Wide-Angle Frequency Scanning Metasurface Antenna Fed by Spoof Plasmonic Waveguide

We propose novel wide-angle frequency scanning metasurface antenna and antenna array fed by spoof plasmonic waveguide. The spoof plasmonic waveguide adopts the ultra-thin corrugated metallic strip which can support and propagate spoof surface plasmon polariton (SSPP) with high efficiency. By compensating the momentum difference between SSPP and free-space propagation modes via constant phase gradient, metasurface antenna can exhibit wide-angle and continuous radiation property according to the Generalized Snell’s law. A prototype is designed, fabricated and measured, which consists of a wideband polarization rotating transmissive phase gradient metasurface (TPGM) placed a certain distance above spoof plasmonic waveguide. Such antenna works in a wideband range from 6.8 to 14.0 GHz, and the main beam can continuously scan from the forward direction to the backward direction with high efficiency. The measured results show that the scanning angle can reach 55°, which agree well with the simulated and theoretically calculated results. Furthermore, the proposed metasurface antenna can be combined easily as an antenna array with a simple feed network. Compared to single metasurface antenna, the antenna array exhibits the pencil-shaped directional radiation property, and the average gain of antenna array is obviously enhanced. The proposed frequency scanning metasurface antenna is of great value in the design of conformal antenna and the large aperture antenna array, especially on curved faces.


I. INTRODUCTION
Surface plasmon polaritons (SPPs) are highly localized electromagnetic (EM) wave bounded along an interface of metal and dielectric medium at the optical frequency band [1], [2]. Due to their unique properties of field enhancement and energy localization in sub-wavelength scales, SPPs have important potential applications in super resolution imaging, bio-sensing, photonics, etc [3]- [5]. However, SPPs cannot be excited effectively on flat metallic plate at lower frequencies.
To overcome this obstacle, artificial structured surfaces decorated by periodic grooves, slits, or holes have been studied to support and propagate spoof SPPs (SSPPs), which have similar properties of SPPs at optical frequencies [6]- [9].
The associate editor coordinating the review of this manuscript and approving it for publication was Lin Peng .
Recently, the ultra-thin corrugated metallic strip (CMS) fed by a coplanar waveguide has been investigated by both simulation and experiment, which can realize broadband and high-efficiency conversion from the guided wave to SSPPs [10], [11]. Phase gradient metasurface can also convert a propagating wave into SSPP via pre-defining wave vector along the surface [12], [13]. Especially, SSPP meta-coupler based on a transmissive phase gradient metasurface (TPGM) can convert the incident propagating wave to the eigen-mode SSPP with high conversion efficiency of 73% according to the near-field and far-filed experiment, which can suppress both decoupling and surface initial reflections [14], [15].
According to the principle of reversibility, SSPPs can also be converted into free-space radiation, which has potential applications in leaky-wave antennas [16]- [21], [25]- [29]. Continuous beam scanning leaky-wave antenna can be VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ achieved by circular patches loaded periodically along one side of the SSPP transmission line. That is because it can achieve an additional momentum to modulate the SSPP wave into a spatial wave [11], [16]. Cui et al demonstrated that periodically-modulated spoof plasmonic waveguide structures composed of non-uniform CMS have the ability of converting SSPPs to spatial wave [17]. Recently, a wideangle frequency scanning antenna is proposed based on a hybrid mode of a quasi-TEM mode and a SSPPs mode [18]. The wide-range scanning in a narrower band can be realized due to the nonlinear dispersion curve of SSPPs [18]- [21]. However, above antennas cannot modulate polarization characteristic. Phase gradient metasurface is a two-dimensional planar array composed of sub-wavelength element structure, which can freely control the polarization mode, wave-front, propagation direction, propagation mode [22]- [24]. Therefore, it also can use the phase gradient as an additional momentum for converting the slow-wave mode to a fast-wave mode and then to modulate the SSPPs to radiation wave. In 2015, Cui et al have achieved efficient conversion of surface-plasmon-like modes to spatial radiated modes by controlling phase modulations produced by the reflective phase gradient metasurface (RPGM) [25], [26]. Qu et al have also proposed a frequency scanning antenna by decoupling SSPP via RPGM [27], [28]. The radiation angles are modulated by both the nonlinear phase gradient and the wave vector of SSPP. In order to achieve wideband frequency scanning characteristic and higher efficiency, SSPP planar antenna based on wideband TPGM is proposed [29]. The antenna consists of TPGM placed a certain distance above an SSPP guided wave structure. In addition, it has a polarization conversion characteristic by using polarization conversion TPGM as both the radiating and receiving panel. However, this antenna is also fed by rectangular waveguide, which will lead to a larger antenna profile and feeding structure in lower frequency regime, especially in the meter-wave regime. Furthermore, the dispersion of SSPP on the metallic grating structure cannot be modulated flexibly by changing parameters of the metallic grating structure, which will limit the working bandwidth and scanning range.
In this paper, we propose an alternative method for designing frequency scanning metasurface antenna fed by spoof plasmonic waveguide. The configuration of such antenna is composed of a TPGM placed a certain distance above a spoof plasmonic waveguide. The spoof plasmonic waveguide adopts the CMS fed by coplanar waveguide here and the dispersion of SSPP on CMS can be controlled easily by height of strip. By compensating the differences between wave vector of SSPP on the ultra-thin CMS and spatial propagating modes by using transmissive phase gradient, the SSPP can be converted to directional radiation. Compared to the existing planar leaky-wave antenna, our antenna has several advantages. Firstly, since the radiation angles are mainly regulated by the nonlinear dispersion of SSPP on CMS and wideband phase gradient, it can be easier to realize a continuous wide-angle and wideband frequency scanning. Secondly, it has a polarization conversion characteristic by using polarization conversion metasurface. Thirdly, owing to low loss, low profile and simple fabrication, the method also possesses potential applications in plasmonic waveguide communication. Lastly, since the TPGM doesn't need the electrical connection, it is easy to combine the large aperture antenna array with simple feed network. To verify this idea, wide-angle and wideband directional radiation antenna and antenna array are designed and measured by using wideband linear polarization conversion TPGM as radiated panel. Both simulated results agree well with the measured results.

II. DESIGN PRINCIPLE
The working principle of our designed frequency scanning metasurface antenna is based on transmissive phase gradient modulation on eigen-mode SSPP. In essence, SSPPs are non-radiative EM modes, whose wave vector k sspp is larger than that in free space k 0 . The SSPP mode can be converted into a radiated mode when wave vector of SSPPs is reduced to radiation region under certain conditions. Fortunately, TPGM can provide pre-defined phase gradient on the surface, which can compensate the wave vector difference between k sspp and k 0 at the same frequency. That means perfect momentum matching can be achieved. According to the working principle analyzed above, the schematic diagram of metasurface antenna is shown in Fig. 1, which consists of TPGM and spoof plasmonic waveguide. Here, the spoof plasmonic waveguide also adopts the CMS fed by coplanar waveguide. It has been demonstrated that waves on CMS have similar EM behavior as TM-mode SSPPs [10]. According to the generalized Snell's law [22], the radiation angle of frequency scanning antenna can be calculated by a formula as [26]- [28] where the ξ is the transmissive phase gradient. The radiation angle is mainly controlled by the nonlinear dispersion of SSPP on the CMS and phase gradient generated by TPGM, both of which can be designed separately. Therefore, a wideband and wide-angle beam scanning from backward to forward can be easily achieved by elaborate designing of CMS and phase gradient ξ .

III. DESIGN OF FREQUENCY SCANNING METASURFACE ANTENNA A. DESING OF TRANSMISSIVE PHASE GRADIENT METASURFACE
In order to validate this idea, we design a wide-angle and wideband frequency scanning metasurface antenna. For comparison to SSPP planar antenna fed by rectangular waveguide [29], we adopt the same TPGM as radiated panel.
Such TPGM can generate wideband and constant phase gradient along the x direction from f = 6.8 GHz to 14 GHz [15]. As shown in Figure 2(a), the sub-unit cell is composed of two-layer F4B dielectric (ε r = 2.65, tanδ = 0.001, t = 3mm) and three metallic layers. When the y-polarized wave normal incidence, it can efficiently convert into wideband x-polarized transmission wave due to both multiple plasmon resonances and Fabry-Perot-like resonances [30]. Here five different sub-unit cells are selected to construct the super cell. The optimized parameters of the super cell are (l, d, g) = (7.7mm, 1.2mm, 0.2mm), (7.7mm, 2.6mm, 0.2mm), (7.5mm, 3.55mm, 0.15mm), (7.6mm, 2.3mm, 0.2mm), (7.8mm, 3mm, 0.2mm), respectively. The other parameters are the same with q = 6mm, m = 2mm, b = 0.2mm and w = 0.2mm. Figure 2(b) shows the cross-polarized transmission phase for a y-polarized normally incident EM wave. It can be clearly seen that the phase change step is 2π/5 along each sub-unit cell from 6.8 to 14.0 GHz. Therefore, these five sub-unit cells compose a super cell to realize 2π phase change and the phase gradient can be calculated as ξ = 2π/L = 209.3 m −1 .
In order to generate phase gradient along the −x direction, the 5 sub-unit cells are arranged in reverse to that of Ref [15], as shown in Fig.2(c). Since the phase gradient can hold nearly constant in a wide frequency range, the radiation angles are mainly modulated by the dispersion of SSPP, which is different to Ref [27], [28]. The total size of the TPGM here is set to be 300 mm×30 mm, consisting of 10 super unit cells along x axis and 5 unit cells along y axis.

B. DESING OF SPOOF PLASMONIC WAVEGUIDE
The spoof plasmonic waveguide is carefully designed, as shown in Fig.3, which has a similar structure as discussed in Ref [10]. It has been confirmed to be able of converting the guided EM wave to SSPPs in a wideband with high efficiency due to good momentum and impedance matching. The spoof plasmonic waveguide consists three parts, in which l 1 = 8 mm, l 2 = 56 mm, and l 3 = 300 mm. The whole spoof plasmonic waveguide is printed on flexible dielectric substrate F4B (ε r = 2.65 and tanδ = 0.001) with the thickness 0.2mm. Note that the spoof plasmonic waveguide does not have metallic back sheet. The part I is the coplanar waveguide as shown in Fig. 3(b), which is designed to achieve 50 impedance with the dimensions w c = 3 mm, g c = 0.25 mm, and h c = 4.7 mm. The part II is the transition with gradient grooves and flaring ground to achieve good momentum and impedance matching as shown in Fig.3 8 mm, and h 9 = 1.9 mm, respectively. The curve of flaring ground is described as where C 1 = (y 1 − y 2 )/(e αx 2 − e αx 1 ) , C 2 = (y 2 e αx 1 − y 1 e αx 2 )/(e αx 1 − e αx 2 ) α = 0.05, and (x 1 , y 1 ) and (x 2 , y 2 ) are the start and end points of the curve. The part III is the double-side CMS without the metallic back is shown in Fig. 3(a) and inset of Fig. 3(d). The desired dispersion relation of CMS for the TM-polarized waves propagating along the x direction can be approximately obtained as following [10]: where p, a, and h are the period, width, and depth of the grooves, respectively. When 0 < k 0 h < π/2, k sspp shall be a real number and k sspp > k 0 . Hence, the dispersion relation of SSPP on the CMS can be easily tuned by the depth of grooves. Thus, propagation characteristics of SSPP on CMS can be manipulated easily through changing the depth of grooves h. Here, the parameters of CMS are set to p = 2.5mm, a = 1.0 mm, h = 1.9mm and s = 0.9mm, respectively. Figure 3(d) gives dispersion curves of SSPP on the CMS and TPGM-modulated CMS, respectively. As shown in Fig. 3(d), the red curve shows the dispersion relation of SSPP on the CMS, while the black curve gives the dispersion relation in free space. It can be seen that the wave vector of SSPP k sspp is much larger than that of the free space k 0 . The constant phase gradient ξ designed along -x direction can compensate momentum difference between the SSPP and propagating waves. The blue curve shows the dispersion of SSPP on the modulated CMS by the transmissive phase gradient ξ . Within the band of 6.8-14.0 GHz, the modulated dispersion relation (blue curve) is shifted from the slow-wave region to the fastwave region (the green zone), which means the SSPPs on the CMS can be decoupled to directional radiation wave in this region according to the generalized Snell's law. Here, only zero-order dominant mode of SSPP dispersion is utilized to realize the desirable radiation [28]. It can also be seen that our antenna has the capability of scanning from backfire to endfire. Since waves on CMS have similar EM behavior as TM-mode SSPPs with E-field mainly distributed in the xoz plane [10], [28], the TPGM is placed above the spoof plasmonic waveguide with a certain distance for the mode matching. The distance is set to d M = 1.6 mm by optimization.

C. RESULTS AND ANALYSIS
We carry out both simulation and experiment to verify the properties of metasurface antenna designed above. The simulations are carried out by using Computer Simulation Technology (CST) Microwave Studio. Fig.4(a) illustrates the sample of whole antenna and the inset view shows TPGM and spoof plasmonic waveguide structures in detail. As can be seen from the front view of the sample, the spoof plasmonic waveguide is placed in the middle of the TPGM. Simulated and measured results of the reflection coefficient are shown in Fig.4(b). In the 6.8-14.0 GHz frequency range, S 11 is nearly lower than -10 dB, which means that most energy of SSPP is radiated out. The experimental result is almost in agreement with the simulated one and the little deviation may result from fabrication and measurement error.
In order to further verify the working principle of the metasurface antenna, we monitor the near-field and far-field distribution of the antenna at f = 10.0 GHz as shown in Fig.5. From the electric field E z distribution shown in    5(a), we observe that incident EM wave is firstly coupled to high-efficiency TM-mode SSPPs with the E-field mainly distributed in the xoz plane and then the SSPPs propagate along the CMS. When SSPPs transmit to the TPGM, the high-efficiency directional radiations are generated by gradually decoupling SSPP on the CMS by phase gradient. In addition, since we use the linear polarization rotation TPGM as the radiated panel, the TM-mode SSPP finally converts into the y polarization radiated wave with the radiation angle of 16 • at f = 10 GHz. This can be validated by electric field E y distribution shown in Fig.5(b). Fig.5(c) and (d) show the 3D radiation realized gain pattern of metasurface antenna and antenna without the TPGM above the spoof plasmonic waveguide. By comparison, it can be seen obviously that the  high-efficiency directional radiation is achieved with the peak realized gain of 16.5 dB by metasurface antenna as shown in Fig.5(c).
We have also simulated and measured normalized H-plane radiation patterns at different frequencies as shown in Fig. 6. It can be seen that the main lobe in the H-plane can continuously steer from backward to forward as frequency changes from 6.8 GHz to 14.0 GHz, which proves that SSPP can be decoupled to directional radiation wave. The measured results agree well with the simulated results. Fig. 7 presents H-plane main lobe direction of antenna obtained by theoretical calculation (red line), simulation (black line) and measure (blue star), respectively. The theoretical calculation is based on Eq. (1) by using dispersion relation of SSPP on CMS with h = 1.9 mm as shown in Fig. 3(d). It can be seen from the theoretical calculation that the antenna is capable of scanning from −10 • to 45 • in the frequency range of 6.8-14.0 GHz, which is in good agreement with the measured and simulated results. Fig. 8 shows simulated results of the peak gain curve, radiation efficiency and total efficiency of the designed metasurface antenna in the band of 6.8-14.0 GHz, respectively. As can be seen from Fig. 8(a), the average gain of the SSPP planar antenna is 15.8 dB and the maximal gain reaches 17.9 dB at f = 12 GHz. As can be seen from Fig. 8(b), the total efficiency and radiation efficiency of designed antenna are greater than 80%, with the maximal total efficiency of 92%, which is higher than that of the beam scanning of patch array in Ref [11]. Hence, the designed antenna has properties of high gain and high efficiency.

IV. METASURFACE ANTENNA ARRAY
As we can see from Fig.2(c), sub-unit cells of TPGM as the radiated panel do not require the electrical connection, they can be divided and assembled easily. Therefore such antenna is convenient for transportation and application. Inspired for this, we can combine some single metasurface antennas to an array with a simple feed network. To validate this idea, we use eight identical metasurface antennas designed above (marked as A-1) to assemble an array along y axis, as shown in Fig.9(a) (marked as A-8). The sample of metasurface antenna array A-8 is shown in Fig.9(b), which has a larger size of 300 mm×240 mm, consisting of 10 super-unit cells along x axis and 40 unit cells along y axis. The antenna array is fed individually by eight spoof plasmonic waveguides. And the spoof plasmonic waveguides are fed simultaneously by eight coplanar waveguides. Therefore there is no mutual couple between the ports. The SSPP can also be decoupled by TPGM. According to antenna array theory, the radiation pattern of uniformly linear antenna array depends on the array pattern. Its normalized array pattern can be expressed as where ψ denotes phase difference in the radiation of adjacent units and N denotes the number of unit antenna. According to Eq. (3), the larger the number of unit antenna N , the narrower the radiated beam of antenna array becomes. The simulated and measured results of the reflection coefficients are shown in Fig.9(c). The simulated S-parameters are all nearly lower than −10 dB in the 6.8-14.0 GHz frequency range, the bandwidth of which are the same with the single antenna A-1. Note that the S-parameters have little difference to each other due to the less mutual near-field coupling. The measured result agrees with the simulated ones. We also monitor the far-field radiation of the antenna array at f = 10 GHz as shown in Fig.9(d). We can see that the radiation pattern of antenna array changes from sector-shaped to pencil-shaped compared to single antenna A-1 as shown in Fig.5(c). High-efficiency directional radiation is achieved due to antenna array along y axis. The peak of realized gain enhances to 26.2 dB, which is higher than that of the single antenna A-1 with 9.7 dB at f = 10 GHz. Therefore the gain can be further improved by antenna array. Fig.10 gives the H-plane main lobe direction results of metasurface antenna A-1 and antenna array A-8 by simulation and measure, respectively. It can be seen obviously that the directional radiation angles of antenna array A-8 follow the Generalized Snell's low. Besides, the radiation angles of   either simulated or measured results are in good agreement with that of the single antenna A-1. We also compare the peak gain of metasurface antenna A-1 and metasurface antenna array A-8 by simulation. Compared to the antenna A-1, the gain of antenna array A-8 has been apparently enhanced, as shown in Fig.11(a). It can be seen that the average gain of antenna array A-8 is 25.2 dB, which is larger than that of single antenna A-1 with 9.2 dB. At f = 12.5GHz, the realized gain reaches its maximum with 27.5 dB. The total efficiency of antenna array A-8 exceeds 80% almost in the whole working frequency range, as shown in Fig.11(b). Furthermore, the radiation efficiency of antennas array is nearly the same as the antenna A-1. Therefore, the antenna array A-8 also has a high efficiency property.

V. CONCLUSION
We propose and demonstrate a method of achieving wide-angle frequency scanning metasurface antenna and antenna array fed by spoof plasmonic waveguide. With the modulation of constant phase gradient supplied by TPGM, SSPP on the CMS can be decoupled and directionally radiated out. The wideband and wide-angle frequency scanning metasurface antenna can be achieved by independently designing of the spoof plasmonic waveguide and wideband TPGM. Both the simulated and measured results verify the working principle. The proposed metasurface antenna works in a wideband from 6.8 to 14 GHz, and the beam can scan not only in the forward directions but also in the backward directions, in which the scan angle reaches 55 • . The total efficiency of the designed antenna can exceed 80% and the highest total efficiency of antenna can achieve 92%.