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Abstract
We propose a chiral metasurface (CMS) that exhibits asymmetric transmission (AT) of double circularly and linearly polarized waves at the same frequency band. In order to realize the manipulation of electromagnetic (EM) waves in the whole space, the unit cell of CMS consists of three layers of dielectric substrate and four layers of metal patches. The Z-shaped chiral micro-structure and a grating-like micro-structure are proposed and designed to achieve AT. The simulated results show that the x-polarized wave that is incident along one direction can be transmitted into the right-hand circularly polarized (RHCP) wave and the left-hand circularly polarized (LHCP) wave that is incident along the opposite direction can be reflected as the LHCP wave in the frequency band of 4.69GHz-5.84 GHz. The maximum chirality response can be reflected by AT and circular dichroism (CD) and they can reach up to 0.38 and 0.75, respectively. In addition, we also produced the sample of CMS, and the experimental results are in good agreement with the simulated results.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metasurface is the two-dimensional (2D) manifestation of metamaterials, in which the unit cell is arranged in 2D plane as periodic or aperiodic [1–3]. Because of the advantages of ultrathin profile and easy processing, it has been widely studied in recent decades [4–7]. Asymmetric transmission (AT) refers to the phenomenon that electromagnetic (EM) waves incident on metasurface in opposite directions to produce different functions [8–12]. This kind of device with AT effect has important application in satellite communication, radomes and optical system.
The metasurface with AT characteristics can improve the space utilization, thereby miniaturizing the devices. The chiral micro-structure of the metasurface [13–16] is equivalent to breaking the symmetry so that the metasurface exhibits AT properties [17–19]. The metasurfaces exhibit the asymmetric transmission in the frequency of microwave, THz and even optical [20–24]. And in order to increase the working bandwidth, graphene is used to realize the tunable AT [25–30]. Moreover, the spiral metasurface based on a hybrid structure of metal and VO2 is proposed to achieve AT function with tunable bandwidth and large angle of incidence at THz frequency [31]. The wavefronts of both linearly polarized [32–37] and circularly polarized [25–28] waves can be controlled by the metasurface with AT. For example, a chiral metasurface (CMS) is designed for AT in dual band. The concept of multiple models and the transmission matrix is introduced to explain the function of AT for linearly polarized waves in the terahertz range [32]. A three-layer CMS is proposed, which realizes the AT function of linearly polarized waves in three frequency bands. In addition, it is still stable when the incident angle reaches 40° [33]. The PIN diodes are installed on the split resonant ring to form a tunable CMS, which realizes the function of AT in multi-band. The physical mechanism is analyzed by the transmission matrix [34]. A transmission-reflection -integrated coding metasurface is illustrated to control the wavefront of linearly polarized waves with three different functions [35]. A bi-layered CMS exhibits high efficiency AT for mutually orthogonal linearly polarized waves in the certain frequency range, and can also keep a stable response with the large incident angle [36,37]. Recently, a CMS composed of L-shaped silver nanostructures simultaneously reflect and transmit mutually orthogonal circularly polarized waves for y-polarized incident wave from 200THz to 261THz. And the chirality is increased by engraving another smaller L-shaped silver nanostructure to achieve the more perfect polarization conversion [38]. A three-layered metasurface using the Fabry-Perot like resonance cavities is proposed to achieve broad dual-band AT of circularly polarized waves [39]. However, there are few reports on the simultaneous control of AT of linearly polarized waves and circularly polarized waves [40,41].
In this paper, we propose an AT device based on CMS, which can transmit the right-hand circularly polarized (RHCP) wave when the x-polarized wave is incident along the + z axis and reflect the left-hand circularly polarized (LHCP) wave when the LHCP wave is incident along -z axis from 4.69 GHz to 5.84 GHz. The AT is realized for incidences with the linear- and circular-polarizations simultaneously. The relative bandwidth is 21.8% with the axial ratio less than 3 dB and the total transmittance can reach up to 0.8 in the working frequency band. The CMS is able to find the potential applications in antenna systems due to the advantages of miniaturization and multi-functionalization.
2. Design and analysis
A multi-layer CMS is designed and given in Fig. 1. Figure 1(a) shows the schematic of the function and every layer of CMS that can achieve AT effect. The CMS can transmit the RHCP wave for the x-polarized incident wave along the + z (forward) and reflect the LHCP wave when the LHCP wave is incident along the -z axis (backward). As shown in Fig. 1(b), the proposed CMS consists of three layers of Rogers RT5880 dielectric substrate (ɛr=2.2 and tanδ=0.0009) and four cooper layers (σ=5.8×107S/m) that are I, II, III and IV. The unit length of the CMS is p=15 mm and the thickness of each dielectric substrate layer is 2 mm. Besides, the thickness of each copper layer is 0.035 mm. Figures 1(c) and (e) show the layers I and III of the cooper patches with chiral micro-structure. The layers II and IV of the unit cell are given in Figs. 1(d) and (f), respectively. By optimizing the structural parameters, the dimensions we get are as follows: l1=17 mm, l2=6.5 mm, l3=6.3 mm, l4=4 mm, l5=12 mm, w1=2 mm, w2=1 mm, s1=3.1 mm, s2=1.6 mm, s3=12 mm.
Fig. 1. Schematic diagram of CMS and the unit cell. (a) AT effect of CMS. (b) The unit cell for CMS. (c) I layer. (d) II layer. (e) III layer. (f) IV layer.
The copper patch is in the x-y plane and the EM waves transfer along the z axis when the CMS is placed in a rectangular coordinate system. The CMS can realize the function of the transmissive polarization converter and the reflective PB (Pancharatnam-Berry) metasurface when transmitted along the -z and + z axis. Here, the CMS is equivalent to a two-port network, and the Jones matrices for transmission along the + z axis and reflection along the -z axis can be respectively given by [42]
Fig. 2. Simulated results of CMS. (a) Transmission coefficient of linearly polarized waves. (b) Transmission phase and phase difference. (c) Axial ratio and total transmittance. (d) Reflection coefficient. (e) Transmission coefficient for the x-polarized wave and the circularly polarized waves. (f) AT of the x-polarized wave.
It is obvious that T can reach up to 0.8 in the frequency band of achieving the polarization manipulation and the transmitted energy reaches 80% of the incident energy. Meanwhile, AR is below 3 dB, indicating that the transmitted EM wave is the circularly polarized wave. Figure 2(d) shows the reflection curves of the circularly polarized wave incident along the -z axis. The values of r+- and r− are very small at operating frequency band means that the RHCP incident wave has almost no reflection. While values of r-+ and r++ are almost 1 and less than 0.2, respectively, indicating that the LHCP wave incident along the -z axis is reflected from 4.69 GHz to 5.84 GHz for LHCP incident waves. The simulated curves of the x-polarized incident wave along the + z axis are shown in Fig. 2(e). The energy of the transmitted RHCP wave is greater than 0.8 and that of the transmitted LHCP wave is less than 0.2 in the working frequency band, which further indicates that the incident x-polarized wave is transmitted into the RHCP wave. Since the CMS exhibits different functions for the EM waves incident along the + z and -z axis, we simulated the transmitted energy curve of the x-polarized wave incident along the + z and -z axis and plotted the curve of AT. The equation of AT is shown as follows [41]
Where Tb x and Tf x respectively represent the transmitted total energy when the x-polarized wave is incident in the back (+z) and the front (-z) directions. In Fig. 2(f), the value of AT in the working frequency band can reach up to 0.38 and can be maintained at a high level, indicating that the CMS realizes the excellent characteristic of AT.Figure 3(a) shows the contour distributions of the current of the IV layer and side view of current distributions of the unit cell. The left and right parts are the current distributions after the incident of the x- and y-polarized waves along the + z axis, respectively. It can be seen that the current intensity with the x-polarized wave is much less than that with the y-polarized wave when the wave is incident on the grating-like micro-structure. It can also be seen from the side view that the current intensity of I, II and III layers after the incident y-polarized wave is much less than that after the incident x-polarized wave. So, the y-polarized wave is reflected and the x-polarized wave is transmitted. This is also consistent with the curves of Fig. 2(a). Figure 3(b) shows the surface current of chiral micro-structures as the x-polarized incident wave along the + z axis. It obvious that the presence of chirality breaks the consistency of the surface currents on the two layers, causing the surface currents of the two layers to flow perpendicular to each other. Figure 3(c) shows the surface current of II layer as the x-polarized incident wave along the + z axis. The current flows along the long side of the copper patch. From the previous current analysis, it is known that the two copper patches with chiral micro-structure can be equivalent to two mutually perpendicular polarizers, then the strip copper patch placed at an angle of 45° between the I and III layers acts as a 45° polarizer that increases the transmittance of linearly polarized waves and also introduces a phase delay. By optimizing the parameters of II layer so that the phase delay reaches 90°, the x-polarized wave incident along the + z axis can be transmitted into the circularly polarized wave. The schematic diagram of the LHCP wave being reflected as the LHCP wave is shown in Fig. 3(d). According to the reciprocity theorem, it can be concluded that the LHCP wave will be converted into the y-polarized wave after passing through the I, II and III layers in turn. The current in IV layer mainly flows along the y direction from the current distributions in Fig. 3(e) when the LHCP wave is incident along the -z axis. Therefore, IV layer plays an important role in reflecting the y-polarized wave. From the previous current analysis and simulated results, the x-polarized wave can be transmitted into the RHCP wave after passing through the I, II and III layers. Moreover, the reflected y-polarized wave by IV layer can be transmitted into the LHCP wave after passing through the III, II and I layers in turn because the copper patch structure of three layers has axial symmetry.
Fig. 3. Surface current distributions on CMS at 5.1 GHz. (a) Surface current distributions after orthogonal linearly polarized waves are incident along the + z axis. (b) Surface current distributions of copper patches with chiral micro-structure under incident x-polarized wave. (c) Surface current distributions of the 45° polarizer under incident x-polarized wave. (d) Surface current distributions after the LHCP wave is incident along the -z axis. (e) Surface current distributions of IV layer under incident LHCP wave.
Figures 4(a) and (b) are schematic diagrams of comparison between grating-like micro-structure and general grating structure. The grating-like micro-structure is different from the general grating structure in that the copper patch along the x axis is longer. As shown in Figs. 4(c) and (d), the bandwidth of AR for grating-like micro-structure with relative bandwidth of 21.8% (4.69-5.84 GHz) is broader than that for general grating structure with relative bandwidth of 16.1% (4.63-5.44 GHz). Consequently, the grating-like micro-structure can slightly improve the polarization conversion efficiency of y-polarized wave, so that the RHCP wave can be transmitted in a wider frequency band.
Fig. 4. Comparison diagrams with grating-like micro-structure and general grating structure. (a) Grating-like micro-structure. (b) General grating structure. (c) Transmission coefficient of two contrasting structures. (d) Axial ratio of two contrasting structures.
In addition, we discuss the influence of different incident angles on the CMS, where phi and theta represent the azimuth angle and the elevation angle of incidence, respectively. Figures 5(a) and (b) respectively show the spectra of the x-polarized wave transmitted into the RHCP wave with different phi and theta. It is obvious that the transmittance has a periodic change of 180° with the change of phi, and there is about 120° in a cycle without destroying its function. With the shift of theta, there is a 90° periodic change, and the working frequency band is divided into three sections outside the range of -15°∼+15° and is widened. Figures 5(c) and (d) respectively show the spectra of the LHCP wave transmitted into the LHCP wave with different phi and theta. It can be seen that there is insensitivity to phi, which is due to the characteristics of circularly polarized waves. As theta shifts from -90° to 90°, there is still a 90° periodic change, the working frequency band is divided into three sections and the high frequency part increases sharply with the increase of the angle. Figures 5(e) and (f) show the spectra of circular dichroism (CD) with different phi and theta, respectively. The formula for CD is given as follows [43]
Fig. 5. Spectrogram of transmission coefficient, reflection coefficient and CD varying with phi and theta. (a) Transmitted RHCP wave with different phi. (b) Transmitted RHCP wave with different theta. (c) Reflected LHCP wave with different phi. (d) Reflected LHCP wave with different theta. (e) CD with different phi. (f) CD with different theta.
3. Fabrication and measured results
Fig. 6. Experimental environment and results. (a) Experimental environment and details of the processed sample. (b) Measured and simulated curves of transmission coefficient. (c) Measured and simulated curves of transmission phase and phase difference. (d) Measured and simulated curves of transmission coefficient for the x-polarized wave and the RHCP wave. (e) Measured and simulated curves of reflection coefficient for LHCP wave.
To experimentally verify the AT performance of the proposed CMS, we fabricated a sample composed of 28 × 28 unit cells with a size of 420 × 420 × 6 mm3. The sample is laminated by three-layer dielectric substrate and four-layer copper patch. Figure 6(a) shows the experimental environment and the details of the top and bottom of the sample. In order to measure the transmission coefficient, we placed the fabricated sample between two linearly polarized horn antennas, firstly. Two horn antennas were connected to the vector network analyzer through the coaxial cable, so that one of the horn antennas launched the x-polarized wave, and the other horn antenna received the linearly polarized wave. The amplitude and phase results in measurement are shown in Figs. 6(b) and (c). After that, we changed the receiving antenna to a circularly polarized horn antenna and the launching antenna is placed in the state of positive and oblique incidence. The incident angle is set at 0° and 15° respectively, and the measured energy of the RHCP wave is shown in Fig. 6(d). Then the reflection coefficient is measured, we placed two circularly polarized horn antennas directly in front of the sample, and absorbing materials are placed between the two horn antennas to prevent mutual interference of signals. One of the horn antennas launches the LHCP wave, and the other horn antenna receives the LHCP wave. Similarly, the incident angle is set at 0° and 15° respectively, and the measurement curve is shown in Fig. 6(e). From the measured results, AT function is achieved that the x-polarized wave incident on the bottom of the CMS is transmitted into the RHCP wave and the LHCP wave incident on the top of the CMS is reflected as the LHCP wave from 4.69 GHz to 5.84 GHz. It can be seen from Figs. 6(b), (c), (d) and (e) that the experimentally results are in good agreement with the simulated results, and the small discrepancies between measurement and simulation come from the processing error and the measurement error of the sample.
Table 1. Comparison With Other Chiral Metasurface
In Table 1, a comparison between the proposed CMS in this paper and other chiral metasurface. It is obviously that the CMS in this paper can be thinner and achieve more functions. The electrical size of the designed CMS is not very large, which has the role of miniaturization to some extent.
4. Conclusion
In summary, we proposed a multi-layer chiral metasurface that can realize the AT effect. Through the design of grating-like and chiral micro-structure, the metasurface realizes the function of AT that the x-polarized incident wave along the + z axis can be transmitted into the RHCP wave and the incident LHCP wave along the -z axis can be reflected into the LHCP wave from 4.69 GHz to 5.84 GHz. AT and CD can reach up to 0.38 and 0.75, respectively. Furthermore, we verify the function of the proposed chiral metasurface by measurement samples. The chiral metasurface is simple to fabricate and improves the utilization of space, which can effectively reduce the volume of devices working in the microwave, THz and even optical frequency bands.
Funding
China Postdoctoral Science Foundation (2019M650098); National Postdoctoral Program for Innovative Talents (BX20180375); Youth Innovation Team of Shaanxi Universities (202022); National Postdoctoral Program for Innovative Talents (2019K219); Natural Science Foundation of Shaanxi Province (2017JM6025, 2020JM-350); Young Talent fund of University Association for Science and Technology in Shaanxi Province (20170108); National Natural Science Foundation of China (61603412, 61701523, 61801508).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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