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Four-Way Spoof Surface Plasmon Polaritons Splitter/Combiner 98 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 29, NO. 2, FEBRUARY 2019 Four-Way Spoof Surface Plasmon Polaritons Splitter/Combiner Shi-Yan Zhou, Sai-Wai Wong , Senior Member, IEEE, Jing-Yu Lin , Student Member, IEEE, Lei Zhu , Fellow, IEEE,YejunHe , Senior Member, IEEE, and Zhi-Hong Tu , Member, IEEE Abstract— Corrugated metal with period grooves supports mode [2], [3]. Recently, there are many types of SSPP spoof surface plasmons polariton (SSPP) wave in microwave transmission lines reported [4], [5]. These designed SSPPs- and terahertz ranges. In this letter, a four-way SSPP splitter/ based devices have the virtues of the controllable ultra-wide combiner with oval-ring periodic structures is presented by using a single connection junction. Up to the authors’ best operating frequency bandwidth [6], low transmission loss [7], knowledge, a four-way SSPP splitter/combiner is never reported and low crosstalk loss [8]. in the literature. In order to verify the performance of the The multiple way splitter/combiner [9]–[11] plays an impor- mentioned SSPP splitter/combiner, we have manufactured and tant role in the microwave components and integrated circuits, tested the presented SSPP splitter/combiner. The simulation and which can be used in array antennas, phase shifter, power measurement results demonstrate that the designed conversion between a single coplanar waveguide junction and four SSPP amplifiers, and so on. The SSPP devices have attracted great waveguides is high efficiency. The SSPP splitter equally separates interest in recent years. The radio frequency front-end com- the electromagnetic wave into four paths. Hence, the proposed ponents of SSPP antennas [12], [13] and SSPP amplifier [14] four-way splitter is expected to develop the SSPP active and have been investigated. In addition, two two-way SSPP power passive components and relative integrated circuits. dividers [15], [16] have been designed. Those SSPP power Index Terms— Coplanar waveguide (CPW), four-way splitter/ dividers consist of one composite SSPP waveguide and two combiner, spoof surface plasmon polariton (SSPP). symmetrical branches. However, those reported methods are not convenient to separate the SSPP transmission line into I. INTRODUCTION more channels. URFACE plasmon polariton (SPP) is the confined wave In this letter, we present an SSPP splitter to realize the four Sthat propagates along the interface between substrate and equal divisions of SPP EM waves in a single printed circuit metal in optical frequency. The SPP wave has the abil- board metal layer. In previous studies, the proposed two- ity to confine electromagnetic (EM) fields to subwavelength way power divider [16] split one oval-ring SSPP waveguide dimensions, enabling the fabrication of miniaturized photonic into two half oval-ring branches. To increase the transmission devices and highly integrated optical circuits [1]. However, paths, we design the conversion section with four half oval- the metal with uniform perfect electric conductor property ring SSPP waveguides by using single coplanar waveguide does not support the SPP at low frequencies. In order to take (CPW) connection junction. The simulation and measurement advantage of the unique property of SPP in the microwave and show that the proposed four-way splitter equally divides an terahertz bands, the conductors with periodic holes or grooves EM wave into four ways in the operating frequency band. The are used to propagate spoof surface plasmon polariton (SSPP) proposed four-way SSPP splitter/combiner has the potential to further facilitate the study of the SSPP components and Manuscript received September 28, 2018; revised November 15, 2018; systems in microwave and terahertz frequency bands. accepted December 5, 2018. Date of publication January 4, 2019; date of current version February 14, 2019. This work was supported in part II. DISPERSION GRAPH by the Shenzhen Science and Technology Programs under grant JCYJ 20180207154824493, in part by the Natural Science Foundation of Guang- As is shown in Fig. 1, the dispersion curves of the SSPP dong Province under Grant 2018A030313481, in part by Shenzhen University transmission lines and the light line are calculated by the Research Startup Project of New Staff under Grant 20188082, in part by the NTUT-SZU Joint Research Program under Grant 2018009, and in part by eigenmode solver of the commercial software CST. The the Fundamental Research Funds for the Central Universities under Grant SSPP transmission line consists of cascaded half oval-ring- 2017ZD044 and Grant 2017ZD055. (Corresponding author: Sai-Wai Wong.) shaped cells, whose material is copper with the thickness S.-Y. Zhou, J.-Y. Lin, and Z.-H. Tu are with the School of Elec- tronic and Information Engineering, South China University of Technology, of 0.017 mm. The thickness of the underlying substrate is Guangzhou 510640, China. 0.8 mm, and the relative permittivity is 2.55 (1 + i0.0029). S.-W. Wong and Y. He are with the College of Information Engineer- At the low frequency, the wave vectors kx of the light line and ing, Shenzhen University, Shenzhen 518060, China (e-mail: wongsaiwai@ ieee.org). the SSPP transmission lines are very close to each other. As the L. Zhu is with the Department of Electrical and Computer Engineering, frequency increasing, the curves of the SSPP transmission University of Macau, Macau 999078, China. lines gradually move away from the curve of the light line. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. This implies that the wavelength of the SSPP transmission line Digital Object Identifier 10.1109/LMWC.2018.2886318 is shorter than the wavelength of the light line. Due to this 1531-1309 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. ZHOU et al.: FOUR-WAY SSPPS SPLITTER/COMBINER 99 Fig. 3. (a) Photography of the designed four-way SSPPs splitter/combiner. (b) Simulated near-field distributions at 7.5 GHz. Fig. 1. Dispersion graph of light line and designed SSPP transmission lines with a long axis radius of different lengths. Periodic SSPP cells are constructed which is described in Fig. 2(a). The whole structure is sym- by half oval rings, in which rx = 1.2, dy = 0.5, and p = 2.1 (all in mm). metric along the center vertical axis. Fig. 2(b) marks the dimensions of four identical transition sections at four output ports. Fig. 2(c) marks the dimensions of the 1-to-4 transition section at input port. The transition section is required to match the wave vector and the impedance between the CPW mode and the SSPP mode. As illustrated in Fig. 1, the cutoff frequency gradually decreases when the long axis radius of the ellipses increases. Thus, several oval-ring cells with gradient long axis radius are adopted to match the wave vector. In order to match the impedance, the stepped impedance is added between the CPW and the SSPP waveguides. And then, compare with the design of two-way splitter in [16], the flaring ground parts are replaced by two SSPP waveguides with gradient long axis radius. The CPW can be thought of as two slot lines, feeding EM energy to four SSPP waveguides. Meanwhile, the SSPP transmission line with double gratings fed by a slotline is reported in [17]. The power division ratio is controlled by the angle θ1 as shown in Fig. 2(d). When the angle θ1 is equal to 36.4°, an equal power division is obtained. Meanwhile, when the θ1 increases, the power division ratio of port 2/3 to port4/5 increases. IV. EXPERIMENTAL RESULTS Fig. 2. Configuration of a four-way SSPPs splitter/combiner. (a) Front view (left) and cross-sectional view (right) of the four-way splitter, l = 168.6, In order to verify the properties of the designed four-way w = . = . = . = . = . = . 107 7, W1 52 7, W2 98 0, W3 52 7, W4 17 65, k 9 65, SSPP splitter/combiner, we have fabricated and tested the L1 = 15, L2 = 3.66, L3 = 15, L4 = 16.78, θ2 = 45°, R2 = 28, R3 = 24, and R = 24. (b) Dimensions of the conversion section connected to port 5, circuit. As shown in Fig. 3(a), the five ports of the proposed 4 S2 = 5.0andD2 = 0.4. (c) Dimensions of the conversion section connected four-way splitter are connected to 50- SubMiniature version = . = . = . = . θ = to port 1, feed W 2 3, c 6 0, S1 5 0, D1 0 4, and 1 36°. A connectors. The simulated near-field distribution of the (d) Relationship between the angle θ1 and the power division ratio of port 2/3 to port 4/5 (all in mm). proposed splitter is shown in Fig. 3(b). The simulated electric field of Ez component is 2 mm above the copper layer. It is obvious that the fed EM energy is divided into four-branch feature, SSPP devices make a contribution to the development equal division. The S-parameters of the SSPP splitter are mea- of miniaturization and high integration. Moreover, as the long sured by the Agilent N5230A vector network analyzer. We first axis radius of the ellipse increases, the cutoff frequency of the tested the transmission coefficient |S21|. The port 1 and port 2 SSPP transmission lines gradually drops to a lower frequency. are connected to the vector network analyzer. Meanwhile, Wherein, the cutoff frequency of the SSPP transmission line the rest ports (port 3, port 4, and port 5) are connected to three is 12.5 GHz when the long axis radius is 5.2 mm. 50- matching loads, respectively. According to the same method, the |S31|, |S41|,and|S51| are subsequently obtained. III. FOUR-WAY SSPP SPLITTER/COMBINER Fig.
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