 Strain pulses returning to the surface from buried interfaces or other sub-surface acoustically inhomogeneous regions are detected as a series of echoes.For example, strain pulses propagating back and forth through a thin film produce a decaying series of echoes, from which one may derive, in particular, the film thickness, the ultrasonic attenuation or the ultrasonic dispersion.The original detection mechanism used in picosecond ultrasonics is based on the photoelastic effect.