Generation of ultrasound by an expanding plasma

The use of pulsed lasers to generate ultrasound in the non-contact regime is a well documented technique. A variety of lasers have been used to generate ultrasound in a range of materials, but all so far have allowed the laser to impinge directly on the sample or on a constraining layer in contact with the sample. It is also well known that the plasma generated at the surface of a sample when hit by a fast pulse of laser light is the most efficient mechanism for generation of longitudinal ultrasonic waves. One of the factors that has limited the use of lasers in ultrasonic generation is the fact that they can damage the surface of a sample. This damage can range from pitting or cracking the surface to the removal of a surface coating, such as a paint. We report here a technique that uses the plasma created by a focused pulsed laser beam to generate ultrasound without the sample actually being illuminated directly by the laser light.