Study of Surface Acoustic Wave propagation using Synchrotron Radiation X-Ray Topography

Bernard Capelle (1) , Yves Epelboin (1) , Alain Soyer (1) , Jacques Détaint (1)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés

X-ray topography is an accurate and sensitive method to image the strain fields existing in crystals, particularly those created by bulk or surface acoustic waves (BAW or SAW). The time structure of the synchrotron radiation allows to image progressive waves having a frequency multiple of that of the synchrotron. The main purpose of this study is the understanding of the exact nature of the information obtained about SAW by means of X-Ray topography. To this end we will consider experimental and computed images obtained for SAW propagating on ST-cut quartz and YZ-LNB. Most experiments were made using wide X-ray beams. Depending on the diffraction conditions, the so-called "translation" topographs seem often as being good maps of the displacement existing near the surface of the SAW device. These images may be considered as the result of a process of spatial integration of section topographs, more simply related to the X-ray diffraction process and to the displacement gradients of the acoustic field. These section topographs, corresponding to an extremely thin X-ray beam, contain, in fact, more information about the SAW. Quite intricate section topographs of low amplitude SAW propagating on YZ lithium niobate and ST-cut quartz were experimentally obtained using the Bragg diffraction geometry (reflection). Simulations of these section topographs gave information about the displacement gradients existing near the surface (most intense contrast) and about the depth dependence of this gradient (weak contrasts). In the summation process, leading to the "translation" topographs, only the intense contrasts (representing the largest strains) contribute significantly while the weaker ones are blurred into a grey level. Thus, the main contrast in translation topographs arise mainly from the gradients of the component of the displacement along the diffraction vector, at (or very near) the surface. These derivatives are proportional to the displacement so that these topographs represent essentially, with a different phase, the corresponding displacement. When the amplitude of the SAW becomes large enough, a kinematical diffraction process explains that the most intense contrast generated near the surface is enlarged proportionally to the local amplitude. This allows a simple method to measure the transverse amplitude variations of the surface modes. The two other elastic components of the SAW are imaged using the Laue diffraction geometry (transmission) with appropriate diffraction vectors. The corresponding section topographs obtained with ST-cut quartz are also complex and again, simulated images were used to understand the contrasts observed. Similarly the summation process, leading to the translation images, is dominated by the largest displacement gradient and so leads to a representation of the lateral displacements existing near the surface. Small velocity (phase) variations most probably induced by the dislocations present in the synthetic quartz crystal, were observed in the sections images.

Mots clés: Surface Acoustic Waves; Synchrotron radiation; X-ray topography; piezoelectric waves

Domaines: Science des matériaux [cond-mat.mtrl-sci]

Bernard Capelle, Yves Epelboin, Alain Soyer, Jacques Détaint. Study of Surface Acoustic Wave propagation using Synchrotron Radiation X-Ray Topography . 2014. ⟨hal-01273414⟩