Influence of diffraction on low thermal diffusivity measurements with infrared photothermal microscopy

We consider a photothermal microscope using infrared detection for local thermal diffusivity measurements. In order to extract a reliable thermal diffusivity estimate from the recorded temperature profile, we show that diffraction arising from the Cassegrain objective of the microscope must be taken into account. Indeed, the apparent temperature profile is deformed significantly when the thermal diffusion length approaches the size of the diffraction pattern. This is especially a problem for low diffusivity samples because avoiding high-noise figures at very low modulation frequencies implies small thermal diffusion length measurements. The measurement of the impulse response of the microscope allows us to correct the recorded profile for diffraction and detector size effects. In particular, whereas the amplitude profile is smoothed only in the excitation beam area, the phase profile is globally distorted, yielding up to a 60% thermal diffusivity overestimate if neglected. Furthermore, nonlinear infrared emission contributes to enhance the impact of diffraction. Validation of our model is done with a Y2O3-doped ZrO2 sample by comparing the estimation at different modulation frequencies and also with the well-established photothermal beam deflection technique.