Photothermal Techniques in Material Characterization Applications

Abstract

Photothermal (PT) effects are produced by the heating a sample and its surroundings due to the absorption of electromagnetic radiation, and is a type of energy conversion. Using a laser beam for PT generation, many PT effects can be measured, including refractive-index gradients, acoustic emission, surface deformation, reflectivity changes, desorption, and “grey-body” infrared emission, providing useful techniques for materials characterization. These PT material probing or characterization techniques generally rely on the use of high-sensitivity detection methods, involving the use of “probe” laser beams, transducers, or infrared detectors to monitor the effects caused by PT heating of a sample. Many of these PT effects occur simultaneously, e.g., PT heating of a sample in air will produce temperature rise, photoacoustic waves and refractive-index changes in the sample and in the adjacent air, infrared thermal radiation increase, etc., all at the same time. Thus, the choice of a suitable PT effect for detection will depend on the nature of the sample and its environment, the light source used, and the measurement desired. We have previously reviewed [1,2,3] the various experimental arrangements and detection schemes for the different PT effects, and given a summary of possible applications. We have shown that it is possible to obtain high PT generation efficiencies using a short-pulsed laser beam; various sensitive detection schemes have also been developed to monitor the resultant temperature or pressure transients in the sample or its vicinity.