 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.