Directivity patterns of laser-generated ultrasound in aluminum

Directivity patterns of laser-generated longitudinal and shear waves in aluminum have been determined experimentally, for laser power densities both above and below the threshold for plasma formation on the target’s surface. Comparison of experimental results with theory has allowed generation mechanisms to be identified. Thermoelastic mechanisms predominate in the absence of a plasma, where stresses parallel to the surface are a dominant source characteristic. At higher power densities, momentum transfer from ablating material becomes more important, leading to a source with normal drive components.