An ultrathin transparent metamaterial polarization transformer based on a twist-split-ring resonator
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In this paper, an ultrathin transparent metamaterial polarization transformer using a circular twist-split-ring resonator (TSRR) was proposed and investigated experimentally and numerically. The experimental and simulated results exhibit an asymmetric transmission only for forward and backward propagating linearly polarized waves. An incident linearly polarized wave can convert its polarization nearly completely to the cross direction after transmission under certain conditions. The simulated spatial evolution of the electric field further indicates that the twist structure functions as a perfect polarization transformer at certain frequencies.
Electromagnetic (EM) metamaterials (MMs) have attracted enormous interest in recent decades due to their unique responses to EM radiation, which are generally not encountered in their natural forms [1, 2]. MMs are usually periodically arranged with artificial materials that can be composed of dielectric elements or structured metallic components [2, 3]. Polarization is an important characteristic of EM waves and could be applicable in many areas, such as antennas, astronavigation, and communication [4, 5, 6, 7, 8]. Thus, it is highly desirable to efficiently control the polarization of EM waves . Conventionally, optical gratings and dichroic crystals can be used to convert polarization by employing, e.g., the Brewster and birefringence effects . However, the conversion efficiency is very low. Recently, chiral metamaterials (CMMs) have attracted much attention, due to their optical activity, circular or elliptical dichroism, and negative index [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. CMMs are artificial materials that lack any planes of mirror symmetry; they exhibit cross-coupling between the electric and magnetic fields at resonance [10, 15]. Thus, the similar design of chiral MMs could be used to convert polarization of incident EM waves, resulting in, e.g., giant optical activity  and polarization rotation . The asymmetric transmission (AT) of polarized light is an important research aspect of planar CMMs and has become a hot topic in recent years [18, 19, 20, 21, 22, 23, 24, 25]. The AT effect of circular or linear polarization has been studied for different MM structures, where the partial conversion of the incident EM wave is asymmetric for opposite propagation directions, which mainly originates from the interaction of EM radiation with the structural two-dimensional (2D) chirality in the MMs [23, 24, 25, 26, 27]. Thus, we can manipulate the polarization of EM waves by employing the AT effect, such as converting its polarization completely to the cross direction, and realizing circular or elliptical polarization after transmission under certain conditions [15, 23, 24, 25, 26, 27]. Menzel et al. and Kang et al. introduced the design criterion of AT for polarized EM radiation theoretically with a 4×4 matrix analysis based on the classical model of optical activity [21, 23, 24].
In this paper, we demonstrate experimentally and numerically an AT phenomenon for linearly polarized EM waves only and a linear polarization transformation. Based on the design criterion of the AT effect of MMs, the designed MM polarization transformer is composed of 90∘ twist-split-ring resonators (TSRRs) on both sides of a dielectric slab. A polarization conversion ratio (PCR) of 95 % in the experiment and 97 % in simulation can be achieved. Calculations on the spatial evolution of field distributions provide an intuitive picture of the interlayer coupling for polarization transformation inside the slab. Furthermore, when using a loss-free dielectric substrate as the middle spacer, the AT parameter can achieve a value of nearly unity.
2 Design, simulation, and experiment
Based on the above analysis, to achieve polarization transformation, we can design a similar CMM structure. The metallic resonant structure in each layer has the same arbitrary pattern; however, when the first layer structure is fixed, the structure in the second layer should be rotated clockwise 90∘ along the propagation direction of the EM wave. Thus, any planes of mirror symmetry can be broken.
For the experiments, the designed structures were fabricated into a 20×20 unit cell sample (200 mm × 200 mm × 1.272 mm) using the conventional printed circuit board (PCB) process with 36 μm thick copper patterns on both sides of a FR-4 (lossy) substrate. A photograph of a portion of a fabricated circular TSRR structure MM sample is shown in Fig. 1(b). The complex transmission/reflection coefficients of the experimental measurements were carried out in an EM anechoic chamber . An Agilent PNA-X N5244A vector network analyzer connected to two standard gain broadband linearly polarized horn antennas that produced microwaves in the range of 7–12 GHz was used to measure the MM sample. All components of the EM wave transmission/reflection (the complex Jones matrix) for different polarizations were measured by changing the orientation of the two horn antennas.
3 Results and discussion
The remarkable difference between the two cross-polarization transmissions may contribute to the polarization conversion of the designed structure. Thus, when the y-polarized (x-polarized) wave is normally incident to the structure along the −z (+z) direction, the wave is well coupled to the structure and is converted mostly to the x-polarized (y-polarized) wave due to the cross-coupling between the two metallic layers when passing through the designed MM. However, along the opposite direction, the y-polarized (x-polarized) wave can hardly be coupled to the structure, resulting in a weak transmission [18, 25]. In other words, the incident y-polarized (x-polarized) wave is well converted to a transmitted x-polarized (y-polarized) wave when passing through the designed MM, while the incident x-polarized (y-polarized) wave will be mostly forbidden to transmit for propagation along the −z (+z) direction. Thus, the designed MM can be used as a transparent polarization transformer for linearly polarized EM radiation. Figures 2(c) and (d) show the experimental and simulated reflection (the complex Jones matrix) for propagation along the −z direction; the cross-polarization reflections rxy and ryx are very small and below 0.15 in the entire frequency range. While the co-polarization reflection ryy reduces to a minimum of about 0.1 at a frequency of 9.5 GHz, rxx is near unity both in the experiment and the simulation across the whole frequency range. These results of the reflection further verify that the incident x-polarized (y-polarized) wave will be forbidden to transmit for propagation along the −z (+z) direction. That is, the coupling of the split rings among each other is very low for this polarization, and nearly without magnetic coupling, resulting in a high reflectivity, and no polarization transformation can be observed.
In conclusion, we first analyzed the design criterion for the AT effect of MMs, and then demonstrated experimentally and numerically an AT phenomenon for linearly polarized EM waves only and an ultrathin linear polarization transformer based on a TSRR-structured MM with a thickness smaller than λ/26. An incident linearly polarized wave can convert its polarization nearly completely to the cross direction after transmission under certain conditions. The PCR was achieved to a maximum of 95 % in the experiment and 97 % in the simulation around a resonance frequency of 9.65 GHz. The calculated snapshots of the EM wave evolution of wave fields propagating inside the structure illustrate the physical mechanism of the AT effect and polarization transformation of our design. Further simulated results indicate that we can obtain a high performance MM polarization transformer by using a loss-free or low-loss dielectric substrate. Such a design may find potential application in optical isolators, microwave wave plates, or other EM control devices.
This work is supported by the National Natural Science Foundation of China (51207060).
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