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Scanning Acoustic Microscopy: Review Of Recent Developments

Scanning acoustic microscopy, where the object to be imaged is scanned across the waist of a tightly focussed acoustic beam, has advances to the point where the resolution in water is comparable to that of a high quality optical microscope. The resolving power of the acoustic microscope can be further improved by resorting to a fluid which has a lower sound velocity than water. The two practical possibilities are cryogenic liquids, and higher pressure gases. Progress on both these fronts will be discussed. Improving the resolving power is only one aspect of our research effort: it is equally important to obtain quantitative information from the recorded images. We are working; on several schemes whereby it is possible to record the spatial frequency transmittance or reflectance of the object at the point being investigated and hence deduce the sound velocity and density on a microscopic scale. In order to make accurate predictions on the elastic parameters in certain experiments - such as the observation of stress near a crack tip - it is necessary to measure the phase of the acoustic beam with great accuracy. For this purpose, we have developed a differential interference contrast acoustic microscope which is the acoustic analogue of the optical nomarsky system. Finally, the great success of acoustic microscopy has led to the invention of many new forms of microscopy. Two such forms will be described - photodisplacement and photothermal microscopy - which are based on the microscopic measurement of some thermometric property. The use of such techniques to image current flow in integrated circuits and record optical absorption spectra of biological samples with a lateral resolution of a few microns will be described.