By scanning the surface of a specimen cooled to liquid-helium temperature with the electron beam, ballistic phonons are generated which can be used for acoustic imaging. The anisotropy of the ballistic phonon propagation caused by the phonon focusing effect has been observed in recent experiments. The simultaneous operation of two or more small-area phonon detectors during the scanning process appears promising for three-dimensional acoustic imaging of structural inhomogeneities even far from the specimen surface. The principles of this scheme for acoustic imaging are discussed and the spatial resolution limits are estimated.
Huebener, R. P. and Metzger, W.
"The Ballistic Phonon Signal in Low Temperature Scanning Electron Microscopy,"
Scanning Electron Microscopy: Vol. 1985
, Article 13.
Available at: https://digitalcommons.usu.edu/electron/vol1985/iss2/13