characteristics of the one-way absorber along different directions and compare the results in Fig. 4. When an x-polarized EM wave propagates through such device, it has a measured absorption of 88.8% along the positive direction and a measured energy transmittance of 83.0% along the negative direction (see Fig. 4(a)); meanwhile when the incident wave is y-polarized, it has a measured energy transmittance of 83.0% along the positive direction and a measured absorption of 83.8% along the negative direction at 9.87 GHz (see Fig. 4(b)). From Fig. 4, we also note that the measured results coincide with the simulation results well, except a slight shift of the absorption peak. The slight mismatch is probably caused by the dimension errors in the circuit printing process and uncertainty of the measured value of the dielectric constant of substrate. These measured results provide a direct experimental demonstration for the previous analysis. The idea of combining metamaterials with different functionalities into one design with new property is proved to be feasible. The total thickness of the device in the propagation direction is 5.2 mm, which is only about one-sixth of the operational wavelength.

 

Fig. 4 Absorption and transmission of (a) the x-polarized, or (b) the y-polarized incident wave through the polarization selective one-way absorber.

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To intuitively illustrate the physical mechanism of the polarization selective one-way absorber, we investigate the spatial distribution of electric fields located at different planes along the propagation direction at the resonant frequency (9.87 GHz). To clearly demonstrate the working mechanism, the air spacer’s thickness is slightly enlarged for the simulation. The results are shown in Figs. 5(a) and 5(b), which demonstrates that along the positive direction the x-polarized wave is absorbed while along the negative direction the y-polarized wave is absorbed. Meanwhile, the device is almost transparent to the corresponding cross-polarized wave with nearly full energy transmission, respectively. The functionality of the left slab as a polarization rotator, as well as the right slab as a bi-directional absorber is verified, which is the foundation of the proposed device. The electric field distribution coincides with the experiment results and has proved the mechanism of the designed polarization selective one-way absorber.

 

Fig. 5 Calculated spatial distributions of electric fields in a unit-cell for (a) x-polarized, or (b) y-polarized wave at 9.87 GHz incident from both directions. The arrows indicate the direction of the electric field vector.

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4. Conclusions

In summary, we present an electrically thin polarization-based one-way microwave absorber for linearly polarized EM wave through composite metamaterial. For an axially linearly polarized wave incident from two opposite directions, the device behaves significantly different, absorbent or transparent. Experimental measurement has validated the performance that the absorption peak for x-polarized and y-polarized wave is about 88.8% and 83.8%, respectively, while the transmission peak is about 83.0%, at around 9.87 GHz, respectively. We believe that such absorber can find applications in designing asymmetric electromagnetic devices and polarization control devices. The proposed concept may be scaled down to be applied at other frequency band, such as terahertz or even optical range. Following the similar design procedure, EM devices can obtain new functionality by utilizing composite metamaterials, which may extend the application field of metamaterials.