For small spot sizes approaching the optical diffraction limit, for example "D"~1 µm, it may be necessary to consider the three-dimensional nature of the problem. In this case acoustic mode-conversion at surfaces and interfaces and acoustic diffraction play an important role, resulting in the involvement of both shear and longitudinal polarizations. The strain pulse separates into different polarization components and spreads out laterally (for distances >"D"²/Λ) as it propagates down into the sample, resulting in a more complicated, three-dimensional strain distribution.