10. Ultrasonics, Surface Waves
A coherent laser probe provides the basis for the recording of complex SAW distributions. Using three or more scans, the surface wave velocity can be deducep with an accuracy of a few parts in 105. Such measurements are sufficiently sensitive to detect small changes in surface characteristics; as an example, results will be presented on the effect of doping a Si surface. It is also possible to improve the accuracy of the basic elastic constants of the material by reference to the velocity characteristic. Of particular importance is the fact that this technique provides evidence on the effective value of these constants close to a surface; it is, therefore potentially useful for surface layer characterisation. Surface defects can be detected by means of scattered waves in both the forward as the reverse directions. In principle, either can be used to "image" the defect. Using both these components, the defect size and location can be determined with improved accuracy.
Ameri, S.; Ash, Eric A.; Htoo, U.; Murray, D.; and Wickramasinghe, H. K., "Laser Detection and Imaging Techniques for Surface Examination" (1980). Proceedings of the DARPA/AFML Review of Progress in Quantitative NDE, July 1978–September 1979. 55.