The exact depth for the stress generation depends, in particular, on the material involved and the optical pump wavelength. In metals and semiconductors, for example, ultrashort-timescale thermal and carrier diffusion tends to increase the depth that is initially heated within the first ~1 ps.  Acoustic pulses are generated with a temporal duration approximately equal to the acoustic transit time across this initially heated depth, in general greater than the optical absorption depth. For example, the optical absorption depths in Al and GaAs are ~10 nm for blue light, but the electron diffusion depths are ~50 and 100 nm, respectively. The diffusion depth determines the spatial extent of the strain pulse in the through-thickness direction. The main generation mechanism for metals is thermal expansion, whereas for semiconductors it is often the deformation potential mechanism. In piezoelectric materials the inverse piezoelectric effect, arising from the production of internal electric fields induced by charge separation, may dominate. When the optical spot diameter "D", for example "D"~10 µm, at the surface of an elastically isotropic and flat sample is much greater than the initially heated depth, one can approximate the acoustic field propagating into the solid by a one-dimensional problem, provided that one does not work with strain propagation depths that are too large (~"D"²/Λ=Rayleigh length, where Λ is the acoustic wavelength). In this configuration—the one originally proposed for picosecond ultrasonics—only longitudinal acoustic strain pulses need to be considered. The strain pulse forms a pancake-like region of longitudinal strain that propagates directly into the solid away from the surface. 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. The use of both shear and longitudinal pulses is advantageous for measuring elastic constants or sound velocities. Shear waves may also be generated by the use of elastically anisotropic solids cut at oblique angles to the crystal axes.