Photothermal refraction is a novel thermo-optical technique that measures absorbance from localized regions within larger samples. In this technique, a modulated pump laser beam and a coplanar continuous-wave probe laser beam cross at right angles within the sample. Absorbance of the pump laser beam generates a time-varying localized temperature rise within the sample. This heated region acts as a thermal cylindrical lens that periodically defocuses the probe beam and results in a change in the probe beam center intensity. Since the pump and probe beams interact only in their intersection volume, a localized measure of sample absorbance is produced. A high spatial resolution image of sample absorbance may be made by recording the photothermal refraction signal as the sample is scanned through the intersection of the laser beams.
Dean S. Burgi,
Thomas G. Nolan,
Jon A. Rusfelt,
Norman J. Dovichi,
"Photothermal Refraction For Scanning Laser Microscopy," Optical Engineering 23(6), 236756 (1 December 1984). https://doi.org/10.1117/12.7973377