Abstract
The crossed‐beam thermal lens is used for the measurement of minute optical absorbance within very small probe volumes. In this technique, a cylindrically‐symmetric temperature rise is produced within a sample from absorbance of a modulated pump laser beam. This heated region is probed at right angles with a second, cw laser beam. Defocusing of the probe beam caused by the heated sample is detected as a periodic change in the far‐field probe beam‐center intensity. Since the two beams only interact in their intersection volume, very small regions may be probed with tightly focused beams. By recording the thermal lens signal as an inhomogeneous sample is scanned through the probe volume, an image is generated based upon the absorbance of the sample. Furthermore, by recording the phase of the signal, it is possible to generate an image based upon thermal diffusivity. Images of histopathological and geological samples are presented.
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