Optical generation and detection of elastic waves

Since the first work on the generation of elastic waves without mechanical contact in a solid (e.g. by impact of an electron beam or electromagnetic waves), experimental studies have progressed thanks to the use of more powerful optical sources such as lasers, and more sensitive detectors. Theoretical models have been developed to explain the generation mechanisms. Compared with traditional methods (piezoelectric transducers), photoacoustic generation offers several advantages: in addition to requiring no mechanical contact, the position and shape of the source can be modified. Elastic waves can be generated in materials heated to high temperatures. This technique is currently used for non-destructive testing, measurement of elastic constants, acoustic emission, spectroscopy and microscopy.

In most experiments, the solid is irradiated with light pulses. Volume and surface waves are generated in this way. These waves are detected either by conventional transducers (piezoelectric, capacitive, electromagnetic-acoustic) or by optical methods. Optical measurements, which are the subject of the second part of this article, have the advantage of being made at a distance with a wide bandwidth, without disturbing the acoustic field. The combination of optical generation and detection is potentially very important in the field of non-destructive testing. Examples of applications are given at the end of the article.