Some Aspects of Lateral Waves Generation and Detection by Acoustic Microscopy Using V(z) Technique

In the non scanning configuration, the characteristic response V(z) of the acoustic microscope treated as a signature is a valuable method for measuring quantitatively on a microscopic scale the elastic properties of a sample and for explaining the contrast observed in surface images [1]. This signature is obtained by recording the reflected acoustic signal variation V as a function of the sample defocus z and arises from an interference between a bulk wave specularly reflected at the normal direction and generally a leaky Rayleigh wave generated near and at the critical angle θ R at the liquid sample interface and reradiated in the liquid medium at precisely θ R. V(z) is periodic and its periodicity is directly connected to the leaky Rayleigh wave velocity VR in the material. In a recent work [2], we have pointed out that depending on both the material and the liquid loading, other modes in addition to the leaky Rayleigh mode can contribute with a significant efficiency to the interference mechanism in V(z) signature. These modes result in waves known as “longitudinal lateral waves” or “surface skimming bulk waves” generated at the longitudinal critical angle θ L and involving radiation of energy in the liquid. They propagate along the interface with a velocity equal to that of the sample longitudinal bulk wave. Owing to the complex reflectance function R(θ) variation, we have shown that longitudinal lateral waves appear when a phase transition exist at θ L.