A Critical Analysis of the Thermoacoustic Microscope

A one-dimensional analysis of the mechanisms by which acoustic waves are generated in the thermoacoustic microscope is presented. Thermal waves and acoustic waves are treated as eigen-modes of the system, so that there is no acoustic wave generated by thermal waves in the bulk. It is then shown that there are four channels by which the incident energy can be converted into acoustic energy which can produce an image. One is purely acoustic, and the others all involve mode-conversion of thermal waves to acoustic waves. An important consequence is that thermoacoustic microscope images can reflect different combinations of physical parameters than do images made with microscopes based on pure thermal wave scattering. We comment on the ability of scattering mechanisms to exhibit resolution which is much better than either the thermal or acoustic wave length.