High resolution photothermal laser probe

At frequencies near 1 GHz, the acoustic wavelength in argon gas becomes half that of visible light, making possible collinear Bragg scattering of laser light by the sound. This acousto-optic interaction has been used to detect photothermally generated acoustic power at the surface of solids. This new technique provides a noncontacting probe for the detection of 100-1000-Å thermal waves with optical resolution. The essential features of the theory behind this new photothermal probe are presented along with a description of the experimental implementation. High resolution images of implanted silicon are included which clearly demonstrate the sensitivity of the probe to variations in the thermal characteristics of solids.