Picosecond Ultrasonics

When an ultrashort light pulse (duration: ~ 100 fs, energy: <1 nJ) is absorbed at the surface of a thin metal film, the resulting thermal expansion of the surface results in the generation of a strain pulse (composed of longitudinal acoustic phonons) that propagates into the film and the substrate. For a 15 nm Al film grown on a Si substrate this pulse will have a peak frequency and bandwidth both ~ 100 GHz, and a strain amplitude of about 1x10-4. Reflections from buried interfaces will return to the surface, where they can be detected by means of a time delayed optical probe pulse. This phenomena was first studied in depth by researchers in Humphrey Maris’ group at Brown University in the late 1980’s, and is currently used in a thin film metrology technique (with nm resolution) that sees widespread use in the microprocessor industry.