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A transient grating technique has been used to measure film thicknesses in five multilayer film structures. Through-plane acoustic responses are observed through time-resolved diffraction that reveals their return to the free surface after partial reflections within the structure. Optical heterodyne detection with a controlled reference field amplitude permits optimization of the signal/noise ratio relative to detection of the signal against the far stronger incident probe field or against a dark background (homodyne detection). In-plane acoustic waves are also generated and observed.
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Slayton et al. (2001) studied this question.
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