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 reexamined 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.