Infrared photothermal heterodyne imaging in thermally thick medium for thermo-optic property characterization

Measuring temperature fields in semi-transparent media requires the knowledge of material thermo-optic properties. While the current techniques are well established for isothermal thin films, they remain unexplored for thick media where temperature gradients need to be considered. This study proposes to generalize the current methods of infrared heterodyne photothermal imaging to thick media by combining transmittance measurements and thermo-optic modeling to measure simultaneously the thermoabsorbance and thermoreflectance coefficients in semi-transparent media. In this work, we demonstrate the validity of our methodology followed by the measurements of the thermo-optic properties of several materials: soft polymers (polydimethylsiloxane) and borofloat glasses.