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Spatially resolved acoustic spectroscopy (SRAS) is an acoustic microscopy microstructural-crystallographic characterization technique commonly used in the study of crystalline or polycrystalline materials. The technique can provide information about the structure and crystallographic orientation of the material. Traditionally, the information provided by SRAS has been acquired by using diffraction techniques in electron microscopy.

Laser ultrasound

In a SRAS measurement two lasers are used, one for the generation of acoustic waves and one for the subsequent detection of these waves.

An optical amplitude grating, illuminated by the generation laser, is imaged on to the specimen surface. The incident light is

Microstructure imaging

Orientation mapping

Having measured the SAW velocity in multiple directions the challenge is then to convert this information into the measurement of crystallographic orientation. The direct solution of this inverse problem is difficult.

The acoustic velocity of a known material in a known orientation can be calculated analytically. We use a method outlined by Farnell. This method is used to calculate SAW and pseudo-SAW velocities on different crystallographic planes by using iterative search procedures and the known materials' elastic constants. The method requires the iteration through different SAW velocities in a specified range to find out which velocities satisfy the boundary conditions. Using this method we can calculate the velocity surface for any crystallographic orientation.

The output from the model gives us an idea of w which wave modes e.g. surface waves, pseudo surface waves and leaky modes that may propagate for that orientation. We have to work out which of these our experiment will be sensitive to and that depends on he type of detector used. for example our current detector is sensitive only to the out of plane motion of the waves, so we calculate the out of plane motion for all of the modes found during the search and choose the dominant mode. This then gives us an indication of the expected velocity surface for that crystallographic orientation that will be measured with our experiment.

To find an orientation from experimental data we fit our experimentally obtained velocity surface to a database of velocity surfaces in all possible orientations until we get a good match.