Acoustic microscopy at cryogenic temperatures

The acoustic microscope operating in superfluid helium now has a resolution of 500 Angstroms. The images from this instrument are superior to those in an optical microscope and exhibit some features that rival those of the SEM. Immediate progress should permit us to move up in frequency by a factor of two and improve the resolution accordingly. Another mode of imaging is described which exploits thermal phonons in solids and holds promise of providing information on the interior details of solid structures.