Elastic band gaps for surface modes in an ultrasonic lithium niobate phononic crystal S. Benchabane a *, A. Khelif a , L. Robert a , J.Y. Rauch a , T. Pastureaud b and V. Laude a a Institut FEMTO-ST, D´ epartement LPMO, CNRS UMR 6174, Universit´ e de Franche-Comt´ e, Besan¸con,France b TEMEX, 399 Route des Crˆ etes, BP 232, F-06904 Sophia Antipolis Cedex, France ABSTRACT If a number of experiments aiming at demonstrating fundamental properties of phononic crystals have been successfully implemented, a need for enlarging both the research and the application fields of these structures has more recently risen. Surface acoustic waves appear as appealing candidates to set a new ground for illustrative experiments involving some different physical concepts from those usually observed when dealing with bulk waves. The possibility of a direct excitation of these surface waves on a piezoelectric material, and their already extensive use in ultrasonics also make them an interesting basis for phononic crystal based, acoustic signal processing devices. In this work, wave propagation in a square lattice, piezoelectric phononic crystal consisting of air holes etched in a lithium niobate matrix is both theoretically and experimentally investigated. The crystal was fabricated by reactive ion etching of a bulk lithium niobate substrate. Standard interdigital transducers were used to characterize the phononic structure by direct electrical generation and detection of surface waves. A full band gap around 200 MHz was experimentally demonstrated, and close agreement is found with theoretical predictions. Keywords: Phononic crystals, surface acoustic waves, lithium niobate machining 1. INTRODUCTION Photonic crystals have proved their relevance as actual optical devices over the past decade, 1–3 leading to an increasing interest in their elastic or acoustic counterparts, the so-called phononic crystals. These are also two- or three-dimensional periodic structures made of two materials with different elastic constants which can give rise to absolute stop bands under specific geometrical conditions. In addition to the usual band gap material behavior, such as guiding or filtering as observed in electromagnetic structures, phononic crystals exhibit some specificities pertaining to elastic waves inherent properties. The strong anisotropy of the propagation and the usually mixed shear and longitudinal wave polarization combine with the intrinsic effects of the crystal periodicity to affect wave scattering. In particular, strong effects such as total internal reflexion phenomena can occur due to the very large contrast that can be obtained between the elastic parameters of the constitutive materials. Void inclusions in a solid matrix, for instance, provide us with a realistic way to achieve these sharp changes in the propagation conditions. If significant research effort has already been devoted both theoretically and experimentally to phononic crystals, even leading to the demonstration of novel acoustic phenomena, 4–6 first demonstrations of their physical properties have generally remained in the audible and ultrasonic range, with millimetric or larger wavelengths. 6–10 Although phononic crystals have recently moved to the hypersonic region 11 and now appear as interesting candidates for high frequency signal processing applications, most of the theories and experiments related to this topic have often remained devoted to bulk acoustic waves (BAW) for which the external boundaries enclosing the phononic crystal do not play a significant role in wave propagation. Elastic stop bands for the well-known surface acoustic waves (SAW) have only been studied more recently. 12–16 SAW are guided along the surface of a solid or liquid material and are hence confined in the direction normal to the surface. As a consequence, a two-dimensional periodic structuration of the surface can be expected to provide at least to a certain extent the same phononic properties for SAW as would a three-dimensional phononic crystal for BAW. Phononic devices relying on surface acoustic waves in piezoelectric materials are of particular interest as they can *sarah.benchabane@femto-st.fr, www.femto-st.fr, www.lpmo.edu