Shear wave generation by mode conversion in picosecond ultrasonics: Impact of grain orientation and material properties

Abstract

Picosecond ultrasonics has seen wide application in the investigation of interfaces in semiconductors and 3D imaging in biological systems. Shear ultrasonic waves are important for elastic constant and grain orientation measurement in ceramic materials. In this study, we investigate the impact of grain orientation and material's elastic properties on the generation efficiency of shear waves by mode conversion at a transducer thin film/substrate interface. The solution of acoustic wave equations suggests that crystal grain orientation has a strong impact on the generated shear wave amplitude. This dependence is found to be closely related to the magnitude of longitudinal component of wave displacements. The applicability of analytical model is validated by the experimental results from time‐domain Brillouin scattering. Moreover, material properties determine acoustic wave amplitudes based on the acoustic mismatch model and, particularly, large elastic anisotropy defined by Zener ratio favors strong shear waves. In the light of this analysis, several recommendations on suitable grain orientations and film materials are made to facilitate shear wave detection.

Photoacoustic 3-D imaging of polycrystalline microstructure improved with transverse acoustic waves

“…Generation efficiency of transverse acoustic waves has been recently studied in Ref. [68] . By considering multiple crystallite orientations, an estimate of the Brillouin frequency (BF) range can be obtained for each acoustic mode ( Table 1 ).…”

Optical polarization perturbed by shear strains of ultrasonic bulk waves in anisotropic semiconductors: Multiphysics modeling and optoacoustic validation

“…On the other hand, it is worthy of attention that Eq. (7) requires further generalization if the interested orientation along the z -axis is not symmetric [51] . Additionally, is the phase velocity of the longitudinal mode in the MS-substrate along the z -axis, which is given by where represents the element in the third row and third column of the silicon stiffness matrix with respect to the global coordinate system (G-CSYS), as shown in Fig.…”

“…This has motivated recent endeavors to explore and make use of the shear strains induced by OA-UBW in crystalline materials, with paradigms including nondestructive evaluation of microscopic elastic properties of monocrystalline silicon [50] , calibration of the crystalline grain orientation of transparent ceria ceramic substrates [51] , and sound-velocity based work function analysis for semiconducting two-dimensional electronic materials [52] . Success of these applications has further inspired the use of shear strains for characterization of MS. Gusev [53] theoretically demonstrated that the plane shear waves can be generated in a submicron layer made of an elastically isotropic materials, by means of a laser-induced transient grating, to quantify the shear rigidity and shear viscosity of the layer.…”