Low-Temperature Acoustic Microscopy

Abstract

Abstmt-The resolution of the acoustic microscope is presently limited by the sound wavelength in the coupling fluid between the lens and sample. Cryogenic fluids offers two advantages over room temperature fluids for use in acoustic microscopy: low sound speed and low acoustic attenuation. Liquid nitrogen, argon, and helium have been used for microscopy, and they are all described. In liquid nitrogen and liquid argon, images have been obtained at frequencies as high as 2.8 GHz with a corresponding wavelength of 3000 A . A nonlinear effect was discovered in these liquids (as well as water) that improves the resolution of the microscope beyond the linear diffraction limit. Liquid helium emerges as the “ultimate” fluid for high-resolution acoustic microscopy because of its near zero acoustic attenuation at very low temperatures. Operating at temperatures less than 0.2 K, imaging with 300-A wavelength sound has been achieved. Applications include detection of thermal phonon emission from surfaces and general purpose high-resolution imaging with excellent sensitivity to slight topographical features.