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Abstract
Metasurfaces possess excellent capabilities to flexibly manipulate electromagnetic waves in multiple frequency domains, which show great potential application in multispectral stealth. Herein, a broadband surface waves coupler based on the design of thin Pancharatnam–Berry (PB) phase gradient metasurfaces (PGMs) of thickness 0.12λ0 is proposed to reduce infrared emission and microwave reflection simultaneously. Low infrared emission results from the high filling ratio of the indium-tin-oxide (ITO) on the surface, and low microwave reflection results from the conversion from propagating waves to surface waves. Intriguingly, this design is also capable of acting as a simple circular polarized (CP) discriminator because orthogonal CP waves are coupled into surface waves propagating along opposite directions. A proof-of-concept prototype is simulated and measured to validate the effectiveness of our methodology. The results indicate that the broadband surface waves coupler shows low infrared emissivity less than 0.28 from 3 to 14 µm and has microwave reflection reduction larger than 10 dB in 7.3-9.5 GHz. The exceptional performances of the proposed broadband surface waves coupler make us believe that our design offers an alternative strategy for multispectral stealth and multifunctional application.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Multispectral compatible stealth has become a hot research issue recently in response to the rapid development of detection technologies in infrared, visible light, microwave and other domains [1–4]. In particular, radar-infrared bi-stealth technology is one of the most valuable studies [5–6]. Actually, low electromagnetic reflection can make the targets realize radar stealth, and low infrared emission is desired for infrared stealth, which means high reflection based on Kirchhoff’s law [7–8]. Obviously, it is difficult to achieve infrared and radar stealth concurrently because of the contrary implementation requirements. Some efforts have been devoted to settle this problem, such as the designs of microwave absorbing materials with low infrared emissivity coating [9], composite photonic crystal structures [10] and so on [11–13], but aforementioned designs still have limited microwave absorption bandwidth or relatively high infrared emissivity.
Metasurfaces, two-dimensional arrangements of artificial subwavelength meta-atoms, have attracted enormous interest and shown practical applications owing to their strong capabilities to control electromagnetic waves’ amplitude, phase and polarization state [14–18]. In recent years, a series of metasurfaces are proposed to achieve radar-infrared bi-stealth, which can effectively reduce infrared emission and microwave reflection at the same time, but these designs suffer from some inherent defects [7–8,19–23]. For example, multilayered absorbing metasurfaces usually have the disadvantages of large thickness, complex fabrication and causing temperature rising for the stealth target [7,19–22], and coding metasurfaces scatter electromagnetic waves into the incident half-space, which is not conducive to radar stealth in practice [8,23]. On the other hand, phase gradient metasurfaces (PGMs) gradually replace traditional gratings and prisms couplers to efficiently couple propagating waves into surface waves [24–27], and Pancharatnam–Berry (PB) PGMs can couple circular polarized (CP) waves into hybrid modes surface waves within a broad bandwidth utilizing the intrinsic dispersionless characteristic of PB phase [28–29]. Therefore, if an exotic PB PGM can couple incident waves into surface waves and has low infrared emissivity synchronously, it will achieve low infrared emission and microwave reflection at relatively broad bandwidth, and resolve some deficiencies of existing schemes. This innovative idea inspires us to design the novel radar-infrared bi-stealth metasurface.
In this work, a fire-new surface waves coupler is proposed to reduce infrared emission and microwave reflection simultaneously, as illustrated in Fig. 1. Low microwave reflection is attributed to the efficient conversion of incident waves into surface waves, and low infrared emission is derived from the reflection of indium-tin-oxide (ITO) thin film on the surface of surface waves coupler. Both simulations and experiments verify that the infrared emissivity of our design is about 0.28 from 3 to 14 µm and the microwave reflection reduction is larger than 10 dB in 7.3-9.5 GHz. Also, this radar-infrared bi-stealth surface waves coupler can not only be validated for arbitrary linear polarized (LP) waves, but also couple left-handed circular polarized (LCP) waves and right-handed circular polarized (RCP) waves into surface waves propagating along opposite directions in the broad bandwidth, which can be considered as a simple CP discriminator.
Fig. 1. Schematic view of the proposed broadband surface waves coupler with low infrared emission and microwave reflection.
2. Structure design and analysis
The meta-atom of the proposed PB PGM is composed of infrared shielding layer (ISL) and surface waves coupling layer (SWCL), and the two layers are separated by FR4 dielectric with a relative permittivity of 4.3(1-j0.025), as shown in Fig. 2(a). The ISL consist of ITO square patches with the sheet resistance of 6 Ω/sq etched on the polyethylene terephthalate (PET). The relative permittivity of PET is 3.0(1-j0.06). Theoretically, the infrared emissivity $\varepsilon $ of the whole meta-atom depends on the filling ratio of ITO square patches in the ISL, and corresponding calculation formula can be written as follow [6,30]
where ${\varepsilon _I}$ and ${\varepsilon _P}$ are the emissivities of ITO and PET respectively, and ${f_I}$ and ${f_P}$=1-${f_I}$ are the filling ratio of ITO square patches and PET in the surface of each meta-atom’s ISL. According to the design dimensions, the filling ratio of ITO square patches is ${f_I}$=81% and that of PET is ${f_P}$=19%. Moreover, the emissivities of ITO and PET are about 0.1 and 0.8 respectively. Consequently, the calculated infrared emissivity of this PB PGM is around 0.23. With the assistance of the commercial software CST MICROWAVE STUDIO, we find the ISL with low infrared emission can also efficiently transmit LP and CP waves into corresponding co-polarized waves in 6-11 GHz, which is helpful for us to utilize the phase gradient of SWCL to couple incident waves into surface waves in microwave band, as depicted in Fig. 2(b) (${t_{yy}}$ and ${t_{xx}}$ represent co-polarized transmission amplitudes under y-polarized and x-polarized incidence respectively, and ${t_{LL}}$ and ${t_{RR}}$ represent co-polarized transmission amplitudes under LCP and RCP incidence respectively).Fig. 2. Designs and simulations of meta-atoms. (a) The schematic diagram of meta-atom composed of ISL and SWCL, where m=0.9 mm, n=1 mm, p=6 mm, d1=0.175 mm, d2=1 mm, d3=3 mm, l1=5.8 mm, l2=4 mm, l3=1 mm, w=0.2 mm. (b) Simulated co-polarized transmission amplitudes of meta-atom’s ISL under LP and CP incidence. (c) Simulated co-polarized reflection amplitudes and phases of meta-atom under LP incidence and co-polarized reflection amplitudes under CP incidence.
The SWCL consists of the metallic N-shaped structure etched on the F4B substrate with a relative permittivity of 2.65(1-j0.001), and the grounded metal is added at the bottom to block all transmission. The thickness of metallic N-shaped structure and grounded metal is 0.017 mm. We employ the Jones matrix to analyze the reflection amplitudes and phases of the whole meta-atom under LP and CP illumination [31–33]. Because the cross-polarized reflection amplitudes under LP incident waves approach zero, the Jones matrix can be written as $R_0^{LP} = \left( {\begin{array}{cc} {{R_{xx}}}&0\\ 0&{{R_{yy}}} \end{array}} \right)$ based on LP basis. Here, ${R_{xx}} = {r_{xx}}{e^{i{\varphi _{xx}}}}$ and ${R_{yy}} = {r_{yy}}{e^{i{\varphi _{yy}}}}$ denote x-polarized and y-polarized incident waves’ co-polarized reflection coefficients respectively, where ${r_{xx}}$ and ${r_{yy}}$ represent corresponding co-polarized reflection amplitudes, and ${\varphi _{xx}}$ and ${\varphi _{yy}}$ represent corresponding co-polarized reflection phases. After rotating the meta-atom with an arbitrary angle $\theta $, $\; R_0^{LP}$ will be transformed as follow
where $M(\theta )= \left( {\begin{array}{cc} {\cos \theta }&{ - \sin \theta }\\ {\sin \theta }&{\cos \theta } \end{array}} \right)$ is the rotation matrix. Besides, the Jones matrix is described as $R_\theta ^{CP} = \left( {\begin{array}{cc} {{R_{RL}}}&{{R_{RR}}}\\ {{R_{LL}}}&{{R_{LR}}} \end{array}} \right)$ based on CP basis, where ${R_{RL}}$ and ${R_{LR}}$ denote cross-polarized reflection coefficients under LCP and RCP incidence, and ${R_{LL}}$ and ${R_{RR}}$ denote co-polarized reflection coefficients. According to the relationship between LP and CP waves, $R_\theta ^{CP}$ can be calculated as follows:In order to couple CP waves into surface waves, a PB PGM composed of 6${\times} $30 super cells is constructed, which has a size of 180${\times} $180 mm2, as shown in Fig. 3(a). Each super cell consists of five similar meta-atoms which are obtained by rotating meta-atom I counterclockwise with the rotation angles changed from 0$^\circ $ to 144$^\circ $ with a variation step of 36$^\circ $, as depicted in Fig. 3(b). From meta-atom I to meta-atom V, the co-polarized reflection phases have a decreasing interval of about 72$^\circ $ for LCP incident waves and an increasing interval of about 72$^\circ $ for RCP incident waves in 6-11 GHz, which are consistent with the results of Jones matrix analyses, as shown in Figs. 3(c)-3(d). In the light of generalized Snell’s law, the parallel wave vector ${k_{/{/}}}$ of reflected waves can be expressed as [14,34]
where ${k_0}$ represents the wave vector in free space and $\alpha $ represents the incident angle. $\nabla \varphi $ is the phase gradient of the super cell, which can be written as $\nabla \varphi = d\varphi /dx$. Here, two adjacent meta-atom’s phase difference $d\varphi $ is 72$^\circ $, and the distance $dx$ is equal to meta-atom’s period p. The incident waves will be coupled into surface waves by PB PGM composed of super cell when ${k_{/{/}}}$ is greater than ${k_0}$. More particularly, the condition of coupling surface waves will become $\nabla \varphi > {k_0}$ if the incident angle $\alpha $ is 0. The super cell we designed can satisfy this condition below 10 GHz because its phase gradient $\nabla \varphi $ is 209 m-1, which means the PB PGM can convert CP incident waves into hybrid modes surface waves in 6-10 GHz. And it's worth noting that the surface waves converted from LCP waves and RCP waves will propagate along the opposite directions, which results from the facts that the meta-atom has opposite phase shift for orthogonal CP waves after rotation and the surface waves propagate along the direction of co-polarized reflection phase decrease [24–25].Fig. 3. Designs and simulations of super cell and PB PGM. (a) The top view of PB PGM. (b) The top view of super cell, which consists of meta-atom I-V. (c) Simulated co-polarized reflection phases of meta-atom I-V under LCP incidence. (d) Simulated co-polarized reflection phases of meta-atom I-V under RCP incidence.
In the simulations of the PB PGM, all the boundary conditions are set as perfectly matched layer. Under the normal illumination of LCP waves, hybrid modes surface waves are induced on the surface of the PB PGM and propagate to the left, and the simulated results at 8.5 GHz are shown in Figs. 4(a)-4(b). For RCP incident waves, hybrid modes surface waves are excited and propagate to the right, as shown in Figs. 4(c)-4(d). Because arbitrary LP waves can be considered as the composition of LCP waves and RCP waves with equal proportion [35], hybrid modes surface waves will be coupled and propagate to the both sides when arbitrary LP waves illuminate onto the PB PGM. Compared with a metal plate of the same size, the simulated reflectivity of the PB PGM under y-polarized incidence is reduced by more than 10 dB in 7.3-9.5 GHz, and the reflection reduction is larger than 24 dB at about 9 GHz, which is depicted in Fig. 5(a). The reason for low reflection is that the PB PGM convert y-polarized waves into hybrid modes surface waves and propagate them to the two sides from 7.3 GHz to 9.5 GHz, which is also demonstrated by the simulation results shown in Figs. 6(a)-6(d). And we can observe from Figs. 6(e)-6(h) and Fig. 5(a) that the phenomena of x-polarized incident waves are basically same as y-polarized incident waves, which proves that the designed PB PGM can couple arbitrary LP waves into surface waves and achieve low reflection in 7.3-9.5 GHz. The deviation of x-polarized and y-polarized incident waves’ performances mainly results from the limited simulation accuracy and different arrangements of super cell in x and y directions.
Fig. 4. Simulations of PB PGM under CP incidence at 8.5 GHz. (a)-(b) Simulated Ez and Hz field distributions under LCP incidence. (c)-(d) Simulated Ez and Hz field distributions under RCP incidence.
Fig. 5. The reflection reduction spectra of PB PGM under y-polarized and x-polarized incidence. (a) Simulated reflection reduction spectra. (b) Measured reflection reduction spectra.
Fig. 6. Simulations of PB PGM under LP incidence. (a)-(b) Simulated Ez and Hz field distributions under y-polarized incidence at 7.3 GHz. (c)-(d) Simulated Ez and Hz field distributions under y-polarized incidence at 9.5 GHz. (e)-(f) Simulated Ez and Hz field distributions under x-polarized incidence at 7.3 GHz. (g)-(h) Simulated Ez and Hz field distributions under x-polarized incidence at 9.5 GHz.
3. Experiment and discussion
To further validate the performance of the surface waves coupler we designed, a prototype with the same size as the simulation model is fabricated, as shown in Fig. 7(a). The ISL is processed by laser etching technique, and the SWCL is processed by printed circuit board technology. And the ISL and SWCL are respectively pasted on the two sides of FR4 dielectric substrate. An experimental system is constructed to measure the reflectivity and electric field distribution, which mainly includes a vector network analyzer, a scanning coaxial probe and a pair of LP horn antennas. In order to test the reflection of the prototype, we measure the reflection of a metal plate which has the same dimension as the prototype for normalization at first, and then we obtain the measured results of the prototype under the normal illumination of y-polarized and x-polarized waves, as shown in Fig. 7(b). It is clear in Fig. 5(b) that the reflection reductions of both polarizations are basically larger than 10 dB in 7.3-9.5 GHz, and the measured results shown in Figs. 7(c)-7(f) indicate that the coupler converts y-polarized and x-polarized incident waves into surfaces waves in this broad frequency band. All these results illustrate that the broadband surface waves coupler we designed is able to couple arbitrary LP waves into surface waves and realize low microwave reflection in 7.3-9.5 GHz. The discrepancies between the simulated and measured results mainly come from the sample assembly and experimental system construction.
Fig. 7. Prototype, microwave experimental setups and results. (a) The fabricated prototype of the surface waves coupler. The details of the ISL are observed by optical microscope. (b) The setups for measuring the reflection. (c)-(d) Measured Ez field distributions with a range of 120×50 mm2 under y-polarized incidence at 7.3 GHz and 9.5 GHz. (e)-(f) Measured Ez field distributions with a range of 120×50 mm2 under x-polarized incidence at 7.3 GHz and 9.5 GHz.
In the light of the Kirchhoff’s law, the infrared emissivity is equal to the absorptivity. Hence, the infrared emissivity of the fabricated sample can be calculated by measuring its reflection spectra because its transmission can be ignored. We use the FTIR spectrometer to measure the reflection of the prototype from 3 to 14 µm, and the calculated emissivity given in Fig. 8(a) is less than 0.28 in the entire test band. As shown in Fig. 8(b), a TSS-5X infrared emissivity meter is also used to obtain the average infrared emissivity, and the measured results is 0.28. which is basically consistent with the result measured by FTIR spectrometer. The reason why the measured infrared emissivity is slightly higher than the theoretical calculation by Eq. (1) is mainly due to the imperfect fabrication. In order to further verify the infrared performance, the prototype is photographed by infrared thermal imager G120EX working from 8 to 14 µm at about 80°C. As we can see from Fig. 8(c) and Fig. 8(d), the infrared radiation of the prototype with ISL is obviously weaker than that of the prototype without ISL, which shows that ISL can effectively reduce the infrared radiation of the prototype. In brief, the above experimental results prove that the proposed broadband surface waves coupler has low infrared emission in the band of 3-14 µm.
Fig. 8. Infrared experimental results. (a) The infrared emissivity from 3 to 14 µm obtained by FTIR spectrometer. (b) The TSS-5X infrared emissivity meter and corresponding test result. (c)-(d) The infrared thermal image of the prototype with ISL and prototype without ISL at about 80°C.
4. Conclusion
In this paper, we propose a broadband surface waves coupler which realizes low infrared emission from 3 to 14 µm and low microwave reflection in 7.3-9.5 GHz. The surface waves coupler can couple arbitrary LP incident waves into hybrid modes surface waves to reduce microwave reflection. The high filling ratio of the ITO arranged on the surface of ISL reduces the infrared emissivity. Besides, the broadband surface waves coupler can also be used as a CP discriminator because orthogonal CP waves can be coupled into hybrid modes surface waves and propagate along the opposite directions. The results of simulations and experiments basically accord with theoretical predictions and validate the feasibility of our design. Compared with other radar-infrared bi-stealth schemes, our design has the advantages of simple construction, small thickness and low scattering into the incident half-space. We firmly believe that the methodology of this exotic surface waves coupler will have practical application prospects in multispectral stealth.
Funding
National Natural Science Foundation of China (12004437, 61901508, 61971435, 62101589); National Key Research and Development Program of China (2020JQ-471).
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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