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Abstract
The independent tailoring of electromagnetic waves with different circular-polarized (CP) wavefront in both reflection and transmission channels is of broad scientific and technical interest, offering ultimate degrees of freedom in designing advanced devices with the merits of functionality integration and spatial exploitation. However, most metasurfaces only provide dependent wavefront control of dual-helicity in a single channel, restricting their applications to limited practical scenarios. Herein, we propose a full-space dual-helicity decoupled metasurface and apply it to assemble a multi-folded reflective antenna (MFRA) in the microwave regime. A multilayered chiral meta-atom is designed and optimized to reflect a particular helical wave while allowing the orthogonal helical wave to penetrate through, with simultaneous full span of phase modulations in both channels. When a uniform reflection and a hyperbolic transmission phase profile is imposed simultaneously on the metasurface in a polarization-selective manner, it can be engineered to conduct specular reflection for one helical wave and convergent transmission of the other helical wave. Combining the proposed metasurface with a metallic plate as a bottom reflector and an integrated microstrip patch antenna in the center of metasurface as a feed, a MFRA is realized with a low profile, high efficiency, and high polarization purity in a broad frequency band. The proposed design method of the dual-helicity decoupled metasurface and its antenna application provide opportunities for high-performance functional devices, promising more potential in future communication and detection systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metasurfaces, planar arrays composed of spatially engineered subwavelength-sized structures, have emerged as a promising platform for advanced control of electromagnetic (EM) waves by manipulating the transmitted or reflected waves in terms of amplitude, phase, and polarization states at will [1–9]. With an upsurge of researches on metasurfaces, a number of exotic physical phenomena and functional devices have been successfully implemented from microwave to terahertz and optical regions with the advantages of low profile, low loss, and simple fabrication, such as anomalous reflection/refraction [1,10], meta-lens [11,12], wave plates [13,14], beam shapers [15,16], holographic imagers [17,18], etc.
Among the methods of realizing phase-modulated metasurface, geometric phase (also termed as Pancharatnam-Berry (PB) phase) metasurface is a convenient way for its simple design strategy that only a single meta-atom with different orientation angles can achieve continuous full span of phase modulation required for kaleidoscopic wavefront manipulation [19,20]. However, the mechanism of the PB phase brings two major restrictions to phase modulation, inherently limiting their freedom to manipulate the wavefront. One is the conjugated coupling that the phase profiles for left-handed circular polarization (LHCP) and right-handed circular polarization (RHCP) excitations are exactly opposite [21–24]. For instance, if a focusing PB phase profile is designed for one particular helical wave, it turns to divergent for the orthogonal helical wave [25,26]. To resolve this bottleneck, researchers then propose a hybrid method combining both geometric and propagation phases to achieve independent phase control of two orthogonal helicities [27–30]. The other is that the fixed helicity conversion caused by the path on the Poincaré sphere determines the PB phase metasurface can only work in cross-polarized transmission or co-polarized reflection [8,19,31,32]. As a result, these two factors limit the multifunctional capacities of phase-modulated metasurfaces, especially for the full-space multifunctional devices. So far, a common way to realize diversified functionalities is superimposing different sets of functional meta-atoms into one metasurface, which inherently suffers from low efficiency and low polarization purity because of the existence of mutual interferences [33,34]. More importantly, the majority of proposed multifunctional devices work on a single side of metasurface with all reflection or transmission, leaving the other side unused [35,36]. Hence, it is still rare to achieve full-space wavefront manipulation with independent phase modulation for dual helical waves in an ultrathin metasurface [37–39], and to further apply the multifunctional metasurface to practical devices such as the multi-folded reflective antennas [40–42].
Here, we report a concept of full-space dual-helicity decoupled metasurface with independent phase modulation for two orthogonal helical waves. Based on this metasurface, a multi-folded reflective antenna (MFRA) is proposed for practical application. To decouple the two orthogonal helical waves, chirality, an additional phase resource that can effectively generate circular dichroism, is utilized for independent phase modulation separately in reflection and transmission channels. With this in mind, a multilayered meta-atom composed of two coupled slotted patch resonators is optimized with high-efficiency co-polarized reflection for LHCP incidence and co-polarized transmission for RHCP incidence. The meta-atom acts like a perfectly reflective PB structure in reflection channel for that the reflection phase shift is exact twice the rotation angle of bottom patch resonator. While in transmission channel, the meta-atom is different from the PB structure, and the transmission phase shift is linearly modulated simultaneously by the rotation of two patch resonators. Therefore, 360° range of phase coverage can be realized and independently controlled in both channels. The meta-atoms are engineered to configure a transmissive focusing meta-lens for RHCP incidence and a reflective meta-mirror for LHCP incidence in a single metasurface for MFRA application. By setting the designed metasurface as the main radiator, and introducing a metallic plate as the bottom reflector, a MFRA is finally assembled with low profile, high aperture efficiency, and high polarization purity. Compared with conventional low-profile antennas based on resonant property, such as Fabry-Perot cavity (FPC) antennas, the designed MFRA outperforms in bandwidth and gain performances to some degree. With these advantages, the proposed full-space dual-helicity decoupled metasurface and its application in MFRA are expected to offer an untapped platform toward satellite communication, radar systems, the next generation of mobile communication systems, and so on.
2. Full-space dual-helicity decoupled metasurface
2.1 Concept and meta-atom design
As schematically depicted in Fig. 1(a), the metasurface with independent control of reflected and transmitted wavefront can be achieved under the illumination of two orthogonal helical incidences. For convenience, the right side of metasurface, where contains the incident and reflected waves, is denoted as “Region 1”, while the left side of metasurface containing the transmitted wave is denoted as “Region 2”. As a proof of concept, two functions are designed as co-polarized transmission with focusing beam under RHCP illumination and co-polarized meta-mirror with specular reflection under LHCP illumination, which are illustrated as red trace and blue trace, respectively. Therefore, the proposed metasurface can perform distinct and independent phase-modulations for dual helicities in full space, which can be described by the two Jones matrices ${R_{cp}} = \left[ {\begin{array}{{cc}} {{r_{RL}}}&{{r_{RR}}}\\ {{r_{LL}}}&{{r_{LR}}} \end{array}} \right]$ and ${T_{cp}} = \left[ {\begin{array}{*{20}{c}} {{t_{LL}}}&{{t_{LR}}}\\ {{t_{RL}}}&{{t_{RR}}} \end{array}} \right]$. Here, the first subscript of the elements indicates the polarization states of the reflected or transmitted wave while the second subscript indicates that of the incident wave. The subscripts L and R represent the LHCP wave and the RHCP wave, respectively. To impose distinct phase profiles for two functions (denoted as F1 and F2) into different CP channels, the phase-modulated coefficients can be described as
where ${\Phi _{{F_1}}}$ and ${\Phi _{{F_2}}}$ character the designed phase profiles for distinct functions under the illuminations of two circularly-polarized (CP) waves. The proposed metasurface is composed of multilayered structure, which can provide additional freedom of modulating the transmission phase profile independently without interfering with the reflection phase profile when illuminated by incident waves propagating along + z-direction. Thus, the fixed phase relation of the orthogonal helicities is decoupled, and then independent phase profiles of ${\Phi _{{F_1}}}$ and ${\Phi _{{F_2}}}$ can be integrated into a single metasurface. Moreover, the phase profiles of full-space channels can be easily controlled for other distinct functionalities, which are not limited to that mentioned in this article.
Fig. 1. Schematic view of the proposed full-space dual-helicity decoupled metasurface and the constituent meta-atom. (a) The working principle of the metasurface device with independent phase control to generate focusing meta-lens in co-polarization transmission channel for RHCP incidence and meta-mirror in co-polarization reflection channel for LHCP incidence, respectively. (b) The perspective view of constituent meta-atom consisting of two CP patches with optimized geometric parameters of (unit: mm) p = 16, h = 2.5, r1 = 5.1, l1 = 8.2, w1 = 1.2, s = 2.8, d1 = 0.6, and d2 = 1.
To decouple the correlated phase of two orthogonal helicities and then independently manipulate the full-space wavefront, our strategy can be described as follows. First, a kind of chiral structure is utilized to distinguish the helicities of incidences and then assign them to different channels. In this case, LHCP incidence is assigned to the reflection channel while RHCP incidence to the transmission channel. Then, additional freedom should be generated to manipulate the transmission phase profile without interfering with the reflection one. Based on this, here, we consider a meta-atom composed of two circularly-polarized patch resonators connected with a metallic via, as shown in Fig. 1(b). The meta-atom consists of three metallic layers, separated by two layers of dielectrics. A metallic plate is set in the middle to serve as the shared ground. By carefully designing the size of rectangle slot and shift distance between the patch and the center of meta-atom (where is also the location of the via), both patch resonators can act as good RHCP microstrip patch antennas at desired frequencies. Based on the theory of antenna and wave propagation, for this meta-atom, the RHCP incidence propagating along + z-direction can be received and transferred to the other patch resonator through the metallic via. Then, the received energy will be radiated to the free space as RHCP wave, and the phase can be modulated by rotating the receiving and radiating patch. For the LHCP incidence, it cannot be received and as a result be reflected. It is noted that the reflected wave possesses the same helicity as incidence, showing the potential to carry the reflective PB phase when rotating the receiving patch. Therefore, the phase coupling between LHCP and RHCP waves is broken and two distinct functions can be designed independently in full space using this meta-atom.
For the proposed chiral meta-atom, two Jones matrices in the linear base can be employed to describe its EM characteristics [43], which can be calculated as ${R_{lp}} = \Lambda {R_{cp}}{\Lambda ^{ - 1}} = \left[ {\begin{array}{{cc}} {{r_{xx}}}&{{r_{xy}}}\\ {{r_{yx}}}&{{r_{yy}}} \end{array}} \right]$ and ${T_{lp}} = \Lambda {T_{cp}}{\Lambda ^{ - 1}} = \left[ {\begin{array}{*{20}{c}} {{t_{xx}}}&{{t_{xy}}}\\ {{t_{yx}}}&{{t_{yy}}} \end{array}} \right]$ based on the aforementioned two Jones matrices in the circular base, where $\Lambda = \frac{1}{{\sqrt 2 }}\left[ {\begin{array}{*{20}{c}} 1&1\\ j&{ - j} \end{array}} \right]$, and the subscripts denote x- or y-polarization. Considering the pre-designed functions which are depicted in Fig. 1, the meta-atom is required to be perfectly reflective under LHCP illumination while perfectly transmissive under RHCP illumination. Thus, the required Jones matrices in the circular base are ${R_{cp}} = \left[ {\begin{array}{*{20}{c}} 0&0\\ 1&0 \end{array}} \right]$ and ${T_{cp}} = \left[ {\begin{array}{*{20}{c}} 0&0\\ 0&1 \end{array}} \right]$, with the corresponding matrices in the linear base can be calculated as ${R_{lp}} = \frac{1}{2}\left[ {\begin{array}{*{20}{c}} 1&{ - j}\\ { - j}&{ - 1} \end{array}} \right]$ and ${T_{lp}} = \frac{1}{2}\left[ {\begin{array}{*{20}{c}} 1&j\\ { - j}&1 \end{array}} \right]$. These matrices are then used to analyze the mechanism of independent phase modulations in full-space channels. Upon the co-polarization reflection for LHCP incidence, the receiver patch combined with the ground acts as a PB structure, and nearly zero current is excited on the surface of the radiator located at the other side of the meta-atom. If the receiver patch is rotated along the center of the meta-atom with an orientation angle of θb in the xoy plane, the reflection Jones matrix can be written as
On the other hand, when the receiver is rotated along the center of the meta-atom with an orientation angle of θb in the xoy plane, the transmission matrix is written as
Based on Eq. (3–5), it can be concluded that for the LHCP incidence towards the reflection channel, its reflection phase is controlled by θb. While for the RHCP incidence towards the transmission channel, the transmission phase is determined by θb and θu jointly. Therefore, the fixed phase coupling between two orthogonal polarizations is successfully broken, and independent manipulation of wavefront can be applied for full space on both sides of the meta-atom. Furthermore, apart from the example shown here, the proposed design strategy of meta-atom can be applied for an arbitrary combination of input and output helicities in full-space channels, just by changing helicities of the receiver and radiator.
To show the meta-atom’s ability for the designed functions, full-wave simulations are then performed using the commercial software of CST Microwave Studio. Periodic boundary conditions are applied to both x- and y-directions with Floquet ports. The center working frequency of the proposed meta-atom is 8.7 GHz, and the optimized physical parameters denoted in Fig. 1(b) are as follows. The radius of the patch is r1 = 5.1 mm. The size of the rectangle slot etched in the patch is l1 = 8.2 mm and w1 = 1.2 mm. The shift distance between the patch and the center of meta-atom is s = 2.8 mm. The dielectric substrate has a relative permittivity of 2.2 + 0.001i and thickness of h = 2.5 mm. The radiuses of metallic via and circular slot etched in the middle ground are d1 = 0.6 mm and d2 = 1 mm, respectively. Besides, the metallic layer is made of copper (σ = 5.96 × 107 S/m) with a thickness of 0.018 mm. The total thickness of the proposed meta-atom is 5.1 mm or 0.15 operating wavelength. Finally, the periodicity of the meta-atom is p = 16 mm or 0.46 operating wavelength. The miniaturization of meta-atom can be achieved to reduce the period length by using substrates with a higher permittivity, however with certain sacrifices of efficiency and bandwidth performance. Figures 2(a) and 2(b) show the simulated results of amplitude responses under normal LHCP and RHCP incidences, respectively. It is obvious that at the center frequency around 8.7 GHz, the amplitude of co-polarization reflection for LHCP incidence $|{{r_{LL}}} |$ is close to unity whereas that of other coefficients are suppressed to nearly zero, approaching the condition of forming the high-efficiency reflection channel. Similarly, the amplitude of co-polarization transmission for RHCP incidence $|{{t_{RR}}} |$ is close to one and other coefficients are nearly zero, validating the high-efficiency transmission channel. The results show a good agreement with the above theoretical analysis.
Fig. 2. Amplitude performance of the meta-atom. Simulated reflection and transmission coefficients of the meta-atom when illuminated by normal (a) LHCP and (b) RHCP incidences. Simulated angular stability of (c) co-polarization reflection coefficient under LHCP illumination and (d) co-polarization transmission coefficient under RHCP illumination versus incidence angle and frequency.
In the MFRA application, good angular stability is a key point for high gain and aperture efficiency. Hence, we also investigate the angular performance of meta-atom concerning a wide range of incidence angles, as shown in Figs. 2(c) and 2(d), in which the black dotted lines denote the contour lines when the amplitude is exact 0.9. It showcases that under the normal illumination, the bandwidths of efficiency higher than 0.9 are 18.6% (8.3-10 GHz) and 14.5% (8.3-9.6 GHz) for $|{{r_{LL}}} |$ and $|{{t_{RR}}} |$, respectively. When the incidence angle increases to as large as 50°, the bandwidths drop apparently but the maximum efficiencies around 8.7 GHz are still greater than 0.9, showing stable angular performance.
As for the phase modulation of the meta-atom, the simulation results are shown in Fig. 3. Firstly, the orientation of upper patch (radiator) is fixed along x-axis with no rotation, while the bottom patch (receiver) is rotated with an angle θb, as shown in Fig. 3(a). The reflection phase delay $\varphi ({{r_{LL}}} )$ and transmission phase delay $\varphi ({{t_{RR}}} )$ with the rotation angle θb changing from 0° to 360° in the frequency range from 7.5 GHz to 10.5 GHz are depicted in Figs. 3(b) and 3(c), respectively. It is obvious that the PB phase is introduced in the reflection channel for the phase delay satisfying $\varphi ({{r_{LL}}} )= 2{\theta _b}$. On the other hand, the transmission phase delay has the inverse trend with different slope, which can be written as $\varphi ({{t_{RR}}} )={-} {\theta _b}$. As shown in Fig. 3(d), the orientation of bottom patch (receiver) is fixed along x-axis with no rotation, while the upper patch (radiator) is rotated with an angle θu. Clearly, from 7.5 GHz to 10.5 GHz, the reflection phase delay $\varphi ({{r_{LL}}} )$ is immune from the changing of θu, whereas the transmission phase delay satisfies $\varphi ({{t_{RR}}} )= {\theta _u}$, as shown in Figs. 3(e) and 3(f). The simulated results agree well with the theoretical predictions. Therefore, once the geometric sizes of meta-atom are determined, a continuous 2π coverage of phase modulation can be obtained and manipulated independently in full-space channels, just by tuning the orientation angles of the patches separately on the two sides of the meta-atom. Then, the distributions of the parameters θb and θu are loaded to the corresponding meta-atoms to construct the full-space dual-helicity decoupled metasurface according to the pre-designed phase profiles ${\Phi _{{F_1}}}$ and ${\Phi _{{F_2}}}$. Thus, in this strategy for designing metasurface, we apply two degrees of freedom contributed by two rotation parameters to independently engineer two phase profiles in both the reflection and transmission channels, along with the advantages of thin thickness, high efficiency, and high channel purity.
Fig. 3. Phase performance of the meta-atom. (a) Schematic of meta-atom with rotation of bottom patch. Simulated (b) co-polarization reflection phase under LHCP illumination and (c) co-polarization transmission phase under RHCP illumination versus rotation angle θb of bottom patch. (d) Schematic of meta-atom with rotation of upper patch. Simulated (e) co-polarization reflection phase under LHCP illumination and (f) co-polarization transmission phase under RHCP illumination versus rotation angle θu of upper patch.
2.2 Metasurface design and its performance
Based on the above analysis on the mechanism and unique features of the proposed meta-atom, here, here a full-space metasurface operating as a co-polarized reflective meta-mirror for LHCP incidence and simultaneous a co-polarized transmissive focusing meta-lens for RHCP incidence is simulated, fabricated, and experimentally verified for MFRA application. To achieve the functionality of meta-mirror reflecting the incident plane LHCP wave to the specular direction without flipping the spin state, the reflection phase profile ${\Phi _{{F_1}}}$ is designed as uniform distribution without any disorder and then applied to the bottom interface of metasurface. On the other hand, the target phase profile for transmission functionality of planar focusing metasurface with hyperbolic profile generated by an m × n matrix can be implemented as
where p is the meta-atom size, f indicates the focal length of the devised metasurface. Taking the preset focal length of f = 140 mm and the operating wavelength of λ = 34.1 mm at the designed center frequency of 8.7 GHz, the intuitionistic colormap of the focusing phase profile is calculated and depicted in the inset of Fig. 1(a). Corresponding the phase blocks to concrete meta-atoms, we configurate a full-space dual-helicity decoupled metasurface as shown in Figs. 4(a) and 4(b). The metasurface sample has a size of 256 mm × 256 mm (7.4λ × 7.4λ) with 16 × 16 meta-atoms.Fig. 4. Photograph of the (a) bottom and (b) upper layers of the fabricated metasurface. Simulated results of the designed metasurface. (c) Simulated and measured curves of reflection angle with respect to the incident angle. The scattering intensity of one sample with incident angle of −40° is illustrated in the inset. (d) Simulated and measured efficiency of specular reflection for various reflection angles. (e) Simulated (upper panels) and measured (bottom panels) normalized power distribution on the (e) xoz and (f) xoy plane for the transmissive focusing meta-lens functionality.
The numerical simulations are performed in the CST Microwave Studio with incidence of the CP plane waves along + z-direction and open (add space) boundary conditions set in all directions. As the first illustrative example, the metasurface is illuminated by a LHCP wave under various incident angles. To demonstrate the meta-mirror which is able to reflect the LHCP incidence to specular direction while keeping the spin state, the simulated and measured results of the co-polarized specular reflection are depicted in Figs. 4(c) and 4(d). The scattering intensity of one sample with incident angle of −40° is shown here. When the incident angle of the LHCP wave varies from 0° to −50°, explicit measured specular reflection is obtained with the reflection efficiency dropping slightly from nearly 1 to 0.77. The measured results agree well with the simulated ones, indicating that high-efficiency meta-mirror working for wide range of angles is obtained.
Furthermore, if we change the helicity of the incident plane wave to RHCP, the corresponding phase profile ${\Phi _{{F_2}}}$ will converge the transmitted wave to the preset focal point. As shown in Fig. 4(e), the normalized power of the co-polarized output along propagation direction in the xoz plane shows that the energy is successfully focused into the preset focal plane (z = 140 mm) at 8.7 GHz. The simulated and measured full widths at half maximum (FWHM) of normalized intensity profiles at the focal spot are 24 mm (0.7λ) and 31 mm (0.9λ), respectively, as shown in the inset of Fig. 4(e). The slight differences are mainly from the fabrication and measurement tolerances. Figure 4(f) shows the power distribution on the surface of xoy focal plane with a distance of 140 mm to the metasurface. Apparently, the power on the surface converges to the center point, indicating a good performance of the designed meta-lens. Unambiguously, the measured results agree well with the simulated ones. Based on the above analysis and the design concepts, the proposed full-space dual-helicity decoupled metasurface is capable of realizing independent wavefront manipulation for co-polarization reflection and co-polarization transmission for circular waves.
3. Full-space metasurface for MFRA application
3.1 Concept and design of the MFRA
As two typical high-directivity low-profile antennas, the folded reflectarray antenna (FRA) and the folded transmitarray antenna (FTA) are schematically shown in Figs. 5(a) and 5(b). The basic principle to reduce the antenna profile while maintaining high gain performance is to fold the ray path between two planar structures. Clearly, the ray path is folded once for the FRA while twice for the FTA during the propagation, as denoted by the red dotted circles. According to the equivalent optical path, the heights of the FRA and the FTA can be reduced to around f/2 and f/3, respectively. Therefore, the more times the ray path is folded, the lower profile can be achieved for these types of antennas. Although many FRAs and FTAs with low profile and various functions have been reported [40–42,45–48], for most folded-type high-gain antennas, the folded times of ray path are limited to twice in FTAs and the total profiles are difficult to be reduced to less than the working wavelength.
Fig. 5. The operation mechanisms and side views of (a) the FRA, (b) the FTA, and (c) the assembled MFRA based on the proposed full-space dual-helicity decoupled metasurface.
Here, we aim to realize a high-efficiency MFRA with a triple-folding of the ray path. As shown in Fig. 5(c), the full-space dual-helicity decoupled metasurface introduced in Section 2 can be employed as the main radiator in the configuration of MFRA, allowing the RHCP wave pass through while reflecting the LHCP wave. A metallic ground is then placed below the metasurface working as the bottom reflector, which reflects the CP waves with simultaneous helicity conversion. An RHCP microstrip patch antenna depicted in the inset of Fig. 5(c) is integrated into the center of metasurface as the feed source. The operating mechanism of MFRA can be described as follows. Firstly, an RHCP spherical-like wave emitted from the microstrip feed antenna reaches the ground and is then reflected to the specular direction with polarization conversion due to the intrinsic characteristic of metallic surface for CP waves. The reflected LHCP wave impinges on the metasurface and then produce a co-polarized specular reflection. Afterward, a third time specular reflection is achieved when the reflected LHCP wave reaches the ground. Finally, the reflected RHCP wave propagates and passes through the metasurface with little loss and simultaneous transmission phase compensation to form a high-gain radiation beam in the far-field region. Therefore, as denoted by the black dotted circles in Fig. 5(c), totally three times of specular reflections are realized through the whole process. After setting a proper distance H between the metasurface and ground, a virtual focal point can be calculated and located below the metasurface with an equivalent focal length of f based on the same optical path, which is the same as that of the meta-lens proposed in Section 2, and it is used for the calculation of required transmission phase compensation. Since the optical path is triple-folded, the relationship between the height of MFRA and the equivalent focal length of metasurface can be described as
which indicates that the proposed MFRA has a very low profile. Compared with the traditional transmitarray/reflectarray antennas [49,50], FRAs [45], and FTAs [40], whose heights are usually equal to f, f/2, and f/3, the proposed MFRA can achieve a much lower profile with the total height reduced by approximately 75%, 50%, and 25%, respectively. In addition to the lower profile, the high-efficiency transmission and reflection of the proposed metasurface combined with the ideal reflection efficiency of metallic surface contribute to the high aperture efficiency of the proposed MFRA. The high polarization selectivity of the metasurface between different CP waves also results in remarkably suppressed cross-polarization level in a wide bandwidth.3.2 Experimental verification of antenna performance
To validate the feasibility and performance of the proposed design of MFRA, a bottom reflector composed of metallic plate is fabricated, as shown in Fig. 6(a). With the combination of the preciously fabricated full-space dual-helicity decoupled metasurface (shown in Figs. 4(a) and 4(b)), the final MFRA sample is assembled, and several nylon columns are used to fix the whole structure, as depicted in Fig. 6(b). Based on the reversibility of optical path, the spherical wave emitted from the focal point can also be totally converted into plane wave by a focusing metasurface when the constituent meta-atoms are lossless and not sensitive to the incident angles of the EM waves. For the meta-atom proposed in this work, although slight amplitude deterioration and phase shift will happen with the increasing of incident angle, high directivity of the MFRA can still be maintained with negligible interference. When a carefully designed microstrip patch antenna is integrated into the center of the metasurface, triple-folded ray tracing is formed between the two planar structures, greatly reducing the whole profile of the MFRA compared with conventional high-gain antennas based on meta-lens. The integrated feed, schematically shown in the inset of Fig. 5(c), are designed with a radius of r2 = 5.2 mm. The displacement values of the feed point are optimized as sx = 3.2 mm and sy = 1.1 mm to achieve good impedance and axial ratio (AR) matching. The geometries of the slots are l2 = 3.3 mm and w2 = 1.2 mm. The metallic plate, which is designed with the same size of metasurface, is etched on the top of a substrate with a relative permittivity of 2.2 + 0.001i and a thickness of 2 mm. Finally, the distance between the bottom reflector and metasurface, which is the profile height of the MFRA, is determined as H = 35 mm according to Eq. (7).
Fig. 6. Fabricated sample and test scenario. (a) The fabricated bottom reflector. (b) The assembled MFRA composed of metasurface and bottom reflector. (c) The test scenario.
The designed MFRA is verified through numerical simulation and measurement in a microwave anechoic chamber to avoid unwanted interference reflected from the surroundings, as shown in Fig. 6(c). Firstly, the reflection coefficient of the whole antenna is shown in Fig. 7(a), and the measured S11 is less than −10 dB from 8.2 to 10.3 GHz, with a relative bandwidth of 22.7%, indicating that most energy is radiated successfully in a wide frequency band. To show the far-field radiating performance of the assembled MFRA, the simulated and measured realized gain of RHCP wave and AR from 7.5 to 10.5 GHz are depicted in Fig. 7(b). It explicitly showcases that RHCP wave occupies the co-polarization component, and the measured maximum gain is 24.5 dBi at 8.7 GHz, corresponding to the peak aperture efficiency of 40.7%. The gain variation is less than 3 dB from 8.2 to 9.8 GHz (17.8% fractal bandwidth). Furthermore, a 3 dB AR bandwidth is utilized to evaluate the polarization purity performance of the antenna. Clearly, the AR level is suppressed below 3 dB across the entire operating band from 7.5 to 10.5 GHz (33.3%). Figures 7(c) and 7(d) show the simulated and measured co-polarized far-field radiation patterns of the MFRA in the E-plane (xoz plane) and H-plane (yoz plane) at the center frequency of 8.7 GHz. Apparently, the measured results agree well with the simulated ones and demonstrates that the MFRA has a directive beam with 3 dB beam width of 8° and a side lobe level of −13.7 dB. Compared with the radiation patterns of single feed, a gain enhancement of 17.4 dB can be achieved with the load of metasurface. In addition, with the merit of metasurface in terms of good polarization purity, the cross-polarization level is suppressed to −41.5 dB at 8.7 GHz. Moreover, the profile of the MFRA is significantly shrunk to 35 mm (or one wavelength), which is only 0.14 times of the aperture of MFRA, enabling the antenna to be more compact and more applicable in many applications, such as satellite communication, radar and detection systems, etc.
Fig. 7. Performances of the MFRA. (a) Reflection coefficient S11 and (b) realized gain of RHCP and AR versus frequency of the MFRA. Far-field RHCP radiation patterns of the feed and MFRA on the (c) xoz and (d) yoz plane at 8.7 GHz.
For a clear demonstration of the advantages of the proposed MFRA, we perform its comparison with other reported FRAs and FTAs [40,46,47] in Table 1. Obviously, with the triple-folded ray tracing, the proposed MFRA possesses the smallest value of H/D which is only equal to 0.14, indicating a lower profile. More importantly, the planar feed is ingeniously integrated into the MFRA, without any additional vertical space to set feed source, such as horn antennas or waveguides. Notably, the proposed MFRA possesses not only a low profile, but also outstands in other aspects. Based on the above analysis, the proposed MFRA herein obviously realizes good comprehensive radiation performances in terms of polarization purity, impedance and gain bandwidths, as well as aperture efficiency.
Table 1. Comparison with other folded-transmitarray/reflectarray antennasa
4. Conclusion
In conclusion, we propose a high-efficiency full-space dual-helicity decoupled metasurface that can independently encode distinct wavefront for two orthogonal incident helicities separately in transmission and reflection channels, realizing desired dual functionalities in full space. The chiral meta-atom with receive-and-radiate functionality is employed to optimize the metasurface and the design principle for independent manipulation of phase profiles in both the reflection and the transmission channels has been revealed. As design examples, metasurface simultaneously acting as a reflective helicity-preserved meta-mirror and a transmissive focusing meta-lens is realized. As a practical application, the proposed full-space metasurface is then applied to devise a MFRA with a low profile, high aperture efficiency, and high polarization purity. By assembling the metasurface and a metallic plate with face-to-face configuration and integrating a microstrip patch antenna as the feed into the center of metasurface, triple-folded ray tracing is achieved before the wave is finally radiating, realizing a reduced low profile. Consistent numerical and experimental results validate the good performances of the final assembled MFRA in terms of an aperture efficiency of 40.7%, a 3 dB gain bandwidth of 17.8% and a 3 dB AR bandwidth of 33.3%. In addition, the proposed MFRA possesses a compact profile as low as one wavelength, which is more promising and applicable in miniaturized communication and detection systems. Furthermore, the design principle of proposed metasurface and MFRA can be extended to millimeter-wave regions, offering an untapped platform toward the application of next generation wireless communication.
Funding
National Natural Science Foundation of China (61731010, 61801207, 62071215, 91963128); National Key Research and Development Program of China (2017YFA0700201); Priority Academic Program Development of Jiangsu Higher Education Institutions; the Fundamental Research Funds for the Central Universities and Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Wave.
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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