Non-linearity in photothermal radiometric imaging

Non-linear effects occurring in photothermal microscopes based on principle of photothermal radiometry are studied as a function of the laser power and of the modulation frequency. For stainless steel, a second harmonic component of up to 11 % of the fundamental amplitude is found. Phase shifts of up to 20 degrees are observed as a function of the modulation frequency. The nonlinearity is mainly due to Stefan-Boltzmann's law. The experimental results are in good agreement with a theoretical model based on the static and oscillating temperature distribution