In the early 1990s the method was extended in scope at Nippon Steel Corp. by direct sensing of the picosecond surface vibrations of the film caused by the returning strain pulses, resulting in improved detection sensitivity in many cases. Advances after the year 2000 include the generation of picosecond acoustic solitons by the use of millimeter propagation distances and the generation of picosecond shear waves by the use of anisotropic materials or small (~1 μm) optical spot sizes. Acoustic frequencies up to the terahertz range in solids and up to ~ 10 GHz in liquids have been reported.