Photothermal measurements using a localized excitation source

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Abstract
Optical‐beam deflection (OBD) photothermal imaging uses spatially localized excitation to observe spatial variations in the sample surface temperature. This paper analyzes OBD signals produced by localized excitation in terms of three‐dimensional thermal diffusion in the sample and in the fluid region in contact with the sample. The dependence of the signals on the local optical absorption coefficient, on gas/sample thermal properties, on modulation frequency, and on the probe/excitation beam radii are discussed with special attention being given to determining the spatial resolution possible for OBD imaging. A criterion for photothermal ’’saturation’’ appropriate to localized optical absorption is developed. Finally, a new variant of the OBD technique is introduced, which is especially adapted to studying optical and thermal boundaries in the plane of the sample. Some comparisons between theory and experiment are provided which illustrate transverse thermal diffusion.