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Abstract
Multifunctional electromagnetic (EM) metasurfaces are capable of manipulating electromagnetic waves with kaleidoscopic functions flexibly, which will significantly enhance integration and applications of electronic systems. However, most known design schemes only realize the reflection or transmission functions under a specific angle range, which wastes the other half EM space and restricts wider applications of multifunctional metadevices. Herein, an encouraging strategy of broadband and wide-angle EM wavefronts generator is proposed to produce two independent functions, i.e., antireflections for transverse electric (TE) waves and retroreflection for transverse magnetic (TM) waves, which utilizes band-stop and bandpass responses of the metasurface, respectively. As a feasibility verification of this methodology, a three-layer cascaded metasurface, composed of anisotropic crossbar structures patterned on the two surfaces of a dielectric substrate with sandwiched orthogonal metal-gratings, is designed, fabricated, and measured. Both the simulated and experimental results are in good accordance with theoretical analyses. This full-space metasurface opens up a new route to multifunctional metasurfaces and will further promote engineering applications of metasurfaces.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Acting as the two-dimensional (2D) form of metamaterials, metasurfaces [1,2] are typically formed by periodically arranging subwavelength resonant building blocks (meta-atoms) on a single plane. Despite their lightweight and facile fabrication, they can flexibly manipulate electromagnetic (EM) wavefronts [3–5] owing to inherently unique physical characteristics. After years of ongoing researches, various metasurfaces with excellent EM performance have become versatile platforms for modulating the intrinsic properties of EM waves with high spatial resolution, including phases, amplitudes, and polarizations [6–8]. To date, metasurfaces have unlocked numerous fascinating effects such as anomalous refractions and reflections [9,10], vortex beam generation [11–13], achromatic focusing lenses [14–15], and colorful hologram [16–19] in microwave and optical fields. With the help of the linear phase gradients utilizing scaling-induced propagation phase (resonance phase) [20,21] and rotation-induced phase (Pancharatnam-Berry phase) [22,23], a large number of anisotropic and cascaded metasurfaces are used to design band or polarization dependent meta-devices with distinct phase profile [24–27], endowing that arbitrary shaping of EM wavefronts.
Unfortunately, most existing metasurfaces operate either in reflection or transmission modes [28–30] due to the mutual interference between diverse functions, which not only leaves the other half-place resource unexploited, but also limits diversified performances in applications. In the field of transmissive metasurfaces especially involving AR, many efforts have been devoted to utilizing multistage interference to enable through-wall metalens [31] or achieving unusual Brewster effects under TE-incidences based on angle-independent impedance matching [32]. Nevertheless, so far, it’s a difficult work for the majority of AR studies to take into the transmission bandwidth and efficiency under ultra-wide incident angles account. In parallel, when it comes to reflective metasurfaces, retroreflection of oblique incidence plane wave among these applications has been of particular interest. Some feasible methods including anomalous gratings diffraction [33] and genetic algorithm [34] are demonstrated to deal with the problem of finite channel and single polarization respectively. Depending on the operation frequency and application scenarios, whether gratings period or group cells arrangement inevitably introduce excessive computation. It’s worth noting that EM waves cannot penetrate the dielectric due to the metal plate. To further improve the aperture utilization and transmission-reflection integration, full-space metasurfaces [35,36] are developed as a solution to cover resources blind region. To achieve full-space manipulation of EM waves, several feasible strategies based on multiplexing techniques of polarizations, frequencies, or even propagation directions [37–40] have been proposed for independent control of transmission and reflection channels. For example, a chiral transmission-reflection switched metasurface was proposed to transmit right-handed circular polarized (RCP) waves and reflect left-handed circular polarized (LCP) waves simultaneously, which could act as an integrated meta-device for orbital angular momentum (OAM) generators or metalens, while working in relatively narrow bandwidths [37]. Additionally, anomalous reflection, radar cross section (RCS) reduction and OAM generation can be achieved independently at two different microwave frequencies under orthogonal polarization states by the metasurface composed of dual-band elements [38]. More recently, a metasurface multiplexed with both incident propagation directions and polarizations was designed to generate four independent holographic images in reflection and transmission channels [39]. However, these current works are constrained in limited angular domain due to those only considering the normal incident condition, which may significantly hinder potential applications in tailoring EM wavefronts. Encouragingly, based on theory of dispersion engineering and Fresnel zone plate, some novel meta-devices, i.e., a single-layer metalens which is capable of focusing at three wavelengths and with different incident angles into the same point [41], ultrathin metalens with wide-angle beam steering ability beyond ± 60° [42], and terahertz metasurface zone plates converting arbitrary polarizations to a fixed polarization [43] are fabricated to multiplex and even manipulate polarization. All these works provide another option for wide field-of-view detection and vectorial imaging, which avoids the drawbacks of the aforementioned researches to a certain extent. Furthermore, although reconfigurable metasurfaces with switchable components and/or phase change materials (PCMs) extend multifunctional metasurfaces into real-time control of EM waves [44–46], they usually require a complex layout and control system, which dramatically increases the loss and cost of this system. To enhance the capability of information channel and achieve full-space EM control with more degrees of freedom, simply-designed integrated meta-devices with polarization independence, wide angle, full-space EM control still need to be in-depth analyzed.
To overcome the above limitations and complete above requirements, here we propose a strategy to construct full-space metasurfaces for generating wide-angle directional EM wavefronts, whose meta-atoms are composed of anisotropic crossbar structure on upper and lower surfaces and sandwiched orthogonal metal-grating structures. Through modulating the geometric profiles of sandwiched orthogonal metal-grating structures and anisotropic crossbar structure, independent transmission-reflection amplitude and phase control can be obtained simultaneously in full-space for different polarization states. In addition, the corresponding high transmission or reflection rate and inter/intra-element coupling under two incident modes give birth to low crosstalk in co-polarized states in the same frequency channel, which ensure operation efficiency and precision. Furthermore, based on integration of Fabry-Perot like constructive interference, generalized Snell’s law, and effective medium theory, the mingled modulator evolves into a multi-channel, polarization-multiplexed, full-space modulator for diversified multitasking implementations. Compared with some other successful research [47–49], the angle resources in manipulation of EM wavefronts in space domain are fully excavated, providing an additional operational degree of freedom. An integrated meta-device of broadband and wide-angle antireflective coatings for TE-polarization and three-channel retroreflector for TM-polarization are established to demonstrate our theoretical designs, as described in Fig. 1. Both the simulated and measured results verify that the proposed metasurface can independently and effectively manipulate EM wavefronts in full-apace, which may yield a lot of applications in modern communication systems.
Fig. 1. Schematic diagram of the proposed grating-assisted full-space metasurface, which can generate wide-angle directional EM wavefronts. (a) Broadband and wide-angle transmission enhancement under y-polarization illumination. The transition from red to purple denote the wide-angular domain where the incident angle covers from -80° up to 80°. Destructive interference combined with Drude resonance in the right depict synergy of dual mechanisms for enhancing transmission; f1 ∼ f2, and f3 ∼ f4 represent the transmission bands resulted from different mechanisms. (b) High-efficiency retroreflection under x-polarization illumination. The cylindrical areas described by red, green, and purple represent efficient retro-reflective channels at the same frequency at different incident angles of 0°, and ±55° independently, while rectangular regions depict coding sequences of “0101……”, and “1010……”.
2. Meta-atom design and basic principles
2.1 Design and simulations of full-space EM wavefronts generator
By way of combining frequency selective surface (FSS) [50,51], plenty of spatial filtering structures are designed to transmit in-band EM waves and reflect or absorb those out of the band, while they are difficult to achieve polarization isolation and shared aperture in the same frequency band. Inspired by polarization selectivity [52] of metal gratings, a three-layered anisotropic structure which consists of two layers of ceramic matrix composites (CMC) whose permittivity is εr = 3.6(1 + 0.002j) with the thickness of d = 1.5 mm is designed, utilizing the commercial software CST Microwave Studio, and three cross metal patches with different sizes etched on these two substrates, as denoted in Fig. 2(a) and 2(b) respectively. The specific metal gratings shown in intermediate layer in Fig. 2(c) are equivalent to Drude resonators, whose effects in region I and region II are transmission mode and reflection mode respectively. Besides, the same cross structures in upper and lower layers provide amplitude and phase shift compensation simultaneously so as to broaden transmission bandwidth and angular domain for TE-polarization, and enhance multichannel backscattering enhancement for TM-polarization.
Fig. 2. Design and simulations of the meta-atom. (a) Schematic view of the single meta-atom. (b) Front views of three layers (I, II, III) of cascaded structure. (c) Equivalent permittivity function of Drude model. (d) surface currents distribution and transmission/reflection amplitudes at normal incidence under the illumination of TE/TM-polarizations, respectively.
To validate the operating principle of our design and explain it in detail, the underlying mechanism of achieving different reflection/transmission amplitude is analyzed by effective medium theory. The effective medium theory [53] refers to that transmission performance of materials can be qualitatively characterized via deriving their equivalent EM parameters. Therefore, we can tune the equivalent parameters into expected values, such as positive, negative, or zero value, and then flip the way EM waves propagate (such as reflection or transmission) via adjusting the period of the metal gratings and changing the structure parameters. As aforementioned, by adding the metal gratings along orthogonal directions, the effective relative permittivity can be modified by the following formula
where εeff(ω), ωp, ω and γ are the effective permittivity, the plasma frequency, the working frequency, and the loss factor (collision frequency) of the system, respectively. The plasma frequency ωp is governed by where neff is effective density of electrons, meff is effective mass of electrons, e is the electron charge, and ε0 is the permittivity of vacuum. Furthermore, the plasma frequency existing in metal gratings (the specific derivation process is given in Supplement 1 Note S1) can be expressed as where r and a are wire radius, and spacing between adjacent wires. As a specific proof-of-principle example, a full-space metasurface integrated with reflection for TM-polarization and transmission for TE-polarization was designed in Ku-band (central frequency point f0 = 15GHz). Because of subwavelength structure (the period is between 1/3 and 1/5 of operating wavelength), the length p1 and p2 of meta-atom along x-direction and y-direction are set as 6.1mm and 5mm.For the transmission fields, in the light of impedance matching [54], to completely eliminate reflection upon the air-dielectric interface, εeff should meet the requirements of having a value of 1.0. Therefore, according to Eq. (1) and (3), it’s concluded that ωpm needs to be set around f0. The optimized geometrical parameter ω21 of longitudinal metal gratings is 0.2mm. First of all, combined with Eq. (3), ωpm1 is approximately calculated as 21.2GHz, which is slightly higher than expected frequency. To further elaborate the operating mechanism of the proposed meta-atom composed of double-layered dielectric and middle gratings, the surface currents and transmission amplitudes are calculated via full-wave simulations with periodic boundary conditions in x- and y-directions and open conditions along the z-direction. The upper right region in Fig. 2(d) depicts transmission amplitudes of CMC with and without embedded metal gratings. It’s clearly seen that with embedded gratings, the transmission amplitudes of CMC vary from 0.5 to 0.95 at normal incidence in Ku-band. Though the average transmission amplitude (black symbol line) of pure CMC is 0.8, increasing co-polarized transmission amplitudes (red symbol line) indicate good transition of impedance matching, in keeping with Drude resonator. On the other hand, the monitored surface currents denoted by red arrows on the longitudinal metal gratings are apparently more intense and hence plays the main role for TE-polarization, as shown in upper left region in Fig. 2(d).
For TM-polarized incidence, according to the theory of EM wave transmission [55], due to the non-magnetic property (µr≈1.0) of the dielectric material, if the effective permittivity of CMC εeff is much less than zero (the dielectric is equivalent to a metal plate at this time), it can achieve efficient co-polarized reflection. And the optimized geometrical parameter of transverse metal gratings is ω22 = 1.2mm. Furthermore, ωpm2 is approximately calculated as 40.1GHz according to Eq. (3), which is a lot higher than expected frequency. Thus, most EM waves are reflected by the dielectric, and the average reflection amplitudes obtained by simulation (lower right panel in Fig. 2(d)) are above 0.96, indicating that the feasibility of this design architecture. As shown in lower left region in Fig. 2(d), the monitored surface currents denoted by red arrows on the transverse metal gratings are obviously stronger and make a major contribution for TM-polarization. Therefore, we can obtain independent regulatory function of transmission and reflection for different polarizations.
According to the previous mechanism analysis and meta-atom design, embedded metal gratings perform polarization isolation effect under the illumination of different polarized waves. On this basis, we further employ double-layered metal cross-resonators with identical geometry at the top/bottom dielectric surface to dramatically enrich EM manipulation characteristics and realize directional wavefronts. As a proof of concept to illustrate that, we integrate wide-angle EM anti-reflector and three-channel retroreflector into a metasurface utilizing proposed meta-atoms.
2.2 Broadband and wide-angle EM anti-reflector for TE-polarization
To overcome the limitations of previously demonstrated antireflection metasurfaces (only embedded middle gratings) with a relatively narrow frequency range (that is because that tuned equivalent permittivity eventually tend to be the same as the dielectric), here we exploit specifically tailored dispersions of the top/bottom metasurface layers (metal cross-resonators) to immensely expand antireflection operational bandwidth in resonant region I illustrated by Fig. 2(c). In the simulations, the geometric parameters of additional meta-atoms in layer I and III are carefully optimized to reach satisfactory antireflection performance. Due to anisotropy of this structure, it’s indicated that the short metal wires along y-direction determine transmission efficiency for TE-polarization, and detailed parameters are depicted as: w11 = 0.25 mm, w12 = 0. 5 mm, l11 = 2.1 mm, and l12 = 2.4 mm.
Figure 3(a) and (b) delineate the numerically simulated transmission spectra varying against frequencies and incident angles for CMC without and with the proposed metasurfaces, where broadband and wide-angle antireflection is observed from the correspondingly enhanced transmission peaks (i.e., I, f1 = 14.3 GHz with T1 = 0.98, and II, f2 = 16.4 GHz with T2 = 0.975, at θi = 60°). Further Inspecting transmission amplitudes in Fig. 3(b), transmission channel II and I are coupled due to their close resonant frequencies under θi = 0°, 20°, 40°. Note that when comparing to pure dielectric film, the present antireflection metasurface structure allows for a flat and broadband performance (in almost entire Ku-band). Additionally, the 1dB-bandwidth is significantly fulfilled in 13.46-16.85 GHz at θi = 60°, enhancing average transmission amplitude of CMC by 60%. Especially for the incident angle 80°, the average transmission amplitude in the whole Ku-band is increased by nearly 40%. Furthermore, the surface currents on the meta-atom are calculated at a typical incident angle 60°, as shown in Fig. 3(c) and (d). It can be clearly observed that surface currents are mainly concentrated in layer II at 14.3 GHz, while in layer I and III at 16.4 GHz, respectively, which reveals great agreement with theoretical analysis and numerical simulations.
Fig. 3. Broadband and wide-angle EM anti-reflector. (a), (b) Simulated transmission amplitudes under different incident angles for a CMC dielectric without and with the designed metasurfaces. (c), (d) Monitored surface currents at an incident angle of 60° at 14.3/16.4 GHz, respectively.
In order to intuitively demonstrate the directional modulation of EM waves by metasurface, the simulated far-field radiation patterns of an EM anti-reflector of 120 × 60 meta-atoms of 732mm×300mm in size plotted in Fig. 4, where Fig. 4(a)-(d) show far-field radiation patterns at different incident angles and frequencies. As desired, whether EM waves are incident at 60° or -60° in the forward half-space (regions z > 0), the simulated single beam is exactly accumulated in the propagating direction of it and transmitted into backward half-space (regions z < 0), which coincidences well with the theoretical prediction.
Fig. 4. Simulated 3D far-field radiation patterns and corresponding 2D far-field radiation patterns in the cutting-plane of the xoz-plane of the proposed metasurface. (a) at an incident angle of 60° at 14.3 GHz, (b) at an incident angle of 60° at 16.4 GHz, (c) at an incident angle of -60° at 14.3 GHz, and (d) at an incident angle of -60° at 16.4 GHz.
As previously analyzed, transmission channel I depicted in Fig. 3(b) results from reduction of equivalent permittivity of this dielectric by metal gratings around its resonant frequency. However, when operating frequency ω increases away from ωpm, the tuned equivalent permittivity εeff will converge on εr, as shown in Fig. 2(c). Therefore, double-layered metal cross-resonators are introduced into the dielectric film to drive resonant frequency ωpm of metal gratings to lower frequencies in order to focus on f0 and expand transmission bandwidth. On the other hand, frequency difference between transmission channels I and II denoted in Fig. 3(b) is controlled to achieve weak Drude resonant effect at resonant frequency of II as far as possible, which means that middle metal gratings almost make no difference to transmission of that. In order to further elucidate the transmission enhancement mechanism, double-layered metal cross-resonators at top/bottom metasurface layers are considered as two effective interfaces with zero thickness for the propagating waves, while CMC plate forms a subwavelength waveguide array and can be treated as an effective medium that can be computed numerically. Additionally, the near-field interactions of middle metal gratings to upper and lower resonant metallic structures can be neglected due to enough frequency difference and dielectric thickness. Then we can further calculate overall transmission response based on interference except layer II between interface I and III forming a Fabry-Pérot-like cavity, with spacer medium given by CMC substrate.
Figure 5(a) shows the whole antireflection system, where overall transmission are superpositions of multiple transmissions at the two interfaces [56]
Fig. 5. Schematic view and simulated results of multi-reflection/multi-transmission within the double-layered metasurface cavity. (a) Illustration of interference model and the associated variables, (b) Transmission amplitude t and (c) phase θ, ϕ spectra of the double-layered metasurface under 60° incidence. (d) semi-analytically calculated and simulated transmission amplitude spectra for this metasurface cavity under 60° incidence.
2.3 High efficiently three-channel retroreflector for TM-polarization
On the basis of reflection of middle metal gratings, double-layered metal cross-resonators are established as another control variables to achieve amplitude and phase distributions for three-channel retroreflector for TM-polarization. To achieve high-efficiency three-channel retroreflection, an appropriate gradient and near-unity reflection amplitude are required. Therefore, we implement resonant region II of metal gratings and generalized Snell’s law. When a flat metasurface is illuminated by a plane wave at incident angle θi, the direction of the reflected waves is calculated by
where ki is the wave vector of the incident waves. $\nabla {\phi _x}$ is the provided phase gradients along x-axes. Herein, the propagation direction of the reflected waves is opposite to that of the incident waves. Thus, $\nabla {\phi _x}$ is set to meet the conditionIn accordance with Eq. (6), a multi-channel retroreflector with mirrored symmetrically phase distribution is proposed. The operating frequency is 15 GHz, and the phase profile is fixed as “0-π-0”, which only needs two kinds of meta-atoms. Similarly, due to low interference between orthogonal polarizations, we next optimize the geometric parameters of the short metal wires along x-direction in layer I and III, where detailed dimensions are listed by Table 1.
Table 1. Four parameters values and corresponding codes of two meta-atoms
To investigate the reflection characteristics of the proposing three-channel retroreflector, we use finite intergation technique (FIT) in CST Microwave Studio with the same boundary conditions as before. Figure 6(a) illustrates the amplitude and phase response of the proposed meta-atoms at incident angles of ±55°, in which reflection amplitudes are above 0.95 and a phase difference of approximately 180° is obtained at 15 GHz. Noting that an arbitrary form of phase distribution can be theoretically acquired by meticulously tailoring meta-atomic size. The results above indicate the flexiblility of phase manipulation. Because of the shared apeture structure, this three-channel retroreflector maintains invariable dimensions as before. As shown in Fig. 6(b), the required phase distribution, in accordance with the aforementioned theoretical calculation, generating a sequance of “010101……” or mirror mode “101010……” along x-direction that can deflect main lobe with specified angle in xoz plane. The reflection amplitudes of CMC without and with the proposed coding meta-atoms varying against frequencies and incident angles are compared in Fig. 6(c) and (d), wherein a clear reflection enhancement can be observed. Under the incidence of TM-polarized waves at different angles, the average reflection amplitude is around 0.9, enhancing average reflection amplitude of CMC by 40%.
Fig. 6. High efficiently three-channel retroreflector. (a) Reflection amplitude and phase responses under illumination of TM-polarized waves for proposed two meta-atoms under incident angles of ±55°. (b) Phase distribution of the three-channel retroreflector. (c), (d) Simulated reflection amplitudes under different incident angles for a CMC dielectric without and with the designed metasurfaces.
The intergrated array models with open (add space) boundary conditions in all directions use the finite-different time-domain (FDTD) technique in CST Microwave Studio to analyze far-field characteristics. Figure 7 shows the simulated 3D scattering patterns and corresponding 2D RCS patterns for TM-polarized waves incidence, impining on the front side of the proposed retroreflector toward -z direction. It can be obeserved that the incident waves are precisely redirected to direction in which the waves come. A metallic sheet of the same size as that of the proposed retroreflector is uesd for comparison; the direction of the relfected waves follows the classic reflection law indicating spectular reflection. The 2D bistatic RCS patterns of the proposed retroreflector and the metallic plate on xoz-plane at 15 GHz are illustrated in Fig. 7. Three distinct reflected beams appear at θi = -55°, 55°, and 0° at the cutting plane of φ=0° for TM-polarized waves at incident angles of θi = -55°, 55°, and 0°, which is in good agreement with the theoretical prediction. In addition, compared with the metallic plate, the retroreflections of TM-polarized waves under incident angles of θi = ±55° are enhanced by more than 35dBsm at 15 GHz. The absolute efficiency of retroreflection can be calculated using the following formula [57]
where $|{{\zeta_{_{_{meta}}}}{\theta_r}} |$ and $|{{\zeta_{_{_{copper}}}}{\theta_r}} |$ represent the linear value of RCS of the metasurface and copper under θr when illuminating at θi, respectively. Therefore, the calculated retroreflection efficiencies are $\zeta _{ - 55}^{\textrm{meta}} = 10exp (\textrm{ - 0}\textrm{.2}/10)/10exp (\textrm{0}/10) = 98.0\%$, $\zeta _{55}^{\textrm{meta}} = 98.0\%$, and $\zeta _0^{\textrm{meta}} = 95.1\%$, which demonstrates high-efficiency three-channel retroreflection.Fig. 7. Performance of the proposed retroreflector for TE-polarized waves at 15 GHz. Simulated 3D scattering pattern of the conceived retroreflector and the identical metallic reference plate and corresponding comparison of 2D RCS patterns of the metasurface and the metallic plate cutting on xoz-plane at an incident angle of (a) θi = -55°, (b) θi = 55°, and (c) θi = 0°.
3. Experiment verification and discussion
To experimentally verify the strategy and demonstrate the performance of the meta-device, a metasurface sample of size 732mm×300 mm is fabricated by using the standard print circuit board (PCB). Precisely, the metallic structures are etched on two identical CMC substrates, and each substrate is 1.5 mm thick with a relative permittivity of 3.6. And then, the elaborate boards are aligned in accordance with the double-layered design of simulation with adhesives, which are next reinforced through hot press. For the sake of comparison, a dielectric plate of 3 mm thick is also fabricated utilizing the same CMC board. The photography of the sample and a zoom in view of the top meta-atoms are denoted in Fig. 8(a). The measurement procedure is divided into three parts and is carried out in a microwave anechoic chamber. During the transmission experiment, a pair of linearly-polarized horn antennas operating in Ku-band are separately used as the transmitter and the receiver of EM waves. As shown in Fig. 8(b), the prototype is fixed on the foam substrate, and placed in the center of the rotatable stage. The two antennas are connected to the two ports of Agilent E8363B network analyzer, and are used for measurement in the microwave anechoic chamber to avoid the unwanted reflections from the environment. By rotating the fixed prototype and switching horn antennas, the transmission coefficients are measured with the incident angle varying from 0° to 80°. Figure 8(c) and (d) depict the measured transmission amplitudes of CMC without and with the proposed metasurface under the illumination of TE-polarized waves. For the EM anti-reflector, with the assistance of transmission channel II and I, the transmission bandwidth and angular domain in Ku-band are dramatically extended when comparing with CMC board. More concretely, the average transmission amplitude at θi = 60° is enhanced by over 40% of that. The slight transmission deviations including frequency shift and amplitude discrepancy between the simulated and measured results are attributable to the approximate estimation in the experimental setup, uncertainties in the constitutive parameters, and finite size of the prototype. Overall, the measured results show good agreement with numerical simulation, which proves our design’s feasibility.
Fig. 8. Samples, transmission experimental setups and experimental results. (a) Photography of the fabricated meta-device with the inset showing the meta-atom. (b) Transmission experimental setups. (c), (d) Measured transmission amplitudes of CMC and the EM anti-reflector varying against θi in Ku-band under TE-polarized incidence.
Next, the reflection and far-field detection system presented in Fig. 9(a) and (b) are implemented to measure the reflection coefficients and performance of three-channel retroreflection. Two ports of the vector network analyzer connect to a pair of LP Ku-band horn antennas. When rotated by θi in xoz-plane with the sample fixed in front of them, the corresponding reflection amplitudes can be obtained. Figure 9(c) plots the measured results for the fabricated retroreflector, in which the average reflection amplitudes are more than 0.94 especially at 15 GHz though EM reflection deteriorates at lower incidence angles. In the process of retroreflection measurement, the receiver horn antenna is placed over 2 m away from experimental platform to keep the incident waves to be quasi-plane waves, whereas the transmitter horn antenna and fabricated sample are held on experimental turntable which can be rotated to obtain the results varying with the incident angles. When TM-polarized waves illuminate onto the samples at θi = -55°, 55°, and 0° at the cutting xoz-plane, three distinct peaks (black symbol line) emerge at θr=θi, demonstrating the considerable multi-channel retroreflection, and giving birth to a sharp enhancement of back scattering by 20dBsm compared with the metal plate (red symbol line) of the same size, as shown in Fig. 9(d)-(f). Despite of the slight disagreement between the measured and simulated results which might result from imperfect matching of measurement environment, the experiments show the effectiveness and stability of our proposed high efficiently three-channel retroreflector.
Fig. 9. Reflection and far-field experimental setups and experimental results. (a), (b) Reflection and far-field experimental setups. (c) Measured reflection amplitudes of the EM retroreflector varying against θi in Ku-band under TM-polarized incidence. (d-f) Measured bistatic RCS under different incident angles for TM-polarized waves at 15 GHz.
4. Conclusion
In summary, we have proposed a grating-assisted strategy of precisely controlling the transmitted and reflected wavefronts in full-space by changing the polarization states and incident angles. Double-layered anisotropic meta-atoms are elaborately arranged at the designated linear polarization state to compensate for amplitude and phase distributions. The broadband and wide-angle EM wavefronts generator of antireflections for TE waves and retroreflection for TM waves is fabricated as a proof-of-concept. The simulated and measured results are conducted in microwave regime that effectively validate the feasibility of method and manifestation of transmission-reflection independent manipulation.
We also envision that by extending the amplitude or phase responses of the meta-atom to more states and individually encoding each meta-atom with other functional sequences, this polarization-multiplexed full-space metasurface will provide a more general and flexible platform in multifunctionalities integration and engineering applications. Furthermore, changeability and extensibility of our scheme can be achieved via introducing novel coding sequences or utilizing mature and dynamic methods, such as PIN or varactor diodes, liquid crystals, electro-optic media, and micro-electromechanical system. For example, a reconfigurable full-space polarization-switchable device can be further developed through loading PIN diode into the embedded middle gratings for transmission/reflection conversion. Significantly, our strategy improves the utilization and integration of metasurfaces, and provides a degree of freedom for incident angles, which has important application value in the field of radar detections, information communications, and optical imaging.
Funding
National Natural Science Foundation of China (61971435, 52272101); National Key Research and Development Program of China (SQ2017YFA0700201); Graduate Scientific Research Foundation of Department of Basic Sciences.
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.
Supplemental document
See Supplement 1 for supporting content.
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