Complex analysis of reflection coefficient for the characterization of layered materials using a line-focus-beam acoustic microscope

In the material characterization by a line-focus-beam acoustic microscope, objective of the measurement is the phase velocity and attenuation of leaky surface acoustic waves. This paper investigates how the analytical property, namely poles, zeros and branch cuts, of an acoustic reflection coefficient affects the measurement of the leaky Rayleigh wave and other modes excited on a specimen with layered structure. A pseudo-Sezawa wave is suitable for the accurate measurement, if fd(f: frequency, d: layer thickness) is large. On the other hand, a Rayleigh wave is recommended for the measurement of a specimen with a thin layer. We also find that the pole and zero reach to a close vicinity of the longitudinal branch point when the thickness approaches to zero, and this is responsible for a velocity difference between a bulk longitudinal wave and a leaky surface skimming compressional wave. A specimen with a “fast” layer on a “slow” substrate is also theoretically investigated