Transmission Acoustic Microscopy Investigation

Abstract

The nature of acoustic contrast, i.e. the connection of the amplitude and phase of the output signal of the acoustic microscope with the local values of the acoustic parameters of the sample (density, elasticity, viscosity) is a central problem of acoustic microscopy. Two approaches have become established to the analysis of formation of the output signal of the microscope. In one of these approaches (C. Quate, 1978; A. Atalar, 1979; H. Wikramasinghe, 1979) the converging beam is regarded as an ensemble of plane waves and its interaction with the sample is described by the methods of Fourier optics. The output signal is expressed in an integral manner through the coefficient of reflection at the immersion liquid-sample interface (reflection mode) or through the coefficient of transmission through the sample (transmission mode). The coefficients of reflection and transmission carry the basic information concerning the acoustic properties of the sample. Such an approach provides a possibility of constructing a strict quantitative theory of output signal formation for objects with plane boundaries. Mathematical difficulties confine the applicability of this approach, in the main, to numerical calculations. The ray approach (R. Wilson and R. Weglein, 1984; W. Parmon and G. Bertoni, 1979), based on geometrical optics, appears to be descriptive. Within the framework of this approach the focused beam as an ensemble of rays interacting with the object. The incident rays are reflected and refracted on the object boundaries in the usual manner some of the rays, falling on the interface at definite angles, excite side waves in the sample; outflowing surface waves or waveguide modes (of Sezawa or Lamb type), etc. Propagating along the surface of the sample, such waves are re-radiated again in-to the immersion liquid and participate in the formation of the output signal of the microscope.