ABSTRACT
We propose to use shear waves instead of longitudinal waves in a novel scanning tomographic acoustic microscope (STAM) in which the specimens are solid. When a specimen with a shear modulus is immersed in the microscope's water bath, mode conversion takes place at the water-solid interface. The shear wave energy is detectable and can be used for image reconstruction. Although wave transmission in most solid specimens is limited to about 20 degrees for longitudinal waves, it is about twice that for shear waves. Also, velocities of shear waves are lower than those of longitudinal waves and hence the wavelengths at the same frequency are smaller. For these and other reasons we can expect that for many specimens the resolution of a shear-wave STAM to be substantially better than that of a longitudinal-wave STAM. We use computer simulation in order to compare the operation of a shear-wave STAM with that of the conventional longitudinal-wave STAM. We have simulated tomographic reconstruction for each. The corresponding critical angles of incidence are computed and tomographic reconstructions of a particular solid specimen is obtained by using the back-and-forth propagation algorithm (BFP). Our simulation results show that shear-wave STAM has better resolution than longitudinal-wave STAM.
ABSTRACT
Acoustic microscopy is capable of providing high-resolution images of small objects. When such a microscope operates in the transmission mode, it produces simply a shadow-graph of all the structures encountered by the acoustic wave passing through the object. The resultant images are difficult to comprehend because of diffraction and overlapping of complex structures. Scanning tomographic acoustic microscopy (STAM) overcomes these difficulties and produces unambiguous micrographs of objects of substantial thickness and complexity. STAM uses the back-and-forth propagation algorithm to reconstruct tomograms of various layers to be imaged. When these layers are physically close to one another, ambiguities appear in the reconstructed images. Using an iterative algorithm eliminates these ambiguities and resolves layers that are only two wavelengths apart.
