 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.