Investigation of Leaky-Wave Propagation in Rotated Y Cuts of LiNbO3

Abstract

LiNbO, is examined as to its suitability for ex- tended time delays using the wraparound concept. In this connection the propagation characteristics of surface waves on rotated (about the X axis) I' cuts are investigated for propaga- tion directions such that the sagittal plane is in the neighbor- hood of the YZ plane. These crystal cuts and directions of propagation are of considerable interest inasmuch as they contain planes of propagation which exhibit large electrome- chanical coupling and favorable anisotropy (extended beam collimation). These same orientations also contain directions of propagations in which leaky wave modes can appear. Leaky-wave propagation is possible for directions of propaga- tion which do not lie in the YZ plane for a range of rotated Y cuts, wherein the outward surface normal makes an angle of between -5 and -85 (or equivalently - 175 to -95) with respect to the positive Y axis of the crystal measured toward the positive Z axis. Electric probe measurement of piezo- electric surface waves have been made on rotated Y cuts which support leaky-wave propagation. Velocity profiles on three rotated Y-cut planes were obtained for each of the surface- wave, shear-wave, and leaky-wave solutions. general, the surface-wave mode is evanescent in the transverse direction so long as its velocity is less than the lowest bulk shear-wave velocity in the substrate material.5 Since the sur- face wave propagation velocity on a cylindrical surface in- creases as the ratio of the radius to wavelength approaches unity,' the lowest shear wave will be the lowest velocity mode at some point and the evanescent condition is no longer satis- fied. Mode conversion is then a distinct possibility, unless the particle displacements of the two modes are orthogonal to each other, thus preventing mode coupling. On the other hand, there exists an upper limit on the radius of curvature if mode conversion, leading to significant attenua- tion, developes along all or part of the propagation path in the curved region. In that case, and for purely practical reasons, the curved path length must be minimized by reducing the curvature radius as much as is possible, subject only to the lower limit criteria discussed previously. The large functional bandwidth and favorable anisotropy inherent in YZ LiNbOj renders it an attractive candidate for wide band information storage. It is shown in this paper that the angle between the helical propagation path (sagittal plane) and the YZ plane in LiNhO, must be small if significant losses on the curved portions of the path are to be avoided. These additional losses arise from the coupling between the surface wave and the lowest bulk shear wave in one quadrant of rotated Y cuts, where the velocity of the latter is lower than that of the surface wave and the particle motions of the two waves have a common component and are therefore coupled. EXPERIMENTAL DATA