On dipole source representation for laser-generated sound

It is normally assumed that at the thermo-optical regime of laser-generated ultrasound in solids the equivalent thermo-optical source can be modelled as a mechanical dipole comprising two horizontal forces applied to the surface in opposite directions. In the present paper this assumption is re-examined on the basis of the rigorous theory that takes into account all acoustical, optical and thermal parameters of a solid material and all geometrical and physical parameters of a laser beam. It is shown that the dipole-source representation is rather limited, especially in respect of generated longitudinal waves. A general criterion has been established under which the dipole-source representation gives predictions with acceptable errors.