Using an optical technique ultrafast vibrations of the surface of thin opaque films in the 0.1-THz range have been directly observed in the time domain. We propose a new type of phonon detection by monitoring the surface velocity, and demonstrate an exceptional sensitivity to atomic-scale inhomogeneities by observing an anomalous reflection from a buried film of monolayer order in thickness.

• Received 29 June 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.1668