0885-3010 (c) 2018 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TUFFC.2018.2832174, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control Influence of Coupling between Rayleigh and SH SAWs on Rotated Y-cut LiNbO 3 to Their Propagations Yulin Huang, Student Member, IEEE, Jingfu Bao, Member, IEEE, Xinyi Li, Student Member, IEEE, Benfeng Zhang, Student Member, Gongbin Tang, Student Member, IEEE, Tatsuya Omori, Member, IEEE, and Ken-ya Hashimoto, Fellow, IEEE Abstract— This paper investigates influence of coupling between Rayleigh and SH SAWs on rotated Y-cut LiNbO3 to their electromechanical coupling. For the purpose, a COM model including the coupling between two SAWs is developed, and the coupling influences to their excitation in addition to electromechanical coupling factors are discussed. Then its validity is confirmed by comparison of its results with the FEM analysis. Comparing with conventional COM shows this extended COM model could fit the complex value area much better in dispersion curve analysis. After that, variation of COM parameters with the device design is discussed. Then, all COM parameters are fitted by polynomial equations and the procedures to find the optimal rotation angle are obtained in terms of the SH mode suppression. It indicates that the optimal angle changes rapidly at certain Cu thickness, which is due to decoupling between two SAW modes. At last, structure based on low cut LiNbO3 is discussed and the validity of this COM model is further confirmed. Keywords—mode coupling; LiNbO3; COM; K 21 I. INTRODUCTION This paper is an extended vision of [1]. Suppression of the spurious surface acoustic wave (SAW) response is mandatory to design SAW devices on  degree rotated Y-cut LiNbO3 [2]-[15]. At  ~ 128°, the Rayleigh SAW is the main mode and the shear-horizontal (SH) SAW should be suppressed [2],[4],[8],[13],[14],[15]. [15] designed a This work was supported by the National Natural Science Foundation of China and the China Academy of Engineering Physics (U1430102). X.Y. acknowledges the support of the Japanese Government (MEXT) for the scholarship through the Super Global University Project. (Corresponding author: Y. Huang) Y. Huang and X. Li are with the Department of Electronic Engineering, University of Electronic Science and Technology of China, No.2006, Xiyuan Ave, West Hi-Tech Zone 611731, Chengdu, Sichuan, China, and Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan (e-mail: huangyulin@chiba-u.jp; kimilee56@gmail.com). J. Bao is with the Department of Electronic Engineering, University of Electronic Science and Technology of China, No.2006, Xiyuan Ave, West Hi-Tech Zone 611731, Chengdu, Sichuan, China (e-mail: baojingfu@uestc.edu.cn). B. Zhang and G. Tang are with the Department of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China, and Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan (e-mail: bfzhang@chiba-u.jp; gongbin.tang@chiba-u.jp) T. Omori is with the Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan (e-mail: omori@faculty.chiba-u.jp). K. Hashimoto is with the Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan, and the Department of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail: k.hashimoto@faculty.chiba-u.jp). Digital Object Identifier small sized band 20 SAW duplexer with low acoustic velocity Rayleigh SAW. They optimized the cut angle of LiNbO3 at 120° for their multilayer electrode structure by Finite Element Method (FEM) simulation. While at  ~ 15°, the SH SAW is the main mode and the Rayleigh SAW should be suppressed [3]-[7],[9]-[12]. [12] realized a SAW resonator having moderate K 2 (8%) and zero temperature coefficient of frequency (TCF) with complete suppression of the Rayleigh-mode response by properly setting the top shape of the SiO2 layer. However, it is not easy because the electromechanical coupling factor K 2 for SH SAW changes rapidly with , and its optimal  in terms of the SH SAW suppression is also depending on the electrode material and thickness h. In [1] the authors studied the relation of K 2 for SH SAW on rotated Y-cut LiNbO3 substrate with its structure parameters, and found that the optimal  in terms of the SH SAW suppression always locates close to the situation where the phase velocities of these two SAWs are close to each other. This fact implied that coupling between these two SAWs plays an important role on variation of K 2 which changes significantly with  and h. This paper investigates influence of the coupling between Rayleigh and SH SAWs to their K 2 in detail. For this purpose, the Coupling-of-mode (COM) theory [16] is extended to include the coupling between two SAWs. The paper is composed as follows: In Section II, it is shown how K 2 for SH SAW changes with  and h for rotated Y-cut LiNbO3, and described why the authors thought the variation is related to the coupling between two SAWs. In Section III, it is discussed how the coupling influences the two modes excitation in addition to propagation. In Section IV, SAW dispersion curves are derived by FEM, and COM parameters including two SAW coupling are determined by fitting for the Cu grating/ degree rotated Y-cut LiNbO3 substrate structure. Validity is confirmed by their comparison, and the parameters are express by the function of Cu thickness hcu and . In Section V, the condition for complete SH-SAW suppression is derived in a closed form, and it is demonstrated how the two SAW coupling influences to the optimal  with hcu in terms of the SH suppression. Discussions are given for the case where  is close to 128 o and the Rayleigh SAW is the main mode from Section III to Section V. In Section VI, discussions are extended to the case where  is close to 15 o and the SH