The coupled influence of sample heating and diffraction on thermal diffusivity estimate with infrared photothermal microscopy

We consider the influence of sample heating on spatially resolved thermal diffusivity measurements with a photothermal microscope using infrared detection. The continuous component of temperature induced by an intensity modulated focused laser beam results in a sensitivity amplification in the laser beam area and contributes to enhance the effect of diffraction on the phase profile, yielding a thermal diffusivity overestimate if it is not considered. A model including diffraction and nonlinear infrared emission is presented. For moderate heating, the influence of sample heating on the phase shift and on the amplitude of the infrared signal as seen by the detector is calculated. By using this model for fitting successively the amplitude and the phase data, no significant change in thermal diffusivity value is found on a homogeneous Y2O3-doped ZrO2 sample when the modulation frequency is changed. Validation of our model is done by comparing experimental amplitude data with the expected profiles obtained with the experimental absorbed power and with the reference thermal conductivity estimate for cubic zirconia. (C) 1998 American Institute of Physics.