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Abstract
Metasurfaces have provided unprecedented degrees of freedom in manipulating electromagnetic (EM) waves and also granted high possibility of integrating multiple functions into one single meta-device. In this paper, we propose to incorporate the retroreflection function with transmission function by means of metasurface design and then demonstrate a dual-polarization multi-angle retroreflective metasurface (DMRM) with bilateral transmission bands. To achieve high-efficiency retroreflections, the compact bend structures (CBSs), which exhibit high reflections around 10.0 GHz in X band, are added onto the substrate of the DMRM. Two selected metasurface elements are periodically arranged so as to form 0-π-0 phase profile. By delicately adjusting the periodicity, high-efficiency retroreflections can be produced for both TE and TM-polarized waves under both vertical incidence and oblique incident angles ±50.0°, with an average efficiency of 90.2% at the designed frequency. Meanwhile, the two metasurface elements exhibit high transmission properties and minor phase disparities in S, C and Ku bands, resulting in bilateral transmission windows. Prototypes were designed and fabricated. Both simulated and measured results verified our design. This work provides an effective means of integrating retroreflection functions with other functions and may find applications in target tracking, radomes and other sensor integrated devices in higher frequency or even optical frequency bands.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Retroreflections is to eflect the received EM waves back along the source direction as much as possible, causing enhanced backscattering RCS obviously [1]. The application of retroreflections [2–4] has the ability to make the small RCS target exhibits a large shape in the detection game process of the other party, resulting in confusions and lures. Therefore, it can be regarded as another form of stealth technology [5–8], having attracted considerable interest with regard to enhancing survivability. Traditional retroreflectors, including cat's eye [9,10] and corner reflectors [11–13], are not amicable to be taken advantage of in aircrafts due to relatively bulky volume and heavy mass, limiting the application of retroreflectors in stealth technology. Therefore, lightweight, planarization and even conformability or reconfigurable are the practical requirements in application scenarios for innovative retroreflective devices.
Since its birth, metamaterials have intrigued extensive attention of many scholars and researchers due to their versatile control abilities for EM waves, and metasurface, acting as its two-dimensional (2D) form, has unique advantages of lightness and planarization, greatly promoting the speed and quality of their applications into practices [14–19]. With the profound development on metasurfaces, it has onrushing researches and applications in the fields of abnormal reflectors [20–22], polarization converters [23–26], meta-lenses [27–29], holographic imagings [30–33] and so on. In terms of backscattering enhancement, the metasurface also presents its extraordinary control ability for incident EM waves. In the early stages of the study, D. Lipuma et al. [34] proposed a passive structure to reduce the profile of the dihedral corner reflector whose opening angle was greater than 90.0°, realizing a similar performance to that of the traditional dihedral corner reflector in the small incident angle range at 9.0 GHz. To achieve complete planarization, we utilized the phase gradient metasurface in the previous stage to obtain retroreflections under a single incident angle in the whole X-band, with the sacrifice of working efficiency [35]. With the intensive study of metasurface design, V. S. Asadchy et al. [36] revealed the possibility of realizing retroreflections in a number of propagation directions simultaneously on the basis of planarization, accompanying with high backscattering enhancement efficiency. What is more, to realize retroreflections under the incident angular domain as continuous as possible, Amir Arbabi et al. [37] adopted cascaded two metasurfaces to achieve spatial Fourier transform and a spatially varying momentum respectively. Shaohua Dong et al. [38] proposed and achieved an asymmetric transmission loss-assisted metasurface based on the exceptional point theory and combined with the high-order diffractions for source waves, providing more abundant design frameworks for follow-up researchers. These excellent works mentioned above only consider the retro-reflection design for EM waves located in the upper half space, ignoring transmission performances in other frequency bands. With the development of retroreflective devices and the traction of more practical applications, in addition to effectively realizing retroreflections in the specific frequency bands, combing high transmission outside the band will greatly promote integration and miniaturization in retroreflectors.
Inspired by this, copious versatilities should be integrated into ordinary retroreflectors, e.g., the transmission windows could be unfolded into the lower and higher frequency bands respectively of the operational band where retroreflections materialized by metasurface, greatly enriching the functionality and adaptability of retroreflective devices. In this paper, we design and propose a dual-polarization multi-angle retroreflective metasurface (DMRM) with efficient retroreflections in X band accompany by two transmission windows in S and C and Ku bands, respectively. Specifically, multi-angle retroreflections including vertical incidence and oblique incident angle ±50.0° with average 90.2% efficiency at the optimal frequency point of 10.3 GHz in X band, also called high reflection coefficient band, are customized by binary coding the two structural elements constituting the metasurface to form a 0-π-0 phase profile. Thanks to the independent and flexible control ability for transverse electric (TE) and transverse magnetic (TM) polarized waves simultaneously of the choreographed compact bend structure (CBS) acting as the element of metasurface, the retroreflections are effective for both TE and TM waves, maintaining a high degree of consistency. To enhance the confusion and inducement of the device to opposite radars, the transmission windows are unfolded in the lower and higher frequency bands of the operational band to reduce the specular reflections of the proposed device and keep it in a low detectability state, so as to present a comprehensive effect of low detectability in the two transmission bands while high detectability in the intermediate band as a whole, just as denoted in Fig. 1 where beams with different colors from tangerine to purple represent frequencies from low to high. The fruition of transmission window requires the CBS to have efficient transmission coefficient in S, C and Ku bands, and meanwhile the transmission phases of the elements in these frequency bands should be tailored as close as possible. As an example, X band is selected as the intermediate frequency band, S and C bands as the lower frequency band, Ku band as the higher frequency band, and 10.0 GHz as the central frequency point of the retroreflection performance. Theoretical analysis, simulation, fabrication and measurement are carried out, both simulated and measured results verifying the feasibility of the design idea. This design idea provides a feasible framework for the application of retroreflective devices in the field of integration and incorporation, and also has potential values during the design of sensor devices in higher frequency or even optics.
Fig. 1. Schematic illustration of the DMRM: In the proposed intermediate frequency band with high reflection coefficient, there exist efficient retroreflections under the incident angle of ±50.0° and vertical incident, as denoted by green arrows in the green beam. In the low and high frequency regions at both ends of the intermediate band, as denoted by tangerine and purple arrows and their beams, there are transmission windows, which can reduce the specular reflection of the DMRM.
2. Results and discussion
2.1 Underlying mechanism
For a typical reflective metasurface, the wave-vector conservation law along the direction of the metasurface between the free-space wave-vector k0, incident angle θi and reflected angle θr can be obtained according to the generalized Snell’s law.
where kxi = k0·sinθi, kxr = k0·sinθr, ζ=2π/P are incident, reflective wave-vector components and the phase gradient parallel to the metasurface, and integer m = 0, ±1, ±2…… represents the diffraction orders. Parameter P is the period of the supercell constituting the metasurface, which is equal to the sum of the periods of the structural element, i.e., P = n×p, where n and p represent the number and period of the element respectively. To avoid confusion, the x axis is defined as the positive direction in this paper, that is, when the EM waves are incident from the left side, its incident angle is negative, and vice versa. To achieve the same manipulation abilities for the left and right EM waves, it is necessary to ensure that the phase gradient provided by the metasurface is symmetrical about the y-o-z plane, that is, the supercell of the metasurface is composed of two structural elements to form a 0-π-0 phase profile, leading to n = 2 and m=±1. When the EM waves are incident on the metasurface from the left to form the retroreflection, that is θr=-θi, meanwhile condition |k0·sinθr|≤k0 allows only a propagating m=+1 diffraction order. Vice versa, m=−1 corresponds to the case of EM waves from right side. In this case, according to Eq. (1), the corresponding incident angle is θc = 32.15° under the critical condition that the abnormal reflection angle θr = 90.0°. Note that the scattered waves also obey the wave-vector relation as Eq. (1) when the incident angle θi is smaller than the critical angle θc = 32.15°, and the condition |k0·sinθr|≤k0 allows only a propagating 0th diffraction order, i.e., m = 0. Thus, there only exists specular reflection when the incident angle below the critical angle. Therefore, from the point of view of the relationship between periodic constant P and diffraction order m, it can be concluded that when the incident angle θi is in the range from 32.15° to 90.0°, the metasurface can achieve −1st- order diffraction efficiently, and when the incident angle is in the range from 0.0° to 32.15°, only the 0th-order diffraction exists.To construct the designed metasurface, the CBS proposed in this paper is a typical sandwich structure, it is composed of two ultra-thin F4B dielectric substrate with permittivity εr = 2.65(1 + 0.001j) layers with compact bend metal curves as denoted in Fig. 2. The air layer with thickness th2 is interspersed between these two F4B layers, forming a substrate-air-substrate laminated model, and here we define it as layer1-layer2-layer3. The compact bend curve whose specific contours are denoted in Fig. 2(a)ii and (b) is evolved on the basis of the metal short-line dipole model [39], with a common underlying mechanism where strong resonances emerge along the direction parallel to the incident electric field, and the compact bend curve has higher spatial utilization. Whether it is a compact bend curve or a short line, it can be equivalent to a lumped circuit model of inductance L and capacitance C which represent the associated inductance and capacitance between adjacent resonant elements respectively based on the equivalent circuit theory [40], denoted as iii panel in Fig. 2(a). The impedance of the metasurface formed by the periodic arrangement of structural elements can be obtained from the following equation, in which the dielectric substrate acts as the transmission line.
when the equivalent impedance Z reaches the minimum, it is corresponding to the resonant frequency ω0=$1/\sqrt {LC}$ of the equivalent circuit. Obviously, compared with the short line, the equivalent inductance Lcbc and capacitance Ccbc of the compact bend curve are improved, causing the red shift of resonant frequency from ωsl to ωcpc shown in iv panel in Fig. 2(a). The equivalent model is simple enough and it is also necessary to comprehensively optimize the reflection coefficient, transmission coefficient and phase of structural elements to achieve the designed metasurface utilizing the CST Microwave Studio full-wave analysis method, which will be further described in detail later instead of the equivalent circuit used for specific numerical analysis. Thus, we only use the equivalent circuit model for qualitative analyses here. The single-layer structural element is obtained by rotating the compact bend curves 90.0° around its geometric center, having excellent independent manipulation characteristics for mutually orthogonal TE and TM waves owing to the fact that the two bend curves are orthogonal to each other as denoted in Fig. 2(b) and (c). To verify the design concept, the proposed structural element should have such EM characteristics, that is, it should have efficient reflection coefficient near the central frequency point 10.0 GHz, and have efficient transmission effect in its lower and higher frequency bands. Under the condition just mentioned, the structural element constituting the proposed metasurface should have tailored phase characteristics, that is, the reflection phase difference should meet the binary distribution to form the 0-π-0 phase profile in the reflection frequency domain, and the transmission phase difference should be as close as possible in the transmission frequency bands to hold a circumstance in which the energy of the transmitted waves are not weakened as much as possible with unchanged direction simultaneously. This is the basic requirement of magnitude and phase for the metasurface structure element proposed by the design concept. Two structural elements, sub1 and sub2, are finally adopted to form the supercell based on careful and rigorous optimization design, which are then periodically arranged to form the DMRM. The geometric parameters and specific dimensions of the two elements are denoted in Fig. 2(b), (c) and Table 1 respectively.Fig. 2. Structural elements constituting the DMRM: (a) The evolution of ii compact bend curve from i short line and their iii equivalent circuit and iv redshift spectrum. The schematic illustrations of (b) sub1 and (c) sub2 and their geometric parameters of each layer are denoted in i ii and iii panels respectively. (d) Simulated results of structural elements: The i & ii reflection and iii & iv transmission coefficient and phase of two elements under i & iii TE and ii & iv TM waves.
Table 1. The specific dimensions of each layer consisting of the two structural elements.
When TE and TM waves incident at an oblique angle of −50.0° from the left side, the reflection coefficient and phase, as well as the transmission coefficient and phase of the two structural elements are analyzed utilizing the frequency-domain solver in CST, and the simulated results are shown in Fig. 2(d). It can be seen from the simulated results that the two structural elements have high reflection coefficients at 10.0 GHz with phase differences 174.4° and 197.2° under TE and TM waves respectively, and both have high transmission coefficient, maintaining low transmission phase difference in the ranges from 2.0 GHz to 6.2 GHz in S and C bands and 13.3 GHz to 14.9 GHz in Ku band under the incident of TE polarized waves and ranges from 2.0 GHz to 9.0 GHz in S and C bands and 12.2 GHz to 17.6 GHz in Ku band under the incident of TM polarized waves. From the analyses based on the simulated data, the following conclusion can be drawn that the magnitudes and phase differences of the two elements for reflected and transmitted waves meet the basic requirements raised by the theoretical framework, and then the two structural elements are arranged periodically to obtain the DMRM with a 0-π-0 phase profile in the reflection frequency domain.
2.2 Realization of DMRM
So far, the theoretical analyses and ingenious structural element of the DMRM with bilateral transmission windows have been completed, and the basic design prerequisites have been met. Herein the metasurface composed of two basic element is proposed to verify the design concept, with the specific size 300.0mm×300.0 mm. The simulated exploration mainly includes three aspects, that is, investigating the EM regimentation abilities of the metasurface around the central frequency point 10.0 GHz, the transmission windows in the low frequency S and C bands and high frequency Ku band under the illumination of two orthogonal linear polarized waves. A metal plate with the same area (SAMP) is selected for overall comparison to more intuitively estimate the fantastic manipulation abilities of the metasurface for EM waves, utilizing the time-domain solver in CST.
Firstly, the EM manipulation performances of the DMRM under incident angle θi=−50.0° around 10.0 GHz are investigated. Simulated results of the four selected frequency points under TE waves in Fig. 3 indicate that compared with the SAMP, the metasurface has obvious manipulation abilities for diffracted waves whose main lobe directions are along the source, forming backscattering enhancements. The abnormal reflection angles corresponding to 9.4 GHz, 10.0 GHz, 10.3 GHz and 10.9 GHz are −59.0°, −50.0°, −46.0° and −40.0° with bistatic RCS 13.4dBm2, 16.1dBm2, 16.6dBm2 and 15.4dBm2 respectively, which are highly consistent with the theoretical directions −59.8°, −50.0°, −46.2° and −39.8° calculated according to the Eq. (1). Compared with the SAMP with bistatic RCS −15.9dBm2, −13.8dBm2, −12.0dBm2 and −12.1dBm2, DMRM has obvious backscattering enhancement effect and the enhanced values are 29.3 dB, 29.9 dB, 28.6 dB and 27.5 dB respectively. In addition to the reflection information, the near field states of the transmitted waves denoted as iv and v panels in Fig. 3(a), (b), (c) and (d) reveal that there exists almost no transmitted energy under the action of TE waves around 10.0 GHz and most of the incident waves are reflected by the DMRM and their abnormal reflection directions are highly consistent with the source direction, compared with the metal plate. To further consider the abnormal reflection efficiency of the DMRM, the efficiency calculation is carried out according to the Eq. (3) [41].
where |${\zeta _{\textrm{DMRM}}}{\theta _r}$| and |${\zeta _{\textrm{SAMP}}}{\theta _r}$| represent the linear RCS of the DMRM and SAMP respectively corresponding to the main-lobe diffraction angle, under the same incident state. For example, retroreflection efficiency at 10.0 GHz from simulated results in Fig. 3(b) is $\zeta _{10.0}^{ - 50.0} = 10\exp (16.1/10)/10\exp (16.9/10) = 83.2\%$. The other three retroreflection efficiencies corresponding to the four frequency points of 9.4 GHz, 10.3 GHz and 10.9 GHz are 51.3%, 89.1% and 61.7% respectively. Thus, it can be generalized that the best working frequency point of the DMRM under the TE polarized waves is 10.3 GHz. The DMRM also has excellent backscattering enhancement performances around 10.0 GHz under TM polarized wavs, which is proved by the simulated results in Fig. 4. From the far-field simulated results, it can be seen that the abnormal reflection angles of the DMRM under TM polarization are −52.0°, −49.0°, −45.0° and −38.0° respectively, corresponding to four frequency points 9.8 GHz, 10.0 GHz, 10.3 GHz and 11.0 GHz, which are highly consistent with the theoretical abnormal reflection angles −52.9°, −50.0°, −46.7° and −38.8°. Compared with the SAMP, the backscattering enhancement values of the DMRM are 29.4 dB, 29.2 dB, 32.1 dB and 26.6 dB, respectively, with the efficiencies of 34.7%, 56.2%, 91.2% and 58.9%. Similarly, the optimal operating frequency of the DMRM under the TM waves is also 10.3 GHz.Fig. 3. Simulated results of DMRM under TE polarized waves with incident angle −50.0° around 10.0 GHz. The information illustrated by i to v panels is the same at (a) 9.4 GHz, (b) 10.0 GHz, (c) 10.3 GHz and (d) 10.9 GHz. The i far-field curves of the DMRM and the SAMP. The three-dimensional (3D) far-field diagrams of the ii DMRM and iii SAMP. The near-field Ey distributions of the iv DMRM and v SAMP.
Fig. 4. Simulated results of DMRM under TM polarized waves with incident angle −50.0° around 10.0 GHz. The information illustrated by i to v panels is the same at (a) 9.8 GHz, (b) 10.0 GHz, (c) 10.3 GHz and (d) 11.0 GHz. The i far-field curves of the DMRM and the SAMP. The 3D far-field diagrams of the ii DMRM and iii SAMP. The near-field Ex distributions of the iv DMRM and v SAMP.
The 0-π-0 phase profile formed by sub1 and sub2 around 10.0 GHz has symmetry, that is, the EM waves incident from the right and left side should have the same performance. The simulated results in Fig. 5(a) and (b) verify this point of view. When EM waves incident from the right side with an oblique incident angle 50.0° at 10.3 GHz, both TE and TM polarized waves have efficient retroreflections where the backscattering enhancement of the metasurface is 28.8 dB and 31.9 dB with 93.3% and 87.1% efficiencies respectively. The phase profile fails under the condition that the metasurface is perpendicular to the incident EM waves, resulting in the specular reflection for the incident waves, which forms a state of enhanced backscattering as a whole. It can be seen from the simulated results in Fig. 5(c) and (d) that the scattering patterns of the metasurface under the vertical incidence whether TE or TM waves are almost consistent with that of the SAMP, with the efficiency of 85.1% and 78.4%.
Fig. 5. Simulated results of DMRM under (a) & (c) TE and (b) & (d) TM polarized waves with incident angle (a) & (b) 50.0° and (c) & (d) vertical incidence at 10.3 GHz. The information illustrated by i to v panels is the same: The i far-field curves of the DMRM and the SAMP. The 3D far-field diagrams of the ii DMRM and iii SAMP. The near-field (a) & (c) Ey or (b) & (d) Ex distributions of the iv DMRM and v SAMP.
The wave-transmitting performances of the DMRM in bilateral frequency bands are further explored based on analyses of the reflection characteristics in the intermediate frequency band. Firstly, the EM characteristics of the metasurface in the lower frequency S and C bands are analyzed, and the simulated results under the TE and TM polarizations denoted in Fig. 6 and Fig. 7 respectively. It can be seen from the far-field information of i, ii, and iii panels in Fig. 6 and 7(a), (b), (c) and (d) that the beam energy of the specular reflected waves on the DMRM are significantly weaker than that of the SAMP, whether for TE or TM polarized waves. The far-field curves in i panels in Fig. 6 and 7 exhibit that the bistatic RCS of the DMRM is completely lower than that of the SAMP in the selected four frequency points. In addition, it can be seen from the iv, and iv panels in Fig. 6 and 7(a), (b), (c) and (d) that the transmission directions of the transmitted waves through the DMRM have hardly changed its intended transmission directions. Secondly, the EM characteristics of the DMRM in Ku band are analyzed. The simulation information of the metasurface under TE and TM states are shown in Fig. 8 and Fig. 9 respectively. Similarly, it can be analyzed from its far-field data that the specular reflection energy of the DMRM is significantly lower than that of the SAMP. The near-field results confirm that the transmission directions of its transmitted waves have hardly changed. Similar to the conclusion of high reflection band, the EM incident waves on the right side of the metasurface has the same scattering patterns as incident on the left side in theory, which is determined by the composition of two structural elements, so it will not be repeated here.
Fig. 6. Simulated results of DMRM under TE polarized waves with incident angle −50.0° in S and C bands. The information illustrated by i to v panels is the same at (a) 2.0 GHz, (b) 3.4 GHz, (c) 4.8 GHz and (d) 6.2 GHz. The i far-field curves of the DMRM and the SAMP. The 3D far-field diagrams of the ii DMRM and iii SAMP. The near-field Ey distributions of the iv DMRM and v SAMP.
Fig. 7. Simulated results of DMRM under TM polarized waves with incident angle −50.0° in S and C bands. The information illustrated by i to v panels is the same at (a) 2.0 GHz, (b) 4.3 GHz, (c) 6.6 GHz and (d) 8.9 GHz. The i far-field curves of the DMRM and the SAMP. The 3D far-field diagrams of the ii DMRM and iii SAMP. The near-field Ex distributions of the iv DMRM and v SAMP.
Fig. 8. Simulated results of DMRM under TE polarized waves with incident angle −50.0° in Ku band. The information illustrated by i to v panels is the same at (a) 13.3 GHz, (b) 13.8 GHz, (c) 14.3 GHz and (d) 14.9 GHz. The i far-field curves of the DMRM and the SAMP. The 3D far-field diagrams of the ii DMRM and iii SAMP. The near-field Ey distributions of the iv DMRM and v SAMP.
Fig. 9. Simulated results of DMRM under TM polarized waves with incident angle −50.0° in Ku band. The information illustrated by i to v panels is the same at (a) 12.2 GHz, (b) 14.0 GHz, (c) 15.8 GHz and (d) 17.6 GHz. The i far-field curves of the DMRM and the SAMP. The 3D far-field diagrams of the ii DMRM and iii SAMP. The near-field Ex distributions of the iv DMRM and v SAMP.
However, it is worth noting that, different from the performance of the DMRM in the low frequency band, the reflected EM waves in the four frequency points selected in Ku band reveal the condition that the specular reflection energy of the metasurface is indeed lower than that of the SAMP, while the bistatic RCS in other directions is not always lower than that of the metal plate. The reason for this condition can be explained according to the Eq. (1). For a designed metasurface, it is obvious that the period of its structural element has also been fixed. Then, with the increase of frequency and the decrease of wavelength, the structural element will become relatively large in the view of high-frequency EM waves. Then the energy based on grating diffraction, that is, the grating lobes termed in the microwave field, will be produced, which reasonably explains the reason for this condition. As for how to reduce and eliminate the energy of grating lobes, we can appropriately bring into a certain loss [38] in the metasurface and consume the energy through high-order diffractions with multi-times, which will be deeply explored in the future research process. Here, the specular reflection energy of the DMRM in Ku band is lower than that of the SAMP, explaining and verifying the design concept.
3. Experiment
To further verify the accuracy of design concept and simulated results, as an example, the samples are fabricated by printed circuit board (PCB) technology, and the far-field data are measured in the anechoic chamber. The size of the sample is 500.0mm×500.0 mm with the geometric dimensions of each structural element and the thickness of the F4B substrates are consistent with the values determined during the design process. The PMI foam with a thickness of th2 = 8.0 mm is sandwiched into the two metasurface layers, mimicking the air layer, and these three layers as a whole form the DMRM. The bistatic reflection coefficients of the DMRM in X band and transmission coefficients in S, C and Ku bands are measured in the microwave anechoic chamber where the sample and the transmitting antenna (TA) are fixed on a rotatable experimental platform, and the relative angle between the sample and TA determines the incident angle and polarization state. The receiving antenna (RA) is placed outside the rotating platform, and far-fields of the bistatic reflection coefficients of the DMRM can be measured by rotating the platform, as shown in Fig. 10(a)-(d) and Fig. 11(a)-(b).
Fig. 10. Measurement verification: Measured reflection results of the DMRM under a & c TE and b & d TM waves with incident angle a & b −50.0° and c & d 50.0° at i 9.4 GHz, ii 10.0 GHz, iii 10.3 GHz and iv 10.9 GHz in subgraph a & c and i 9.8 GHz, ii 10.0 GHz, iii 10.3 GHz and iv 11.0 GHz in subgraph b & d.
Fig. 11. Measurement verification: Measured transmission results of the DMRM under a TE and b TM waves with incident angle −50.0°at i 4.8 GHz, ii 6.2 GHz, iii 13.3 GHz and iv 14.3 GHz in subgraph a and i 4.3 GHz, ii 6.2 GHz, iii 12.2 GHz and iv 14.0 GHz in subgraph b. e The i-iii photos of the fabricated sample and iv experimental platform in anechoic chamber.
Measured results normalized by the SAMP under TE and TM waves in X band are obtained utilizing the experiment method just mentioned, and four frequency points are taken for each polarization for summary as denoted in Fig. 10(a)-(d). It can be seen from the measured results in Fig. 10(a)-(d) that the DMRM sample has significantly different EM manipulation abilities from the SAMP for the reflected waves under the two orthogonal linear polarized oblique incident waves with angle −50.0° and 50.0° denoted as Fig. 10(a) and (b) and (c) and (d) respectively around the designed frequency 10.0 GHz, and the main lobe directions of the abnormal reflected waves are biased towards the direction of the source waves. Specifically, the abnormal reflection angles of the DMRM under TE waves at 9.4 GHz, 10.0 GHz, 10.3 GHz and 10.9 GHz with incident angle −50.0° and 50.0° are −60.0°, −51.0°, −47.0°, −41.0° denoted as Fig. 10(a) and 59.0°, 50.0°, 46.0°, 42.0° denoted as Fig. 10(c) respectively, which are highly consistent with the theoretical abnormal reflection angles of -(+)59.8°, -(+)50.0°, -(+)46.2° and -(+)38.8° calculated according to the Eq. (1). Compared with the SAMP, the backscattering enhancement values under incident angle −50.0° and 50.0° are 27.8 dB, 32.1 dB, 32.5 dB, 24.3 dB and 33.3 dB, 35.5 dB, 32.9 dB, 24.3 dB respectively, from where can be seen that the optimal working frequency point of the DMRM under TE waves is 10.0 GHz.
It can be seen from the measured results illuminated by TM waves in Fig. 10(b) and (d) that the abnormal reflection angles corresponding to 9.8 GHz, 10.0 GHz, 10.3 GHz and 11.0 GHz of the DMRM under incident angle −50.0° are −53.0°, −50.0°, −47.0°, −39.0° and incident angle 50.0° are 52.0°, 49.0°, 46.0°, 39.0° respectively with accompanying backscattering enhancement values 23.2 dB, 29.1 dB, 26.8 dB, 22.2 dB and 23.9 dB, 27.8 dB, 23.4 dB, 22.4 dB, being highly consistent with the abnormal reflection angles -(+)52.9°, -(+)50.0°, -(+)46.2° and -(+)39.8° calculated theoretically. Measured results reveal that under two orthogonal linear polarized waves, the optimal working frequency of the DMRM is 10.0 GHz, which is highly consistent with the theoretical concept and simulated results, further proving the feasibility of the proposed idea. It can be seen from the measured results that when EM waves are incident on the metasurface from the left and right sides the diffraction patterns are not completely symmetrical, which is not totally consistent with the theoretical analyses, and meanwhile there still exist slight differences between the measured and simulated results. The reason for this phenomenon results from sample errors, permittivity tolerance of F4B substrates and testing errors.
To further verify the bilateral transmission windows of the fabricated sample, the transmission performances in S, C and Ku bands are analyzed under the illumination of TE and TM waves denoted as Fig. 11(a) and (b) respectively. The measured results are also carried out in the microwave anechoic chamber, taking the position where the TA and RA are completely relative as the angle zero point, and the relative position between the sample and the TA determines the incident angle. The sample is measured and the real gain of the horn is taken as the normalized comparison. From the measured results under TE and TM polarization in Fig. 11, it can be seen that the metasurface has well transmission windows in S, C and Ku band, being highly consistent with the theoretical analysis and simulated results.
4. Conclusion
Artificially woven structure elements form different scattering characteristics at high reflection frequencies and transmission bands utilizing the dispersion characteristics of the metasurface appositely, so as to improve the survivability in the radar detection process of the other side. Based on this design idea, we propose the DMRM with bilateral transmission windows, which has well expected performances. The DMRM is composed of two well-designed customized structural elements which have high reflectivity around the design frequency point 10.0 GHz, forming the 0-π-0 phase profile, so as to realize the symmetrical characteristic that the EM waves incident from the left and right sides of the metasurface has the same functional characteristics. An example is designed and fabricated, and simulated and measured results exhibit the backscattering enhancement of the DMRM reachs an average of 30.4 dB at the optimal operating frequency 10.3 GHz relative to the SAMP under the TE and TM waves with an average retroreflection efficiency 90.2%. Meanwhile, the feasibility of forming bilateral transmission windows in the transmission bands is revealed, through the numerical simulation analyses of four frequency points in the lower frequency S and C bands and higher frequency Ku band. The theoretical framework of reflection in the intermediate band and transmission in the bilateral bands proposed in this paper still has more potential for expansion. For example, the loss absorption or scattering cancellations of the transmitted waves utilized cascaded metasurfaces can maintain low detectability continuously in the ultra-wide wave-transmitting band, thus forming a metasurface device with absorption outside the working band and achieving designed functions within the working band. Therefore, the design concept proposed in this paper has potential application value and can be expanded in integrated and miniaturized devices in high frequency or even optics.
Funding
National Natural Science Foundation of China (51802349, 61801509, 61901508, 61971435, 62101588, 62101589); National Key Research and Development Program of China (2017YFA0700201); Research Fund for Young Star of Science and Technology in Shaanxi Province (2021JQ-363); Air Force Engineering University (KGD080920016).
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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