Optimized, Omnidirectional Surface Acoustic Wave Source: 152° Y-Rotated Cut of Lithium Niobate for Acoustofluidics.

Abstract

Here, we propose an optimized Y -rotated cut of lithium niobate (LN) for multidirectional surface acoustic wave (SAW) propagation, simultaneously minimizing the anisotropic effects while maximizing the electromechanical properties of this cut of LN. The goal is to offer a piezoelectric material suitable for acoustofluidics applications that require greater flexibility in wave generation and propagation than the currently ubiquitous 128° Y -rotated X -propagating cut. The 128YX LN cut is known to most effectively generate Rayleigh SAW along the x -direction alone. Any SAW veering from this propagation direction is affected by beam steering and changes in resonance frequency and electromechanical coupling coefficients, consequently limiting the use of LN in various acoustofluidics applications, where more diverse configurations would be beneficial. The L2 -norm of these properties was evaluated under rotational transformation to produce a physical model with closed governing equations for 40-MHz surface wave propagation on the surface of a piezoelectric material. This was then utilized to obtain the surface wave velocity and coupling coefficient of the specific Y -cut LN with respect to the propagating direction. Next, the averaged coupling coefficients of various Y -cuts of LN in all propagating directions were calculated and integrated to simultaneously minimize anisotropy and maximize the electromechanical properties of the LN substrate. A 152° Y -rotated cut was found to be the optimal choice under these constraints, enabling multidirectional SAW propagation with greater coupling and lower variation in wave performance for SAW generated across the surface in any direction. Compared with the 128YX LN cut, this cut provides a 66.5% improvement in the in-plane isotropy and a 37.0% improvement in the average electromechanical coupling for in-plane SAW propagation. Experimental devices operating at the frequency of 40 MHz were designed, fabricated, and tested on the surface of this 500- [Formula: see text]-thick specific cut of LN and served to verify the supporting analysis and the superior isotropic properties of the 152° Y -rotated cut in generating SAW.