Theory of the photopyroelectric method for investigation of optical and thermal materials properties

The photopyroelectric (PPE) method is a spectroscopic- and thermal-property investigation technique based on a photothermal effect, consisting of the heating of a sample via nonradiative deexcitation processes following absorption of radiation. The resulting temperature increase is measured by a pyroelectric transducer placed in thermal contact with the sample. The theory for a periodically modulated excitation is developed, assuming a one-dimensional heat-flow process in a model cell composed of six layers and having a finite reflectance at the interface between the sample and the pyroelectric material. Special effort was devoted to the exhaustive analytic and numeric analysis of the general expression for the PPE signal and its special cases, as a function of dimensionless thermal and optical parameters of the sample. The great diversity of the typical response types is shown to originate in the interplay between saturation, optical absorption, and optical or thermal transmission effects. The usefulness of the theory relies on its ability to predict and to interpret the results which can be obtained in a large variety of PPE experimental configurations.